William G. Pierce

The Heart Mountain break-away fault, northwestern Wyoming

The Heart Mountain break-away fault was the last of the four phases of the Heart Mountain detachment fault to be discovered and is the only fault of this type yet recognized and described. The descriptive term, “break-away fault,” was introduced in 1960 for the steep surface along which the rocks of the Heart Mountain decollement fault mass separated from the stationary block on the west. The break-away fault, located near the northeast entrance to Yellowstone National Park, has a known linear extent of 37 km. The northern one-third trends north, and the remainder trends southeast and south. Rocks on its western side are Paleozoic carbonate rocks and Eocene volcanic rocks; the volcanic rocks are pre–Heart Mountain fault in age. The rocks on its eastern side are younger and consist of Eocene volcanic rocks which were deposited against and over the fault. Thus the break-away fault is unusual in that one side is a fault surface and the other side is a depositional surface, and in that respect is similar to parts of the bedding-plane phase of the Heart Mountain detachment fault. Slickensides and brecciation in the Madison Limestone adjoining the break-away indicate horizontally directed compressive shear. Movement was probably right lateral, and then laterally southeastward away from the fault at an angle of about 45°. To the north, in Wolverine Pass and in the valley of Soda Butte Creek, the break-away fault extends down to a bed in the basal part of the Bighorn Dolomite where it joins or terminates at the bedding plane phase of the Heart Mountain detachment fault; movement of upper-plate rocks away from the break-away fault increases from about 0.25 km at the northernmost trace of the fault to more than 1.5 km on the south side of Soda Butte Creek valley. Several kilometres southeastward, in Cache Creek valley, the Heart Mountain break-away fault terminates downward in volcanic rocks of the undivided Lamar River and Cathedral Cliffs Formations and joins a low-angle fault in those rocks. This low-angle fault probably steps downward to the east where it reaches the Heart Mountain detachment fault surface. Three kilometres farther to the southeast, the break-away fault is presumably represented by a low-angle fault in these volcanic rocks that probably also steps down to the northeast beneath the volcanic cover to join the Heart Mountain detachment fault. Erosion in this area probably has removed rocks that contained the high-angle part of the fault. The south end of the break-away fault is overlapped and concealed by the Wapiti Formation. It presumably continues southward, but it probably does not join a normal fault near Black Mountain, although Voight (1974) has suggested that it does.

Reef Creek Detachment Fault, Northwestern Wyoming

The Reef Creek fault is in northwestern Wyoming, a few miles east of the northeast border of Yellowstone National Park. It lies within the area covered by the more extensive Heart Mountain fault. Like that fault, it is a decollement or detachment fault in which strata became detached along a basal shearing plane and moved laterally on a slightly inclined fault surface. At the most northwesterly exposures the Reef Creek fault parallels the bedding; southeastward it cuts upward across the bedding as a transgressive fault and becomes a fault in which the allochthonous blocks moved on the surface of the ground. The fault blocks consist chiefly of the Madison Limestone of Mississippian age and volcanic tuffs and breccias of the Cathedral Cliffs Formation of early or middle Eocene age. The Reef Creek fault blocks are scattered over a 7- by 14-mile area, but a considerable part of the scattering is due to “piggy-back” transportation on the Heart Mountain fault blocks. The Reef Creek fault is younger than the Cathedral Cliffs Formation and older than the middle Eocene early basic breccia. Consequently, the rocks transported by it were emplaced either in late early Eocene or in early middle Eocene time. Both the Reef Creek fault and the South Fork fault are older than the principal movement along the Heart Mountain fault. The movement of all three fault masses was southeastward, and the mechanics of their emplacement is believed to have been similar. Movement was due in part to gravity, but considering the low slope involved, gravity alone seems to be inadequate. The shaking motion of many earthquakes is suggested as a contributing force which acted in conjunction with the constant force of gravity.

Jura Tectonics as a Décollement

For many years the structure of the Jura Mountains was interpreted as a decollement whose origin was related to the Alps; in recent years, however, this mode of origin has been questioned. Most of the alternative explanations recognize a decollement to some extent, but attribute it to movement of the basement beneath. Surface and subsurface data are here reviewed to show that the Jura deformation was produced in a gliding sheet, in which the forces of gravity and inertia were generated within the total moving mass. Features of the folded Jura which support the decollement hypothesis are: (1) Nowhere are rocks older than Middle Triassic exposed, which strongly suggests that the folding does not extend to the older rocks. (2) Subsurface data in the Lons-le-Saunier region clearly show that the external border of the Jura has moved northwestward over the eastern margin of the Bresse Basin. (3) Lower Jurassic rocks rest on Upper Jurassic along a horizontal fault 1234 m deep in the Risoux well near the middle of the Jura. (4) The tabular areas, with their absence of folds, are expectable in a decollement. (5) High-angle tear faults, interpreted as not extending into the basement, are normal features of a decollement sheet. A continuous decollement around the southwestern end of the Swiss Plain can reasonably be inferred, connecting the internal Jura, the Saleve, and the Subalpine folds as part of the decollement mass. Elsewhere, the internal border of decollement extends southeastward into the Molasse basin for an unknown distance and probably underlies the entire basin; if it does, a causal relation to the Alps is indicated.

