Lower-Middle Cambrian Boundary Research
Southern Great Basin
and South China
1993-2005


DR. LINDA B. McCOLLUM
GEOLOGY DEPARTMENT,
EASTERN WASHINGTON UNIVERSITY
CHENEY, WASHINGTON 99004-2439

A national mandate for the reform of undergraduate science education has been laid out over the last decade, with the National Science Foundation (NSF) persistently calling for increased emphasis on undergraduates participating in scientific research. It was imperative that the geology department and Eastern Washington University be in the vanguard of this emerging trend, so Linda B. McCollum applied for and received NSF funding in the form of two NSF RUI (Research at Undergraduate Institutions) grants, the first in 1992 and the second in 1999. To date, undergraduate students have participated in several field expeditions followed by classroom instruction and laboratory projects, with the students then presenting their work at scientific meetings.

What follows is a running commentary on the history and results from an ongoing, decade long, research endeavor into the nature of the Lower to Middle Cambrian faunas and facies of the western United States. This document is aimed primarily at future participating undergraduate students in the Cambrian Research Class, so they might get a feel for the history of the project on which they will be building. Both published articles and unpublished data can be found within several links, thus making it easily available to my students. Use of this data for other purposes is unlawful under the copyright laws, without written permission. If you're interested in joining in on this project or using some of the unpublished data, please email me <lmccollum@mail.ewu.edu

Faculty Expertise (see vita)
Dr. McCollum's expertise is in lower Paleozoic paleoecology, and she has written papers analyzing the community structure of Middle Devonian muddy bottom epeiric sea fauna just outboard of the Catskill Delta in New York (McCollum, 1988, 1991). She made several contributions to the stratigraphy and depositional environments of the Cambrian System in the Great Basin (McCollum and McCollum, 1984; McCollum and Miller, 1991), including a series of regional paleobiogeographic maps (McCollum and McCollum, 1988; McCollum, Robison, and Rees, 1988), prior to initiating this project.

Once this project began, Dr. McCollum joined two international subcommissions in order to facilitate scientific communication with colleagues interested in extinction and recovery research. The first, in 1994, was the International Geological Correlation Project 335, which concentrated on biotic recoveries from mass extinction events. The second, a year later, was the International Subcommission on Cambrian Stratigraphy, which is involved in establishing international boundaries, many of which are based on trilobite extinction events. Membership in these international groups allowed her to attend field conferences in South America, Europe, Africa, and Asia over the last several years. Drs. McCollum and Fred Sundberg presented a paper at the 1st International Palaeontological Congress in Sydney, Australia in July, which was the first visit to that continent for either of them.

Scientific Problem
The first task was to define a scientific problem in paleoecology which would merit funding in the highly competitive arena of national grantsmanship, where less than a fourth of all proposals receive funding. The second task was to involve undergraduate students in a meaningful experience, from data collection, processing, analyzing, and presenting of the conclusions. The third task would be to tempt other researchers to apply their expertise on specific aspects of a project which would involve defining overall biodiversity and completeness of the stratigraphic record in a relatively unstudied time period.

The key to a successful coalition between undergraduates and faculty-oriented field research was in proposing a well-defined research project which encompasses the fundamental concepts of scientific research and has the allure to capture undergraduate students' imaginations. Certainly mass extinction events have captured the public interest and the topic seems to fascinate earth science students, when discussed in class. The geologic record is an archive of speciation and extinction, providing a deep-time perspective on factors governing global patterns. Global extinctions and recoveries certainly have the essential elements of a great detective story, i.e., mass death; who died, who survived, and why. The top five mass extinction events had already received a great deal of study, as well as the trilobite (biomere) extinction events of the Late Cambrian. However, one of the earliest extinction events, on which the North American Early-Middle Cambrian series boundary was defined by Walcott (1890), remained virtually unstudied. This series-based biomere extinction event ended the long reign of the olenelloid trilobite clade and would be a good topic for involvement of undergraduates in faculty research.

What do we want to do and where should we do it
The most logical approach to a multi-year study of the Lower-Middle Cambrian boundary interval was to take a transect across the Cambrian shelf somewhere along an early Paleozoic continental margin. A transect would have the advantage of documenting the facies and faunal changes associated with depth changes across the shelf. As to location, two problems were immediately evident. The first problem is that the Lower-Middle Cambrian series boundary is defined by the last occurrence (LAD) of Olenellus in Laurentia and on the first occurrence (FAD) of Paradoxides in Gondwana (actually, a number of island continents probably existed during the early to middle Cambrian) and that these to index fossils did not occur in the same region, thus the boundary could be highly diachronous. The second problem was that few continuously fossiliferous sections across the boundary exist in either of these early Paleozoic continents. The only two regions known to us that had the potential for a detailed study of the Lower-Middle Cambrian boundary were in the southern Great Basin of Nevada and eastern California and in the Guizhou Province in South China. We began the first phase of our study in the southern Great Basin because of the accessibility to well-exposed sections, the nearby availability of logistical support, and relatively current literature on the Cambrian. This transect would begin near the craton and progress oceanward, ending in the outer shelf facies. If funding permitted, we would continue our second phase of this boundary research in South China.

The Lower-Middle Cambrian boundary interval within the southern Great Basin and Mojave Desert region was known to occur within six geologic formations; the Bright Angel Shale, the Cadiz Formation, the Pioche Shale, the Carrara Formation, the Monola Formation, and the Emigrant Formation. The cratonal sections include Bright Angel Shale in the Grand Canyon region of northern Arizona and the Cadiz Formation in central Mojave Desert in southern California. The Pioche Shale occurs in the cratonal to inner shelf sections in eastern Nevada and western Utah. The Carrara Formation, which occupied a middle shelf or central miogeoclinal position, occurred over a vast area of the southern Great Basin in southern Nevada and the Death Valley region of California. The Monola Formation occurs in the Saline Range and the White-Inyo Mountains region of eastern California and was deposited outboard of the Death Valley Carrara Formation and inboard of the outer shelf Emigrant Formation. The Emigrant Formation is restricted to Esmeralda County, Nevada and represents the deepest water outer shelf environment in which the Lower-Middle Cambrian boundary could be located. My husband Mike and I had already measured and collected at least one section in each of these formations during the early 1980’s under a contract with Gulf Oil Exploration and Production Company, Houston, Texas.

The cratonal sections in the Grand Canyon (McKee and Resser, 1945) and Lake Mead region (Hardy, 1986) are virtually unfossiliferous between the last occurrence of Lower Cambrian olenelloids and the first occurrence of the Middle Cambrian Albertella (Pack and Gayle, 1971). Evaluation of the facies patterns within the boundary interval of the Pioche Shale in the Virgin Mountains and at Frenchman Mountain in southeastern Nevada, and the Bright Angel Shale at the Grand Wash Cliffs near Meadview, Arizona, revealed that an erosional unconformity occurs at the Lower-Middle Cambrian boundary interval in this region (McCollum, 1999). The Lower-Middle Cambrian boundary had not yet been constrained in the Cadiz Formation, so we decided to begin our research outboard of these crational sections.

Therefore, our study would begin in the inner shelf facies of the Pioche Shale, which contains a moderately fossiliferous Lower to Middle Cambrian boundary interval in the type area, located near the historic mining town of Pioche, Lincoln County, Nevada (Merriam, 1964; McCollum and McCollum, 1990). The basic facies pattern seen in the Pioche Shale was of thin, but regionally persistent, limestone layers separating coarsening upwards clastic intervals. This alternating siliciclastic and carbonate facies pattern reflected regional environmental changes in sealevel and were coincident with changes in the trilobite faunal assemblages and sequence boundaries. Detailed documentation of these facies and faunas seemed ideally suited for study by undergraduate students.

First NSF Grant
The first NSF RUI grant EAR-9218892 was entitled "Pattern of Extinction and Replacement at the Lower-Middle Cambrian Boundary Event in the Great Basin, Western U.S." for $49,285. The Lower-Middle Cambrian series boundary had been established at the trilobite (biomere) extinction of the olenelloid trilobite fauna in Laurentia, a lower Paleozoic continent composed of present-day North America, Greenland, and portions of northwest Scotland and part of the southern Andes Mountains. The major questions to be resolved were 1) what died; 2) what survived; 3) what is the evolutionary pattern of recovery; and 4) could the cause (smoking gun) of the trilobite extinction be resolved through analysis of the sedimentary record or 5) is the sedimentary record so incomplete that the extinction is merely a stratigraphic artifact. In theory, it sounded like a lot of fun, and the undergraduate students were very enthusiastic. In reality it was a lot more work than anticipated, with some issues still unresolved.

The goal was now to determine the biodiversity of a limited stratigraphic interval which included the Olenellus trilobite biomere extinction within a relatively small geographic region. For the next few years, my undergraduate students and I would work on the Pioche Shale in Lincoln County, Nevada. This research project would include gathering field data, laboratory preparation and application of the latest techniques on data analysis. Undergraduate students would gain a firsthand knowledge of the fundamental aspects of sedimentary geology and paleontology; including a basic knowledge of taxonomy, taphonomy, sedimentology, and sequence stratigraphy.

Results of First NSF Grant
This initial study of the extinction and replacement at the Lower-Middle Cambrian boundary within the Pioche Shale is nearing completion. My students and I have measured two dozen sections in four mountain ranges, with over 5,000 fossil specimens having been collected, curated, and described. Six peer-reviewed papers describe the faunas and amend the stratigraphy of the Pioche Shale (Eddy and McCollum, 1998; Palmer, 1998, Sundberg and McCollum, 1997, 2000, 2002, 2003a), and abstracts report on other aspects of our study (Beaver and others, 2001; McCollum, 1994, 1995, 1998, 1999; McCollum and McCollum, 1994; Sundberg and McCollum, 1999; Walkley and others, 1994).

Our work on the Pioche Shale resulted in a four fold increase in the trilobite diversity (45 genera, 102 species), in defining several new biozones, naming formal members which included emendation of the lithostratigraphy, and proposing a sequence stratigraphy. For a complete listing of known and described faunas from the Pioche Shale, see Pioche Shale faunal list. Mark Webster (UC Riverside) plans to complete his faunal list of the Lower Cambrian olenelloid trilobites of the Delamar Shale Member by the late summer, 2004. Two trilobite range charts for the Pioche Shale are also provided, see Lower Cambrian and Middle Cambrian.

LOWER-MIDDLE CAMBRIAN PIOCHE SHALE RESEARCH IN NEVADA

Stratigraphy of the Pioche Shale prior to this study
The type section of the Lower and Middle Cambrian Pioche Shale occurs in the Pioche Hills located in eastern Lincoln County, Nevada. The 75 km long outcrop belt occurs within a mountain chain which trends southward along the west side of the Bristol, Highland, Chief, and Burnt Springs Ranges to Lime Mountain in the Delamar Mountains (see geologic map of Lincoln County in Tschanz and Pampeyan, 1970). The pinyon pine and juniper forests of the north give way to sage and Joshua trees to the south. Merriam (1964, p. 4-5) gives a brief history of stratigraphic investigations of the Cambrian in this region up to that time, so this narrative begins after that.

