BALANCED-FILLED CONDITIONS FOR THE LOWER PART OF THE JURASSIC PORTLAND FORMATION, NEWARK SUPERGROUP, HARTFORD BASIN

Jurassic lakes of the Hartford Basin were initially interpreted to be fresh and alkaline. Later, higher salinities were identified through rare evaporites, mudcracked nearshore facies with no bioturbation, saline biomarkers, and arid-climate paleosol formation. Lowermost shales of the Portland Formation represent the youngest part of the rift basin fill (Sinemurian to Toarcian) and their characterization will constrain lake evolution during the final subsidence phase. In order to better constrain the paleochemistry of the Portland lakes, we targeted laminated black shale and gray mudstones representing perennial lake facies for detailed geochemical and isotopic analyses. These facies were sampled in order to characterize both lateral and vertical changes in 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 0.52 wt. % (n=33) and are associated with disseminated pyrite. Values of C and S less than the averages (2.07 and 0.52 wt. %, respectively) are consistent with fresh water conditions, while values greater than average may indicate saline and alkaline conditions. Saturated hydrocarbons are typical of lacustrine deposits; they show a smooth distribution of n-C12 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) normalized to average Upper Crust values is consistent with secondary mineralization and/or anoxic conditions associated with a chemically- stratified lake. In particular, the relative enrichments of Cu and Mo found in laminated intervals, away from fractures and obvious macroscopic diagenetic features, are consistent with organic molecular markers suggesting a microbial population and anoxia. Combining all the geochemical indicators suggests 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.