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W. G. Pierce

Publications and source records attributed to W. G. Pierce.

16 recordsLinked to original sources
USGS1987

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

T wo 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.

Wyoming↗
USGS1986

The South Fork detachment fault, Park County, Wyoming: discussion and reply ( USA).

Blackstone (1985) published an interpretation of South form detachment fault and related features. His interpretation of the area between Castle and Hardpan transverse faults is identical to mine of 1941. Subsequent detailed mapping has shown that the structure between the transverse faults is more complicated than originally envisioned and resurrected by Blackstone. The present paper describes and discusses geologic features that are the basis for my interpretations; also discussed are differences between my interpretations and those of Blackstone. Most data are shown on the geologic map of the Wapiti Quadrangle (Pierce and Nelson, 1969). Blackstone's 'allochthonous' masses are part of the South Form fault. Occurrences of Sundance Formation, which he interpreted as the upper plate of his 'North Fork fault', are related to Heart Mountain fault. Volcaniclastic rocks south of Jim Mountain mapped as Aycross Formation by Torres and Gingerich may be Cathedral Cliffs Formation, emplaced by movement of the Heart Mountain fault. - Author

Contributions to Geology - University of Wyoming, ↗
USGS1973

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. © 1973 Geological Society of America.

Geological Society of America Bulletin↗
USGS1966

Jura tectonics as a décollement

For many years the structure of the Jura Mountains was interpreted as a décollement 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 décollement 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 décollement 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 décollement. (5) High-angle tear faults, interpreted as not extending into the basement, are normal features of a décollement sheet. A continuous décollement around the southwestern end of the Swiss Plain can reasonably be inferred, connecting the internal Jura, the Salève, and the Subalpine folds as part of the décollement mass. Elsewhere, the internal border of décollement 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. © 1966, The Geological Society of America, Inc.

Geological Society of America Bulletin↗
USGS1963

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. © 1963, The Geological Society of America, Inc.

Wyoming↗