The case for tectonic denudation by the Heart Mountain fault—A response

Two basic concepts pertaining to the history of the Heart Mountain fault of northwestern Wyoming have recently been challenged; one, that there was tectonic denudation, and two, that volcanic rock of the Wapiti Formation was deposited on the exposed fault surface. Tectonic denudation is believed to have occurred as a consequence of the upper plate having broken into numerous blocks that separated as movement progressed along a nearly horizontal fault surface, thus leaving the fault surface exposed between blocks. Volcanic rocks of the Wapiti Formation were then deposited both on the exposed fault surface and against and over the upper-plate blocks. Two formations of Eocene volcanic rocks are involved. The older volcanic unit, the Cathedral Cliffs Formation, and the Paleozoic carbonate rocks are part of the upper plate of the Heart Mountain fault and moved with it, whereas the younger Wapiti Formation was deposited on the fault surface after movement had ceased. In an alternate interpretation recently advanced by T. A. Hauge, subdivisions of the Absaroka Volcanic Supergroup, of which the Cathedral Cliffs and Wapiti Formations are units, are not recognized. The upper plate of the Heart Mountain detachment is interpreted as having been a single, continuous allochthon composed largely of volcanic rocks with small amounts of Paleozoic rocks. During Heart Mountain faulting, extension of the once-continuous slab of Paleozoic sedimentary rock is alleged to have been accompanied by the formation of ten or more grabens, now filled predominantly by Absaroka volcanic rocks. This interpretation further proposes that the volcanic rocks were emplaced while the separating blocks of Paleozoic strata were still moving and that the basal part of the volcanic rock between these blocks is in fault contact rather than depositional contact with the strata beneath the Heart Mountain fault. Many lines of geologic field evidence indicate that the Wapiti Formation is younger than the Heart Mountain fault and was deposited on the technically denuded fault surface. (1) Wapiti rocks bury the break-away fault. (2) Fault breccia at the base of the upper-plate carbonate blocks is composed entirely of carbonate fault breccia and has no volcanic component. (3) Small blocks of upper-plate rocks have been displaced by gravity from the upper part of the allochthon to the detachment fault surface. (4) Eocene stream-channel deposits locally cut into the surface of tectonic denudation and also have been displaced on the Heart Mountain fault. (5) The volume of Wapiti Formation filling the spaces between allochthonous blocks in proportion to the volume of those blocks is much too great for the Wapiti to have been allochthonous. (6) Clastic dikes of carbonate fault breccia penetrate Wapiti volcanic rocks. (7) Some of these clastic dikes of fault breccia contain Precambrian xenoliths and wood phenoclasts requiring surface exposures of the fault breccia before injection as dikes. (8) Wapiti volcanic rocks having chilled borders are in tightly bonded contact with upper-plate Paleozoic rocks. (9) Faults present in the upper-plate blocks do not penetrate the overlying Wapiti Formation. (10) Volcanic fault breccia is absent where volcanic rocks overlie carbonate fault breccia. (11) A mound of carbonate fault breccia is not mixed with overlying Wapiti Formation. The continuous allochthon interpretation is based on several erroneous assumptions that cannot be supported by field observations. (1) Faults to transport and emplace the Wapiti Formation onto and along the Heart Mountain fault do not exist. (2) The contact between volcanic rocks and the allochthon west of Corral Creek at the west end of Cathedral Cliffs, cited by Hauge as a fault in an extending allochthon, is a depositional contact. (3) The volcanic rock adjoining allochthonous Paleozoic rocks north of Pilot Creek cannot be part of an extending allochthon because (a) it is Cathedral Cliffs Formation, which is pre–Heart Mountain fault, and (b) its direction of movement is horizontal rather than down dip, as required in an extending allochthon. (4) Most of the igneous dikes were intruded after the Heart Mountain fault movement ceased, and so they could not accommodate significant extension of the upper plate. (5) Striae reported as indicating fault emplacement of volcanic rock (Wapiti Formation) on the Heart Mountain fault actually lire flow features, formed as the Wapiti Formation was deposited on the exposed fault surface. Tectonic denudation is the only model that is consistent with evidence observable in the field. Although the process by which tectonic denudation was accomplished remains enigmatic, tectonic denudation remains a constraining fact in any model for the origin of the Heart Mountain fault.

