Discussion
Features given below are diagnostic to which sedimentary environment ?
Tectonic Setting : Typically in fault grabens, downwarped basins with internal drainage or limited outflow.
Geometry : Coursing upward from laminated shales, marls, limestone to rippled and cross-bedded sandstone, sequence shows cyclic alternation of lamine.
Sedimentology : Finely lamined mudstones rich in kerogen with marl and freshwater limestone, fluvial muds and sand with cross-beds, wave ripples and climbing ripple drift, association of regular sequence of carbonate and evaporites with desicussion and mudcracks.
Fossils : Nonmarine ostracods, diatoms, molluscs and fish and insects.
Answer: C
The environment is lacustrine.
Every feature in the description points to a lake. Fault grabens and basins with internal drainage are typical lake basins. Finely laminated, kerogen-rich mudstones (oil shales) form in quiet, stratified lake water. Marl, freshwater limestone and evaporites with mudcracks record lake chemistry and drying of its margins. Nonmarine ostracods, diatoms, molluscs, fish and insects are lake fossils.
Glacial deposits are poorly sorted tills without such fine laminae. Eolian deposits are well-sorted, large-scale cross-bedded sands with few fossils. Alluvial fans are coarse, poorly sorted gravels at mountain fronts, not laminated organic mudstones.
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