Author Question: What sedimentary structure is illustrated in this photograph? What direction (toward the left or the ... (Read 43 times)

strangeaffliction · **on:** Jul 16, 2018

What sedimentary structure is illustrated in this photograph? What direction (toward the left or the right) were sediment-transporting currents flowing when the sediment was deposited?

What will be an ideal response?

Question 2

Think about a modern coastal lagoon with mudflats that are alternately submerged and exposed each day by rising and falling tides.

With this environment and tidal processes in mind, what clues would you look for to identify the deposits of an ancient lagoon in the geologic record? How would you distinguish an ancient coastal lagoon mudstone from a floodplain deposit, where mud collects during river floods?
What will be an ideal response?

Tabitha_2016 · **Reply #1 on:** Jul 16, 2018

Answer to Question 1

Cross-bedding is illustrated and the sediment-transporting currents appear to be moving from right to left.

Answer to Question 2

Mud and organic-rich sediment are common to both coastal lagoons and floodplains. Both environments are alternately wet and dry, so the rocks may contain mud cracks. However, lagoons may contain marine fossils while floodplain fossils would be terrestrial plant and animals. If there is cross-bedding, it would show transport in two different directions in the coastal lagoon, associated with the incoming and outgoing tide.