Cyclic sedimentation and the origin of analcime-rich Upper Triassic Lockatong Formation, west-central New Jersey and adjacent Pennsylvania

Abstract

The Upper Triassic Lockatong Formation is a huge lens (3500 ft x 25 mi x 90 mi) of lacustrine deposits composed mainly of massive dark gray or reddish brown argillite and subordinate platy dark gray mudstone and black shale arranged in asymmetrical short cycles of 2 kinds. One variety, averaging 17 to 22 ft in thickness, consists of several feet of black shale succeeded by platy carbonate-rich mudstone in the lower part and massive calcareous mudstone (argillite) in the upper part. Lenses of thin-bedded fine-grained sandstone commonly occur in these cycles. A second, more common kind of short cycle, averaging 7 to 14 ft in thickness, contains less detrital and more chemical sediment. The lower beds are platy carbonate-rich mudstone and marlstone, some of which are varved. The upper part is massive carbonate- and analcime-rich mudstone (argillite), which contains as much as 7% soda and as little as 47% silica. Analcime, which constitutes as much as 35 to 40% of the argillite, or its Na-rich colloidal precursor were primary products of sedimentation in a lake from whose water calcium and magnesium carbonates had been extracted during deposition of the chemical variety of short cycle. Progressive reduction in water depth and decrease in redox potential associated with the development of a distinctive sequence of sedimentary structures through the course of a short cycle presumably resulted from a cyclic variation in rainfall accompanied by periodically accentuated compaction imposed on lacustrine mud accumulating in a continuously sinking basin. Varve counts of black mudstone suggest control by the 21,000-year precession cycle. Groups of detrital and chemical cycles occur in rhythmic couples 325 to 350 ft thick. These apparently resulted from alternating wetter and drier phases of a long cycle, producing either through-flowing drainage in the lake basin and a group of detrital short cycles, or a closed basin and a group of chemical short cycles characterized by the concentration of Na in the upper part of each cycle. In some chemical cycles reddish brown mud accumulated in an oxidizing environment induced by saline waters that inhibited organic growth.