Selective dolomitization in Arabian carbonate rocks

الملخص EN

Selective dolomitization is a distinctive petrographic feature observed in the Upper Permian Khuff and Upper Jurassic Jubaila and upper Hanifa carbonate rocks exposed in Central Saudi Arabia.

In broader terms, an intimate affinity exists between dolomite replacement and origi¬nal micrite matrix in the limestone, so that the former steadily increases with the increased abundance of the latter.

In closer terms, however, each one of the carbonate constituents in limestone (micrite matrix, allochems and sparry calcite cement) has shown variable susceptibility to dolomite re¬placement.

In general, the Khuff and Jubaila limestones were differently affected by dolomitization in both intensiveness and texture of the replacing dolomite.

The original depositional constituents of the Khuff rocks were usually af¬fected by complete (indiscriminate) dolomitization resulting in extremely fine-textured dolomite, whereas those of the Jubaila and adjacent rocks were affected by partial (selective) or complete dolomitization resulting in coarser-textured dolomite.

It seems that the general contrast in apparent dolomite behaviour in the examined rocks is mainly attributed to whether dolomitization occurred earlier or later than the diagenetic cementation of carbonate sediments.

Subsequently, early and late phases of dolomitization might be envisaged to have respectively antedated and postdated the ncomorphic transformation of carbonate sediments from aragonite into cal¬cite.

Selective dolomitization could be explained in the light of certain factors which may separately or collectively influence the dolomite replacement of each carbonate constituent.

The three main factors are : pre-dolomitization mineralogy, grain size and permeability of the original calcium carbonate sediments and rocks.

A general order for the sequence of variable suscepti¬bility to dolomitization of the various carbonate constituents is established for the examined rocks.