ISOTOPIC AND GEOCHEMICAL ANALYSES OF THE LACUSTRINE SHALES OF THE JURASSIC PORTLAND FORMATION, NEWARK SUPERGROUP, HARTFORD BASIN

The Jurassic lakes of the Hartford Basin were described and interpreted in the 1960s to early 1980s to be fresh and alkaline. In the late 1980s to early 1990s, evidence for a higher salinity in these siliciclastic-dominated lakes included rare evaporites, mudcracked nearshore facies with no bioturbation, saline biomarkers, and arid-climate paleosol formation. Detailed geochemical and isotopic analyses of the lowermost shales of the Portland Formation, the youngest part of the rift basin fill (Sinemurian to Toarcian in age), finally refines the interpretation of the paleochemistry of lake waters during the final subsidence phase of this rift basin.

Cores were drilled in the distal basinal area through the Portland Formation and underlying Hampden Basalt in the Hartford city area for a river diversion project. Thirty-five cores along a three-kilometer transect have a horizontal separation of cores of about 100m. Black shales from the lower part of the Formation were sampled in order to characterize lateral and vertical changes in the organic and trace element geochemistry. Organic carbon contents range from 0.10 – 7.55 wt. % with an average of 2.07 wt. % (n=33). Carbon isotopic values of bulk organic matter average -28.4‰ with a range from –24.7 to –30.6‰ reflecting variations in productivity, provenance of organic matter, and episodes of anoxia and dysoxia. Total sulfur contents range from 0.02 – 2.21 wt. % with an average of .52 wt. % (n=33) and are associated with disseminated pyrite. The saturated hydrocarbons are typical of lacustrine deposits; they show a smooth distribution of n-C₁₄ through n-C₃₅ alkanes, low amounts of pristane and phytane, and an abundance of branched and cyclic alkanes. Enrichment of redox sensitive trace elements (V, Zn, Mo, U) is consistent with anoxic conditions associated with a chemically stratified lake. Preliminary interpretations support conditions ranging from fresh to saline lake environment with restricted bottom waters consistent with balanced filled conditions for the lower part of the Jurassic Portland Formation.