Lacustrine facies in response to millennial-century-scale climate changes (Lake Hayk, Northern Ethiopia)

The different response of depositional systems to changes in accommodation causes lacustrine shorelines to develop a marked along-strike variability, which is well known in modern lakes but poorly documented in the fossil record. The late Holocene shoreline deposits of Lake Hayk (Southern Wollo, Ethiopia) provide an outstanding example of this variability. The succession is time-calibrated through numerous radiocarbon dates and provides a detailed record of climate-driven, lacustrine oscillations that occurred during the past 3500 yr. These oscillations are also recorded in coeval East Africa lakes and promoted three main highstand phases, which occurred at about 3250-3000, 2600-950, and 650-160 Cal yr BP. Oscillations in lake levels are documented in fluvio-deltaic systems and colluvial fan deltas with stromatolite biostromes along low- and high-relief lake margins, respectively. Along low-relief margins, most rises in lake level were recorded by aggradation of fluvio-deltaic or marsh successions, which range in thickness from 2 to 6 m. Along the high-relief margins the same lake-level rises are documented by 0.2 to 4.5 cm thick, wave-winnowed, sandy lag deposits. Development soils, which stabilized the steep slopes and prevented their erosion, allowed the occurrence of nondepositional transgressions. Isotope data show that stromatolite biostromes formed during phases of intense evaporation and lacustrine contraction, which are also recorded by fluvial erosion in adjacent areas. Changes in accommodation can be differently documented in nearshore lacustrine successions as a consequence of a marked along-strike variability of shoreline depositional dynamics.