Great Disturbances in the Force

Arches National Park, located in east-central Utah, is a 74,000-acre park with the greatest concentration of rock arches in the world (figure 1). The arches were formed by a unique set of circumstances involving Middle Pennsylvanian to Late Triassic salt movement, Middle Pennsylvanian to Late Cretaceous sedimentation, and Tertiary and Quaternary folding, faulting, erosion, and salt dissolution (only a brief summary of the processes is given in this guide; for more detail see Doelling, 2000). Most arches are formed in three geologic units: the Dewey Bridge Member of the Carmel Formation, the Slick Rock Member of the Entrada Sandstone, and the Moab Member of the Curtis Formation (figure 2). These three members were formerly all included in the Entrada Sandstone. Recently, other geologists and I determined that these members are more appropriately assigned to the formations stated above. Until officially changed, these new assignments are considered informal.

www.e-education.psu.edu/geosc10/l11_p3.html
Dinosaur National Monument lies in western Colorado and eastern Utah. The key rocks were deposited in swamps and and along rivers during the Jurassic in the middle of the Mesozoic, and are called the Morrison Formation. Before the modern Rockies were raised, sluggish streams flowed across the basins of this region, with numerous low, wet floodplains. Dinosaurs flourished. After some died, their bodies were washed up on sandbars, where their bones were buried before they were consumed by scavengers and gnawers. Over time, minerals carried in groundwater reacted chemically with the bone, depositing silica.

coloradogeologicalsurvey.org/colorado-geology/colorado-points-of-geological-interest/pogi-interactive-map/garfield/dinosaur-national-monument/
The world-famous Douglas Quarry Visitor Center, which sits on the Utah side of DNM, preserves 1,400 dinosaur bones left in place as they were deposited 145 million years ago. More than half of all the different kinds of dinosaurs that lived in North America in the late Jurassic Period are found in this quarry. Besides the huge plant-eating sauropods Apatasaurus, Camarasaurus, Barosaurus, and Diplodocus, which compose three-quarters of all of the fossils in the exposed quarry wall, the dinosaurs of the quarry include other plant-eaters ranging from large to small, with a few meat-eaters as well.

This centerpiece visitor center also houses exhibits about the bones and their burial and preservation, a preparation laboratory, research facilities, and a bookstore operated by the Intermountain Natural History Association (open during the summer) that specializes in dinosaur and natural history books. A second visitor center near Dinosaur, Colorado, features an orientation movie and a bookstore.

pubs.usgs.gov/pp/1035a/report.pdf

The Triassic and Jurassic rocks of the Western Interior were deposited on a stable shelf that sloped mainly westward in Triassic time and mainly northwestward in Jurassic time.

In areas where fossils or age dates are lacking, the unconformities are essential for establishing correlations.
The Jurassic unconformities are designated J-0, J-l, J-2, J-3, J-4, and J-5. Of these, the J-2 surface is the best preserved and most widespread. It extends throughout the Western Interior and truncates the older unconformities in different parts of this area. Consequently, the J-2 surface is discussed and illustrated in much more detail than the others.

The J-0 unconformity is at the base of the Glen Canyon Group and the equivalent Nugget Sandstone over wide areas of Nevada, Arizona, New Mexico, Colorado, Utah, Wyoming, and Idaho. The J-0 unconformity in most of this area lies at the top of the Chinle Formation or the equivalent Popo Agie Formation.

The J-0 unconformity is truncated by the J-2 unconformity at the base of the Twin Creek Limestone a short distance to the north of Garns Mountain [II] in Idaho, and by the J-l surface at the base of the Gypsum Spring Member of the Twin Creek Limestone in northwestern Wyoming. The J-2 unconformity at the base of the Entrada Sandstone or Canyon Springs Sandstone Member of the Sundance Formation terminates the J-0 unconformity in south-central Wyoming, western Colorado, and northwestern New Mexico (B-ff, C-C, A-A', and map B, pi. 1).I n southwestern Utah and southern Nevada the J-0 unconformity is an erosion surface that bevels across older parts of the Chinle Formation.

The J-l unconformity is recognized mainly in Wyoming and adjacent parts of Idaho and Utah, where it is the contact between the overlying Gypsum Spring Formation (locally Gypsum Spring Member of the Twin Creek Limestone) and the underlying Nugget Sandstone or older rocks.

the J-2 surface of unconformity extends from Buckhorn Wash [U12], in the San Rafael Swell (fig. 1), central Utah, eastward to Ralston Reservoir [C32], near Denver, Colo., and from Zuni, N. Mex. [NM20], at least as far north as Redwater Creek [SD1] in the Black Hills, S. Dak.

