Chemostratigraphy of Madeira Abyssal Plain Miocene-Pleistocene turbidites, Site 950

Abstract

Miocene to Holocene sediments on the Madeira Abyssal Plain (MAP), northeast Atlantic, are dominated by thick-bedded distal mud turbidites. These turbidites record the history of sedimentary source areas and slope failure on the margins of the Canary Basin since ~15 Ma. Major elements and selected trace elements have been determined in 488 turbidite samples collected between 0 and 325 mbsf (Miocene-Pleistocene) at Site 950, on the western MAP. Carbonate and Ti/Al ratio data have been plotted against a detailed sedimentary log to demonstrate the distribution of turbidite chemofacies through the succession. Three major turbidite groups, recognized previously from Quaternary cores, are confirmed to continue through the older sediment record on the plain. Organic-rich, volcanic, and calcareous turbidites are clearly differentiated on chemostratigraphic logs. Organic-rich turbidites dominate both volumetrically and numerically. They have been deposited since the middle Miocene (~15 Ma) and may be subdivided into three geochemically distinct subgroups, the relative importance of which has changed through time. The oldest sediments are Al rich, reflecting more kaolinitic compositions; two K- and Mg-rich subgroups become dominant upward, implying a trend toward more chloritic and illitic clay-mineral assemblages. These changes indicate an increasing importance of northerly source areas on the northwest African continental slope, and/or climatic changes promoting mineralogical shifts in sediments on the margin. The onset of significant volcanic turbidite deposition occurred in the mid-late Miocene, ~14-16 Ma, with the deposition of low-Ti sediments derived from the vicinity of an evolved volcanic source, possibly the slopes of the Canary Islands off Lanzarote or Gomera. A major change toward more basaltic sources occurred in the late Pliocene (~3.5 Ma), possibly associated with the early development of La Palma. Wide ranges in trace-element compositions and a shift toward less Ti-rich compositions indicate the continued existence of multiple sources with increasing volcanic fractionation since that time. Calcareous turbidites have been deposited regularly since the Miocene, but underwent a major decrease in their volcaniclastic component ~3.5 Ma. This is interpreted to indicate the subsidence and draping of the seamount chains to the west of the MAP, which are believed to be the source area for these turbidites.