Geology, sedimentology, geochemistry and provenance of Late Quaternary turbidites, Madeira Abyssal Plain
The Madeira Abyssal Plain (MAP) consists of three connected sub-basins, the deepest lying at around 5400m. Late Quaternary sediments consist of very poorly graded metre-scale mud-dominated turbidites with thin decimetre-thick sand/silt basal facies, interbedded with centimetre-thick pelagic clays, marls or oozes. Individual turbidite basal facies commonly display a complex repetition of sand/silt to mud fining upwards packages generally separated by synsedimentary deformational shear structures. Sedimentological, mineralogical and geochemical data demonstrate that MAP turbidites differ from 'model' fine-grained turbidites, having been deposited by hyper-concentrated turbidity currents. Turbidite emplacement points and palaeoflow directions are deduced from isopach and basal facies maps. Four consistent turbidite compositional groupings are indicated by modal analysis, mineral chemistry, and an extensive bulk geochemical (> 500 samples) investigation. Petrographic and mineral chemistry data is utilised to determine mineralogical assemblages for basal sands which are shown to be highly sensitive provenance indicators. Bulk geochemical data have been subjected to Cluster and Principal Components Analysis, which allow the definition of geochemical fingerprints for individual beds and sediment packages. This has enabled the correlation of turbidites over > 500 km. These data represent the first demonstration of a high resolution basin-wide cheinostratigraphy. In addition, proximal to distal geochemical variation within individual turbidites is employed as a proximality indicator which is consistent with sedimentological data. Evidence from sedimentological, mineralogical and geochemical studies demonstrate that turbidites originated from four distinct source areas, and were supplied from numerous points along the plain margin: (1) volcanic turbidites derived primarily from the Canaries to the east; (2) calcareous turbidites which originated from the Great Meteor-Cruiser Seamount Chain to the west; (2) organic-rich turbidites emplaced from the northeast and derived from the lower continental slope of NW Africa north of the Canaries; (4) organic-rich turbidites emplaced primarily from the south, which originated from the continental slope of West Africa south of the Canaries. Turbidite emplacement was coincident with times of rapid sea-level rise and fall, and was controlled by sedimentation on the lower continental slope and seamounts, and by volcanism on the Canaries.