Mixed Carbonate-Siliciclastic Sedimentation At The Margin Of A Late Miocene Tidal Strait, Lower Colorado River Valley, South-Western Usa

The processes and deposits of tide-dominated river deltas and estuaries are well-understood, but the sedimentary dynamics of tide-dominated straits and seaways are relatively little studied. Although recent depositional models have started to fill this gap, many aspects of tidal strait sedimentation such as interaction with strait-margin alluvial fans and marginal marine systems remain poorly understood. This paper presents a study of the late Miocene basal carbonate member of the Bouse Formation exposed along the lower Colorado River, where prior studies have suggested a tidal influence on deposition. This study explores the applicability of tidal strait models in a continental oblique-rift setting, and tests hypotheses for depositional processes and environments through detailed analysis of sedimentary facies, cross-bedding architecture and palaeocurrent data. Mixed carbonate-siliciclastic facies on the west margin of the southern Blythe Basin record sedimentation in alluvial fans and fan-fringing tidal flats at the retreating margin of a transgressive tidal strait. Pre-Bouse normal faults established a narrow, tectonically confined basin architecture that led to amplification of tidal currents. Basin-margin deposits pass laterally and up-section into high-energy cross-bedded grainstone facies that record southward migration of compound dunes in the dune-bedded strait zone of a shallow (ca 25 m) tidal strait. These findings provide the basis for recognition of strait-margin facies in other settings where complex facies associations result from along-strike variations in palaeobathymetry and current velocity. The results support a tidal origin for the southern Bouse Formation and provide new evidence for post-Miocene uplift of the lower Colorado River Valley and adjacent areas.