Continental Margins from Viewpoint of the Petroleum Geologist

Major features of the earth’s surface are its continents and its ocean basins—which in turn reflect fundamental differences between its continental crust and its oceanic crust. The broad zone of contact between continental and oceanic domains has been called the continental margin. Geomorphically, the margin is expressed by the continental slope, but also includes the seaward part of the submerged continental shelf and the landward part of the continental rise. Geologically, the margin includes the zone of lateral change in the lithosphere marking the oceanward limits of characteristic continental crust. Many of the most exciting events in the history of our planet have taken place at this contact zone, between continental and oceanic crust, and between continents and oceans; the continental margin represents the stage where, throughout earth history, this drama has been played. Important elements of the continental margin are the outer shelf, the borderland, the marginal plateau, the slope, the base of the slope, the rise, and the marginal trench. The nature and the origin of these features and their sediments are of vital importance to petroleum geology. Of particular interest to the petroleum geologist are the sediment-rich, semi-enclosed basins or seas associated worldwide with the continental margin; the structural barriers and reefs commonly developed near the rim of the continental slope; the numerous manifestations of gravity tectonics and diapirism on the slope; and the growing evidence for impressive vertical movements associated with continental margins. Great advances in our understanding of the processes active at the continental margins have come from the subsea geologic and geophysical studies of the last decades, and rapid additional progress may be expected from the stimulus of “the new global tectonics”; but current hypotheses are still largely in a developmental stage. Factual data are still woefully inadequate, and the need for additional continuous refraction seismic profiles across the margin is critical. Moreover, continuing studies are needed, not only to better know the margins of the present, but also to better identify the margins of the past. Of special interest are the extent and age of transmarginal features. For the petroleum geologist, it is significant that through the ages the continental margin has been the great mixing bowl in which has been brewed most of the world’s petroleum and from which most of its petroleum production to date has been derived. The continental margin should be the fruitful meeting ground of the petroleum geologist, the geologist of the oceans, and the student of earth history.