Mud, predominantly silty clay, accounts for at least 95% of the Holocene fill of the western Alboran basin in the western Mediterranean Sea, 110 km east-southeast of the Strait of Gibraltar. This mud has a multiple origin; it is not deposited from a simple rain of pelagic material onto the basin floor. The mud distribution is closely related with topography, as determined from cores and high-resolution seismic records. Total thickness of the Holocene section is approximately 2 m on basin slopes, but increases to over 4 m (sedimentation rates up to 30 cm/1,000 years) in the near-horizontal basin plain at a depth of 1,500 m. Thickest mud deposits are localized in lows where thin sand layers most abound, and major mud transport paths appear to be similar to sand dispersal pa terns. Mud is hemipelagic in composition: lutite with planktonic and benthonic Foraminifera, deep- and shallow-water ostracods, and plant fibers. Components are, at least in part, near-shore in origin.

X-radiographs show that 10-20% of the mud in the cores is parallel- and cross-laminated, indicating the importance of bottom traction transport. Bottom currents also have truncated the top of sand layers and concentrated microfossils in thin laminae. A few graded mud units are probably mud turbidites. The predominant "trigger mechanism" of the fine-grained turbid flows is floods that seasonally inject material at fluvial point sources along the mountainous Moroccan and Spanish margins. Homogeneous mud layers with scattered microfossils, comprising more than half of the Holocene fill, reflect a more regular deposition from less dense suspension layers. The suspensate is also in part extrabasinal, derived from low-density Atlantic surface water entering at the Strait of Gibraltar and de ser Mediterranean water circulating at depth.

End_of_Article - Last_Page 365------------