Abstract
The Greenwich slice of the Hamburg klippe, a Taconic allochthon in eastern Pennsylvania, is part of an ancient subduction complex that formed during early Paleozoic crustal convergence in the central Appalachian orogen. The dominant lithologies of the Greenwich slice are arranged in fault-bounded, coarsening-upward sequences of pelagic deposits (Lower to Middle Ordovician) overlain by hemipelagic and turbidite deposits (Middle Ordovician). These sequences record advance of a site on oceanic lithosphere from a pelagic realm toward, and ultimately into, the trench in Middle Ordovician time. The trench-fill deposits of the Greenwich slice are characterized by structurally coherent, sandstone-rich zones adjacent to mudstone-rich horizons of mesoscopically deformed sandstone and scaly mudstone. Deformation of sandstone ranges from minor variations in bed thickness to disruption so severe that beds cannot be traced for any appreciable distance across the outcrop. Deformed sandstone beds typically contain minor, but conspicuous, dewatering structures and lack internal primary sedimentary structures. All sandstone beds display various proportions of microgouge and pressure-solution zones. Microscopic analysis of the scaly mudstone reveals (1) reorientation of platy grains along scaly cleavage surfaces, (2) the presence of microstriations and micro-steps on scaly cleavage surfaces, and (3) microfractures filled with sparry calcite and quartz, some of which truncate scaly cleavage surfaces.
Consideration of the structural evolution of these deposits suggests that initial accretion-related deformation and dewatering were localized along weak mud-rich horizons, whereas stronger sand-rich zones remained coherent. Increased pore-fluid pressures of sand beds within mud-rich zones resulted in intergranular ductile flow and stratal disruption of the sand. Interlayered mud became the stronger sediment by virtue of its greater cohesive component of strength and failed brittlely, resulting in development of scaly cleavage. Subsequently, pore-fluid pressures within the deforming mud-rich zones became sufficient enough to generate a fracture porosity. Progressive deformation resulted in less of a strength contrast between the strong sand-rich and deformed mud-rich horizons. Eventually, both early deformed and coherent sand beds failed brittlely by grain granulation and pressure solution. The variations in style of deformation of the Greenwich slice trench-fill deposits are probably reflective of variations in porosity and state of compaction of the accreted sediment.
