A geologist is working in an area with exposed sedimentary rocks and finds a well exposed sequence of stratified rocks.
As he works through a thick sequence of limestone, he sees abundant large clam fossils that produce an interlocking texture suggesting a reef, but within the limestone section these clams disappear and the overlying limestone is made up primarily of fragments of corals. He now moves 50 km away and finds limestones interbedded with shales, and the limestones contain the same clam fossils he had seen previously 50 km away. As he continues through this section, the rocks become entirely limestone, and the clams disappear again with coral fossils above forming a reef structure. He concludes ________.
A) clams have nothing to do with reefs, so he needs to go back to the first section to determine what he missed
B) this is impossible; the rocks are different at the same place the fossils change
C) sea level must have risen abruptly in a great catastrophe to produce the coral fragments scattered over 50 km
D) the horizon where the clams disappear occurred at the same time in the two sections when the clams went extinct
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