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.

The Eastern California shear zone (ECSZ; southwestern USA) accommodates ~20%–25% of Pacific–North America relative plate motion east of the San Andreas fault, yet little is known about its early tectonic evolution. This paper presents a detailed stratigraphic and structural analysis of the uppermost Miocene to lower Pliocene Bouse Formation in the southern Blythe Basin, lower Colorado River valley, where gently dipping and faulted strata provide a record of deformation in the paleo-ECSZ. In the western Trigo Mountains, splaying strands of the Lost Trigo fault zone include a west-dipping normal fault that cuts the Bouse Formation and a steeply NE-dipping oblique dextral-normal fault where an anomalously thick (~140 m) section of Bouse Formation siliciclastic deposits filled a local fault-controlled depocenter. Systematic basinward thickening and stratal wedge geometries in the western Trigo and southeastern Palo Verde Mountains, on opposite sides of the Colorado River valley, record basinward tilting during deposition of the Bouse Formation. We conclude that the southern Blythe Basin formed as a broad transtensional sag basin in a diffuse releasing stepover between the dextral Laguna fault system in the south and the Cibola and Big Maria fault zones in the north. A palinspastic reconstruction at 5 Ma shows that the southern Blythe Basin was part of a diffuse regional network of linked right-step­ping dextral, normal, and oblique-slip faults related to Pacific–North America plate boundary dextral shear. Diffuse transtensional strain linked northward to the Stateline fault system, eastern Garlock fault, and Walker Lane, and southward to the Gulf of California shear zone, which initiated ca. 7–9 Ma, implying a similar age of inception for the paleo-ECSZ.

Marine straits and seaways are known to host a wide range of sedimentary processes and products, but the role of marine connections in the development of large river systems remains little studied. This study explores a hypothesis that shallow marine waters flooded the lower Colorado River valley at ∼ 5 Ma along a fault-controlled former tidal straight, soon after the river was first integrated to the northern Gulf of California. The upper bioclastic member of the southern Bouse Formation provides a critical test of this hypothesis. The upper bioclastic member contains wave ripple-laminated bioclastic grainstone with minor red mudstone, pebbly grainstone with HCS-like stratification and symmetrical gravelly ripples, and calcareous-matrix conglomerate. Fossils include upward-branching segmented coralline-like red algae with no known modern relatives but confirmed as marine calcareous algae, echinoid spines, barnacles, shallow marine foraminifers, clams, and serpulid worm tubes. These results provide evidence for deposition in a shallow marine bay or estuary seaward of the transgressive backstepping Colorado River delta. Tsunamis generated by seismic and meteorologic sources likely produced the HCS-like and wave-ripple cross-bedding in poorly-sorted gravelly grainstone. Marine waters inundated a former tidal strait within a fault-bounded tectonic lowland that connected the lower Colorado River to the Gulf of California. Delta backstepping and transgression resulted from a decrease in sediment output due to sediment trapping in upstream basins and relative sea-level rise produced by regional tectonic subsidence.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5740426

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