ABSTRACT The deposits of Pleistocene Lake Tecopa include lacustrine, alluvial, eolian, and groundwater discharge deposits of the Tecopa basin in southeastern California. Stratigraphic sections measured in the Tecopa basin and detailed sedimentary facies analysis were used to interpret the depositional settings and track the evolution of sedimentary processes in the basin during the Pleistocene. The early Pleistocene (ca. 2.4–1.0 Ma) deposits of the Lake Tecopa beds record deposition in small saline, alkaline lakes and playas with surrounding mudflats and sandflats and adjacent alluvial fans. Ancestral Amargosa River gravels are first observed in fluvial deposits in the northern part of the basin at ca. 1.0 Ma and correspond with lake expansions (Glass Mountain [GM] lakes) during deposition of the uppermost Glass Mountain ash beds. Several oscillations in lake level followed the post-GM lake decline, culminating in the basin-filling Lava Creek (LC) lake, which reached its acme during deposition of the 0.63 Ma Lava Creek B ash bed. The post–Lava Creek B strata reflect primarily alluvial, fluvial, eolian, and groundwater discharge depositional processes, punctuated in the youngest part of the section by basin-filling lakes (high lake 1 and 2). The Lava Creek B ash bed and older lacustrine strata exhibit extensive zeolitization and clay authigenesis, characteristic of saline, alkaline lake deposits, but the post–Lava Creek B ash bed lacustrine strata have only minor zeolite and clay alteration, suggesting fresher water conditions and a change in the hydrologic state of the basin. Sedimentological observations along with shoreline elevation data provide evidence for intermittent spillover of basin-filling lakes after ca. 0.63 Ma. Subtle tectonic deformation influenced sedimentary processes in the Tecopa basin throughout its history. Episodes of uplift and tilting of Lake Tecopa strata during the middle Pleistocene in the southern part of the basin along the Tecopa Hump likely controlled the sill elevation for spillover of the lake, creating accommodation space for late Pleistocene basin-filling lakes. Ultimately, decreased uplift could not keep pace with increased discharge resulting from high effective moisture during latest middle Pleistocene pluvial periods, and Lake Tecopa drained, most likely during or immediately after marine oxygen isotope stage (MIS) 10 (ca. 0.3 Ma). The deposits of Lake Tecopa provide a detailed record of Pleistocene paleoclimate from ca. 2.4 to 0.3 Ma that demonstrates Milankovitch-scale tuning and clarifies the amplitude of Pleistocene climate change in the southern Great Basin of North America.

The Late Jurassic (157–150 Ma) Morrison Formation of the Western Interior of the United States contains abundant altered volcanic ash. On the Colorado Plateau, this formation accumulated behind and downwind of a subduction-related volcanic arc along the western margin of North America. The ash in these distal fallout tuffs probably drifted eastward from coignimbrite ash clouds related to collapse calderas. Altered volcanic ash is particularly abundant in the Brushy Basin Member of the upper part of the Morrison Formation. In one 110-m-thick section in eastern Utah, 35 separate beds were deposited in a 2.2 m.y. period. Alteration occurred when glassy volcanic ash fell into fluvial and lacustrine environments, where it was diagenetically altered to various mineral assemblages but most commonly to smectitic clay. Periodically, ash fell into saline, alkaline lakes, and diagenetic alteration of the glassy ash produced a crudely zoned deposit on the Colorado Plateau. Altered volcanic ash beds in the outermost part of the lacustrine deposits are argillic (with smectitic clay), whereas zeolitic (clinoptilolite, analcime) and feldspathic (K-feldspar and albite) alteration dominates the interior zones. Feldspathic ash layers contain secondary silica, and consequently immobile element (e.g., Al, Ti, and high field strength elements) abundances were strongly diluted in these rocks. In contrast, the argillic ash beds experienced strong SiO 2 depletion, and, as a result, they are enriched in the relatively immobile elements. The compositions of the zeolitic ash beds are intermediate between these two extremes and experienced the least alteration. As a result of these changes, immobile element concentrations are less reliable than ratios for determining the original magmatic composition of the ash. Most of the altered ash (regardless of type) was also depleted in water-soluble elements like the alkalies, U, and V. The latter two elements were oxidized during diagenesis of the ash, became soluble, and were partially leached away by groundwater. Locally, U and V in groundwater were reduced upon contact with organic materials and formed important ore deposits. Several aspects of the mineralogy and geochemistry of the altered volcanic ash beds yield information about their original magmatic compositions. The volcanic ash beds typically have small phenoclasts of quartz, sanidine, plagioclase, biotite, zircon, apatite, and Fe-Ti oxides. Titanite is present in ∼40% of the ash beds; pyroxene and amphibole were found in less than 5%. Phenocryst assemblages, mineral compositions, inferred high f O 2 , rare earth element patterns, and immobile element ratios all suggest the parent magmas for the altered tuffs were subduction-related dacites and rhyolites. Small numbers of tuffs have Fe-rich biotite, amphibole, and/or clinopyroxene; both pyroxene and amphibole are alkali rich. These tuffs lack titanite, but some contain anorthoclase and F-rich apatite. Combined with enrichments in Nb and Y, these features show some tuffs had an A-type character and were related to some type of within-arc extension. Paleowind directions, and distribution, radiometric ages, and compositions of the volcanic ash beds and of plutons in the western United States suggest that the most likely eruption sites were in the subduction-related Jurassic magmatic arc, which extended across western Utah and central Nevada and southward into the Mojave of California and southern Arizona (present-day coordinates). Pb isotopic compositions show that at least some of the ash was erupted from magma systems (now exposed as plutons) in the Mojave Desert. We conclude that a brief ignimbrite flare-up from 157 to 150 Ma, but focused on the time period from 152 to 150 Ma, in this region may have been driven by slab steepening and conversion to a strike-slip boundary after a preceding phase of folding and thrusting. The presence of ash beds with A-type characteristics mixed with those that have more typical subduction signatures confirms that the Late Jurassic was geologically a transitional time in North America when subduction was changing to transtensional movement along the western plate boundary.