Geological Survey Circular 217 Preliminary Report on the White Canyon Area, San Juan County, Utah

Rocks in the White Canyon area consist of sedimentary formations ranging in age from Carboniferous to Jurassic(?); the entire section is more than 2,000 feet thick. These beds are on the southwest flank of the Elk Ridge anticline and have a regional dip of 2° to 3° SW. The nearest exposures of igneous rocks are in the Henry Mountains, about 7 miles to the west.

Gregory (1938, pp. 40-56) has described most of the rocks exposed in the area. The general stratigraphic features of the formations are summarized in the table referred to on the following page, part of which is taken from Gregory.

General sections of the rock formations in the White Canyon area, Utah

Carboniferous rocks

Hermosa formation

The Hermosa formation of Pennsylvanian age is exposed only in Dark Canyon, and was not examined during this investigation. Gregory (1938, p. 40) described the formation as interbedded limestone, sandstone, and shale, ranging from 400 to 1,000 feet in thickness.

Permian(?) rocks

Rico formation

The Rico formation of Permian(?) age, like the Hermosa formation, is exposed only in Dark Canyon, and was not studied by the authors. Baker (1946, pp. 32-36) has described the Rico formation of the Green River Desert-Cataract Canyon region north of White Canyon as comprising approximately 575 feet of interbedded sandstone, shale, siltstone, and limestone, with sandstone predominating.

Permian rocks

Cutler formation

The Cutler formation in southeast Utah consists of six units, namely, the Halgaito tongue, the Cedar Mesa sandstone member, the Organ Rock tongue, the De Chelly sandstone member, the Hoskinnini tongue, and the White Rim sandstone member. Of these, only Cedar Mesa, Organ Rock, and White Rim members are present in the White Canyon area. The upper part of the Organ Rock tongue is a cliff-forming red sandstone similar to the Hoskinnini tongue, and may be the equivalent of the Hoskinnini.

Cedar Mesa sandstone member.--The Cedar Mesa sandstone member of the Cutler formation forms the floor of White Canyon and has been trenched by streams to depths of 50 to 200 feet. It comprises thick beds of light-cream colored, cross-bedded fine grained calcareous sandstone with local thin beds of red shale near the top. The cross bedding is eolian type and is foreset to the southeast. In Dark Canyon the Cedar Mesa sandstone member directly overlies the Rico formation with apparent conformity.

Organ Rock tongue.--The Organ Rock tongue of the Cutler formation is composed of 200 to 300 feet of pale and dark reddish-brown micaceous siltstone and very fine grained sandstone, the upper 50 to 100 feet of which forms a cliff. Gray sandstone beds are present in the upper 50 feet of the formation near the western end of the mapped area. The upper cliff-forming sandstone may be the equivalent of the Hoskinnini tongue.

White Rim sandstone member.--The White Rim sandstone member, described by Baker (1946, pp. 44-48) and Hunt (1952?), occurs only in the lower part of White Canyon, where it is a light-colored fine-grained sandstone, less than 10 feet thick. It thins and pinches out a few miles east of the Colorado River.

Triassic rocks

Moenkopi formation

The Moenkopi formation of Early Triassic age comprises 200 to 350 feet of thin-bedded dark-brown to grayish-red siltstone, brown to yellow shale, and grayish-red to light-gray sandstone. Many of the sandstone beds contain abundant clay balls. In the western part of the mapped area the lower part of the Moenkopi contains lenticular beds of light-gray sandstone, and beds of dark-gray, petroliferous cherty conglomerate. The lowest bed of the cherty conglomerate forms the base of the formation and fills channels cut into the underlying Organ Rock tongue. The lower beds of the Moenkopi formation and upper beds of the Organ Rock tongue are crumpled locally by small folds as much as 5 feet high and 5 to 10 feet wide.

