What causes the light and dark layers in some oil shales?

The alternating light and dark layers found in some oil shales, often referred to as varves, are primarily attributed to annual cycles of organic matter deposition and accumulation.1

These laminar structures formed in the quiet waters of lakes or shallow seas. The layering resulted from the periodic settling of different materials:
* Dark layers: These are typically associated with higher concentrations of organic matter.21
* Light layers: These formed when carbonates precipitated from the water or when clay minerals and other silicate minerals were transported into the basin as extremely fine detritus.1

In general, the color of shale is determined by its composition; a higher organic content typically results in a darker color, while calcareous shales—those containing a large percentage of calcite—tend to be light gray or yellowish.2