WYOMING ROCKIES 



373 



| east-west, and the other north-northwest to divide the uplift into four 

 units. In three of these units, the beds rise toward a high point near the 

 northeast, beyond which they drop off abruptly. Most of the flexures have 

 ruptured to produce faults of moderate displacement. Thom ( 1923 ) and 

 later Blackstone ( 1940 ) have concluded that the faults curve under the 

 uplifted blocks at depth and have resulted from horizontal compression. 

 The pliable sedimentary veneer flexes first over the scarp of the crystalline 



4 rocks and later, when displacement becomes sufficient, it breaks to reveal 



i the deep-seated fault. See Fig. 24.10. 



Big Horn Basin 



The Big Horn basin is underlain in its deepest parts by 2500 to 3200 feet 

 of Paleozoic strata, by about 1500 feet of Triassic and Jurassic strata, by 



I 7000 to 9000 feet of Cretaceous strata, and in the central and western 

 parts by several thousand feet of Paleocene and Eocene strata. For a re- 



| view of the formations, see Wyoming Geological Society Seventh Annual 

 Field Conference Guidebook, 1952. As indicated in earlier parts of this 

 book, the Wyoming region, including the Big Horn basin, was a shelf 

 area of sedimentation until Cretaceous times, when considerable sub- 

 sidence occurred adjacent on the east to the active Cordilleran geanticlinal 

 area that extended through Utah and eastern Idaho. See the paleotectonic 

 maps of the Early and Late Cretaceous. 



With the elevation of the ranges surrounding the Big Horn Basin, its 

 sediments were thrown into many folds, all trending in a northwest direc- 

 tion. The Early Tertiary strata probably obscure many folds in the central 

 part of the basin, for the anticlines and synclines are known only in a 

 broad marginal belt. Those on the east side have steep flanks facing the 

 Big Horn Mountains. The major anticline on the west side, the Rattle- 

 snake Mountain, is asymmetrical toward the west, but the smaller folds 

 do not have any regular symmetry. Some have steeper flanks toward the 

 basin, some are about symmetrical, and some are dome shaped. The anti- 

 clines and domes are nearly all oil or gas producing. Two of the anticlines, 

 especially, are cut by numerous, small, high-angle faults in a transverse 

 direction. These are the Elk Basin and Garland anticlines in the northern 

 part of the basin. The deepest part of the Big Horn basin, according to 



e 



5 

 4000 J 



4O00 J 



6 



^~- , 5'' , "«!»"ir>' , '^v'J • 



A- > ,.Tiv.-r.^.:,V//,/,/ 



m 



m m m m 



PRE- CAMBRIAN ORDOVICIAN MISS. MISS." PENN- 



CAMBRIAN DEADWOOD BIGHORN MADISON PENN. PERM 



GRANITE AMSDEN TENSLEEP 



8. SCHIST , EMBAR 



* — ' / \ — "■iff*' J* ^^^ =1=rr ^=iri^ 



TRIASSIC JURASSIC CRETACEOUS 

 CHUG- SUNDANCE CLOVERLY 

 WATER MORRISON 



IOOO' 2000' 



P-C 



PC 



/ 



Fig. 24.10. Cross sections of the frontal faults of the tilted blocks of the Pryor Mountains. The 

 lower diagrams, A to E, illustrate the theory of origin. Taken from Blackstone, 1940. 



