FLUVIAL ARCHITECTURE AND CYCLICITY IN CAMBRIAN BRAIDPLAIN STRATIGRAPHY, MIDDLE MEMBER WOOD CANYON FORMATION, MARBLE MOUNTAINS, SOUTHEASTERN CA, USA

Fluvial architecture of the middle member Wood Canyon Formation (mmWCF) in the Marble Mountains, southeastern CA, has revealed new details about the sedimentary structures, surfaces, and cyclicity found within the Cambrian sheet-braided stratigraphy. Environmental interpretations presented here combine UAV imagery, a ~0.5 km wide 3D digital outcrop model (DOM), photography at ground level, and field orientation measurements to identify fluvial architectural elements. Exposed channel elements can be traced in detail for 10s of meters along strike. Channel elements (CH) are dominantly 40-200 cm thick, containing ~10-30 cm thick trough cross-bedded downstream accreting (DA) elements and thin (5-10 cm thick) trough cross-bedded DA and laterally accreting (LA) elements near the top of channels. Bounding surfaces of the 3^rd order that define each channel are associated with laminated ~1 cm thick silt to very-fine-sand layers, possibly representing channel abandonment. Channel edges are rare in the outcrop, but those observed terminate at a shallow angle (~10º).

The stratigraphy preserved across the outcrop is not laterally continuous due to avulsion and erosion from nesting of channels. Differences in the types and thicknesses of facies across the outcrop indicate that the channels in this system were relatively thin, migrating within a larger channel belt. Potential 4^th order surfaces occur between the bases of 5-10 m thick bundles of channels (CB) and 1-7 m thick interbundle (IB) strata typically covered in talus. The 4^th order surfaces are readily visible on the DOM, and are traceable for ~100 m. Where no cover is present, IB units are red or tan, fine-to-medium-sand, cross-stratified sandstones. Atypical cross-bedding can be found in multiple IB intervals, defined by broad meter-scale westward-migrating troughs with tightly-spaced (2-5 mm) foresets, well-sorted medium-to-fine sand, and an absence of mud chips. At least 5 alternating CB and IB units form a cyclic pattern that may represent the migration of km-scale braidplain channel belts. In this proposed cycle, thicker channels with mud intraclasts and pebble lags in the CB units represent active channel belts. The IB units represent interfluves, where increased stabilization allowed for more intense chemical weathering before burial.