370 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Thrust or glide surface follows bedding 



Glide surface 

 across beds 



Glide surface here 

 former land surface 



CRANDALL CR. 



WINDY DETACHED 

 M ™: / TV 



SLIDE BLOCKS 



rifiKA 



• % /*/ W »/ » 



'MMM'tf"','ii 



'tp£ 



HEART MTN. THRUST (GLIDE) 

 ^ S U_R_FA_CE _ _J*^ 



HEART 

 Tw. MTN - 



10 



Fig. 24.8. Generalized section along Clarks Fork to Heart Mountain showing nature of thrust 

 (glide) surface and detached glide blocks. After Pierce, 1957. Tv, mostly early basic breccia 

 (middle Eocene); Tc, Crandall conglomerate (early (?) Eocene); Tw, Willwood fm. (early Eocene); 



1000 to 4000 feet. The maximum thickness of the series in any continuous 

 section is 6500 feet (Rouse, 1937). 



The pyroclastic rocks were erupted through hundreds of small vents 

 and from a few volcanoes of moderate size. The basalt sheets are all 

 fissure eruptions. 



The breccia series have been divided into formations better suited for 

 mapping purposes by Hay (1954) and Wilson (1959), and the succession 

 of formations thus established is given under the column, Wood River, 

 in Fig. 24.9. 





BIG HORN 

 BASIN 



ABSAROKAS 

 (OLD TERMINOLOGY) 



ABSAROKAS 

 (WOOD RIVER) 



WASHAKIE 

 MTS. 



MIOCENE 







Rhyolite 



w 



8 Granodiorite 



m Andesite 



3 



*j Dacite 



h Rhyodacite 





OLIGOCENE 





Late basalt flows 

 Late basic breccia 

 Late acid breccia 



Wiggins fm. 

 (volcanics) 





u 



u 

 o 

 M 



UPPER 





Tepee Trail 



fm. 



Ay cross fm. 



7 



? 



MIDDLE 



Tat. man fm. 



Early basalt flows 

 Early basic 

 breccia 



Pitchfork fm. 

 (80% andesite) 



LOWER 



Willwood fm. 



Early acid breccia 



Willwood fm. 



Wind River fm. 

 Indian Meadows 



PALEOCENE 



■ ■ 



Polecat 

 Bench fm. 









* Crandall conglomerate of Heart Mountain region is Lower Eocene. 



Fig. 24.9. Tertiary formation of Big Horn basin and Absaroka Mountains. 



Pal 



Mes 



Mes 



Mes, Mesozoic and Paleocene strata; Pal, Permian to Ordovician strata. The Heart Mountain 

 thrust blocks are from the Madison, Threeforks, Jefferson, and Big Horn formation. Vertical 

 scale in thousands of feet. 



Plutons of the Absaroka Range 



In the northern Absaroka Range stocks, laccoliths, plugs, cone sheets, 

 and radial dike systems occur and are closely related to the volcanic 

 centers. The magma of the radial dikes moved horizontally outward. 

 The rocks in general show a normal differentiation series from Olivine 

 gabbro and basalt through diorite and andesite to sodic syenite and 

 trachyte (Parsons, 1939). 



A number of intermediate felsic stocks are known in the central area 

 of the southern Absarokas, and these occur in striking alignment. The 

 zone may have served first for the breaking through of the volcanic con- 

 duits and then later for the stocks that cut the breccias ( Rouse, 1937 ) . 



The kinds of post- Wiggins intrusive rocks which Wilson ( 1959 ) mapped 

 in the Wood River area are listed in Fig. 24.9. 



Breccia Series of Yellowstone Park 



The general region of Yellowstone Park was a basin in the time of 

 accumulation of the first volcanics, the early acid breccias. Although 

 partly surrounded by higher topographic features, it was a rugged surface 

 like that of the early Absaroka Range but somewhat lower. It is estimated 

 that in places about 1000 feet of the early acid breccias accumulated 

 ( Howard, 1937 ) . Then the voluminous early basic breccias were erupted. 

 These include breccias, agglomerates, tuffs, and flows of a more basic 

 character with basalt predominating. They reach a maximum thickness 

 of 4000 feet and have a wide distribution from the Absaroka Range 

 through northern Yellowstone Park to the Gallatin Range in Montana 



