62 



SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1921. 



has been noted by a number of obseiTers. 

 Powell,-' Walcottr"- Dake,"' hm\-' Gregory," 

 Slimier,^'' and Longwell -' have all cited evidence 

 of its presence over a wide area and of its con- 

 siderable magnitude. 



SHINAKUMP CONGLOMERATE. 



The Shinai-ump conglomerate forms a marked 

 stratum tlu'oughout northerii Arizona and 

 southern Utah. It is thin but resistant and in 

 many places caps mesas or forms prominent 

 benches. (See PI. X.) 



The name was originally given by Powell -* 

 to the middle member of his ''Shinammp 

 group." 



In om- aj-ea only one detailed section was 

 made, that north of Vu'gin City (section 13, 

 p. 73), where the Shmanmip conglomerate was 

 fountl to consist of thi-ee members — (1) at the 

 base a brown sandstone 75 feet thick, with 

 lines and lenses of pebbles of chert, tpiartz, 

 silicified wood, and rarely of igneous rock and 

 with fossil logs; (2) gi'ay and green shale with 

 some fossil wood, 20 feet thick; and (3) at the 

 top gray platy sandstone with a ivw pebliles, 

 20 feet thick. The base of the lower sandstone 

 is irregular and is marked by a discontinuous 

 layer of dark shale. This plants is an evident 

 disconformity, but of what significance it is 

 difficult to say. At the Harrisburg dome (sec- 

 tion 14, p. 73) the Shinarunip conglomerate is 

 about 100 feet thick and consists of sandstone. 

 It seems to be fairly uniform over most of the 

 area examined, but 15 miles northwest of St. 

 George it is less than 50 feet thick and not as 

 resistant as usual. To the east, on the Kanab- 

 Fredonia road, the conglomerate is 40 feet 

 thick. 



The Shinarump locally contains crusts of 

 iron and manganese oxides which are liigh in 



»i Powell, J. W., Report on the geoloio' of the eastern portion of the 

 Uinta Mountains, U. S. Geog. and Geol. Survey Terr., 2d div., ISTtj. 



" Walcott, C. D., The Permian and oilier Paleozoic groups of the 

 Kanab VaUey, Ariz.: Am. Jour. Sci., 3d ser., vol. 20, pp. 221-22,5, Isso. 



" Dake, C. L., The pre-.Moeniopi (pre-Permian?) unconformity of 

 the Colorado Plateau: Jour. Geology, vol. 2><, pp. til-74, 1920. 



"Lee, \V. T., General stratigraphic break between Pennsylvanian 

 and Permian in western America (abstract): Geol. Soi'. .\merica Bull., 

 vol. 28, pp. 169-170, 1917. 



>5 Gregory, H. E., Geology of the Navajo country; I'. S. Geol. Survey 

 Prof. Paper 93, p. 30, 1917. 



" Shimer, H. W., Permo-Triassic of northwestern Arizona: Geol. Soc. 

 America Bull., vol. 30, p. 494, 1919. 



>' Longwell, C. R., Geology of the Muddy Movmtains, Nev., with a 

 section to the Grand Wash CliUs in western Arizona: Am. Jour. Sei„ 

 5th ser., vol. 1, p. 49, 1921. 



s» Powell, J . W ., Report on the geology of the eastern portion of the 

 llinta Mountains, pp. 53, 6S-69, U. S. Geog. and Geol. Survey Terr., 2d 

 div., 1876. 



manganese l)ut do not occur in sufficient 

 amount to have a commercial value. The 

 fossil logs are locally replaced in part by copper 

 sulphides instead of silica. 



CHINLE FORMATION. 



The name Chinle formation was given by 

 Gregory ^^ to the beds above the Shinarump 

 conglomerate and below the Wingate sand- 

 stone. Gregory was able to distinguish four 

 persistent divisions, which are, in ascending 

 order, (1) dark-red, light-red, chocolate-col- 

 ored, or rarely gray shales (70 per cent) and 

 shaly sandstone (30 per cent), with brown 

 conglomerate of limestone and clay pebbles; 

 (2) shales and "marls" with rare calcareous 

 sandstone, all lenticular, exceedingly friable, 

 red, ash-colored, and purple, with character- 

 istic limestone conglomerate; (3) gray, pink, 

 and purple cherty limestone and light to dark 

 red shale in alternating bands; (4) red, brown, 

 pink, or rarely gray calcareous shales and 

 shaly sandstones, with a few thin bands of 

 limestone and limestone conglomerate. In a 

 composite section given by Gregory division 1 

 is 203 feet tliick; 2, 450 feet; 3, 214 feet; and 4, 

 315 feet. 



In our area only one section of the Chinle 

 formation was measured, though the formation 

 is a prominent member of the stratigraphic 

 series. (See PI. XI, A.) Tliis section (No. 13, 

 p. 73) is near Mrgin City and aggregates 995 

 feet in tliickness. No limestones were observed. 

 The basal member consists of 260' feet of 

 variegated, bluish-gray, greenish-gray, mauve, 

 red, and rarely brown ''gumbo" clay shale 

 and contains fossil wood. Upon this rests 

 25 feet of variable coarse arkosic cross-bedded 

 sandstone banded with gray, white, and 

 mauve and containing fossil wood. This sand- 

 stone is overlain by 420 feet of brick-red shale 

 and sandstone, forming a slope. Next comes 

 a massive medium-grainetl cross-bedded ripple- 

 marked clifl'-forming mauve sandstone 90 feet 

 thick. Above this sandstone lies about 200 feet 

 of brick-red to deep-red shale and sandstone. 



The banded gray, white, and mauve sand- 

 stone is locally known as the "Sdver Reef 

 sandstone," as it is said to be the zone which 

 m the Silver Reef, near Leeds, Utah, contains 

 the fossil logs impregnatetl with silver minerals 



» Gregory, H. E., Geology o( the Navajo country: U. S. Geol. Survey 

 Prof. Paper 93, pp. 42-43, 1917. 



