The environments of particular deposits
Because the top layer is marine, you're being asked to think of the sequence being "coastal" in origin, where there is ocean, coastal plain, and inland mountains. In this scenario, two things are happening in tandem: the mountains are eroding and sea level is rising.

As the mountain erodes, boulders and cobbles (eroded from steep slopes) bury trees and form coarse conglomerates and source material for the coastal plain deposits, which are the middle unit of your sequence. As sea level rises due to local subsidence or a global sea level rise, the marine environment marches steadily inland depositing sands and then limestones.

Assuming no unconformities, yours is a textbook example of a continuous depositional sequence!