THE HEART MOUNTAIN OVERTHRUST, WYOMING 537 



over the entire eastern edge of Absaroka Range, perhaps for 125 

 or 150 miles. 



On the basis of exposures in the mountain region, Dake was 

 able to conclude that the overthrust took place after the deposition 

 of his Fort Union ( ?) (early Eocene) beds and before the outbursts 

 of the "early basic breccias (Neocene or upper Miocene) " described 

 by Hague. The McCullock Peak outhers show that the overthrust 

 is probably post-Bridger (middle Eocene). 



It may be recalled that Richards and Mansfield (15a) concluded 

 that the Bannock overthrust in southeastern Idaho was developed 

 before the deposition of Wasatch beds (lower Eocene). Similarly 

 Veatch (i6a) concluded that the Absaroka overthrust in south- 

 western Wyoming was developed after the deposition of the Almy 

 and Fowkes beds (lower Wasatch) and before the deposition of 

 the Knight formation (upper Wasatch, lower Eocene). On the 

 north, Willis (17a) concluded from physiographic rather than 

 stratigraphic evidence that the Lewis overthrust was developed in 

 mid-Tertiary time, and was completed before the Miocene epoch. 

 Although further work will undoubtedly determine more closely 

 the periods at which these four overthrusts were developed, it 

 appears highly probable at present that, although they lie in a 

 belt scarcely 500 miles long, they did not take place simultaneously. 

 The Lewis overthrust may have been nearly simultaneous with 

 the Heart Mountain overthrust, however. 



A brief reconnaissance by the writer in the region west of Cody 

 studied by Dake confirmed his conclusion that the overthrust beds 

 were deeply eroded before the outbursts of the early basic brec- 

 cias of the Yellowstone Park region (upper Miocene). Evidence 

 obtained by the writer previously between Owl Creek and Wood 

 River, however, indicates that there was conformable deposition 

 of the Bighorn Basin Wasatch and the overlying tuffs and breccias 

 of that region, which are here tentatively correlated with the 

 "early acid breccias" of the Absaroka Range (lower Eocene). 

 It is concluded that the overthrust took place after the deposition 

 of the "early acid breccias" and before the outbursts of 'early 

 basic breccias" and is, therefore, middle Eocene or early OHgocene 

 in age. 



