Stratigraphic Evidence for Tectonically Driven Subsidence during Deposition of the Middlebury Formation (middle Ordovician), Western Vermont

The Middlebury Formation is a Middle Ordovician limestone deposited on the margin of Laurentia during the collision of the Ammonoosuc Arc with Laurentia. Tectonic processes such as subsidence can have an effect on the forearc basin morphology such that the sedimentation in the basin is influenced and the stratigraphy will reflect such tectonic processes. The purpose of this study is to examine the lithofacies and stratigraphy of the Middlebury Formation to determine what effect, if any, the onset of the Taconic Orogeny had on the deposition of the Middlebury Formation. The field area of interest for this study is to the west of Middlebury, Vermont and further south between Orwell and Sudbury, Vermont. Cross sections along B-B’ and C-C’ show that the overall structure of the field area is characterized by asymmetric overturned folds that are present on multiple scales, as well as four eastward dipping thrust faults. Understanding the orientation and geometries of these structures is important because it enables a cross section to be completed from which each field locality can be placed in its relative position in the stratigraphy of the Middlebury Formation. Additionally, the cross sections are the basis for a thickness calculation of the Middlebury Formation of ± 450 feet. Three lithofacies are present in the stratigraphy of the Middlebury Formation and they indicate that the Middlebury Formation was deposited in “quiet” water on the inner ramp of the Laurentian margin that experienced occasional high-energy events. Lithofacies A is the dominant lithofacies and is present throughout the stratigraphy and is a horizontally and cross-laminated mudstone that contains dolomitic horizons that are recognized by their beige mottled weathering pattern. Lithofacies B is present at the base of the stratigraphy and is calcareous sandstone to arenaceous mudstone that contains horizontal and cross-laminations. Lithofacies C is a heterogeneous limestone that contains wackestone-mudstone couplets and in some localities near the top of the stratigraphy, it is very fissile and shale seams are present. Lithofacies B is interpreted as being deposited on the inner ramp and is the most proximal lithofacies in the Middlebury Formation and Lithofacies A represents a laterally contiguous environment. The shaley horizons of Lithofacies C are interpreted as being deposited on the inner ramp laterally contiguous with the Glens Falls Formation. This gradual change in lithofacies throughout the Middlebury Formation suggests that a rapid change in base level did not occur during the deposition of this formation. Instead this gradual change reflects a transgression.