Doctoral Dissertations

Date of Award

12-1992

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Steven G. Driese

Committee Members

Robert J. Elias

Abstract

The Silurian Red Mountain Formation (RMF) was deposited along the southernmost extent of an interior seaway. Paleogeographic reconstructions differentiated the region into four major physiographic elements: 1) the vestiges of Taconic "highlands", located to the present-day east; 2) an elongate interior seaway; 3) an extensive epicontinental sea that periodically inundated the North American craton; and 4) an expansive ocean to the present-day south. The strata of the Red Mountain Formation in northwest Georgia and northern Alabama are predominantly composed of mixed siliciclastic and carbonate strata interbedded with minor ironstone. Five lithofacies were differentiated within the Red Mountain Formation: 1) a basal sandstone fades; 2) a heterolithic sandstone and mudstone fades; 3) an amalgamated fine-grained sandstone facies; 4) a cross-bedded conglomeratic sandstone facies; and 5) a silty, fossiliferous bioclastic limestone facies. The lithologic character of each facies and the lateral and vertical distributions of the different facies in the Red Mountain Formation are, for the most part, similar throughout the study area. There are, however, some notable regional variations. First, a thick succession of the cross-bedded conglomeratic sandstone facies can be identified in the the more eastern regions of the study area but is very thin or absent in the more central and western portions of the area. Secondly, the rocks in the northern exposures are much less fossiliferous than the rocks in the exposures to the west and south. Facies maps indicate at least three depositional stages within the Red Mountain Formation: 1) a basal transgressive sequence; 2) a lower sandstone-dominated shallow-marine depositional sequence; and 3) an overlying mudstone-dominated depositional sequence. The paleoshoreline probably existed to the east of the study area, extending approximately northeast-to-southwest. The strata within each depositional sequence along the eastern border of the basin were deposited under more proximal conditions. Strata within depositional sequences that accumulated further west were deposited at a greater distance from the shoreline and closer to the edge of a carbonate-rich environment on the western flank of the seaway. The basal transgressive sequence, composed of the basal sandstone facies, is interpreted to have formed during the glacio-eustatic sea-level rise that has been identified at the beginning of the Silurian. The sandstone-dominated shallow-marine depositional sequence developed on the shelf during the early stages of the sea-level rise and persisted from early to late Llandovery time. The sequence is predominantly composed of fine-grained siliciclastic and mixed carbonate and siliciclastic deposits which were dispersed across the shelf by storm-related, bottom-directed flows as well as fairweather currents. A thick wedge-shaped conglomeratic body that formed during westward-directed progradational event developed just below the middle part of the sequence as tidal inlet deposits (ie., conglomeratic sandstone facies) were superimposed on the shelf. The mudstone-dominated shelf sequence includes all strata above the uppermost sandstone body. I hypothesize that depositional conditions during formation of this part of the Red Mountain Formation precluded significant accumulations of the amounts of fine sand required to produce extensive storm deposits or well-developed shoals. A sea-level curve, constructed from lithostratigraphic and biostratigraphic information, indicates that at least two distinct regressive events occurred during the deposition of the RMF. The earliest event is represented by the progradational pulse that occurred in the sand-dominated shelf sequence. A second "shallowing" event, represented by the uppermost sandstone body, is biostratigraphically well constrained. Palynomorph samples taken above and below the sandstone body yield palynomorphs which existed during the late Landovery (C4). I have not assigned water depths (ie., "highstands") to the rocks which were deposited between the sea-level "lowstands" because I am concerned about the reliability of the sedimentological and paleontologica criteria that are being used to assess paleobathymetric and paleoenvironmental conditions. The depositional conditions during the accumulation of mudstones, often associated with sea-level highstands (e.g., see Aigner, 1985), require corroborating information to establish the conditions under which they were deposited (e.g., ichnofossils, taphonomic character of skeletal debris, etc.). In the middle part of the Gadsden section, for example, conditions of the seabed during the deposition of a succession of mudstone have been determined from the orientation of solitary rugose corals imbedded in the strata. Equal-area plots on the orientations of preserved corals indicated that they had been reworked by wave activity as well as undirectional flow. In addition to their orientations, the most mechanically stable positions for curved corals were determined. Most of the corals in the mudstone had been preserved in mechanically stable positions and thus had been deposited on firm or hard substrates. The sedimentological character is described for two horizons containing Costistricklandidia lirata, a species used in construction of paleontologically-based sea-level curves for the Early Silurian (Llandovery). Stratigraphic and sedimentologic characteristics of the rocks associated with the brachiopods suggest water depths of 15-150 m during times of low rates of terrigenous influx. The Costistricklandid assemblages from the lower horizon are composed of extremely large individuals associated with a diverse population of large favositid and syringoporid corals and therefore are interpreted to have lived in a protected environment. In an overlying horizon, costistricklandids occur in growth position at the base of a thick siliciclastic interval. These brachiopods lived in a storm-dominated environment and were buried in situ by the rapid influx of sediment associated with a passing storm.

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