Stirling Quartzite Research Articles

Superposition of two kinematically distinct extensional phases in southern Death Valley: Implications for extensional tectonics

Abstract Geologic mapping in southern Death Valley, California, demonstrates Mesozoic contractional structures overprinted by two phases of Neogene extension and contemporaneous strike-slip deformation. The Mesozoic folding is most evident in the middle unit of the Noonday Formation, and these folds are cut by a complex array of Neogene faults. The oldest identified Neogene faults primarily displace Neoproterozoic units as young as the Johnnie Formation. However, in the northernmost portion of the map area, they displace rocks as young as the Stirling Quartzite. Such faults are seen in the northern Ibex Hills and consist of currently low- to moderate-angle, E-NE–dipping normal faults, which are folded about a SW-NE–trending axis. We interpret these low-angle faults as the product of an early, NE-SW extension related to kinematically similar deformation recognized to the south of the study area. The folding of the faults postdates at least some of the extension, indicating a component of syn-extensional shortening that is probably strike-slip related. Approximately EW-striking sinistral faults are mapped in the northern Saddlepeak Hills. However, these faults are kinematically incompatible with the folding of the low-angle faults, suggesting that folding is related to the younger, NW-SE extension seen in the Death Valley region. Other faults in the map area include NW- and NE-striking, high-angle normal faults that crosscut the currently low-angle faults. Also, a major N-S–striking, oblique-slip fault bounds the eastern flank of the Ibex Hills with slickenlines showing rakes of <30°, which together with the map pattern, suggests dextral-oblique movement along the east front of the range. The exact timing of the normal faulting in the map area is hampered by the lack of geochronology in the region. However, based on the map relationships, we find that the older extensional phase predates an angular unconformity between a volcanic and/or sedimentary succession assumed to be 12–14 Ma based on correlations to dated rocks in the Owlshead Mountains and overlying rock-avalanche deposits with associated sedimentary rocks that we correlate to deposits in the Amargosa Chaos to the north, dated at 11–10 Ma. The mechanism behind the folding of the northern Ibex Hills, including the low-angle faults, is not entirely clear. However, transcurrent systems have been proposed to explain extension-parallel folding in many extensional terranes, and the geometry of the Ibex Hills is consistent with these models. Collectively, the field data support an old hypothesis by Troxel et al. (1992) that an early period of SW-NE extension is prominent in the southern Death Valley region. The younger NW-SE extension has been well documented just to the north in the Black Mountains, but the potential role of this earlier extension is unknown given the complexity of the younger deformation. In any case, the recognition of earlier SW-NE extension in the up-dip position of the Black Mountains detachment system indicates important questions remain on how that system should be reconstructed. Collectively, our observations provide insight into the stratigraphy of the Ibex Pass basin and its relationship to the extensional history of the region. It also highlights the role of transcurrent deformation in an area that has transitioned from extension to transtension.

Pleistocene lakes and paleohydrologic environments of the Tecopa basin, California: Constraints on the drainage integration of the Amargosa River

