RESEARCH within the ANTLER and
LINDA B. McCOLLUM
MICHAEL B. McCOLLUM
For over a decade, we taught part or all of our spring
geology field camp within the deformed rocks of the Antler and Sonoma Orogens,
with mapping projects in the
The first geology field camp mapping project in the Antler
orogen was restricted to several square miles in the Galena and Iron Canyon
drainage system on the west side of Battle Mountain and took place during the
spring of 1985 and 1986. In spring of
1987, we relocated our spring field camp to the
By 1994, shrinking enrollments and departmental budget cuts caused us to discontinue teaching our own field camp, and our graduate program was shelved. Therefore, the only opportunity to involve students in our field-oriented research would be during the summer between their junior and senior years. Our research interests became focused on the more elementary aspects of stratigraphic research and we selected a project in the Lower-Middle Cambrian boundary interval (see Cambrian Research).
The purpose of this document is to make a single record of our observations, including some of the unpublished database, concerning the nature of the sedimentary rocks within the region of the Antler and Sonoma Orogenic belt. Many of our observations and interpretations were presented at GSA meetings, so a brief summary exists as a series of published abstracts and an occasional short paper. We even suggested a different tectonic scenario to account for the emplacement of these orogenic belts (McCollum and McCollum, 1989) and the factual database is presented for the first time in this document.
BATTLE MOUNTAIN, LANDER COUNTY, NEVADA
Both the Antler and
Objective of our
The objective to doing research at Battle Mountain was to 1) resolve the apparent age discrepancy within the Scott Canyon Formation (Lower Cambrian archaeocyathids reported by Roberts, 1964, and Devonian microfossils reported by Jones and others, 1978), 2) attempt to establish an internal stratigraphy within the Scott Canyon Formation and deduce its depositional history, 3) ascertain the nature of the contact between the Scott Canyon and Harmony formations, since the contact had been previously mapped as the Dewitt thrust, a structure justified by the supposed differences in structural history and presumed juxtaposition of formation consisting of facies thought to deposited in widely separated environments, such as the cherts and greenstones in the Valmy and quartzofeldspathic sandstones in the Harmony (a notion already challenged by Suczek, 1981), and 4) see whether the structural history of both formations were shared or distinctly different.
In the spring of 1985, our geology field camp began mapping
a two-square-mile area between
The geology of the area to be mapped consisted of two
At the scale of our mapping, most lithologies within the
Scott Canyon Formation could be mapped out individually, thus separating
cherts, sandstones, mudrock, and volcanic layers. Tracing out key beds resulted in a much
greater appreciation for mid-scale folding, which had previously been ignored
in the larger scale mapping. In
addition, it allowed us to establish a stratigraphic sequence within the
Results of our
Age discrimination within the lower Paleozoic deep water and
oceanic rocks has always been difficult, and this is well reflected in the history
of geologic mapping within the
Results of the detailed geologic mapping were 1) a small fault-bounded block of chert bearing a Devonian microfauna at the mouth of Scott Canyon is interpreted as a down dropped block of Slaven Chert; 2) the more heterolithic portion of the Scott Canyon Formation resembled the Valmy Formation, and this was confirmed by the presence of Middle Ordovician radiolarian and conodont faunas within a prominently bedded chert layer; 3) a stratigraphy existed within the portion of the Scott Canyon Formation (now transferred to the Valmy Formation) which consisted of a massive gray quartzite overlain by a heterogeneous mudstone-chert-greenstone and minor quartz sandstone facies overlain by a mudstone containing Lower Cambrian fossiliferous limestone olistoliths, overlain by the Harmony-Elder Sandstone (this stratigraphic sequence can be traced along the Valmy-Harmony outcrop belt its entire length from the Sonoma Range southward to Hall Creek); 4) the limestone lenses containing the Lower Cambrian archaeocyathids reported by Roberts are interpreted as olistoliths within a mudstone interval that occurs at the presumed top of the Valmy Formation, adjacent to the Harmony Formation, throughout their mutual outcrop length; 5) the contact between the Valmy and the Harmony is clearly depositional and well exposed in our map area; 6) although small-scale folding in the heterolithic and thin-bedded layers of the Valmy Formation are absent in the more massive quartzofeldspathic sandstones of the Harmony, the mid- to large-scale folding has affected both formations, which is particularly well exposed just north of Galena Canyon, where both formations are clearly infolded; 7) there is no evidence that the Dewitt thrust exists in our map area, and no compelling reason that the Valmy and Harmony Formations should be separated structurally; 8) there is no faunal evidence for the age of the Harmony at Battle Mountain or elsewhere, although the Silurian age of the Elder Sandstone is factual; 9) the Late Cambrian-Early Ordovician dates from microfossils reported by Madden-McGuire and others (1991) is widely believed to be based on fraudulent data and should not be cited according to the senior author; 10) the Upper Cambrian shelly faunas reported from strata mapped as Harmony in the Osgood Mountains and Hot Springs Range, as reported in Hotz and Willden (1964), are from olistolithic units within upper Paleozoic formations (McCollum and McCollum, 1991a) and can no longer be considered conclusive evidence of age or of faunal provenance; and 11) the reason the Harmony and Elder Sandstone never co-occur in the same mountain (although both occur with the Valmy Formation) is that they are different names applied to the same unit, lithologically and stratigraphically.
