M. B. Royo y S. E. Wendt (ed.). Homenaje a Aída Barbagelata. In Memorian, Buenos Aires, Actualidad Producciones, 1994, 2 tomos, 461 y 555 pp.

Fil: Nápoli, Juan Tobías. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

M. B. Royo y S. E. Wendt (ed.). Homenaje a Aída Barbagelata. In Memorian, Buenos Aires, Actualidad Producciones, 1994, 2 tomos, 461 y 555 pp.

Fil: Nápoli, Juan Tobías. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Geochemistry and ages at DSDP Holes 81-552A, 81-553A and 81-555

Glauconite-rich sediments have been encountered at two horizons during drilling in the southwest Rockall Plateau. The younger of these horizons lies at the base of a deep-sea ooze sequence and is of early or middle Miocene age. Glauconite formed in situ during periods of nondeposition related to strong bottom-water currents, in water depths of as much as 2500 m - five times greater than previously accepted limits for glauconite formation. The older horizon, of early Eocene age, is a record of the major transgression coincident with the separation of Rockall and Greenland. Isotopic age dating of the Miocene glauconites gives results in relatively close accord with their biostratigraphic age. However, an Eocene (NP12) glauconite gives a highly discrepant date (36.5 m.y. ago). One possible explanation is that the Eocene glauconites have continued to evolve after burial by the diagenetic uptake of potassium from the surrounding mud matrix, a possibility denied to the Miocene glauconites by the relative scarcity of available potassium in the nannofossil-foraminiferal ooze matrix.

Analysis of organic matter from DSDP Leg 81 Holes

Organic geochemical and visual kerogen analyses were carried out on approximately 50 samples from Leg 81 (Rockall Plateau, North Atlantic). The sediments are from four sites (Sites 552-555), Pleistocene to Paleocene in age, and represent significantly different depositional environments and sources of organic matter. The Pleistocene glacial-interglacial cycles show differences in sedimentary organic matter based on Rock-Eval pyrolysis, organic phosphorus, and pyrolysis/mass-spectrometry analyses. Glacial samples contain more organic carbon, with a larger proportion of reworked organic matter. This probably reflects increased erosion of continental and shelf areas as a result of low sea level stands. Inter glacial samples contain a larger proportion of marine organic matter as determined by organic phosphorus and pyrolysis analyses. This immature, highly oxidized marine organic matter may be associated with the skeletal organic matrix of calcareous organisms. In addition, Rock-Eval data indicate no significant inorganic-carbonate contribution to the S3 pyrolysis peak. The Pliocene-Miocene sediments consist of pelagic, biogenic carbonates. The organic matter is similar to that of the Pleistocene interglacial periods; a mixture of oxidized marine organic matter and reworked, terrestrial detritus. The Paleocene-Oligocene organic matter reflects variations in source and depositional factors associated with the isolation of Rockall from Greenland. Paleocene sediments contain primarily terrestrial organic matter with evidence of in situ thermal stress resulting from interbedded lava flows. Late Paleocene and early Eocene organic matter suggests a highly oxidized marine environment, with major periods of deposition of terrestrially derived organic matter. These fluctuations in organic-matter type are probably the result of episodic shallowing and deepening of Rockall Basins. The final stage of Eocene/Oligocene sedimentation records the accelerating subsidence of Rockall and its isolation from terrestrial sources (Rockall and Greenland). This is shown by the increasingly marine character of the organic matter. The petroleum potential of sediments containing more than 0.5% organic carbon is poor because of their thermal immaturity and their highly oxidized and terrestrial organic-matter composition.