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Diagenesis of the Middle–Upper Jurassic Carbonate Platform (Amran Group), Yemen

By
Khalid A. Al-Thour
Khalid A. Al-Thour
Department of Earth and Environmental Sciences, Sana’a University, P.O. Box 2027, Sana’a, Yemen email: hayssan@xj.net.ye.
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Published:
January 01, 2000

Abstract

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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Contents

SEPM Special Publication

Middle East Models of Jurassic/Cretaceous Carbonate Systems

Abdulrahman S. Alsharhan
Abdulrahman S. Alsharhan
Faculty of Science United Arab Emirates University, P.O. Box 17551, Al Ain, U.A.E.
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;
Robert W. Scott
Robert W. Scott
Precision Stratigraphy Associates, RR3 Box 103-3, Cleveland, OK 74020, U.S.A.
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SEPM Society for Sedimentary Geology
Volume
69
ISBN electronic:
9781565761919
Publication date:
January 01, 2000

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