The interpretation of our two square miles of detailed
mapping, supplemented by numerous field observations, is surprisingly different
from those USGS geologists that have mapped hundreds of square miles of this
terrane. Simply put, the lower Paleozoic
oceanic sequence is remarkably similar from mountain range to mountain
range. The co-occurrence of the Valmy
and Harmony formations at
Our student mapping project at
Again, both the Antler and
Objective of our
The objective to doing research at Osgood Mountain was to ascertain 1) the age and nature of the contact between the “Harmony” Formation and the adjacent upper Paleozoic Formations since this was the only area where a formation previously found only within the Roberts Allochthon was emplaced into a sequence deposited after the presumed lower Mississippian age emplacement of the Roberts Mountain Thrust, 2) the Antler overlap sequence is present in the Osgood Mountains, but deposited on the Cambrian Osgood Mountain Quartzite, a shelfal unit thought to be part of the autochthon and the Antler overlap is shown being overthrusted by the Cambrian Preble Formation, and 3) there was a renewed interest in this mountain by economic geologists employed by mining companies interested in the revival of gold mining along the old Getchell Trend, and many of these companies promised access to their properties and copies of letters refining age assignments and internal mapping of both the surface and subsurface in order to aid us in mapping this range as accurately as possible. In fact it was a mining geologist (Doug McGibbon) that we meet near the crest of the Osgoods in the fall of 1985 that first brought our attention to the fact that much of the mapping by Hotz and Willden (1964) needed to be redone at a larger scale.
Our spring geology field course began mapping the
Results of our
Research in the
Some of the results of our detailed geologic mapping in the Osgood Mountains were already noted in McCollum and McCollum, (1989a,b; 1990, 1991a,b) and include 1) the recognition of that a through going fault (Osgood Mountain suture) separated the parautochthonous lower Paleozoic outer shelf rocks, which were unconformably overlain by the upper Paleozoic Antler overlap sequence, from the upper Paleozoic continental margin to slope oceanic rocks, 2) that the upper Paleozoic section consists of four mappable formations (Farrel Canyon, Goughs Canyon, “Harmony”, and Adam Peak) ranging in age from early Mississippian to the early Permian, 3) that this upper Paleozoic formational sequence was infolded, but not separated by several thrusts as depicted by Hotz and Willden (1964), 4) that the Upper Cambrian faunas were recovered from megabreccia blocks and smaller olistoliths within the “Harmony” Formation had microfaunas as young Pennsylvanian in age, and that large blocks (some were of typical Harmony facies, others were from the Battle and Etchert Formations) derived and shed basinward from the Antler orogenic highland (including material from both below and from the Antler overlap facies), 5) that the Ordovician rocks exposed along the east side of the mountain range seem to grade northward from a shale facies typical of the Comus Formation into the shale, chert and volcanic facies typical of the Valmy Formation, 6) that the limestone boulders containing early Ordovician trilobites occur as olistoliths in a mudstone facies adjacent to the greenstones mapped as Valmy Formation by Hotz and Willden (1964, p. 23), and 7) our mapping confirmed the fact that in the region including the Osgood Mountains, Hot Springs Range, and Edna Mountains, the lower Paleozoic of both the allochthon and the autochthon share a similar to identical structural history and both are unconformably overlain by the Antler overlap sequence.