The Pioche Shale is a mixed siliciclastic and carbonate assemblage, 300-330 meters thick within the area of the type section. In a study of the Cambrian rocks of the Pioche Mining District, Merriam (1964) divided the Pioche Shale into four informal shale members and two formal members, in ascending order, D-shale, Combined Metals Member, C-shale, Susan Duster Limestone Member, B-shale, and A-shale. The Lower-Middle Cambrian boundary was placed at the contact between the Combined Metals Member and the overlying C-shale member by Merriam (1964, Table 4).

The only member of the Pioche Shale to be studied in detail, prior our work, was the Combined Metals Member. Milos Velechovsky, a Masters student at SUNY, Stony Brook, began his study of the Combined Metals Member under the supervision of Pete Palmer and later under the direction of Dick Robison (Univ. of Kansas). Velechovsky (1985) measured and described nine sections of this member, documented the existence of eight lithofacies and their faunal content and concluded that the mixed carbonate-terrigenous facies were deposited in a shallow subtidal environment.

Most of the body fossils previously reported from the Pioche Shale was from the fissile shale and bioclastic limestone intervals, which constituted less than 10% of the total lithologic thickness of the formation. A.R. Palmer (in Merriam, 1964) listed a dozen genera and 16 species of trilobites from collections made in the Pioche Shale, assignable within several faunal assemblages, ranging from the Lower Cambrian Olenellus Biozone through the Plagiura-Poliella to the Albertella Biozone. Palmer (1954) had redescribed and illustrated some of Walcott's 1886 trilobites from the Pioche Shale, although some species do not appear on the faunal lists in Merriam (1964) due to the lack horizon and locality data. Therefore, relocation of Walcott's faunal localities would be put on our first years "to do list" (see Walcott mystery).

Summary of our Research on the Pioche Shale Facies
We decided from the beginning of our research on the Lower-Middle Cambrian boundary interval to include all of the Pioche Shale faunas and facies in our study. Emendations of the member nomenclature and additions to the existing biozonal schemes would be addressed as each interval was described and then published at the beginning of the articles on the trilobite taxonomy. A comprehensive study of the sequence stratigraphy along a shelfal transect would be published separately after the regional study of the other shelfal sections found within the Carrara, Emigrant and Monola formations had been completed. We have, however, presented talks at GSA meeting outlining the criteria being used in establishing the sequence stratigraphy of the Lower-Middle Cambrian boundary interval in the southern Great Basin (McCollum, 1998, 1999; and Sundberg, McCollum, and McCollum, 2001).

All of our work on the Pioche Shale is either published or in press. New members have been described, replacing the informal member designations found in Merriam (1964), new biozones have been introduced and all of the trilobite faunas described in papers Eddy and McCollum, 1998; Palmer, 1998; Sundberg and McCollum, 1997, 1998, 2000, 2002, 2003a; Webster (in progress). The last student project in the Pioche Shale involving undergraduates was a detailed petrologic and faunal study of the Susan Duster Limestone Member (Beaver and others, 2001), although student work in this region still continuous with the study of the overlying Lyndon Limestone and Chisholm Shale.

Summary of our Research on the Pioche Shale Faunas
Despite the relatively low percentage of fossiliferous rocks, the reported faunal diversity of the Pioche Shale in Merriam (1964) turned out to be a gross underestimate, and consequently, the time and effort that was spent in describing new faunas was considerably longer than anticipated. Faunal diversity, evolutionary lineages, and the rate of speciation could only be ascertained after all of the faunas, including those new to science, had been described. This would require specialists for each of the groups of taxa to be involved in this research.

Fortunately, two preeminent trilobite taxonomists, A.R. "Pete" Palmer (Institute for Cambrian Studies, Boulder, CO) and Frederick A. Sundberg (Los Angeles County Museum), joined in on the task of describing the youngest Lower Cambrian and oldest Middle Cambrian trilobites, respectively. Work continues on the Lower Cambrian trilobites of the Pioche Shale by Mark Webster (U.C.L.A.). Even with all of this assistance, it has taken years to clear the backlog of trilobite species new to science, with the trilobite diversity now reaching 45 genera and 102 species. All of the taxonomic and phylogenetic work needed to be published prior to the final analysis of the extinction, survival and replacement within the Lower-Middle Cambrian boundary interval.

Trilobites:
Description of the Lower Cambrian, pre-extinction trilobite fauna is progressing well. Five new species of olenelloids (Palmer, 1998a), a new species of Oryctocephalites (Sundberg and McCollum, 1997), and a new genus and species of ptychopariid (Sundberg and McCollum, 2000) have been described from the youngest Lower Cambrian fauna. Mark Webster is describing several additional olenelloid species from the older horizons in the Pioche Shale. At present, the Lower Cambrian trilobite diversity of the Pioche Shale stands at 11 genera and 25 species.

Fred Sundberg and I began our work with a study of the oryctocephalid trilobites, including a phylogenetic study to help sort out the relationships of this important group (Sundberg, 2000). Oryctocephalids are found above and below the olenelloid extinction, and have a wide geographic distribution in the circum-Pacific region. We ended up describing three genera and six species of oryctocephalid trilobites from the Lower-Middle Cambrian boundary interval from several formations across the southern Great Basin, including the first recorded occurrence of Oryctocephalus indicus from Laurentia (Sundberg and McCollum, 1997). The discovery of O. indicus within our study area was to have a profound effect on the course of our future research, including an international aspect to our work.

Fred and I then went on to study the survivor fauna, which was composed of two distinct, low diversity ptychopariid trilobite assemblages, which fell between the range zones of the Olenellus and Plagiura-Poliella zones of Lochman-Balk and Wilson (1958). We therefore established two new lower Middle Cambrian biozones to accommodate these assemblages, the Eokochaspis nodosa Biozone and the overlying Amecephalus arrojosensis Biozone; described seven species assigned to three genera of ptychopariids; and introduced three new formal members to replace the informal D-, C-, and B-shale members of the Pioche Shale (Sundberg and McCollum, 2000).

Our final study of the early recovery faunas within the Middle Cambrian Poliella denticulata Biozone (replaces Plagiura-Poliella Zone) began with a study of the kochaspids, an informal group within the ptychopariid trilobites. We named one new genus, three new species, and redescribed several existing genera and species (Sundberg and McCollum, 2002). Fred is currently completing a cladistic study of the kochaspids, which is essential to the understanding of speciation within the recovery fauna (Sundberg, in press). A second paper (Sundberg and McCollum, 2003a) describes the remaining trilobite taxa, which include two new genera and six new species. This will bring the total number of trilobite taxa in the Poliella denticulata Biozone to 17 genera and 35 species.

The only thesis project to date was a description of the recovery fauna within the Albertella Biozone, which is the youngest faunizone in the Pioche Shale. Our students, my husband Mike, and I collected almost 3,000 specimens from the 100 meter thick interval at the top of the Pioche Shale, exposed in several mountain ranges in Lincoln County, Nevada. A graduate student, Julie Eddy, and I documented 21 genera and 34 species of trilobites, including 4 new genera and 7 new species, in addition to establishing the Grassy Spring Member to replace the informal A-shale member (Eddy and McCollum, 1998).

The trilobite diversity of the early Middle Cambrian portion of the Pioche Shale in our study area now stands at 36 genera and 77 species, including 8 new genera and 25 new species. Only the Glossopleura Biozone faunas remain to be studied before the trilobite diversity of the Delamaran Stage is completely documented. Work on this last biozone could commence as early as the summer of 2004, funds permitting.

The only other study of Pioche Shale trilobites was by Fritz (1968) from the northern Egan Range, about 150 kms north of our study area. After some updating of his genera based on the recent faunal literature, he has 6 genera and 8 species from the Olenellus Zone, nothing from the Plagiura/Poliella Zone, 18 genera and 34 species from the Albertella Zone, and 11 genera and 15 species from the Glossopleura Zone. The Glossopleura Zone fauna begins at or near the base of the Lyndon Limestone, which overlies the Pioche Shale in the study area (Eddy and McCollum, 1998). In September, 2002, the sixth Cambrian Research Class made addition collections of Glossopleura faunas from the type section of the Lyndon Limestone in the Highland Range.

Other Faunas:
The non-trilobite faunas are also an important element of the Pioche Shale fauna, and were in need of greater documentation. The inarticulate brachiopods within the boundary interval had already been described by A.J. "Burt" Rowell (1980), although Pete Palmer has discovered a new Eothele species from the youngest Early Cambrian. This find led to a restudy of the inarticulate brachiopods across the biomere boundary by Sarah Rieboldt (1999), a graduate student at U.C. Berkeley. Fred sent some articulate and inarticulate brachiopods collected during our study to Rex Hanger (University of Wisconsin-Whitewater) in July, 2001.

Gerd Geyer (University of Wuerzburg) has completed the identification of several species of helcionellid and stenothecoidid molluscs we sent him from the Pioche Shale. Bruce Lieberman (University of Kansas) has identified the Lower-Middle Cambrian boundary soft bodied faunas that we collected from the Comet Shale Member, Pioche Shale, in the Highland Range and Pete Palmer collected from the Chief Range. This fauna includes two anomalocarid species (Anomalocaris pennsylvanica and A. cf. saron), bivalved crustaceans (Canadaspis perfecta, ?Perspicaris dilatus and Tuzoia guntheri, T. dunbari, T. getzi, T. polleni), a priapulid worm (Ottoia), Chancelloria, and Protospongia (Lieberman, 2000; 2003). Hyolithids are fairly common, and were sent to Loren Babcock (Ohio State University) for identification. This would complete a rather extensive taxonomic study of the diverse groups within the Lower-Middle Cambrian boundary interval in the Pioche Shale, which was necessary prior to a definitive statement being made on the patterns of extinction, survival, and recovery (McCollum, in prep.).

Over 70% of the Pioche Shale lithology is composed of bioturbated sandy siltstone and quartz sandstone intervals. Although a diverse trace fossil assemblage occurs in these bioturbated sediments, no systematic study had ever been made. Our colleagues from Spain, Eladio Linan and Jose Antonio Gamez-Vintaned, both from the Universidad de Zaragoza, identified 28 genera and 32 species of ichnofossils from the Pioche Shale and other formations during a 10 day field excursion in the summer of 1997. Their assessment of ichnofauna diversity should encourage even more study in the future.

Pioche Shale Sections outside the Study Area
The Lower-Middle Cambrian boundary interval within the Pioche Shale to the north and south of our study area remains poorly constrained. Fritz (1968), in his study of the trilobite faunas in the Pioche Shale within the northern Egan Range, White Pine County, Nevada, found an 80 meter thick barren interval between the highest occurrence of Lower Cambrian olenelloids and the lowest occurrence of Middle Cambrian trilobites of the Albertella Zone. There is a similar faunal gap in western Utah (Hintze and Robison, 1975) and at Frenchman Mountain (Pack and Gayle, 1971), just to the east of Las Vegas, in southern Nevada. My husband Mike and I have also measured and collected several sections of Pioche Shale within these areas, but were unable to close the faunal gap at the boundary interval. Therefore the first three Middle Cambrian biozones established by Fred and I in the type area, are not present in the Pioche Shale in the surrounding region.

