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 lo gs. 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‐lat e 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 frac tionation 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.