Gary G. Lash

Diverse melanges of an ancient subduction complex

ABSTRACT Three lithologically and structurally diverse melanges occur within an early Paleozoic (Early-Middle Ordovician) subduction complex in the central Appalachian orogen. Type I melange, characterized by horizon of variabls y deformed sandstone and scaly mudstone that alternate with coherent sandstone-rich sequences, is interpreted to reflect accretion-related deformation of water-saturated trench deposits. Type II melange, composed of exotic radiolaria-bearing mudstone clasts in a scaly mudstone matrix, can be explained by remo biliza-tion and mixing of inner-trench slope sediments. Type III melange is a poorly sorted polymict assemblage of native lithology clasts in a scaly mudstone matrix. Evidence o f forceful injection of matrix mud into clasts and inferred discordant contacts between melange and surrounding bedded deposits suggest that the type III melange formed from mud diapirism. The close association o f these melanges points out the diversity of tectonic and sedimentary processes previously documented from modern convergent margins that may be reflected in older sub-duction complexes. INTRODUCTION

Origin of Shale Fabric by Mechanical Compaction of Flocculated Clay: Evidence from the Upper Devonian Rhinestreet Shale, Western New York, U.S.A.

ABSTRACT Most workers attribute the preferred grain orientation that defines the shale fabric to the mechanical collapse of a high-porosity clay microfabric under increasing overburden pressure. Scanning electron microscope analysis of samples of the Upper Devonian (Frasnian) Rhinestreet shale of western New York State collected from near early diagenetic carbonate concretions provides further evidence of this mechanism. Specifically, mudstones recovered from pressure shadows immediately adjacent to lateral edges of concretions are characterized by an open fabric of randomly arranged clay domains--the cardhouse fabric. Laterally equivalent shale samples collected only 0.2 to 0.3 m distant the pressure shadows, however, reveal a low-porosity, strongly oriented shale fabric in which clay particles mold around incompressible quartz silt and euhedral pyrite grains and pyrite framboids. These observations suggest that the organic Rhinestreet shale accumulated as water-rich flocculated clay. The open clay fabric observed in pressure-shadow mudstones was preserved by the incompressible concretions during burial. Flocculated clay beyond the shielding effect of the concretions, however, collapsed during progressive burial to form the strongly oriented shale fabric.

Recognition of trench fill in orogenic flysch sequences

Ancient trench-fill deposits within erogenic eugeosynclinal sequences are best recognized by their stratigraphic position above older hemipelagic and pelagic deposits. Such coarsening-upward sequences reflect migration of an abyssal site toward and ultimately into the trench. Axial transport and continuity of turbidites may help to better define trench geometry and sedimentation patterns but alone are not strong evidence for trench sedimentation. Likewise, sandstone petrography, because it is influenced by such variables as tectonic and geologic peculiarities of the source area and transport of detritus from distant terrenes, is not a useful tool for recognition of trench deposits. Melanges and broken formations are best considered as evidence for gravity transport of material onto the trench floor or tectonic deformation of trench fill when they are part of a coarsening-upward sequence.

Sedimentology and possible paleoceanographic significance of mudstone turbidites and associated deposits of the Pen Argyl Member, Martinsburg Formation (Upper Ordovician), eastern Pennsylvania

Abstract The Pen Argyl Member, the stratigraphically highest unit of the Upper Ordovician Martinsburg Formation of eastern Pennsylvania, consists of the three mudstone facies: laminated mudstone, uniform mudstone, and black shale. These deposits conformably overlie a coarsening-upward sequence of fine-grained and classic sandstone turbidites of the underlying Bushkill and Ramseyburg Members, respectively. Petrographic and X-radiographic analysis suggests that laminated and uniform mudstone facies beds accumulated from muddy turbidity currents. Interbedded black shale facies beds, however, are interpreted to reflect slow, gravitational settling of sediment in a density stratified, anoxic basin. The marked transition from sandstone and siltstone turbidite deposits of the Ramseyburg Member to mudstone turbidite and black shale deposits of the Pen Argyl Member may reflect the combined effects of a Caradocian rise in sea level (increased organic-carbon production; lack of sand-size sediment) and a tropical to subtropical location of the basin (density stratification). Such a model, however, does not explain adequately the very thick laminated and uniform mudstone facies beds common only to the Pen Argyl Member. An alternative hypothesis involves amplification of global eustatic- and climatic-related oceanographic effects by local or basinal influences. The very thick beds of the laminated and uniform mudstone facies, by comparison with lithologically similar deposits of the eastern Mediterranean, are interpreted to have been ponded in a topographically restricted basin. Accordingly, deposition of the Pen Argyl Member may record a significant change in basin morphology from a wide extensive basin to a topographically restricted or enclosed anoxic basin.