The Pioche Shale in Clark County, Nevada, has not been divided into members mainly because it is composed only of siliciclastics and is much thinner (ranging from 160 meters at Sheep Mountain to 125 meters at Frenchman Mountain and to less than 100 meters in the Virgin Mountains) than sections to the north in Lincoln County, Nevada. We have identified three fairly distinct Lower Cambrian faunas in the Frenchman Mountain section which allow us to correlate this section to established members in the Carrara Formation to the west. The oldest fauna is characterized by the olenelloid genus Bristolia, and occurs between eight and 16 meters above the base of the Pioche Shale. A second Lower Cambrian fauna, characterized by the olenelloid genus Biceratops, occurs in a two-meter-thick interval of light olive gray shale, approximately 30 meters above the base of the Pioche Shale. Immediately above this is a three-meter-thick, grayish red shale interval containing a Nephrolenellus multinodus fauna, overlain by a 12-meter-thick, olive gray shale interval containing Olenellus gilberti. Above this is a 60 meter thick barren interval of lenticular, channeled quartz sandstones and interbedded, highly bioturbated siltshales. The uppermost 18 to 20 meters of the Pioche Shale is a grayish green fissile shale containing Albertella schenki.

The distinctive red and green color banding in the Lower Cambrian Biceratops and Nephrolenellus faunal interval were easily located in each of our sections of the Pioche Shale in the Virgin Mountains and in the Bright Angel Shale of the Grand Wash Cliffs, Arizona. This olenelloid interval was always overlain by a channeled quartz sandstone, one to three meters thick, which shows bidirectional low-angle cross-bedding and oscillation ripples on the upper surfaces. This sandstone has a sharp basal contact and appears to have been deposited as a transgressive sand sheet on an erosional surface which downcuts the deeper subtidal facies of the Lower Cambrian in an easterly (cratonward) direction (McCollum, 1999). The regional extent of this disconformity at the Lower-Middle Cambrian boundary has profound implications on our study of the extinction and recovery interval.

Another cratonal region of Cambrian rocks occurs in the Mojave Desert and unmetamorphosed sections exist in the Marble and Providence mountains. My husband and I measured sections across the Lower-Middle Cambrian interval in the Cadiz Formation in these mountains during March, 1994. We weren't able to recover any basal Middle Cambrian faunas, so do not include this area in our research. Subsequently, in June of 1999, Mark Webster collected the highest bed of olenelloids and found early Middle Cambrian ptychopariids just a few meters higher in the Cadiz Formation at the southern end of the Marble Mountains.

Fred Sundberg later identified these ptychopariids from the Cadiz Formation as Mexicella robusta Sundberg and McCollum, 2000. This species is found in the Amecephalus arrojosensis Biozone, about 30 meters above the last olenelloids in the Pioche Shale in our study area. Absent from both the Cadiz Formation and the Monola Formation is the Eokochaspis nodosa Biozone faunas, which are present in both the Pioche Shale and Carrara Formation.

 

Undergraduate Field Research Results 1993-1995

Earth science and geology majors were selected from the Historical Geology course to participate in a formal course entitled Cambrian Research Class. This course included two weeks in early September of data gathering in the block-faulted mountains of the southern Great Basin, Nevada and California. The field phase was followed by laboratory preparation and class presentations during the fall quarter, and the results presented at a Geological Society of America meeting. A brief summary of each year’s field research on the Lower-Middle Cambrian boundary interval is presented below.

1993
In preparation for the first fall's field work involving undergraduate students, my husband Mike and I spent the first week of May, 1993, looking at potential Lower-Middle Cambrian boundary sections with A.R."Pete" Palmer (Institute for Cambrian Studies) who was accompanied by his wife, Pat. Fortunately, Pete had resumed his long standing interest in collecting and describing the youngest olenelloid trilobite fauna in the Pioche Shale that spring and had coupled this with some volunteer support work for the USGS in their BARCO geologic mapping project in the Caliente area. We rendezvoused in Caliente and began by looking at several Pioche Shale sections before heading west to look at a Cambrian section in the northern Groom Range and in the Quinn Canyon Range. What Pete already knew and we soon realized, was that a great deal of work still remained to be done on both the facies and faunas of the in the southern Great Basin and it would take a cooperative effort over many years to complete.

After visiting over a dozen potential sections, we decided that the fall research class should be directed towards an effort to define the amount of stratigraphic and faunal variation between the inner and outer shelf along an east to west transect. The two sections selected were at Oak Springs in the northern Delamar Mountains and a section 65 miles to the west, in the northern Groom Range. The Oak Springs section of the Pioche Shale was well known to Pete Palmer, and he had included it in his Biomeres and Biomere Boundaries GSA field trip in 1984 as the best Lower-Middle Cambrian boundary section known to him.

The northern Groom Range section had been originally brought to my attention by Jack Kepper ten years earlier, but it had not received much attention because of its remoteness. But what formational name should we use in this section, Pioche or Carrara? The Carrara Formation nomenclature had been used by Palmer and Halley (1979) in their southern Groom Range (most of the Groom Range was now included within the Groom Lake restricted zone or Area 51) and Belted Range (this range is within the restricted Nellis Bombing Range) sections, but upon our visit to the northern Groom Range, Pete Palmer called into question the appropriateness of using the established nomenclature in this region. We soon realized that a new Cambrian formational scheme should be erected for the Groom and Belted Ranges, but until that could be done, we would have to retain the Carrara Formation in this area.

The first Cambrian Research Class began work in September and we were joined by Pete Palmer, who was to lend his expertise as stipulated in the terms of my NSF grant. We decided to spend the first week quarrying boundary intervals at two or three sections, while establishing a detailed stratigraphy for the Pioche Shale. We choose sections which were within easy commuting distance from Caliente, a small railroad town which had both camping and shower facilities.

We spent the first few days at the Oak Springs section, where the students were paired up in teams which were assigned to one or more of the Pioche members, with one team member having the responsibility for the lithostratigraphy and the other for the biostratigraphy. In addition, each team took an hour shift at quarrying the boundary and recording all collections taphonomic data. A few days were spent quarrying and collecting the boundary interval in the Highland Range and another day spent at another boundary section just a mile north of town, in Antelope Canyon. The results were presented as a poster session at a GSA meeting the following spring, which is now displayed at the end of the Geology corridor.

The data gathered from quarrying these sections strongly supported Pete's observation that a sudden demise of the olenelloids, with no decrease in either diversity or species dominance through the sampled interval at the top of the Nephrolenellus assemblage-zone. A 70cm thick ribbon limestone occurred directly above the olenelloid shale facies and we began to refer to this distinctive carbonate ledge as Pete's boundary limestone. Pete informed us that this limestone contained only a single ptychopariid species which Fred and I later named Eokochaspis nodosa Sundberg and McCollum, 2000.

We moved our camp to the northern Groom Range during the second week and began digging pits in the shale in order to find the highest occurrence of olenelloids. We found that the boundary interval here differed in several ways from what we seen in the Pioche Shale, including the following observations 1) the coarser clastics and oncolitic limestones typical of the Gold Ace Member, Carrara Formation and the Combined Metals Member, Pioche Shale were absent, 2) a twelve meter thick coarsening upward facies interval of bioturbated mudrock and sandy siltstone, containing only a few rare olenelloid specimens, occurs above the highly fossiliferous shales containing the Nephrolenellus assemblage-zone fauna, 3) above this  interval of coarser clastics are two, thin argillaceous limestone layers, separated by a half meter of shale containing two undescribed olenelloid species in the basal centimeter, which are not found in the underlying Nephrolenellus fauna, 4) immediately above the last olenelloid layer were beds containing blooms of inarticulate brachiopods and hyolithids, 5) an effaced ptychopariid occurs within and above the highest olenelloid beds between the first and second argillaceous limestone, but does not occur in the post-olenelloid shales in the Pioche Shale, 6) there is an 8 meter barren shale interval separating the second argillaceous from a third silty limestone, no such barren interval exists above the last occurrence of olenelloids in the Pioche Shale, 7) the third argillaceous limestone contains a few rare specimens of Eokochaspis nodosa, the same species which occurred in the boundary limestone directly overlying the last olenelloids in the Pioche Shale.

It became clear to me (McCollum, 1994) after this first field season, that it was not going to be easy to ascertain whether the olenelloids went out with a whimper (gradual) or a bang (sudden). Martin Keller (Univ. Erlangen, Germany) and John Cooper (UC Fullerton) suggested that the thin ribbon limestones in the shale intervals could have chronologic significance and that we should trace them out regionally. Upon doing this and finding out that some units were being cutout between these limestones, Mike and I began to suspect that the stratigraphic record at the boundary in the Pioche Shale may not be as complete as we all had first hoped. Pete Palmer saw no direct evidence at the outcrop level that the stratigraphic record was not complete across the boundary interval in the Pioche Shale and a few years later proposed (Palmer, 1998b) a formal Laurentian stage (Delamaran) and series (Lincolnian), whose basal boundary he established within the very pit that we had excavated at the Oak Springs section.

It was during our work on the Pioche Shale at the Oak Spring section that the local BLM office volunteered to aid in site access and preparation of the Lower-Middle Cambrian boundary interval in two mountain ranges. This led to assistance agreements for paleontological sites the following year, one in the northern Groom Range under my direction, and the other in the Chief Range under the direction of Pete Palmer. The Chief Range site at Ruin Wash has yielded numerous complete olenelloids, along with lightly scleritized fauna, but has been closed to public collecting because of extensive environmental damage due to quarrying and overcollecting by fossil dealers. The remote Groom Range site remains unaffected, except for extensive road damage due to subsequent monsoonal rains.

1994
My husband and I scouted out some other promising sites during Spring break in late March and early April. We measured boundary sections in the Cadiz Formation in the Marble and Providence Mountains, accompanied by John Cooper (UC Fullerton) and Paul Myrow (Colorado State Univ.). We also measured boundary sections in the Carrara Formation within the Nevada Test Site with the assistance of James Cole (USGS). We ended this trip with BLM staff in the northern Groom Range setting up the paleontological site, which would include road construction and site preparation.

We returned in late May and co-taught our geology field camp, spending two weeks mapping portions of the Delamar Mountains and the northern Groom Range. We spent a few days with Pete Palmer, who was finishing up his initial study of the Ruin Wash BLM site in the Chief Range, looking at boundary sections at Patterson Pass in the southern Egan Range and recently prepared northern Groom Range site. Fred Sundberg also joined us for a few days.

It was during the geological mapping exercise in the Delamar Mountains, just a mile southwest of the Oak Springs section, that the students discovered a well exposed boundary interval, which they informally named Hidden Valley. This new section was to provide countless specimens in the years to follow and be cited in many of the published reports. Geologic mapping in the northern Groom Range was made a little easier because the BLM had done site preparation and constructed an access road, at government expense, earlier that spring. One major accomplishment here was the recognition that much of the Middle and Upper Cambrian was composed of outer platform to ramp carbonates just inboard of the basinal facies of the Emigrant Formation exposed in the Belted Range to the west.

