--> ABSTRACT: Miocene Reefs in Western Mediterranean, by Mateu Esteban; #91030 (2010)

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Miocene Reefs in Western Mediterranean

Mateu Esteban

Coral reefs were particularly abundant and well developed during the late Tortonian and Messinian in southeastern Spain, the Balearic Islands, Italy, Sicily, Algeria, and Morocco. These reefs occurred just before and during the deposition of the thick Messinian evaporite units in the basinal areas and disappeared completely from the Mediterranean during the early Pliocene. Most of the coral reefs are fringing terrigenous coastal fan complexes with very small lagoons and show progradation of less than 2 km. Some of the reefs occur on, or are intercalated with, Neogene volcanics or Messinian evaporites. Barrier-reef complexes are less common, have extensive lagoons behind them, and show complex progradational geometries more than 10 km wide. Excellent outcrops allow detaile reconstruction of paleogeography and sea level changes. Progradation predominated during phases of relative sea level drops and stillstands, while significant retrogradation occurred during sea level rises.

The coral reef wall framework is commonly less than 20 m thick and is dominated by Porites and, locally, Tarbellastraea. Older Miocene reefs are less well developed, but show greater diversity of corals and reef organisms. Younger Miocene reef complexes occurring in open ocean settings are formed by only one branching coral genus (Porites or, locally, Tarbellastraea) with branching colonies up to 7 m high. Halimeda sands are particularly abundant in the upper reef slopes with occasional intercalations of red algae pavements that most likely coincide with episodes of terrigenous influx.

Messinian events led to drastic variations in sea level, water chemistry, and nutrients and coincided with high sedimentation rates in coastal areas. To survive these adverse conditions, coral reefs grew rapidly by developing successful colonial morphology and intense carbonate cementation. Some Messinian coral reefs present marked cyclicity, with repeated intercalations of brackish and hypersaline carbonates and large stromatolites. The northern Betic strait (southern Spain) and the southern Riffean strait (northern Morocco) were the restricted connections with the Atlantic Ocean. These seaways present significant differences in reef development, associated facies, and diagenesis. The amount and patterns of continental freshwater influx into the Mediterranean basins and their mixing ith the Atlantic influxes was probably one of the most important sets of interrelated factors controlling reef development and diagenesis during the late Miocene.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.