SEDIMENTOLOGY AND STRATIGRAPHY OF THE LOWER JURASSIC PORTLAND FORMATION (NEWARK SUPERGROUP), HARTFORD BASIN

The Triassic-Jurassic rift system along the eastern North American coast was formed by the breakup of the supercontinent Pangaea. This Newark rift system is composed of a series of half grabens extending from South Carolina to Nova Scotia (about 2300km). The sedimentary fill and associated tholeiitic lava flows and diabase plutons of these rift basins are collectively known as the Newark Supergroup with sedimentary depositional paleoenvironments interpreted as rivers, lakes, and alluvial fans. One of these rift basins, the Hartford basin, is a half graben bounded by a west-dipping normal fault on its eastern side with east-dipping sedimentary strata. The Hartford basin is approximately 140 km long and 20-30 km wide containing a 4-7 km thick fill of red beds and mafic rocks. The sedimentary rocks are generally composed of reddish brown to deep brown sandstones and siltstones with lesser amounts of conglomerate and thin black shales. This basin is also known for numerous paleontologic sites that contain dinosaur tracks and fossil fish.

The sedimentary fill of the Hartford basin is divided into four sedimentary formations interbedded with three basaltic flows. The Portland Formation is the youngest sedimentary formation (Sinemurian-Toarcian in age) 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. Identified facies include black shales, stratified mudrocks, ripple cross-laminated sandstones to mudrocks, trough cross-bedded sandstones, 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.