Geological Survey Bulletin 1244-J Carmel Formation of The Zion Park Region Southwestern Utah - A Review

ESTABLISHMENT OF THE SAN RAFAEL GROUP

Stratigraphic studies in eastern Utah, particularly in the area of the San Rafael Swell (Gilluly and Reeside, 1928), give the basis for subdividing that part of the Jurassic sequence assigned to the San Rafael Group. In the San Rafael Swell this group, composed of limestone, sandstone, shale, and gypsum, lies between the Navajo Sandstone and the Morrison Formation. Gilluly and Reeside named, in ascending order, the Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville Formation, and constituted them as the San Rafael Group. The name Carmel, from a locality near Mount Carmel, Utah, studied by H. E. Gregory and L. F. Noble, was adopted in 1926 at a conference of H. E. Gregory, R. C. Moore, James Gilluly, and J. B. Reeside, Jr. (U.S. Geol. Survey, Press Bull. 6064, March 30, 1926). Type sections for the other formations of the group were established in the northern part of the San Rafael Swell (Gilluly and Reeside, 1928). Although the name Carmel was adopted in 1926, a section at the type locality was not described until 1931. At that time Gregory and Moore (p. 73, 74) placed the lower boundary at the top of the Navajo Sandstone and the upper boundary at the top of a thin fossiliferous limestone (unit 20, fig. 2) and tentatively identified two units above it as Summerville (?) Formation and Morrison (?) Formation.

REVISIONS

Later, Gregory (1933, p. 15) restricted the Carmel Formation in the Zion Park region by placing the upper boundary at the top of the limestone and calcareous shale sequence (units 1-11, fig. 2). He assigned the beds above this sequence to "undifferentiated Jurassic(?)" and stated that they probably represent the rest of the San Rafael Group and the overlying Morrison Formation. Gregory (1950a, p. 91) explained that redefining the upper boundary of the Carmel was the result of reexamining numerous outcrops in southern Utah and concluding that the abrupt change in sedimentation at the top of the limestone and calcareous shale sequence marked a more appropriate position for the formation boundary.

Inasmuch as continuous tracing of the formations in question is impossible and satisfactory evidence had not been found for precise correlation between the Zion Park region and the San Rafael Swell, there was still doubt about the relation of the type Carmel to the Carmel of the San Rafael Swell. Additional information from studies in south-central Utah led Gregory (1950a) to another revision in which part of the sequence that had previously been included in the Carmel was assigned to the Entrada Sandstone (unit 12, fig. 2) and the Curtis Formation (units 14-20, fig. 2). The beds of Jurassic age above unit 20 were named Winsor Formation. First reference to the Winsor was made in a report describing the geology of central Kane County, Utah (Gregory, 1948, p. 235), but the source of the name was not explained until later (Gregory, 1950a, p. 98) and the type locality was designated in a separate paper (Gregory, 1950b, p. 42). The type locality is in Winsor Cove, an open area in the valley of Muddy Creek, just west of Mount Carmel.

During the 1950's, some authors who described the San Rafael Group in areas adjacent to the Zion Park region suggested revisions of correlation that involved the Carmel Formation. These authors are not cited here, however, because a review of the overall stratigraphy of the Carmel Formation is not within the scope of this report. Stokes and Holmes (1954) used the nomenclature of Gregory (1950a) in a fence diagram that includes the Zion Park region, but they pointed out that the Curtis Formation of southwestern Utah might be a facies of the Carmel of the San Rafael Swell.

Detailed mapping and regional stratigraphic studies by members of the U.S. Geological Survey have yielded new information and given a firmer basis for correlation of Jurassic rocks of the Colorado Plateau. Wright and Dickey (1963a, b), after studying the regional stratigraphy of the San Rafael Group, concluded that the Entrada Sandstone, Curtis Formation, and Winsor Formation of the Zion Park region should be included in the Carmel Formation because all Jurassic strata above the Navajo Sandstone near Mount Carmel are correlatives of beds within the Carmel Formation as it was first recognized in the San Rafael Swell.

Despite the Carmel problems in the Zion Park region, the name has been used logically and consistently in most other parts of the Colorado Plateau. Carmel is a well-established and useful name and its continued use in the Zion Park region seems appropriate. The author, in mapping near Mount Carmel, used those limits for the Carmel prescribed by Wright and Dickey (this report, fig. 2) and also divided the formation into four members. The boundaries of these members are the same as those for the four widespread lithologic subdivisions described earlier in this report. Informal descriptive member names are assigned to all except the one that has previously been called Winsor Formation (see fig. 2).

The Carmel Formation of the Zion Park region, as described by Wright and Dickey (1963a, b), includes Gregory's Winsor Formation. Consequently, several recent publications concerning these rocks do not use Winsor Formation, and a clarification of its status should be made. The author feels that it would be advantageous to retain the name Winsor in the Zion Park region and herein changes the rank of the Winsor to a member of the Carmel Formation. The name Winsor should not be used east of the Paunsaugunt fault (see fig. 1) where it has been applied to beds that are considerably younger than the beds included in the Winsor near Mount Carmel (J. C. Wright, written commun., 1966).

Although no type section was designated by Gregory, the description of the Winsor Formation in his section 13 (Gregory, 1950a, p. 126) might be considered as typical Winsor and it was measured in, or very near, Winsor Cove. Therefore Gregory's section is here designated as the typical section of the Winsor. The two units which comprise the Winsor described in section 13 are the same as units 21 and 22 in the section describing the type Carmel (Gregory and Moore, 1931, p. 73; this report, fig. 2).