Thermal maturity of Jurassic shales from the Newark Basin, U.S.A.: Influence of hydrothermal fluids and implications to basin modeling

Organic geochemical investigations were conducted on a series of cores that systematically sampled the uppermost Jurassic strata from the northern Newark Basin. Each sedimentary unit consists of fluvial red sandstones and siltstones with cyclic deposits of interbedded black lacustrine shales and gray deltaic siltstones. In a suite of organic-rich shales from the Boonton, Towaco and Feltville Formations, organic maturation parameters were used to determine aspects of the thermal history of the Newark Basin. Comparisons of model calculations and measured maturities support³⁹ Ar/⁴⁰ Ar-geochronometer studies that indicate a hydrothermal event occurred ∼175 Ma ago. An increase in the regional geothermal gradient to ∼7.5°C/100 m for ∼5 Ma best conforms to the organic geochemical observations. Biomarker compounds in Boonton and Towaco strata should have been relatively unaltered by this regional event, but anomalous molecular distributions in the organic-rich rocks may have resulted from localized heating by hydrothermal fluids. The effects of this interaction would be very subtle and may be indistinguishable from variations caused by differences in organic facies. Within this uncertainty, sterane and hopane isomerization and steroid aromatization reactions advanced in the Boonton and Towaco Formation strata primarily because of burial and normal geothermal heating that followed the hydrothermal event. Biomarker kinetic models indicate that ∼2400 m of Boonton and post-Boonton strata were eroded after basinal uplift commenced ∼50 Ma ago.