Quaternary geochemical stratigraphy of the Kedong-Olorgesailie section of the southern Kenya Rift valley

摘要

This study uses a variety of geochemical parameters to reconstruct a Quaternary environmental history of the Kedong-Olorgesailie sector of the southern Kenya Rift Valley. Lacustrine, wetland and fluvial deposition took place between about 1.9 and 0.72Ma atMunyuwa Gicheru in the northern part of the study area. Sedimentation continued about 40 km further south to produce the lake, wetland, fluvial and pedogenic deposits of the Olorgesailie Formation between about 1.2 and 0.5 Ma. Faulting subsequently led to basin breakup at Olorgesailie and the development ofMiddle to Late Pleistocene cut-and-fill fluvial deposits, as well as pond, wetland and terrestrial sediments that were locally altered by pedogenesis. Tufas and calcretes also accumulated during this latter time interval. Sedimentation in these different depositional sequences reflects several long-term trends, including: 1) an increase in carbonate deposition; 2) a decrease in biogenic silica; and 3) decreased chemical weathering indices (Chemical Index of Alteration, Vogt ratios), which imply increasing Quaternary aridity and/or erosion of catchment soils. The biophile elements Br and S also increase upwards, suggesting pulses of erosion of organic soils. Plants permineralized by silica became widespread after about 0.5 Ma, providing evidence that groundwater bodies were becoming silica-enriched, possibly as a result of deeperwater circulation along active faults. Several fault-aligned fossil hot spring deposits with siliceous vent, pedogenic and swamp facies have been identified at Emerit in the central part of the Kedong-Olorgesailie rift. Tectonism also provided pathways for meteoric water that led to the formation of carbonate springs in the Olorgesailie area. REE patterns in clastic sediments are stable throughout the Quaternary, reflecting the dominant trachytic source rocks, but these patterns become variable in later Pleistocene calcretes and tufas documenting changing hydrochemical conditions. The geochemical evolution of the area partly reflects climate change through time. However, it also documents the influence of Quaternary rifting processes. Rift faulting affected the geochemistry of the basin sediments by 1) causing changes in the depth of fracture systems that controlled groundwater movements and sourcing of fluids, 2) changing accommodation space in which sediments could accumulate, and 3) altering relief and thus rainfall and evaporation patterns across the rift system, as well as erosion and weathering rates.