The Catskill Delta complex is interpreted to be the aggregate of delta-alluvial wedges and associated facies that developed in the central Appalachians and on adjacent parts of the stable craton from the Early-Middle Devonian transition to the Middle Mississippian during the Acadian orogeny.Recent interpretations of the Acadian orogeny suggest that it probably was related to oblique convergence and transcurrent movement along a major strike-slip fault zone separating the eastern margin of the North American landmass from a linear continental fragment called the Avalon terrane. Distribution of clastic wedges and basinal deposits resulting from this orogeny support a general southwestward progression of orogeny and indicate that the major clastic wedges emanated from areas near promontories on the continental margin during successive phases of Acadian deformation. Three and possibly four such tectophases have been noted. Each tectophase appears to represent increased convergence or possible collision between a specific continental promontory and the Avalon terrane, but some delta development occurred continually along many parts of the orogen in response to each tectophase. The four tectophases are: (1) Collision near the St. Lawrence promontory during the Early-Middle Devonian transition with initiation of the Catskill Delta complex represented by the Needmore and Esopus shales and associated clastics near promontories. (2) Southward migration of deformation and collision near the New York promontory during the Middle Devonian with the development of a large peripheral basin having an east-dipping, western paleoslope. This basin was filled with cyclic delta clastics and carbonates of the Hamilton Group and Tully Limestone. (3) Southward migration of deformation and collision near the Virginia promontory during the LateDevonian to earliest Mississippian accompanied by intense clastic influx of the Geneseethrough-Canadaway groups. As a result, the basin was progressively filled from the east so that basinal environments migrated westward out of the peripheral basin and onto adjacent parts of the stable craton. Eventually the basin was filled and a regional west-dipping paleoslope was established. (4) Migration of deformation southward from the Virginia promontory during the Early to Middle Mississippian as basinal environments in cratonic seas were destroyed and Pocono and equivalent clastic wedges essentially filled the epicontinental sea. Middle Mississippian carbonates mark the end of the Acadian orogeny and Catskill Delta complex.

No abstract

Dark, commonly black, fìssile shales represent the basinal facies of the Catskill Delta complex. This dark-shale facies originated in the Appalachian Basin during the Emsian (late Early Devonian) and subsequently spread beyond the Appalachian Basin thus suggesting the importance of regional controls such as paleogeography, paleoclimate, and tectonic regime.The Catskill delta debouched into a subsiding peripheral basin on the eastern margin of a nearly enclosed, equatorial sea. Enclosure prevented deep-water incursions from other water bodies and enhanced the production and concentration of organic matter. Existence in a warm, rainy equatorial belt increased organic productivity and probably formed a nearly permanent thermohaline water stratification that prevented vertical circulation and resulted in anaerobic bottom conditions.Episodes of uplift and deformational loading in the rising Acadian Mountains resulted in closely following or concomitant periods of subsidence through isostatic adjustment in the adjacent peripheral basin. Hence, episodic Acadian uplift not only formed the source of the Catskill clastic wedge, but was also responsible for forming the peripheral basin that received the wedge. Basin subsidence isolated deltaic sediments from other parts of the sea, enhanced water stratification, and augmented net transgression. Rapid aggradation of sediments in nearshore areas due to transgression and the formation of a rainshadow due to uplift in the mountains caused decreased sedimentation on the delta, while concomitant basin subsidence produced abrupt deepening, strong water stratification (bottom anoxia), and migration of clastic-deficient (darkshale) basinal environments shoreward. During intervening periods of tectonic quiescence, subsidence eventually halted, and erosion outstripped the effects of uplift in the mountains, so that the rainshadow was destroyed and abundant clastic influx resumed. As a result, the delta prograded rapidly across former basinal environments. Decreasing subsidence, shallower water, and abundant clastic influx destroyed basinal, bottom anoxia, so that marginally-to highly-oxygenated bottom conditions resumed throughout all delta environments. Five such major cycles of dark, basinal shales alternating with coarser clastics occur in the Catskill Delta complex of the Appalachian Basin. The progressive westward migration of successive cycles reflects the westward migration of Acadian deformation and accompanying basin infilling. 65

No abstract

Sedimentological and geochronological studies along a north–south traverse across the Bangong‐Nujiang suture zone (BNSZ) in Gaize, Tibet provide evidence for a Late Triassic–Jurassic accretionary wedge accreted to the south margin of Qiangtang. This wedge, preserved as the Mugagangri Group (MG), records evidence for the northward subduction of the Bangong‐Nujiang Ocean (BNO) beneath Qiangtang. The MG strata comprise two coarser intervals (lower olistostromes and upper conglomerates) intercalated within sandy turbidites, which are consistent with timing and forearc stratigraphy during subduction initiation predicted by geodynamic modelling. Following the model, the northward subduction of the BNO beneath Qiangtang and subsequent arc‐magmatism are inferred to have begun, respectively, at ca. 220 Ma and ca. 210 Ma, with respect to depositional ages constrained by youngest detrital‐zircon ages. The initiation of arc‐magmatism is also supported by provenance transition reflected by sandstone detrital modes and age patterns of detrital zircons. Previously, evidence for an incipient arc was lacking, but the timing of Late Triassic BNO subduction and related arc‐magmatism is coincident with an important Late Triassic magmatic event in central Qiangtang that probably represents the ‘missing’ arc. Other Qiangtang events, such as exhumation of the Qiangtang metamorphic belt as a source area, and development of the Late Triassic Nadigangri deposits and bimodal volcanism, are more easily explained in the tectonic context of early northward subduction of the BNO beneath Qiangtang, beginning at about 220 Ma.