An Overview of Miocene Reefs from Mediterranean Areas: General Trends and Facies Models
Mateu Esteban, 1996. "An Overview of Miocene Reefs from Mediterranean Areas: General Trends and Facies Models", Models for Carbonate Stratigraphy from Miocene Reef Complexes of Mediterranean Regions, Evan K. Franseen, Mateu Esteban, William C. Ward, Jean-Marie Rouchy
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Abstract: Miocene carbonates in the Mediterranean are dominated by organic buildups of rhodalgal and coral-reef facies with local stromatolitic mounds, ahermatypic coral mounds and oyster banks and occur in a wide variety of tectonic settings and substrates. Regional chronostratigraphic correlation is in a state of flux, but it appears that coral reef development was extensive during the climatic optimum of the Chattian-Aquitanian, Langhian and Late Tortonian-Messinian times corresponding to global 2nd-order highstands or supercycles of relative sea level. The coral reef provinces of the Mediterranean reflect the transition between Early Miocene open-oceanic, humid-tropical conditions and Late Miocene landlocked, semi-arid and marginally subtropical environments. This is interpreted as a reflection of the global cooling trend and the northward displacement of the European plate, but also the increasing involvement of the rising Alpine foldbelts in controlling the climatic trends. The evolution of the connecting seaways with the Atlantic and Indo-Pacific domains are critical in the development of the Mediterranean Miocene carbonates.
Most facies models in the region are based on superb outcrops of Upper Miocene carbonate complexes, which are largely applicable to lesser known Lower and Middle Miocene carbonates. Narrow platforms with fringing reefs are predominant; lagoonal facies are poorly developed and commonly with variable amounts of terrigenous mixing. Extensive carbonate platforms with barrier reefs and lagoons occur in Oligocene-Lower Miocene carbonates but are very scarce or ephemeral in Upper Miocene platforms. The best outcrops show excellent preservation of depositional morphologies (platform slopes, reef buttresses and spur-and-grooves, reefal patches, skeletal sand bodies and lobes) and allow detailed paleogeographic reconstructions. Depositional sequences of different orders of magnitude display a basic stacking pattern consisting of vertical aggradation, progradation and offlaping (downstepping); faithfully reflecting inferred relative sea-level oscillations. Coral diversity decreased from Early to Late Miocene times; part of the Messinian coral reefs are essentially monogeneric (Porites, one of the main reef builders in all Miocene times). There are also Messinian reefs with 3-5 coral species; variations in diversity reflect local conditions rather than age or basin-wide events. The largest Upper Miocene reef complexes tend to be monogeneric and show good vertical zonation in colonial morphologies. However, these vertical zonations cannot be generalized for an entire basin or even a single reef complex.
Upper Miocene coral reefs developed before, during and after the repeated deposition of basinal evaporile units and marine marls, resulting in complex wedge-on-wedge geometries of difficult correlation. Some of the Messinian coral reefs in the western Mediterranean exhibit peculiar features: exuberant monogeneric coral branches coated by cyanobacterial crusts, locally associated with giant stromatolitic domes, algal blooms and diatomitic marls. The peculiar look of some Messinian reefs is interpreted to be a result of the influx of cold, nutrient-rich Atlantic waters and their interaction with dense, warmer Mediterranean waters with a tendency to eutrophic, stressed marine conditions. These features are considered part of the scenario referred to as the Messinian crises, leading to major salinity variations and evaporile deposition in the basin.
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Miocene carbonates are intensively explored and locally exploited for hydrocarbons in parts of the Mediterranean regions. The outcrop models presented in this publication provide excellent analogs for the highly productive Miocene carbonates from Iran, Iraq and Gulf of Suez and for smaller reservoirs in other localities. Lessons learned in the outcrops of the Mediterranean regions are applicable as well to Miocene carbonate reservoirs. The Miocene outcrops in Mediterranean regions can serve as models for the relationships between carbonate reservoirs, pre-evaporitic basinal sediments, and overlying evaporites. Additionally, the Miocene carbonate rocks exposed in the Mediterranean regions serve as important analogs for ancient carbonate-rimmed basins with or without basinal evaporites.