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
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
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
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
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
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
The cratonal sections in the
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
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
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
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
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
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 (
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
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
(
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
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
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
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
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 (
Gerd Geyer (
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
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
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
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
Another cratonal region of Cambrian rocks occurs in the
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
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
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
The northern
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
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
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
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
We returned in late May and co-taught our geology field camp, spending two
weeks mapping portions of the
It was during the geological mapping exercise in the
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
We were very fortunate to gain access to the
1995
The third Cambrian Research Class
began that September by resuming the study of the boundary interval at the
Less then a mile north of Caliente was a third boundary section, exposed in
the steep north side of
After a few days in the Caliente area, we moved camp to the northern
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
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
THE LOWER-MIDDLE CAMBRIAN BOUNDARY STUDY
EXPANDS FROM THE
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
1996-98
The first NWI grant covered expenses to the Second Cambrian International
Subcommission Field Conference trip in
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
In May, 1998, Fred, Mike and I then went on to measure and describe
The third NWI grant covered the expenses of Fred Sundberg and me on our
1999
In early June, 1999, I co-chaired (with Stew Hollingsworth) a full day
symposium at the GSA meeting in
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
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
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
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
2002
I spent two weeks at the end of June, 2002, with an undergraduate student,
collecting faunas from several sections in the southern
In late July, my husband Mike and I traveled to see Dr. Richard A. Robison
at the
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,
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
In August of 2003, the seventh
Cambrian Research Class began measuring and collecting the Rennie Shale and
lower Lakeview Limestone in northern
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
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
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,
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.
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Last updated 11/08