Abstract The Tecopa basin in eastern California was a terminal basin that episodically held lakes during most of the Quaternary until the basin and its modern stream, the Amargosa River, became tributary to Death Valley. Although long studied for its sedimentology, diagenesis, and paleomagnetism, the basin’s lacustrine and paleoclimate history has not been well understood, and conflicting interpretations exist concerning the relations of Tecopa basin to the Amargosa River and to pluvial Lake Manly in Death Valley. Previous studies also did not recognize basinwide tectonic effects on lake-level history. In this study, we focused on: (1) establishing a chronology of shoreline deposits, as the primary indicator of lake-level history, utilizing well-known ash beds and new uranium-series and luminescence dating; (2) using ostracodes as indicators of water chemistry and water source(s); and (3) correlating lake transgressions to well-preserved fluvial-deltaic sequences. During the early Pleistocene, the Tecopa basin hosted small shallow lakes primarily fed by low-alkalinity water sourced mainly from runoff and (or) a groundwater source chemically unlike the modern springs. The first lake that filled the basin occurred just prior and up to the eruption of the 765 ka Bishop ash during marine isotope stage (MIS) 19; this lake heralded the arrival of the Amargosa River, delivering high-alkalinity water. Two subsequent lake cycles, coeval with MIS 16 (leading up to eruption of 631 ka Lava Creek B ash) and MIS 14 and (or) MIS 12, are marked by prominent accumulations of nearshore and beach deposits. The timing of the youngest of these three lakes, the High lake, is constrained by a uranium-series age of ca. 580 ± 120 ka on tufa-cemented beach gravel and by estimates from sedimentation rates. Highstand deposits of the Lava Creek and High lakes at the north end of the basin are stratigraphically tied to distinct sequences of fluvial-deltaic deposits fed by alkaline waters of the Amargosa River. The High lake reached the highest level achieved in the Tecopa basin, and it may have briefly discharged southward but did not significantly erode its threshold. The High lake was followed by a long hiatus of as much as 300 k.y., during which there is evidence for alluvial, eolian, and groundwater-discharge deposition, but no lakes. We attribute this hiatus, as have others, to blockage of the Amargosa River by an alluvial fan upstream near Eagle Mountain. A final lake, the Terminal lake, formed when the river once again flowed south into Tecopa basin, but it was likely short-lived due to rapid incision of the former threshold south of Tecopa. Deposits of the Terminal lake are inset below, and are locally unconformable on, deposits of the High lake and the nonlacustrine deposits of the hiatus. The Terminal lake reached its highstand at ca. 185 ± 21 ka, as dated by infrared-stimulated luminescence on feldspar in beach sand, a time coincident with perennial lake mud and alkaline-tolerant ostracodes in the Badwater core of Lake Manly during MIS 6. A period of stillstand occurred as the Terminal lake drained when the incising river encountered resistant Stirling Quartzite near the head of present-day Amargosa Canyon. Our studies significantly revise the lacustrine and drainage history of the Tecopa basin, show that the MIS 6 highstand was not the largest lake in the basin as previously published (with implications for potential nuclear waste storage at Yucca Mountain, Nevada), and provide evidence from shoreline elevations for ∼20 m of tectonic uplift in the northern part of the basin across an ENE-trending monoclinal flexure.

Influence of textural parameters on detrital-zircon age spectra with application to provenance and paleogeography during the Ediacaran−Terreneuvian of southwestern Laurentia

Multiple sedimentary structures of different scales and representative of various flow conditions were sampled from single, sand-dominated, fluvial channels in the Neoproterozoic (Ediacaran) upper member of the Stirling Quartzite and the Cambrian (Terreneuvian) middle member of the Wood Canyon Formation. Sampling was designed to determine if sorting of detrital-zircon populations by their textural properties affects detrital-zircon age spectra in fluvial sandstones. Grain sizes of the sampled host sandstones range from fine to coarse sand, and sorting ranges from moderately well to poorly sorted. Four Wood Canyon Formation samples contain detrital-zircon grains of similar size, whereas detrital zircons in samples of the Stirling Quartzite differ greatly in size between two samples. Paired Kolmogorov-Smirnov, overlap, and likeness statistical tests demonstrate that all samples from each channel are similar. Comparing size, sphericity, and roundness of detrital-zircon grains to their ages yielded no linear correlation. Sorting of sediments by their textural properties did not affect detrital-zircon age spectra in either unit. In this study, different sedimentary structures within a single fluvial channel do not control the age distribution of detrital zircons. Furthermore, the two braidplain systems (Wood Canyon Formation, Stirling Quartzite) have substantially different detrital-zircon age spectra, indicating that the fluvial environment in general is not responsible for selecting specific age populations. The dominant ca. 1.1 Ga Grenvillian peak in the middle member Wood Canyon Formation results from provenance, not sampling bias. Uplift and erosion of Grenvillian rocks along the Ouachita rift flank during the earliest Cambrian may have generated a pulse of sediment that entered a northwestward-flowing braidplain.