The second Cambrian Research Class began that September by resuming work on the Pioche Shale. Pete Palmer again lent his faunal expertise and we were joined for a few days by M.N. "Peggy" Rees (UNLV), whose expertise in the Cambrian depositional environments of the Great Basin was welcomed. The students quarried the Lower-Middle Cambrian boundary from two sections, Comet Mine and One Wheel Canyon, in the Highland Range. A significant find was a lightly scleritized fauna in the survival fauna, including anomalocarid grasping appendages, several Tuzoia, and a single well preserved Canadaspis. The BLM completed some remedial work on the boundary site in the northern Groom Range just as this year's field class ended.

We were very fortunate to gain access to the Belted Range section discussed in Palmer and Halley (1979), which is deep inside the restricted Neillis Air Force Bombing Range. Jim Cole (USGS) made the arrangements for Mike and I, plus Pete Palmer, M. N. “Peggy” Rees (UNLV) and Cathy Summa to visit the site on the second weekend in December. The Carrara Formation is quite similar to that found to the east in the northern Groom Range, but above it is a deeper water facies referred to the Emigrant Formation. The next day we met up with Steve Rowland (UNLV) and Fred Sundberg and traveled to the Mormon Mountains to look at a Pioche Shale section originally described by Wheeler (1943, p. 1804-1806).

1995
The third Cambrian Research Class began that September by resuming the study of the boundary interval at the Hidden Valley section discovered that spring in the Delamar Mountains. Our relationship with Pete Palmer had become strained due to differing views of the completeness of the boundary interval and he declined to participate with our students in the fall field research program. We were joined by Fred Sundberg and began discussing a partnership in describing the trilobite faunas within the Lower to Middle Cambrian boundary interval. Fred was finishing up a manuscript on the phylogeny of the spiny oryctocephalid trilobites, so I contributed large numbers of a Lower Cambrian oryctocephalid which the students had collected from the Hidden Valley section and a few rare oryctocephalid specimens from the basal Middle Cambrian Pioche Shale in the Highland Range. This was followed up by having Fred visit me at EWU the following January, where he could examine numerous older collections housed there.

Less then a mile north of Caliente was a third boundary section, exposed in the steep north side of Antelope Canyon in the southern Chief Range. The students measured and collected the boundary interval from the base of the Combined Metals Member to the top of the Susan Duster Limestone Member. They discovered two additional ribbon limestone containing silicified trilobites above Pete's boundary limestone. These limestones, each containing a distinct ptychopariid species, were instrumental in our attempts to establish a high resolution stratigraphy in the faunal crisis and recovery interval.

After a few days in the Caliente area, we moved camp to the northern Groom Range, and began work in the expanded bulldozer trench site. The students were able to examine the 44 meter thick olenelloid-bearing clastic interval and made numerous faunal collections. The most important accomplishment at this locality was a detailed collection made in the half meter shale interval between the two silty limestones. Bed by bed collections revealed the details of the opportunistic species blooms occurring just above the Nephrolenellus faunal extinction.

But what had we observed about the Lower-Middle Cambrian boundary interval in the Pioche Shale during our first three years of study was that there was some evidence for missing section below Pete's boundary limestone. First, there is an abrupt disappearance of olenelloids in the Pioche Shale vs. the gradual disappearance during an opportunistic species bloom interval in the Groom Range section. Second, there was regional evidence that an extensive coarsening upward clastic sequence, typical of a highstand systems track, was present in the shelfal sections at the end of the olenelloid-bearing strata outboard of the Pioche Shale outcrop belt. The absence of proximal clastics in the inboard shelfal region could be explained either by sedimentary bypass or removal by erosion. The problem was complicated by two factors in the local Pioche Shale study area: 1) a local north-south facies shift nearly parallel to depositional strike, and 2) a lack of east-west sections needed to document progressive erosional thinning towards the craton. Although there is sharp contact at the base of the boundary limestone and some local evidence for thinning at the top of the underlying olenelloid shales, based on a progressive northward disappearance of small limy layers, no definitive conclusion could be reached concerning the nature of the boundary in the local area.

1996
A year's extension of my NSF grant allowed us to have a fourth Cambrian Research Class, which was moved to the second week of August in order to stay within the grant's first of September expiration date. We began this undergraduate field study by looking at the Albertella to Glossopleura Zone rocks and faunas in Spence Shale Member, Langston Formation in northern Utah and southern Idaho, under the guidance of the Gunther clan in Brigham City. Our group then headed south, visiting the excellent dinosaur museum in Price, and then across the San Rafael Swell through Capitol Reef and Grand Stairway-Escalante Monument, and then westward to Pioche, Nevada. We continued our research on the sequence boundaries within the Pioche Shale for several days and then on August 18, we were joined by Pete Palmer in an underground tour of the Pan American Mine, which is located in the Combined Metals Member on the west side of the Highland Range.

THE LOWER-MIDDLE CAMBRIAN BOUNDARY STUDY EXPANDS FROM THE SOUTHERN GREAT BASIN TO SOUTH CHINA AND BEYOND

Northwest Institute Grants, 1996-1998
I received three $7500 Northwest Institute grants from EWU. The periods of these grants ran from July 1 to June 30 of the following year. The first grant was to finish up our work in the Pioche Shale. The second grant was to continue work in the Carrara Formation, and the third grant was to expand our work on the Lower-Middle Cambrian boundary interval to include South China.

1996-98
The first NWI grant covered expenses to the Second Cambrian International Subcommission Field Conference trip in Spain during September, 1996. It also covers a trip with Isabel Montanez and Dave Osleger (UC Davis) to work on sections on the Nevada Test Site, the northern Groom Range, and the Oak Springs and Highland Range sections in May, 1997. A week in mid-June was spent with Nigel Hughes (UC Riverside), his graduate student Mark Webster, and Loren Smith (USC), in working on the Lower-Middle Cambrian boundary interval in the Pioche Shale.

The second NWI grant covered expenses of Eladio Linan and his graduate student Jose-Antonio Gamez-Vintaned (Universidad de Zaragoza) to join us in a study of the ichnofauna of the Lower-Middle Cambrain boundary interval across the southern Great Basin during July, 1997. In August, I attended the Second Trilobite Conference in Toronto, and went on a field trip which included the Lower-Middle Cambrian interval in the southern Canadian Rocky Mountains. In September, Mike, along with Bill Fritz and Stew Hollingsworth, discovered a well-exposed Lower-Middle Cambrian boundary interval at the base of the Emigrant Formation within Claton Ridge near Split Mountain, Esmeralda County, Nevada.

In May, 1998, Fred, Mike and I then went on to measure and describe Carrara sections in southern Nevada and the Death Valley region of California. We later met up with William H. Fritz and Stewart J. Hollingsworth to look at a newly discovered section of the boundary interval in the Emigrant Formation at Split Mountain. In mid-June, I met up with Isabel Montanez and her graduate student, Kirsten Tambo, to collect isotopic samples across the boundary interval in both the Carrara and Emigrant formations (Tambo and Montanez, 1998).

The third NWI grant covered the expenses of Fred Sundberg and me on our South China trip in mid-August, 1998. In late September and early October, Mike and I participated in a field trip concentrating on the Lower-Middle Cambrian boundary interval across the southern Great Basin. This trip was attended by R.A. Robison (University of Kansas), A.R. Palmer, Bill Fritz, Stew Hollingsworth, and Tony Runkel. During the Christmas break, Mike and I measured and described sections in the Virgin Mountains and Grand Wash Cliffs.

1999
In early June, 1999, I co-chaired (with Stew Hollingsworth) a full day symposium at the GSA meeting in Berkeley, CA. entitled “Cambrian of the Cordillera: A symposium in honor of A.R. Palmer, which included thirteen presentations. I presented a talk on the cratonal Lower-Middle Cambrian (McCollum, 1999) and was co-author on two other presented papers (Montanez and others, 1999; Sundberg, and McCollum, 1999). After commencement, I joined husband Mike and Fred Sundberg, who were quarrying faunas within the Emigrant Formation at Split Mountain, Nevada. In October, Fred and I presented a poster session on the near synchronicity of trilobite extinctions at the Lower-Middle Cambrian boundary of eastern Gondwana and Laurentia (Sundberg and McCollum, Yuan, and Zhao, 1999) at the national GSA meeting in Denver, CO.

My second NSF grant became effective in the fall of 1999, in time to cover my expenses for the Fifth Field Conference of the Cambrian Stage Subdivision Working Group, International Subcommission on Cambrian Stratigraphy. Fred Sundberg and Linda McCollum co-led this trip to the western U.S. and presented both oral and written papers, including the description of a proposed Lower to Middle Cambrian international boundary section at Split Mountain in western Nevada (McCollum and Sundberg, 1999; Sundberg, McCollum, Yuan, and Zhao, 1999; Sundberg, Yuan, McCollum, and Zhao, 1999a, b). A trench in the basal 20 meters of the Emigrant Formation exposed a highly fossiliferous mudrock section that contained faunas from four biozones, starting at the top of the Olenellus through the Eokochaspis nodosa, the Amecephalus arrojosensis, and the Oryctocephalus indicus horizon near the base of the Poliella denticulata Biozone. The uppermost few meters of mudrock and sandy beds is barren, but occupies the stratigraphic position of the Albertella biozone. The basal several meters of silty limestone which overlies the basal mudrock interval, contains a Peronopsis bonnerensis/Glossopleura Biozone fauna.

Second NSF Grant
The undergraduate involvement in research continues under a three-year NSF RUI grant EAR-9973180, entitled "International Correlation of Lower to Middle Cambrian Trilobite Faunas between South China and the U.S.", for $110,000. The goal of this research grant was establish an international correlation within the Lower-Middle Cambrian boundary interval between Laurentia and Gondwana. This would be accomplished by detailed biostratigraphic collecting from the outer shelf section in the western US and in South China. This grant allowed undergraduate students to continue research on the Lower-Middle Cambrian boundary interval in Nevada and California during the next three summers. This project also includes the collaborative effort of Dr. Frederick A. Sundberg and two Chinese colleagues, Dr. Zhao Yuan-long, Guizhou University of Technology, and Dr. Yuan Jin-liang, Nanjing Institute of Geology and Paleontology.

Results of Second NSF Grant
This three year study resulted in the complete documentation of the Lower-Middle Cambrian trilobites found in the outer shelf sections within the Monola and Emigrant Formations in the southwestern Great Basin (Sundberg and McCollum, 2003b) and several papers authored with our Chinese colleagues on the international index fossil Oryctocephalus indicus (McCollum and Sundberg, 2002; Sundberg, Yuan, McCollum, and Zhao 1999a, 1999b), including a proposal to establish this species as a GSSP, which stands for "Global boundary Stratotype Section and Point" (Zhao, Yuan, McCollum, Sundberg, and others, 2001) in South China.

Undergraduate Field Research Results 2000-2003

2000
Two weeks of field research began in mid-June, 2000, on a detailed study of the basal Middle Cambrian outer shelf faunas of the Monola Formation in California and the Emigrant Formation in Nevada. The faunal interval studied was from the post-olenelloid extinction to the first occurrence of Oryctocephalus indicus, and the results of this research were presented by the undergraduate students at the national GSA meeting held in Reno, NV that November (Beaver, Larsen, Newton, Schulz, and McCollum, 2000). In addition, Fred and Linda also presented a paper at this meeting on a comparison of the Middle and Upper Cambrian carbonate platform section in Nevada and Utah with the highly condensed, outer shelf section of the Emigrant Formation (McCollum and Sundberg, 2000).

