Mixed clastic-carbonate lake margin systems: An example from the Triassic of East Greenland

Lake margin deposits are the subject of increased study, but this is often focussed on either clastic or carbonate/microbial dominated end members. This study examines the interaction of clastic and carbonate systems. The Upper Triassic Edderfugledal Formation in East Greenland provides superb exposures through a carbonate dominated lacustrine succession. Fluctuations in lake level, interpreted as a response to cyclic, orbitally forced, climatic variance resulted in a highly mobile lake shore zone. The response of the shore zone environment to these fluctuations in lake level, and the interaction of both clastic and carbonate components, are documented in this study. A general trend from more arid to more humid conditions is recognised through the Edderfugledal Formation. This trend is reflected in a transition from more ephemeral lacustrine conditions with low sediment input to conditions where lacustrine episodes were more prolonged and clastic input was increased. Deposits reflecting more ephemeral conditions are dominated by extensive post-depositional disruption including desiccation, pedogenic processes and evaporite precipitation. These effects increase towards the lake margins where exposure was most common and most prolonged. Increasingly humid conditions and the associated longer-lived lacustrine developments and increased clastic sediment input resulted in a very different form of lake margin. During transgressive phases sediment input was pushed back to the lake margin allowing extensive microbialite development. Ooidal shoals developed in shallow water beyond the extent of clastic input. The lakeward migration of the ooidal shoals and the progradation of clastic systems eventually stifled the microbialites prior to the next transgressive event. In a mixed clastic-carbonate lacustrine setting the interaction of sediment supply and production are key factors in governing facies development and these are in turn predominantly controlled by lake-level change and lake margin bathymetry.