SHORTER CONTRIBUTIOIfS TO GEINEEAL GEOLOGY, 1021. 



shows that a change simihxr to that described 

 above occurs westward from the typical area 

 toward the Muddy Mountains. The Coconino 

 sandstone thins and loses its identity, and the 

 Supai formation changes largely from red to 

 gray. 



Nothing was observed to indicate an uncon- 

 formity beneath or within the Supai formation, 

 though continiious tracing might disclose it. 



Schuchert" in 1918 called attention to the 

 thinning of the Coconino sandstone northward 

 and northwestward from the typical area, 

 suggesting, however, that the component sands 

 came from that direction. 



KAIBAB LIMESTONE. 



The '' Upper Aubx-ey " or ''Aubrey limestone" 

 of the older literature of southern Utah and 

 northern Arizona was named tlie Kaibab lime- 

 stone by Darton,' the name Aul)rcy bemg re- 

 tained in its broad sense, as a group term, to 

 include the Supai formation, the Coconino sand- 

 stone, and the Kaibab limestone. In the Shi- 

 numo cjuadrangle, Aiiz.' (see map, fig. 10, 

 No. 23), the Kaibab limestone is 520 feet thick 

 and consists, in descending order, of a cherty 

 gray limestone 75 feet thick, a white crystalline 

 limestone 200 feet thick, a soft calcareous sand- 

 stone, locally a conglomerate of soft sandstone 

 pebbles, 20 feet thick, a red and white calcareous 

 sandstone 135 feet thick, a buff crystalline sili- 

 ceous limestone 40 feet thick, and a calcareous 

 white sandstone 50 feet thick. The 200-foot 

 limestone and the 40-foot limestone form clifTs. 



In most of the exposures seen in our work 

 the Kaibab limestone shows a fivefold topo- 

 graphic and lithologic division — (1) a lower soft 

 member consisting of gypsum, gray and yellow 

 shale, soft gray sandstone, and subordinate 

 amounts of thin-bedded dark-drab limestone; 

 (2) a lower clifT-forming member of gray mas- 

 sive limestone with much brown to black con- 

 cretionary chert; (3) an upper soft member 

 with much the same character as the lower 

 one; (4) an upper massive cliff-forming lime- 

 stone which is similar to the lower one but con- 

 tains more chert and which from Bright Angel 

 Creek to southwestern Utah shows tower-like 

 erosion forms along its upper cliff face; (5) a 

 topmost member (PI. IX, (7), less resistant 



« Schucbert, Charles, On the Carboniferous of the Grand Canyon of 

 Arizona: Am. Jour.Sci.,4thser.,vol.45,pp.:J47-ytVJ, I'Jls. 



' Darton, N. H., A reconnaissance of ]iarts of northwestern New Mexico 

 and northern Arizona: U. S. Geol. Survey Bull. 435, p. 2S, 1910. 



8 Noble,!,. F., The Shinumoquadrangle, Grand Canyon district, Ariz.: 

 U. S. Geol. Survey Bull. 549, p. 70, 1914. 



than the underlying tieds and highly variable in 

 composition and thickness, consisting of shale, 

 gypsum, and limestone. The limestone of the 

 top member is at some places arenaceous, at 

 others partly silicified, at still others filled with 

 masses of light-colored chert that breaks into 

 flat platy fragments; at many places the upper 

 layers contain many small angular fragments of 

 chert. In color it is light gray, yellowish brown, 

 pink, and rarely a sugary white. The sand- 

 stone is gray to yellow, calcareous, and locally 

 gypseous — that is, it has a gypsum cement. 

 The shale may be gi'ay, yellow, or rarely red. 

 It is usually gypseous and in some places sandy. 



These divisions of (he Kaibab limestone vary 

 much in thickness from point to point, and it . 

 seems unlikely that exactly the same beds enter 

 into the same divisions at all localities. How- 

 ever, over as long a stretch as that along the 

 Hurricane fault scarp from a point some dis- 

 tance south of Black Rock Canyon to Virgin 

 Canyon — a distance of 25 miles — the lower four 

 divisions are continuously exposed, though 

 varying in thickness from place to place. The 

 uppermost division is present locally but at a 

 distance can not be distinguished from the 

 overlying basal Moenkopi beds. 



The lowest division is thin or lacking at sev- 

 eral localities but is usually from 75 to 100 feet 

 thick. The lower cliff-forming division ranges 

 from 150 to 230 feet in thickness in the sections 

 examined. The upper slope-forming division 

 ranges from SO to 285 feet in thiclvness. The 

 upper cliff-forming division is variable, ranging 

 from 185 to 455 feet. A thick limestone breccia 

 occurs in the lower part of this unit in Virgin 

 Narrows, below St. George (section 19, p. 75). 



The uppermost member is composed of pecu- 

 liar and characteristic rocks. It may be rec- 

 ognized, in spite of its variability, wherever it 

 occurs, and as it is a definite unit between the 

 upper cliff-forming limestone of the Kaibab 

 and the basal beds of the Moenkopi formation 

 it is here named the Harrisburg gypsiferous 

 member, from its occurrence in the Harrisburg 

 dome, 8 miles east of St. George. A section 

 measured here (section 14, p. 73) shows a tliick- 

 ness of 280 feet. This member may be absent 

 from some of the sections examined, but in 

 others it reaches a tliickness of nearly 300 feet. 

 It is apparently the same unit as that desig- 

 nated "Super-Aubrey betls" by Huntington 

 and Goldthwait.-' The uppermost limestone 



'Huntinglon, Ellsworlh, and (ioldlhwait, J. W., The Hurricane 

 fault in the ToquerviUc district, Utah: Harvard Coll. Mus. Comp. Zool. 

 Bull., vol. 42, p. 203, 1904. 



