DISTINGUISHING BETWEEN EROSIONAL AND DEPOSITIONAL SHELF SURFACES UNDERLYING FORELAND BASIN DEPOSITS: THE KEY TO DETERMINING OROGENIC TIMING IN THE TACONIC OROGEN

In western new England and eastern New York, the Cambro-Ordovician shelf strata are capped by black shales deposited in the Taconian foreland basin. These shales and the underlying shelf were subsequently overridden by the Taconic allochthons, Champlain thrust sheet, and other major thrust sheets of the Taconic orogen. All major attempts to date the onset of Taconian orogenesis have relied on the timing of the shelf-shale transition, based mostly on the age of the shelf strata because fossil evidence from the shales is sparse.

It has recently become evident that the pre-orogenic shelf was deeply dissected by submarine canyon systems during a major Mid-Ordovician eustatic event, and that the top of the shelf strata locally represent an erosional surface rather than a depositional surface. Distinguishing between the erosional and depositional shelf surfaces is critical for accurately dating orogenic onset. Although we have not been able to distinguish between depositional and erosional shelf surfaces in the field, regional analysis of the stratigraphic systems allows that distinction to be made. Based on our analysis, final portions of the active outer shelf were built during Chazy and Black River (Mid to Late Ordovician) time and sat atop the remnants of the dissected earlier shelf strata. These late shelf buildups are overlain by thin layers of shale, whereas the intervening canyons are filled with thick shale deposits. Where older shelf strata is overlain by shale, it represents incised and foundered portions of the shelf; most of the shelf loss occurred in late Whiterockian (early Middle Ordovician) time. Thus, by distinguishing between eustatically controlled shelf surfaces (i.e. in the canyons) and tectonically controlled shelf surfaces (i.e. atop the remnant active shelf), we conclude that orogenic onset occurred during Late Ordovician, rather than the Early to Mid Ordovician timing proposed by previous workers.