Stratigraphy and the World Tectonic Model

Abstract

The character and relationships of sedimentary formations are clues to the roles of sedimentation and tectonism in the geological cycle. The sites of sediment (and volcanic) accumulation—depositional basins—that are preserved in the record are all to a greater or lesser extent determined and controlled by earth movements. In the tectonic framework of sedimentation we see the combination of variously moving or relatively immobile tectonic elements in and around a depositional basin. There are about 600 depositional basins in the world today. They vary greatly in size and shape, in their contents and in the nature of their growth or shrinkage. Within the basin itself the nature and thickness of the sediments may reflect the contemporary tectonic behaviour of the earth’s crust locally. The contrast between sedimentary formations of stable shelves and platforms and of active tectonic regions and mountain belts is striking. Isostatic adjustment by uplift following orogeny explains the long-continuing role of orogenic belts as sources of sediment for new basins of deposition, but the initiation of the orogenic cycle has presented a problem. In recent years the new plate-tectonic models of the earth’s crustal behaviour have enabled us to understand a possible means by which some of the largest and longest-enduring basins of deposition have originated and evolved.