In broad outline the Heart Mountain fault of Wyoming is a nearly horizontal thrust whose overriding sheet was derived from a source without any known roots, and whose frontal part has ridden across a former land surface. The suggestion is here made that this thrust and the near-by South Fork thrust are detachment thrusts or decollements, that is, they are sheets of sedimentary rocks which have broken loose along a basal shearing plane, have moved long distances probably by gravitational gliding, and have been deformed independently from the rocks below the fault plane.

The present remnants of the Heart Mountain thrust sheet include more than 50 separate blocks which range in size from a few hundred feet to 5 miles across and which are scattered over a triangular area 30 miles wide and 60 miles long. The rock formations represented in the thrust blocks comprise a very limited stratigraphic range, none being older than the Bighorn dolomite (Ordovician) and none younger than the Madison limestone (Mississippian). The maximum stratigraphic thickness of the formations involved is 1,800 feet, but these include the most competent group of beds in the sedimentary sequence in this area.

In the northwestern part of its known extent the Heart Mountain thrust plane follows the bedding of the rocks and lies at the base of the massive and resistant Bighorn dolomite and above the underlying Grove Creek formation (a thin unit at the top of the Cambrian sequence). Near the center of the area here described this bedding thrust plane changes abruptly to a shear plane that cuts stratigraphically upward across the Bighorn and younger formations; the thrust plane then passes southeastward onto and across a former land surface. The present thrust remnants on this surface are separated blocks that rest on rocks ranging in age from Paleozoic to Tertiary.

In the area of the bedding thrust the displaced sheet was broken into numerous blocks which became detached from one another by movement, with large spaces or gaps separating them. Thus by tectonic denudation the thrust plane was exposed at the surface. Associated with the events accompanying the thrusting was the rapid formation of a stream channel deposit, here named the Crandall conglomerate. Next there followed the deposition of the “early basic breccia.” This blanket of volcanic rock, which is now in the process of being eroded, has preserved much of the geologic record pertaining to the development of the Heart Mountain thrust since middle Eocene time.

The concept is here advanced that, near the close of early Eocene time, the Heart Mountain thrust originated as a detachment or shearing-off of strata at the base of the Bighorn dolomite. Near Dead Indian Hill the advancing southeastern edge of this bedding thrust sheet passed upward into a shear thrust and thence southeastward onto and across the land surface as an erosion thrust.

The South Fork thrust sheet, which underlies and is slightly older than the Heart Mountain thrust sheet, likewise has the character of a detachment thrust in that the plane of the thrust sheet extends downward to a stratigraphic horizon in the Sundance formation, but goes no farther. In three test wells which started in the South Fork thrust sheet, the plane of the thrust was found at depths of 550 to 1,040 feet, and the beds below are essentially undeformed.

Characteristic features of the South Fork thrust mass, which suggest a detachment thrust (decollement), are: (1) tightly folded anticlines and synclines and overturned, recumbent, and faulted folds; (2) the base of the thrust mass is in most places at or near a stratigraphic horizon; (3) so far as known, it has no “roots” from which it could have come as a deep-seated thrust; (4) the thrust mass contains no rocks from below the plane of detachment. Although the South Fork thrust mass reacted to deformation quite differently from the Heart Mountain thrust blocks, the differences are readily accounted for by the great lithologic differences of the rocks of the two sheets.

To test further the proposed interpretation for the Heart Mountain and South Fork thrusts, additional field observations should be made to shed more light on the mechanics of the deformation.

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