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 sedimentological and ichnological 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 comprising: Facies Association 1siliciclastic-rich heterolithic facies, lime mudstone with desiccation cracks, and plant debris rich carbonate silt interpreted as siliciclastic-rich tidal flats; Facies Association 2well-sorted gravels, siliciclastic-rich sandy strata, lime mudstone with desiccation cracks, and sandy microbial micrite interpreted as tidal-channel deposits; Facies Association 3carbonate-rich heterolithic lime mudstone to well-sorted, cross-bedded bioclastic grainstone interpreted as intertidal to shallow subtidal deposits; and Facies Association 4lime 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 and 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 percentage 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.

Tectonostratigraphic record of late Miocene–early Pliocene transtensional faulting in the Eastern California shear zone, southwestern USA

“…2). Geologic maps were compiled from previous work (Homan, 2014;Gootee et al, 2016;O'Connell et al, 2021;Dorsey et al, 2018;Gardner and Dorsey, 2021) and new mapping conducted for this study. Stratigraphic relations were characterized by detailed geologic mapping, measuring sections using a Jacob's staff, correlation of key contacts and marker beds, and construction of geologic cross sections and stratigraphic panels to illustrate important stratal architectures and fault geometries.…”

“…2). In the western Trigo Mountains, bioclastic facies overlie Miocene alluvial fan conglomerate along a sharp, quasi-planar, laterally continuous disconformity (O'Connell et al, 2021). In the southeastern Palo Verde Mountains, a thin basal travertine unit is encrusted on steep irregular paleotopography in Miocene volcanic rocks, and bioclastic facies onlap onto travertine-encrusted local relief.…”

“…It is clast supported and well sorted and displays tabular to trough cross bedding formed in beach ridges, gravelly barchan dunes, delta mouth bars, and small Gilbert deltas (Fig. 5B; Dorsey et al, 2018;O'Connell et al, 2021). Carbonate-dominated facies (unit Tbbc) overlie and are interbedded with unit Tbbg and contain a wide range of sedimentary structures, facies associations, and carbonate-siliciclastic mixtures that record deposition in low-to high-energy tidal Monocline at depth (Fig.…”

“…Carbonate-dominated facies (unit Tbbc) overlie and are interbedded with unit Tbbg and contain a wide range of sedimentary structures, facies associations, and carbonate-siliciclastic mixtures that record deposition in low-to high-energy tidal Monocline at depth (Fig. 9A) channels, bars, and intertidal to shallow subtidal flats (O'Connell et al, 2017(O'Connell et al, , 2021Dorsey et al, 2018;Gardner and Dorsey, 2021). The youngest unit of the basal carbonate member consists of laterally extensive fine-grained carbonate micrite and shale (marl; Fig.…”

“…Research Paper Crossey et al, 2015;O'Connell et al, 2017O'Connell et al, , 2021Dorsey et al, 2018;Gardner and Dorsey, 2021). Interpretations for specific stratigraphic units are summarized below.…”