161 Preliminary Report


Leg 161 was the second in a two-leg ODP program to address both tectonic and paleoceanographic objectives in the Mediterranean Sea. The paleoceanographic program concentrated on reconstructing Atlantic-Mediterranean water exchange and the paleoceanography of the western Mediterranean during the late Cenozoic. Tectonic studies focused on the origin and tectonic evolution of the Alboran Sea as a well-defined example of an extensional basin developed in a collisional setting.

The main focus of the paleoceanographic program during Leg 161 was documentation of the timing of sapropel formation in the Tyrrhenian Sea and of circulation patterns in the western Mediterranean, as well as monitoring of the Atlantic-Mediterranean water exchange. Secondary objectives were to determine environmental conditions during the onset of evaporitic conditions and the reestablishment of open-marine conditions during the earliest and latest Miocene, and the evolution of the Mediterranean's hydrography during the onset of Northern Hemisphere glaciation, about 3.0 m.y. ago. Because the Mediterranean is a semi-enclosed, landlocked basin with only restricted water exchange with the open ocean, the composition of these sediments is especially sensitive to climate change, and environmental signals are preserved in great detail. To achieve these goals, three drill sites where chosen: Site 974 in the Tyrrhenian Sea, Site 975 on the Menorca Rise, and Site 976 in the western Alboran Sea. Sites 977, 978, and 979 in the eastern and southern Alboran Sea were primary tectonic sites but also included paleoceanographic objectives.

APC and XCB drilling at multiple offset holes ensured continuous recovery of Pliocene-to-Pleistocene sequences at these sites. In the Alboran Sea, major unconformities were observed in the Pliocene and Miocene sections. Sedimentation rates varied between 3 and 4 cm/k.y. in the Tyrrhenian and southern Balearic seas, and 15 and 30 cm/k.y. in the Alboran Sea. At Sites 974 and 975, we recovered up to 38 organic-rich layers (ORL's), which resemble the type sapropels found in the eastern Mediterranean in that they are discrete, dark layers with sharp upper and lower boundaries. Total organic carbon concentrations of these layers varies between 0.8% and 2.5%; maximum concentrations of >6% are reached in the Tyrrhenian Sea. At the Alboran sites, more than 40 ORL's were found, but here they are more dispersed and have gradational upper and lower contacts. In addition, some are more than 3 m thick, reflecting higher sedimentation rates. Interstitial- water profiles show that brines were present in the deepest parts at all sites. In the Tyrrhenian and Balearic seas, these brines likely are derived from dissolution of evaporites that are known to occur below the cored sediments. No major evaporite series are known in the Alboran Sea, however, and the brines there are preliminarily interpreted as a paleo-fluid of Messinian age or represent a lateral flow of brines resulting from dissolution of evaporites in other parts of the area. At Site 975 on the Menorca Rise, we retrieved an upper Miocene evaporite sequence that consists of finely laminated gypsum, limestone, and marls. This sequence, together with sediments from the Miocene/Pliocene transition in the Alboran Sea, is important for establishing the transition from restricted to open-marine conditions.

The prime tectonic objective of the Alboran Sea drill sites was to develop a better understand the dynamics, kinematics, and deformation of continental lithosphere margins. The following problems were explored: the origin of extensional basins developed on former collisional orogens; the dynamics of the collapse of collisional ridges which result in extensional basins surrounded by arc-shaped orogenic belts; and a determination of the actual or sub-actual collisional processes.

Sites 976, 977, 978, and 978 in the Alboran Sea focused on the tectonic goals. Site 976, in the western Alboran Basin, penetrated through the Pleistocene and Miocene sedimentary cover and recovered 258.97 m of high-grade metamorphic basement, yielding information on the origin and evolution of the Alboran Sea. Metamorphic conditions indicate that these rocks underwent a significant decrease in pressure, accompanied by constant, or possibly increasing, temperature. Sites 977 and 978, in the eastern Alboran Basin, yielded information on the late Miocene to Holocene subsidence history and rifting evolution of the basin. Site 979 penetrated a zone of syn- and post-sedimentary folds on the flank of the Alboran Ridge (the main compressional feature of the Alboran Sea), yielding information on the age and nature of the later stages of compressional tectonic reorganization of the Alboran Basin.

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