Lower Miocene coastal lagoon carbonates and Evaporites of Rabigh Area, Red Sea Coast, Saudi Arabia

الملخص EN

The carbonate-evaporite sequence in Rabigh area forms Al Jahfah Formation that is conformably overlying the siliciclastic sequence of Al Haqqaq Formation.

Field examination of Al Jahfah Formation indicates that the thickness of the carbonate rocks is limited (< 15m) in contrast to > 50 m thick for the evaporite rocks (mainly in the quarries).

The carbonate rocks are well exposed due south, east and north of the evaporite rocks.

Microscopic examination of the studied rocks indicates the existence of the following carbonate microfacies types; (a) dolomitic foraminiferal packstone, (b) dolomitic oolitic wackestone, (c) dolomitic intraclastic wackestone, (d) dolomitic mudstone, and (e) boundstone.

The recorded evaporite microfacies types are: (a) porphyroblastic gypsum, (b) granoblastic gypsum, (c) alabastrine gypsum, (d) satin spar gypsum veins, (e) secondary anhydrite, and (f) micritized microbial laminae.

The diagenetic processes that affected the carbonate rocks during early diagenesis are: Micritization, aggrading neomorphism, compaction, dissolution, early cementation and dolomitization.

Displacive growth of gypsum nodules are assumed to be formed during early diagenesis.

The alteration effect of burial stage of diagenesis is more pronounced on the evaporite rocks.

This is attributed mainly to their solubility, where gypsum is converted to anhydrite.

The uplift stage of diagenesis is characterized by late cementation of the carbonates and hydration of burial anhydrite to secondary gypsum rocks.

Due to solar heating, the secondary gypsum dehydrates to felted anhydrite on outcrop.

Field, sedimentary and petrographic criteria point to the formation of the studied carbonate and evaporite rocks in shallow coastal lagoon.

The carbonate rocks were formed at the periphery of the lagoon, whereas the evaporite rocks were formed in the central, continuously subsiding part of the lagoon.

Therefore, a bull's eye distribution pattern of the carbonate-evaporite rocks is inferred.