Geochemistry, trace elements and minerals at DSDP Holes 80-550B and 80-551

The Leg 80 basalts drilled on the Porcupine Abyssal Plain 10 km southwest of Goban Spur (Hole 550B) and on the western edge of Goban Spur (Hole 551), respectively, are typical light-rare-earth-element- (LREE-) depleted oceanic tholeiites. The basalts from the two holes are almost identical; most of their primary geochemical and mineralogical characteristics have been preserved, but they have undergone some low-temperature alteration by seawater, such as enrichment in K, Rb, and Cs and development of secondary potassic minerals of the "brownstone facies." K/Ar dating fail to give realistic emplacement ages; the apparent ages obtained become younger with alteration (causing an increase in K2O). Hole 551 basalts are clearly different from the continental tholeiites emplaced on the margins of oceanizing domains during the prerift and synrift stages.

Organic facies of Cenozoic and Cretaceous sediments at DSDP Leg 80 Holes

The organic facies of Cenozoic sediments cored at DSDP Sites 548-551 along the Celtic Sea margin of the northern North Atlantic (Goban Spur) is dominated by terrestrially derived plant remains and charcoal. Similar organic facies also occur in the Lower and Upper Cretaceous sections at these sites. Mid-Cretaceous (uppermost Albian-Turonian) sediments at Sites 549-551, however, record two different periods of enrichment in organic material, wherein marine organic matter was mixed with terrestrial components. The earlier period is represented only in the uppermost Albianmiddle Cenomanian section at the most seaward site, 550. Here, dark laminated marly chalks rich in organic matter occur rhythmically interbedded with light-colored, bioturbated marly chalks poor in organic matter, suggesting that bottom waters alternated between oxidizing and reducing conditions. A later period of enrichment in organic material is recorded in the upper Cenomanian-Turonian sections at Sites 549 and 551 as a single, laminated black mudstone interval containing biogenic siliceous debris. It was deposited along the margin during a time of oxygen deficiency associated with upwelling-induced intensification and expansion of the mid-water oxygen-minimum layer. In both the earlier and later events, variations in productivity appear to have been the immediate cause of oxygen depletion in the bottom waters.