THERMAL HISTORIES OF THE NEWARK AND TAYLORSVILLE BASINS: WHY THICK TRIASSIC SOURCE ROCKS ARE OVERMATURE

Late Triassic- Early Jurassic Newark Supergroup basins have been past targets of petroleum exploration, but the major lacustrine black shale-dominated formations are generally overmature. New borehole and surface vitrinite reflectance data for the Newark basin shows the maturity range of the late Triassic (Carnian) Lockatong Formation, not affected by diabase contact metamorphism, to be 1.9- 3.4% mean random reflectance (Ro). In the Taylorsville basin Carnian Port Royal Formation and older Doswell Group, reflectances range from 0.5- 2.9%, but maturities in the oil window (0.5- 1.3%) are confined to a small rider block adjacent to the border fault and fluvial equivalents on the hanging wall basin edge.

Patterns of vitrinite reflectance plus modeling show that while thermal evolution of both basins is due to advective modification of basement conductive heat flow by basin-scale groundwater systems, there are material differences between the Newark and paleoequatorial Taylorsville basins in 1) basement heat flow and 2) basin hydrostratigraphy. Basal basin heat flow is a relict of position relative to the Alleghanian orogenic metamorphic/ thermal axis which was also the locus of post-orogenic collapse. The Newark Supergroup half graben are here recognized as one phase in the collapse of the late Paleozoic orogen, succeeding early post-orogenic ductile exhumation and preceding initiation of Atlantic seafloor spreading. The Taylorsville basin is located on the metamorphic axis and had a background geotherm of ~45°C/km, modified by a syn-rift gravity-driven groundwater system producing crossbasinal variation in geothermal gradient (40-55°C/km). The Newark basin, west of the metamorphic axis, had a background gradient of ~25°C/km. Syn-rift steady state heated groundwater flow through basal basin fluvial strata conductively heated overlying formations to ~35°C/km. The interbasin differences in advective heat flow patterns are due to latitudinal climate influence on basin syn-rift coarse sedimentation and resulting hydrostratigraphic architecture.

Change in stress regime due to post-rift inversion produced transient advective events and exhumation that both cooled the basin-scale groundwater systems and variably eroded 0.4-3.1 km from the Taylorsville basin and >6 km from parts of the Newark basin.