354 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



South to Eost 



Tort Union 



formotion 



(Tlul 



Tongue River member [Ttr> 



lebo shole member (TD 



Tuilock member (Tt) 



St Mory River 

 for motion 



(Ksm) 



Hell Creek formotion (Knc) 



Niobroro shole 

 Coriile shole 



y "Bovdoin sond'^f 

 S "pniiiips sond" ~% 



Greenhorn limestone (Kg) 



Frontier formotion (KO 



Belle Fourche shole (Kbf) 



Mowry shole (Km) 



[1 ThermopoliS Mu0t )y ss , memDer 

 i >. shole (Kt) 



Newcostle sondstone (Kn) 

 ond Skull Creek shole ik 5 ) 



-CONTOURED HORIZON • 



<^GreyDuii sondstone member J> 

 \ of Cloverly formotion (Kor) ^ 



Foil River ss = 1st Cot Cr sond 

 so-colled "Dohoto sond" 



Kootenoi ond Cloverly formotions (Kk) 



X ^'Mou 



Fuson shale 

 IKIul 



,. .Kk-Cb) ^UJI 



Cut Bonk sond ^ 



I™. (KK-sh) , J Pryor cgl memDer \ 



V Sunburst sond" \ } of Cloverly fm, (Kpn ^ 



Lokota sandstone 



IKI) 



UHCOHFOftUI 



Ul 



Morrison formotion (Jit 



Swift formotion (j$«) 



Sundance formation us) 



Piper formotion (jp) 



-^UNCOHFOHMITT- 



>< 



Fig. 23.3. Stratigraphic diagram of Cretaceous and Paleocene formations of Montana. Repro- 

 duced from Dobbin and Erdman, 1955. 



Most of the anticlines and domes of the central zone of uplifts are 

 asymmetric and can be considered as flexures. The Cat Creek anticline 

 which extends eastward from the Judith Mountains, is a good example. 

 Thorn (1923) considers the flexures due to draping of the flexible sur- 

 ficial strata over faulted blocks of the more brittle Precambrian rocks 

 below. See lower diagram of Fig. 23.3. 



Central and eastern Montana and northern Wyoming are the sites of a 

 remarkable succession of formations that bridge the Cretaceous and Terti- 

 ary periods. The position of the boundary of the two stratigraphic sys- 

 tems has been a matter of lively argument and study for many years. The 

 chart of Fig. 23.4 shows the age assignments of the various formations 

 on the Structure Contour Map of the Montana Plains by Dobbin and 

 Erdman ( 1955 ) . The youngest formation generally reported upturned 

 in the flexing is the Fort Union, which is regarded as Paleocene. It is 

 known that the Big Horn Mountains arch had already risen and been 

 stripped to the Precambrian by Fort Union time, and that further uplift 

 occurred soon afterward. Since the Big Horn arch extends into south- 

 central Montana, in the proximity of the east-west flexures and related 

 domes just to the north, it is possible that the central Montana structures 

 came into existence during the same period, viz., immediate pre-Fort 

 Union and again in post-Fort Union. 



The laccoliths, dikes, and stocks are nearly all in Cretaceous strata; only 

 in the Bearpaw Mountains has a deposit as young as the Teritary in asso- 

 ciation with the volcanics been reported. The oldest volcanics there rest 

 mostly on the Fort Union formation, but in places a cobble layer inter- 

 venes which may be Eocene or even Oligocene in age. At least the oldest 

 extrusions, which are older than the stocks of the area, are younger than 

 the cobble layer, and therefore at least lower Eocene in age, and perhaps 

 younger (Pecora, 1941). 



ZONES OF EN ECHELON FAULTS 



Characteristics and Structural Relations 



Three zones of en echelon faults occur in central and south-central 

 Montana. The Cat Creek fault zone is at the north, the Lake basin fault 

 zone in the middle, and the Nye-Bowler zone on the south, just north of 