Five students participated in the fifth Cambrian Research Class for an additional two weeks of field work in early September, 2000. The focus was on the faunas and environments of a deepening upward carbonate, the Susan Duster Limestone Member, Pioche Shale. Sundberg and McCollum (2000, p. 605) noted that the Susan Duster Limestone Member was deposited above a regional unconformity, and therefore represents a transgressive deposit. The undergraduate students did a petrologic study of this thin (four to six meters thick) carbonate interval, comparing half a dozen sections to determine the regional variability and they presented their results at the Cordilleran section GSA meeting in Los Angeles the following April (Beaver and others, 2001).

2001
Fred presented a paper on the Lower-Middle Cambrian sequence stratigraphy of the southern Great Basin (Sundberg, McCollum and McCollum, 2001) at the March meeting of GSA in southern California. Dr. McCollum participated in the seventh field conference of the Cambrian Stage Subdivision Working Group, ISCS, in South China in late summer of 2001. The Subcommission was now seriously considering our proposal that the first occurrence of Oryctocephalus indicus be used in the establishment of the lowest stage and series boundary within the Cambrian System. We presented a candidate locality for establishment of the GSSP in South China, and presented a scientific assessment (Zhao and others, 2001) of its applicability, as set forth in the International Stratigraphic Code. Dr. McCollum also collected shale samples from the Lower-Middle Cambrian boundary interval of the Kaili Formation as part of Isabel Montanez's carbon isotope study. Isabel presented a paper on the carbon isotope data on the Lower-Middle Cambrian boundary interval at the national GSA (Montanez, Nest, McCollum, McCollum, and Osleger, 2001).

2002
I spent two weeks at the end of June, 2002, with an undergraduate student, collecting faunas from several sections in the southern Great Basin. We were joined by Dr. James Sprinkle (Univ. Texas at Austin) and Mark Webster (UC, Riverside) during the first few days collecting in Esmeralda Co. We spent several additional days on collecting Glossopleura Zone faunas in the northern Groom Range, where we were joined by Val and Glad Gunther, from Brigham City, Utah. The Albertella to Glossopleura zone contact was investigated at Current Gap, southern White Pine Range, on our last day out.

In late July, my husband Mike and I traveled to see Dr. Richard A. Robison at the Univ. of Kansas and transferred Doug Campbell's thesis collections from the Langston Formation of Utah and Idaho to EWU for research. In August, Drs. McCollum and Sundberg (2002) presented a paper on correlation of the Lower-Middle Cambrian boundary interval in the circum-Pacific region based on oryctocephalid trilobites at the First International Palaeontological Congress in Sydney, Australia. In late October, we presented three papers at the national GSA meeting in Denver (Hollingsworth, McCollum, McCollum, and Fritz, 2002; McCollum, Sundberg, and Montanez, 2002; Montanez, Nest, Gragg, McCollum, McCollum, and Osleger, 2002)

The sixth Cambrian Research Class began a two week study in mid-September. In the first week, we recovered Glossopleura Zone trilobites from the Lyndon Limestone and Chisholm Shale in the Pioche and Caliente area, Nevada. The second week was spent collecting Albertella Zone faunas from the Carrara Formation north of Pahrump. Upon our return to EWU, the students participated in a study of a newly recognized Cambrian section just five miles west of campus. The area, on an air force recreational site adjacent to Clear Lake, had been previously mapped as Precambrian Wallace Formation until Robert Derkey and Mike Hamilton found trilobites in a reddish shale in May, 2002. The fauna they (Hamilton, Derkey, and McCollum, 2003) collected included Glossopleura, Zacanthoides, Amecephalus, plus inarticulate brachiopods and hyolithids. This age fauna had been known from the Lakeview area of northern Idaho, but had not been previously reported from Washington. Plans to map the geology of the Medical Lake 7 ½ minute quadrangle, which includes the Clear Lake Cambrian exposures, in the late spring and early summer of 2004 are progressing.

2003
The final two major trilobite studies by Fred and myself were published during the year. The first was published in the March issue of JP and this was the fifth and final paper on the Pioche Shale trilobites. In this paper we introduced a new species based biozone, documenting 17 genera and 35 species, including two new genera and six new species (Sundberg and McCollum, 2003a). The second paper, published in the September issue of Palaeontology, described 16 genera and 25 species of Early and Middle Cambrian polymeroid trilobites of the Emigrant Formation, Esmeralda County, Nevada, and the Monola Formation, Inyo County, California (Sundberg and McCollum, 2003b).

Mike and I made two trips to northern Utah and southern Idaho during late June and early July, 2003 to begin working on the Glossopleura Biozone trilobites from the Spence Shale. The Gunther family kindly loaned us there rather extensive (over 4,000 specimens) collection of trilobites from the Spence Shale in northern Utah and southern Idaho and the type section of the Chisholm Shale. Much more field work is anticipated for the upcoming year.

In August of 2003, the seventh Cambrian Research Class began measuring and collecting the Rennie Shale and lower Lakeview Limestone in northern Idaho. We found Albertella occurring from the base of the Rennie Shale to near the top. An open shelf Peronopsis bonnerensis fauna within the lower Lakeview Limestone, originally described by Resser (1938) and now being redescribed by Fred Sundberg and myself, occurs at the cement adit along the shore of Pend Oreille Lake, about a mile north of Lakeview.

In the fall of 2003, Fred and I headed up an International Subcommission on Cambrian Stratigraphy (ISCS) Working Group to correlate, evaluate, and hopefully establish the foundation for a Global Stratotype Section and Point (GSSP) within the Early/Lower to Middle Cambrian interval. At present, our working group consists of over two dozen scientists from around the world representing a wide range of expertise; from paleontology and stratigraphy, to geochemistry. You can visit this website at: http://www.ewu.edu/mccollum/workinggroup/

STRATIGRAPHIC STUDY ACROSS THE LOWER-MIDDLE CAMBRIAN BOUNDARY

It became apparent during the first few years of study that there were several third order stratigraphic sequences within the Pioche Shale that were bounded by erosional disconformities. The goal of the Cambrian Research class was expanded to better document the stratigraphic record within the Pioche Shale by focusing on suspected sequence boundaries. Criteria for recognizing sequence boundaries at the outcrop level in a mixed clastic and carbonate depositional environment were discussed and then applied by the students during these two week field excursions. The most difficult sequence boundaries to ascertain were those at diastemic boundaries where significant erosion had not occurred below them and there was little sedimentologic evidence found at the outcrop.

However, it became obvious that the upper portion of the highstand sequence tracts at the top of the shale members were being progressively removed in a cratonward direction by carefully comparing stratigraphic sections in the Highland and Chief ranges to those in the Pioche Hills to the east. In the sections south of the Highland Range, the two oncolitic limestone layers within the Combined Metals Member were often karsted at their basal contact with the underlying quartz sandstones and that a truncation surface was present neat the top of the highest oncolitic bed. Thus we had good evidence that more then one sequence existed with the CM Member. We finally documented at least six erosional bounded sequences at the base of regionally extensive limestone layers, most at or near member boundaries in the Pioche Shale. The next step would be to make a comparison between our sequences and those just proposed by Adams (1995) in the Carrara Formation.

Stratigraphy of the Carrara Formation
The Lower and Middle Cambrian Carrara Formation, which is exposed in numerous mountain ranges in the southern Great Basin in Nevada and California, occupies a medial shelf position just to the west of the inner shelf Pioche Shale. The Carrara Formation consists of a mixed carbonate and clastic facies, totaling about 400 meters in thickness and ranging in age from the upper part of the Olenellus Biozone through the Glossopleura Biozone. Therefore, it is age equivalent to all of the Pioche Shale, in addition to the overlying Lyndon Limestone and Chisholm Shale to the east.

The most comprehensive study of the Carrara Formation, which has an extensive outcrop area in the southern Great Basin, was by Palmer and Halley (1979), which included over a dozen measured sections and a description of 38 genera and 95 species of trilobites which were assigned to nine trilobite "zonules". Unfortunately, Palmer and Halley (1979, p. 58) were only able to constrain the Lower-Middle Cambrian boundary faunally to within a 50 meter interval within the Pyramid Shale Member in only a single section, and the barren interval at the boundary was much thicker in their other Carrara sections. The Lower-Middle Cambrian boundary in the Carrara Formation would have to be better constrained faunally if the rather extensive middle shelf region were to be included in our study.

Palmer and Halley (1979) also described nine members, based on alternating clastic and limestone facies, although not all of these members could be distinguished in all of their sections. They noted that when these members are paired, they constitute three complete and one partial sedimentary cycle, which they equate to Aitken's (1966) Grand Cycles in the Cambrian of the Canadian Rocky Mountains. Adams (1993, 1995) used their Grand Cycle scheme as the basis for his sequence stratigraphic model for the Carrara Formation, defining two partial third-order sequences and three complete third-order sequences.

William H. Fritz, who had previously correlated three of the Lower Cambrian Grand Cycles into the Great Basin (Fritz, 1975), expressed serious doubts to us (personal communication, September 1997) as to whether the facies within the Carrara meet the criteria for regionally recognizable Grand Cycles and therefore could not serve as a template for establishing regional sequences. We had also found that many of the Carrara "Grand Cycles" were not traceable into the Pioche Shale and that the sequences we had established were not coincident with those which Adams (1995) had described in the Carrara Formation.

Summary of our Research on the boundary interval in the Carrara Formation
The first chance my husband Mike and I had at attempting to ascertain whether the Lower-Middle Cambrian boundary could be precisely located in sections of the Pyramid Shale Member, Carrara Formation, was during Christmas break of 1993-94. We visited several boundary sections and the results were somewhat encouraging, although the shale intervals in some of the Carrara sections were pervasively cleaved. One observation, first made in the Pioche Shale, was that a color change in the shales, from a lighter to a darker shade of green, coincided with the Lower-Middle Cambrian boundary.

Since then, we've measured and described over twenty additional boundary interval sections of the Carrara Formation and found that the Death Valley region held the greatest promise in adding substantive data to our study of the Lower-Middle Cambrian boundary. Eokochaspis nodosa, the trilobite species that defines the base of the Middle Cambrian, has now been recovered from the Bare Mountain, Echo Canyon, Emigrant Pass, Pyramid Peak, and southern Nopah Range sections. This species is often silicified in a very thin, lenticular siderite layer found at the base of the darker green shales between 20 to 23 meters above the base of the Pyramid Shale Member.

A comment about defining the Pyramid Shale Member seems warranted at this point. Palmer and Halley (1979) did not define the upper and lower contacts of the Pyramid Shale Member in their discussion of this member, but in the discussion of the underlying Gold Ace Member and overlying Red Pass Limestone. They mention that the contact between the Gold Ace Limestone and Pyramid Shale members is very sharp and that the base of the Red Pass Limestone Member is placed at the first limestone bed 0.5 m thick or thicker, even if there are shales above this. However, it is evident that Palmer and Halley (1979) were very inconsistent in the placement of member boundary in regards to these limestones and this greatly affected the thickness of these two members as compiled on their isopach maps. It would have been much wiser if they had used Reynolds (1971) original definition of the Red Pass Limestone, and proposed another member to include the strata found between the Susan Duster Limestone equivalent and the base of the cliff forming Red Pass Limestone.

