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Deep-Sea Drilling Project Sites 225 and 227 were located in the main trough of the Red Sea, 10 and 3 mi, respectively, east of the axial trough in the vicinity of the Atlantis II Deep. Coring was continuous. The sedimentary section included 3 units. Unit 1 (0-175 m) is carbonate and nannofossil ooze and chalk, with minor variable detrital admixture. It is of early Pliocene to Holocene age. Unit 2 (175-195 m) is claystone, rich in dolomite and pyrite, and is earliest Pliocene. Unit 3 (195-? m) is layered anhydrite and halite, with interbeds of black shale. Dolomite is present in the shale and between anhydrite nodules. Unit 3 is of late Miocene age.

Scanning electron micrographs show the dolomites in Units 2 and 3 to have euhedral planar crystal faces and to show penetration twinning, indicating them to be authigenic. The overall percentage of dolomite increases from less than 10% at the top of Unit 2 to, locally, more than 80% within the evaporite.

Geochemical signatures differ between the dolomites of Units 2 and 3. The dolomites in Unit 2 contain excess calcium. The dolomites of Unit 3, generally of finer crystal size, achieve stoichiometric composition and show better ordering. Both types of dolomite are interpreted as early diagenetic, with the calcium-magnesium ratios being indicative of paleosalinity conditions, analogous to the dolomites of the Zechstein and the Triassic Keuper Formation.

Stable carbon and oxygen isotope data show the environments of deposition of dolomite of both units to have been restricted, but to differing degrees. The situation is directly analogous to the dolomites of the Mediterranean basin.

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