Determining Depositional Heterogeneity through Clay Mineralogy and Particle Size Analysis of an Early Miocene Paleo-forest Paleosol, Hiwegi Formation, Rusinga Island, Kenya

William H. Horner¹, Lauren A. Michel¹, Daniel J. Peppe¹, Steven Driese¹, Kieran P. McNulty², Thomas Lehmann³, Holly M. Dunsworth⁴, and William E. H. Harcourt-Smith^5
1Department of Geology, Baylor University, One Bear Place #97354, Waco, TX 76798-7354, [email protected]
²Department of Anthropology, University of Minnesota, 395 Hubert H. Humphrey Center, 301 19th Avenue South, Minneapolis, MN 55455
³Abteilung Paläoanthropologie und Messelforschung, Forschungsinstitut und Naturmuseum Senckenberg, Senckenberganlage 25, Frankfurt, D-60325, Germany
⁴Department of Sociology and Anthropology, University of Rhode Island, 507 Chafee Building, 10 Chafee Road, Kingston, RI 02881
⁵Department of Vertebrate Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024

Understanding the selective pressures that helped drive early hominoid evolution requires an understanding of environmental dynamics and the habitats available to early Miocene primates. Fossil sites on Rusinga Island (Lake Victoria, Kenya) have represented some of the best sites for understanding this relationship between environmental dynamics and hominoid evolution. This has been due to the quantity and quality of fossil specimens of early apes, such as the catarrhines Proconsul and Dendropitchecus macenssi, preserved amongst evidence of the habitats in which they lived. Whereas a significant amount of prior research focused on the paleontology of these primates, less work focused on the paleoenvironment, and the results of paleoenvironmental studies are contradictory. A recently discovered in situ fossil forest that stretches ~5km across three different fossil localities represents a unique opportunity to reconstruct the paleoenvironment in which early apes lived.

This study examined clay mineralogy and grain size of paleosols at the three contemporaneous localities. These analyses were used to assess variations in paleoweathering and/or in depositional environment within and between sites to test for environmental variability across the landscape. X-ray diffraction (XRD) of oriented clay aggregates was performed to determine both temporal and spatial variations in clay mineralogy as a proxy for weathering. Clay mineral suites varied with paleosol depth, as expected for a soil profile, and were largely consistent spatially within and between localities. This proxy indicated uniform paleoweathering at this time-slice. Particle size analyses were conducted using a Mastersizer grain size analyzer and were used as a proxy for depositional energy. Grain size results indicated variation with depth and between localities. This proxy suggested variability in depositional energy across the landscape. These results, coupled with our sedimentology and paleopedology data, suggested that during this particular interval of time the early apes Dendropithecus and Proconsul most likely lived in a relatively uniformly forested environment with variations in depositional energy.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013