Seismic facies of the Madeira Abyssal Plain: A correlation between seismic reflection profile and borehole data

Abstract

Seismic profiles across the Madeira Abyssal Plain show a relatively simple seismic stratigraphy in which an irregular diffractive acoustic basement is overlain by distinctive seismic units, reflecting a great thickness of ponded turbidites overlying pelagic drape. Within the uppermost ponded turbidite unit, a number of distinct, continuous, and laterally extensive reflectors are recognized. Sites 950 through 952 were drilled into these reflectors and allow dating of the beginning of large-scale turbidite emplacement on the abyssal plain and identification and dating of previously recognized seismic reflectors with a good degree of certainty. The extent and probable volume of the distinct turbidite packages can now be quantified. The Madeira Abyssal Plain overlies oceanic crust of Cretaceous age. Five distinct seismic units, separated by prominent, continuous, laterally extensive reflectors, can be identified. The lowermost of these (Unit B), which directly overlies acoustic basement, is a variably stratified unit and contains reflectors that generally show low coherency and onlap onto basement highs. At Site 950, the upper part of Unit B consists of red pelagic clays, with thin calcareous turbidites and ash layers, of late Eocene to Oligocene age. Unit A overlies Unit B with clear unconformity, marked by a conspicuous basinwide seismic reflector (Reflector 4). Unit A is a variably stratified unit and can be divided into four seismic units, A0 through A3, separated by prominent reflectors of regional extent. These units consist of thick, ponded turbidites with pelagic intervals. Many turbidites are basinwide in extent and can be correlated between drill sites. Four main types of turbidites are recognized: volcanic-rich turbidites derived from the Canary Islands, organic-rich turbidites derived from the Northwest African Margin, calcareous turbidites derived from seamounts to the west of the plain, and turbidites of 'intermediate' character. Organic-rich turbidites are the dominant type, although volcanic-rich turbidites are numerous in Units A0 through A2. Conversion of two-way traveltime to depth using shipboard sonic log data suggests that thick volcanic-rich and 'intermediate' character turbidites of wide lateral extent commonly correspond to strong seismic reflectors, and that acoustically transparent intervals within Unit A correspond to intervals of predominantly organic-rich turbidites. The base of Unit A is the regionally important Reflector 4 that correlates with a distinctive calcareous bed at all three drill sites dated at 16 Ma. The seismic units can be laterally mapped using an extensive data set of seismic reflection profiles and the minimum volumes of sediments deposited within individual seismic units calculated, giving values for sediment accumulation on the plain per unit time. The data show that since the inception of the abyssal plain in the middle Miocene (16 Ma), a minimum of 19,000 km3 of sediments (turbidites and hemipelagites) have been deposited.