HIATUSES IN THE TACONIC STRATIGRAPHIC SUCCESSION: GEOLOGY OF WHAT'S NOT THERE

Unconformities are a geological and philosophical quandary. They are key stratigraphic markers, but it is difficult to interpret events that are directly represented by a hiatus. By examining strata above and below an unconformity as well as trends associated with the unconformity, insight towards the history of the unconformity is gained. Within the Middle to Upper Ordovician strata of northeastern North America, there are two types of unconformities: submarine and subaerial. Submarine unconformities are present between mixed carbonate-siliciclastic strata (Trenton Group) and overlying black shale (Collingwood Shale of Ontario, "Utica" Shale of NY, Antes and Martinsburg Shale of PA). Across these unconformities, there are dramatic lithologic changes. Strata below the unconformities are predominantly limestone, recording distal shelf to slope conditions. Strata above the unconformities are composed of predominantly hemipelagic mud and typically contain closely spaced volcanic ashes. The upper surfaces of the underlying carbonates and lower portions of the overlying shale are typically rich in authigenic minerals (pyrite crusts and phosphate or pyrite nodules). Hiatuses associated with these unconformities are greatest towards the basin center. These unconformities grade to conformity towards the craton (west) and towards the orogen (east). These unconformities resulted from high subsidence rates coupled with sediment bypass, submarine erosion and dissolution. Thus, the locations of these unconformities in conjunction with the timing and duration of the associated hiatuses help delineate the dynamic history of the Appalachian basin during the Taconic orogeny. There are two subaerial unconformities in the Taconic siliciclastic succession. Hiatuses associated with these unconformities reach their greatest extent towards the orogen. The lowest subaerial unconformity is in the Maysvillian, Bald Eagle-Oswego interval of east-central Pennsylvania. Although the unconformity is fairly localized, lithologic patterns across the unconformity are consistent with a sequence-bounding unconformity, in part the response to eustatic fall. Likewise, the geographically-widespread Cherokee unconformity between underlying Upper Ordovician red beds and overlying Lower Silurian quartz arenite records a sequence boundary.