Tectonic Vs. Eustatic Unconformities: Examples from Upper Ordovician and Lower Silurian Rocks, Southern Appalachians
Joachim Dorsch, Steven G. Driese, Anne R. Gogola, J. C. Bolton, Richard K. Bambach, 1994. "Tectonic Vs. Eustatic Unconformities: Examples from Upper Ordovician and Lower Silurian Rocks, Southern Appalachians", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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The Martinsburg-Reedsville, Juniata-Sequatchie, and Tuscarora-Clinch-Rockwood-Red Mountain Formations, (Upper Ordovician to Lower Silurian) comprise the major part of the Taconic clastic wedge in southwestern Virginia, eastern Tennessee, northeastern Alabama, and northwestern Georgia. This clastic wedge constitutes the fill of a peripheral foreland basin which formed in response to thrust loading. The well-known disconformity at the base of the Tuscarora Fm. in the eastern Valley and Ridge of Virginia was recently traced as a paraconformity to the northwest (i.e., basinward). A white, thick-bedded "upper" Tuscarora abruptly overlies the Martinsburg or Juniata Formations. with a distinctive basal conglomerate in the southeastern Valley and Ridge; a yellowish, thin-bedded, more mudstone-rich "lower" Tuscarora interpreted as facies-equivalent for parts of the Juniata Formation, occurs beneath the "upper" Tuscarora in the northwestern Valley and Ridge. This regional unconformity, with maximum lacuna towards the southeast, divides the clastic wedge into two parts. The nature of the unconformity along and across structural strike, its regional extent and disappearance towards Tennessee, and the overall tectonic setting all favor a tectonic origin for this unconformity. Basin rebound following the cessation of thrusting, concomitant with erosional/tectonic unloading of the Taconic orogen farther to the southeast, is suggested as the causal mechanism for the erosional unconformity. This interpretation contrasts markedly with stratigraphic relationships observed in eastern Tennessee, northeastern Alabama and northwestern Georgia, where the equivalent stratigraphic interval exhibits a small lacuna between the Juniata-Sequatchie and Clinch-Rockwood-Red Mountain Fms. Basin development was not related to collisional tectonism, but represents a "left-over moat" of the Blount-phase. Paleosols occur within the upper Juniata Fm. in the eastern Valley and Ridge, sharply overlain by a 0.6- to 3-m thick, very fine- to medium-grained and phosphatic, highly bioturbated, transgressive sandstone of marine shoreface origin that occurs in the basal Clinch, Rockwood and Red Mountain Fms. This regionally correlative sandstone, informally known as the "basal transgressive sandstone" (bts), was deposited along ravinement surfaces during erosional shoreface retreat; it therefore documents a lowstand followed by a eustatic rise. To the west, the bts overlies open-marine shale and limestone deposits containing a typical Ashgillian brachiopod fauna; shelf strata above the bts contain a Lower Llandovery (A1-2) brachiopod fauna. Hence, the magnitude of the lacuna appears decrease in a northwesterly direction, where the Ordovician to Silurian boundary is nearly conformable.
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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.