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Abstract

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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