Tectonic, Climatic, and Biotic Modulation of Lacustrine Ecosystems—Examples from Newark Supergroup of Eastern North America
Paul E. Olsen, 1990. "Tectonic, Climatic, and Biotic Modulation of Lacustrine Ecosystems—Examples from Newark Supergroup of Eastern North America", Lacustrine Basin Exploration: Case Studies and Modern Analogs, Barry J. Katz
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Rift-related early Mesozoic lacustrine strata of the Newark Supergroup provide a background for exploration of general concepts of lacustrine paleoecology and stratigraphie architecture. The large-scale tripartite sequence of depositional environments (i.e., fluvial basal part, deep lacustrine middle part, and shallow lacustrine or fluvial upper part), commonly seen in lacustrine deposits, including the Newark, can be quantitatively modeled as the result of relatively simple interaction between basin filling and subsidence. Whereas tectonic processes produced the rifts and the maximum depths of the lakes they contain, high-frequency fluctuations in the depths of rift lakes are largely controlled by climate. Milankovitch-type climatic cycles caused by variations in the Earth's axis and orbit produce lake-level cycles with periods of 21, 41, 100, and 400 k.y. The magnitude and mode of these lake-level changes are governed by position within the climate system and by orography.
Three major classes of lacustrine facies complexes are recognized in the Newark Supergroup. These are, in order of increasing overall dryness, the Richmond, Newark, and Fundy types. Each is characterized by different suites of highstand and lowstand deposits, different sedimentary cycle types, and different amounts of organic carbon-rich rocks. Organic-carbon content of the strata is largely a function of ecosystem efficiency, which, in turn, responds to lake depth. Finally, the long-term trends in evolution of bioturbators must be taken into account because they affect not only the carbon cycle and oxygen state within lakes but also our ability to interpret the metabolic state of ancient lake systems.
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Lacustrine Basin Exploration: Case Studies and Modern Analogs
Lacustrine environments are a major contributor of petroleum source rocks. Lacustrine source rock prediction is, however, influenced by numerous, complex variables governing lake sedimentation. Current predictive capability can be improved by attempting to map essential climatic variables to limit in space and time the area of lacustrine source rock exploration. Climatic characteristics that govern lake occurrence and the potential for stratification have been investigated with a General Circulation Model of the atmosphere for the present and for the mid-Cretaceous. In this analysis, the distribution of areas with a positive water balance first is used as an indicator of the distribution of areas conducive to lake formation. Second, the distribution of areas that experience large annual climatic variations is used as an indicator of the distribution of lakes that are less likely to be stratified and, hence, less likely to be sites of high organic-carbon preservation. Four factors used to define large climatic variations include (1) seasonal temperature cycle in excess of 40°C; (2) seasonal temperature extreme of less than 4C°; (3) average seasonal differences in precipitation minus evaporation balance in excess of 5 mm/ day; and (4) distribution of mid-latitude winter storms. Evidence is presented to support the capability of climate models that add insight into lacustrine source rock prediction by simulating geographic regions conducive to lake development and to stratification and organic-carbon preservation