ALBERT

Description: The book is devoted to stratigraphy of Cretaceous deposits from high latitudes of the southern hemisphere (subantarctic part of the ocean), as well as to geological and climatic Cretaceous history of the area. Correlation with Cretaceous sediments from warm water regions is carried out. Description and photos of characteristic species of planktonic and benthic foraminifera and calcispherulides are given.

Description: The resistivity, porosity, and cation exchange capacity of 36 basaltic samples recovered in Hole 504B during four DSDP and ODP legs have been measured in the laboratory at room temperature and atmospheric pressure. The presence of chlorites and particularly smectites as alteration products of basalt phases is reflected by high values of cation exchange capacity (CEC). Whereas the massive units of Layers 2A and 2B are defined by high and uniform CEC values, the more fractured and altered pillows are characterized in the entire hole by even higher values of CEC and a large variability. The lowest CEC values, measured for the massive units of Layer 2C, are due to changes of basalt alteration fades with depth and the related decreasing abundance of smectites with increasing depth in the oceanic crust. The porosity and the apparent formation factor (computed from resistivity measurements made with a fluid salinity similar to that of seawater) are related by an inverse power law similar to Archie's formula, with m close to 1.0 and a as large as 9.1. A review of the literature shows that such a low m value equates to current conduction in cracks and microcracks present throughout the rock. The presence of these microstructures reflects the extensional regime under which the rock formed at the ridge axis, and they were conserved by precipitation of clay minerals attributable to intense hydrothermal circulation. The comparison of these results to similar studies of mid-ocean ridge basalt physical properties indicates that m tends to increase to values close to 2.0 with age. Such values are, in fact, similar to those found for sedimentary rocks and probably reflect an increased tortuosity of the conducting pore space with increasing age. An inverse relationship also relates CEC and apparent formation factor, indicating that surface conduction of clay minerals plays an important role during downhole electrical experiments. This provides a plausible key to the paradox of low permeability and high apparent porosity obtained from comparing the in-situ experiments conducted in Hole 504B.

Description: The geochemistry of the REE (rare earth elements) in oceanic sediments is discussed, based mainly on samples from DSDP Holes 530A and 530B, Leg 75, and Hole 525A, Leg 74. The proposed mechanisms for incorporation of the REE into the marine carbonate phases are adsorption, chiefly onto the carbonate minerals and on Sc, Hf, and Ta-rich Fe-Mn hydroxide flocs as carbonate coatings. The Ce anomaly of marine carbonate was used as an indicator of paleo-ocean water redox conditions: the bottom water of the Angola Basin was in a reducing condition in the Cretaceous. At ca. 54 My, the South Atlantic water condition became oxidizing, similar to the present seawater redox condition. This change was related to the improvement of circulation due to the widening of South Atlantic and the subsidence of water circulation barriers such as the Walvis Ridge and perhaps the Romanche Fracture Zone. The younger (Eocene-Recent) and older (Albian-Santonian) argillaceous sedimentary rocks from 530A (denoted as YSAB and OSAB respectively) show different degrees of Eu depletion with a transition period in between. The REE patterns of OSAB suggest a basaltic origin. The possible sources are Kaoko basalt in Southwest Africa or Namibia and the basaltic Walvis Ridge itself. The decrease in the area covered by Kaoko basalt due to erosion, the subsidence of the Walvis Ridge, and the improvement of water circulation led to changes in the Eu anomaly from Campanian to Paleocene, and resulted in the YSAB REE pattern. Changes in the Sm/Eu, La/Th, Th/Yb, Ti/Al2O3, FeO/Al2O3, and Hf/Al2O3 ratios suggest changes of average source rock composition from and esite to granodiorite. The REE abundances and patterns of younger sediments in the Angola Basin (YSAB) are very similar to those observed in NASC, PAAS, and ES sediments. The YSAB REE abundances and patterns may represent the average REE distribution of the exposed African continental crust. The strong resemblance of REE distributions of YSAB, NASC, PAAS and ES suggests thorough REE mixing from different sources and the uniformity of the average crustal compositions of different continents: Africa, North America, Australia, and Europe.