Lakes of the Jurassic Portland Formation, Newark Supergroup, Hartford Basin

Zerezghi, Simret Ghirmay¹, Elizabeth H. Gierlowski-Kordesch¹, Peter A. Drzewiecki² (1) Ohio University, Athens, OH (2) Eastern Connecticut State University, Willimantic, CT

The eastern coast of North America was within a zone of active rifting during the Triassic-Jurassic as the break-up of Pangaea proceeded. The Hartford rift basin of Connecticut and Massachusetts is part of series of half grabens, containing mostly sedimentary fill called the Newark Supergroup, that were deposited during this rifting event which formed the Atlantic Ocean. The basin contains a 4-7 km thick fill of sedimentary rocks and associated tholleiitic basalt and diabase. Interpreted depositional paleoenvironments represented in the basin fill include lakes, playa lakes, rivers, sheetflood plains, soils, and alluvial fans. The Portland Formation is the youngest sedimentary formation and is exposed along the eastern half of the basin. Only its well-exposed coarser facies close to the eastern border fault have been studied in great detail. Finer-grained facies are only exposed in limited outcrops along stream beds and in small quarries. However, an opportunity to study the finer-grained facies of the Portland Formation in the central portion of the basin is now possible with cores recovered from the city of Hartford. This study is based on 20 out of 35 drilled cores across a 3km transect of the Park River tunnel project in the downtown area. Around several hundred meters of thickness of the lower portion of the Portland Formation and portions of the underlying Hamden Basalt were measured and correlated from core material to establish stratigraphic and sedimentologic patterns. Identified facies include black shales, stratified mudrocks, ripple cross-laminated sandstones to mudrocks, trough cross-bedded sandstones, massive red mudstones, and red shales, all interpreted as part of a semi-arid lake-playa-alluvial plain system, similar to that of the underlying East Berlin Formation. The rift fill patterns of this basin are similar to other rift basins where paleoclimatic and tectonic factors, including subsidence, sill height, and variable rainfall input, control deposition