Extensive Cambrian Braidplain Sedimentation: Insights from the Southwestern Cordillera, U.S.A.

Abstract

The Lower to Middle Cambrian time-transgressive coarse terrigenous clastic rocks that form the basal part of the Cordilleran cratonal (Tapeats Sandstone) to miogeoclinal (Wood Canyon Formation, Zabriskie Quartzite) succession have long been though of as products of marine sediment dispersion processes. The authors' recent facies analyses and reinterpretation suggest, instead, that those strata record an extensive continental braidplain which interacted with marine environments. Marine deposits are distinguished by the presence of trace fossils and abundant wave- and tide-influenced sedimentary structures. The most abundant braidplain facies are those assigned a fluvial origin. Coarser, more poorly sorted deposits are feldspathic, whereas finer, better sorted deposits are quartzitic. Beds typically are wedge- and channel-shaped and their width ranges inversely to grain size; channels in conglomeratic layers are relatively narrow (< 20 m wide), whereas those in sand-dominated intervals are shallow (< 1 m) and wide (tens of meters) with broadly concave-up bases. Trough cross-strata and flat laminations are common with planar cross-strata less common. Basal lags and cyclic arrangement of sedimentary structures are rare, although thinning- and fining-upward intervals are developed locally. Paleocurrent indicators across the entire region are west directed and of low variance. They suggest that the dominant mode of sedimentmore » transport was by down-channel migrating dunes and that subordinate processes included sheet flow (flat laminated sandstones) and eolian deflation (textural bimodality). These consistent paleocurrent directions attest to the remarkable longevity (Early through Middle Cambrian) of an extensive, westward-draining fluvial system.« less