Masters Theses

Date of Award

12-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Steven G. Driese

Committee Members

Richard W. Arnseth, Thomas W. Broadhead

Abstract

The sedimentology of the Clinch Sandstone (Lower Silurian) was analyzed at Powell Mountain, southwestern Virginia in order to ascertain the paleoenvironmental factors leading to its deposition. At this location, the Clinch is divided into the lower Hagan Shale Member and upper Poor Valley Ridge Sandstone Member, each of which is further subdivided into four facies representing distinctive depositional environments. Facies A/HS , a thick calcareous quartzarenite, directly overlies the subaerially exposed tidal flat deposits of the Upper Ordovician Juniata Formation. Sub-facies B1/HS and B2/HS of the Hagan are differentiated by subtle differences in lithology, sedimentary structures, bedding geometry and mineralogical character. Sub-facies B1/HS is composed of very thinly interbedded, very fine-grained, calcareous quartzarenite and interbedded shale. Sub-facies B2/HS consists of very thinly interbedded, very fine-grained, calcareous quartzarenite, wackestone/packstone units and interbedded shale. Facies C/HS is a medium to dark-gray, fissile to slightly blocky shale. Three individual units comprise Facies D/HS consisting of a bryozoan packstone, a fossiliferous quartzarenite and a ferruginous sandstone conglomerate.

The Poor Valley Ridge Sandstone is also divided into four facies. Facies A/PVR consists of medium-bedded, compound cross-stratified, fine-grained quartzarenite containing intercalations of shale less than 2 can thick. Ubiquitous trough cross-stratification is enclosed within large-scale second-order bounding surfaces. Facies B/PVR is composed of a heavily bioturbated, fine to coarse-grained, clay-rich, quartzarenite. Facies C/PVR is composed of fissile to slightly blocky shale. Facies D/PVR is the lowermost facies within the Poor Valley Ridge, and it constitutes the transitional contact with the underlying Hagan Shale. The lithology of D/PVR consists of interbedded subarkose to quartzarenite and intervening shale, with the former exhibiting mineralogical, textural, geometrical and sedimentological changes upward within this facies. Capping the Clinch Sandstone at Powell mountain is the maroon and gray mudstone, siltstone and sandstone of the basal Rose Hill Formation. Facies of the Hagan Shale represent deposition within laterally contiguous subenvironments on a shallow-marine shelf. Deposition took place within the transition zone and on the middle shelf, both above and below fairweather wave base.

Facies A/HS is interpreted to represent the shallowest, most nearshore deposits of the Late Ordovician, Early Silurian marine transgression. Facies B/HS represents deposition in a marine, sublittoral, storm-shelf setting. The thin sandstone and wackestone/packstone units show several features consistent with deposition from episodic, high-energy storm events punctuating the background sedimentation of mud and fine silt. Facies C/HS represents similar but more prolonged periods of quietwater sedimentation than the interbedded shale of B/HS. Facies D/HS represents deposition by rippled or megarippled sand shoals which occasionally prograded into the offshore area of Hagan deposition.

The four facies of the Poor Valley Ridge Sandstone are interpreted to have been deposited in various subenvironments within a marine-shelf sand wave setting. The large-scale, second-order bounding surfaces of Facies A/PVR represent foreset planes of giant sand waves; internal trough cross-stratification was formed by the migration of sinuous crested dunes capping these large, flow-transverse bedforms. Facies B/PVR represents fairweather deposits during times of reduced sand wave activity. Facies C/PVR represents suspension-fallout deposits which collected in the troughs between sand waves. The upward coarsening/thickening sandstone-and-shale sequence of Facies D/PVR is interpreted as a transitional deposit representing the distal zones of Facies A/PVR. A zone of small sand waves was the first of the Poor Valley Ridge progradational clastic wedge which covered Hagan sediments during the Lower Silurian transgression. As progradation culminated later in the Silurian, tidal flat and offshore sand shoal deposits of the Rose Hill Formation covered the area.

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