There are big differences in North American depositional environments between th
ID: 288281 • Letter: T
Question
There are big differences in North American depositional environments between the Mississippian and Pennsylvanian periods. While non-US geologists consolidate these periods into the “Carboniferous,” US geologists felt a division was needed because of very different rock types and deposition within the Mississippian and Pennsylvanian. Can you hypothesize the different types of organisms that proliferated during these periods, and whether they contributed to the types of rocks that dominate each period?
Explanation / Answer
The division of the Carboniferous into the Mississippian and Pennsylvanian period in the US is done based on the difference of the stratigraphy. the Mississippian period saw the deposition of carbonates while the Pennsylvanian saw the deposition of the coal-bearing formations.
The Mississippian rocks were deposited when there was a shallow sea in the region. A large amount of marine calcareous organisms lived in the oceans which when died sunk to the ocean floor and consolidated to form the Mississippian rocks.The marine organisms primarily included crinoids and lime-encrusted green algae. In the shallow sea, there was also the deposition of high energy oolites. This high amount of carbonate supply primarily from the crinoids and other shallow marine organisms lead to the deposition of the shallow marine Mississippian limestones.
The end of the Mississippian and the start of Pennsylvania is marked by upliftment of the region. This upliftment meant the replacement of the shallow seas by a number of floodplains and deltas. This time of environment did not support the marine organisms and there were only a very few left. Rather, now the more terrestrial environment supported plants like the lycopsids, fern, sphenopsids, etc. These plants died and were deposited in the swamps of the deltaic environments. It was the anaerobic decomposition of these carbon-rich lives that lead to the developed of the coal seams of the Pennsylvanian period.
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