Pilot Study within the Monola and Emigrant Formations
The outer shelf sections of the Monola and Emigrant formations were next on our study of the Lower-Middle Cambrian boundary interval. My husband Mike and I had measured and collected several boundary sections within these formations in the early 1980's, and found the lower Middle Cambrian to be relatively fossiliferous. In fact, the survivor and recovery faunas in the open shelf environment were two to three times more diverse than what we had seen in the shelfal Pioche Shale and Carrara Formation. The moderately diverse faunas of the outer shelf environment often contain a mixture of endemic and cosmopolitan trilobites which could serve as a template for correlating Laurentian biozones with other continental biozonal schemes. Sundberg and McCollum (1997) noted that some of the early Middle Cambrian oryctocephalid species were cosmopolitan and that they could be useful in correlation between Laurentia and the Asian portion of Gondwana. This observation would serve as the foundation for the second NSF grant proposal.

Stratigraphy of the Monola Formation
The Monola Formation was named by Nelson (1965) for a 366 meter thick mixed clastic and carbonate facies with occurred between the lower Cambrian Mule Spring Limestone and the Middle to Upper Cambrian Bonanza King Dolomite in the White-Inyo Mountain region of eastern California. Clem Nelson reported early Middle Cambrian trilobites ranging up to the Glossopleura Zone. From the basal 3 meters of Monola Formation in the Saline Valley area, Clem listed Syspacephalus laevigatus and an unidentified species of Oryctocephalus which turned out to be O. indicus (Sundberg and McCollum, 1997).

The Monola Formation in the type area of the Inyo Range is only sparsely fossiliferous, with only a few Amecephalus arrojosensis (Alokistocare sp. in Nelson, 1965) specimens from Nelson’s lower member, Ogygopsis sp. from the top of the middle limestone member, an Oryctocephalus sp. from the dark shale at the base of the upper member and species of Ogygopsis and Glossopleura from higher in the upper member. The Saline Valley section mentioned by Nelson (1965) was fossiliferous to the lower shale beds of the Monola Formation, but the specimens were highly distorted.

Luckily, my husband Mike and I had found a highly fossiliferous section of lower Monola Formation in the northern Saline Range in December, 1980 which would provide the first detailed early Middle Cambrian faunal picture of a medial to outer shelf environment in the western US. The lower 15 meters of shale and mudstone with interbedded bioclastic limestone beds held an Amecephalus arrojosensis Biozone fauna and the overlying 5 meter thick fissile shale contained an Oryctocephalus indicus fauna. Just above this is a 35 meter thick interval of dark siltshale and lighter calcareous mudstone containing Paralbertella in the lower portion and Ptarmiganoides near the top (Sundberg and McCollum, 1997). In June, 2000, Fred Sundberg, myself and several undergraduates students spent four days recollecting the Lower Monola Formation in the Saline Range and found several additional taxa, some new to science. At the conclusion of our study of the basal 22 meters of the Monola Formation in the northern Saline Range section, we had documented 11 genera and 17 species of trilobites (Sundberg and McCollum, 2003b).

Stratigraphy of the Emigrant Formation
The Emigrant Formation was named by Turner (1902) for exposures shale and thin bedded limestone near Emigrant Pass in Esmeralda County, Nevada. The most comprehensive study of the Emigrant Formation and its faunal content was by Albers and Stewart (1972) in their study of the geology and mineral deposits of Esmeralda County. They noted that the Emigrant Formation outcropped in eleven different areas, ranged in age from earliest Middle Cambrian to latest Late Cambrian, divided the formation into three informal members and estimated the total thickness to be in the range of 1,500 to 1,800 meters.

Albers and Stewart (1972, fig. 6) presented three of the eleven Emigrant sections in the graphic correlation chart from which Mike and I choose the Goldfield Hills section as the most complete and fossiliferous to began our study. We had measured and collected this section in February, 1981 and made additional collections in March, 1986, and again in May, 1993. The data collected during these trips was included in Sundberg and McCollum (1997).

We made other measured sections of the Emigrant Formation during this initial study, including the type section just south of Emigrant Pass in December, 1980; the Paymaster Ridge section of Albers and Stewart (1972) and a section just north of Railroad Pass in the southern Montezuma Range, in April, 1986. None of these sections were over 400 meters in thickness, but they did not provide much in the way of faunal control, so faulting could not be ruled out. The thickest section shown by Albers and Stewart (1972) was the Goldfield Hills section, at just under 500 meters and they thought that the shale member in this section was much thicker than elsewhere.

What was needed was an unfaulted, fossiliferous section of the Emigrant Formation to use as a template to compare the other section to. Such a section or sections did exist on either side of Split Mountain at the Clayton Ridge locality listed in Albers and Steward (1972), but it wasn’t until September, 1997 that we realized that. It was during a trip to the northwestern side of the Montezuma Range to look at the possibility of establishing a sequence stratigraphy of the lower portion of the Dyran Stage, that Mike, along with William Fritz and his partner, Stew Hollingsworth noticed a thick limestone cliff to the west in Clayton Ridge. The cliff turned out to be the thickest section of Lower Cambrian Mule Spring Limestone in the region, but more importantly, a well exposed and fossiliferous section of basal Emigrant Formation occurred just across the dry creek bed. Mike measured the section and collected faunas from a thin limestone at 10 meters and a limy concretion layer at 15 meters above the base of the Emigrant Formation.

The faunas collected from the two limy horizons in the lower Emigrant Formation contained several trilobite species previously known only from South China and Australia, so additional collecting seemed warranted and in May, 1998, we returned to Split Mountain with Fred Sundberg, Bill Fritz, and Stew Hollingsworth. We discovered Oryctocephalus indicus just above the 15 meter limy lenticular layer and Peronopsis bonnerensis from the silty limestone interval several meters thick beginning about 22 meters above the base of the Emigrant Formation. In June, Isabel Montanez (UC Davis) visited the section and began collecting samples for a regional isotopic study of the Lower-Middle Cambrian boundary interval. This section and its faunas were of such interest that it was included in an informal field trip of the boundary interval in early October, 1998; participants included (besides the regular Esmeralda group of Bill, Stew, Mike and myself) Nigel Huges (UC Riverside), Pete Palmer (Cambrian Research Institute), Richard Robison (Univ. of Kansas) and Tony Runkel (Minnesota Geological Survey). In September of the following year, Fred Sundberg and I led an international group of two dozen scientists into this site.

Stew Hollingsworth and I chaired a Cambrian symposium in tribute to Pete Palmer at the GSA meeting in Berkeley, California in early June, 1999. After the meeting, Mike and Fred Sundberg drove to the Split Mountain section and I joined them the following week after commencement. We made detailed collections from the shale facies and decided that the Oryctocephalus indicus horizon at Split Mountain should be considered for a global stage boundary (Sundberg, Yuan, McCollum, and Zhao, 1999a, 1999b). The Split Mountain section was written (McCollum and Sundberg, 1999) up as field stop locality for the Fifth Field Conference of the Cambrian Stage Subdivision Working Group of the International Subcommission on Cambrian Stratigraphy and over two dozen participants visited the section in September, 1999. After this visit, the Split Mountain section became one of the leading contenders for an international boundary stratotype (GSSP) and the trilobite faunas across this boundary were completely documented by Sundberg and McCollum (2003b).

Fred and I returned to recollect both Monola and Emigrant sections with undergraduate students in June, 2000. At Split Mountain, we measured a second and more complete section of the Emigrant Formation and collected younger faunas from several horizons. The unfaulted section measured just over 400 meters in thickness, and ranged from the top of the Lower Cambrian Olenellus Zone to the Lower Ordovician Cordylodus intermedius Zone at the top (McCollum and Sundberg, 2000). An ever thinner and more condensed section of the Emigrant Formation exists at Albers and Stewart (1972) Weepah Hills locality, where Mike and I collected a lower Ordovician trilobite and conodont fauna from the Cordylodus proavis Zone at 275 meters above the formation base in October, 2001.

Sequence Stratigraphy of the Delamaran and upper Dyran stages
Sequence stratigraphy is a conceptual model, set forth by stratigraphers from Exxon in the mid 1970's, to analyze stratigraphic successions in terms of genetically related packages of strata bounded by regional unconformities (and their correlative conformities). This model includes a nomenclature of system tracts which encompass sediments deposited as a result of changing sea level, accommodation space (space available for sediment accumulation) and source material (sediment supply). Students who need additional details about sequence and genetic stratigraphic approaches may wish to consult recent texts such as Miall (2000, Chap. 6) or visit [ http://www.aseg.org.au/publications/Mul/ss1.htm ] and [ http://www.uga.edu/~strata/sequence/seqStrat.html ].

The approach advocated by genetic stratigraphers was to first establish where the deepest water facies and faunas occur in the section. In clastic dominated marine shelfal facies, the deepest facies are the regionally extensive fissile shale intervals. In the Cambrian, the deeper water fissile shale facies is characterized by a lack of a large and diverse benthic infauna and the presence of a high diversity benthic fauna including sponges, hyolithids, brachiopods and a trilobite assemblage composed of both endemic and cosmopolitan species, with a higher percentage of complete specimens. The actual depth of these regionally extensive fissile shales varies greatly, from just below normal wave base in the inner shelf to hundreds of meters in the other shelf regions. It is not unusual to find tempestite beds and sedimentary features associated with wave impingement in these fissile shale successions, indicating that most were deposited within storm wave base.

The Pioche Shale had not been studied in terms of sequence stratigraphy prior to our research, so we began from scratch. We already knew that the Pioche Shale is composed predominantly of inner shelf clastics and that there were several geographically extensive fissile shale horizons, each constituting a maximum flooding event within a single sequence. The plan was simple 1) determine the horizon of the maximum flooding interval and establish the underlying transgressive and overlying highstand system tracts, and 2) attempt to locate horizons where regionally extensive erosional breaks occur, thus defining the boundary between each sequence. Once this was accomplished, we could correlate stratal sequences across the Cambrian paleoshelf and ascertain the completeness of the stratigraphic and faunal record. This was a crucial step in determining the pattern of extinction and recovery in the Lower-Middle Cambrian interval.

We found that the fissile shale facies, in which the maximum flooding interval occurs, constituted less than 30% of the total thickness of the Pioche Shale and that these shales occurred as discrete horizons within the formation. The Grassy Spring Member had two extensive shale horizons, each above a prominent limestone marker bed. The Log Cabin Member had thin shale at its base overlain by a siltshale to hackly mudrock interval that correlated to a 25 meter thick fissile shale interval in the northern Groom Range section. The 30 to 34 meter thick Comet Shale had two fissile shale intervals, the lowest and thickest having one or more thin ribbon limestones in the lower portion and a second fissile shale with thin bioclastic limestones beginning about 30 meters above the member base. The Delamar Member has one or more fissile shale horizons in the upper half of this clastic sequence. All of these shale horizons would be found in the Carrara Formation as well.

