Late Pleistocene and Holocene sedimentation in the Persian Gulf—Gulf of Oman

Abstract

Studies of thirteen piston and gravity cores from the Persian Gulf and Gulf of Oman, collected during a 1977 ‘Atlantis II’ cruise, showed that distinct and correlative sedimentary units can usually be distinguished in the Late Pleistocene and Holocene sediments. The units from the Persian Gulf are: 1. Unit 1 — olive-grey marl of variable thickness (50 to > 100 cm) that was deposited within the last 5000 or 6000 years B.P. The sediments are very silty and contain abundant biogenic constitutents; carbonate content is about 55%. The most characteristic and diagnostic mineral of this unit is high magnesian calcite. 2. Unit 2 — light greenish-grey carbonate mud with occasional intercalated lithified carbonate layers. The mud is basically composed of clay-sized aragonite needles, and carbonate content is > 70%. In the coarser size fraction oolites and pellets, often of black color, are abundant. This unit is commonly about 10–100 cm thick and was deposited between 6000 and 12,000 years B.P. 3. Unit 3 — greyish-brown detrial silt deposited between ∼12,000 and at least 30,000 years B.P. Carbonate content ranges between 35 and 59%. Calcite and dolomite, including Ca-dolomite, are the main constitutents. In the lower part of this unit the sediments are highly compacted (water content <20%). These sedimentary units are related to the major changes in the environmental conditions of the Persian Gulf. Unit 3 was deposited when the Persian Gulf was essentially a large river valley crossed by the ancient Shatt-al-Arab River which discharged directly into the Gulf of Oman. Unit 2 was deposited in a period of rising sea level and the establishment of marine conditions within the Persian Gulf. Unit 1 has been deposited since the time that sea level has essentially reached its present level. In the Gulf of Oman terrigenous sediments dominate and the main constituents are chlorite and illite. Carbonate content decreases toward the central part of the basin to less than 20% and consists of calcite with little dolomite. The presence of aragonitic material in certain sedimentary intervals clearly demonstrates transport of shallow water sediments by turbidity currents into the deeper basin. Sedimentation rates for the central basin of the Gulf of Oman are in the order of ∼50 cm/1000 years.