Provenance of the Neoproterozoic Johnnie Formation and Stirling Quartzite, southeastern California, determined by detrital zircon geochronology and Nd isotope geochemistry

Detrital zircon geochronology coupled with Nd isotopic data constrains the provenance of the Neoproterozoic Johnnie Formation and Stirling Quartzite in the southern Nopah Range, southeastern California, and provides data to test a model of the early tectonic evolution of the southwestern Laurentian continental margin. A total of 228 detrital zircon grains from four samples of quartzose arenite, were analyzed by sensitive high resolution ion microprobe – reverse geometry (SHRIMP-RG). To supplement the provenance information derived from these grains, whole-rock Nd isotopic compositions of four coarse- and fine-grained samples were analyzed from the Rainstorm Member of the Johnnie Formation. Paleo- and Mesoproterozoic zircons dominate the lower Johnnie Formation and upper Stirling Quartzite, whereas the middle Johnnie Formation and lower Stirling Quartzite have a greater proportion of late Grenville-aged detritus, lesser amounts of older ∼1400Ma Mesoproterozoic grains, and a scattering of Paleoproterozoic grains. Archean-aged zircons occur in both samples of the Johnnie Formation and both samples of the Stirling Quartzite. We interpret this distribution to indicate a source area composed of a mixture of local and distal provenance components. Sedimentary rock Nd isotopic compositions are characterized by low ɛNd (0) values that range from −16.1 to −20.0, are independent of grain size or 147Sm/144Nd ratios, and suggest derivation from Nd Provinces 2 and 3 (Yavapai and Mazatzal provinces). Shifting provenance patterns from the Johnnie Formation and superjacent Stirling Quartzite and Wood Canyon Formation indicate an evolving provenance recorded by a general up-section increase of late Grenville-aged detritus. These data support a Laurentian provenance for Johnnie Formation detritus composed of contributions from both distal source areas in the cratonic interior and local source areas in the rift shoulder, which is consistent with its deposition as part of a post-rift, early-stage, passive-margin succession.

The Shuram and subsequent Ediacaran carbon isotope excursions from southwest Laurentia, and implications for environmental stability during the metazoan radiation