We have had the great good fortune to have our geologic mapping at Battle Mountain and in the Osgood Mountains critiqued in the field and then put into a regional perspective by several key players who contributed directly to establishing the major tenets of the Paleozoic orogenies in the western US. Most of the interest came as a result of a poster secession (McCollum, Buchanan, and McCollum, 1985) in which we presented our mapping of the Galena Canyon area at Battle Mountain showing that the Lower Cambrian faunas used to date the Scott Canyon Formation by Roberts (1964) occurred as limestone olistoliths in a mudstone found above the highest chert layer and just below the depositional contact with the overlying Harmony Formation. Although it was not mentioned it in our abstract, we were thinking that, based on our observations that the Devonian Scott Canyon Formation was in conformable contact with the younger Harmony Formation, that the Harmony must be the lateral and time equivalent of the Mississippian(?) Inskip Formation as mapped in the East Range.
In late spring, 1986, Walt Snyder visited us at camp in Galena Canyon and spotted microfossils (which turned out to be an Ordovician radiolarian and conodont assemblage) in a regionally extensive chert bed just below the olistostromal unit (containing the Lower Cambrian archaeocyathids) found in depositional contact with the Harmony Formation. This find was to profoundly change our ideas concerning the age of the Scott Canyon Formation, since there were only two mappable cherts in this area. We had written Chris Suczek in 1985 to send us a locality map of where she had recovered the Devonian microfossils reported by Jones and others (1978) as coming from the Scott Canyon. Her locality map showed that all of her samples can from a chert dominated facies within a relatively small area just at the mouth of Scott Canyon on the north side of Galena Canyon. We had previously mapped this Devonian chert unit as being fault bounded on all sides by high angle normal faults that had extensive brecciation and mineralization (several pits and adits were dug along these fault zones). We were beginning to suspect that the Devonian cherts were a down dropped segment of the Slaven Chert found to the east in the northern Shoshone Range and that the rest of the Scott Canyon Formation, consisting of massive gray quartzite, mudrocks, chert and volcanics should be reassigned to the Valmy Formation.
On July 1, 1986, we spent the day with Ralph Roberts, viewing critical contacts in the lower Paleozoic rocks near the mouth of Galena Canyon that proved that his “Scott Canyon” (now Valmy based on Ordovician microfossils and Slaven based on Devonian microfossils) Formation was both interbedded and infolded with the Harmony Formation (and by inference, the Harmony probably correlates to the Silurian Elder Sandstone). Dr. Roberts expressed his surprise that these relationships we were pointing out had escaped notice since so many geologists had visited this area over the several decades since he finished his mapping. He gave us a brief review of the early geologic mapping in this area which gave us a much needed prospective on the formulation of ideas regarding the Antler Orogen.
On September 13, 1986, Keith Ketner joined us for the same tour of this critical section. Keith was also an important player as he and the late Fred Smith had determined the age of the emplacement of the Roberts Mountain Thrust as early Mississippian. He later came to challenge that age and revisited many of the sites used to determine the age of thrusting within the Antler Orogen.
On June 12, 1987, we came across Elizabeth Jones, a grad student of Davy Jones, looking at chert beds in the Farrel Canyon Formation along the western slopes of the Osgood Mountains. Liz informed us that she wished to do her own thing and didn’t want our input despite our having just completed mapping in the Adam Peak quadrangle. About two weeks later were met up with Davy Jones and he assured us that Liz was only interested in studying the microfossils in the Paleozoic cherts and he thought what kind of information would be very useful in our stratigraphic and structural interpretations that were to accompany our geologic mapping. Neither of them ever consulted with us, or even acknowledged our work, as they proceeded on their own, so we finally withdrew from this region in 1993 until they had finished up.
On June 25, 1987, we recollected both Ordovician and Devonian chert samples with David Jones. Dr. Jones thought we should present the new faunal data along with our conclusions that the Scott Canyon Formation was actually a structural amalgamation of the Ordovician Valmy and the Silurian Slaven Chert. The reassessment of both the ages and formational assignments were presented that October at the national GSA in Phoenix (McCollum, McCollum, Jones, and Repetski, 1987).