Defining the sequence boundaries and establishing system tracts within them was not to be as easy as it first seemed. It took many hours of walking out suspected sequence boundaries along the outcrop belt to document actual sedimentary deposits or structures normally associated with a basal onlap and ascertain whether any downcutting of the underlying strata had occurred. The mountain ranges themselves are oriented nearly parallel to the craton, thus sedimentary patterns are reflecting little of their dip component and the progressive erosional downcutting expected to occur in a cratonward direction was thus hidden from view. Also, the Cambrian paleoslope was very low, so the amount of erosional relief or downcutting was never more than a few meters within the outcrop belt along a mountain front.

The present study of the sequence stratigraphy on the Delamaran Stage began within the type boundary section proposed by Palmer (1998b) at the base of the Comet Shale Member, Pioche Shale, Lincoln County, Nevada. The Delamaran Stage encompasses five biozones (interval between the Olenellus and Ehmaniella biozones) beginning at the base of the Middle Cambrian Comet Shale Member and ends at the base of the Burrows Dolomite Member, Highland Peak Formation. An erosional unconformity, characterized by a highly karsted interval at the top of the Peasley Limestone, is a sequence stratigraphic boundary which may be coincident with the end of the Delamaran Stage. At least 65 valid genera are present within the Delamaran Stage.

Much of the field work on the Pioche Shale and overlying formations had been completed by 1995 and we had defined several sequences. Our sequences did not correlate with sequences proposed by Adams (1995), which were based on the Grand Cycles proposed by Palmer and Halley (1979), for the Carrara Formation. This was a problem that necessitated a reevaluation of sequence and system tract criteria used for the Carrara Formation in the context of a regional study of all Cambrian formations in the southern Great Basin which contained strata laid down during the upper Dyran and Delamaran stages. We began by measuring and collecting several sections in the Carrara Formation which resulted in numerous additional faunal collections leading to a much tighter biozonal control of the Middle Cambrian portion of the Carrara Formation than that of the original study by Palmer and Halley (1979). We also included sections from the Bright Angel Shale, Cadiz, Emigrant, and Monola formations, in order to complete an across the shelf transect.

After 1995, we began studying boundary sections outside of our main area in order to present a shelfal transect of the upper Dyran and Delamaran Stage sequence stratigraphy from the craton to the outer shelf. Each of the regions presented unique problems in the application of sequence stratigraphic principles. The cratonal sections had lower rates of subsidence, resulting in less accommodation space and longer periods of emergence and erosion, resulting in missing system tracts and amalgamated bounding surfaces. The outer region were typified by greater accommodation space and a much lower sedimentation rate, resulting in a deep water condensed facies in which sequence and system tract boundaries were difficult to establish.

Despite the difficulties, our ongoing study of the regional facies patterns within the Lower-Middle Cambrian boundary interval has yielded some interesting results. We were able to define regionally extensive erosional surfaces as sequence boundaries, as well as establishing maximum flooding intervals in which we could place systems tract boundaries. Fred Sundberg drafted up a number of our measured sections and produced several computer-generated graphics depicting transects of the boundary interval across and parallel to the shelf (Sundberg, McCollum, and McCollum, 2001). A brief listing of the stratigraphic sequences we defined in the upper Dyran and Delamaran stages can be found at SSCDGB.

The challenges faced in reconstructing a shelfal transect remains daunting. Isolated block faulted mountain ranges, often administered by different governmental agencies, each with its own peculiar set of restrictions, are one aspect of the problem. Another aspect is that differential Tertiary extensional rates, resulting in transcurrent faulting along the Garlock and Death Valley-Furnace Creek fault zones and within the Las Vegas shear zone, have disrupted the original facies patterns to such an extent that juxtaposition of crustal sections is not uncommon. We have therefore decided to present our work on a pre-Cenozoic palinspastic reconstruction of the southern Great Basin, as recently presented in Snow and Wernicke (2000).

 

Sequence Stratigraphy of the Laurentian lower Dyeran Stage
Most recent authors believe that Grand Cycle boundaries are sequence boundaries, so it was not surprising that Mount and Bergk (1998) adopted this idea, with some modification, in their study of the Lower Cambrian sequence stratigraphy in the southern Great Basin. Grand Cycles begin with a lower siliciclastic half-cycle which is overlain by a carbonate half-cycle that is always placed at the top of the Highstand System Tract, despite the presence of karsting or pebble lags at the base of the carbonate half cycle. This fact, coupled with the lack of erosional features at the presumed sequence boundary at the base of the siliciclastic half-cycle, makes it all the more surprising that none of the authors present much evidence to support or challenge their hypothesis that Grand Cycles are stratigraphic sequences.

We have strong evidence, from numerous horizons in dozens of sections, that the “upper” carbonate half-cycle is often the initial deposit over an extensive erosional surface and thus forms the basal unit within the Transgressive System Tract. The maximum flooding interval is within a regional shale facies at or just above the “lower” siliciclastic half-cycle, making this unit the highstand system tract. If the Grand Cycle definition could be changed to a lower carbonate half-cycle overlain by an upper siliciclastic half-cycle to correspond to a stratigraphic sequence, then the problem would disappear. It’s probably worth a short paper in Geology to get this mess straightened out. For now, all sequence boundaries in the Lower Cambrian appear to begin at the base of extensive limestone intervals, such as the lower and upper limestone members of the Poleta Formation, the basal limestones of the Saline Valley Formation, and the base of the Mule Spring Limestone.

Our work on the Lower Cambrian Dyeran Stage in Esmeralda County, Nevada and Inyo County, California, is in its very early stages. We have benefited immensely over the last few years from participating in the biannual field excursions with our friends and colleagues William and Judith Fritz and Stewart and Mary Hollingsworth. The Dyeran Stage coincides with the Olenellus Biozone and begins within the middle member of the Poleta Formation, includes all of the overlying Harkless Formation and Mule Spring Limestone, and the basal one and a half meters of the Emigrant Formation. Data from a preliminary study of the Poleta Formation in the outer shelf and slope environments was presented by Hollingsworth and others (2002).

 

Summary of our Research on the Lower-Middle Cambrian Boundary Interval over the Past Decade (1993-2002)

Pete Palmer made it quite clear to us from the beginning of our research in 1993 that he believed the sedimentologic record of the Lower-Middle Cambrian boundary interval was complete and that facies changes within the boundary interval reflected only minor shifts in the depositional environment. A discussion of Pete's data and conclusions can be found in the introduction to his paper on the terminal Early Cambrian extinction of the Olenellina (Palmer, 1998a). As a side note, Pete also proposed a scheme of formal stages and series for Laurentia that year, including a new Stage (Delamaran) and Series (Lincolnian) at the Lower-Middle Cambrian boundary at Oak Springs (Palmer, 1998b) and quickly added his new Cambrian "ages" to the GSA 1999 Geologic Time Scale.

It has been suggested, most recently by Peters and Foote (2002), that the highly variable rock record may distort the apparent timings of taxonomic origination and extinction at both the outcrop scale and globally. They note that the principles of sequence stratigraphy imply that the last occurrences of taxa should cluster artificially at temporal gaps and shifts in depositional environments resulting from eustatic sea level variations and basin infilling. They conclude that all but the very major extinction events can be explained entirely as stratigraphic artifacts due to an imperfect sedimentologic record. Could this also be the case for the olenelloid extinction?

It seemed highly unlikely that the Laurentian olenelloid extinction was just a stratigraphic artifact, because 1) olenelloids were the dominant, long-ranging trilobite group, who’s generic and species diversity appeared to be as great or greater at the end of their range as it had been any time in their existence and 2) they left no possible descendants (Palmer, 1998a). However, the central question still remains; as to how completely the extinction, crisis, and recovery interval is preserved in the stratigraphic record in our study area. Does our data fit the pattern proposed in the extinction and recovery paradigm, and if it doesn't, why not?

The pattern that has emerged, from numerous studies of extinction and recovery intervals, is that 1) a barren interval occurs immediately above the extinction assemblage; 2) a species-poor assemblage of surviving taxa occurs above the barren zone; and 3) within the surviving taxa zone are opportunistic blooms of single species, presumably taking advantage of the empty ecological niches and the absence of effective predation (see Hart, 1996, Erwin, 1998). This pattern had been documented in Upper Cambrian biomeres (Stitt, 1975, 1977; Loch, Stitt, and Derby, 1993; Palmer, 1984), and it could reasonably be assumed that such a pattern should exist at the Lower-Middle Cambrian boundary interval biomere.

Comparing the faunal and sedimentological data gathered during our research of the Lower-Middle Cambrian boundary interval in the southern Great Basin has given us considerable insight as to the nature of the olenelloid extinction and the completeness of the stratigraphic record within the shelfal environment. We'll first outline the results from three year period (1994-96) of student excavations within olenelloid extinction interval in the type area of the Pioche Shale and in the Carrara Formation of the northern Groom Range. We'll begin with the youngest olenelloid fauna and continue through the extinction, crisis, and recovery interval in the inner to outer shelf sections. Then we will discuss the stratigraphic record in terms of sequence stratigraphy.

The boundary interval in the Pioche Shale was quarried to a depth of three meters at Oak Springs by the First Cambrian Research Class. The two genera and six species of Lower Cambrian olenelloids showed no sign of diminishing diversity or individual body counts (Walkley and others, 1994). Palmer (1998) quarried the boundary interval at the Ruin Wash and Hidden Valley sections and also reported that olenelloid species diversity did not diminish prior to their extinction in the Pioche Shale. However, there is an abrupt lithologic change at Pete's point of "extinction" in the Pioche Shale from the olenelloid-bearing shales (some with very thin limy nodular layers) to a 70 cm thick ribbon limestone containing a silicified Middle Cambrian ptychopariid, Eokochaspis nodosa (Sundberg and McCollum, 2000).

The Groom Range quarry data was clearly different, both in terms of the sedimentologic record and the faunal content. The highest olenelloid-bearing layer occurred 165 meters above the formational contact with the underlying Zabriskie Quartzite and 55 meters below a 4 meter thick ledgy limestone equivalent to the Susan Duster Limestone Member, Pioche Shale. The olenelloid extinction is within a 50 cm thick clastic interval between two thin (20-30 centimeter thick), argillaceous limestones. This half-meter clastic interval begins above the lower argillaceous limestone with a 15cm thick sandy, glauconitic mudstone which has an upper undulating layer of pelletal glauconite up to a centimeter in thickness. Above this glauconitic bed is a 15cm thick fissile shale whose basal 2 cm are densely crowded with two new species of olenelloids and rare specimens of an effaced ptychopariid, which occurs both within and above the olenelloid horizon. The rest of the shale interval consists of bedding plane surfaces covered with inarticulate brachiopods and hyolithids. The latter are sometimes perfectly aligned, with shells barely touching but not overlapping. This thin shale interval appears to consist of opportunistic species blooms in a late crisis or post-crisis stage. The uppermost 20 cm is composed of two thin (1 cm thick) sandstone layers above a sandy mudstone, which is faunally barren except for Planolites.

A regional study of the ichnofauna across the Lower-Middle Cambrian boundary was accomplished by Eladio Linan, Jose Antonia, Fred Sundberg and myself in July, 1997. We found that the ichnofauna diversity was facies controlled, fissile shale having the least diversity and smallest species, but the total diversity didn’t change much. This was in keeping with other benthic faunas, except trilobites, which showed little effect on species diversity across the Lower-Middle Cambrian boundary.