The J-2 unconformity extends throughout the Western Interior. All the older unconformities are truncated by the J-2 surface in one part of the area or another. The areal extent and stratigraphic position of the J-2 surface and its relationship to other surfaces are shown in the five restored sections and the two paleogeologic maps on plate 1. At several localities, younger unconformities truncate and destroy the J-2 surface. The Cretaceous Dakota Sandstone, for example, rests on Precambrian rocks in north-central New Mexico (Muehlberger and others, 1960), and the J-2 surface in this area has been destroyed. In north-central Colorado, near Hot Sulphur Springs [C25], the J-5 surface at the base of the Morrison Formation has eroded the J-2 surface, removing the chert pebbles commonly associated with the J-2 surface. Pebbles found at the base of the Morrison at that locality are mainly quartz derived from the underlying Precambrian.

The J-2 surface differs from nearly all the others in that throughout most of its extent it is associated with chert pebbles, shown in all our restored sections by a triangular symbol. The unconformity and(or) the associated pebbles have not been recognized everywhere, but have been described from widely scattered localities. The pebbles have been reported in the subsurface and on the outcrop at the base of the Sawtooth, Piper, Sundance, and Twin Creek Formations in Montana, South Dakota, Wyoming, Idaho, and Utah.

An erosion surface (the J-2 unconformity of this report), as well as the associated chert pebbles, has been recognized at the top of the Navajo Sandstone in southern Utah and northern Arizona by Peterson (1973), Peterson and Barnum (1973a, b) and by Zeller and Stephens (1973). The J-2 surface in this area is discussed in more detail by Peterson and Pipiringos (1978).
The pebbles have been recognized in northwestern New Mexico by Moench and Schlee (1967, p. 7) and by Green (1974, p. D4).

Not generally recognized, heretofore, is that the pebbles reported in all these scattered places overlie a single widespread surface of erosion. Also the chert pebbles that characterize the unconformity are of limited variety that can be identified with certainty as to source. Recognizing and extending this surface into areas where it had not previously been known solves many old correlation problems. Chert pebbles in general are incorporated in rocks directly on the erosion surface but in a few places they occur within a few centimeters to a meter or two above it.

Locally, the pebbles are incorporated in dike-like injections of the pebble-bearing units into the underlying unit. Some of these injections are wedge-shaped and undoubtedly occupy widened joints as at Horse Creek, Wyo. [W61], Four Mile Canyon, Colo. [C27] (Pipiringos and O'Sullivan, 1976, Iocs. 8, 22), and Burns, Colo. [C29].

Others occupy irregularly shaped cavities in the underlying unit which were hollowed out by erosion or weathering as at Arches National Monument, Utah [U20] (Pipiringos and O'Sullivan, 1975, fig. 4f).

The chert pebbles are of four principal kinds:

(1) pebbles banded in dark and light shades of gray, derived from the Gypsum Spring Formation (see Pipiringos, 1968, fig. 18);

(2) pink and red pebbles, most bleached by sunlight to shades of white, pale red, cream, and light gray, and eroded from the Glen Canyon Group, mainly from the Navajo Sandstone (see Baker and others, 1936, pis. 13-14; Pipiringos and O'Sullivan, 1975, fig. 6);

(3) dark-red pebbles, some banded yellow, derived from the Petrified Forest Member of the Chinle and its equivalents; and

(4) dark-gray and black, a few dark-red, and locally some banded blue and tan chert pebbles derived from Paleozoic rocks.

At places in northwestern Colorado near Precambrian terranes, metamorphic rock fragments are found in rocks directly above the J-2 unconformity (Pipiringos and others, 1969, p. N17). Frederickson, De Lay, and Saylor (1956, p. 2132-35, 2141-2144) identified several varieties of pebbles derived locally from Paleozoic rocks that are associated with the J-2 surface in the Canon City area, Colorado.
An estimated 80 percent of the pebbles associated with the unconformity were eroded from the Gypsum Spring Formation and from the Glen Canyon Group. Of the remainder, about 10 percent came from the Chinle Formation, 8 percent from Paleozoic rocks, and 2 percent from Precambrian rocks.
The chert pebbles that were derived from the Gypsum Spring Formation came from bedded chert lenses and nodules in marine limestone beds occurring near the top of the formation.
The chert pebbles derived from the Glen Canyon Group came principally from cherty limestone lenses that formed in ephemeral lakes among dunes prior to consolidation of the Navajo Sandstone. Similar limestone lenses are present, but scarce, in the Kayenta Formation and in the Wingate Sandstone.
Pebbles from the Chinle Formation are derived from bedded chert that formed in lakes that existed during deposition of the Petrified Forest Member and the ocher member of the Chinle Formation.
All these cherts are composed almost entirely of fibrous chalcedony, but the different types can be distinguished from each other both in thin section and in hand specimen.
Most pebbles on the J-2 surface that were derived from Paleozoic and Precambrian rocks are nondescript. A possible exception are the unique pebbles found on the east side of the Bighorn Basin. These chert pebbles, probably eroded from Paleozoic rocks, are alternately banded tan and milky blue. Parallel contacts between the thin bands are interrupted where digitations of blue chert fill what seem to be minute shrinkage cracks developed in the tan bands.

