MACROSTRATIGRAPHY OF NORTH AMERICA

Abstract

The geological record is a three-dimensional mosaic of gap-bound rock bodies that, at any given scale of temporal resolution, were emplaced more or less continuously. At any geographic location, the irregular alternation of processes responsible for the formation and destruction of rock bodies results in the preservation of hiatus-bound rock packages that have a distinct time of first occurrence (base, or oldest portion), a time of last occurrence (top, or youngest portion), and a suite of defining characters (e.g., lithologies, thickness, fossils, etc.). Many important aspects of the geologic record can be quantified by compiling the stratigraphic ranges of such gap-bound rock packages. These include the quantity and spatial and temporal distribution of preserved rock, rates of rock formation, sequence stratigraphic architecture, and area-weighted rates of expansion and contraction/erosional truncation of rock emplacement settings. This approach to characterizing the rock record is scalable, permitting application to records encompassing entire continents and hundreds of millions of years as well as individual basins and geologically short time intervals. To illustrate the utility of this approach and to provide a new high-resolution analysis of the temporal structure of the geologic record, gap-bound rock packages in the continental United States and southern Alaska were compiled directly from the American Association of Petroleum Geologists Correlation of Stratigraphic Units of North America (COSUNA) charts. The COSUNA charts were assembled at a temporal resolution of approximately 1-3 million years (m.yr.) in the Phanerozoic and contain 4173 gap-bound rock packages. Many important aspects of the North American geologic record are revealed by the temporal distribution of gap-bound rock packages, including rock quantity, long-term rates of sediment accumulation, and basin turnover. The durations of gap-bound sedimentary successions are approximately exponentially distributed, with a mean duration of 25.2 m.yr. and a median duration of 16.9 m.yr. The probability of initiation and truncation among sedimentary packages does not increase or decrease substantially during the Phanerozoic, but these parameters do vary on shorter timescales in response to tectonically and glacioeustatically driven changes in sea level. The largest increase in the rate of sediment truncation occurs at the end-Permian, which marks a clear and fundamental temporal discontinuity in the sedimentary record of North America. Smaller discontinuities occur at the end-Ordovician, the end-Triassic, and the end-Cretaceous. Lithologically, Cambrian-Mississippian sedimentary successions are dominated by carbonates, and post-Paleozoic successions are dominated by terrigenous clastics. The quantity of preserved rock, the carbonate/siliciclastic ratio, and the dominant lithology comprising terrigenous clastics all vary substantially from interval to interval during the Phanerozoic, indicating that processes governing the formation and destruction of sedimentary rocks vary on timescales of <5 m.yr. © 2006 by The University of Chicago. All rights reserved.