FRESHWATER LIMESTONE IN AN ARID RIFT BASIN, A GOLDILOCKS EFFECT

The Olduvai Basin (3ºS), situated just west of the East African Rift System in northern Tanzania, was infilled during the last 2 million years with lavas, air fall tuffs, fluvial-lacustrine sediments and localized carbonates. Milankovitch (precession) cycles are archived in the sediments. Olduvai is west of the 2500 m high Ngorongoro Volcanic Highland, which acts as a rain shadow, trapping moisture-laden westerly winds. Rainfall (1200-2000 mm/yr) on the Highland moves as groundwater into the basin and exits at the base of the slope and at faults. Annual precipitation in the basin ranges from 250-700 mm/yr and estimated evapotranspiration is ~2500 mm/yr indicating a negative hydrologic balance. Despite persistent aridity, extensive deposits of freshwater limestone punctuate the stratigraphic record. Three limestones (1.84, 1.80, and 1.36 Ma) were studied to determine their origin using field relations, sedimentary structures, fossils, petrography, stable isotope and geochemistry. Our analysis indicates that the freshwater carbonates form periodically when the paleoenvironmental and paleoclimatic conditions are just right, i.e. a “Goldilocks effect”. The conditions include a hydrogeologic setting with groundwater flow greater than evaporative loss under the prevailing climatic conditions. The three limestones formed under a range of climatic conditions (dry, intermediate and wet) and in two different hydrogeologic settings: (1) groundwater discharging from faults or (2) slope margin seepage. Limestones deposited during drier times of the precession cycle appear to be limited to groundwater discharge near faults. Only carbonates formed during wetter times precipitated from groundwater slope seepage. Plant-rich wetlands developed during intermediate to wet portions of the precession climate cycle. Stable isotope signatures indicate the likely processes involved in carbonate precipitation. In fault-controlled limestones, δ¹³C covaries strongly with δ¹⁸O, suggesting an evaporation-dominated system. Where wetlands are present, d¹³C may be buffered by biological activity while d¹⁸O reflects evaporative processes. In both contexts, the continuous flow of groundwater with surface evaporation and CO₂ degassing optimizes the conditions for limestone formation in this arid environment.