Geochemistry and isotopes at DSDP Hole 79-546

At Site 546, below the Mazagan Escarpment at a water depth of 4 km, 36 m of salt rock was cored from the top of one of a field of salt domes. The core was studied by thin section and a variety of geochemical procedures. The salt rock contains 0.1 to 3% carnallite and lesser amounts of sylvite and polyhalite, which with the corresponding high level of bromide place it within the potash evaporite facies. The bromide profile is of a dominantly marine evaporite deposited in moderately shallow brine which, however, was not repeatedly desiccated. A mineralogical argument suggests that the brine surface was not below sea level. An average of about 5% elastics, with dispersed anhydrite, darken the salt rock to deep shades of red, brown, and gray green. Most of the included materials are in highly deformed boudins or dispersions in the salt rock that has also undergone cataclasis in a subsequent, probably tectonic, deformation. The salt rock is slightly deficient in anhydrite, and the usual separate beds and laminae of anhydrite are virtually absent. Stable isotope ratios of sulfur and oxygen in the sulfate are clearly derived from sea water of Permian to Scythian age, in contrast to the late Triassic or Early Jurassic age of evaporites onshore in Morocco and Portugal and the corresponding evaporites offshore Maritime Canada. In contrast to those evaporites off the axis of Atlantic rifting, the salt at Site 546 may have been deposited in a very early central rift fed by marine waters from Tethys through the Gibraltar or South Atlas fracture zones.

Pyrolyzable carbon, temperature maximum, production index and lithology for DSDP Leg 96 holes

Sediments from Deep Sea Drilling Project Sites 615, 617, 618, 619, and 620-623 were subjected to pyrolysis. The sediments are immature with respect to petroleum generation as determined by production index values of less than 0.1 and Tmax values of 460-480°C. The amount of pyrolyzable organic matter was moderately low as compared to typical petroleum source rocks. The immature organic matter present does not appear to contain a significant proportion of woody material as shown by the low gas-generating potential. Typical overbank sediments from Sites 617 and 620 generally show higher P2 values (500-800 µg hydrocarbon per g dry weight sediment) than typical channel-fill sediments from Sites 621 and 622 (P2 = 450-560 µg/g). Tmax for both types of sediment remained very constant (462-468 °C) with a slight elevation (+ 15°C) occurring in samples containing lignite. The highest P2 values occurred in sections described as turbidites. Very low P2 values (about 50 µg/g) occurred in sands. P2 values for shallower sections of basin Sites 618 and 619 tended to be higher (900-1000 µg/g) and decreased in deeper, more terrigenous sections of Site 619. Preliminary experiments indicate that microbiological degradation of sediment organic matter causes a decrease in P2. Pyrolyzable organic matter from lower fan Site 623 appears to increase with depth in two different sediment sequences (40-85 and 95-125 m sub-bottom). Organic matter type, as shown by pyrolysis capillary gas chromatography (GC) patterns, was generally the same throughout the well, with much more scatter occurring in the deepest sections (130-155 m sub-bottom). One major and two minor organic matter types could be recognized in both fan and basin sites drilled on Leg 96.