Sedimentation on the Madeira Abyssal Plain: Eocene-Pleistocene history of turbidite infill

Abstract

The sedimentary infill of the Madeira Abyssal Plain is analyzed in detail from the upper Eocene to Holocene at Sites 950, 951, and 952. In addition to the three turbidite groups (organic, volcanic, and calcareous) described in previous publications, gray nonvolcanic, brown and volcaniclastic turbidite groups were also recognized. Site 950 shows the longest sequence beginning with emplacement of two coarse volcaniclastic turbidites in the late Eocene. This was followed by a long interval of pelagic clay deposition until at least the end of the Oligocene. During this time volcanic ash was added from the now-extinct Cruiser/Hyeres/Great Meteor volcanic seamounts to the west. A hiatus in the lower Miocene rock is associated with the deposition of three coarse calcarenites at Site 950, also believed to be from the seamounts. The uppermost calcarenite is a clear marker bed at 16 Ma. Sites 951 and 952 comprise thick sequences of relatively thin organic turbidites through the lower Miocene sequence, representing early infill of the fracture zone valleys in which they were drilled. Many sequences of flows can be correlated between all three sites from the middle Miocene to Holocene, although a series of brown turbidites occurring during the late Miocene (6.5-13 Ma) at Site 950 is less easy to trace. Volcanic turbidites are rare or very thin prior to 7 Ma, but common and thick after this time. Gray nonvolcanic turbidites are found in four limited intervals between 10.5-14.6 Ma, 5.6-7.4 Ma, 3.8-4.2 Ma, and 1.4-1.2 Ma, and include the thickest turbidites of all (11 m at Site 952). The pelagic interbeds reveal the history of the carbonate compensation depth (CCD) which was shallower than the abyssal plain depth until ~8 Ma. Then the CCD deepened slightly, deepening again at ~5 Ma and finally entered the Pliocene/Pleistocene oscillatory mode at ~3.5 Ma. After this time alternating clays and marls or oozes were deposited.