Discontinuity surfaces that recorded superposition of marine hardground and subaerial exposure stages are common in the Middle Cretaceous of northern Oman. These surfaces formed during periods of rapid sea-level drop. The marine hardground stages are dominant in the field, whereas the subaerial exposure stage is documented in circumstantial petrographic, geochemical, and biological evidence. The record of a shoaling phase prior to exposure is commonly subtle and incomplete; supratidal deposits are conspicuously absent. Porosity in the limestones underlying the discontinuities is rearranged during subaerial exposure and subsequent burial and hence the permeability of large volumes of limestone is affected at a variety of scales. During marine hardground stages, carbonate cements, iron oxides, and manganese occludes some of the existing pore space. During burial, these intervals may thus have acted as either seals or efficient conduits of fluid flow. The surfaces under study in the Shu'aiba, Nahr Umr, and Natih formations are spaced ten to few tens of meters apart and many of them were traced laterally over distances of 100 kilometers and more between sections at Jebel Akhdar and in the Foothills. This implies that they play an important, but poorly understood role in compartmentalization of carbonate reservoir rocks.