HINTZE, GEOLOGY OF WASATCH MOUNTAINS, UTAH 135 



In the mine workings on this hill, the discontinuity of the beds seen 

 on the surface is also shown. No regular structure can be followed very 

 far within the quartzite, or overthrust zone. The deeper workings which 

 drift far to the westward finally enter the limestones below the thrust 

 mass, and here the dip is regular to the east. The thrust contact dips 

 strongly to the east on the surface, but deeper it gradually flattens out. 



The age of the overthrust is not positively known, but there can be 

 little doubt that it occurred during, or at least at the close of one of the 

 periods of folding in late Mesozoic time. The folding of the Wasatch 

 is generally assigned to the close of the Cretaceous, but King^® has de- 

 scribed an unconformable contact between the local Dakota beds and the 

 Jurassic and older sediments exposed along Mountain Dell road in the 

 upper part of Parley's Canyon. The difference in dip of the beds is 

 given as about 30 degrees, and the Cretaceous strata rest on the truncated 

 edges of all of the older Mesozoic and Paleozoic formations, but else- 

 where the Cretaceous is described as conformable with the older series, 

 and this relation is the commonly accepted one. More work will have 

 to be done to settle this question. If there was important folding at the 

 close of the Jurassic, the overthrust in the Cottonwood region could have 

 occurred then. It certainly took place before the intrusive action oc- 

 curred in this district, as is evidenced by the independent manner in 

 which the dikes cut through the basal series and overthrust blocks. This 

 event followed or accompanied a period of northeast-southwest fracturing 

 and faulting which preceded the period of mineralization. Still later, 

 important faulting transverse to this first fracture line occurred, of which 

 the Superior fault is the best known example. The overthrusting, there- 

 fore, appears to have happened along with or following the first dynamic 

 disturbance in the region. Later warping has deformed the thrust sur- 

 face and tilted the masses at a high angle. 



Farther north in the range, Blackwelder^^ has described similar struc- 

 tures which he thinks were made at the same time that the Paleozoic 

 rocks were folded, which is generally assigned to the close of the Cre- 

 taceous period, but he says "It seems to be a fact that the Lower Eocene 

 (Wasatch) sediments cover the outcrop of the overthrusts in several 

 places, thus indicating that the folded and overthrust structures had been 

 deeply eroded." It is quite likely that these two districts less than fifty 

 miles apart suffered overthrusting at the same time and that whatever 

 period is deduced for one will be found to be the same for the other. 



38 C. King : U. S. Geol. Expl. 40th Par., Vol. I, p. 304. 



3»E. Blackwelder : "New Light on the Geology of Wasatch Mountains," Bull. G. S. 

 A., Vol. 21, p. 539. 



