WYOMING ROCKIES 



369 



roots, and whose frontal part has ridden across a former land surface. The 

 suggestion is here made that this thrust and the near-hy 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 inde- 

 pendendy 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 (Ordo- 

 vician) 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. See Fig. 24.8. 



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 accompany- 

 I ing 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. 



Pierce ( 1960 ) has more recently recognized the break-away point of the 

 detached slide blocks. 



NATURE OF BEDDING AND SHEAR THRUST 



Mcculloch 

 peaks 



Fig. 24.7. Origin of Heart Mountain thrust, after Pierce, 1957. Dotted area is postulated area 

 of bedding plane gliding of extensive sheet; zone marked shear thrust is where glide sheet cut 

 across beds, and the area to east is where detached blocks glided 20 and 40 miles to form 

 Heart Mountain and McCulloch peaks. South Fork thrust is older than Heart Mountain thrust, 

 and Logan Mountain and Sheep Mountain are detached remnants of Heart Mountain thrust rest- 

 ing on South Fork sheet. 



ABSAROKA RANGE AND YELLOWSTONE PARK 



Breccia Series of the Absaroka Range 



The Absaroka Range and Yellowstone Park comprise a large volcanic 

 i area which is made up of pyroclastic rocks and lavas. Two groups have 



been distinguished, each of which is composed of a lower acid breccia, 

 a middle basic breccia, and an upper series of basalt sheets. Altogether, 

 they are known as the breccia series. See columnar sections in the chart 

 of Fig. 24.9. The early acid breccia was probably erupted just before the 

 Heart Mountain thrust occurred, and the succeeding breccias and flows 

 accumulated on a rugged surface, the local relief of which ranged from 



