MIOCENE STEWART VALLEY, NEVADA: THE BEST LITTLE TERRESTRIAL ECOSYSTEM IN THE NEOGENE OF NORTH AMERICA

If best is equivalent to most complete and diverse, then the rocks and fossils in Stewart Valley (SV), Nevada, certainly preserve one of the best terrestrial paleoecosystems known from the Neogene of NA. Numerous excellent studies show that major structural depressions in west central Nevada were associated at various times with extension in the Walker Lane. These structures began at ~17 Ma and occur in an area ~400 km NW-SE by 200 km NE-SW, and become younger to the W-NW. In the SV basin the various lithologies, sedimentary structures, fossil content, and geometry of the rock units indicate deposition in a swallow linear depression formed on the NE flank or apron of a waning andesitic volcanic center. The initial area of deposition was probably no more than 600 km2, but after a brief period of non-deposition at ~13.5-13.0 Ma the area was integrated into a larger region of primarily fluvial/lesser lacustrine deposition extending ~140 km SE of the original basin. Stewart Valley basin ss records a sequence lasting from basin initiation at ~17 Ma to its termination at ~10 Ma with the deposition of unroofed basement rocks associated with initiation of Basin and Range structure. During this time the continually changing physical settings and their associated biotas preserve a rich and varied concentration of the terrestrial ecosystem of this area. Fossils/fossil evidence include ichnofossil mud borings, near-shore tufa deposits surrounding standing tree stumps (since removed), diatoms, algal mats, lignites, two major horizons of plants, with the lower containing ~20 Fam/35 ssp and the upper with ~26 Fam/50 ssp, in situ standing individual trees and a small petrified forest, a deltaic and near shore freshwater molluscan fauna of ~14 Gen/35 spp, fish, feathers, two major vertebrate faunas, the Barstovian with ~37 Fam/53 Gen and the Clarendonian of ~31 Fam/45 Gen, and what has become known as the “crown jewel” of the fossil suite, an excellently preserved insect fauna of some 14 Ord/40 Fam distributed among ~4000 fossil specimens.