The Claron / Wasatch Formation
Distribution and regional relationsExcept for scattered outcrops of younger rocks, the Claron formation constitutes the Tertiary of eastern Southwest Utah. It marks the southwestern limit of a sedimentary mass that through an expanse of more than a thousand square miles is a recognizable stratigraphic unit and a major element in the topography of Utah. In the High Plateaus it is displayed as the Pink Cliffs-brightly-colored high walls, marvelously decorated with carving, "the glory of all rock work." Naturally a formation with such outstanding scenic features attracted the attention of the pioneer geologists and of later students of the plateau country. Thus in the first known reference to the Tertiary of the Markagunt Plateau, Dutton(1) speaks eloquently of the "Pink Cliffs series."
"In the Tertiary beds of the plateau . . . the coloring is always striking and vies in brilliancy with the Triassic beds. The highest member is frequently almost snow-white, with a band of strong orange-yellow beneath it. But the great mass of color is a pale rosy-pink. When the sun is low and sends his nearly level beams of reddish light against the towering fronts and mazes of buttresses, alcoves, and pinnacles, they seem to glow with a rare color, intensely rich and beautiful-flesh-of-watermelon color is the nearest hue I can suggest."
Though comparable in many respects with exposures in north central Utah, the Claron formation of the Markagunt Plateau most closely resembles that in the Paunsaugunt Plateau, which has been mapped recently and described in considerable detail.(2) In these two plateaus the formation exhibits about the same range of variability in bedding, composition, texture, and color; the chief difference is the greater amount of shale and the lesser amount of coarse conglomerate in the Markagunt. In the two plateaus also the topography of the Claron is similarly expressed: relatively flat surfaces and sheer walls, alcoves, and buttresses of large dimensions, benches at various levels, and a bewildering display of small-scale erosion forms. As features in the Utah landscape, the "painted amphitheater" in Bryce Canyon National Park on the eastern face of the Paunsaugunt Plateau is matched in attractiveness by the "shining bowl" in Cedar Breaks National Monument on the western rim of the Markagunt.
In distant views the Claron formation appears as a sequence of roughly parallel beds that differ merely in thickness and color. In near views it is seen to comprise three parts, roughly defined by color and manner of erosion: a basal, generally red, unit of conglomerate and sandstone 5 to 150 feet thick; an intermediate sequence of pink limestone and calcareous shales, 500 to 900 feet thick; and a top series of white limestones. However, exposures along the Pink Cliffs show that these subdivisions, based on gross composition and color, have no persistent boundaries and that within them the composition, texture, and continuity of beds is far from uniform. Within a half-mile along strike thick, massive, nearly pure limestone may grade into thinly laminated calcareous shale or terminate in sandstone, and in places lenticular masses of conglomerate lie within the limestone several hundred feet above the base of the formation.
The basal conglomerate in the Claron formation consists of exotic pebbles in a matrix of calcareous sandstone or calcareous-ferruginous silt-stone. In places this coarse material forms a wel-defined bed, but commonly it appears as wedges and stubby lenses of closely packed pebbles about which individual pebbles are sporadically distributed. At the exposures examined the conglomerate consists chiefly of quartzite, chert, and limestone pebbles, 1/2 to 4 inches in diameter, polished, partly rounded, and variously colored. Less common are fragments of white quartz, slate, slabs of sandstone, sand aggregates, chips of shale and limestone, and various igneous rocks. Of the sandstones intermingled with or overlying the conglomerates, some are regularly bedded and consist of well-sorted coarse and fine, round grains of clear quartz. Others are lenticular, cross-bedded, and include sand concretions and bits of chert, limestone, manganese, and iron.
