The Curtis & Summerville Formations
(MIDDLE JURASSIC)
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Because of its color and its manner of erosion, the Curtis formation in Southwest Utah is a conspicuous topographic feature. It is a band of white rock (locally known as "the white ledge") easily traceable across country and, because it is relatively resistant to erosion, it projects as buttresses from canyon walls. Charcteristically the formation comprises three divisions: a basal unit of white gypsum, commonly more than 60 feet thick, a middle unit of red and brown sandstones 6 to 30 feet thick, and a top unit of whitish-tan limestone 20 to 50 feet thick. The middle unit is not everywhere present and in places a thin wrinkled sheet of limestone lies below the gypsum. At most outcrops studied the gypsum is a single unlaminated bed, much of it anhydrite, and provides material suitable for making plaster. At Squaw Creek about half of it is roughly bedded. At other places parts of it, particularly near its base, are definitely stratified: thick beds of amorphous gypsum are separated by paper-thin, green-white gypsiferous shale, commonly crumpled and otherwise distorted. The sandstone, perhaps properly classed as siltstones and mudstones, are evenly laminated, very fine grained, loosely cemented, calcareous, and include thin irregular beds of gypsiferous shale and lenses of conglomerate made of sand and lime pellets, nodules, and concretions, and, at one exposure, a few shale fragments. The limestone at the top of the Curtis formation is thinly, evenly foliated, very hard, and on weathering strews the surface with angular chips. Many of its beds are ripple marked and some show "worm tracks" and nodules suggestive of algae. Fossils from the limestone in a branch of Stephens Canyon are identified by J. B. Reeside, Jr., as Pentacrinus asteriscus Meek and Hayden, Ostrea strigilecula White, Camptonectes extenuates Meek and Hayden, Arctica sp., Panope n. sp., and Trigonia; indeterminate. Equivalent strata in Parunuweap Valley yielded Dosinia jurassica (Whitfield) and undetermined species of Ostrea, Neritina, Ostracoda, Trigonia, and Pentacrinus. At its type locality, San Rafael Swell, the Curtis contains five species of fossils that belong to the fairly well-known fauna of the upper part of the Sundance formation. From a study of all available material, Reeside (1) concludes that "the fossils in the Curtis place it below the middle of the Upper Jurassic (mainly Argovian)."
A petrographic study of beds considered characteristic of the Winsor formation shows angular and subangular grains of quartz (about 90 percent of the rock), calcite or dolomite, biotite, zircon, magnetite, and clay materials. It also shows that, except for relative amounts of iron, the composition and texture of the red beds and the white beds are alike.
The age of the Winsor formation has not been substantiated by fossil evidence, as none of the numerous exposures in Utah has yielded fossils. However, because of its stratigraphic position, its gross similarity with known Jurassic formations below and its marked contrast with the Cretaceous formations above, it is assigned to the Jurassic-the youngest formation of that age in southwestern Utah. With no apparent break in sedimentation, it overlies the Curtis formation (Upper Jurassic) and is set off from the overlying Cretaceous strata (Dakota sandstone of Upper Cretaceous age) by a regional unconformity that obviously represents a long period of time during which strata probably of considerable thickness were deposited, then stripped away by erosion. In any event, tracing along the base of Markagunt Plateau and across Zion National Park shows that the Winsor formation of Southwest Utah is equivalent in age and mode of deposition to the Winsor at its type locality, Winsor Cove, near Mount Carmel, Utah.(2)
1. Baker, A. A., Reeside, J. B., Jr. and Dane, C. H., Correlation of the Jurassic formations of parts of Utah, Arizona, New Mexico, and Colorado; U. S. Geol. Survey Prof. Paper 183, pp. 8, 58, 1936.
2. Gregory, H. E., The Zion Park Region, Utah; U. S. Geol. Survey Prof. paper- [awaiting publication].
SOURCE
-from Geology of Eastern Iron County, Utah by Herbert E. Gregory
As mapped by Gilluly and Reeside the Summerville at its type locality, the San Rafael Swell, comprises "thin-bedded chocolate-colored sandstone, earthy redbrown sandstone and shale, some gypsum, and a little limestone" that conformably overlie the fossiliferous Curtis formation and unconformably underlie strata referred to the Morrison. In the Kaiparowits region Gregory and Moore 13 found few outcrops that could definitely be correlated with the Curtis and experienced difficulty in fixing the stratigraphic limits of a series of red and variegated sandstones and shales comparable in position and mass composition with the Summerville. Many measured sections were therefore classed as "San Rafael group and Morrison formation undifferentiated."
In his studies of the Moab region Baker 14 found the Curtis formation absent and not represented by an unconformity, recognized the Summerville (25 to 65 feet thick) and correlated it with "the thick and distinct Summerville formation in the San Rafael Swell", and placed the top of the Summerville "at the base of the first ledge of white sandstone in the Morrison", where an "apparent unconformity" was observed. In the San Juan country strata in the position of beds reasonably assigned to the Summerville are little like those at the type locality, and the contact of the Summerville and Morrison is somewhat arbitrarily drawn. In cliffs bordering the San Juan River, in Butler Wash and northward, the formation includes red, brown, white, and yellow sandstones and mudstones; red and variegated shale; some conglomerate of quartz, lime pellets, and mud balls; and a few thin lenses of limestone, gypsum, and large chert concretions. The sandstones are foliated or cross-bedded, lenticular, and weakly or firmly cemented with lime and iron. Most of the shale is gypsiferous and flaky; some of it is wrinkled or cut by tiny faults. All the beds tested are very calcareous. The evidence is insufficient to determine the conditions under which the Summerville was deposited. The relatively large quantities of chert suggest marine origin, but the lenticular bedding and the capricious changes in texture are more characteristic of stream deposits (pl. 14, A).
An unconformity at the base of the Summerville that might represent the absent Curtis formation was not demonstrated. In Allen Canyon and in upper Butler Wash lenses of coarse white sandstone fill shallow channels in the Entrada, but at Tank Mesa and on the back of the Comb monocline east of Arch Canyon the Entrada seems to grade regularly upward into interbedded white sandstone and red shale. Baker 35 reports "that the Curtis formation grades laterally into the Summerville formation near Dellenbaugh Butte, on the Green River." The upper limit of the Summerville (?) along the San Juan is more clearly marked. The con of the brightly colored calcareous and gypsiferous sandstones and shales assigned to the Summerville (?) with the overlying massive, cliff-making buff and white sandstone classed as basal Morrison (Bluff sandstone member) is characterized by an abrupt change in lithology and in most places by and unconformity in bedding that suggest regional erosion before the upper beds were laid. down. The significance of this relation is uncertain in view of the fact that somewhat similar breaks in continuity of sedimentation appear above and below the contact. The division plane may prove to lie at one of several horizons within 500 feet of Summerville (?) or of Morrison sediments. The strata here called Summerville (?) are the "crinkled beds" of Lee 11 and his coworkers, who correlated them with the Carmel formation.
SOURCE
-from The San Juan Country - A Geographic and Reconnaissance of Southeastern Utah by Herbert E. Gregory
