380 

 Powder River Basin 



Ko5 /^Crok.Co.Ch.^Cq 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Hartville: Uplift 

 Gq t Ch 



^^»!i.™i!:!';!?i!il!i'Vi:i';iiiiii;i'!i;i;ii\i! 



m ■ i ■ \ u i ' ) >'i i itTi i ■' i s \ w ( i'ii tP i i :'■ i i'i i -i i nTTTM 



n wiiMiTTrrnti^ 



Agr Cq+Ch 



Aw 1 Ta Agr i 



High Plains 

 Tci 



2 3 4 5 M , 



i Miles 



Hanna Basin 

 Th 



5ADDLEBACK H1LL5 

 ANTICLINE 



Carbon 



Basin 

 .Th 



KTmb 

 Miles 



Fig. 24.14. Upper diagram, cross section of the Hartville uplift from the Powder River basin 

 southeastward. Aw, Algonkian Walen group; Agr, granite intrusive into Walen group; Cg, 

 Guernsey fm.; Ch, Hartville fm.; Co, Opeche red ss.; Cmk, Minnekahta Is.; Jm, Spearfish (?), 

 Sundance, and Morrison fms.; Kd, Dakota ss.; Kgs, Graneros sh.; Ta, Miocene Arikaree fm. After 



The lower cross section of Fig. 24.14 shows the unconformity at the base 

 of the Hanna formation, which, according to Dobbin et al. ( 1929 ) : 



. . . occupies the central portion of the Hanna and Carbon Basins and con- 

 tains most of the coal mines in this area. It rests uncomfortably on the Ferris 

 formation and transgresses across all underlying formations at least down to the 

 Cloverly and possibly down to the granite. It consists of alternating conglomer- 

 ate, sandstone, shale, and coal beds, and its base is marked by a thick con- 

 glomeratic sandstone and locally by massive conglomerate. 



The pebbles of the conglomerate are abundantly of Precambrian 

 derivation, and formations from Tensleep to Mesaverde are also repre- 

 sented. According to the map and cross sections of Dobbin's report, the 

 principal folds of the basin were formed in post-Ferris and pre-Hanna 

 time, and then accentuated in post-Hanna time when the Hanna forma- 

 tion was appreciably folded. Rut during the first episode of folding, the 

 Medicine Row Range was vigorously uplifted and considerably eroded to 

 furnish much of the debris for the basin. The Medicine Row Range had 



W. S. T. Smith, 1903. Lower diagram, cross section from the Hanna basin to the Carbon basin, 

 after Dobbin, Bowen, and Hoots, 1929. Ks, Steel sh.; Kmv, Mesaverde fm.; Kl, Lewis sh.; Ktmb, 

 Medicine Bow fm.; Tf, Ferris fm.; Th, Hanna fm. 



been gently uplifted in Pierre time, and the Precambrian may have been 

 cut into at that early date, but certainly it was widely exposed after the 

 second uplift and during the deposition of the Hanna formation. 



The Cenozoic history of the north flank of the Hanna basin and south 

 flank of the Sweetwater Range, together with the thrust structure, is 

 vividly shown in block diagrams by S. H. Knight and reproduced in Fig. 

 24.15. His comments are are follows: 



The upper diagram represents an early stage in the deformation of the 

 Basin and depicts conditions as they are believed to have existed during early 

 Paleocene time. Erosion has breached the Paleozoic and Mesozoic rock succes- 

 sions and Precambrian rocks are exposed along the crest of the rising Sweet- 

 water Arch. Rock debris derived from the entire succession is accumulating 

 on the shallow Basin floor. Just when, in terms of the local sequence of forma- 

 tions, the central portion of the Sweetwater Arch rose above base level and 

 erosion began to feed debris into the Basin is a question which still remains 

 to be answered. The writer subscribes to the concept that the Sweetwater Arch 

 and the Medicine Bow Mountains may have risen as islands out of the 



