Abstract: Preliminary Results from Leg 44 Deep Sea Drilling Project in Western North Atlantic

W. E. Benson, R. E. Sheridan, P. Enos

Drilling three areas of the lower part of the Atlantic continental margin provided definitive data on the sedimentary buildup of this province. Hemipelagic muds overlie a flat-lying middle Miocene reflector of the lower continental-rise hills drilled at Site 388. The inclined reflectors under the hills, therefore, must have originated after the late Miocene if they were produced as folds at the toe of a large slump gliding over the middle Miocene reflector. However, delicate burrowing structures and normal stratigraphic transitions indicate that the reflector is not a shear plane. The internal structures observed on reflection profiles could be interpreted as depositional foreset beds of mud-wave ridges, but local slumping of muds piled high by contour currents is also po sible.

Sites 389, 390, and 392 on the Blake nose found Eocene just below a veneer of manganese-nodule gravel. Fossiliferous Eocene and Paleocene nanno ooze is interlayered with hard chertified limestone, especially near the upper Paleocene contact. A major angular unconformity between Campanian and Albian oozes was cored, indicating erosion by strong ocean-bottom currents. Hard limestones of a reef-complex facies of Barremian and older age marked the termination of the reef environment on the Blake nose. The 2,600-m depth of the Blake nose reef complex is attributed simply to a cessation of reef building as the overall margin subsided. The reef complex maintained itself farther south and west to form the 1,500-m lip of the Blake Escarpment thought to be built of Upper Cretaceous limestone. N structural downfaulting between the two structures appears to be required by the data.

Site 391 in the Blake-Bahama basin discovered a 500-m thick Miocene intraclastic chalk deposit formed by turbidite and gravity-debris flows. Shallow-water limestone pebbles from the Bahama Banks, and resedimented nanno ooze from the Blake Plateau flushed down to the basin many times throughout the Miocene. Tectonic activity may have stimulated these flows, especially movements along Great Abaco fracture zone.

Horizon A in the Blake-Bahama basin is the hiatus between the Miocene chalk and Cretaceous (Aptian-Albian) black sapropelic clay, and Horizon Beta is correlated with the contact with the underlying Neocomian limestone. The Early Cretaceous-late Jurassic nannofossil biostratigraphy is well developed at Site 391 where this transition was well cored.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA