Growth structures : examples of integrated sedimentological and structural-geological basin analysis

Abstract

This thesis is based on research in the interface oftwo geological disciplines, sedimentology (the study of sediments, like sandstone and limestone) and structural geology (the study of rock deformation). Sedimentation is influenced by cosmic and climatological processes, sea-level change, and deformation of the earth's crust. Prerequisites for the generation of sediments are uplift and erosion in source areas, and subsidence and deposition in basins. Contrary to these vertical motions, deformation is often acting (sub)parallel to the earth's surface, i.e., horizontally (tension, compression, wrenching). To understand the influence of the deformation on sedimentation, the horizontal effects of deformation have to be translated into vertical ones. Growth structures are deformations active during sedimentation. They cause irregularities in the pattern of basin infilling. They represent the first deformation of sediments, and are generally overprinted by several later phases of deformation. It, therefore, requires stepwise restoration and elimination of these later structures to detect the growth structures. Since sediments can be dated, in a relative sense with fossils and absolutely (with the decay of radioactive elements in minerals), growth structures can be calibrated chronologically. This allows to determine variations in the rate of displacement, direction, and intensity of the deformation process, which is often difficult to detect directly by structural analysis. The study of growth structures, therefore, comprises both the analysis of the mechanism of sedimentation and that of deformation. This kind of research is generally done separately by different scientists with diverse aims, and at different scales. Combination in retrospect of such data easily leads to misinterpretations. The thesis emphasizes the advantage of integrated analysis from the very beginning of a research project in the combined fields of sedimentology and structural geology. The printed part considers that interrelationship with examples from research in mobile belts of a wide range of ages, carried out with colleagues and many MSc students: the early Archaean (± 3450 million years ago) of NW Australia (Pilbara) and South Africa (Barberton Mountains), the Proterozoic (± 1750 Ma) of the Mount Isa area in Queensland (NE Australia), the Carboniferous (± 300 Ma) of the Cantabrian Mountains in NW Spain, and the Eocene of the southern Pyrenees (± 50 Ma). The results of these studies have been published in the period 1989- 1998. A choice of five articles forms, on CD, part of this thesis.