Abstract

Alternations in the Red Peak Member of the Chugwater (Triassic) Formation of western Wyoming consist of: 1) a massive siltstone; and 2) a less massive, fissile and non-fissile, poorly sorted siltstone, commonly with interbeds of well sorted siltstone, silty claystone, sandstone, and mudstone. Commonly, the massive siltstone can be divided into 3 parts on the basis of bedding characteristics; the alternations are then asymmetrically cyclical. From base to top, these cycles contain: 1) a platy to flaggy siltstone (about 1 ft); 2) a massive siltstone; i) a platy to flaggy siltstone (about 1 ft); and 3) a less massive, fissile and non-fissile, poorly sorted siltstone, with interbeds of well sorted siltstone. The 1, 2, 1 part of the cycle corresponds to the massive siltstone of the alternations. Forty-nine alternations average 10.1 ft in thickness, ranging from 4.3 to 17.1 ft. The massive siltstone parts average 8.1 ft. The less massive, fissile and non-fissile, poorly sorted siltstone and mudstone parts average 2.0 ft. The alternations begin 20 to 75 ft below the base of the Alcova Limestone Member, and extend downward 200 to 425 ft in the sequence. This is 35 to 55% of the total thickness of the Red Peak Member. Seventy-five to 85% of the Red Peak Member in western Wyoming is silt-sized sediment, with almost 45% of the total volume of silt in the alternations. Alternations in the Brunswick (Triassic) Formation of New Jersey consist of: 1) a resistant ledge of predominantly poorly sorted siltstone, with sandstone interbeds; and 2) less resistant mudstone and poorly sorted siltstone, with minor silty claystone and sandstone interbeds. Twenty-five measured alternations average 8.4 ft in thickness, ranging from 3.5 to 15.8 ft. The massive, ledge-forming part of the alternations averages 3.6 ft; the less massive part averages 4.8 ft.

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