An Ancient Mega-Lake and Mega-Delta in Southern Africa:Hydrogeophysical Investigations Across the Okavango Delta

摘要

The near juxtaposition of the world's largest saltpan complex within the Makgadikgadi Basin and the huge wetlands of the inland Okavango Delta has fascinated explorers and scientists for more than a century.Early observations suggested that the Makgadikgadi Basin once contained a huge lake,popularly referred to as Paleo Lake Makgadikgadi,and several authors have speculated that this lake may have extended into the area currently occupied by the Okavango Delta.Through a commercial contract,the Botswana Department of Geological Survey has acquired a helicopter time-domain electromagnetic (HTEM) data set across the entire Okavango Delta.The HTEM data are of extremely high quality.Our inversions of these data return electrical resistivity models containing three principal layers: (i) an upper heterogeneous layer that is largely resistive,(ii) an intermediate-depth conductive layer,and (iii) a lower resistive layer.The upper heterogeneous layer corresponds to the waters and sediments of the active Okavango Delta.According to borehole logs and groundwater sampling,the electrically conductive layer comprises a combination of clay and saline-water-saturated sand.We interpret these sediments to have belonged to a northwestern extension of Paleo Lake Makgadikgadi,such that the total area of the paleo lake would have exceeded that of Earth's most extensive fresh-water body today (i.e.,Lake Superior).Information from high-resolution seismic refraction/reflection experiments and boreholes suggest that the lower resistive layer is composed of fresh-water sediments,probably representing a paleo Okavango delta,lying on top of basement rock.If this interpretation is correct,then the clay layers of the Paleo Lake Makgadikgadi sediments must be acting as an aquitard protecting the fresh-water sediments of the paleo delta below from salt-water contamination above.