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

Abstract

Pliocene to Recent lacustrine and playa deposits in the southwestern UnitedStates 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 are 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 fine-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 evaporite components. Lithofacies dominated by siliciclastic components include the siltstone, fine-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, fine-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 are 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 Owlshead 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

Tephra layers of blind Spring Valley and related upper Pliocene and Pleistocene tephra layers, California, Nevada, and Utah: Isotopic ages, correlation, and magnetostratigraphy

“…The combined chronostratigraphy developed for the stratigraphic section in Confidence Hills of Death Valley, based on data presented in Troxel and others (1986), Pluhar and others (1992), Beratran and Murray (1992), and in this report ( fig. 18), provides new age estimates for three Reunion magnetostratigraphic events.…”

“…Deformed lake beds of silt, clay, mud, and sand intercalated with gravel, gypsiferous beds, and halite in the Confidence Hills of southern Death Valley, southeastern California (Troxel and Butler, 1986;Troxel and others, 1986;Pluhar and others, 1992;Beratran and Murray, 1992), contain multiple tephra layers that correlate with tephra layers in several other sections discussed above and below ( figs. 1, 18; table 7).…”

“…Above the basal tuffs at the Confidence Hills locality is the Huckleberry Ridge ash bed (CHA86 BT-2; CH88007; table 7). This ash bed (actually an indurated tuff at this locality) is used as the reference point for correlation of the measured sections of Troxel and others (1986) and B. W. Troxel (written commun, 1988) with that of Beratran and Murray (1992) (fig. 18).…”

“…Magnetostratigraphy determined by Pluhar and others (1992) in "Confusion Canyon" of Holt and Kirschvink (1995), and tephrostratigraphy of the same section, from Beratan and Murray (1992). The Huckleberry Ridge ash bed, sample 07, is common to both measured sections (Beratran and Murray, 1992;Troxell and others, 1986; this report) and is used as a reference for correlating the two sections and other tephra layers. Solid horizontal lines in tephrostratigraphy column are tephra layers.…”

Two-stage formation of Death Valley

“…These low hills are composed of clastic sediments of the Confi dence Hills Formation (Wright and Troxel, 1984), with an age range of 1.7-2.2 Ma (Knott et al, 2005). Like the Black Mountains sections, these sediments show little evidence of syndepositional tec tonism, yet are now highly deformed, with almost vertical dips in places (Beratan et al, 1999). This deformation is associated with the Southern Death Valley strike-slip fault (Dooley and McClay, 1996) and gives a qualitative indication of the large amount of post-1.7 Ma motion there has been on this fault.…”