Stratigraphic and geomorphic analyses reveal that the regional drainage basin of the modern Amargosa River formed via multistage linkage of formerly isolated basins in a diachronous series of integration events between late Miocene and latest Pleistocene–Holocene time. The 275-km-long Amargosa River system drains generally southward across a large (15,540 km 2 ) watershed in southwestern Nevada and eastern California to its terminus in central Death Valley. This drainage basin is divided into four major subbasins along the main channel and several minor subbasins on tributaries; these subbasins contain features, including central valley lowlands surrounded by highlands that form external divides or internal paleodivides, which suggest relict individual physiographic-hydrologic basins. From north to south, the main subbasins along the main channel are: (1) an upper headwaters subbasin, which is deeply incised into mostly Tertiary sediments and volcanic rocks; (2) an unincised low-gradient section within the Amargosa Desert; (3) a mostly incised section centered on Tecopa Valley and tributary drainages; and (4) a west- to northwest-oriented mostly aggrading lower section along the axis of southern Death Valley. Adjoining subbasins are hydrologically linked by interconnecting narrows or canyon reaches that are variably incised into formerly continuous paleodivides. The most important linkages along the main channel include: (1) the Beatty narrows, which developed across a Tertiary bedrock paleodivide between the upper and Amargosa Desert subbasins during a latest Miocene–early Pliocene to middle Pleistocene interval (ca. 4–0.5 Ma); (2) the Eagle Mountain narrows, which cut into a mostly alluvial paleodivide between the Amargosa Desert and Tecopa subbasins in middle to late Pleistocene (ca. 150–100 ka) time; and (3) the Amargosa Canyon, which formed in late middle Pleistocene (ca. 200–140 ka) time through a breached, actively uplifting paleodivide between the Tecopa and southern Death Valley subbasins. Collectively, the interconnecting reaches represent discrete integration events that incrementally produced the modern drainage basin starting near Beatty sometime after 4 Ma and ending in the Salt Creek tributary in the latest Pleistocene to Holocene (post–30 ka). Potential mechanisms for drainage integration across paleodivides include basin overtopping from sedimentary infilling above paleodivide elevations, paleolake spillover, groundwater sapping, and (or) headward erosion of dissecting channels in lower-altitude subbasins. These processes are complexly influenced by fluvial responses to factors such as climatic change, local base-level differences across divides, and (or) tectonic activity (the latter only recognized in Amargosa Canyon).

Abstract This one-day field trip gives an overview of the geologic framework of this region, with emphasis on its Quaternary geomorphologic and hydrogeologic aspects. Its route (Fig. 1 ) is northwest from Las Vegas, along an arm of Las Vegas Valley, the problematic trace of the Las Vegas shear zone, to where the shear zone disappears into the Tertiary volcanic field of southern Nevada. Enroute, we pass several late Pleistocene/early Holocene groundwater discharge areas that have been long inactive, and visit a typical one. Then we go west to one of many warm springs in Ash Meadows, a major regional groundwater discharge area, noting exposures of 2-3-m.y.-old volcanic-ash beds within a few m of the surface, indicating that the Amargosa Desert basin has been a pediment, not a depocenter, for at least the last 1 m.y . Thence, we travel south along the Amargosa River to Tecopa Valley, the site of pluvial Lake Tecopa and the chief objective of this trip. Its badlands expose a very detailed record of Quaternary climatic change (many pluvial-interpluvial cycles and consequent erosion and groundwater cycles) and tectonism for the last 2.5 m.y. This record is relevant to climatic, groundwater, erosion, and tectonic issues at the Yucca Mountain, Nevada, proposed nuclear-waste repository. It also has important evidence on the significance of episodes of regional pedimentation versus alluvial-fan aggradation during the Quaternary in the Great Basin. After 8 stops in Tecopa Valley, we return to Las Vegas, briefly viewing more extinct groundwater-discharge areas of late Pleistocene-early Holocene age in Chicago and Pahrump Valleys.

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.