Stratal architecture and sedimentology of a portion of the Upper Cambrian Hickory Sandstone, central Texas, U.S.A.

Abstract

Fluvial and coastal depositional environments may have been quite different before the development of land plants in the late Silurian. Rapid drainage of terrestrial surfaces, flashy rivers with poorly stabilized banks, coarse sediment loads supplied to coasts from landscapes dominated by physical weathering, and the prevalence of epicontinental seas are expected to have altered depositional patterns and associated preserved Facies. Quarries in the Upper Cambrian Hickory Sandstone located in central Texas provide an exceptional opportunity to examine the sedimentology of deposits of this age in order to interpret sedimentary environments. During quarrying, vertical walls, one half-kilometer long and several tens of meters high, are blasted back a few tens of meters at a time and then the rubble excavated, exposing successive outcrops in walls that are perpendicular to the regional paleocurrent direction. The deposits are characterized by sheet-like bedsets dominated by unidirectional cross-stratified sandstones interpreted to have formed in coastal areas fed by bedload dominated rivers. Thinner heterolithic and clay beds locally separating cross-stratified bedsets are commonly bioturbated by marine organisms. Presence of tidal features, such as abundant mud drapes, concave-upward cross-stratification and sparse herringbone cross-stratification, also suggests marine influence during deposition. Detailed mapping of stratal geometry and Facies across these exposures shows a complex internal architecture that can be interpreted in terms of growth and superposition of bars within shallow fluvial channels and adjacent shallow marine areas along the coast. Detailed 3D reconstruction of bars and channels reveals a range of processes including growth, coalescence, and erosion of bars during channel migration, switching and filling of channel segments, and mouth bar growth as channelised flows decelerated seaward. Sedimentary Facies, stratal geometry and ichnofossils suggest that these deposits were formed in a braid-delta system fed by lowsinuosity bedload-dominated rivers. Basinal processes were controlled by the shallow epicontinental sea, dissipating wave action and strengthening tidal currents.