The Petrology of Lower Pennsylvanian Sewanee Sandstone, Lookout Mountain, Alabama and Georgia

Abstract

ABSTRACT A petrologic study of texture, including apposition fabric and mineralogical composition was made of the Lower Pennsylvanian Sewanee Sandstone in order to deduce petrogenesis. Field studies of stratigraphic and primary structural characteristics supplemented these investigations. Results indicate that the Sewanee, a quartzose sandstone, was formed under tectonically rather stable conditions in transitional depositional environments. Its sediments were mainly derived from the east in regions of the present Piedmont by a south-southwesterly regional sediment transport system. The source rocks were composed mainly of igneous plutonic rocks with metamorphic rocks of secondary importance; pre-existing sediments were only minor contributors. No textural indication of deep burial is to be fo nd and diagenetic changes are confined to some welding and suturing of adjacent quartz grains and the development of sericitic intergrowths between grains and matrix. A method for sampling and measuring the preferred orientation of sand grains is described. The statistical study of the variability of the mineral constituents of the Sewanee Sandstone, both locally and regionally shows that the rock body is extremely homogeneous. Regionally, the mean directions of both preferred grain orientation and cross bedding are fairly consistent and coincident with each other. The variations of apposition fabric, however, are rather greater locally than regionally. This is related to the environment of deposition.