Abstract: Sedimentation and Diagenesis of Lacustrine Sequences (Upper Triassic) Lockatong Formation, Newark Rift Basin, Eastern USA

Mohamed El-Tabakh

The lacustrine sediments of the Upper Triassic Lockatong Formation in the Newark basin (930 m) were deposited in rift lakes which occupied broad areas during their high-stand water levels. These deposits exhibit a general upward-shallowing sedimentary pattern as well as a smaller-scale depositional cyclicity that also shallowed upwards. Sedimentation within the rift lakes

was apparently controlled by climatic conditions, although the basinal-bounding faults also were active during sedimentation. This study utilizes a continuous cored sedimentary sequence (1220 m) which is composed of successive upward-shallowing lacustrine cycles (1 to 5+m thick). The lake sequences follow a repetitive pattern of cyclicity which began with deep, fresh-water deposition and then passed upward into sediments that were the product of syndepositional evaporative concentration. The identified lithologies making up the successions are, in order from base to top, organic-rich black shale, sparry euhedral carbonates (interlocking), gray mudstones, siltstones and evaporite-bearing mudstones.

The diagenetic minerals present include analcime, dolomite, calcite, and ankerite. Most of authigenic minerals are identified within the massive gray, and evaporite-bearing mudstone lithologies. Petrographic investigations suggest that analcime formed as an early diagenetic mineral, by the alteration of lake clays largely during interaction with alkaline lake and groundwaters. Dolomite was penecontemporaneously formed during evaporative concentration of shallow ground waters. Ankerite, however, was formed as a later diagenetic mineral within the calcite-rich layers. Formation of most of the diagenetic changes appear to have been controlled by the early paleoenvironmental conditions and is not a pervasive overprint of the entire formation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994