Mixed Terrigenous-Carbonate Rocks in Jurassic Arapien Shale of Central Utah

Abstract Mixed terrigenous-carbonate rocks form a significant suite in the Jurassic Arapien Shale of central Utah. They are arranged in alternating or cyclic sequences with micrite at the base. Major rock units are: (1) clayey and silty micrite (3 ft thick); and (2) mixed terrigenous-carbonate rocks (9 ft thick). The micrite is characterized by thin laminae and ripple marks; the mixed rocks by low-angle ( 50 percent terrigenous grains of sand-size) contains up to 12 percent ooliths. Limestone (>50 percent allochemical and orthochemical carbonate constituents) contains 15-71 percent ooliths. Grains range from 2φ to 4φ and are dominantly subrounded. Sorting is generally moderate (0.50-1.00φ). The sandstone is subarkose. The limestone is dominantly sandy oolitic sparite, sandy oosparite, sandy sparite, and oosparite. The sandstone and limestone are intimate mixtures with no natural break between them. In thin section the ooliths are spherical or subspherical—in some instances elongate—and are very fine to fine in grain size. Twenty-two nuclei types are recognized, but quartz nuclei and carbonate-centered ooliths are the major types. The majority of nuclei are enclosed by a single radial shell of sparite. Some ooliths have two shells of radial sparite; rarely there are three. The subarkose and the limestone have had the same paragenesis. The general sequence, from oldest to youngest event, is (1) thin clay rinds and hematite staining; (2) quartz and feldspar overgrowths; (3) rare pressure solution; (4) recrystallization of micrite to microspar and spar; (5) pore-filling by sparry calcite; (6) calcite replacement of quartz and other detrital grains; (7) clay minerals from alteration of feldspars and rock fragments into pore spaces; and (8) hematite as a stain and pore filling. The intensity of some of the diagenetic stages is different in the limestone. Much of the coarse clastic material in the Arapien Shale was cannibalized from the underlying Navajo Sandstone. Minor contributions were derived from volcanic and metamorphic source rocks. The micrite was deposited near wave base in the Arapien sea. The mixed rocks were dominantly deposited on platform edges (most common) and in open marine settings. Shoal, beach, tidal bar, lagoon, and tidal flat settings are represented.