Our efforts on the boundary interval focused on the outer shelf facies of the Emigrant and Monola formations during the 1998-2000 period. We found that these boundary sections were highly condensed, but contained a fossiliferous and diverse fauna. An undergraduate student at the Tennessee Technological University (now at Ohio State Univ.), Jih-Pai Lin, worked as a field assistant on two of our trips and did a preliminary study of the inarticulate brachiopods from the Glossopleura and Ehmaniella zones within the lower Emigrant Formation carbonate facies (Lin, 2001).

In the fall of 2001, we began a detailed study of the Pyramid Shale Member of the Carrara Formation in the Grapevine and Funeral Mountains just to the east of Death Valley, California and several sections in western Nevada. The Death Valley sections included the type Pyramid Peak section near Travertine Point, three sections located in Echo Canyon, and two sections in the Grapevine Mountains, one in Titanothere Canyon and the other in Titus Canyon. The western Nevada sites include two sections in the Pahrump Hills (also known as the Montgomery Hills), and a section at Mount Montgomery near Johnnie.

We began our study of the Pyramid Shale Member at the Titanothere Canyon section where we measured 72 meters of clastic strata between the top of the Gold Ace Member and the base of the type Red Pass Limestone Member. The basal coarsening upward sequence is 37.5 meters thick, beginning with a 12 meter thick olive gray fissile shale containing the Nephrolenellus multinodus fauna, overlain by a 10 meter interval of shale with thin limy bioclastic (olenelloids) and sandstone beds, overlain by 15.5 meters of unfossiliferous, but heavily bioturbated, hackly mudstone and glauconitic quartz sandstone. The next sequence begins with a 5 cm thick lenticular pebble bed whose matrix is composed of yellowish coarse-grained calcite containing an Eokochaspis species which is separated by only 2.5 meters of bioturbated mudstone from the overlying sequence. This 23 meter thick sequence begins with a 10 cm thick pebble bed (which has a carbonate matrix loaded with frosted quartz grains to granules), followed by 10 meters of pale reddish brown shale containing Kochina?  walcotti,  Mexicella antelopea and hyolithids (a fauna found within the third ribbon limestone of the Comet Shale Member, Pioche Shale, within the E. nodosa Biozone), overlain by a 13 meter thick interval of unfossiliferous shales, mudstones, and sandstones (some having interference ripples on the upper surface). The highest sequence is an 8 meter thick greenish gray, unfossiliferous fissile shale whose upper surface is truncated by channeled quartz sandstone.

In the type section for the Pyramid Shale Member, we observed thin (60cm or less thick) sideritic pebble beds at the base of several coarsening upward sequences within the 50 or so meters of this predominantly clastic member. The basal pale olive gray clastic sequence contains a Nephrolenellus multinodus fauna and is composed of a 13 meter thick fissile shale, with a few thin bioclastic carbonates several meters above the base and thin quartz sandstones in the upper few meters. The second olive gray clastic sequence begins with the thickest (60 cm) and lowest of the sideritic pebble layers. Its clasts are imbricated and overlain by a 10 meter thick barren fissile shale interval with thin sandstone beds, some with rippled surfaces, in the upper half. At 23 meters, the third clastic sequence begins with a very thin (several centimeters) bored pebble bed containing silicified specimens of the basal Middle Cambrian Eokochaspis nodosa. E. piochensis? occurs within the basal few meters of grayish green fissile shale within the overlying 20 meter thick coarsening upward clastic sequence, which culminates with a channel sand composed of frosted quartz grains interbedded in a coarse-grained calcite matrix. The fourth clastic sequence also begins with a thin, discontinuous pebble bed which is overlain by several meters of fissile shale containing an Amecephalus arrojosensis fauna. It seems reasonable to interpret these pebble beds as transgressive lag deposits at the base of condensed sequences.

In the Grapevine Mountains, to the north of the Funeral Mountains, exposed in the drainage walls of Titus Canyon, is an overturned Cambrian section which is a virtually identical section to the type Pyramid Shale Member. All three transgressive pebble lags are present at exactly the same stratigraphic interval. The overall thickness of this member within Titus Canyon is within a meter or two of the of the type section.

In Echo Canyon, we measured and described three different outcrop exposures of the Pyramid Shale Member. We found that the section recorded by Palmer and Halley (1979) did not contain the transgressive pebble lag deposits, although a discontinuous limy bed one centimeter thick, approximately 22 meters above the base of the Pyramid Shale Member, contained silicified specimens of Eokochaspis nodosa. There was also a marked color change from the lighter olive grays below to the darker greenish grays above the Lower-Middle Cambrian boundary. A very prominent pebble lag occurs at this horizon in the two Pyramid Shale Member sections up canyon, along with the higher, frosted quartz, channeled sand deposit at the base of the fourth sequence which marks the base of the Amecephalus arrojosensis Biozone. A bored pebble lag also occurs at the Lower-Middle Cambrian boundary in the Eagle Mountain section. The lowest pebble lag, so prominent in the type section of the Pyramid Shale and at Titus Canyon, was absent in all of the Echo Canyon sections.

Certainly, the transgressive pebble lags in the Pyramid Shale Member, Carrara Formation, are relatively thin and lenticular in their distribution. They occur sporadically at the base of regionally extensive, fissile shales in coarsening-upward sequences. They are coincident with changes in trilobite zones and appear to correlate to silty ribbon and nodular limestones found within the Comet Shale Member, Pioche Shale. It is evident that these pebble lags form a series of single parasequence sets within a condensed sequence which begins at the base of the Pyramid Shale Member and ends at an erosional surface below the lowest coarse-grained limestone layer. There is a lack of direct evidence at the outcrop level as to whether these pebble lags were deposited above erosional surfaces, or whether hiatal gaps in the sedimentary record exist below them.

However, we had documented a north to south change in facies patterns in the Combined Metals and Comet Shale (C-shale of older usage) members of the Pioche Shale that might best be explained by missing section at the top of the Lower Cambrian (which would include most or all of the highstand systems tract) immediately below a post-extinction ribbon limestone. The presence of a coarsening upward facies, typical of a highstand tract, at the top of the olenelloid sequence within the medial to outer shelf sections of the Carrara Formation at Groom Range and the Death Valley region gives regional support to our contention of missing (eroded or not deposited) section in the nearshore region. The "youngest" olenelloid fauna reported by Palmer (1998) from the Pioche Shale is also present in the Groom Range, where it is overlain by an opportunistic species interval which has two undescribed olenelloid species at the base. The absence of an opportunistic species interval and an overlying barren interval in the Pioche Shale does support my contention that the nearshore record is incomplete (McCollum, 1994, 1995; McCollum and McCollum, 1994). A discussion of just how complete the sedimentary record is at biomere extinctions, emphasizing the Lower-Middle Cambrian biomere, is the focus of a talk by McCollum, Sundberg and Montanez (2002).

Chemostratigraphy of the Lower-Middle Cambrian Boundary Interval
Several younger extinctions have been tied to sharp isotopic excursions of carbon and strontium, which includes a few studies in the Upper Cambrian (see Saltzman and others, 1995, 1998; Perfetta and others, 1999). These sharp isotopic excursions are thought to be related to a rapid drop in oceanic productivity, sometimes coupled with worldwide sea level changes, and may be the "smoking gun" in the extinction of the Lower Cambrian olenelloid trilobite fauna. Isabel Montanez and her husband Dave Osleger (University of California, Davis) are currently working on a carbon isotopic study of the Lower-Middle Cambrian boundary interval in western North America (Montanez and others, 1999, 2000). They have found a significant carbon isotopic anomaly within the Lower-Middle Cambrian boundary interval, indicating that a major paleoceanic and probable climatic change preceded the trilobite (biomere) extinction at the end of the Early Cambrian.

Isabel and her students are continuing their work on this interval in the southern Great Basin. They have collected samples across the Lower-Middle Cambrian boundary interval in the Pioche Shale, Carrara Formation, and Emigrant Formation. I collected samples from the boundary interval of the Kaili Formation, South China, during the summer of 2001 to be included in this isotopic study, and the preliminary results were presented by Montanez and others (2002). Hopefully, this data will form the basis for an isotopic study of the interval in the Circum-Pacific region.

REFERENCES

Adams, R.D., 1993, Sequence-stratigraphic analysis of mixed carbonate-siliciclastic Cambrian sediments, Carrara Formation, southwest Basin and Range, California and Nevada. Unpub. Ph.D. dissertation, MIT, 750 p.

Adams, R.D., 1995, Sequence-stratigraphy of Early-Middle Cambrian Grand Cycles in the Carrara Formation, southwest Basin and Range, California and Nevada.  In Haq, B.U., ed., Sequence Stratigraphy and Depositional Response to Eustatic, Tectonic and Climatic Forcing, p. 277-328.

Albers, J.P., and Stewart, J.H., 1972, Geology and mineral deposits of Esmeralda County, Nevada. Nevada Bureau of Mines and Geology Bull. 78, 80 p.

Beaver, N.A., Jr., Larsen, J.C., Newton, J.B., Schulz, C., and McCollum, L.B., 2000, Results of undergraduate research on the early Middle Cambrian biostratigraphy of the Monola and Emigrant Formations, western Great Basin. Geological Society of America Abstracts with Programs, v. 32, no. 7, p. A456.

Beaver, N.A., Jr., Boothe, B.S., Hoover, R.L., Koenig, M.C., Newton, J.B., McCollum, L.B., and Sundberg, F.A., 2001, Stratigraphy of the Middle Cambrian Susan Duster Limestone Member, Pioche Shale, eastern Nevada. Geological Society of America Abstracts with Programs, v. 33, no. 3, p. A48.

Eddy, J.D., and McCollum, L.B., 1998, Early Middle Cambrian Albertella Biozone trilobites of the Pioche Shale, southeastern Nevada. Journal of Paleontology, v. 72, p. 864-887.

Erwin, D.H., 1998, After the end: Recovery from extinction. Science, v. 279, p. 1324-1330.

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Hamilton, M.M., Derkey, R.E., and McCollum, L.B., 2003, Early Middle Cambrian (Glossopleura Biozone) trilobites in a steptoe surrounded by Columbia River Basalt near Spokane, Washington. Geological Society of America Abstracts with Programs, v. 35, p. 159.

Hardy, J.K., 1986, Stratigraphy and depositional environments of Lower and Middle Cambrian strata in the Lake Mead region, southern Nevada and northwestern Arizona. Unpub. MS thesis, UNLV, 309 p.

Hart, M.B., ed., Biotic Recovery from Mass Extinction Events. Geological Society Special Publications, 392 p.

Hintze, L.F., and Robison, R.A., 1975, Middle Cambrian stratigraphy of the House, Wah Wah, and adjacent ranges in western Utah. GSA Bulletin, v. 86, p. 881-891.

Hollingsworth, J.S., McCollum, M.B., McCollum, L.B., and Fritz, W.H., 2002, Outer shelf to slope deposits of the Lower Cambrian Poleta Formation, Western Great Basin, Nevada. Geological Society of America Abstracts with Programs, v. 34, no. 6, p. 138.

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