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

Abstract

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.