caprock

Caprock, or cap rock, is a relatively hard, resistant layer of rock that forms a protective covering over less resistant sediments at the top of a stratigraphic sequence. This caprock slows erosion of the underlying strata and often shapes the landscape into features such as mesas, plateaus, and escarpments. The composition of caprock varies and can include durable sandstones, limestones, dolostones, cherts, and evaporites, as well as other relatively impermeable or well-cemented rocks that are more resistant to weathering than the rocks beneath.

Role in fluids: Caprock acts as a barrier to vertical fluid movement. It can function as a seal in groundwater systems, trapping water above less permeable layers or allowing perched aquifers. In petroleum geology, a caprock seals a reservoir rock beneath and helps trap oil and gas, preventing upward migration.

Formation and occurrence: Caprock forms where a harder lithology overlies softer strata or becomes cemented and resistant to erosion. It is common in arid and semi-arid regions and on tablelands or mesa tops, such as the caprock on mesas in the Colorado Plateau. In karst regions, caprock may protect soluble rocks until breaches expose underlying cavities.

Erosion and landscape: If the caprock is breached or removed, the underlying rocks erode more rapidly, often creating cliff faces and exposing layers beneath. Thus caprock controls both hydrology and topography.