Sedimentary geology is most interesting when you examine a whole series of rocks

Question

Sedimentary geology is most interesting when you examine a whole series of rocks deposited in layers one on top of the other because, like pages in a book, they tell you a story. On a hike up a long hill you will find the following succession of horizontal rock layers:

(1) At the bottom are gray shales containing fossils and a thin layer of coal.

(2) Then comes a transition of a quartz sandstone displaying abundant cross-beds and even some ripple marks. The grains are medium-sized at the base but become fine near the top.

(3) This is followed by a transition to more shales, but no plant fossils are visible and there is no coal. Instead, there are ancient seashells of various types

(4) Finally, at the top of the hill come various layers of gray and light tan limestones, some of which display various marine fossils, including abundant corals.

How did the environments of deposition change at this locality?

What one thing could have occurred to cause these changes?

(please answer)

Explanation / Answer

Answer- the sequence of above of question is given below

shale with fossil and little coal layer at the bottom followed by transition of quartz sand that display cross bedding and ripple marks. and grain size is from medium grained to fine grained. followed by more transition of more shale with no fossil and no coal and presence various seashell and corals. and at the top there is presence of grey and light tan limestone with presence of marine of fossil

by the above interpretation represent the transition zone (land-water) or terrestrial - marine environment. it charcterized by the action of river water as well as by the action of ocean water. In this environment regression is followed by transgression movement. At the bottom layer sediment input from land region so there is presence of coal layer. it was followed by transgression movement takes place, at sea water moves towards land side and deposit it sediment (limestone, shale) which are of marine origin and also there is presence of marine fossils and sea shell. that shows the movement of seawater towards land.

when the regression process takes place then sediment brought by river deposited means sediment are of fluvial deposit. when river energy is low then it was followed by the transgression movement sea water moves towards land region and it cause deposition of the shelf sediment with marine fossil and seashell.

Transgression movement cause the migration marine fossil and sea shell along with shale and limestone deposit on land.

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