Mesozoic Carbonate Buildups
The opening of the proto-North Atlantic in the early Jurassic facilitated the western extension of Tethyan marine facies across and around the Moroccan Meseta. Paleolatitudes favored the establishment of a carbonate-evaporite facies assemblage broadly comparable to that found around the southern margin of the present-day Arabian Peninsula.
Continential facies, including alluvial fan and channel systems, occasionally preserved aeolian facies and intermittently developed playa evaporites, are best developed within the Dogger. Coastal sabkha facies and attendant dofomitized intervals are commonly represented throughout the Jurassic, especially along the Atlantic Coast of the Meseta. Further seaward these interfinger with an intertidal and lagoonal complex protected from the open marine realm by intermittently developed oolite shoals and coral-algal reefs. Open marine facies are poorly represented onshore and in wells.
Reefcrl buildups are best developed in the Lias on the Tethyan flank of the Meseta and are especially significant in that they comprise some of the earliest recorded scleractinian reef builders. The buildups are up to 3 km (1.9 mi) long, .25 to I km (.2 to .61 mi) wide and up to 150 m (492 ft) high, comprising a variety of corals, both massive and branching, various algae, and bound largely by bryozoa, sponges and encrusting coralline algae. A host of foraminifera, molluscs, brachio-pods and numerous reefal borers are also present. Comparisons with modern reefs are understandably inviting. Elsewhere, the corals occur as extensive low thickets of predominantly branching and stick-like forms with only occasional poorly-developed patch reef buildups. Bivalves, either alone or in association with corals and stromatoporoids, also form mounds and buildups, especially in the more restricted inlets.
Reef slope and associated basinal facies are commonly silicified while the reefs themselves are locally prone to massive dolomitization. The resultant increase in propensity to fracture and attendant porosity enhancement within the reefs, coupled with a potential indigenous hydrocarbon source in the adjacent lagoonal and intertidal facies, has resulted in their becoming a prime target of petroleum exploration.
Figures & Tables
Carbonate buildups have long been a focus of intense geological study. An underlying reason is the importance of carbonate buildups as significant hydrocarbon reservoirs. This core workshop is intended to provide a “hands on” look at the subsurface geologic record created by carbonate buildups with emphasis on lithofacies, stratigraphy of buildups and their surrounding deposits, geometry, “reef”-building and sediment-producing organisms, and diagenesis and porosity evolution