Sedimentology and evolution of the Martinsburg Formation (Upper Ordovician) fine-grained turbidite depositional system, central Appalachians



The Upper Ordovician Martinsburg Formation of eastern Pennsylvania consists of mudstone, siltstone, and sandstone turbidites that accumulated in a tectonically active foreland basin. The mudstone-rich Bushkill Member, the stratigraphically lowest unit of the Martinsburg in this area, grades upward into approximately equal proportions of mudstone, siltstone, and sandstone of the Ramseyburg Member. Many of the turbidites of these units are arranged in small-scale (1–9 m) fining-upward sequences that are interpreted as reflecting the influence of external or allocyclic controls such as variations in the local rate of sea-level rise and/or variations in the intensity of tectonic activity in shelf/nearshore or hinterland areas rather than more commonly cited autocyclic mechanisms. The thick (approximately 2000 m) Bushkill-Ramseyburg coarsening-upward sequence records progradation of a muddy turbidite depositional system along the axis of the foreland basin. Although this sequence accumulated during a Caradocian eustatic rise in sea-level, sedimentation rates landward of the shoreline were apparently great enough to allow for long-term seaward progradation of the shelf source. The paucity of depositional lobe-like facies (coarsening-upward sequences) in the Bushkill Member allows for tentative comparison of the progradational Bushkill-Ramseyburg system with the active fan lobe of the Mississippi Fan.

Progradation of the Bushkill-Ramseyburg system ceased abruptly when mudstone turbidites and laminated black shale of the upper unit of the Martinsburg, the Pen Argyl Member, accumulated. The great thickness of some mudstone turbidite beds of the Pen Argyl Member is interpreted to record topographic confinement of the central Appalachian foreland basin, which may have helped to preclude continued progradation of the Bushkill-Ramseyburg turbidite system.