5.2 Lacustrine environments

Hydrological regimes of lakes

A hydrologically open lake basin Supplied from a river (foreground) with outflow to the sea in the distance

Lacustrine environments Clastic lakes have coarser deposits in shallower water and finer deposits in the deeper lake

Lacustrine (lake) environments Lake delta and shoreline deposits

Thermal stratification in a freshwater lake

A sedimentary log of clastic freshwater lake deposits Shallow lake deposits. Beach and/or lake delta sands Shallow lake deposits. Muds and wave- rippled sands Deeper lake deposits. Laminated dark shales and thin turbiditic sands and silts. 10s metres

Thin-bedded clastic freshwater lake deposits

Thin-bedded clastic freshwater lake deposits, including wave ripple lamination

Fine-grained freshwater lake deposits

Lacustrine environments – distinctive features Lake level of hydrologically closed lakes can vary with climate – lake margin can dry out

Lacustrine environments – distinctive features Lake level of hydrologically closed lakes can vary with climate – lake margin can dry out

Sedimentary structures formed by desiccation Mudcracks form by mud drying out: desiccation cracks

Sedimentary structures formed by desiccation Mudcracks preserved in ancient dry lake deposits

Erosion of desiccated mudrocks produces clasts of mud: mudclasts Sedimentary structures formed by desiccation

Lake sedimentation patterns Lakes do not have strong currents – water mass is stable Epiliminion – warm, buoyant, oygenated water Hypolimnion – cold, denser water, anoxic conditions - separated by a thermocline

Lake sedimentation patterns Stratification of the lake waters results in anoxia of the deep lake floor Organic-rich deposits preserved, no lake-floor fauna