STRATIGRAPHIC ARCHITECTURE OF THE JUNIATA FORMATION (CENTRAL APPALACHIANS) AND ITS RELEVANCE TO THE HIRNANTIAN GLACIATION

Uppermost Ordovician (Cincinnatian) strata in the eastern United States were deposited during a critical time in Earth’s history: the world was in transition from a greenhouse climate to a short, intense cooling period known as the Hirnantian glaciation. Few studies have sought to recognize this event in the stratigraphic record of Cincinnatian strata (the siliciclastic Juniata Formation to the northeast and the carbonate Sequatchie Formation to the southwest) in the Appalachians. This study focuses on detailed sequence analysis and involves stratigraphic and sedimentological examinations of the Juniata Formation. In general, the Juniata Formation is dominated by red sandstones, siltstones, and shales that show a repetitive cyclicity. The formation displays a regional trend of storm-dominated deposits at the base of the formation followed by multiple shallowing-upward parasequences that increase in shale content up-section. A typical parasequence consists of cross-bedded or massive sandstone at the base grading into an interval of interbedded, bioturbated sandstone and siltstone-mudstone and capped by red mudstone (vertisol?). Parasequences vary in thickness from 1 to 10 feet on average.

In addition to this temporal trend, the Juniata Formation changes character from north to south. Red mudstone and siltstone dominate in the northeast whereas the quantity and diversity of bioturbated sandstones increase to the southwest. Individual parasequences are thicker and more well-defined in the northern than in the southern sections. In the southern sections, packages of interbedded sandstone and mudstone, averaging about 30 feet thick, display an overall upward trend of increasing mudstone. The differences in the cycles from north to south are likely the result of a change from shallow coastal plain deposition to near-to-offshore shelf deposition, respectively. There are no body fossils present, but trace fossils are abundant and include Skolithos, Planolites, Cruziana, and Monocriterion, among others. These ichnofacies show a trend of increased size up-section, which could be the result of increased oxygenation in the basin. Stacked upward-shoaling parasequences in the Juniata Formation favor high-frequency sea-level fluctuations possibly related to waxing and waning of the Hirnantian glaciation.