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