Research Article| July 01, 2011 The Shuram and subsequent Ediacaran carbon isotope excursions from southwest Laurentia, and implications for environmental stability during the metazoan radiation Charles Verdel; Charles Verdel † 1Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA †Current address: Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA; e-mail: cverdel@ku.edu Search for other works by this author on: GSW Google Scholar Brian P. Wernicke; Brian P. Wernicke 1Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA Search for other works by this author on: GSW Google Scholar Samuel A. Bowring Samuel A. Bowring 2Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2011) 123 (7-8): 1539–1559. https://doi.org/10.1130/B30369.1 Article history received: 08 Jul 2010 rev-recd: 10 Oct 2010 accepted: 12 Oct 2010 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Charles Verdel, Brian P. Wernicke, Samuel A. Bowring; The Shuram and subsequent Ediacaran carbon isotope excursions from southwest Laurentia, and implications for environmental stability during the metazoan radiation. GSA Bulletin 2011;; 123 (7-8): 1539–1559. doi: https://doi.org/10.1130/B30369.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Current understanding of secular changes in the carbon isotopic composition of mid- to late Ediacaran carbonates suggests a relatively long, steady recovery of the global ocean from the Shuram negative excursion, followed by a smaller negative excursion at the Precambrian-Cambrian boundary. New radiometric, stratigraphic, and carbon isotope data from thick exposures of the upper Johnnie Formation in the Panamint Range of eastern California, combined with data from carbonate-rich facies of the Stirling Quartzite in the Funeral Mountains, confirm an Ediacaran age for these strata and provide a more complete record of isotopic variations during this time interval than previously determined from SW Laurentia and other key sections around the globe. A siltstone in the lower part of the Johnnie Formation yielded a detrital zircon grain with an age of 640.33 ± 0.09 Ma, lowering the maximum radiometric age constraint on the Johnnie Formation by >400 m.y., consistent with an Ediacaran age based on chemo- and biostratigraphic data. In contrast to previous C isotope compilations from this region, which were generally based on relatively thin portions of the Cordilleran miogeocline near its depositional hinge, the more basinward exposures exhibit a recovery from values near –12‰ to 0‰ within the upper part of the Johnnie Formation. Details in the shape of the chemostratigraphic profile through the upper Johnnie Formation closely match those in profiles through the Wonoka Formation in South Australia (which lies above the basal Ediacaran global stratotype section and point) and the Shuram-Buah interval in Oman, confirming temporal correlation and suggesting genesis through changes in the isotopic composition of the global ocean. The Shuram excursion in SW Laurentia is followed by at least three smaller Ediacaran to earliest Cambrian isotopic excursions recorded within, from oldest to youngest, the uppermost Johnnie Formation, the middle Stirling Quartzite, and the lower Wood Canyon Formation. These data indicate that the negative excursion associated with the base of the Cambrian is not a unique post-Shuram event, and that post-Shuram, pre-Cambrian animal evolution occurred in an environment of repeated large-magnitude fluctuations in the carbon isotopic composition of the global ocean. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Provenance of quartzite clasts in the Eocene–Oligocene Sespe Formation: Paleogeographic implications for southern California and the ancestral Colorado River

Quartzite clasts in fluvial Sespe Formation conglomerates of the Santa Monica Mountains were compared petrographically with potential source rocks in the Basin and Range and Transition Zone provinces to determine their provenance. Modal analysis indicates that Sespe orthoquartzite clasts are mainly quartzofeldspathic and derived from source rocks with craton-interior provenance. Such clasts were probably derived from Stirling Quartzite and Wood Canyon Formation sources in the northern Mojave Desert. The Mazatzal Peak Quartzite of central Arizona is the probable source of quartzolithic orthoquartzite clasts containing abundant detrital jasper grains. Sespe metaquartzite clasts are mainly from sources in the southern Mojave Desert, but some may be from the Yavapai terrane of central Arizona. The dual provenance of orthoquartzite clasts suggests that the Sespe paleoriver was a bifurcating system tapping both Mojave Desert and Sonora Desert sources. After compensating for Neogene displacement along the San Andreas fault, a paleogeographic reconstruction shows that there is a general spatial coincidence between Sespe (and other Cenozoic) paleodelta deposits around Los Angeles that contain exotic clasts, and the inferred location of the mouth of the Colorado River (assuming that it existed during the Eocene). Thus, the Colorado River may be much older than previously thought (as old as late Paleocene), and perhaps was once part of an ancient fluvial connection with the Mojave Desert interior.

Ediacaran fossils from the southwestern Great Basin, United States

Ediacaran fossils from the southwestern Great Basin may help constrain regional Vendian-Cambrian biostratigraphy and provide biogeographic links between facies in this region and elsewhere. Locally, trace fossils suggest the Vendian-Cambrian boundary occurs within or below the upper third of the lower member of the Wood Canyon Formation. Ediacaran soft-bodied and tubular fossils, including the frondlike fossil Swartpuntia and tubular, mineralized or agglutinated fossils similar to Archaeichnium Cloudina Corumbella, and Onuphionella occur in the lowermost Wood Canyon Formation. Discoidal forms referred to Nimbia occur in both the lowermost Wood Canyon Formation and the underlying strata of the Stirling Quartzite. These fossils occur directly below Lower Cambrian trace fossils, including Treptichnus pedum, and confirm the persistence of the Ediacaran biota to near the base of the Cambrian. These faunas may also help strengthen previously proposed correlation schemes between the two main facies belts of the southwestern Great Basin (the Death Valley and White-Inyo facies), because a nearly identical Vendian-lowest Cambrian succession of faunas occurs in both regions. Lastly, lack of cosmopolitan Ediacaran faunas in these strata suggests a paleobiogeographic link between the southwestern U.S. and southern Africa in Vendian time.

