INVESTIGATING SEDIMENTATION FROM MEGA PALEOLAKE DECEPTION NEAR GUMARE, NW BOTSWANA USING SEDIMENTARY, CHEMICAL AND ISOTOPIC PROXIES.

The existence of Quaternary paleolakes in the Kalahari Basin of Southern Africa has long been studied, the largest and oldest of which is Paleolake Makigadikgadi. Recent Shuttle Radar Topography Mission data suggests an earlier and larger paleolake, Paleolake Deception. This study presents additional sedimentological, geochemical, and isotopic evidence from the region to evaluate the existence of Paleolake Deception and the geologic history of the paleoenvironment. Multi-proxy geochemical and sedimentological data is needed to support the existence of Paleolake Deception, as opposed to geomorphic evidence alone. Grain size distribution, bulk density, magnetic susceptibility, TIC, TOC, metal content, and inorganic C and O isotopic composition were measured for a 6.5 m sediment profile from Gumare, Botswana, Africa. Proxies are constructed from the measured data and compared to similar research to evaluate lacustrine, aeolian or fluvio-deltaic sedimentation.

A change in sediment composition is observed in shifting trends of bulk density and magnetic susceptibility. A change in sediment source is observed as variation in sand and clay content, indicating a shift from the distal Angola highlands to the more local Kalahari sands. Standard deviation values range from 1.8-2.5 φ, and thus indicate poor to very poor grain sorting. Weight percent of organic carbon remains below 1% for the depth profile, except for anomalous spikes at 620 cm, 520 cm, 500cm, 400 cm, and 300 cm. Above 240 cm, the amount of inorganic carbon decreases steadily and below 240 cm, the amount varies widely from 3% to 0.2%. δ¹³C increases from 580-310 cm, and sharply decreases from 310-110 cm. δ¹⁸O values markedly increase above 210 cm as a result of increased aridity. Below 240 cm, the trends of δ¹³C and δ¹⁸O are similar, and thus correspond to a closed basin system. Evidence of evaporite deposits, and lake level shallowing, are observed in the trends of Ca, K, and Mg, and correlation with measured TIC.

A combined analysis of the above proxies suggests the presence of lacustrine sedimentation below 400 cm, and reinforces previously established topographic evidence for the existence of Paleolake Deception. These results highlight the need for multi-proxy geological data in order to interpret the paleoenvironmental history of a region.