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