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

Abstract

Mixed carbonate-siliciclastic deposits provide unique insights into hydrodynamic processes that control sedimentation in tidal systems. This study presents sedimentologic an¬¬d ichnologic data from the upper Miocene¬ to lower Pliocene Bouse Formation, which accumulated during regional transgression at the margin of a tidal strait near the north end of the ancestral Gulf of California. The basal carbonate member of the Bouse Formation records deposition in a tide-influenced, compositionally mixed carbonate-siliciclastic system dominated by salt marsh, tidal flat, and channel environments. The basal carbonate member is an overall deepening up succession of facies associations (FA) comprising: FA1 siliciclastic-rich heterolithic facies, lime mudstone with desiccation cracks, and plant debris rich carbonate silt interpreted as siliciclastic-rich tidal flats; FA2 well-sorted gravels, siliciclastic-rich sandy strata, lime mudstone with desiccation cracks, and sandy microbial micrite interpreted as tidal-channel deposits; FA3 carbonate-rich heterolithic lime mudstone to well sorted, crossbedded bioclastic grainstone interpreted as intertidal to shallow subtidal deposits; and FA4 lime mudstone interpreted as shallow to deep subtidal low-energy deposits that record the end of tidal conditions in the basin. Trace fossils include marine forms Gyrolithes, Teichichnus, Thalassinoides, and non-diagnostic forms Arenicolites, Cochlichnus, Conichnus, Lockeia, Planolites, Skolithos, and Treptichnus (known from marine, brackish, and freshwater environments). The diminutive size of trace fossils reflects brackish conditions created by mixing of freshwater and seawater. This study provides evidence for a late Miocene to early Pliocene humid climate in southwestern North America, in stark contrast to the modern hyperarid climate. Factors that controlled the relative percent of mixed carbonate and siliciclastic sediment include siliciclastic input from local rivers, in situ carbonate production, current energy, degree of tidal mixing, and relative sea level. Pronounced facies variability at bedform, outcrop, and basin scale documented in this study appears to be an important characteristic of mixed carbonate-siliciclastic deposits in tidal depositional systems.

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

“…The northern Bouse Formation, which all workers agree formed in a lake (House et al, 2008;Pearthree & House, 2014;Crossey et al, 2015), lacks coarse-grained bioclastic facies and marine fossils. In contrast, the southern Bouse Formation contains both cross-bedding produced by migration of high-energy tidal bedforms (Buising, 1990;Turak, 2000;O'Connell et al, 2017O'Connell et al, , 2020 this study) and abundant marine fauna (Winterer, 1975;Taylor, 1983;McDougall, 2008;McDougall & Miranda-Mart ınez, 2014). The presence of tidal structures associated with marine fossils is a basic criterion widely used to confirm the tidal origin of ancient deposits (Ainsworth et al, 2012).…”

“…In this model, the basal carbonate member is interpreted to pre-date arrival of the earliest through-flowing Colorado River and its sediment load (McDougall & Miranda-Mart ınez, 2014;Dorsey et al, 2018). Abundant low-diversity marine and brackishwater fossils (Metzger, 1968;Smith, 1970;Buising, 1990;McDougall, 2008;McDougall & Miranda-Mart ınez, 2014), and marine trace fossils such as Teichichnus and Thalassinoides with diminutive form (O'Connell et al, 2020), provide evidence for brackish water conditions during deposition. Evidence from process sedimentology and Fourier transform analysis of rhythmically bedded bioclastic deposits supports a hypothesis that the basal carbonate member in the southern Blythe Basin was deposited by tidal currents in a tide-dominated marginal marine setting (O'Connell et al, 2017;O'Connell et al, 2020).…”

“…Abundant low-diversity marine and brackishwater fossils (Metzger, 1968;Smith, 1970;Buising, 1990;McDougall, 2008;McDougall & Miranda-Mart ınez, 2014), and marine trace fossils such as Teichichnus and Thalassinoides with diminutive form (O'Connell et al, 2020), provide evidence for brackish water conditions during deposition. Evidence from process sedimentology and Fourier transform analysis of rhythmically bedded bioclastic deposits supports a hypothesis that the basal carbonate member in the southern Blythe Basin was deposited by tidal currents in a tide-dominated marginal marine setting (O'Connell et al, 2017;O'Connell et al, 2020). Tide-generated and wave-generated cross-bedding has also been documented in Milpitas Wash, AZ, and the Palo Verde Mountains (Fig.…”

“…The palaeogeographic reconstruction shows that the eastern margin of the tidal strait, east and south-east of Cibola, was occupied by a broad low-gradient tidal flat system with a large well-developed network of tidal channels (Fig. 15;O'Connell et al, 2020). The wide, low-gradient setting on the eastern basin margin contrasts with the steep basin-margin morphology and narrow basin-margin facies belt at the western margin of the tidal strait in the Dashed heavy lines are inferred syn-depositional faults, some of which likely were subsurface structures.…”

“…While a marginal-marine setting is suggested by marine fossils and tidal-like sedimentary structures (Smith, 1970;Buising, 1990;Turak, 2000;McDougall, 2008;McDougall & Miranda-Mart ınez, 2014), some studies have used Sr and stable isotopes and elevation profiles to argue that the Bouse basal carbonate formed in a series of lakes that were isolated from the ocean (Spencer & Pearthree, 2001;Spencer & Patchett, 1997;Roskowski et al, 2010;Spencer et al, 2013;Pearthree & House, 2014;Bright et al, 2016Bright et al, , 2018. Recent work has identified tidal sedimentary facies, trace fossils and laminated rhythmites in carbonate deposits of the southern Bouse Formation (O'Connell et al, 2017;Dorsey et al, 2018;Gardner, 2019;O'Connell et al, 2020), providing new evidence for tidal processes. Thus there currently is no consensus on the depositional environment for this unit, and the palaeogeography remains poorly understood.…”