Diverse Geology Of Atlantic-Type Continental Margins And Some Possible Implications For U.S. Offshore Atlantic

ABSTRACT Recognition of the plate structure of the lithosphere and consequent appreciation of the high lateral mobility of continents has revolutionized geology in the last decade, but understanding that Atlantic-type margins have formed at continental rupture has been followed by increasing awareness of along-strike diversity in their development. Factors generating this diversity include: the distribution of hotspots within the continent at rupture; distribution and pattern of failed-rift systems at the continental margins and related distribution of continental headlands and embayments; the occurrence of ridge and transform sectors of the continental margin; occurrence of deltas, prograding down-failed rift systems; distribution of salt deposits formed at ocean opening; temporal variations in rates of clastic sediment supply; and oceanic and atmospheric circulation. Geological development of Atlatic type margins also varies with climate. Subsidence at Atlantic-type margins is controlled mainly by age, rate of sediment supply, and eustatic sea level. At the present stage in the study of Atlantic-type continental margins, general understanding of the basic processes that operate during development has been achieved, but because-of the variety of the processes involved, Atlantic margins show great along-strike diversity so that there is less need now for refinements of general continental margin models than for comprehensive analysis of the best known areas to see how the interaction of the different processes can produce varied results. Thorough mapping of the distribution of failed-rift systems around the oceans seems an essential first step in understanding diversity. INTRODUCTION Knowledge of the geology of the continental margins has mushroomed as the attention of the petroleum industry has turned to offshore exploration and production in increasingly deeper waters. The idea that Atlantic-type margins have been produced by continental rupture and by the development of oceans on the sites of rift-valleys has proved both demonstrable and useful in the light of the information explosion represented by millions of miles of marine seismic lines and thousands of exploration wells. In this paper emphasis is on the varied ways in which intercontinental rift systems may develop through rupturing of the continental margin into mature ocean boundaries and on the diverse influences that affect the geological evolution of these mature boundaries. TRANSITIONAL CONTINENTAL BOUNDARIES Consideration of oceanic borders in the North and South Atlantic leads to the conclusion that it is generally inappropriate to attempt to locate the boundary between continent and ocean (as shown, for example, in Fig. lA) to within a few kilometers at a place defined by such criteria as bathymetric depth as, for example, Carey8 and Bullard et al.2 did, or at particular gravity or magnetic -anomalies, for example, as Rabinowitz did. Although such criteria are useful in helping to determine the pre-Atlantic opening fit of the continents on a gross scale, the existence of lithosphere that is neither typically continental nor typically oceanic on most traverses across Atlantic continental margins (for example, Fig. lB) renders the precise location of the continent/ocean boundary unrealistic. Two kinds of transitional lithosphere are distinguished close to Atlantic continental margins.