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The AAPG/Datapages Combined Publications Database

Wyoming Geological Association


Geology of Yellowstone Park Area; 33rd Annual Field Conference Guidebook, 1982
Pages 181-183

Relation of Volcanic Rocks to the Heart Mountain Fault

William G. Pierce


In a paper in this volume, Hauge concludes that the Heart Mountain fault movement did not produce tectonic denudation. He also believes that the volcanic rocks associated with it — some heretofore regarded as pre-faulting and others as post-faulting — are still all pan of the fault movement. His studies emphasize analysis of tectonic deformation of the volcanic rocks rather than stratigraphic data for structural interpretation.

The stratigraphic controls which are used to determine the relations of the volcanic rocks to the Heart Mountain fault are the Cathedral Cliffs Formation which is pre-faulting and the Wapiti Formation which is post-faulting. There is general agreement that the Cathedral Cliffs Formation is pre-faulting, for it is part of the upper plate. Some of the relations indicating the Wapiti Formation is post-faulting in age are: (1) The break-away fault, which is part of the Heart Mountain fault, cuts the Cathedral Cliffs Formation but is overlain by the Wapiti Formation which is several thousand feet thick and is not cut by the fault. The Wapiti Formation is clearly younger than the Heart Mountain fault along the 22 mile extent of the break-away fault. (2) The upper plate of the Heart Mountain fault broke up into numerous blocks which separated as movement continued. Large open spaces formed between the blocks which are now filled with Wapiti Formation. The volume or amount of Wapiti Formation filling the spaces between blocks is much too great to have been derived from the upper plate. However, i f the Wapiti was deposited in the open spaces following the faulting, there is no need for a volume relationship to the upper plate. (3) The fault breccia at the base of many of the upper plate blocks has been examined and found to be composed entirely of carbonate fault breccia. If the open spaces between the upper plate blocks were filled with volcanic rocks which were also moving as part of the upper plate, there should be a fair proportion of volcanic material in the fault breccia, but it is lacking. (4) Near Pilot Peak, steeply dipping, tectonically emplaced Cathedral Cliffs Formation is overlain by flat-lying Wapiti which in turn is overlain by flat-lying Trout Peak trachyandesite. If the Wapiti was fault emplaced but the overlying Trout Peak was not, can it be assumed that it is just a coincidence that the stratification is parallel in the two formations here? Such an assumption is questionable, for there are similar examples elsewhere in the area of the Heart Mountain fault. (5) It seems to me that a rapidly accumulating body of volcanic flows, pouring out over an irregular layer of fault breccia is likely to produce features similar in some respects to those produced by sliding mass of the same volcanic material. There is a need to look for diagnostic differences between them.

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