TRANSGRESSION VERSUS THE CATSKILL DELTA: UNIQUE LATEST MIDDLE DEVONIAN LITHOFACIES IN THE NORTHERN APPALACHIAN BASIN

The Catskill Clastic Wedge of the Appalachian Basin is the best-studied example of Paleozoic delta progradation and basin fill; as such, it forms the North American Devonian siliciclastic reference succession. The most pronounced progradation of the Catskill Delta resulted from the latest Givetian through end-Devonian Tectophase 4 of the Acadian Orogeny (sensu Ver Straeten). These progradational facies have traditionally been subdivided using the magnafacies model, with the shelfal magnafacies further split based on modern deltaic sub-environments (delta platform, pro-delta, etc.). It has long been recognized that the Catskill Delta evolved through the Late Devonian and the distribution and sedimentology of delta sub-environments varied due to the interplay of sediment supply, subsidence, and sea level fluctuations among other factors.

This interplay was perhaps most pronounced at the onset of Tectophase 4 which was coincident with the Taghanic Onlap, a global sea level rise, and recorded in lowermost Genesee Group strata. At this time, basin-ward transport of sediment from the rejuvenated Acadian Orogeny into the rapidly subsiding foredeep was met by transgressive black shale; this resulted in a unique suite of lithofacies on a proto-shelf, combining features of delta platform, estuarine, and distributary channel deposits. Lithologic features of this proto-shelf include sideritic and phosphatic concretions reworked into shell-rich tempestites interbedded with channelized sandstones with basal lags comprised of a heterolithic assortment of nodules as well as plant and fish skeletal material. Together, these features suggest an estuarine-like interaction of fresh- and marine waters on a storm-influenced delta platform dissected by distributary channels that bypassed sediment to the nearby proto-slope. Ultimately, increased sediment supply overwhelmed local sea level rise and most of the Genesee Group records progradation of typical magnafacies and deltaic sub-environments.

The recognition of these unique proto-shelf facies provides a better understanding of the interplay of sediment supply, subsidence, and sea level fluctuations on delta dynamics, Future work should look for similar facies in shelf successions equivalent to the repeated younger Devonian black shale intervals.