EDIACARAN FOSSILS FROM THE SOUTHWESTERN GREAT BASIN, UNITED STATES

Ediacaran fossils from the southwestern Great Basin may help constrain regional Vendian-Cambrian biostratigraphy and provide biogeographic links between facies in this region and elsewhere. Locally, trace fossils suggest the Vendian-Cambrian boundary occurs within or below the upper third of the lower member of the Wood Canyon Formation. Ediacaran soft-bodied and tubular fossils, including the frondlike fossil Swartpuntia and tubular, mineralized or agglutinated fossils similar to Archaeichnium Cloudina Corumbella, and Onuphionella occur in the lowermost Wood Canyon Formation. Discoidal forms referred to Nimbia occur in both the lowermost Wood Canyon Formation and the underlying strata of the Stirling Quartzite. These fossils occur directly below Lower Cambrian trace fossils, including Treptichnus pedum, and confirm the persistence of the Ediacaran biota to near the base of the Cambrian. These faunas may also help strengthen previously proposed correlation schemes between the two main facies belts of the southwestern Great Basin (the Death Valley and White-Inyo facies), because a nearly identical Vendian-lowest Cambrian succession of faunas occurs in both regions. Lastly, lack of cosmopolitan Ediacaran faunas in these strata suggests a paleobiogeographic link between the southwestern U.S. and southern Africa in Vendian time.

Evidence of Uppermost Proterozoic to Lower Cambrian miogeoclinal rocks and the Mojave‐Snow Lake Fault: Snow Lake Pendant, central Sierra Nevada, California

Displaced uppermost Precambrian to Lower Cambrian miogeoclinal strata occur within Snow Lake pendant in the central Sierra Nevada. These rocks have been correlated with the Stirling Quartzite, the Wood Canyon Formation, the Zabriskie Quartzite, and the Carrara Formation in the western Mojave Desert and the San Bernardino Mountains (Lahren and Schweickert, 1989; Lahren, 1989). This correlation is based on new, updated, and previously reported data including (1) lithologic similarities, (2) overall stratigraphic sequence, (3) vertical sequence within individual formations, (4) approximate stratigraphic thicknesses, (5) Skolithos in the correct stratigraphie position, (6) depositional environments, and (7) petrographic character and provenance of quartz arenites. The correlation is strengthened by the fact that Snow Lake pendant and the western Mojave share many other close similarities including (1) initial 87Sr/86Sr ratios of associated granitic rocks >0.706, (2) passive margin tectonic setting of Precambrian to Cambrian miogeoclinal rocks, (3) dikes of the Independence dike swarm, (4) possible Lower Triassic overlap sequence, the Fairview Valley Formation, (5) petrographically similar gabbroic complexes of the same age, (6) associated eugeoclinal rocks, and (7) identical(?) pre‐Tertiary structural configuration. New U/Pb zircon geochronology unequivocally shows that dikes at Snow Lake pendant are coeval with the Independence dike swarm of the eastern Sierra and the western Mojave desert and that associated gabbroic complexes in both the Mojave and Snow Lake pendant are the same age. Correlation of Snow Lake pendant with the western Mojave requires about 400 km of dextral displacement of the rocks of Snow Lake pendant, together with associated rocks (Snow Lake block), from the western Mojave Desert along the Mojave‐Snow Lake fault. Displacement most likely occurred after 150 Ma, the age of the Independence dike swarm, and before about 110 Ma, the age of major plutons within the Sierra Nevada batholith. This interpretation, if correct, holds major implications for allochthonous terranes west of Snow Lake pendant, which were probably attached to the Snow Lake block before its northward transport. In addition, a number of Paleozoic and Mesozoic tectonic features in western Nevada and eastern California may have been offset dextrally along the proposed Mojave‐Snow Lake fault.

