57. MARGIN PALEOENVIRONMENTS OF THE NORTHEAST ATLANTIC

Sediments of passive continental margins represent a complex and possibly unique record of the changing paleoenvironment of the oceans, the continents, and also the birth and growth of the oceans. In a broad sense, the paleoenvironment of oceans and continents can be viewed as controlling both the completeness and nature of the lithostratigraphic record shown by the presence or absence of hiatuses, and the lithologic changes between shelf, slope and rise sediments. The sedimentary record is, however, defined by a complex matrix of variables that modify in both space and time not only the accumulation of sediments, but also the relative proportions of terrigenous and biogenous sediments. Among the more obvious of these variables are climate, the altitude of the continents, the fertility of ocean water, and the intensity of ocean circulation. These, and the myriad other factors that fashion the geological record, cannot be separated easily because they may be closely dependent on each other. For example, the climate of the continental hinterland will be determined by its area and orography as well as the climate of the adjacent ocean. In a final, and perhaps extreme analysis, oceanic environments have been said to be shaped by plate tectonics and attendant ramifications (Berger and Winterer, 1974) because the succession of environments in the ocean basins may devolve from changes in ocean basin geometry and inter-basin communication. Although this view is perhaps too general, only a few mechanisms may cause the major variations in the facies of margin sediments. For example, changes in the intensity of the surface and bottom water circulation may lead to changes in sediment accumulation rates, enrichment of waters in nutrients (due to increased residence time), increased biological productivity in upwelling areas and hence to changes in the carbonate compensation depth. Other important factors that may result in profound changes in circulation, and nutrient and oxygen supply, include global changes in sea level, water exchange from higher latitudes resulting from changes in plate motions, and/or the subsidence of sills. Such effects may enhance latitudinal and longitudinal Oceanographic and faunal gradients. Their magnitude and effect may also be variable on a margin that is exposed through subsidence to different water masses, themselves variable with time and depth. In the North Atlantic Ocean, there have been great changes in margin paleoenvironments due not only to local events, but also to global changes in ocean basin geometry and climate. These local events include the construction and subsidence of the various sills between the Arctic Ocean and the Atlantic Ocean, and a complex spreading history that has led to the creation of various parts of its margins at different times and to the isolation of microcontinents such as the Rockall Plateau (Sclater et al., 1977). The complex morphology of the North Atlantic basins and basin margins is likely to have radically influenced the input and distribution of terrigenous sediments and the effects of ocean circulation changes during the Cenozoic that arose from climatic changes. In this paper, we have not attempted a detailed synthesis of Mesozoic and Cenozoic paleoenvironments in the North Atlantic. Such a synthesis, though clearly desirable, would require a thorough analysis of all DSDP and other relevant data for the entire North Atlantic region that lies beyond the scope and purpose of this Initial Report. The paper is, therefore, restricted to a brief review of the present North Atlantic circulation and its influence on passive margin sediment distribution, and a wider discussion of Cenozoic and Mesozoic paleoenvironments and their influence on northeast Atlantic passive margin stratigraphy.