Pliocene-Pleistocene stratigraphy and depositional environments, southern Confidence Hills, Death Valley, California

Abstract

Pliocene to Recent lacustrine and playa deposits in the southwestern United States preserve a sedimentary record of climate change and tectonic activity. The middle Pliocene record is particularly important, because it represents the beginning of the glacial-interglacial climate that characterizes the Quaternary. However, continental sedimentary records of this time period arc scarce. Well-exposed playa margin deposits in the Confidence Hills, southern Death Valley National Monument, California, were deposited between about 2.5 and 1.5 Ma. Detailed chronometric control is provided by tephrochronology and magnetostratigraphy. A new stratigraphic unit, the Confidence Hills Formation, is herein defined. This formation consists of interbedded tine- to coarse-grained siliciclastic strata with abundant gypsum and anhydrite. Ten distinct sedimentary lithofacies were defined within the Confidence Hills Formation based on variations in their siliciclastic and evaporitc components. Lithofacies dominated by siliciclastic components include the siltstone, tine-grained sandstone, coarse-grained sandstone, and interbedded coarse-grained sandstone/conglomerate lithofacies. Those dominated by evaporitic components include the halitic mudstone/siltstone/fine-grained sandstone, gypsiferous mudstone/ siltstone/fine-grained sandstone, banded anhydrite, and thin-bedded to massive anhydrite lithofacies. The siliciclastic component shows an overall coarsening- and thickening-upward trend, from dominantly thinly bedded siltstone in the lower part of the section to dominantly medium-bedded, tine- to medium-grained sandstone in the upper part. Superimposed on this pattern is variation in amounts, types, and textures of evaporite deposits. The lithofacies observed in the Confidence Hills arc readily interpretable using the classification of saline lake subenvironments developed by Hardie et al. (1978). The halitic mudstone lithofacies is interpreted as having been deposited within the center of the saline lake. The anhydrite- and gypsum-bearing lithofacies are interpreted as having been deposited in the saline mudflat subenvironment marginal to the saline lake. The siliciclastic sediment was derived primarily from the Owlshcad Mountains to the west and deposited by streamflood mechanisms in a distal alluvial fan setting. The overall coarsening-upward sequence is interpreted as recording progradation of one or more marginal alluvial fans into a saline lake. The depositional rate was fairly constant at about 29 mm/ka. Major changes in depositional setting occurred roughly every 200,000 yr; smaller scale variations record changes occurring on time scales of tens of' thousands of years (evaporite-rich vs. evaporitc-poor intervals) to years (individual evaporite/siliciclastic couplets). Potential causes of these variations include autocyclic changes within the marginal depositional systems (e.g., lobe switching within the alluvial fan), changes in sediment supply due to fault motion, or changes in the position of the groundwater table reflecting local and/or regional climate variations. Different causes likely acted at different time scales. Larger scale variations do not appear to correlate with events in Searles Lake, and thus arc tentatively interpreted as resulting from autocyclic or tectonic causes. Smaller scale variations may record climate fluctuations. More detailed comparison with the sedimentary records from elsewhere in the region is required before a detailed climate record can be constructed.

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