Paleolimnology of lacustrine rocks in Stewart Valley, Nevada: Evidence for Middle Miocene climatic cooling

Conference · Thu Apr 01 00:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5024963

Starrat, S W ^[1]

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1. Geological Survey, Menlo Park, CA (United States)

Three diatom floras from Middle Miocene (Barstovian and Clarendonian) lacustrine rocks in Stewart Valley, Nevada have been distinguished. The change in floral composition between the two youngest floras may be indicative of climatic cooling over a period of about 3 m.y. (15--12 Ma). Age control is provided by radiometric (K-Ar) and vertebrate fossil data. The oldest flora is dominated by members of the genus Fragilaria'. Although most common in modern-day marshy areas, the laminated nature of the Stewart Valley strata in which this flora is found suggest that large numbers of these diatoms were washed into deeper waters, where they continued to thrive as a significant part of the planktonic biomass. Stratigraphically equivalent rocks elsewhere in Stewart Valley contain abundant clusters of unopened prasinophyte algae. These unopened algal structures are thought to indicate extreme environmental stress. Environmental stress would also explain the presence of several beds of well-preserved fish fossils in stratigraphically adjacent beds. The other tow floras are preserved in a 45-m-thick section of diatomaceous shale, located about 95 m above the flora discussed above. The flora in the lower part of this section is dominated by the genus Aulacoseira (primarily A. granulata). Modern-day members of this genus are common in areas with abundant summer precipitation and mild winters. The flora in the upper part of the section is dominated by Actinocyclus cedarensis Bradbury and Krebs. If A. cedarensis can be considered an ecological analog of the late Pleistocene (glacial) representatives of the genus Stephanodiscus, then its dominant position in the flora may be indicative of a cooling event. This climate trend is also evident in paleobotanical (leaf and pollen) data from Stewart Valley, as well as many other localities across the Great Basin.

• OSTI ID: 5024963
• Report Number(s): CONF-9305259-; CODEN: GAAPBC
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Vol. 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993; ISSN 0016-7592
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
GEOLOGIC DEPOSITS
PALEONTOLOGY
STRATIGRAPHY
NEVADA
PALEOCLIMATOLOGY
ARGON ISOTOPES
ISOTOPE DATING
LAKES
LIMNOLOGY
PALYNOLOGY
POLLEN
POTASSIUM ISOTOPES
SHALES
TERTIARY PERIOD
AGE ESTIMATION
ALKALI METAL ISOTOPES
CENOZOIC ERA
DEVELOPED COUNTRIES
GAMETES
GEOLOGIC AGES
GEOLOGY
GERM CELLS
ISOTOPES
NORTH AMERICA
ROCKS
SEDIMENTARY ROCKS
SURFACE WATERS
USA
580000* - Geosciences