SEQUENCE STRATIGRAPHIC INTERPRETATION OF THE TYPE ITHACA FORMATION (UPPER DEVONIAN, FRASNIAN): RECOGNITION OF DEPOSITIONAL SEQUENCES AT A FORELAND BASIN PRO-DELTA

The Frasnian Ithaca Formation of the Appalachian Basin of New York presents a novel test case for application of sequence stratigraphy to recognize depositional sequences at the pro-delta. These sediments were deposited just below storm wave base, thereby making the study area a sink for sediment bypassed from up ramp locations, and the Sequence Boundary (SB) conformable. The high rate and variable nature of sediment deposition also makes recognition and correlation of these sequences particularly challenging. Lowstand Systems Tract (LST) deposits are recognized as relatively fossiliferous and somewhat coarsely clastic strata inferred to have been deposited during initial sea level rise and sediment sequestration in bays and estuaries; however, this is still a regressive regime. As the rate of sea level rise increased, this area became more sediment starved, resulting in deposition of shell-rich limestones and calcareous siltstones. This is in stark contrast to the surrounding fossil-dilute strata that are typical of the Ithaca Formation. Such inferred Transgressive Systems Tracts (TSTs), commonly contain reworked concretions, phosphate pebbles, and in some cases clasts derived from the underlying deposits of the LST. These beds vary considerably in lithological character over short distances because of the highly variable nature of the local sediment supply, but their general sediment starved nature makes them traceable throughout the study area. The condensed deposits of the TST are commonly overlain by dark gray to black shale beds culminating in a flooding surface (FS) on which the remains of pelagic organisms became concentrated, probably representing the highest rate of sea level rise. This surface separates the TST from the Highstand Systems Tract (HST) and indicates a change from transgression to regression. As relative sea level continued to rise slowly and then ultimately began to fall, the HST muds gave way to turbiditic facies. As the rate at which relative sea level fall increased to a maximum, HST deposits were replaced by thick packages of channelized fine-grained sandstone recording the Falling Stage Systems Tract (FSST). The HST and FSST deposits make up the majority of the thickness of strata in any given depositional sequence. These strata are in turn overlain by the SB and LST deposits.