Marine Transgression: Understanding the Change in Depositional Environment

What was the most likely change in depositional environment that occurred from the Tapeats Sandstone to the Bright Angel Shale and the Muav Limestone?

Sandstone is overlain by shales and limestones, a transgression

Answer:

The transition from Tapeats Sandstone to Bright Angel Shale and then to Muav Limestone indicates a marine transgression. This means there was a shift from a terrestrial to a marine environment, signifying a rise in sea levels which moved the shoreline inland.

The most likely change in the depositional environment that occurred from the Tapeats Sandstone to the Bright Angel Shale and the Muav Limestone was a marine transgression. This conclusion is drawn from the types of rock and their formations in the sequence. Specifically, sandstone, which is a type of sedimentary rock formed in a terrestrial environment, is overlain by shale and limestone. Shale typically forms from silt and clay in calm water environments such as lagoons, deltas, or deep sea where there is slow and gradual sediment deposition. Limestone usually forms in warm, shallow marine environments. Thus, the sequence from sandstone to shale and limestone indicates a transgression, suggesting the sea level rose over time, moving the shoreline farther inland, resulting in a transition from terrestrial to marine environments.