Detailed lateral study of strata associated with the A.L. (Afar Locality) 333 hominin locality provides paleoenvironmental information at geographic scales of hundreds of meters to kilometers as well as insights regarding alluvial deposition and pedogenesis in the middle Denen Dora Member of the Hadar Formation. A.L. 333 is dated at ca. 3.2 Ma and has produced over 260 surface and excavated specimens of Australopithecus afarensis . It represents an unusual source of high-resolution information about the paleoenvironmental context of this hominin. The in situ hominin fossils are associated with the final stages of filling of a paleochannel and were buried prior to the formation of overlying paleosols. Preserved bedding structures in the fine-grained hominin-producing strata provide evidence that the abandoned channel continued to aggrade prior to the onset of sustained pedogenesis. Pedogenic carbonates associated with the hominin level thus postdate the death and burial of the hominins, possibly by centuries to millennia. The reconstructed paleodrainage of the DD-2 sandstone (DD-2s) is oriented south to north and consists of a trunk channel, ~40 m wide and 3–5 m deep, connecting a tributary system south of A.L. 333 to a distributary system to the north, which likely ended on the deltaic plain associated with the basin’s depocenter. The hominin concentration occurs in the upper part of the fill of the trunk channel. The burial of the hominin remains involved fine-grained deposition indicating low-energy, seasonal flood events, and there is no sedimentological evidence for a high-energy, catastrophic flood that could have caused the demise of the hominins.

Abstract A sequence of lacustrine, volcaniclastic, and alluvial sedimentary deposits that record the past million years of the history in the Olorgesailie Basin, southern Kenya, provide an example of how tectonics, climate, and volcanism affect sedimentation in a rift valley. A series of radiometric dates on volcanic materials through this sequence permits relatively fine-scale calibration of the timing and duration of volcanic input to the depositional system, transgressive-regressive cycles of the lake, and intervals of valley cutting and filling. The Olorgesailie Formation, accumulated between 0.992 and 0.493 Ma, consists of relatively pure diatomites, reworked diatomites, primary volcanic and reworked volcaniclastic units, and alluvial deposits (clays, silts, and sands with several well-developed paleosols) that bear a rich archeological and paleontological record. After 0.493 Ma, increased tectonic activity initiated a series of valley cutting and filling cycles that continue into Recent times. A working hypothesis attributes the formation of the paleolake to a barrier on the southwest side of the basin, large-scale lacustrine versus alluvial phases of the Olorgesailie Formation to variations in subsidence rates operating on a time scale of 10 4 -10 5 yr, and transgressive-regressive cycles within these sedimentary packages to wet-dry climate cycles on time scales of 10 3 -10 4 yr. Episodes of volcanism were superimposed on these patterns but did not have significant long-term effects on the depositional system.

Full article available in PDF version.

Abstract The deposits of the Potwar Plateau in northern Pakistan show large-scale changes in fluvial deposition during the period between 14 and 6 my BP in three superimposed formations (Chinji, Nagri, Dhok Pathan) approximately 2700 m in total thickness. Five cross sections spaced throughout this sequence document intervals 30-80 m thick over lateral distances of 3-5 km. These show the architectural relationships of sand bodies, fine-grained facies, pedogenic horizons, and vertebrate fossil occurrences within floodplain deposits between major channel belt sandstones. Vertebrate fossil localities are particularly frequent within the fine-grained fill of different scales of abandoned floodplain channels (crevasse-splay and/or tributary channels). Such channels are larger and more common in the lower part of the sequence where average rates of sediment accumulation were 0.14-0.32 m/1000 yrs. as determined from the Siwalik magnetostratigraphic time framework. Higher average accumulation rates of 0.46-0.48 m/1000 yrs for the upper part of the sequence correspond to fewer fine-grained channel fills and lower fossil abundance. Changes in type and frequency of floodplain channels are attributed to a combination of differences in the fluvial regimen and in sediment accumulation rates. The quality of the vertebrate record is linked to the frequency and/or magnitude of abandoned channels on the sub-Himalayan alluvial plain and thus to large-scale tectonic and climatic controls on the fluvial systems.