--> Abstract: Evidence of Wapiti Volcanic "Blocks" Involved in Heart Mountain Faulting-Implications for the Continuous Allochthon and Tectonic Denudation Models, by D. M. Malone; #91017 (1992).

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ABSTRACT: Evidence of Wapiti Volcanic "Blocks" Involved in Heart Mountain Faulting-Implications for the Continuous Allochthon and Tectonic Denudation Models

MALONE, DAVID M., University of Wisconsin-Madison, Madison, WI

The Heart Mountain detachment in northwestern Wyoming has been one of the most controversial features in the field of structural geology for nearly a century. The traditional Tectonic Denudation model, characterized by catastrophic emplacement of many mountain-size blocks of Paleozoic carbonate rocks and some Eocene volcanic rocks, recently has been challenged. Large volumes of volcanic rocks originally interpreted to be in depositional contact on the detachment plane have been reinterpreted as allochthonous and emplaced noncatastrophically along with the carbonate blocks as a continuous, extending allochthon.

Detailed geologic mapping of the "former land surface" phase of the Heart Mountain fault near Sheep Mountain has revealed the existence of large blocks of well-stratified, proximal facies, Wapiti Formation laharic breccias and sandstones within a complexly deformed matrix directly adjacent to upper-plate Paleozoic blocks. This chaotic unit is laterally persistent and mappable, and is interpreted to have been emplaced catastrophically in a single event during the early middle Eocene.

The volcanic blocks range in size from a few meters to several kilometers in area and are as thick as 150 m. The matrix is a 1 to 20-m-thick veneer of intensely deformed primary volcaniclastic rock. The volcanic matrix has been identified at the base of, and between, volcanic blocks, and adjacent to, but not beneath, Paleozoic blocks.

Catastrophic emplacement favors the Tectonic Denudation model, whereas the recognition of additional allochthonous Wapiti formation volcanic rocks supports the Continuous Allochthon model. Additional field work is planned to try to resolve this dilemma.

 

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)