Mineralogy and geochemistry at DSDP Leg 80 Holes

Cenozoic sediments recovered from Sites 548, 549, and 550 were the objects of mineralogical (bulk sample and <2 - µm fraction) and geochemical (HCl extract) studies. Thin sections of rock pebbles embedded in sediments (upper levels at Site 548, particularly) were examined on a polarizing microscope. This study outlines the vertical and lateral variation and evolution of the sedimentation. In the Paleocene and lower Eocene, the clay fraction is abundant and smectite is practically the sole existing clay mineral. High Mn, Al, Fe, Mg, and K contents were measured in HCl extracts. Through the middle Eocene, carbonates become more abundant - highly dominant at Site 548. Metal contents in HCl extracts are very low. The clay fraction, although dominated at all sites by smectites, becomes richer in illite and poorly crystallized chlorite. At the middle/upper Miocene boundary, a significant decrease in the smectite/(illite + chlorite) ratio occurs at all sites, and this decrease continues into the middle Pliocene. This decrease is marked by an abrupt increase of quartz at Site 548. At the two other sites, carbonates remain highly predominant; HCl extracts reflect the relative abundance of the clay and carbonate fractions. After a brief recurrence of smectite in a high-metal-content interval, illite and chlorite become the dominant clay minerals in the upper Pliocene and the Pleistocene, where numerous variations in mineralogical composition occur in the clay fraction (Sites 548 and 549) or in non-clay components (Site 548). Several pebbles of various nature and origin, encountered in different levels of this interval at Site 548, appear to have an ice-rafting origin. This study points out three main breaks in the general evolution of the sedimentation: the first, corresponding to the lower/middle Eocene boundary, is marked by the increase of carbonates and associated elements; the second, corresponding to the middle/upper Miocene boundary, is marked by a major decrease of the smectite/(illite + chlorite) ratio at all sites and by a massive appearance of quartz at Site 548; and the third, which occurred toward the late Pliocene, is marked by the dominance of primary clay minerals and the arrival of ice-rafted pebbles. Our interpretation of results considers paleohydrological and paleoclimatic phenomena. It is suggested that the major middle/late Miocene break was associated with an increase of the deep bottom-water circulation between the Norwegian Sea and the North Atlantic Ocean, and/or a climatic evolution: humidification and cooling of climate. The changes toward the late Pliocene appear to have been the first effects of the glaciations at the end of Cenozoic.

Messinian and pre-Messinian sediments of ODP Holes 107-652A and 107-654A

Sedimentology, mineralogy, and petrology of the pre-Pliocene sediments drilled at ODP Sites 652 and 654 in the Tyrrhenian Sea (Leg 107) have been studied with emphasis on the lower Messinian to pre-Messinian intervals. Messinian at Site 652 is essentially turbiditic and basinal in character; it was deposited during the syn-rift phase in a strongly subsiding half-graben and is correlatable with emerged coeval sequences; in part with the Laga Formation of the foredeep of Apennines, and in part with the filling of grabens dissecting that chain in the Tyrrhenian portion of Tuscany. The sequence found in Site 654 indicates an upper Tortonian to Messinian transgression accompanying crustal stretching in the western Tyrrhenian Sea and is perfectly correlatable with the so-called "Sahelian cycle" and with "postorogenic" cycles recognized in peninsular Italy and in Sicily.

(Table 1) K-Ar age determinations on biotite from coarse sand within DSDP Sample 72-516F-76-4,107-115

Qualitative petrographic study of selected clastic horizons within the Eocene section of Hole 516F has revealed the presence of abundant fine-grained lithic fragments, probably volcanic, along with coarser fragments of quartz and feldspar apparently derived from a nearby plutonic terrain. In detail, poor sorting, presence of graded bedding, and an abundance of clay suggest these are turbidite horizons locally derived from a mixed volcanic/plutonic terrain, possibly with some direct contribution from contemporary volcanic ash falls. A progressive increase in plutonic versus volcanic components with time is, however, more consistent with an erosional origin for most of this material. Unusual euhedral dark biotite is abundant in several of the lower clastic horizons; it is most easily interpreted as microphenocrysts weathered in situ out of alkalic volcanic ash. Biotite separated from Sample 516F-76-4,107-115 cm, has been dated by the K-Ar method at about 46 Ma. Alkaline volcanoes active on the Rio Grande Rise in the middle Eocene would be the most probable source of this ash and would be consistent with other evidence for potassic, alkaline volcanism along the Rio Grande Rise and at the Tristan da Cunha hot spot.