As in the rest of the Great Basin, mountain building began in the area of the Stewart Valley some 16 million years before the present, in the Miocene Epoch of the Cenozoic. In an area of less than 50 square miles, the rocks in this valley record a complex history of volcanic, fluvial, and lacustrine environments. These environments coexisted, or alternately replaced one another, as they responded to the ever-changing physical conditions dictated largely by the tectonic events of the region. The life of these times, as it interacted with the changing physical setting, is preserved as a fossil record of these same changes. Although this physical and biological record is but a small part of the Earth's history, the wide variety of physical settings in such a small area, coupled with the very diverse fossil assemblages, sets Stewart Valley apart as one of the truly unique terrestrial fossil areas of the world. No other known Miocene area in North America has such an excellently preserved and diverse terrestrial record (Savage and Russell, 1983).
The insects, for which fossil evidence exists from the Upper Carboniferous (360 - 400 million years before present) (Kukalova-Peck, 1987) potentially offer the greatest amount of data for interpretative studies of paleoecology, because of their great diversity and enormous number of species. Because their occurrence as an adequate sample of well-preserved fossils is very rare, they are hardly utilized in this respect. However, because of the probability of its yielding tens of thousands of specimens, Stewart Valley may be uniquely capable of supporting detailed paleoecologic studies of the Middle Miocene, when coupled with the extensive fossil representation of other terrestrial biota.
Following extrusion of the hornblende andesitic lavas of the lower part of the Gilbert Andesite, the landscape was a gently rolling, irregular surface for some distance around the present area, and shallow lakes or ponds developed in which leaves of the Fingerrock Flora were preserved in the accumulating muds.
Regional faulting and warping were initiated about this time (16 myBP) and began to delineate the various graben-horst relationships in this part of Nevada. Associated with this tectonic activity was the eruption of brecciated dacite flows.
Faulting and warping of this area contemporaneously with the eruption of the brecciated dacite flows produced a northwest-southeast drainage system into the Stewart Valley area. Continued faulting and rifting formed the depression in which sedimentary rocks of Units S-1 to 4 and G-1 to 3 accumulated. The basal Unit S-1 began to form some 14 - 15 myBP, and is unconformable with the dacite breccias. The entire sedimentary record of the earlier part of Unit S-1 indicates a time when drainage was being interrupted and small lakes coexisted with fluvial conditions. Anastomosing of smaller lakes and ponds brought about the formation of Stewart Valley Lake.
During the maximum extent of the lake (Units S-2 to 4) much finer material was brought into the lake and distributed quite uniformly. In the deeper part of the lake only thin layers of mud accumulated. These muds (the "paper shales" of Unit S-2) empirically indicate two features: 1) Litte or no coarse-grained material was supplied to this part of the basin., 2) There were no bottom-dwelling organisms or currents to disturb the laminated (thinly bedded) sediments once they were deposited.
The undisturbed laminae, containing complete leaves, fully intact insects, a few feathers, and articulated fish skeletons indicate a lack of scavenging and burrowing bottom dwellers, and suggest that the lake was chemically stratified, and the lower depths were a reducing environment, which greatly retarded the processes of decay and promoted fossilization.
Examination of polished sections of the more silicified parts of the paper shale sequence indicate deposition of couplets in an anerobic environment. Presence of silicified fine-grained laminae may be directly observed in section. In all counts made thus far, laminae number between 110 and 120 per centimeter, with an average of 116. The alternation of light and darker laminae (also containing, presumably, an allochtonous clay fraction) are interpreted as varves in the broader (non-glacial) sense, as glaciers are not involved. Different water viscosities would, however, be expected between winter and summer, and could account for the development of laminae, reflecting the difference in seasonal depositions in even a warm temperate area, such as proposed for Stewart Valley in the Miocene. Lewis (1977) has proposed mechanisms of formation for these lacustrine varves.
During the time of its maximum extent, Stewart Valley Lake was probably about 4 to 5 miles wide, east to west, and 8 to 10 miles long. This period coincides with a time of tectonic quiescence. The shales of Unit S-2 that formed in the deeper part of the lake were replaced toward the lake margins, and in the deeper parts during times of greater sediment influx, by massive-bedded shales and siltstones. The steady increase of these fine to medium-grained clastics ended deposition of the laminated shales of Unit S-2. The massively bedded nature of the mudstones of Unit S-3 indicate fairly rapid deposition, followed in turn by a period of slightly slower accumulation of the diatomaceous mudstones of Unit S-4. A volcanic mudflow (top of Unit S-3) and air fall ashes (Unit S-4) record an increase of pyroclastics that became much more common in the Granny Goose Member.
Filling of the basin, and renewed faulting in the area ended the predominately lacustrine deposition and initiated the predominately fluvial sedimentation of Units G 1-3. The thin pebble conglomerates and cross-bedded sandstones lower in Unit G-1 are gradually replaced by predominately fine-grained reworked or primary volcanic material of Units G-2 and 3. Fossil mammals are commonly found associated with point bar sediments in Unit G-3.
The tectonic and depositional history indicate that starting about 15 myBP, the graben-horst structure was initiated, while sedimentary accumulation began in the basin about 15 myPB. The initial fluvial conditions were quickly followed by lacustrine deposition of the Savage Canyon Member, Units S-1 to S-4. Renewed uplift and an increased supply of primary and reworked volcanic debris ended the lacustrine environment and generally filled the basin with alluvial deposits of the Granny Goose Member, Units G-1 to G-3. Continued rifting and faulting, though intermittent, produced the major structural features of the area and largely controlled the land forms, with subsequent modification by weathering and erosion.
At the present time, The Entomology Department of the California Academy of Sciences has about 731 insect fossils representing about 11 orders and 70 families from the Stewart Valley fossil beds. They are currently being processed, after which the pertinent data will be entered into a database and made available to the public through this webpage.
Last Updated January 2009