Stratigraphy and sedimentology of a late Pliocene intertidal to fluvial transgressive deposit: Bacons Castle Formation, Upper York -James Peninsula, Virginia.

摘要

The Bacons Castle Formation is a tidal to fluvial transgressive unit deposited on the Virginia Coastal Plain during the late Pliocene. Lithofacies recognized within the Bacons Castle form associations that represent distinct depositional systems. Association I consists of sand and mud deposited in a tidal-flat and estuarine system along an embayed shoreline. Association II consists of pebble and cobble gravel and gravelly sand that grade upward into sandy mud that was deposited in a fluvial to estuarine system. Areal distribution of the associations I and II and their distinctive lithologies allow for subdivision of the Bacons Castle into two members, the Barhamsville and Varina Grove Members, respectively.;Deposition occurred on a surface dissected by rivers ancestral to the modern James and Pamunkey Rivers. A regolith and/or soil horizon on the underlying Yorktown Formation indicates that the Yorktown was exposed subaerially prior to Bacons Castle deposition. During Bacons Castle transgression, the antecedent topography was flooded. Depositional environments were localized and lithofacies influenced by the configuration of the topography. Antecedent highs provided shelter for tidal flats from wave and current activity until overtopped by transgression and environments migrated landward. Fluvial deposition was localized to antecedent valleys until the valleys were filled after which the fluvial system migrated unrestricted.;The Bacons Castle rests unconformably on the late Pliocene Yorktown and Chowan River Formations. Based on its transgressive nature and stratigraphic position, the age of the Bacons Castle is estimated to be between 2.3 and 2.1 my, following initial Northern Hemisphere glaciation(s) during the late Pliocene. The composition and texture of the Bacons Castle indicate a source from the Valley and Ridge, Blue Ridge, and Piedmont. The coarse texture and alluvial architecture of the unit indicate high-discharge transport. Sediment textures and transport mechanism suggest a climatic influence of cold-climate mass wastage transported by rivers fed by snow melt, runoff on frozen ground, or increased rainfall with interglacial climatic amelioration. The Bacons Castle represents evidence of the influence of late Pliocene climate on deposition within the Atlantic Coastal Plain.