GEOCHEMICAL AND FAUNAL EVIDENCE SUGGESTS THAT THE UTICA SHALE WAS DEPOSITED IN A RESTRICTED RETROARC FORLAND BASIN

Recent studies in the Taconic hinterland of New York and New England indicate that the Iapetus Ocean closed at ~475 Ma during collision of a Laurentian terrane with a peri-Gondwanan arc. Subsequent closure of the Taconic seaway, in this model, was followed by a reversal to westward-dipping subduction at ~460 Ma. The younger Ordovician sediments of the Taconic foreland basin such as the 453-450 Ma Utica Shale, therefore, must have been deposited in a retroarc basin rather than on the subducting Laurentian margin (as posited by most earlier models). These differing settings might be expected to produce different oceanographic conditions and thus be testable by study of the faunas and paleo-environmental proxies.

Utica graptolite faunas are odd compared to coeval faunas – both from nearby Iapetus (e.g., Exploits Group) and Panthalassa. The initial transgressive Utica graptolite community (~452.9 Ma) included a restricted fauna with very limited duration. Proxy data suggest high surface productivity and anoxic bottom water, but U was strongly depleted and Mo/U suggests limited connection to the global ocean. Subsequently, a basin-wide deepening transformed Utica sediment compositions, produced the maximum backstep seen in coeval Trenton Group facies (the basal Poland Limestone, sequence M5C maximum starvation interval, ~452.7 Ma) and introduced the more cosmopolitan C. americanus Zone fauna. Bottom water conditions remained predominantly anoxic mainly on the east side of the basin. More widely restricted conditions with increased productivity and Mo/U depletion returned at ~451.9 Ma, during late C. americanus Zone time, as the Dolgeville carbonate fan spread down the Taconic basin. Extinction among the former inhabitants and evolution of a few, endemic species led to the characteristic depauperate O. ruedemanni fauna. Accelerated extension at ~ 451.3 Ma drowned the Dolgeville fan and allowed the immigration of the more cosmopolitan D. spiniferus Zone fauna.

These features of the Utica Shale suggest deposition within a relatively shallow, restricted basin with subsidence and oceanographic conditions driven in large measure by thrust loading. This model most closely matches expectations for deposition in a retroarc foreland basin following initiation of westward, flat slab subduction