Chert pebbles derived from the Gypsum Spring Formation and the Glen Canyon Group are angular to subrounded and most of them are wind polished. So-called "dreikanter," supposedly derived from within and indicative of the environment of deposition of the Navajo Sandstone in the San Rafael Swell and Green River Desert of Utah (Baker and others, 1936, p. 52-53, and pis. 13,14), instead mark the J-2 unconformity of that area.

Less common are wind-polished pebbles derived from the Gypsum Spring Formation and associated with the J-2 surface in the Wind River and Bighorn Basins of Wyoming and in the Pryor Mountains of Montana. Most of these pebbles are thinly banded light to medium gray. Many are angular and are enclosed in a black rind less than 0.5 mm thick; they obviously were not transported far before the desert varnish was developed. In some places the chert pebbles in rocks directly above the J-2 surface and associated with it can be distinguished from angular fragments of chert beds directly underlying the J-2 surface only by their desert patina or by their high polish.At many places, as at Dry Muddy Creek [W8] and Douglas, Wyo. [W41], the Gypsum Spring-derived pebbles are well rounded as though worn by wave action on a beach, or more probably by transportation in ancient stream channels; they were later fluted, etched, and polished by the wind. Most of these rounded pebbles are a few centimeters in maximum
diameter, but locally they are as much as 10-13 cm long.



Extensive inter-tonguing has been described in northern Arizona and southern Utah between the underlying Navajo and the overlying Carmel Formation (Wright and Dickey, 1963; Phoenix, 1963, p. 32-33). This reported inter-tonguing, however, occurs above the Navajo Sandstone entirely within units of Middle Jurassic age. The Temple Cap Member of the Navajo Sandstone has been considered to be a tongue extending into the Carmel Formation. Recent stratigraphic studies by Peterson and Pipiringos (1978), however, show that instead of being a tongue of that formation, the Temple Cap is everywhere separated from the Navajo Sandstone by the J-l unconformity. Similarly, the Thousand Pockets, heretofore considered to be a tongue of the Navajo Sandstone, instead interfmgers with the Carmel Formation as much as 60 m above the J-2 unconformity and nowhere joins the main body of the Navajo. The J-2 surface has been traced by us into northwestern New Mexico where its recognition solves a vexing stratigraphic problem. In the Wingate area [NM15], the J-2 surface rests on the Chinle Formation (C-C", pi. 1). Many pebbles and granules associated with the J-2 surface are from the Chinle Formation, but about half the granules are types derived from cherty limestone lenses of the Glen Canyon Group. A sequence of partly crossbedded sandstone immediately overlying the J-2 surface was correlated with the Wingate Sandstone by Harshbarger, Repenning, and Irwin (1957, p. 8).
Inasmuch as this sandstone sequence overlies the J-2 surface, it cannot be an equivalent of the Wingate Sandstone, which is beneath the J-2 surface. The relation of the J-2 surface convinces us that the partly crossbedded sandstone sequence is an equivalent of the San Rafael Group and can be correlated with either the Carmel Formation (O'Sullivan and Craig, 1973, p. 79-81) or the Entrada Sandstone. Moench and Schlee (1967, p. 6) considered it to be the lower sandstone unit of the Entrada. Green (1974) has recently named this sandstone the lyanbito Member of the Entrada Sandstone.

The widespread J-2 surface must have formed in a relatively short period of time. The oldest formation directly above the J-2 surface is apparently the Sawtooth Formation in Montana. The lower part of the middle member of this formation contains ammonites of middle Bajocian age equivalent to the Stephanoceras humphresianum ammonite zone (Imlay, 1956, p. 563; 1967, p. 21; 1973, p. 34). The youngest formation directly below the J-2 surface is the Gypsum Spring Formation of central and northern Wyoming. This formation has not yielded ammonites, but its pelecypod and gastropod fauna (Imlay, 1945; Love, Tourtelot, and others, 1945) are probably of middle Bajocian age (R. W. Imlay, oral commun., 1974). These fossils were collected from a limestone bed that is at most no more than about 5 m below the J-2 surface. Consequently only about 1 million years elapsed between uplift and erosion of the Gypsum Spring and initial deposition of the Sawtooth.
The distribution of the stratigraphic units that directly overlie the J-2 surface (map A, pi. 1) indicates that the J-2 surface was overlapped from west to east and from north to south. Initially, the J-2 surface was buried in the western part of the area by the Carmel Formation and Twin Creek Limestone and in the Williston basin by the Piper Formation. The youngest unit directly above the J-2 surface is the Canyon Springs Member of the Sundance Formation of north-central Colorado (Pipiringos and others, 1969), which is now considered to be of early to middle Callovian age (R. W. Imlay, written commun., 1974).