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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 47 (1963)

Issue: 2. (February)

First Page: 373

Last Page: 373

Title: Cyclic Lacustrine Sediments in (Upper Triassic) Lockatong Formation, Central New Jersey and Adjacent Pennsylvania: ABSTRACT

Author(s): F. B. Van Houten

Article Type: Meeting abstract


Two kinds of cycles: "detrital" and "chemical" can be recognized in the Lockatong Formation, a lacustrine deposit of Late Triassic age in central New Jersey and adjacent Pennsylvania.

"Detrital" short cycles, averaging 17-22 feet thick, comprise several feet of black shale succeeded by platy carbonate-rich mudstone in the lower part and gray massive calcareous silty argillite in the upper. The argillite has a small-scale crenulate fabric produced by crumpled casts of shrinkage cracks and burrows. Thicker, coarser-grained "detrital" cycles contain 3-5-foot layers of thin-bedded, commonly cross-stratified, fine-grained sandstone. Some sandstone has small-scale convolute bedding.

More common "chemical" short cycles average 7-14 feet thick. Lower beds are alternating platy carbonate-rich mudstone and marlstone ½-3 inches thick, extensively broken by crumpled shrinkage cracks. Locally, initial deposits are crystalline pyrite or dolomite (rarely calcite) as much as an inch thick. In the middle, several feet of dark gray mudstone encloses 1-3-inch layers of disrupted gray marlstone fragmented by syneresis. The upper part is massive gray analcime- and carbonate-rich argillite containing as much as 7 per cent soda, as little as 47 per cent silica, and a maximum of 35-40 per cent analcime. The argillite is brecciated on a microscopic scale, probably the product of syneresis. Much of the argillite is also disrupted by crumpled shrinkage cracks irregularly filled ith crystalline dolomite and analcime.

Some thinner "chemical" cycles are reddish brown, especially in the uppermost part of the formation. These contain thin greenish gray beds of mosaic intraformational breccia produced by mud-cracking, and small lozenge-shape pseudomorphs of dolomite and analcime after glauberite(?).

Varve-counts in black mudstone suggest that short cycles resulted from 21,000-year precession cycles.

Groups of "detrital" and "chemical" short cycles in couples 325-350 feet thick apparently resulted from alternating wetter and drier phases of a long climate cycle, producing through-flowing drainage and a group of "detrital" short cycles or a closed basin and a group of "chemical" short cycles.

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