372 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



base is submerged by basalts, which are probably the equivalents of the Late 

 Canyon Basalts. 



Late Canyon Basalts. Erosion was then followed by another period of basalt 

 extrusion, these basalts being the most recent flows of the Park. They are 

 found largely in the northern part of the Park, in Lamar, Yellowstone, and 

 Gardiner valleys, and at a few places on the broad interstream uplands. Other 

 patches are preserved on the uplands in west-central Yellowstone. The basalts 

 of the Snake River Plains, which crowd against the western scarp of the 

 rhyolite plateau, are probably of the same general age. 



BIG HORN RANGE AND BIG HORN BASIN 

 Divisions of Big Horn Range 



Overall the Rig Horn Range is a great anticlinal fold, steep to over- 

 turned to overthrust on the east, and gently dipping on the west. Examine 

 Fig. 24.1. The range is arcuate in plan view and terminates in the Pryor 

 Mountains on the northwest and the Owl Creek Range on the southwest. 

 The Precambrian crystalline rocks on which the Paleozoic strata rest are 

 exposed in three areas in the core of the range, and serve as natural 

 divisions. 



At three points near the center of curvature in the central division along 

 the east front of the range, blocks of the range, including the crystallines, 

 have been thrust out upon the Cretaceous strata. The main block is clearly 

 bounded by tear faults. 



On cross sections published by Hoppin (1961), overturning, thrusting, 

 and a detached slide mass are shown. It appears evident that sharp uplift 

 and upturning of the beds are the primary movements and then that 

 secondary gravity movements have resulted in downhill overturning of 

 the beds, tear faults, and small-scale thrusting. 



As the axis of the great fold in the central division is traced northward, 

 it plunges, and the dips on the northeast flank become gentle. Reyond, in 

 the northern division, the asymmetry is reversed, and the crystalline rocks 

 are exposed close to the southwestern flank. Here steep dips, overturning, 

 and even thrusting to the southwest occur. The northern division is further 

 distinguished by sharp flexures which trend northwestward, northward, 

 and eastward. 



In the southern section, the trend of the mountain axis curves from a 

 north-south direction to a southwesterly one; but in spite of this change, 



the smaller structures within the range maintain the northwesterly direc- 

 tions that dominate the northern division. The marginal folds and faults 

 trend dominantly to the northwest, and the dips are steeper on the south- 

 west sides of these small folds (Rucher, 1934). 



The Tensleep fault cuts across the Rig Horn Range from the town of 

 Tensleep on the west to the Horn on the east, and separates the central 

 from the southern divisions. As the range was uplifted and the central 

 division developed asymmetrically eastward and the southern asymmetri- 

 cally westward, the Tensleep fault came into existence (Wilson, 1938; 

 Demorest, 1941 ) . Relations are complex along the fault, but they point to 

 a downthrow on the south side. 



Laramide History 



The Laramide history of the Rig Horn Range has been summarized by 

 R. P. Sharp ( personal communication ) for the writer. According to him, a 

 series of coarse to bouldery fans ( the Kingsbury conglomerate ) composed 

 primarily of Precambrian debris, were built up along the east base of 

 the central Rig Horn Mountains in Early Tertiary time. This was the 

 section of greatest uplift, and the fan debris was presumably coarser and 

 thicker here than elsewhere. Subsequently, the Paleozoic beds of the 

 mountain front were thrust eastward against and over the gravel, and 

 erosion during the remainder of the Cenozoic has gradually etched out 

 the thickest and coarsest parts of the fan deposits so that they form 

 prominent ridges in the present landscape. At least three periods of Lara- 

 mide uplift of the range are indicated: (a) An uplift which produced the 

 Kingsbury conglomerate. Faulting probably occurred during this uplift. 

 ( b ) A second uplif t, also probably attended by faulting, which deformed 

 the Kingsbury and produced the coarse granite-boulder gravel. This uplift 

 may possibly have been accompanied or closely followed by alpine glacia- 

 tion. ( c ) A third, postgravel, uplift marked by thrust faulting toward the 

 east in the central segment of the range. 



Pryor Mountains 



Northwest of the Rig Horns the Paleozoic strata rise once more by means 

 of two pairs of flexures to form the Pryor Mountains. One pair trends 