During May of 1989, we walked the length of the olistostromal facies containing the Lower Cambrian limestone olistoliths with Roland Gangloff, who collected additional archaeocyathid material. Dr. Gangloff, a leading expert on the archaeocyathida, also concluded that the fauna found here at Battle Mountain was atypical of the Great Basin archaeocyathids he had described in his dissertation in 1975. He was so impressed with our stratigraphic column that he had it reproduced as figure 2 in Debrenne, Gandin and Gangloff (1990) on their redescription of the archaeocyathid collections first reported by Roberts (1964).
In early July, 1989, we returned to Battle Mountain and spent a day with Stan Finney, searching unsuccessfully for any sign of graptolitic facies in the Valmy portion of the Scott Canyon Formation. Dr. Finney noted that he didn’t have any luck finding graptolites in any of the sites in the Valmy Formation listed by Roberts (1964) on the other side of Antler Peak, noting that the typical dark shale facies are just not present in this region. We went on to spend a few days with Stan in the Roberts Creek Mountains going over some of the structural and stratigraphic areas he had been working on.
In April, 1992 (at the end of the GSA meeting in Ogden, Utah) Mike McCollum and A.R. “Pete” Palmer spent a week looking a Cambrian exposures in the northern Great Basin including several important sites where Pete had previously identified Cambrian trilobites in the Antler Orogen. Upon viewing the Upper Cambrian localities in both the Osgood Mountains and Hot Springs Range cited in Hotz and Willden (1964), Pete soon became convinced that the megabreccia limestones containing the Upper Cambrian faunas were olistoliths and he likened them to the Cowhead Breccia in Newfoundland, Canada. As editor of the DNAG series, Pete had a note added at the proof stage of a chapter he had co-authored calling attention to our work in the Osgood Mountains (McCollum and McCollum, 1991a), regarding the age change from Upper Cambrian to Pennsylvanian of the “Harmony” Formation (see Poole and others, 1992, v. G-3. p. 55).
What most puzzled Pete was that the sedimentary nature of these redeposited fossiliferous limestone blocks had not been recognized before, particularly as these rocks had been focus of several subsequent studies including an influential paper on the Late Cambrian continental margin in Nevada by Rowell, Rees, and Suczek (1979). In fact, back in June, 1982, we spent several days looking at Cambrian sections with Burt and Peggy including the Shwin Formation in the Shoshone Range and the Harmony Formation in the Hot Springs Range. The agnostid fauna in the shales of the Shwin Formation were not redeposited and it was difficult to tell what the context of the platy fossiliferous limestone float in the Harmony was at the main collecting site we visited. In all fairness, Chris Suczek (1977) realized that some of the Upper Cambrian boulders were probably olistoliths, but found nothing to suggest the real age of the Harmony Formation in her study area was younger (personal communication at the Spokane GSA, May, 1989).
We returned to Battle Mountain one last time together in late May, 1993 and meet with Phil Signor to take several days comparing the archaeocyathid localities in both the oceanic Antler orogen and shelfal sections. We next traveled to an archaeocyathid site in the Valmy Formation just north of Hall Creek in the Toiyabe Range and found several very large olistoliths near the top of the Valmy Formation containing archaeocyathid faunas similar to those at Battle Mountain. We continued to travel south and visited the Cambrian section in the Mount Callaghan window and the Ravenswood archaeocyathid locality just to the west. Phil later had a student do a Masters Thesis on the Hall Creek faunas (Dorritie, 1998), comparing this archaeocyathid assemblage in a regional context.
On June 28, 1993, Linda McCollum and her graduate student Julie Eddy showed Jeff Doebrich and Ted Theodore some of the more important aspects, including the olistostromal unit just below the Harmony Formation at Galena Canyon, and gave them a copy of our geologic map. So impressive were the exposures there that they later included this as Stop 5 of a field trip road log to the Battle Mountain Mining District (Doebrich and others, 1996). Jeff later incorporated our large-scale mapping onto his 1:24,000 scale mapping of the Galena Canyon quadrangle by Doebrich (1994), but he did not abandon the notion that the Scott Canyon Formation was a valid unit of Devonian age, claiming that the Ordovician age reported by McCollum and others (1987) was possibly from a tectonically imbricated and downfaulted block of Valmy Formation. Jeff also kept the Dewitt Thrust at the base of the Harmony Formation, thus rejecting any idea that the sedimentary contact between the Harmony Formation and the adjacent Valmy or Scott Canyon Formation, that we showed numerous geologists over the years (including Ralph Roberts), exists. We have never understood the loyalty over honesty policy practiced by some USGS geologists, but Jack Stewart made it clear to us many years ago that it does exist and those that ignore this policy of supporting previous conclusions (Jack cited Keith Ketner as a prime example) do the institution a great disservice. Despite this, Stewart (1991, p. 24-25) and Theodore (2000) did present the litany of views concerning both the age and structural context of the Harmony Formation.
