ORDOVICIAN SHALES AND SUBMARINE SLIDE BRECCIAS OF NORTHERN CHAMPLAIN VALLEY IN VERMONT
The fossiliferous, relatively pure Isle la Motte formation and Larrabee and Shoreham members of the Glen Falls formation (lower Trentonian) indicate deposition in shallow, comparatively clear seas. Upward through overlying Cumberland Head and Stony Point formations there is an increasing proportion of argillaceous limestone and calcareous shale. Small-scale current bedding in the Stony Point formation indicates currents from southwest. Next younger Iberville formation (upper Trentonian), 1000 to 2000 feet of rhythmically interbedded clay shale and graded dolomitic siltstone, contains occasional 5- to 10-inch coarser-graded beds with shale fragments, intrastratal crumpling, flow (load) cast, and slump structures. A deeper, muddier bottom subject to repeated turbidity currents is inferred. Uniformly oriented, small-scale current bedding indicates a northeastern source. Lying on Iberville is the Hathaway formation (new name), composed of argillite and bedded radiolarian chert with blocky inclusions (up to 50 feet long) of limestone, dolomite, dolomitic quartz siltstone, sandstone, coarse graywacke, and chert. It is chaotically deformed with extensive flowage and isoclinal folding perpendicular to regional trends, unrelated to tectonic structure of older formations in the area or to its own cleavage. Submarine slides are inferred, possibly from a thrust fault scarp to the east or northeast. This sequence of rocks is complexly folded and sheared, with greater intensity near the major “Logan9s Line” thrusts bounding the area on the east.