GEOLOGY OF THE LOWER GILA REGION, ARIZONA. 



191 



Fossil fresh-water shells have been found m 

 some of these beds. E. L.Jones, jr.,™ who made 

 an examination of the reservation for the 

 United States Geological Survey in 1914, states 

 that these are lake beds. 



Along washes within the mountains and on 

 the borders of the ranges are beds of gravel and 

 sand similar to those of the valley fill. These 

 beds are cut by the present washes. Although 

 they are clearly similar to the material in the 

 modern streamways and were deposited under 

 conditions very similar to those existing to-day, 

 the position of many of these beds indicates 

 that they were laid down in streams whose 

 courses had little or no relation to those of the 

 present streams. The gravel and sand are 

 everywhere somewhat consolidated. In the 

 wash that parallels the new road where it 

 emerges from the mountains on the west side 

 the unconsolidated or slightly consolidated 

 gravel of the valley fill can be seen lapping up 

 on the gently inclined and smooth surface of 

 gravel with a calcareous cement. This gravel 

 is continuous with gravel of the type just de- 

 scribed occui'ring in the mountains proper. 

 Similar exposures were noted near the road 

 between Wenden and Butler Well, on the 

 north side of Cunningham Pass, in the Ilarcu- 

 var Mountams. Gravel of similar appearance, 

 which is being eroded by the present streams, 

 was noted in Osborne Wash, north of Os- 

 borne's Well, in the Buckskin Momitains. 



The partly consolidated detrital beds in the 

 mountains are in places cut by normal faults 

 and tilted to angles of 20° and even more. 

 The best exposures found are in the Gila 

 Bend Mountains. (See PI. XLIV, A.) Tilted 

 blocks of gravel were noted near both of the 

 roads that cross this range, but they are 

 especially well exposed along the part of the 

 old road that lies in the mountains. Out- 

 crops of such material were found also along 

 the large wash followed by the old road on the 

 west side of the moimtains. Slio-ht folding in 

 gravel beds was observed in some outcrops 

 near Woolsey Tanks, along this road. Tilted 

 gravel and sand are exposed at the north end 

 of the Gila Mountains near Dome. Some of 

 the more consolidated alluvium in tlie Dome 

 Rock and Buckskin mountains is probably 

 tilted. Beds of gravel and sand that have 



so Personal cominiinication. 



been disturbed by earth movements doubtless 

 exist elsewhere in the region but were not 

 noted during this investigation. 



It is evident that Quaternary sediments 

 belonging to at least tlu-ee periods of deposition 

 occur in this area. These are (1) the somewhat 

 consolidated beds exposed in and near the 

 mountains, which have been disturbed by 

 faulting, (2) the unconsolidated or only locally 

 consolidated flat-lying valley fill, and (3) the 

 recently deposited material in the washes and 

 the playas of the desert valleys and in the 

 flood plains of the larger streams. This con- 

 clusion is in accord with the results of Lee's 

 work -' in adjoining areas and in portions of 

 the area here considered. He has given forma- 

 tional names to the two older Quaternary for- 

 mations in the vicinity of Colorado River but 

 not to the recent material flooring the river 

 flood plains, etc. The oldest group of gravels 

 and sands he calls the Temple Bar conglom- 

 erate. The unconsolidated material resting 

 upon the Temple Bar conglomerate and 

 exposed in terraced bluffs along Colorado 

 River and elsewhere he calls the Chemehuevis 

 gi-avel. The Temple Bar conglomerate is 

 lithologically similar to the oldest of the three 

 Quaternary formations herein described, but 

 the thicknesses observed by Lee along the 

 upper Colorado are far greater than any 

 found in this region. The two may perhaps 

 be of similar age and history. The Gila con- 

 glomerate, described by Gilbert,-^ is similar to 

 the Temple Bai-, being a thick formation of 

 coarse alluvium in the upper Gila Valley. The 

 correlation of these formations awaits the 

 complete solution of the physiographic history 

 of southwestern ^\i-izona in Quaternary time. 



QUATERNAJRY BASALT. 



Associated with the gravel and sand of the 

 valley fill in places in this area are flows of 

 olivine basalt. Such rock caps the fill, is inter- 

 bedded with it, and cuts it in the form of dikes 

 and other intrusive masses, generally small and 

 irregular. The basalt masses that rise above 

 the present surface of the fill have produced 



21 Lee, W. T., Geologic reconnaissance of a part of western Arizona: 

 U. S. Geol. Survey Bull. 352, pp. 17, IS, 1908; Underground waters of 

 the Salt River valley, Ariz.: U. S. Geol. Survey Water-Supply 

 Paper 136. pp. 111-114, 1905. 



" Gilbert, 0. K., U. S. Geog. and Geol. Surveys W. 100th Mer.Rept., 

 vol. 3, pp. 540-541, 1875. 



