EARTH'S SEDIMENTARY ROCK SHELL: BIG QUESTIONS, MODEST ANSWERS

The Earth's sedimentary rock shell provides fascinating insights into secular changes in the terrestrial hydrosphere,atmosphere, biosphere, and lithosphere as well as detailed information about the evolution of mobile belts. Raymond Siever inspired colleagues and students to approach these topics with open minds and imagination. This led to better understanding of both the geochemistry of sedimentary rocks as well as what sedimentary petrology can tell us about the origin of mountain systems. Beginning in the later 1960's, essential components of classical geosynclines were reinterpreted in the context of plate tectonics. The bulk chemistry of sedimentary rocks filling Phanerozoic mobile belts only crudely mimics that of sedimentary prisms along modern continental margins. Accumulation rates for modern continental rises and terraces are comparable to their supposed analogues, ancient eugeoclines and miogeoclines. Younger (Proterozoic and especially Phanerozoic) sequences are richer in silica, potassium, and calcium; Archean sequences contain more aluminum, sodium, and magnesium. Recycling produces a total volume of sedimentation through time sufficiently large enough to contribute to the growth of continental crust. Plate tectonic models readily permit the sediment fill of various basin types to be predicted and vice versa. Sandstone framework mineralogy expressed using triangular plots developed by Dickinson and colleagues are a critical analytical tool. Case studies from the Central and Southern Appalachians illustrate their effectiveness in better understanding simple, Wilson cycle belts. Mobile belts that arguably consist of problematic displaced exotic terranes, for example, the Newfoundland and Quebec-Maine Appalachians, are less easily deciphered.