Tectonic Controls on the Sedimentary Record of the Early Mesozoic Newark Supergroup, Eastern North America
Joseph P. Smoot, 1994. "Tectonic Controls on the Sedimentary Record of the Early Mesozoic Newark Supergroup, Eastern North America", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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The Newark Supergroup consists of continental sedimentary and igneous rocks of Late Triassic to Early Jurassic age that fill a series of exposed half-graben basins along the eastern coast of North America. The rift basins formed along reactivated Paleozoic faults during extension that later led to the opening of the Atlantic Ocean. The basins had internal drainage systems, with base level largely independent of sea level. In general, each basin exhibits a similar vertical succession of sedimentary environments, although the ages of the deposits may differ from basin to basin: 1) Thin (<200 m), discontinuous immature fluvial conglomerates unconformably overlie Paleozoic or Precambrian rocks; these deposits reflect local provenance and small drainages. 2) Moderately thick sequence (500-1000 m) of conglomerate, crossbedded sandstone, and siltstone reflecting braided-river deposition with greater maturity than the basal conglomerates which they abruptly, and possibly unconformably, overlie. 3) Thick sequence (500-2000 m) of medium to fine sandstone and siltstone reflecting deposits of meandering streams and vegetated muddy plains. 4) Very thick sequence (1000-6000 m) of lacustrine mudstone and siltstone, in moist basins forming cyclic patterns of deep-water to subaerial deposits. In addition to this succession, alluvial-fan deposits intertongue with lacustrine and fluvial deposits along the faulted basin margins.
The vertical succession of depositional environments reflects: 1) the initial erosion surface prior to rifting; 2) basin subsidence and the localization and capture of regional drainages; 3) progressive loss of stream power as fault-related uplift constricted outlets and sediment aggradation lowered gradients; 4) partial or complete hydrographic closure of the basins, as faulting restricted outlets. Fault-related uplift created highlands producing alluvial fans sometime before the end of the third episode of deposition. The timing of these events for each basin probably depends on the relationship of the regional extensional field with the orientation and distribution of Paleozoic faults and preexisting drainages. A progressive vertical decrease in the abundance and thickness of deep-lake laminites within cyclic lacustrine strata may reflect an increase in basin area during subsidence. Three volcanic events extruding large volumes of flood basalt occurred apparently simultaneously in at least three basins and may have been associated with periods of increased subsidence. In several basins, the final stage of sedimentation is fluvial, suggesting a decrease in subsidence or a lowering of the outlet. Tectonic activity may also be reflected by decimeter-scale sequences of grain-size changes in alluvial-fan deposits and by radical shifts in the locations of marginal fluvial deposits. Climatic fluctuation, however, is the most important contributor to development of lake cycles and may also account for some fluvial variability.
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The collected volume begins with a brief perspective by one of the conveners, followed by articles in order of increasing stratigraphic age. Eustatic sea-level changes and tectonic warpings of basins are competing mechanisms for explaining many stratigraphic patterns. The model for sea-level changes should be developed first for a basin, since it is allocyclic and leads to a series of time bands in the strata. The residual effects should then be modeled for tectonic patterns affecting the depositional processes. Doing the reverse limits time constraints on the tectonic warping models and will blur the resolution of detailed time surfaces in the strata. Case histories of situations with both tectonic warping and time surfaces marked by sea-level events will lead to improved interpretations of earth history.