ABSTRACT

Three cores from the southern Blake Basin and adjoining Blake Ridge penetrated several distinct carbonate units and intervening pelagic terrigenous clay units. Structural and textural changes within the carbonate units strongly suggest that these units were deposited by turbidity currents, which presumably originated in the Bahama Island area.

One core from the floor of the Blake Basin had nine turbidite sequences each with the graded, laminated, and pelitic divisions of a typical Bouma cycle, separated from the adjacent sequences by a unit of pelagic terrigenous clay. Determination of the relative amounts of carbonate minerals in samples from the three divisions of the nine turbidites showed no statistically significant mineralogic difference between the graded and laminated divisions although there may be a slight indication of mineralogic change due to size segregation. However, the pelitic divisions contained significantly less aragonite and high-Mg calcite than the other two divisions.

Two cores from the eastern margin of the Basin and the crest of the Ridge showed the same decrease of unstable carbonate minerals in the pelitic division of the only turbidite penetrated at each station. Both of these cores are believed to have penetrated the distal portion of a turbidite.

The changes in the mineralogy, where a loss of unstable minerals occurs, are ascribed primarily to differential solution. Differential solution of the unstable minerals in the pelitic units is enhanced by an increase in the length in time that these units are exposed to sea water, brought about by the slow- rate of deposition of the overlying pelagic clay and bioturbation