Abstract: Lacustrine Sedimentation in Newark Basin, Pennsylvania-New Jersey, and Implications for Uranium Mineralization
Christine E. Turner-Peterson
Sedimentation in the step-faulted Newark basin was caused by alluvial fans that emptied directly into an extensive lake. Carbonate rocks and black mudstone of the Lockatong Formation were deposited in the center of the lake; and gray sandstone, red mudstone, and red siltstone of the Stockton and Brunswick Formations were deposited mainly along the margins of the lake. Fluvial deposition in the basin was minimal.
The Lockatong was deposited in a low-Eh, high pH lacustrine environment, as indicated by the black color, pyrite, and abundant analcime. The Stockton also was deposited in a low-Eh environment, as indicated by the presence of pyrite and alteration of clays to biotite; destruction of organic matter in the marginal-lacustrine sandstones is attributed to the high pH in the lake.
Uranium mineralization in the basin is restricted to the Stockton and Lockatong and occurred in two stages. In stage 1, lacustrine sedimentation in an alkaline lake with a low Eh at the center and a slightly higher Eh at the margins allowed uranium to precipitate syngenetically in the euxinic central parts to form low-grade (0.01 to 0.02%), widespread deposits in the black mudstones. Simultaneously, the high pH of the lake water caused destruction of organic matter and intrastratal leaching of uranium in the nearshore sands. In stage 2, compaction during diagenesis caused injection of reducing fluids from the black mudstones of the Lockatong into the adjacent sandstone facies of the Stockton, lowering the Eh in these sandstones and allowing precipitation of uranium and pyrite around a d near clay clasts. The clay clasts may have localized the uranium by adsorption of uranyl ions onto hematite in the clasts prior to arrival of the reductant, and the availability of iron may account for the localization of pyrite in and near the clasts as well.
A lacustrine interpretation for the Lockatong and most of the Stockton permits construction of a model in which the reductant (from the black mudstones of the Lockatong) and host sandstone (marginal-lacustrine sandstones of the Stockton) are in a close spatial relation and movement of great volumes of water for long distances is not required to account for mineralization.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC