196 



SHORTER CONTRIBUTIONS TO CxENERAL GEOLOGY, 1921. 



present, flooding soutliern Cnlifoinia in the 

 region of the Salton Imsin.'"' Possibly the 

 calcareous beds in the region covered by this 

 report mark the northern extension of this 

 incursion of mai'ine waters. 



There was much normal faulting in Ter- 

 tiary time, some of it on a large scale, and 

 probably there was more than one period of 

 faulting. It resulted in the formation of 

 structural valleys between tlie upthrown 

 blocks. • Folding either did not occur or was of 

 very minor amount. 



QUATERNARY TIME. 



Our knowledge of Quaternary events in this 

 region is more detailed and complete than that 

 of the older geologic periods. Doubtless there 

 were several divisions of Tertiary time besides 

 those mentioned above. The great masses of 

 lava, for example, probably were not all poured 

 out during one continuous period of eruption. 

 There were interruptions and alternations of 

 conditions. The record of these events is so 

 fragmentary and obscure, however, that it was 

 impossible to work out the details of the Ter- 

 tiary history. The record of Quaternary 

 events is naturally much more completelv pre- 

 served, though there is much that is still un- 

 certain oi' entirely unknowm regarding the 

 history of this period. One of the greatest 

 difliculties encountered in interpreting the 

 record is that of differentiating between the 

 older and the younger valley fill, which are 

 lithologically very similar. 



Some uncertainty exists as to the division 

 between the Tertiary and Quaternary in this 

 region. Lee ^" believes that the uplift that 

 initiated the cutting of the Grand Canyon of 

 the Colorado marks the beginning of the 

 Quaternary period. This uplift was very 

 prol)al)ly essentially contemporaneous with 

 that which resulted in the deep cutting of the 

 desert \'allcvs. However, Lee elsewhere" 

 makes the suggestion that the lower portion 

 of the (ill in .Salt River valley may be Ter- 

 tiary, lie c(:)nsiders that this lower portion 

 may be lacustrine in origin and notably 



^■' Ivew, W. S. W., Tcrtiiiry ecliiuoids of Ihc Carrizo Creek region in 

 the Colorado Deserl: California Univ. IJept. Geology Bull., vol. ,s, pp. 

 39-fin, 1(111. 



3fi Lee, W. T., Geologic reconnaissance of a part of western Arizona: 

 U. S. Cieol. Survey Bull. 3,«, pp. 62-<J3, Uliis. 



" Lee, \V. T., Underground waters of Salt River valley, Ariz., U. S. 

 Geol. Survey Water-Supply Paper 136, p. 114, 1905. 



older than the detrital material above it. 

 Deep wells .show that there is a considerable 

 thickness of clay or other fine material be- 

 neath the coarser detritus in Salt River 

 valley. Records of wells in Buckeye and 

 Arlington valleys and at Gila Bend show that 

 similar conditions exist in those localities 

 also. Considerable clay was encountered in 

 several of the Southern Pacific Railroad wells 

 on the Gila west of Gila Bend. It is possible 

 that fossil or other evidence may eventually 

 be found which will show that these beds are 

 Tertiary, but until further facts are dis- 

 covered the most logical conclusion appears 

 to be to consider the deep cutting of the val- 

 leys, original!}' in large part of structural 

 origin, as the first event of the Quaternary 

 period ui the region under discussion. Any 

 sediments, whatever then- origin, l\'ing in 

 these valleys must then be considered of 

 (Quaternary ago. The mere fact that the 

 lower part of the fill is apparently of lacustrine 

 origin does not afl'ect the problem of its age. 

 Beds of unquestionably Quaternary age and 

 very probaljly lacustrine origin occur near 

 Parker, on Colorado River. A temporary 

 lake ■" is believed to have existed in Arlington 

 \'alley m recent geologic time. 



After the valley cutting the conditions were 

 so altered that the streams began to aggrade, 

 and the recently excavated valleys were 

 filled to great depths with detrital material. 

 Basalt flows, the continuation of the basaltic 

 efl'usions at th(- end of the Tertitiry, occurred 

 at this time. As has already been stated, 

 volcanism did nt)! continue as late in this 

 region as it did in some other portions of the 

 Southwest, notably southern California. It 

 continued intermittently to a time consid- 

 erably later than that in which the first valley 

 fill was deposited. 



When the valleys had been very largely 

 filled with detritus, renewed uplift occurred. 

 In places the recently deposited sediments 

 were faulteil and somewhat folded. Degra- 

 dation recommencetl, and much of the material 

 with which the valleys had just been filled 

 was swept out of them. 



Before all of the first valley fill had been re- 

 moved aggradation was resumed and the 



^^ Ros.s, C. P., The lower Gila region, Ariz.: U. S. Geol. Siu^vey Water- 

 Supply Paper — (in preparation). 



