Facies modeling and sequence stratigraphy of an eocene carbonate-phosphorite ramp : damlouk member, ratga formation, Western Desert, Iraq

الملخص EN

The Eocene phosphatic Ratga Formation of the Iraqi Western Desert consists of three members from bottom: Swab, Damlouk and Mugur Members.

This study is focused on the Damlouk Member which is represented by 40 – 50 meters thick sequence of alternating shelf carbonates of mollusk-nummulites association, cherty phosphatic limestone horizons and basinal chalky limestone and marlstone.

This member is an important part of a thick cyclic phosphorites-bearing sequence extends from the Upper Cretaceous to the Eocene.

It is the aim of this paper to shade some lights on the basic characters, origin, and stratigraphic associations, through facies modeling and sequence stratigraphic analysis.

This is done by examining the Damlouk Member sequence in three localities chosen from Wadi Al-Ratga and Wadi Akash around Akashat area at the Western Desert of Iraq.

The Damlouk Member is deposited over a gently sloping ramp system.

Sedimentary facies in seaward direction includes: coastal plain (not well represented in the study area), Nummulite shoals limestone, and the Oyster bank limestone, of the inner ramp.

The middle ramp is represented by gray bioturbated and granular limestone-phosphate horizon.

This facies is occasionally, punctuated by storm-derived shelly-nummulitic limestone lenses.

The intercalation of the phosphatic facies with the basinal marlstone and chalk often occur in the seaward part forming the outer ramp facies.

The latter facies opened up into the basinal marlstone and chalk of the Jaddala Formation or its equivalent.

Both the middle and outer ramp facies are characteristically silicified and contain dark chert nodules and horizons.

The Damlouk Member consists of two sedimentary sequences deposited during the Late Ypresian to Middle Lutetian (Middle Eocene), and developed by two fourth-order eustatic cycles.

This is inferred from facies sequence as well as correlation with the regional and global eustatic curves of the sea level fluctuations.

Both cycles have similar facies type, distribution and stratigraphic evolution.

Each of the two sequences consists of well developed TST and HST which are separated by fingers of basinal marlstone from the adjacent Jaddala representing the MFS.

Each of the two sequences is bounded by Type-2 sequence boundary which is characterized by sharp facies changes.