IRON-SULFIDE KARST FEATURES AND METAVOLACONIC LAYERS IN THE BRIMFIELD FORMATION, EAST HADDAM, CT

The rocks at Devil’s Hopyard State Park in East Haddam, CT were described and mapped at a reconnaissance level as part of an initiative of the Connecticut Geological Survey to make the geology of the state parts accessible to the public. The park is located on a structural dome in the Merrimack Terrane and contains exposures of Silurian Hebron Gneiss and Ordovician Brimfield Formation, the latter of which contains a subunit of amphibolite. Pegmatite intrusions of variable ages are either deformed into boudins and exhibit foliation or are completely undeformed. The Brimfield is a coarse-grained muscovite, biotite schist with rare garnetiferous layers. Main exposures of the Brimfield are on a southwest facing cliff that contains caves that inspired much folklore with the early settlers. The caves form in layers that contain iron-sulfide minerals such as pyrite pyrrhotite. The Fe-sulfides oxidize to form sulfuric acid, which dissolves the other mineral constituents through ground water seepage. Additionally, subtle karst topography is visible by hummocky terrain and closed contours on the topographic map.

The metavolcanic member of the Brimfield is a medium- to coarse-grained amphibolite. On the scale of the state map, it is shown to consists of a main unit, ~12 km long and ~ 500 m thick and a smaller unit, ~1.5 km long and ~400 m thick. In the field, the amphibolite is not a single layer, but consists of a number of thinner layers of amphibolite and biotite gneiss that are interbedded with more typical Brimfield lithologies. In some exceptionally coarse-grained layers, the amphiboles reach 1.2 cm in length. We interpret this metavolcanic member to represent several eruptive episodes. The grain size is a metamorphic phenomenon not related to any previous texture of the igneous protolith.