Units of axes are meters depth in site section. Draw line of correlation so that most data points fall on or near. Least squares principle. Breaks in line of correlation indicate unconformities. Limestone beds within clastic systems.
Black shales indicate anoxia.
Sea level begins to fall everywhere at the same time. Regardless of lithology, the facies sequence will reverse at the same time everywhere. In turn causes changes in climate, which in turn causes periodic changes in lithology. Tell time by counting cycles. Much shorter period than orbital changes.