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2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 6
Presentation Time: 3:15 PM

GEOCHRONOLOGY, STRATIGRAPHY, AND PALEOENVIRONMENTS OF EARLY MIOCENE VERTEBRATE FAUNAS FROM MFANGANO AND RUSINGA ISLANDS, EASTERN LAKE VICTORIA, KENYA


FOX, David L., Department of Earth Sciences, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455-0219, PEPPE, Daniel J., Department of Geology, Baylor University, One Bear Place #97354, Waco, TX 76798, MCNULTY, Kieran P., Department of Anthropology, University of Minnesota, 395 Hubert H. Humphrey Center, 301 19th Avenue South, Minneapolis, MN 55455, HARCOURT-SMITH, Will, Department of Vertebrate Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, DUNSWORTH, Holly M., Department of Anthropology, Northeastern Illinois University, 5500 North St. Louis Avenue, Chicago, IL 60625, DEINO, Alan L., Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, LEHMAN, Thomas, Forschungsinstitut und Naturmuseum Senckenberg, Frankfurt, Germany and ANDREWS, Alexandra L., Department of Geology and Geophysics, Yale University, New Haven, CT 06511, dlfox@umn.edu

Early Miocene deposits of the Rusinga Group from Mfangano and Rusinga Islands (Lake Victoria, Kenya) preserve abundant vertebrate fossils, including several early species of hominoids or apes, which have been collected for over 75 years. We have initiated a study integrating the paleontology, paleoecology, and geochronology of these deposits to better understand climatic and environmental during the first phases of the hominoid radiation in the Early Miocene. Here we focus on the geochronology and stable isotope composition of the youngest and oldest units of the Rusinga Group, the Hiwegi and Wayondo Formations, respectively. Both units are primarily tuffaceous, silt to boulder conglomerate fluvial and volcaniclastic. A new 40Ar/39Ar age determination from the top of the Hiwegi Fm. on Rusinga constrains the age of the Rusinga Group to ca. <18 Ma. Analysis of paleomagnetic samples from the Hiwegi demonstrate a series of geomagnetic reversals that most likely correlate to C5En - C5Dr.2r of the Geomagnetic Polarity Time Scale. The geochronologic results suggest the Rusinga Group is older and represents more time than previously suggested. Paleosols in both formations on both islands are weakly developed and contain dispersed to nodular pedogenic carbonate. Pedogenic carbonate δ13C values record the relative abundance of plant biomass in a soil using the C3 (trees, shrubs, cool-growing season grasses) and C4 (warm-growing season grasses, sedges) photosynthetic pathways. δ13C values from the Wayonodo Fm. on Mfangano range from -10 to -7.5‰ with a mean of -8.9±0.95‰ (VPDB). Accounting for the δ13C of atmospheric CO2, these data imply the early presence of C4 biomass but at a lower percentage (<15%) than indicated by younger Middle and Late Miocene paleosol carbonates in the Tugen Hills to the east. Pedogenic carbonate δ18O values are sensitive to the δ18O value of meteoric water, and hence are a paleoclimate proxy. Wayondo δ18O values range from 23.8-27.4‰ with a mean of 26.1±1.01‰ (VSMOW). These values are similar to those in the Muruyur Formation (16-15 Ma) at the base of the Tugen Hills section. Our isotopic data suggest climatic and, potentially, paleoecological stability during the Early Miocene in East Africa followed by a transition to more open habitats in the Middle and Late Miocene.

Session No. 134

T87. Phanerozoic Paleoenvironmental Evolution of Africa II
Monday, 19 October 2009: 1:30 PM-5:30 PM
Portland Ballroom 255 (Oregon Convention Center)
Geological Society of America Abstracts with Programs. Vol. 41, No. 7, p.362
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