The change of environment from the base of the section up to the thick gray shale indicates .

The change of environment from the base of the section up to the thick gray shale indicates transgression of the sea.

Explanation:

Marine transgression occurs when the sea level rises in relation to the ground, which can cause floods of different intensities. We can say that marine transgression is the advance of sea water on land, when the sea level rises to the point that it cannot be contained. When this happens, a change of environment occurs from the base of the section to the thick gray schist.