WHAT'S UP WITH THE TACONIC SEQUENCE INDIAN RIVER FORMATION?

The nearly barren, burrow-mottled red slates and cherts of the Indian River Formation (IRF) mark a unique transitional interval within the Cambro-Ordovician sedimentary succession of the Taconic allochthons. In 1962, Berry reported early Sanbian graptolites (Nemagraptus gracilis Zone; ~ 458 Ma) from cm-scale black slate interbeds within the IRF strata at several sites in Washington County, NY, and nearby Vermont. The published fauna lists, however, all represent the Climacograptus bicornis Zone (457-453 Ma) or are equivocal; none contain species distinctive of the early Sandbian. Similar faunas are also present in the overlying Mt. Merino Fm. Berry reported a small Sandbian graptolite assemblage from within the upper part of the subjacent Poultney Fm. as well; however, L. Delano mapped that site as Mt. Merino Fm.

More recently, weighted mean ²⁰⁶Pb/²³⁸U dates obtained by Macdonald and colleagues from two ash beds within the IRF in this same area suggest depositional ages of 466-464 Ma. The 466 Ma ages are consistent with the timing of the Barnard volcanic flareup, proposed in the family of models by Macdonald, Karabinos and others to be related to slab breakoff after closure of Iapetus Ocean.

Contacts between the IFR and the Poultney and Mt. Merino formations are generally regarded as conformable. If both sets of age indicators are correct and deposition was more or less continuous, then the IRF must comprise condensed, fine-grained pelagic sediments accumulated over an ~10 myr interval. These features argue for deposition in a sedimentologically isolated setting within a persistent Taconic oceanic tract between the composite Rowe-Moretown terrane and cratonic Laurentia. Deposition of the IRF also coincides with suggested global cooling during the mid to late Darriwilian. Perhaps deposition under arid conditions with relatively low surface productivity and oxic, cool bottom water circulation accounted for formation of the IRF within this oceanic tract. Finally, the well-established westward transport of the allochthons suggests that closure of the Taconic oceanic tract began near the end of Sandbian time and likely involved eastward subduction, in keeping with traditional models, albeit simultaneously with westwardly directed, flat-slab subduction under the arc.