Formation A: Chalk with some beds of limestone. Trace fossils include shallow bu
ID: 292055 • Letter: F
Question
Formation A: Chalk with some beds of limestone. Trace fossils include shallow burrows and surface feeding traces. Body fossils include a variety of marine organisms such as calcareous plankton, fish, snails, oysters, and marine reptiles.
Formation B: Laminated shale. Trace fossils include shallow burrows and surface feeding traces. Body fossils include a variety of marine organisms such as calcareous plankton, fish, snails, oysters, and marine reptiles.
Formation C: A coarsening upward sequence. The lower part of the sequence is laminated claystone with horizontal burrows and surface feeding traces. Above this is silty claystone with more vertically oriented burrows. At the top is a unit of sandstone, mudstone, and coal, which contains abundant plant fossils (mostly angiosperms and ferns), and the bones of dinosaurs, fish, turtles, and large crocodiles.
Formation D: This formation is dominated by mudstone containing abundant root traces, but there are also blanket sandstone beds that show a distinct fining upward sequence. The sequence in these blanket sandstones goes from trough cross bedded conglomerate at the base to sandstone with lateral accretion beds, to sandstone with ripples, to the mudstones that make up most of the formation. In section A, trough cross beds at the base of the sandstones indicate paleocurrents flowing mostly from northwest to southeast; in section B, trough cross beds indicate flow from west to east. Fossils include gymnosperm and angiosperm logs, cycad and fern leaves, and dinosaur bones.
Formation E: This formation is dominated by poorly sorted sandstone and conglomerate, with a few thin beds of mudstone. The unit contains abundant trough cross beds, planar cross beds, and horizontal planar beds. Trough cross beds in section A indicate paleocurrents flowing mostly northwest to southeast. Fossils in the Carter Formation include gymnosperm logs, cycad leaves, and dinosaur bones.
What is the deopositional system for each formation and how did they change overtime?
Explanation / Answer
A. The depositional environment of this formation must have been deep marine so that the chalk and limestones got deposited. Later the basin would have becoming shallower as the fossil assemblage suggests.
B. Shales are also deposited in deep marine condition. And the fossil assemblage suggests that the basin may have become shallower later on.
C. The coarsening upward sequence suggest marine regression condition. The basin earlier would have been deeper and later it became shallower and further the sea completely regretted and the environment became arid/semi arid which is suggested by the horizontal burrows and dinosaurs.
D. The mudstone suggests deeper condition. The root traces suggest it might be a river basin or ocean basin. The overlying sandstone which is dining upward suggest bouma sequence which is formed near the base of continental slope. Ripples and cross beddings suggest shallowing of the basin hence the ocean began regressing. Angiosperm suggest terrestrial environment, cycad and fern suggest tropical to subtropical climate.
E. Poorly sorted conglomerate and sandstone suggests high energy condition. The abundant cross bedds paleocurrent indicates shallow water condition specially fluvial. Cycad leaves suggest tropical to subtropical climate and dinosaurs indicate arid/semi-arid environment.
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