Central Appalachian Foreland Research Articles

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

ABSTRACTThe 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.

The nature of deformation within the outer limits of the central appalachian foreland fold and thrust belt in New York state

Residual strain, a self-equilibrating recoverable strain that remains in rocks even after external forces and moments are removed, is found NNW of the folded Appalachian plateau in the Devonian Onondaga limestone and the Silurian Lockport dolomite and Grimsby sandstone of western New York. This residual strain is manifest upon overcoring by a NNW directed maximum expansion of the limestone and sandstone and a random maximum expansion of the dolomite. Strains, recorded with strain gauge rosettes bonded to outcrops, are as high as 200 μϵ (microstrain). Double overcoring of the sandstone and dolomite relieves smaller strains of the same orientation as the initial overcore. X-ray analysis of the Grimsby sandstone shows that the elastic residual strain locked in quartz grains is characterized by a NE principal extension of 60 μϵ and a 10–30 μϵ NW principal compression oriented 30° counterclockwise from the NNW compression indicated by overcoring. Sonic velocity tests on samples in the lab indicate that Grimsby sandstone is anisotropic with a NNW maximum P-wave velocity of 4.05 km/sec. This anisotropy correlates with the residual strain in Grimsby sandstone. Mechanical twinning of calcite within both the Onondaga limestone and Grimsby sandstone indicates that the rock contains a permanent compressive strain of less than 2% in the NNW direction. The permanent strain becomes progressively smaller in a series of samples from Syracuse to Buffalo, New York. The development of solution cleavage in Onondaga limestone also indicates a NNW compression. No evidence of permanent strain was found in the dolomite. The NNW compression of the limestone and sandstone is normal to the fold axes of the Appalachian foreland fold and thrust belt. This geometric relationship indicates that the residual strain well beyond the outermost Appalachian folds was caused by the same tectonic stresses responsible for folding, the Appalachians during the late Paleozoic. Strain within the Appalachian plateau below the Silurian salt horizon suggests either the presence of a second décollement in, perhaps, Ordovician shales or a general NNW shortening of the crust under the Appalachian plateau.