Proterozoic and Lower Cambrian miogeoclinal rocks of Snow Lake pendant, Yosemite-Emigrant Wilderness, Sierra Nevada, California: Evidence for major Early Cretaceous dextral translation

Detailed stratigraphic and structural studies at Snow Lake pendant have established new evidence for the presence of uppermost Precambrian and Lower Cambrian miogeoclinal strata within the axial region of the Sierra Nevada batholith. Lithologic and stratigraphic data along with trace fossils, a distinctive Triassic overlap sequence (Fairview Valley Formation?), dikes probably related to the Jurassic Independence dike swarm, and structural history suggest that the older rocks of Snow Lake pendant correlate with the Stirling Quartzite, Wood Canyon Formation, Zabriskie Quartzite, and Carrara Formation in the western Mojave Desert and San Bernardino Mountains. This correlation implies approximately 500 km of dextral transport of the rocks of Snow Lake pendant along the proposed Mojave-Snow Lake fault. Movement on the fault probably took place after 148 Ma, the age of the Independence dike swarm, and prior to 110 Ma, the age of plutons in the central part of the Sierra Nevada batholith.

Paleomagnetism of Upper Precambrian sedimentary rocks from the Desert Range, Nevada

About 300 oriented samples were obtained from upper Precambrian and possibly lowest Cambrian strata in a homoclinal sequence conformable with Lower Paleozoic strata in the Desert Range, Nevada. These samples are from miogeoclinal sedimentary rocks of the uppermost Johnnie Formation (upper Precambrian) and overlying Stirling Quartzite (uppermost Precambrian and Lower(?) Cambrian). For about 100 of the samples, thermal demagnetization revealed a characteristic magnetization with a low inclination. The characteristic magnetization of the uppermost Johnnie has two polarities, which allows its direction to be established despite incomplete removal of a secondary component of recent origin. Petrographic and stratigraphic evidence suggest that this characteristic magnetization was acquired penecontemporaneously with deposition. The age of the magnetization probably lies in the range 675–575 m.y. B.P., based on estimates of sedimentation rates in the Desert Range and on proposed lithologic correlations and isotopic age control elsewhere in the Cordilleran Geosyncline. A fine‐grained, red‐purple unit in the middle Stirling Quartzite exhibits a single‐polarity (reversed) characteristic magnetization with a direction roughly like the directions of the uppermost Johnnie or the Lower Cambrian Wood Canyon Formation, which conformably overlies the Stirling. The paleomagnetic pole from the uppermost Johnnie is about 47° from the pole for the Wood Canyon. It seems probable therefore that at least 45° of apparent polar wandering occurred with respect to North America in latest Precambrian time.

Petrology of Late Precambrian-Cambrian Quartzose Sandstones in the Eastern Mojave Desert, Southeastern California

ABSTRACT Petrographic and radiographic analyses were conducted on oriented samples of Late Precambrian-Cambrian quartzose sandstones collected from five cratonal and five miogeoclinal stratigraphic sections in the eastern Mojave Desert, southeastern California. Twelve petrographic variables were measured in each of 494 samples. Cratonal sandstones are dominantly subarkose, whereas miogeoclinal sandstones are subarkose and quartzarenite in almost equal proportions. Sediments were derived chiefly from granitic rocks, gneiss, and schist as well as older sedimentary and metasedimentary rocks. The cratonal source areas were to the east and southeast. Samples collected from six northern sections within the Cordilleran frontal thrust belt have a higher content of undulatory quartz and silica cement and are partially recrystallized when compared with samples from four southern sections. Polynomial time-trend analysis showed considerable variation in mineral composition with thickness at each stratigraphic section. Areal variation of total quartz was estimated from analysis of variance models. The difference was not highly significant for the Stirling Quartzite. Depositional history was inferred for each formation on the basis of petrography, clasticity and sedimentary structures. The Johnnie Formation fines upward, which suggests a marine transgression or change from open to partially restricted shelf environments. The Stirling Quartzite coarsens upward and consists largely of prograding mature shelf sands or deposits of migrating bars and barrier beaches. The Wood Canyon Formation is dominantly subtidal with local evidence for intertidal deposition.

