Abstract

An extensive geochemical database has been used to constrain depositional processes, sediment dispersal patterns and palaeoflow pathways of late Quaternary distal turbidites deposited on the Madeira Abyssal Plain (MAP). Turbidites are meter-thick, mud-dominated units with distinctive litho-, bio- and chemostratigraphic characteristics which have enabled the correlation of individual beds over > 67,000 km 2 . Geochemical profiles for Si, Ti, Al, Mg, Fe, Ca and Zr have been constructed for a representative 5-meter bed (turbidite b ) at 18 core localities distributed across the central part of the MAP. These profiles, combined with grain-size and mineralogical data, demonstrate that turbidites consist of a 1-120 cm, laminated, sand/silt basal layer, rich in volcanic glass, basalt clasts, heavy minerals and foraminifera (Unit 1), overlain by 1-5 m of poorly graded silty muds. Geochemical data additionally indicate the presence of a central layer which is relatively enriched in heavy mineral and flocculated clay-aggregate muds and is typified by high Ti, Fe, Mg and Zr contents (Unit 2). This unit is overlain by sediments containing increasing proportions of calcareous nannofossils and clay minerals (Unit 3) and characterized by enhanced Ca and Al concentrations. Unit 1 thins and fines, whereas Units 2 and 3 thicken and fine toward the deepest part of the basin and into local depressions and cul-de-sacs. It is argued that the majority of the turbidites were deposited from evolving highly concentrated flows, which prevented significant grain-size segregation and enabled the rapid deposition of meter-thick poorly graded muds. The sorted and graded basal silts (Unit 1) were deposited primarily by an initially turbulent flow, which decelerated and matured into a highly concentrated fluid mud turbidity current. Shear at the flow base was responsible for the grading in the upper part of Unit I and in the silty muds of Unit 2. Fluid entrained under the current head, which migrated up through the flow, carried light carbonate and clay minerals toward the turbidite top, leaving a central plug of dense muds overlying sheared sands/silts and silty muds. Isocon maps (produced by joining points of equal chemical concentration) based on mean values for Ca, Ti, Mg and Zr in Unit 2 are used to reconstruct sediment dispersal patterns and palaeoflow pathways. These maps confirm that turbidite b was deposited by turbidity currents which flowed axially southwestward across the central part of the MAP, implying that the Canary Islands and Madeira offer a potential source of sediment for MAP turbidites.

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