Abstract

Sedimentation in the Triassic-Jurassic Newark basin of Pennsylvania and New Jersey was the result of infilling of a rift basin formed prior to continental breakup. Asymmetrical development of facies resulted from differential tectonism in an internally-drained lake basin. The northwestern border is characterized by normal faults with their downthrown sides toward the basin. Recurrent movement along these border faults during deposition maintained a steep slope resulting in the interfingering of low-energy lake deposits of the Brunswick Formation with coarse, poorly-sorted fanglomerates of the Hammer Creek Formation. Fans in the Hammer Creek also reflect sedimentation on a steep, fault-dominated margin; they are restricted in areal extent and are dominated by debris flow units. In contrast, the Stockton Formation, which crops out along the southeastern border of the basin, contains laterally coalescing fans that are dominated by alluvial deposits. Sedimentation along this border was not disrupted by faulting; the development of a marginal-lacustrine sandstone facies within the Stockton can be attributed to a gentle slope of the basin floor along the southeastern margin, which permitted efficient sorting of detritus delivered to the basin.

Other facies in the basin include nearshore-lacustrine deposits of red mudstone, siltstone, and fine-grained sandstone in the Stockton and Brunswick Formations, as well as a basin-center facies of black, analcimic mudstones and carbonates that compose the Lockatong Formation. Lacustrine deposition was the dominant process within the Newark basin, accounting for all of the sedimentary sequence except for the alluvial fan and fluvial facies that occur at the basin margins. Coarse clastics in the basin were apparently delivered from all directions making it unlikely that an outlet existed. The presence of carbonate rocks, analcime, and fish remains suggests that a perennial alkaline lake occupied the closed basin.

Uranium occurs in two facies; the marginal-lacustrine sandstone facies of the Stockton Formation and the offshore-lacustrine black mudstone facies of the Lockatong Formation. Mineralization in the latter facies is considered nearly syngenetic, with uranium being fixed near the sediment-water interface in the reducing, organic-rich basin-center sediments.

Mineralization of the sandstone, on the other hand, occurred some time after deposition and is restricted to places where the marginal-lacustrine sandstones interbed with offshore-lacustrine black mudstones, without the intervening nearshore-lacustrine oxidized facies. It is proposed that the black mudstones served as source rocks of humic substances. Reducing, alkaline conditions within these mudstones would have favored preservation and mobilization of the humic substances so that they could have been expelled along with the pore waters during compaction. The soluble organics may then have been fixed as tabular masses in nearby sandstone beds by organo-clay reactions, thus preventing the movement of the organics great distances from their source. Uranium in ground water flowing through the sandstone beds was concentrated by the organics in the tabular zone. This hypothesis provides for a convenient, local source of the soluble organic carbon that is associated with the tabular uranium deposits.