ABSTRACT

The evolution of a passive continental margin (PCM) isthe result of a combination of high- and low-frequencyinterrelating geologic processes. The term "geologic process"is used here to mean those phenomena and forces thatare recognizable in the present and past as unique and distinct and appear to act alone within the natural world. Different processes have differing intensities (amplitudes) and periodicities (wavelengths). Geologic processes are arranged here in an exponential hierarchy ranging from 10⁰ to 10⁸ yr to aid visualization. Processes are assigned to the hierarchical level in which that process is most obvious, e.g., seasonal changes are obvious on the annual (10⁰-yr) level and PCM evolution on the hundred-million-year (10⁸)level. These assignments are general and crude in that geologic process subtleties may well be obscured.

Low-frequency processes generally have periodicities from 10⁶ to 10⁹ yr. Tectonic and structural processes of such periodicities often give the appearance of greater geologic import than do similarly aged sedimentation, oceanography, and climatic processes. Separating and reuniting of continental plates in a Gondwanaland-type exercise may take 10⁹ yr. Initial rifting as in East Africa lasts 10⁸ years. Similarly, the evolution of an intraplate basin may take 10⁸ yr or multiples thereof. A single subsidence or uplift event may last 10⁶ to 10⁷ yr. High-frequency processes appear to be prominent at periodicities ranging from 10⁰ to 10⁷ yr. As here defined, high-frequency processes are those that are recorded but no longer active in the sediment accumulations and tectonic evolution within a PCM. Sediment formations are the result of wind, weather, and waves, and the forces that create them.

End_of_Record - Last_Page 766-------