HR: 0800h
AN: U21A-0708    [Abstracts]
TI: Discriminating Between Tectonic and Climatic Controls on Early Hominin Paleoenvironments From the Koobi Fora Region, Northeastern Turkana Basin, Kenya: Part I
AU: * Lepre, C J
EM: clepre@eden.rutgers.edu
AU: Quinn, R L
EM: rlquinn@eden.rutgers.edu
AB: Understanding controls on environmental records from Plio-Pleistocene Africa is critical for interpreting human origins. Recent geological studies from East Africa have focused on the relationship between Plio-Pleistocene patterns of hominin evolution, environmental change, and climate preserved in stratigraphic records of sedimentary basins (e.g. deMenocal, 2004; Wynn, 2004). Despite the fact that tectonics is a primary control on sedimentation in East African basins (e.g. Baker, 1986; Frostick, 1997), relatively few studies have either investigated its potential influence on early hominin evolution or attempted to discriminate between tectonic and climate controls on paleoenvironmental change. Presented is a study that explores these issues. Within the Koobi Fora Formation, between 4.0 and 2.5 Ma, environmental change is related to an overall trend of linear rates of tectonic subsidence. However, smaller-scale fluctuations in subsidence rates established lakes during times of increased subsidence followed by the transition to rivers during times of decreased subsidence and basin infilling (Feibel, 1994a, 2000). In contrast, environmental change during the period between 2.5 and 1.5 Ma was forced by changes in half-graben propagation, fault movement, and subsidence. This change is recorded within a stratigraphic sequence that is defined by major (erosional) boundary surface unconformities. The sequence is internally comprised of stable-lacustrine; stable-lacustrine, delta, and ephemeral-lacustrine; and fluvial environments of deposition. This environmental progression defines lowstand, transgressive, and highstand systems tracts respectively. Transition between systems tracts and depositional environments was controlled by rates of tectonic subsidence. The formation of stable-lacustrine environments of deposition during the lowstand systems tract was due to subsidence rates out-pacing sedimentation rates that was associated with a major tectonic event, characterized by uplift in Ethiopia and associated down-warping of the Turkana Basin in Kenya. While the formation of ephemeral-lacustrine and fluvial environments of deposition during the transgressive and highstand systems tract was due to sedimentation rates out-pacing subsidence rates that was associated with the infilling of the stable lake by deltaic environments. Infilling and depletion of accommodation space were primary steps needed to increase the sensitivity of the basin to climate change. Relatively shallow East African sedimentary basins are more sensitive to climate-related changes in evapo-transpiration than relatively deeper basins (e.g. Feibel, 1999). Transgressive and regressive lake margin facies that are associated with the transgressive systems tract suggest that the base level of the ephemeral-lacustrine system periodically oscillated during 2.0 and 1.5 Ma (Feibel, 1994b). The hominin record of Koobi Fora preserves relatively primitive forms between 4.0 and 2.5 Ma and relatively derived forms after about 2.5 Ma (cf. Walker, 2002). Australopithecines are associated with paleoenvironmental change that was forced by linear subsidence rates. The appearance of paranthropines in the region at about 2.5 Ma (cf. Wood, 1991) is associated with an episode of major faulting and paleoenvironmental change that was forced by an exponential decrease in subsidence rates. The appearance of early African Homo erectus in the Koobi Fora Region at 1.8 Ma (e.g. Wood, 1992; Collard and Wood, 1999) coincides with decreased subsidence rates but an increase in the sensitivity of paleoenvironmental organization in the region to climate change.
DE: 9305 Africa
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
SC: Union [U]
MN: 2004 AGU Fall Meeting