Of the rocks that make up the Claron formation, limestone is predominant. From its top to its bottom the formation is calcareous. In fact, the terms "Claron formation" and "Claron limestone" are substantially synonymous. In some cliff sections the limestone appears as undivided masses 100 feet thick and so compact, homogeneous, and fine textured as to justify the local name "pink chalk." Commonly, however, the massive beds are less than 25 feet thick and are separated from each other by thinly laminated hard pure limestone, by sheets and lenses of calcareous sandstone and shale, or by porous material that resembles travertime. In places the rock is brecciated, and includes concretions of hard limestone, isolated pebbles of quartzite, and lenses of conglomeratic sandstone. Many of the thick beds are marked by "honeycombs," and by cavities lined with calcite crystals. On the plateau tops open cracks and sink holes lead downward to tunnels that carry underground water to the cliff faces giving rise to springs. The usual color of unweathered specimens of the limestone, and to a large degree also the sandstones, conglomerates, and shales that make up the Claron formation is red, gray-pink, or nearly white, but over large areas wheathered surfaces of the lower part are prevailingly pink, and of the upper part white. Viewed in detail, the Pink Cliffs present patches and streaks of red, brown, yellow, lavender, and purple, and seemingly innumerable intermediate shades ; at Cedar Breaks, 47 color tones have been distinguished by artists. The coloring matter is chiefly iron and substantially the range in color, especially of the reds and pinks, records the amount of iron in the calcareoussands and silts as originally deposited and the chemical changes induced by weathering. The dense, red, highly calcareous beds contain the most iron; the porous white sandy beds, the least. From some beds the iron once present seems to have been removed by ground water seepage.
Origin and Age
Likewise, Dutton(4) recognized that the fossils and rocks of the Claron are features of fresh-water sediments.
"The Tertiary system of the Plateau Country is lacustrine throughout, with the exception of a few layers near the base of the series which have yielded estuarine fossils. The widely varying strata were accumulated on the bottom of a lake of vast dimension and were derived from the waste of mainlands and mountain platforms some of which are still discernible."
Evidence from various sources compiled elsewhere(5) fail to support Dutton's assumption that the interbedded and laterally intergraded lime-;tone, shale, and conglomerate, highly variable in composition, texture, and structure, that make up the Claron formation were contemporaneously leposited in a "continuous lake." In fact, typical lacustrine sediments in the Tertiary of southern Utah are rare and of small extent. The physical makeup the vertical and lateral distribution of beds in the Claron suggest deposition by streams of fluctuating volume-streams tributary to basins of Efferent sizes and depth, some of them overlapping, others widely separated; some fairly permanent and others filled and dried up in response to periodic -ainf all.
Fossils in the Claron formation are well preserved, but scarce. They represent few species and seem not to be segregated in colonies. The genera most commonly seen are Unio, Physa, and Viviparus, which occur also in the underlying Kaiparowits formation (Cretaceous) and hence, until critically studied, are of uncertain value as time markers. Beds near the top of the formation in Jericho Canyon contain Helix (Macrocyclis) spatiosa Meek and Hayden, and Celliforma spirifer Brown, regarding which Reeside remarks: "Both of these forms have been reported from the later Eocene chiefly. The first is a very large gastropod, probably a land-snail. The second appears to be the larval cell of a bee." Other fossils identified by Reeside are Planorbis utahensis Meek, "not reported below the Claron"; and Bulinus sp. Some of the shale beds retain impressions of leaves. There is a surprising absence of fossil bones.
Though beds classed as Claron in eastern Southwest Utah are unquestionably of Eocene age, neither their fauna nor their physical features serve to place them more precisely in the time scale. That they are not the oldest Tertiary is shown by the presence among the fossils of forms reported to be "later Eocene" in age and by the evidence that erosion was in progress in the Pink Cliffs region while as much as 2,000 feet of Tertiary strata were being laid down in other parts of Utah. Thus the great hiatus in Cretaceous-Tertiary sedimentation of the Markagunt Plateau is represented in the Claron Plateau by an uninterrupted sequence of late Cretaceous, Paleocene, and Eocene strata.(6) Attempts to correlate subdivisions of the Tertiary in the southern High Plateaus with those farther north have yielded no satisfactory results, and it seems probable that the two regions have different Cenozoic histories.
1. Dutton, C. E., Report on the geology of the High Plateaus of Utah; U. S. Geog. Geol. Survey Rocky 2. Gregory, H. E., The Paunsaugunt region, Utah; U. S. Geol. Survey Prof. paper, [awaiting publication]. 3. Gilbert, G. K., Geog. Explor. and Surveys W. 100th Mer 4. Dutton, C. E., Report on the geology of the High Plateaus of Utah, p. 158, 1880. 5. 48 Gregory, H. E., The Paunsaugunt region, Utah; U. S. Geol. Survey Prof. paper [awaiting publication]. 6. Spieker, E. M., Late Mesozoic and early Cenozoic history of central Utah: U. S. Geol. Survey Prof. paper 205-D, 1946.
-from Geology of Eastern Iron County, Utah by Herbert E. Gregory