The Antler Orogeny was named by Ralph J. Roberts in 1949 (Roberts, 2002) from exposures at Antler Peak, Battle Mountain, Nevada and the tenets are largely a construct of USGS geologists mapping in central Nevada. The first regional summary of the Antler Orogen appeared almost a decade later (Roberts and others, 1958). Two decades later, a Penrose Conference on the Antler Orogeny during September 9-14, 1979, in Elko, Nevada, which included field trips and was attended by many of the leading government, academic, and industry geologists working in the west, formulated a consensus on the definition and extent of the Antler orogen and possible tectonic models to account for it (Nilsen and Stewart, 1980). Almost a decade later, in 1989, we instigated and chaired a GSA symposium entitled “Upper Paleozoic Orogenies of Western North America”, and hardly a year goes by that some aspect of these Paleozoic orogenic events are presented at one or more of the GSA meeting. Both the Antler and Sonoma orogenies have found themselves in regional geology and structural textbooks, but seldom is there any sense of controversy over the database (compiled mostly by USGS geologists), although there is some acknowledgement of uncertainty over the correct tectonic setting (academics have done most of the modeling).
The Antler Orogenic belt consists of a structural complex of lower Paleozoic continental-margin, slope and rise sedimentary and volcanic rocks which were transported, deformed, uplifted, eroded, and overlapped by upper Paleozoic orogenic shed and shallow water carbonates and clastics. The Sonoma orogenic belt is a later event, very similar in style and composition, but smaller in geographic extent. There is evidence of complex structural imbrication within some of the oceanic sequences locally, which can be dated as having formed prior to the deposition of the overlap assemblage. Certainly, the eastern portion of the Antler belt was emplaced during the late Devonian and uplifted early in the Mississippian, forming a deep depression along the contact zone which filled with orogenic debris. However, there is absolutely no direct and irrefutable evidence as to the mode of emplacement of these oceanic belts within the continental crust, since the Roberts and Golconda thrusts are Mesozoic (most likely Jurassic) features everywhere they can be accurately dated (Ketner and Smith, 1982; Stahl, 1989; Ketner, 1998; Ketner and others, 2000; Dunston and others, 2001). Keith B. Ketner (USGS) has spent much of his professional life mapping areas critical for dating the age of the Roberts and Golconda thrusts, but his work is seldom, if ever cited.
Almost all previous models of the Antler Orogen have considered the Roberts thrust to be the key essential element in ascertaining that the tectonic setting must be within a compressional regime which resulted in the emplacement of lower Paleozoic oceanic rocks over lower Paleozoic shelfal rocks (Nilsen and Stewart, 1980) and that this emplacement occurred during the Late Devonian and Early Mississippian (Johnson and Pendergast, 1981). These models were always set within an arc setting, differing in whether the arc was obducted, subducted or relatively unaffected, during the Antler orogeny (Moores, 1970; Speed and Sleep, 1982; Burchfiel and Royden, 1991). But would these models apply if the emplacement of the Roberts allochthon over the shelfal facies took place in the mid-Mesozoic? Was it time for a very different view of the Antler Orogeny in terms of its tectonic setting? We thought so and took a shot.
