Carbonate Environments Warm-Up

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Essay Question

Deposition of carbonate sediment is very sensitive to water level and hence exposure index, which can crudely be approximated by supratidal, intertidal, and subtidal settings. Of these three settings, where is most carbonate sediment generated, and by what sort(s) of process(es)?

Once the sediment is generated, it must then be deposited, often after some sort of redistribution by currents. Consider a carbonate shelf or bank in an area of mesotidal range, where shoaling is present near the margin facing the open sea. Describe the environments, nature and energy of currents, and ultimate distribution of carbonate sediment types, from the seaward margin of the bank toward the shoreline, including areas above the shoreline where occasional flooding by storm tides may take place.

How do the cement fabrics produced in the subtidal portion of this spectrum of environments differ from those produced in areas where cementation is taking place above the water table, under conditions of falling sea level?

Where in this area, if at all, is dolomite likely to form?

Multiple Choice Questions

Which of the following show(s) faulty geological reasoning?

formation of dolomite is a replacement process

formation of microspar is a recrystallization process

meniscus cement tends to form in subtidal settings

dissolution hardly ever occurs in the meteoric vadose setting

hi-magnesium calcite cement tends to be elongate in crystal form and is marine

dolomite can either form in settings where mixing of fresh and marine water is occurring, or in settings where hypersaline brines are created, often through evaporitive concentration

In a carbonate depositional environment,

the greatest complexity of facies may occur in the channeled belt of a peritidal setting

the coarsest-grained sediments are likely to accumulate near the edge of the bank, in tidal channels, or in storm deposits of the open shelf

there are few parallels between modern and ancient settings

reefs generally form in areas of high current or wave activity

shallowing-upward sequences are most often generated, as opposed to deepening-upward sequences

autocyclic processes may be as important as eustatic sea-level changes in controlling the position of the water-sediment interface