CARBONATE LAKES ON BASALT FLOWS (MIOCENE KIRI KIRI FORMATION, LAKE TURKANA REGION, KENYA)

The formation of lakes directly upon basalt flows is a rare phenomenon because of the high porosity and permeability of these volcanic rocks. Accumulation of lake sediments can only occur where the regional groundwater table intersects the lava flow; in this case, carbonate sedimentation occurs because of the weathering of Ca-rich feldspars as groundwater flows through and on the basalt. A modern example in Kenya is present at the volcanic area at the southern end of Lake Turkana with a Miocene example preserved approximately 85 km to the west within the Kiri Kiri Formation. Round outcrops of limestone occur within basalt depressions in this formation over a several square kilometer area. Each circular to semi-circular depression filled with limestone ranges on a scale from one meter to tens of meters wide. The thickness of these carbonate pond and lake deposits ranges from decimeters to several meters. Four main facies are present with many vertebrate and invertebrate fossils. Vertebrate remains include a monkey skull, crocodile scutes or scales, turtle shell fragments, and related bones. Invertebrate remains include a diversity of gastropods. Carbonate facies, as determined from thin section petrography, include fossiliferous massive micrite, microbial mounds, rhizolith-rich micrite, and debris-rich limestone. Textures indicate mostly subaqueous conditions with only very rare mudcracked horizons. The high groundwater table maintained these lakes and ponds, establishing a complex ecosystem.