STRATIGRAPHIC ANALYSIS OF PARASEQUENCES FROM THE BANGOR LIMESTONE (MISSISSIPPIAN), ALABAMA

Often massive rock units contain small-scale cyclicity that can be detected through careful lithologic and faunal analysis. We examined small-scale, sequential changes within a fifty-foot section of the Bangor Limestone exposed near Collinsville, AL. The lithology of the section is primarily limestone with occasional chert nodules and a paleosol. The Bangor Limestone at this locality contains abundant pelmatozoans, brachiopods, forams, and bryozoans along with rare corals and echinoids. The section in question was chosen because of the presence of two distinct marker beds that constrain it environmentally. At the bottom of the section are coral beds which contain multiple colonial rugose corals, while a paleosol marks the top of the section. These marker beds suggest an overall shallowing upward cycle culminating in exposure at the paleosol.

The section was measured using a Jacob Staff and samples were collected at 1 foot intervals. After the hand samples were examined lithologically, thin sections were made of every other sample. Each thin section was analyzed under a polarizing microscope to determine overall petrology and point counts were made of fossils and other sedimentary particles. The grains were then analyzed using detrended correspondence analysis (DCA) in order to identify a faunal/sedimentary particle gradient to compare with the petrologic results.

The petrologic stratigraphy is comprised of packstones and grainstones with a few heavily dolomitized samples. When analyzed in thin sections, packstones and grainstones seem to be arranged into several cycles with energy increasing upsection beneath a dolomitic caprock that likely marks an exposure surface. The detrended correspondence analysis indicates a gradient between brachiopods and forams in the lower energy samples and ooids and pelmatozoan ossicles in higher energy samples. The DCA gradient supports the petrology-based sequential boundaries as it shows a shift toward forams and brachiopods in the lower energy section of the parasequences, while showing a shift to pelmatozoans and ooids as energy increases. Therefore, we found that in this massive rock unit there are multiple parasequences which can be observed by a close evaluation of the petrology and distribution patterns of the sedimentary grains.