A TRANSGRESSIVE MIXED CARBONATE-SILICICLASTIC TIDAL FLAT DEPOSITIONAL SYSTEM IN THE 5.0-5.5 MA SOUTHERN BOUSE FORMATION

The late Miocene to early Pliocene Bouse Formation south of Blythe, CA, records depositional paleoenvironments at the north end of the ~5.0-5.5 Ma Gulf of California. Detailed sedimentologic and stratigraphic study shows that the Bouse basal carbonate member is an overall deepening-up transgressive systems tract that contains a complex mosaic of mixed-carbonate and siliciclastic lithofacies, including: (1) basal cobble lag and erosional ravinement surface; (2) well-sorted cross-bedded sandy gravel and grainstone; (3) salt-marsh matted sandy lime mudstone with carbonate reed and grass imprints; (4) upper tidal-flat lime mudstone with green siliciclastic claystone; (5) middle tidal-flat heterolithic lime mudstone, wackestone, packstone, and siliciclastic-rich grainstone with Thalassinoides burrows, flaser, wavy, and lenticular bedding; and (6) lower tidal-flat to shallow subtidal silici-bioclastic grainstone compound tidal dunes with sparse bioturbated lime mudstone interbeds. Cross-strata in facies 6 represent, in vertical succession, a transition from 2D to 3D tidal dunes, with paleocurrent measurements indicating an ebb-tide dominant system. The tidal lithofacies are abruptly capped by a subtidal, offshore low-energy lime mudstone with no tidal signal. We interpret the transgressive deepening-up trend and change to subtidal facies to record a rise in relative sea level and associated change in basin hydrodynamics. Tidal rhythmites are present in most facies, and include well-developed thick-thin couplets with remarkable segregation of carbonate and silicliclastic sediment. Fourier analysis of the silici-bioclastic couplets reveals statistically significant short-term diurnal and semi-diurnal tidal cycles with possible monthly periodicity. ­