Rocks and deposits exposed at the surface in Segment 1 range in age from Precambrian to Quaternary (fig. 12). They consist of igneous intrusive rocks, pyroclastic and extrusive volcanic rocks, and marine and continental sediments, many of which, particularly the older rocks (pre-Mesozoic) have been intensely metamorphosed, folded, and faulted.
The principal water-yielding units are unconsolidated continental clastic deposits of Cenozoic age that partly fill structural basins created by faulting. Volcanic rocks, which are principally lava and pyroclastic flows of Cenozoic age, are important aquifers in scattered areas. Paleozoic limestones and dolomites associated with basin-fill Cenozoic clastic deposits in eastern Nevada are the only older rocks with significant water-yielding potential.
During Precambrian time and the Paleozoic Era, an almost uniform thickness of approximately 40,000 feet of marine sediments was deposited in the Cordilleran geosyncline. This geosyncline was an elongated trough that extended north to south in western North America and included the area that is now eastern Nevada and southern California. Sedimentation was marked by two periods of alternating clastic and carbonate deposition that resulted in the following sequence: quartzite and siltstone, limestone and dolomite, argillite and quartzite, and limestone.
At the end of the Paleozoic Era, volcanoes were active on a grand scale in eastern California and western Nevada. This volcanism marked the beginning of igneous activity that was to become increasingly important during Mesozoic time.
A number of shallow marine invasions inundated parts of the region during the Mesozoic Era. Conditions were such that marine formations alternated with nonmarine deposits derived from erosion of rocks in the continental interior. During this time, a coastal strip as much as 400 miles wide was formed by a combination of marine sedimentation and igneous activity, granitic intrusions, and subaerial volcanism, and was welded to the western margin of the preexisting continental mass.
The early Mesozoic seas spread inland as far as central Utah and Wyoming but were soon blocked by a narrow uplift in central Nevada. During the remainder of the Mesozoic Era, only intermittent subaerial deposition took place east of this uplift. West of the uplift, a thick sequence of Mesozoic marine and continental sediments was deposited, interspersed with lava flows, volcanic breccia, and tuff.
The close of middle Mesozoic time culminated in the first great orogeny in the western part of North America since Precambrian time. As mountain ranges rose, the marine, continental, and volcanic deposits of the Pacific Coast were folded, metamorphosed, and complexly faulted. Intense deformation of the older rocks spread eastward across most of Nevada. Late in the Mesozoic Era, the Pacific coastal region was again downwarped and the sea intruded.
During the Cenozoic Era, volcanic rocks and sedimentary deposits accumulated over wide areas of Segment 1, to thicknesses of as much as 50,000 feet. Early in the Cenozoic Era, the Basin and Range area was a high mountain surface with external drainage. During middle to late Cenozoic time, however, large-scale block faulting formed the Coast Range Mountains, the California Trough, and the Sierra Nevada and caused the Basin and Range structures. These structures are a sequence of alternating horsts and grabens that trend north-south and are reflected in the present-day topography. Volcanism, which still continues today, formed much of the Cascade Mountains.
In late Cenozoic time, the California Trough and the structural basins in the Coast Range were filled with marine and terrestrial deposits that ranged from a few thousand to as much as 50,000 feet in thickness. The grabens of the Basin and Range were filled with continental deposits and minor lava flows to thicknesses of generally less than 2,000 feet, but locally as much as 50,000 feet. The late Cenozoic also was the time of development of basins in the mountains of northern California and Nevada. These basins were filled with clastic sediments and numerous basaltic lava flows.