AN EOLIAN DUNE IN THE EARLY JURASSIC CASS FORMATION PRESERVED BY LAVA FLOWS, POMPERAUG BASIN, CONNECTICUT

A recent quarry-wall exposure of the Early Jurassic Cass Formation in the Pomperaug Basin of Southbury, CT revealed a preserved dune form covered by ~70 m of Orenaug Basalt flows. Fully preserved dunes are rare features, previously unreported in the Pomperaug Basin. The nearly 26 m long and 3.5 m high dune exhibits a sharp contact with the basalt flow that has left the sediment largely undisturbed. The exposure was examined to determine the mechanisms of its preservation and identify any sediment-basalt interaction. Internal characteristics of the dune include ~30-40 cm of ~15° dipping stoss-side beds, high-angle (~23° to NE) foreset beds with numerous reactivation surfaces, and reversely graded ripple cross lamination. The morphology and internal bedforms indicate that the feature is an eolian dune, which supports a previous eolian interpretation for the upper Cass Formation. There are no apparent macro-scale features along the contact at the outcrop. However, petrographic microscopy revealed micro-scale sediment-basalt interaction at the contact and distinctive changes in porosity. Above the contact, little to no sediment was found in the basalt. A few cm below the contact on the lee side of the dune, small, thin (1-2 mm thick) laminae of glassy basalt were identified. Fragments of calcite cemented sandstone within the basalt indicate that the basalt was injected into consolidated sediment. This suggests early cementation that may have aided in the preservation of the dune. Contact alteration on the stoss side of the dune differs from that of the lee side. A green, reduced alteration zone on the lee side extends an average of 8 cm from the contact and contains bands of red hematite in the first 1-2 cm. Porosity is negligible near the contact and increases to ~10 % at the base of the green zone. The stoss side of the dune lacks a significant alteration zone and instead is overlain by ~20 cm of heavily weathered, partially consolidated soil-like material. In addition, calcite cement is rare near the contact. Early diagenesis certainly contributes to the preservation of this eolian dune, yet lava rheology and dune morphology are also important factors. Our observations suggest preservation was due to the gentle advance of low-viscosity lava over a low-relief dry dune surface that possibly was consolidated by a paleosol.