GEOLOGY OF THE LOWER OILA REGION, ARIZONA. 



195 



were noted during the present investigation in 

 the Buckskin Mountains. A number of similar 

 intrusions are known in adjoining regions. 



The pre-Cambrian rocks were considerably 

 metamorphosed during the period between their 

 deposition and that of the Tertiary lavas. The 

 metamorphism probably took place in pre-Cam- 

 brian time, for Paleozoic rocks in adjoining re- 

 gions show no evidence of having been affected 

 by it. There has been no close folding since the 

 deposition of the later pre-Cambrian rocks. 

 Thick masses of these rocks are now exposed 

 which show no folding and little tilting. 

 Faulting took place at some period prior to 

 the eruption of the Tertiary lavas, and it is 

 probable that during that period the major 

 areas of uplift which form the jjresent areas of 

 these rocks may have been blocked out, at 

 least in part. 



TERTIARY TIME. 



The Tertiary was a period of pronounced 

 volcanism, in which great sheets of lava were 

 piled up in flow upon flow. Agglomerate and 

 tuff are associated with the lavas, but in very 

 subordinate amount. Quiet outflows rather 

 than eruptions of explosive violence were the 

 rule. Bancroft ^''' states that volcanic plugs 

 are present in several places in the area in 

 northern Yuma County which he examined 

 and are apparently more numerous near the 

 lower part of Williams River than elsewhere. 

 These plugs may represent remnants of Ter- 

 tiary volcanoes. Plugs of latitic rock occur 

 near Saddle Mountain, west of Quartzsite, in 

 the Dome Rock Mountains, and at a few other 

 places in the region covered by this report, 

 but such remnants of Tertiary volcanoes are 

 rare. Quite possibl}- most of the eruptions 

 were of the fissure type, and no volcanoes, 

 except a few small ones, ever existed here. 

 Probably lava flowed over much of this region 

 during the Tertiary period, covering most of 

 the hills then existing. Apparently, however, 

 some ranges were never capped completely by 

 the lava. The Harquahala, Little Harquahala, 

 and Harcuvar Mountains belong to this class, 

 and portions of the Buckskin Mountains and 

 of some of the other ranges may also have 

 escaped being covered. Felsitic Tertiary in- 

 trusive rocks and possibly some lavas occur 



n Idem, pp. 30-31. 



in the Dome Rock Mountains, but this range 

 consists almost exclusively of rocks of the basal 

 complex. If the range was ever lava capped, 

 all the lava has since been removed by erosion. 

 Comparatively little is known in regard to the 

 geology of the Laguna, Trigo, and Chocolate 

 mountains. Possibly parts of these ranges 

 escaped the general flooding of the region by 

 the sheets of lava. Probalily there was more 

 than one period of extrusion, as has been found 

 to be recorded elsewhere in similar rocks. 

 Much more detailed work is required to deter- 

 mine this point. 



The amount of sedimentary rocks of Ter- 

 tiary age found in the area is small indeed 

 compared to the many hundreds of feet of 

 lavas. Uncjuestionably volcanism rather than 

 sedimentation was the dominant feature of the 

 period. Mucli of the Tertiary sedimentary 

 rock is believed to be of terrestrial origin and 

 was probably deposited under conditions not 

 very different from those of the present day. 

 This fact is better showTi in the exposures of 

 Tertiary formations south of the area covered 

 by this guide, where stream-laid conglomerates 

 occur.^^ 



The calcareous sediments found in several 

 places within this area and in adjoining parts 

 of California tell a very different story. (See 

 pp. 188-190.) These beds were unquestionably 

 laid down in large bodies of quiet .water. They 

 are lacustrine or estuarine. A glance at the 

 map will show that the exposures of these 

 deposits are scattered over a region covering 

 about 2,000 square miles. Only one of them, 

 that near Osborne's Well, is in an area covered 

 by an accurate topographic map, hence the 

 exact altitudes of the others are not known. 

 The best estiniates available, however, show 

 that all. including the California area, are at 

 altitudes of approximately 700 feet above sea 

 level. Unfortunately, the paleontologic evi- 

 dence at hand is not conclusive as regai-ds the 

 character of the waters in which these beds 

 were deposited. It is possible that they were 

 laid down in lakes l.ying between the mountain 

 ranges. Much more probably, however, they 

 were deposited in an estuary or estuaries ex- 

 tending northward from the Gulf of California. 

 In late Miocene or Pliocene time the gulf had a 

 much greater extension to the north than at 



'• Bryan, Kirk, The Papago country, Ariz.: U. S. Geol. Survey Water- 

 Supply Paper — (in preparation). 



