Diagenesis of the Newark Rift Basin, Eastern North America



Late Triassic nonmarine strata in the Newark Rift Basin, eastern North America, accumulated in a subsiding half-graben prior to opening of the Atlantic Ocean. These strata consist, in ascending order, of the Stockton, Lockatong, and Passaic formations. Although different in specific lithology, these formations all exhibit diagenetic fabrics dominated by authigenic albite and analcime. These same minerals have a similar presence in Late Triassic (Newark Supergroup) strata of other rift-related basins to the north, suggesting that related authigenesis is not simply a result of local diagenetic factors.

The basal deposit, the Stockton Formation, is composed of fluvial sandstones and overbank mudstones, with nodular pedogenic calcite (calcrete). During burial, original micrite was first recrystallized into sparry calcite and then later subjected to partial replacement by authigenic albite, which is also present as overgrowths and void fillings in overbank mudstones. The Lockatong Formation contains organic-rich shales, carbonates, and evaporative mudstones deposited under cyclic conditions in laterally extensive lacustrine environments. Analcime comprises up to 40% of these strata by volume, occurring within the matrix, as a replacement of original carbonates and evaporites, and as fillings in macrovoids. The overlying Passaic Formation is made up of massive red mudstones, evaporites, and local calcareous lacustrine sequences. Evaporites are replaced by coarse-grained anhydrite together with some authigenic albite.

A central question concerns the source for the Na, Si, and Al required for albite and analcime authigenesis. It is suggested that, in addition to alteration of primary siliciclastic material, sodium in particular was supplied in two ways: (1) from high concentrations in original evaporative brines and groundwaters (Lockatong and Passaic formations); and (2) from dissolution of associated sodium-bearing evaporites (Lockatong and Passaic Formation) during diagenesis. It is proposed here that basin-sourced, Na-enriched brines circulated through the section over time. As albite is more stable at elevated temperatures relative to analcime, it developed in the lowermost strata of the basin (Stockton Formation). Analcime is more prevalent in the overlying Lockatong Formation.