The Evolution of the Earth's Crust and Sedimentary Basin Development

Abstract

ABSTRACT The earth's crust is a thin, dynamic shell that changes in both thickness and composition through time. It is postulated that the changes that alter the crust are primarily the result of subcrustal processes and lateral interactions of crustal plates. The type of crust that underlies a sedimentary basin determines the physical framework, stability, manner of structuring, and conditions of sedimentation and environment throughout the evolution of the basin. Thus, as sediments are deposited, they record the tectonic history of the basin. These stratigraphic data can be used to develop conceptual, genetic models that put the evolution of oceanic, continental, and transitional types of crust into perspective. An orderly cycle of crustal evolution is proposed which suggests that oceanic crust is thick end and continental crust is thickened and thinned by a number of natural processes. The resulting transitional crustal types, which represent intermediate steps in the continuum, occupy a realm between thin, basic oceanic crust and thick, acidic continental crust. These transitional, somewhat unstable crustal types host most of the world's sedimentary basins. An attempt is made to step back from the detail of the complex interplay involved in the process of crustal genesis and sedimentary basin development and' look at the overall natural system. A set of hypothetical, genetic models depicting the proposed origin of various crustal types is presented so that geophysical data can be compared and tested against them. THE CRUSTAL SETTING OF SEDIMENTARY BASINS Sedimentary deposits greater than 2000 meters thick accumulate in depressions within the major continents or adjacent to their margins (Fig. 1). Basins within the major continents contain a large percentage of Precambrian, Paleozoic, and Mesozoic sediment, which furnishes a record of crustal evolution during these earlier periods of the earth's development. Some of these older rocks have been metamorphosed to such a degree that they can no longer be considered part of the sedimentary section, but rather are more closely related to the underlying crystalline crust. Basins on the marginal shelves, slopes, and rises of the Atlantic and Indian Oceans contain sediments of Mesozoic and Tertiary age and form some of the thickest sediment accumulations in the world. Most of the sedimentary basins around the margins of the Pacific Ocean have formed more recently and furbish valuable clues concerning the Tertiary crustal history of that part of the world. Indeed, many of the Pacific margins are presently in an active stage of basin formation and provide modern analogs to ancient mechanisms of crustal evolution. Most of the world's sedimentary basins overlie crustal types that are neither purely continental nor purely oceanic in terms of thickness and composition. They generally develop upon thinned continental crust or upon accreted, constructed continental crust (Fig. 2).