Crandall Conglomerate, An Unusual Stream Deposit, and Its Relation to Heart Mountain Faulting

The Crandall Conglomerate (Eocene) is a channel deposit, more than 350 ft (100 m) thick, believed to have formed as a result of preliminary movement of the Heart Mountain detachment fault in northwestern Wyoming. Initial movement of the Heart Mountain fault opened a deep rift in which the conglomerate was deposited. The rift was less than a mile (1.6 km) wide and was bordered by 2,000-ft (600 m) cliffs, mostly of Paleozoic limestone. Before the gravel was deposited, unconfined Cambrian shale below the rift was deformed into the Blacktail fold, a sharp anticline without apparent roots, while streams carried away the upwelling shale and cut a channel several hundred feet deep. The debris that accumulated in this channel is the Crandall Conglomerate. Deposition of the conglomerate was followed by Cathedral Cliffs volcanism, by movement on the Reef Creek detachment fault, and by the main movement on the Heart Mountain detachment fault. The main movement on this fault left the lower part of the conglomerate in place but carried the upper part with deposits of the upper plate roughly 15 mi (24 km) southeastward. Most of the deposits of the lower plate rest directly on the Blacktail fold. Of the 15 known deposits of Crandall Conglomerate, five are in place but have been overridden by the upper plate of the Heart Mountain fault, and ten have been transported as part of the upper plate. After this movement, volcanic rocks of the Wapiti Formation blanketed the region.

Cathedral Cliffs Formation, the Early Acid Breccia Unit of Northwestern Wyoming

The name Cathedral Cliffs Formation is proposed for the rocks in the Clarks Fork area of northwestern Wyoming that have long been known by the informal designation “early acid breccia.” In the Clarks Fork area the Cathedral Cliffs Formation is composed of tuffs, with lesser amounts of volcanic sedimentary rocks and breccias. Its thickness ranges from less than 100 feet to about 1500 feet but more commonly is 500–900 feet. The formation is tentatively considered to be late early Eocene or early middle Eocene. It is underlain by rocks ranging from Precambrian to early Eocene(?) and is overlain unconformably by the early basic breccia of middle Eocene age. Low-angle detachment faulting, which involved the Cathedral Cliffs Formation but not the overlying early basic breccia, has made recognition and correlation of the formation difficult. Blocks and masses of Madison Limestone of Mississippian age were emplaced locally on its upper surface by the Reef Creek detachment fault. The Cathedral Cliffs Formation and the Paleozoic carbonate rocks beneath it, as well as the Reef Creek fault masses on its surface, were then transported southeastward by the Heart Mountain detachment fault. As movement on the Heart Mountain detachment proceeded, the large fault mass broke up into smaller blocks, which separated as movement continued. Consequently the Cathedral Cliffs Formation was distributed in a pattern which gives the appearance of isolated occurrences and erosional remnants. The detached blocks of the Reef Creek fault on the upper surface of the Cathedral Cliffs also were scattered more widely than by their original movement on the Reef Creek fault. Soon after the fault-transported segments of the Cathedral Cliffs Formation ceased moving they were buried beneath the early basic breccia. The unconformity between the early acid breccia and the early basic breccia is thus substantiated in the Clarks Fork area; in the time interval represented, the Reef Creek and Heart Mountain fault masses were emplaced. The Cathedral Cliffs Formation is correlated with the early acid breccia in northern Yellowstone National Park and the upper part of the Reese Formation as mapped by Calvert west of Gardiner, Montana. The volcanic-source area probably is not in the central Yellowstone Park region, but somewhere to the north.

The Heart Mountain Detachment Fault: A Volcanic Phenomenon? A Discussion

Neither the dispersed nature of the Heart Mountain fault blocks nor the fault breccia are compatible with the hovercraft mechanism proposed by C. J. Hughes. The upper plate was not emplaced as a coherent sheet; it broke up into numerous blocks soon after movement began, and the blocks became widely separated before movement ceased. If they had been supported initially by high gas pressure, the pressure would have been lost long before the blocks came to rest. The fault breccia was examined at thirty localities; volcanic rock fragments were found only at one, and there they apparently were derived from rocks that are older than the faulting.

ROLE OF FLUID PRESSURE IN MECHANICS OF OVERTHURST FAULTING: DISCUSSION

Rubey and Hubbert (1965) suggest that movement on the Heart Mountain detachment fault of Wyoming may have continued during deposition of the early basic breccia, and believe that some variant of the high-fluid-pressure mechanism played a role in the emplacement of the Heart Mountain fault masses. Field data, however, show that within the area of the bedding-plane phase, the fault masses were fully emplaced before deposition of the early basic breccia. Notably, in the bedding-plane phase of the fault, the early basic breccia rests with depositional contact on a surface of tectonic denudation formed by the fault; the widespread areal pattern of isolated blocks of the upper plate that were not isolated by erosion requires that a tectonically denuded space be created first; and no early basic breccia debris is present in the Heart Mountain fault gouge. Rubey and Hubbert (1965) also suggest that movement on the South Fork detachment fault occurred along planes dominated by gypsum or anhydrite beds. So far no field evidence has been found to support this concept. The gypsum beds are in the very lowermost Jurassic rocks, and as far as can be determined, these beds are not involved in the South Fork detachment. The lowest beds in the upper plate of the South Fork fault are commonly 300 feet above the gypsum beds. Neither gypsum nor cornieule is known to be associated with the fault. The theoretical conclusions that (1) the rate of movement along an emergent thrust is controlled by erosion of rocks from the front of its advancing plate, and (2) that large thrusts can form only when the toe is continually eroded, do not seem to be applicable for the Heart Mountain fault.