Upper Miocene Reef Complex of the Llucmajor Area, Mallorca, Spain
Luis Pomar, William C. Ward, Darryl G. Green, 1996. "Upper Miocene Reef Complex of the Llucmajor Area, Mallorca, Spain", 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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On all the Balearic islands (Mallorca, Menorca, Ibiza, and Formentera), the Upper Tortonian-Lower Messinian part of the post-orogenic sedimentary section is the Reef Complex. This unit is composed of carbonate rocks deposited as progradational reef-rimmed platforms with off-reef open-shelf, forereef-slope, reef-core and back-reef lagoon facies associations. The most extensive progradation of a Late Miocene carbonate platform in the western Mediterranean occurred on the Llucmajor Platform in the area of present-day southwestern Mallorca. Here as much as 20 km of basinward progradation took place during Late Tortonian and Early Messinian time. Core-hole data from the Llucmajor area show that the Reef Complex limestone and dolomite are up to 100 m thick and are spread over a platform of about 15 km by 20 km. The southern and western portions of the Llucmajor Platform coral-reef complex are superbly exposed in high vertical sea cliffs.
The lithofacies units of the Reef Complex are defined on the basis of their lithology, constituents, stratification and geometric relationships. There are two main types of open-shelf lithofacies: (1) dolomitized grainstone-packstone with abundant red algae and (2) packstone-wackestone with planktonic foraminifers. Interfingering landward with the finer-grained open-shelf lithofacies are dolomitized skeletal grainstone, packstone, and wackestone of the reef-slope deposits. These slope deposits are characterized by basinward-dipping clinoforms of variable thickness and lateral extent, depending on configuration of the forereef platform. The slope rocks interfinger landward with massive coral-reef limestone and dolostone. The reef rocks interfinger landward with flat-lying lagoon lithofacies composed of partly dolomitized packstone, wackestone, and grainstone.
The reef framework on the Llucmajor Platform is constructed mainly of only one to two genera of corals, Porites or Porites and Tarbellastraea. Along the southern part of the platform, depositional strike of the reef tracts was N35°W to N60°W, and progradation was toward the southwest. From the perspective of sea-cliff outcrops in this area, there are three main types of reef tracts: (1) discontinuous, mound-like Porites and Tarbellastraea reefs cropping out in the Vallgornera area, (2) more continuous Porites and Tarbellastraea reefs cropping out from Cala Beltrán to Els Bancals and (3) Porites reef tracts cropping out from Els Bancals to Cap Blanc. The youngest reefs of the Llucmajor Platform, which crop out along the western coast, are constructed of Porites and Tarbellastraea, and core data show that most reefs throughout the platform contain both these genera. The most complete sequence of the Porites-framework reef crops out on the high Cap Blanc sea cliff, where the reef is part of an aggradational sequence. Here there are three zones of coral morphology: (1) a lower zone of "dish coral", (2) a middle zone of "branching coral" and (3) an upper zone of "massive coral".
The Reef Complex has a complicated stratigraphy of accretional units, reflecting several orders (probably 4th through 7th) of high-frequency oscillations in relative sea level. These fluctuations in sea level produced the most characteristic facies relationship within the Reef Complex: progradation with vertical shifts (upward and downward) of the reef-core and associated lithofacies. The complex architecture of the Llucmajor carbonate complex can only be adequately defined from the reef-core facies stacking patterns in the dip direction. Changes in stacking patterns allow definition of four systems tracts: "low stillstand," "aggradational," "high stillstand" and "offlapping." These systems tracts are identified in the accretional units of every scale, except the basic accretional unit, the "sigmoid." The fore-reef slope and off-reef open-shelf facies are mainly built up by aggradational systems tracts separated by condensed intervals of fine-grained distal slope and open-platform carbonates. These distally condensed intervals correlate landward with the high-stillstand, offlapping, and low-stillstand systems tracts. Progradation of the reef systems on the southern part of the platform was more extensive during sea-level falls on a gentle depositional profile. The subsequent sea-level rises created wide lagoons, which apparently enhanced carbonate production and downslope shedding of sediment. On steeper topographic gradients, relatively minor reef progradation took place during sea-level falls, and only small back-reef lagoons were created during the subsequent sea-level rises. Barrier reefs with extensive lagoons and patch reefs formed during relative sea-level rises of different orders of magnitude; fringing reefs developed during sea-level falls.
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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.