Shinarump conglomerate

The Shinarump conglomerate, the principal ore-bearing formation in the White Canyon area, rests unconformably on the Moenkopi formation and was regarded by Gregory (1938, p. 49) as the basal conglomerate of the Upper Triassic Chinle formation, The most striking feature of the formation is its extreme lenticularity. Conglomerate of the Shinarump 40 feet thick may pinch out completely in an outcrop less than 2, 000 feet in length. The thicker parts of the formation resulted both from the filling of channels in the underlying Moenkopi formation and from lateral facies change to conglomerate of the overlying shales of the Chinle formation. The formation reaches a maximum thickness of 75 feet.

The Shinarump conglomerate is composed of interbedded yellowish-gray to gray, red, and brown sandstone, conglomerate sandstone, conglomerate, and gray to yellowish-gray clay and siltstone, Sandstone beds, locally conglomeratic, form the top of the formation in most exposures, and range in thickness from less than 1 foot to more than 30 feet. Sandstone constitutes the entire formation along some segments of the outcrop, especially where the Shinarump is less than 10 feet thick.

The sandstone beds of the formation consist of well-rounded to sub-angular quartz and microcline grains and a few zircon, apatite, and tourmaline grains with calcite, clay, and iron-oxide cement. In some places molds of logs remain, the wood itself having been removed. A few logs have been replaced by copper and uranium minerals and iron oxides; other logs have been silicified.

The conglomerate of the formation contains reworked siltstone fragments of the Moenkopi formation, clay balls, carbonized wood, and quartz pebbles. Where all these types of fragments are present it gives the conglomerate a "trashy" appearance. These "trashy" conglomerates are common at the base of many channel scours.

Gray clay and siltstone beds underlie the uppermost sandstone and are interbedded with it in some localities. They are most common in the channel fills, constituting the greater part of the fill in some exposures. These beds range from 2 to 30 feet in thickness. The minerals contained in the clay are principally hydromuscovite, kaolinite, and quartz. Very little montmorillonite has been identified from White Canyon. Many of the clays contain abundant fragments of charred wood, some of which has been completely carbonized to mineral charcoal. The charcoal is porous and may be pulverized easily to a black powder (Waters, A. C., personal communication). Mineral charcoal is fairly common in sedimentary formations that contain lignite coal or other carbonized plant material. In the Shinarump and Chinle formations the presence of volcanic material in the charcoal-bearing beds suggests that the charcoal was produced by falls of hot ash that partly burned the Late Triassic forests.

Chinle formation

The Chinle formation has been divided into three units: upper, middle, and lower. The lower unit of the Chinle ranges from approximately 220 to 245 feet in thickness and locally consists of variegated gray bentonitic clay, and lenticular beds of sandstone and conglomerate. A resistant sandstone and conglomerate bed, 10 to 100 feet thick, forms the top of this unit in many places. This sandstone bed which has been confused with the Shinarump conglomerate by some geologists, has been differentiated from the rest of the lower part of the Chinle and is shown on plate 1 by the symbol kcs.

The middle unit of the Chinle is composed principally of variegated calcareous and bentonitic mudstone and siltstone with minor amounts of cherty conglomeratic claystone. Much of this unit may be altered tuffaceous sediments. Measured sections range from about 300 to 330 feet in thickness.

The upper unit of the Chinle comprises 100 feet of reddish-brown very fine grained calcareous sandstone, siltstone, and mudstone with some thin beds of gray and purplish-gray clay.

Jurassic(?) rocks

Glen Canyon group

Wingate sandstone.--The Wingate sandstone was not studied in detail by the authors in the White Canyon area. Gregory (1938, p. 54) has described the formation as consisting of about 300 feet of reddish-brown fine-grained cross-bedded massive sandstone. The beds form a vertical cliff capping the higher points in the area.

On the south rim of Frey Canyon the lowermost 30 to 40 feet of the Wingate consists of laminated coarse- to fine-grained sandstone with fluvial-type cross bedding. The fine-grained sandstone above these basal beds is probably of eolian origin.

Kayenta formation.--The Kayenta formation caps the Wingate sandstone in many places; it was not examined during the present examination. The Kayenta consists largely of fine-grained red fluvial sandstone.

Navajo sandstone.--Remnants of Navajo sandstone cap the Kayenta in a few places, but are inaccessible in the area studied.