An average of of exclusively
deep-sea sediments, resting upon
oceanic crust, underlies the Madeira Abyssal Plain.
Seismic reflection profiles across the Canary Basin and Madeira Abyssal Plain reveal north-northeast – south-southwest ridge and trough terrain typical of oceanic crust and west-northwest – east-southeast striking
fracture zone valleys that are spaced about part. Because most of the Madeira Abyssal Plain lies within the
Cretaceous Superchron, the oceanic crust underlying it cannot be precisely dated by
magnetic striping. However, interpolation between recognised magnetic stripes estimated an age range of about 75 to 105
Ma for the oceanic crust underlying the central sub-basin. Immediately overlying the oceanic crust is a layer of
hemipelagic sediments. These sediments average in thickness and are expected to consist predominantly of hemipelagic
clay. Overlying the hemipelagic sediments, are alternating
turbidites and thin beds of
pelagic sediment. These sediments initially in filled irregularities on the uneven surface of the hemipelagic sediments to produce a flat plain that later turbidites accumulated. The total thickness of turbidites that have accumulated averages in thickness. In a few deep troughs within the oceanic crust, the total thickness of turbidites may reach . In seismic reflection, the sequence of turbidites varies from being strongly acoustically laminated near the top to poorly stratified to transparent near the base. An individual turbidite often consists of of sediment spread across the entire Madeira Abyssal Plain. It is typically fine-grained, except in the proximal parts of the plain. The deposition of a typical turbidite causes little or no erosion of the underlying bed. A thin bed of fine-grained, pelagic sediment typically separates successive turbidites. Based upon composition, the turbidites have been divided into three groups. First, there are a group of organic-rich turbidites. These turbidites represent organic-rich sediments that turbidity currents transported from two sources, one north and one south of the Canary Islands. These turbidites are typically bicolored turbidite units. Their base is usually olive green where the organic material remains below surface oxidation, and their upper part is pale green where the organic material has been oxidized. Second, there are volcanic turbidites composed largely of sediment derived from either volcanic seamounts or islands. These turbidites represent the distal sediments of turbidity currents generated by massive submarine landslides resulting from the collapse of the flanks of volcanic seamounts or islands within either the
Canary Islands or
Madeira Archipelago. Finally, there are calcareous turbidites derived from submarine landslides effecting one of the Seewarte Seamounts to the west of the Madeira Abyssal Plain. Thin pelagic layers separated individual turbidites. As determined by
microfossils, each individual layer often represents several tens of thousands of years of pelagic sedimentation in a deep sea,
abyssal environment. Depending on the
carbonate compensation depth at the time of deposition, these layers consist either of calcareous ooze, marls, or clay. During the last 2.6 million years within the region of the Madeira Abyssal Plain, carbonate compensation depth has been closely controlled by the general circulation of ocean currents and has oscillated in phase with climatic shifts. During
interglacial periods, the carbonate compensation depth was quite deep. This allowed for the preservation of calcareous
microfossils, e.g.
foraminifera and
coccoliths, in the Madeira Abyssal Plain and the formation of calcareous ooze. Conversely, during
glacial periods, and prior to 2.6 million years ago, the carbonate compensation depth was shallower. This leads either to poor preservation of calcareous microfossils and frequently no preservation of them at all and, respectively, the accumulation of either marl or clay to form pelagic layers. == References ==