We (McCollum and McCollum, 1989a) presented a paper suggesting that the Antler Orogen was a far traveled (from at least as far away as northwestern Canada based on the distinct archaeocyathan assemblages found in the Valmy Formation) terrane that tectonically emplaced oceanic and continental terranes against one another within a transcurrent fault system operating within a transpressional orogen, noting that only in the Jurassic did upper portions of these oceanic terranes get displaced laterally over the shelfal rocks along the Roberts Mountain and related thrusts. In order to reach a large and interested audience, we scheduled our talk after the afternoon coffee break and sandwiched it between two other husband and wife teams speaking on the Antler Orogeny in our GSA symposium on the “Upper Paleozoic orogenies of western North America” which we chaired. The room was filled to capacity for the Burchfiel and Royden talk, emptied (almost everyone stepped outside the room and engaged in pleasant conversation in the hallway during our talk) and then the capacity crowd returned to hear the Silberling and Nichols talk. Despite our strategy of scheduling our talk into an excellent time slot and thereby reaching a relatively captive audience, we didn’t anticipate that virtually no one was even interested in listening to our ideas for a short 10 minutes. Ironically, it was Ralph Roberts that told us just a few years before this (during our only encounter with him), that he thought the time for a totally new interpretation of the Antler Orogeny was not right and we should wait until after the old guard had retired if we wished for a kind reception.
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This document was last updated
SEDIMENTARY ENVIRONMENT AT THE TYPE ANTLER OROGEN AT
McCOLLUM, Linda B., BUCHANAN, John P., McCOLLUM, Michael B., Department of Geology, Eastern Washington Univ., Cheney, WA 99004
The Antler orogeny is a textbook example of a Paleozoic mountain building and crustal shortening event in western North America. A relatively complex geologic history of the type Antler at Battle Mountain, Nevada, is interpreted as distinct thrust plates of Lower Cambrian Scott Canyon Formation, Upper Cambrian Harmony Sandstone, and Ordovician Valmy Formation, overlain unconformably by the Middle Pennsylvanian Battle Formation. Mississippian crustal deformation and emplacement of the Roberts Mountain thrust have previously been thought to characterize the Antler orogen.
Detailed sedimentology studies of the Scott Canyon and Harmony, and the relationship with the overlying Battle Formation at the type section of the Antler orogeny, cast doubt on the previously accepted geologic history. The Scott Canyon is an interbedded sequence of pillow basalts, Late Devonian radiolarian cherts, and mudstone debris flows with numerous limestone olistoliths, many containing undescribed archaeocyathid fauna. The Scott Canyon and overlying Harmony are interbedded and in depositional contact.
The contact of the Harmony with the Battle Formation appears channeled, but otherwise conformable, and the Battle has been interpreted as an alluvial fan facies. The paleoenvironmental interpretation of these sediments in that the Scott Canyon was deposited upon a Late Devonian active continental margin setting, with prograding fan deposits of the Harmony Sandstone, overlain by Middle Pennsylvanian fanglomerates of the Battle Formation. This conformable sequence appears to preclude any major uplift with the type Antler orogen.
THE SCOTT CANYON FORMATION, BATTLE MOUNTAIN, NEVADA: A STRUCTURAL AMALGAMATION OF THE ORDOVICIAN VALMY FORMATION AND DEVONIAN SLAVEN CHERT
McCOLLUM, Linda B., McCOLLUM, Michael B., Geology Dept., Eastern Washington University, Cheney, WA 99004; JONES, David L., Department of Geology, University of California, Berkeley, CA 94720; REPETSKI, John E., U.S. Geological Survey, E-501 U.S. National Museum, Washington, DC 20560
A reevaluation of the type areas for several formations within the Roberts Mountains allochthon (RMA) in the Antler orogen is presently under way. The oldest formation within the RMA was believed to be the Scott Canyon Formation, which contained a late Early Cambrian archaeocyathid fauna. Devonian radiolaria were reported by Jones and others in 1978 from bedded cherts at the type section, suggesting regional equivalency to the Devonian Slaven Chert.
McCollum, Buchanan, and McCollum, in 1985, noted that the Cambrian faunas reported from the Scott Canyon and Harmony Formations occurred in limestone olistoliths within debris flows. Therefore, the only reliable age determination was the Devonian microfauna within the Scott Canyon, and the overlying Harmony was tacitly assumed to be as young as Early Mississippian.
Geologic mapping at a scale of 1:4,800 revealed that part of the type section of the Scott Canyon Formation is a down dropped block of Slaven Chert within the Butte Mine fault system. The remaining Scott Canyon Formation contains a Middle to Late Ordovician microfauna, and is herein assigned to the Valmy Formation. The overlying Harmony now appears to be equivalent to the Silurian Elder Sandstone.