Late Precambrian-Early Cambrian Stratigraphy of Kelso Mountains, Eastern Mojave Desert, California: ABSTRACT

A detailed study has been made of several outcrops of late Precambrian-Early Cambrian strata in the southern Kelso Mountains. These strata, which are part of the predominantly detrital basal sequence of the Cordilleran miogeocline, previously have been described only in reconnaissance. In the Kelso Mountains, these strata have been assigned to 5 regionally recognized formations. The late Precambrian Johnnie Formation, which lies unconformably on metamorphic basement, displays rapid lateral changes in lithology and thickness, perhaps due in part to infilling of uneven basement topography. Of 4 sections of Johnnie exposed, 3 are similar, consisting of 91-141 ft of quartzite and dolomite. Conformably overlying the Johnnie Formation are the late Precambrian Stirling Quartzite (186 ft thick, only the uppermost regional member is present), the late Precambrian-Early Cambrian Wood Canyon Formation (712 ft thick), the Early Cambrian Zabriskie Quartzite (75 ft thick), and the Early Cambrian lower part of the Carrara Formation (70 ft thick). Comparison of these strata with equivalent miogeoclinal strata in the Providence Mountains 8 mi southeast and with Early Cambrian cratonic strata in the Marble Mountains 30 mi south shows that the northeast-trending craton-miogeocline boundary was not an abrupt break (an interpretation based on comparison in Nevada and Utah of thick allochthonous miogeoclinal strata with autochthonous cratonic strata), but rather a relatively broad transition zone of gradual thickening of strata from southeast to northwest. End_of_Article - Last_Page 776------------

Structural Geology of the Specter Range Quadrangle, Nevada, and its Regional Significance

The Specter Range quadrangle, Nevada, lies across the projected trend of the Las Vegas Valley Shear Zone, a right-lateral fault zone with more than 25 miles of displacement. At the north end of the quadrangle a narrow strip of Paleozoic rocks is stratigraphically continuous and nearly continuous in outcrop across the projected trend of the shear zone, but shows no evidence of displacement by a large strike-slip fault. The Las Vegas Valley Shear Zone is interpreted as a deep crustal structure (Longwell, 1960), and in the Specter Range quadrangle most of the displacement on the shear zone at depth is taken up by bending in the overlying sedimentary rocks rather than by fracturing. The Las Vegas Valley Shear Zone should be regarded as a separate structure from the Walker Lane in the Cedar Mountain area. The westward extension of the Las Vegas Valley Shear Zone appears to be south of the western part of the Walker Lane. The most characteristic structural features of the Specter Range quadrangle are the high-angle faults, many of which are topographically expressed. Folding is relatively unimportant and local. Three sets of high-angle faults are recognized on the basis of similar strike: (1) a northwest set, (2) a north-south set, and (3) a northeast set. The north-south set is interpreted as a regional set of normal faults whereas the northwest and northeast sets are interpreted as right-lateral oblique-slip and left-lateral oblique-slip faults, respectively. Evidence indicates that the Johnnie thrust mapped by Nolan (1929) is not present in the Specter Range quadrangle. The contact between the Johnnie Formation and the overlying Stirling Quartzite is conformable in some places and not marked by a thrust. The “tectonic” contact visible at some localities is regarded as bedding-plane slip between competent and incompetent rock types.