Anatomy of an early Paleozoic subduction complex in the central Appalachian orogen

Abstract The Greenwich slice of the Hamburg klippe, a Taconic allochthon in the eastern Pennsylvania Appalachian orogen, is part of a subduction complex that formed during early Paleozoic crustal convergence. It consists chiefly of turbidite sandstone and hemipelagic mudstone of Middle Ordovician age (85%) and sequences of red and light green pelagic mudstone, deep-water limestone, and chert of Early-Middle Ordovician age (14%). Additional minor lithologies include pebbly mudstone and boulder conglomerate and basic intrusive and extrusive rocks. The dominant lithologies of the Greenwich slice are arranged in fault-bounded sequences of pelagic deposits overlain by channelized turbidite sandstone and hemipelagic mudstone. These sequences are interpreted to record migration of a site on oceanic lithosphere from an abyssal plain setting toward and ultimately into a trench by Middle Ordovician time. The channelized turbidite deposits were transported parallel to the continental margin (northeast-southwest) and there are no proximal-to-distal sedimentologic variations along the length of the Greenwich slice. These characteristics suggest that the turbidites accumulated on part of an elongate submarine fan that occupied the floor of the trench or, alternatively, within laterally migrating channels along the floor of a large trench axis channel. Petrographic characteristics of the inferred trench-fill turbidite deposits suggest that the longitudinal dispersal system may have been supplemented by transversely supplied sediment. Calculated sedimentation rates of the Middle Ordovician trench-fill deposits and estimated plate convergence rates are consistent with accumulation of the turbidites as part of a trench wedge rather than a fan.

The Shochary Ridge sequence, southeastern Pennsylvania—a possible Ordovician piggyback basin fill

Abstract The central Appalachian orogen of eastern Pennsylvania is underlain, in part, by three fault-bounded deep water clastic sequences: the Windsor Township Formation of the Hamburg klippe (Greenwich slice), the Martinsburg Formation and the Shochary Ridge sequence. Deep water sandstones of the Windsor Township Formation were deposited in a trench on oceanic lithosphere in early Middle Ordovician time. Clastic detritus of the late Middle-early Late Ordovician Martinsburg Formation accumulated as part of a longitudinal deep sea clastic system in the central Appalachian foreland basin. Deposits of the coeval Shochary Ridge sequence comprise a 1500 m thick coarsening-upward sequence of hemipelagic mudstone and siltstone turbidites. Although devoid of the type of bedding cyclicity implicit to published submarine fan models, the Shochary Ridge sequence may have accumulated as part of a morphologically poorly developed deep sea fan in which submarine channels and depositional lobes never formed or were poorly developed at best. The coarsening-upward nature of the Shochary Ridge sequence probably reflects progradation of its depositional system in response to either an inferred late Middle-early Late Ordovician glacially induced reduction in sea-level and/or an increase in tectonic activity in the hinterland. Sedimentologic and paleocurrent data suggest that deposits of the Shochary Ridge sequence did not accumulate as part of the Martinsburg depositional system. Palinspastic restoration places the Shochary Ridge depocenter to the southeast of the Martinsburg basin; i.e., on the orogen side of the foreland basin. Accordingly, the Shochary Ridge sequence is interpreted to reflect gradual filling of a basin that existed within the active thrust terrane to the southeast of the Martinsburg depocenter. This basin, a “piggyback” or slope basin, formed on Martinsburg strata that had been tectonically incorporated into the northwest migrating thrust terrane as the frontal thrust advanced into the foreland basin.