Mixed carbonate–siliciclastic sedimentation at the margin of a late Miocene tidal strait, lower Colorado River Valley, south‐western USA

Abstract

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.

Mixed carbonate–siliciclastic tidal sedimentation in the Miocene to Pliocene Bouse Formation, palaeo‐Gulf of California

“…Facies associations documented in this study formed on the low gradient eastern margin of the Blythe Basin, in contrast to age-equivalent facies that accumulated on the steeper west margin of the basin in the south-eastern Palo Verde Mountains ( Fig. 1; Gardner & Dorsey, 2021). Different facies models, therefore, are required despite the proximity of deposits.…”

“…The study area is located south-east of Cibola, Arizona, at the eastern margin of the southern Blythe Basin (Figs 1 and 3), where prior geological mapping established the distribution and relative age relations of Miocene to Quaternary deposits (Homan, 2014;Gootee et al, 2016). The Bouse Formation in the studied area formed at the margin of a tidal strait near the north end of the Gulf of California oblique rift, as indicated by marine and brackish-water fossils and a wide range of tidal sedimentary structures (Buising, 1990;Turak, 2000;McDougall et al, 2008;McDougall & Mart ınez, 2014;O'Connell et al, 2017;Dorsey et al, 2018;Gardner & Dorsey, 2021). Although some authors favour an isolated inland-lake model for the Bouse Formation in the study area (Spencer & Jonathan Patchett, 1997;House et al, 2008;Spencer et al, 2008Spencer et al, , 2013Bright et al, 2016Bright et al, , 2018a, the lacustrine model is incompatible with abundant evidence for intertidal, marine to brackish-water fossils and trace fossils, and widespread tide-influenced sedimentary structures (Buising, 1990;Turak, 2000;O'Connell et al, 2017;Dorsey et al, 2018;Gardner & Dorsey, 2021).…”

“…In the south-eastern Palo Verde Mountains west of the Colorado River (Fig. 1C), the basal carbonate member contains an age-equivalent suite of facies that record marine transgression of a steep rocky shoreline followed by subtidal deposition of coarse bioclastic dunes in a high-energy tidal strait (Gardner & Dorsey, 2021).…”

“…The focus of this study is the upper Miocene to lower Pliocene Bouse Formation in its southern exposures along the lower Colorado River region, which accumulated at the margin of a tidal strait near the north end of the ancestral Gulf of California ( Fig. 1; O 'Connell et al, 2017;Dorsey et al, 2018;Gardner & Dorsey, 2021). The basal carbonate member of the Bouse Formation is characterized by compositional mixing (sensu Chiarella et al, 2017), reflecting contemporaneous accumulation of carbonate and siliciclastic sediment at laminae and bed scales (Chiarella et al, 2017).…”

“…The basal carbonate member of the Bouse Formation is characterized by compositional mixing (sensu Chiarella et al, 2017), reflecting contemporaneous accumulation of carbonate and siliciclastic sediment at laminae and bed scales (Chiarella et al, 2017). The Bouse Formation deposits display tidal cyclicity similar to that of the modern Gulf of California (Marinone, 1997;O'Connell et al, 2017), and the tectonic setting and basin geometries are well-constrained (Dorsey et al, 2018;Gardner & Dorsey, 2021). Published analyses, however, are incomplete, and some studies favour a lacustrine origin for the Bouse Formation in its southern exposures along the lower Colorado River region (Spencer & Jonathan Patchett, 1997;House et al, 2008;Spencer et al, 2013;Bright et al, 2016Bright et al, , 2018a.…”