Exercise Depositional Environments 53 PART D Mancos Shale, Cretaceous, Road Cuts

Question

Exercise Depositional Environments 53 PART D Mancos Shale, Cretaceous, Road Cuts along I-70 in Central Utah (Figure 4.10) 1. What is the dominant sedimentary rock type shown in figure 4.10? 2. What does the black color suggest about the en- vironment of deposition? What additional infor- mation is provided by the types of fossils found in the Mancos Shale? 3. Rocks of the Mancos Shale were most likely de- posited in which of the sedimentary environ- ments listed in table 4.1? Explain your answer.

Explanation / Answer

1. The dominant rock type in the figure is Shale.

2. The dark colour in any sedimentary rock mostly indicates the presence of organic matter. An organic content of even 1-2% can impart a dark or black colour to the formation. This also points toward an oxygen-deprived environment during the time of sediment deposition, as any oxygen would lead to the decay of the organic matter and not leave behind any organic content or the black colour. Thus, the Black colour of the formation is majorly due to the presence of Organic content.

Moreover, the fossil content of the shales also indicates a submerged condition with Bivalves which are majorly marine organisms, with a few bottom dwellers indicating an offshore condition of deposition. Ichthyosaurus and Mosasaurs are species of the large marine reptile group. Finally, the presence of Cephalopods, which are exclusively marine species confirms the offshore marine environment of deposition leading to an oxygen-deprived environment of deposition.

3. The rocks of Marcos shale are reddish to black in colour indicating a variable oxygen level, fine-grained well sorted siltstone and bentonite rich mudstones texture, trace fossils of species moving on the seabed, indicate calm offshore conditions, along with the presence of Cephalopods, With thin beds of Siltstones and a graded bedding of shales, which are black in colour. All these indicate a Deep marine environment of deposition.

4. Ooids are generally formed by chemical precipitation of the dissolved calcite on a seed like a small sediment or a piece of a shell. These seeds are washed across and accumulated at the seashore where it forms ooids. These Ooids are common in warm, supersaturated, shallow and highly agitated waters.

Cross beddings are generally formed by the migration of the bedforms such as Ripples or Dunes. Moreover, the presence of Crinoids a marine animal normally associated with the Sun, indicate a shallow marine deposition environment, with frequent floods or highly agitated waters at the seashore leading to the formation of Ooids. Thus the Jurassic Carmel formation was formed in Carbonate Sand Shoal conditions.

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