21. The following stratigraphic column represents a sequence of rocks deposited

Question

21. The following stratigraphic column represents a sequence of rocks deposited during a change in depositional environments. Interpret the processes that deposited the sediment First, list the sedimentary structures; then interpret the flow that produced each one Then give an environmental interpretations for each facies separated by a dashed line. (5 points) 200 PARALLEL-LAMINATED AND BURROWED SANDY SILTSTONE FACIES I60 120 UPPER HUMMOCKY-BEDDED AND BURROWED FACIES 1-1 Burrows LOWER HUMMOCKY-BEDDED FACIES E Planar Lam 40 Hummocky x-strat Dune x-strat (shows- direction of flow) CONGLOMERATIC FACIES

Explanation / Answer

The sedimentary structures observed in the stratigraphic sequences are discussed in brief –

In this section, four different facies can be recognized –

This section can represent a shallow marine setup, where various environmental processes may be at work. Sedimentation started with the deposition of conglomerate, which can be found in various setups from glacial to marine. So the texture, including the grain size, sorting must be studied very well to identify the source of the conglomerate.

Lower and upper hummocky bedded facies are typical of shallow marine setup, a storm event may have resulted into the formation of such structures. Shallow marine settings are a great place for variable organisms. Presences of burrowing organisms support the availability of resources, good living conditions all of which are met by shallow marine settings.

Lastly sandy siltstone facies can be seen, such intercalation of facies are typical of tidal environments, so it can be concluded that the sea may have transgressed.

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