Diagenesis of the Middle–Upper Jurassic Carbonate Platform (Amran Group), Yemen
Khalid A. Al-Thour, 2000. "Diagenesis of the Middle–Upper Jurassic Carbonate Platform (Amran Group), Yemen", Middle East Models of Jurassic/Cretaceous Carbonate Systems, Abdulrahman S. Alsharhan, Robert W. Scott
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The Middle to Upper Jurassic shallow marine carbonate platform (Amran Group) is predominantly limestone to the west and northwest of Sana’a and limestone and dolomite to the east and northeast of Sana’a. Diagenesis of the Amran Group encompasses many processes with conspicuous effects, including cementation, dissolution, neomorphism, and compaction (both physical and chemical), producing secondary microporosity, micritization, and dolomitization. Dolomite cements are common and were precipitated mostly during later diagenesis in cavities and fractures. Replacive dolomitization occurred during shallow burial (small rhombic types) and during burial diagenesis with the formation of saddle dolomite. Integration of field, petrographic, and geochemical analysis (ICP) indicates that lithification of these carbonates occurred during synsedimentary and burial diagenesis, with much of the alteration controlled by eustatic sealevel change and regional tectonism.
Four major subenvironments, in which diagenesis of the Amran Group was operative, can be recognized. (1) Synsedimentary diagenesis is characterized by the formation of isopachous and syntaxiai cements, hardgrounds (with associated borings and burrows, and shelter, fenestral, framework, interparticle, and intraparticle porosity), geopetal structures, and intraclasts, indicating deposition under marine conditions. (2) Shallow burial diagenesis shows other specific features such as leaching, recrystallization, and early dolomitization (both replacive and void–filLing) and mold–filling cements. Moldic and vuggy porosity distribution, early compaction, collapse breccia, and silt deposition indicates that the Amran Group continued to receive meteoric water following sediment stabilization, enlarging some molds and vugs by solution. (3) Deep burial diagenesis is characterized by dissolution, blocky calcite cement, late compaction (fractures and sutured grains), and saddle dolomite. (4) Uplift diagenesis is characterized by reopening of stylolites along fractures and development of dolomitization under meteoric conditions. The occurrence of nonferroari calcite and ferric oxides in rhombohedral zones in dolomite indicates that dedolomitization was driven by oxidation and alteration of ferroan dolomite zones and probably reflects alteration related to recent weathering.
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Middle East Models of Jurassic/Cretaceous Carbonate Systems
This volume will interest tectonic modelers, stratigraphers, sedimentologists, and explorationists. It is the product of the international conference of “Jurassic/Cretaceous Carbonate Platform-Basin Systems, Middle East Models” that was convened in December 1997 jointly by SEPM (Society for Sedimentary Geology) and the United Arab Emirates University in Al Ain, United Arab Emirates. The twenty-three papers present new data and interpretations arranged in three sections: 1) sequence stratigraphy, cyclostratigraphy, chronostratigraphy, and tectonic influences, 2) depositional and diagenetic models of carbonate platforms, and 3) hydrocarbon habitat and exploration/development case studies. New tectonic models of the Arabian Basin, new stratigraphic and sequence stratigraphic reference sections, new geochemical and source rock data, and new reservoir data are presented. New geologic models make this set of papers relevant to geoscientists working outside of Arabia also.