THE ANTLER OROGENY: A TRANSPRESSIONAL OROGEN WITHIN A TRANSCURRENT FAULT SYSTEM
McCOLLUM, Linda B., McCOLLUM, Michael B., Department of Geology, Easter Washington University, Cheney, WA 99004
The Eden Valley terrane, exposed in the Osgood Mountains and Hot Springs Range, consists of a Mississippian to Permian continental slope to ocean basin sequence adjacent to the Antler orogen. The sequence is composed of quartz-rich submarine sand sheets containing chert granules, radiolarian and spicular ribbon cherts, and submarine pillow basalts of the Lower Mississippian Farrel Canyon Formation; submarine basalts, chert pebble conglomerates, and carbonate debris flows of the Upper Mississippian to Lower Pennsylvanian Goughs Canyon Formation; mud-supported olistostromes with boulders derived from the Cambrian Preble Formation (autochthon), the Silurian Elder/Harmony Sandstone (allochthon), and Upper Mississippian bioclastic limestone in the Middle to Upper Pennsylvanian “Getchell” (formerly mapped as Harmony by Hotz and Willden, 1964) formation. The Eden Valley terrane is separated from the autochthon by a high-angle transcurrent fault, and sealed by the Lower Permian Adam Peak Formation. Mesozoic thrust faulting subsequently detached this amalgamated terrane and moved it cratonward several kilometers.
The Eden Valley terrane places severe constraints on the tectonics of the Antler orogeny. Petrology and geochemistry of the submarine basalts show that they lack olivine, and rare earth elements place them in a “within-plate” tectonic regime. This effectively excludes all island arc and rift models thus far proposed for the Antler orogen. In addition, the rather limited geographic extent of the Antler highland and foreland basins, the presence of the highly folded and subsequently detached oceanic rocks of the Roberts Mountains allochthon and the lack of Carboniferous plutonism and volcanism all support a transpressional origin for the Antler orogeny. This orogen appears to be part of a Carboniferous left-lateral transcurrent fault system which extended southward from the Beaufort Sea (Eisbacher, 1983). A minimum displacement of 2500 km is needed to account for an endemic Siberian archaeocyathid fauna found in limestone olistoliths with the Valmy Formation (formerly mapped as Scott Canyon by Roberts, 1964) at Battle Mountain with the nearest similar fauna in the Mackenzie Mountains of northwest Canada.
THE ADAM PEAK
ALLOCHTHON: A COMPOSITE TERRANE IN THE
OSGOOD MOUNTAINS AND HOT SPRINGS RANGE, NORTHER
McCOLLUM, Linda B., McCOLLUM, Michael B., Department of Geology, Easter Washington University, Cheney, WA 99004
In the Osgood Mountains, the Adam Peak allochthon is a composite terrane composed of the lower Paleozoic Osgood terrane of Madden-McGuire and Marsh (1991), unconformably overlain by the upper Paleozoic Antler overlap sequence, and the upper Paleozoic Eden Valley terrane of McCollum and McCollum (1989, 1991). The Eden Valley and Osgood terranes are separated along the Osgood Mountain suture, a high-angle, north-south structure which predates the Adam Peak thrust. Recently completed geologic mapping in the Adam Peak and Eden Valley 7.5-minute quadrangles includes much of the aerial extent of the Adam Peak allochthon in the Osgood Mountains.
In the Hot Springs Range, the upper Paleozoic and Triassic rocks are part of a folded sequence of allochthons whose basal, undeformed decollement rests upon the lower Paleozoic Harmony Formation of the Roberts Mountains terrane. Although the Osgood terrane is not present in the Hot Springs Range, this low-angle decollement appears to be correlative with the Adam Peak thrust. As pointed out by Sosson and other (1984), deformation within the allochthon is post-Upper Triassic, and therefore not related to the Golconda allochthon and deformation during the Sonoma orogeny, as recently proposed by Jones and Jones (1991). However, the Golconda allochthon may be represented by Upper Mississippian to Lower Pennsylvanian lithic quartz sandstones, previously referred to the Havallah sequence in the Hot Springs Range, and erroneously included in the Farrel Canyon Formation by Hotz and Willden (1964) in the Osgood Mountains.
Last updated 2/04