Abstract Detail

The Miocene vegetation and environment of Western North America

ERWIN , DIANE MARIE [1], Myers , Jeffrey Alan [2].

Stewart Valley - An update on the Miocene landscape, flora, and vegetation of west-central Nevada.

The Miocene rocks and fossils of Stewart Valley (SV), Nevada record one of the most complete terrestrial paleoecosystems known from North America. In 1986, it became a Bureau of Land Management Natural Area of Critical Environmental Concern owing to the abundance, diversity and wide geographic extent of fossil vertebrates, invertebrates (Mollusca, insects), algae, ichnofossils, herbaceous and woody plant megafossils and pollen. Such a diverse paleobiota within a section well-dated by mammals and volcanic ash is rare. As evidence mounts suggesting the Miocene Great Basin was a high plateau (the “Nevadoplano”) that underwent extensional collapse resulting in NW/SE trending extensional basins to produce lake-filled valleys, interpretation of the floral and vegetation changes recorded in these Neogene basins has been of especial interest. Initiation of the SV basin began ~17 myBP and coincided with the Mid-Miocene Climatic Optimum (MMCO), a time of elevated global temperature. Wolfe, in 1964, described the two major SV fossil plant assemblages: the lower Fingerrock-Goldyke (FG) and the upper Stewart Valley (SVP) (= Stewart Spring flora). However, additional plant collections, updated floral lists, and a refined chronostratigraphic framework has added to our understanding of these floras. The buff tuffaceous unit containing the FG assemblage lies between the lower and upper volcanic members of the Gilbert Andesite dated at ca. 16.0 Ma. This flora represents warm-temperate mesophytic forest with taxa that inhabited lakeside, riparian, and upland environments. Plants include Equisetum, ferns, Abies (red fir), Tsuga, Chamaecyparis, Picea, Pinus (soft pine), Sequoiadendron, Typha, Poaceae, Juglans, Carya, Quercus (lobed and evergreen oaks), Ulmus/Zelkova, Betula, Alnus, Populus (cottonwood), Platanus, Acer, Marah, Mahonia, Ribes, Cercocarpus, Sorbus and Arbutus. The younger SVP, dated at ~14.5 Ma, is part of the Stewart Valley Formation. It occurs in siliceous shale and mudstones deposited in a lake estimated to be ~8-10 miles (13-16 km) long (NW-SE) by 4-5 miles (6.5-8 km) wide (E-W). Although both floras share taxa, the SV shows the loss of mesic deciduous trees such as the lobed oak, sycamore, maples, and elms, with an incursion of dry-tolerant evergreen taxa such as Garrya, Arctostaphylos, Shepherdia, Quercus (evergreen), Heteromeles sp. and Lyonothamnus. Conifers too increase with the addition of Juniperus, a hard pine, and the earliest megafossil record of pinyon pine. Although this floral shift is consistent with the general trend of decreasing precipitation in the region, temperature estimates for the older FG have ranged from 2-3°C warmer to nearly 1°C cooler than the SVP.

1 - University Of California, MUSEUM OF PALEONTOLOGY, 1101 Valley Life Sciences Building, Berkeley, CA, 94720-4780, USA
2 - Western Oregon University, Earth Science, 345 N Monmouth Ave, Monmouth, OR, 97361, USA

Keywords:
Stewart Valley
Miocene
Nevada
Great Basin
Macro fossils
Paleoclimate.

Presentation Type: Symposium or Colloquium Presentation
Session: C2
Location: Salmon/Boise Centre
Date: Tuesday, July 29th, 2014
Time: 2:15 PM
Number: C2004
Abstract ID:655
Candidate for Awards:None