Date of Graduation

1997

Document Type

Dissertation/Thesis

Abstract

The Tonoloway Limestone (Pridolian) represents a large part of the Siluro-Devonian carbonate interval present throughout most of the central Appalachian Basin. The depositional evolution of this unit is documented in detail on a regional scale using a basin analysis approach which combines facies analysis with applied sequence stratigraphy. Overall results show that deposition occurred over 1-3Ma in an epeiric seaway which changed briefly into a ramp-open shelf-lagoon transition. Results of sequence stratigraphic analysis show that the upper two-thirds to three-fourths of the Tonoloway records one complete Type I depositional sequence (S1), which contains a Lowstand Systems Tract followed upward by Transgressive- and Highstand-to-Regressive System Tracts. The lower one-third to one-fourth of the Tonoloway records the upper part of the Highstand-to-Regressive Systems Tract at the top of the underlying sequence (S1). Each of these systems tracts corresponds to a specific increment on a relative sea-level curve showing an overall regressive-transgressive-regressive pattern. Based on the succession of regional changes represented by the sequence stratigraphy in facies environments and in relative sea-level direction, four major stages of deposition are recognized. Regressive Stage 1 occurred in the initial epeiric basin and corresponds to the Highstand-to-Regressive Systems Tract recorded in the lower part of the Tonoloway. During the Lowstand Stage which followed, siliciclastic sediments from land areas to the east prograded across the restricted lagoon-sabkha deposits during short-term drops in sea-level. By the end of this stage, the southern area had begun to hinge downward toward the southwest, and a ramp-open shelf-lagoon transition began to develop from SW to NE along the basin axis. During the Transgressive Stage, rising sea-level operated simultaneously with ramp development to produce significantly increased water depths on the southwest. Here, depths reached storm wave base (40m) over the ramp and 30m over the open shelf. Carbonate tempestite deposits dominated the ramp, and high-energy skeletal shoal-to-backshoal deposits formed on the shelf. A shallow lagoon persisted northeast of the open shelf, and the SW-to-NE incursion of fresh marine water brought increasingly open marine conditions into this area from the shelf during the Highstand Stage. Following this transgressive peak, the entire area returned to a shallow, restricted epeiric seaway during Regressive Stage 2, and sea-level dropped toward another lowstand as Tonoloway time closed. Comparison of the Tonoloway relative sea-level curve with available eustatic curves indicates that eustatic sea-level activity controlled these stages. Sequence S2 records one complete cycle of eustatic sea level oscillation with a period of 1-3 Ma which began at lowstand in the latest Ludlow and ended in the middle of the Pridoli, with highstand in the early Pridoli. This oscillation is in the range of a third-order Vail cycle. The remainder of the Tonoloway beneath S2 records the regressive portion of the preceding eustatic cycle, which had reached highstand in the late Ludlow. (Abstract shortened by UMI.).

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