John D. Cooper

Late Ordovician mass extinction: A new perspective from stratigraphic sections in central Nevada

Integrated sequence stratigraphic, biostratigraphic, and chemostratigraphic analyses of three stratigraphic sections in central Nevada indicate that Late Ordovician glaciation-induced sea-level fall produced diachronous, stepwise faunal turnover in graptolites, conodonts, chitinozoans, and radiolarians, and also triggered a strong, but transient, positive δ13C excursion. This pattern is very different from that described for most mass extinction events.

Braided Fluvial to Marine Transition: The Basal Lower Cambrian Wood Canyon Formation, Southern Marble Mountains, Mojave Desert, California

ABSTRACT The Lower Cambrian Wood Canyon Formation, recognized over a 39,000 km2 belt through the southern Great Basin and Mojave Desert, was examined in the southern Marble Mountains, eastern Mojave Desert, southeastern California. There it crops out as a well-exposed 90-130 m thick, feldspathic sandstone nonconformably overlying Proterozoic basement. Previous investigators have suggested a tide-dominated, nearshore, shallow marine origin for the entire unit in the eastern Mojave, based on broad-scale stratigraphic trends and localized fossil occurrences in the upper member of the formation. In contrast to a regional approach, the present study uses a detailed lithofacies analysis in a smaller study area to suggest that a fluvial to marine transition occurs within these strata. The middle and upper members of the Wood Canyon Formation are recognized in the southern Marble Mountains and are divisible into six distinct lithofacies that possess gradational contacts where exposed: cross-stratified sandstone (Facies A); mediumpebble conglomerate (Facies B); planar-stratified sandstone (Facies C); sandstone and thin mudstone (Facies D); mudstone with intercalated sandstone (Facies E); and horizontally laminated siltstone (Facies F). Facies A-D, contained within the middle member, comprise the bulk of Wood Canyon sediments and record the depositional characteristics of a transition zone from a distal alluvial braid plain to a tidally influenced, fluvial-dominated braid-delta complex. Cross stratification in the braid-plain sediments displays a strongly unimodal pal ocurrent pattern, suggestive of downstream-migrating sinuous-crested megaripples. Braid-delta sediments, however, show a more dispersed paleocurrent pattern and contain a sparse trace fossil assemblage. A change from fluvial-dominated to marine-dominated processes is recorded in upper member rocks (Facies E and F). Facies E contains many of the traits characteristic of a low- to mid-tidal flat sequence, such as coarsely interlayered bedding, flaser and lenticular bedding, microripple marks, and locally abundant Rusophycus and Planolites trace fossils. Facies F, which dominantly consists of horizontally laminated siltstone, is interpreted as the product of an estuarine lagoon that developed behind and within a discontinuous barrier of stranded braid bars.

Reassessment of the Basal Sauk Supersequence Boundary across the Laurentian Craton-Margin Hinge Zone, Southeastern California

In the Death Valley and Mojave Desert regions of southeastern California, the contact separating the lower and middle members of the Wood Canyon Formation (WCF) is currently interpreted as a regional-scale unconformity coincident with the base of the Sauk sequence. Regional mapping of this surface, however, reveals a nonconformable contact with underlying crystalline basement in cratonic settings and a relatively conformable contact atop a northwest-thickening wedge of off-craton strata that is capped by the lower member of the WCF. Consistent with an unconformity, the progressive loss of three carbonate units within the lower member of the WCF has been attributed to incision by the base of the middle-member WCF. However, fossil evidence and correlation based on carbon isotope compositions of each lower-member WCF dolostone unit reject top-down erosion to describe their loss and overall cratonward thinning. Results from multiple, detailed, measured stratigraphic sections of a conglomerate at the base of or low in the middle-member WCF also do not support a top-down erosion model because the conglomerate has variable stratigraphic position and is absent in some locations. Middle-member WCF conglomerate clasts also reveal variation in composition and grain size across the regions. Sequence-stratigraphic architecture indicates that filling of available accommodation space via a short-period normal regression coupled with high-order sea level fall generated the basal middle-member WCF unconformity. Consequently, the base of the Sauk sequence occurs lower in the stratigraphic section, most likely at the base of the underlying Stirling quartzite.

Sauk Megasequence Supersequences, Southern Great Basin: Second-order Accommodation Events on the Southwestern Cordilleran Margin Platform

Compared to its type cratonal counterpart, the Sauk megasequence in the off-craton southern Great Basin is more fully developed and neatly packaged internally by regional unconformities. The Nopah Range in eastern California superbly and strategically exposes an exemplary and critical baseline Sauk section about 3000-m (9840-ft)-thick that provides an anchor for regional correlation. Beginning at the base of the Lower Cambrian middle member of the Wood Canyon Formation and terminating at the top of the upper Middle Ordovician Eureka Quartzite, the Sauk megasequence can be subdivided physically into five supersequences based on sequence-stratigraphic criteria. Prominent platformwide emergence and erosion events, indicated by unconformities, and succeeding submergence and marine flooding events, recorded by overlying fine siliciclastic packages, coincide with contacts of the traditional lithostratigraphic framework. The five supersequences (Sauk-, , , , and ) and their bounding unconformities reflect the major accommodation events across the platform of the southwestern Cordilleran margin and provide a chronostratigraphic framework guided by, but not dependent on, biostratigraphy. Within this framework, the Las Vegas branch of the great American carbonate bank system, which became established during a Middle Cambrian sea level highstand, rebounded impressively from platformwide interruptions by two supersequence boundary emergence and submergence events, only to meet its final fate from smothering by the voluminous Eureka quartz sand incursion.