Abstract: Mixing of Bottom Sediments in Northwest Atlantic

Sunit Kumar Addy

The geochemistry of sediments from 17 piston and 4 box cores from the abyssal plains and abyssal hills around 60°W and 27°N in the northwest Atlantic was studied by analyses for trace elements including the rare earths, clay mineralogy, grain-size distribution, and total organic carbon. Red clays from the abyssal hills are considered to represent pelagic sedimentation and show average hydrogenous Mn = 6,265, Fe = 17,806, Ni = 97, Co = 68, Cu = 8.1, and P = 619 ppm. In contrast, terrigenous gray clays present as turbidites in abyssal plains have values of Mn = 571, Fe = 7,798, Ni = 15, Co = 14.7, Cu = 11.7, and P = 398 ppm. Average total organic carbon and sand + silt for red clays are 0.33 and 11.6%, respectively, whereas gray clays have 0.81 and 26.1% respectiv ly. In the abyssal plains, a third type--brown clays--overlies or is interbedded with the gray clays. Intermediate values are observed in brown clays, for example, average Mn = 2,046, Fe = 14,472, Ni = 23, Co = 16.3, Cu = 9.4, and P = 445 ppm. Average total organic carbon and sand + silt are 0.53 and 14.8%. These data, together with clay mineralogy and REE patterns, indicate that brown clays are due to mixing of pelagic red clays and terrigenous gray clays in various porportions. The brown clays apparently were deposited between turbidity flows but are not purely pelagic in character, because bottom currents resuspended abyssal-plain sediments, thus mixing the gray clays with a continuous rain of red clays. The observed increase in the optical density of the nepheloid layer directly abov the abyssal plains reflects this resuspension of bottom sediments.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma