tributed the clastic materials that now form the Mesozoic sedimentary rocks and destroyed a large part of the previously deposited Paleozoic rocks. Similarly uplift and subsequent erosion incident to the Mesozoic period of igneous activity further diminished the volume of Paleozoic rock, which now forms isolated blocks that are separated by great distances and are preserved largely because they are infaulted and protected by other formations. MESOZOIC ROCKS SEDIMENTARY ROCKS In the Santa Rita and Patagonia mountains, small portions of which are shown on the geologic map (Pl. IX), Schrader and Hill found a great series of shale, sandstone, and conglomerate, with calcareous layers, aggregating about 6,000 feet in thickness.52 Fossils found in a thin-bedded arenaceous limestone near Mowry indicate, according to T. W. Stanton, that the beds are of Lower Cretaceous (Comanche) age. Because of faunal and lithologic similarities Schrader 5 considers that the Mesozoic rocks of the Santa Rita and Patagonia mountains are equivalent to the Comanche series ("Bisbee group") of the Bisbee quadrangle. At Bisbee there are numerous beds of limestone with a well-developed marine fauna. In the Patagonia region, however, the calcareous layers are few, and the greater part of the beds are red sandstone and shale. 54 What seem to be the same rocks are found in the Tumacacori Mountains. Just east of the Montana mine is a great mass of red sandstone, conglomerate, and shale, dipping to the west at high angles and unconformably overlain by tuff. Both east and west of Arivaca are red and green shales with incipient slaty cleavage, which may be part of the same formation. Red sandy shale and arkosic gray sandstone of a total thickness of 2,140 feet, attributed to Mesozoic time, have been found in the Tucson Mountains.55 Red and green shales with thin quartzitic sandstone are exposed on the east side of the Sierrita Mountains. They may be of Mesozoic age." 56 West of the Tumacacori and Tucson mountains red shale and arkose sandstone were seen at Agua la Vara, faulted against the gneiss of the Coyote Mountains, and again at Pozo Blanco, associated with the limestone already mentioned. In these small outcrops no fossils Schrader, F. C., and Hill, J. M., Mineral deposits of the Santa Rita and Patagonia mountains, Ariz.: U. S. Geol. Survey Bull. 582, p. 53, 1915. Idem, p. 53. Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Ariz. U. S. Geol. Survey Prof. Paper 21, pp. 51 et seq., 1904. Jenkins, O. P., and Wilson, E. D., op. cit., p. 11. *Ransome, F. L., op. cit. (Bull. 725), p. 412. were seen, but it seems not unlikely, because of lithologic similarity, that they are westward remnants of the Mesozoic formations. IGNEOUS ROCKS The igneous rocks which are thought to be Mesozoic are difficult to distinguish from the pre-Cambrian rocks without detailed study. In general they are characterized by lack of sheeting and gneissoid structure and by the fact that many of the granitic types have both orthoclase and plagioclase feldspars. In the Santa Rita and Patagonia mountains the Mesozoic igneous rocks, consisting of granite, granite porphyry, quartz monzonite, quartz diorite, aplitic and lamprophyric dike rocks, syenite, and gabbro, are intruded into the Paleozoic sedimentary rocks but nowhere cut the Mesozoic sedimentary rocks. The period of igneous intrusion is therefore thought to be pre-Cretaceous or at the latest very early Cretaceous.57 At Silver Bell the Mesozoic intrusive rocks consist of alaskite, alaskite porphyry, biotite granite, andesite, and quartz porphyry.58 In the Sierra Estrella Schrader found biotite granite, granitic aplite, and pegmatite, which intrude older gneiss and schist. In the Sacaton Mountains he found mica diorite similarly intruded. These rocks he considers Mesozoic.59 The copper ores of Ajo occur as disseminated deposits in monzonite, or as vein deposits in rhyolite lava and tuff into which the monzonite is intruded. These two types of rock make up the eastern portion of the main part of the Little Ajo Mountains and are older than the conglomerate of the Ajo Peaks and the lava flows of Black Mountain. Older gneiss and schist which resemble the preCambrian complex form the western part of the mountains. The igneous rocks that contain the ores, both the monzonite and the rhyolite into which the monzonite is intruded, seem, because of their lack of intense dynamic metamorphism and their relation to the older crystalline rocks, to belong to the Mesozoic period of igneous activity. In a number of localities in the Papago country were observed granitic and granitoid rocks containing both plagioclase and orthoclase feldspars, which probably belong to the same period of igneous activity. These localities are the Palo Verde Mountains, the mountains in the pass north of Table Top Mountains, the Maricopa Mountains near Estrella, the O'Neill Hills, the Tule Mountains, and the 7 Schrader, F. C., and Hill, J. M., op. cit., p. 57. Stewart, C. A., op. cit., p. 246. Schrader, F. C., unpublished report on the Gila River Indian Reservation, Ariz. 60 Joralemon, I. B., The Ajo copper-mining district, Ariz.: Am. Inst. Min. Eng. Bull. 92, pp. 2011-2028, 1914. Tinajas Altas Mountains. Fractured and altered porphyritic felsite of pre-Tertiary age was seen in the Papago Saguaro National Monument north of Tempe and in Growler Pass. TERTIARY ROCKS SEDIMENTARY ROCKS The sandstone and conglomerate found at a number of localities in this region are thought to be of Tertiary age. The evidence for their age is not definite but consists in their stratigraphic position and their similarity to rocks in the lower Gila region that are associated with calcareous beds from which Tertiary fossils have been collected. The close association of the conglomerate with the great sequence of lavas is probably the best evidence of its Tertiary age. 61 The beds included in this group are probably not strictly synchronous. Wherever their lower members were observed they rest on the eroded surface of the older rocks and are made up of angular débris derived from those rocks. In general the Tertiary sedimentary rocks are composed of coarse, poorly sorted arkosic material, such as is deposited by ephemeral streams in the same region to-day. In some localities so much tuffaceous material is included that it is evident that they were deposited during the volcanic period. Outcrops of the rocks were studied at nine localities, which are shown upon the geologic map (Pl. IX). The beds range in thickness from 50 feet to more than 1,000 feet and are, without exception, dislocated and extensively eroded. Tertiary rocks near Tempe.-One of the largest exposures of the Tertiary rocks forms Tempe Butte and the picturesque hills north of it in the Papago Saguaro National Monument. The northerly hills are composed of arkosic conglomerate in beds 20 to 50 feet thick, consisting mostly of fragments of granite, many of which are 6 inches to 6 feet in diameter. In certain beds there is little or no matrix, the boulders being tightly packed together. In other beds the boulders are embedded in a matrix of coarse sand made up of feldspar and quartz fragments from one-eighth to one-half inch in diameter. Only the finer material shows any sorting, except that there are a few small lenses of sandy laminated clay, and in some beds the larger boulders are ranged along the bottom. The whole rock and many of the constituent pebbles are stained by hematite, and though calcite is the principal cement, hematite doubtless helps to bind the particles together. The peculiar erosion forms of the conglomerate are described on page 91. Just north of Hole in the Rock the conglomerate rests on granite gneiss, and most of the constituent fragments are of this rock. There are also many schist e1 Ross, C. P., op. cit., p. 189. fragments similar to a schist that crops out to the east. South of Hole in the Rock the conglomerate rests on a porphyritic felsite, shattered by minute fracturing and recemented by silica. The conglomerate of this locality is composed largely of fragments of the felsite, and the near-by Quaternary gravel cemented by caliche consists of materials derived from both rocks. The shattered felsite and the Tertiary and Quaternary conglomerates, composed of materials derived from it, are very difficult to distinguish in the field. Near the bridge across Gila River and on the north flank of Tempe Butte is a mass of red arkosic sandstone showing mud cracks. The sandstone appears to be a higher member of the formation to which the conglomerate previously mentioned belongs. At the base of Tempe Butte the sandstone dips 55° SE. and strikes N. 60° E. It is more shaly at the top and is overlain by a thick bed of lava, which caps the butte. Blake considered these rocks to be of Triassic age, because of their lithologic similarity to the Triassic elsewhere.62 Lee 63 considers that they can hardly be Triassic, because the Triassic rests on the Paleozoic rocks in the plateau region to the east with apparent conformity, and assigns them to the Tertiary. In Tempe Butte the upper shaly beds are intruded by a thin basic sill and are interbedded with an amygdaloidal basalt flow and capped by a thick bed of lava. This close association and contemporaneity with lava, which resembles in every way the other Tertiary igneous rocks, seems in the absence of fossils to be good evidence that the beds are of Tertiary age. Conglomerate near Comobabi.-In the pass between North and South Comobabi mountains there is a large area underlain by conglomerate. About half a mile east of the village of Comobabi the conglomerate is well exposed in a cliff (Pl. VIII, B), where it is overlain unconformably by 12 feet of cemented gravel containing fossil bone. (See p. 68.) The dip is 30° NW., and the strike N. 87° E. Boulders 3 and 4 feet in diameter are common; the ordinary size, however, is about 4 to 6 inches, and there is a sandy and pebbly matrix. In angularity of pebbles and lack of sorting this rock is similar to the other outcrops of conglomerate. The material is derived largely from the erosion of porphyritic rocks of andesitic type, but there are many granite boulders. The conglomerate is red but not so thoroughly cemented as in some other localities. Conglomerate in the pass north of Table Top Mountains.-Gray arkosic conglomerate interbedded with lava occurs near the water 62 Blake, W. P., Some salient features of the geology of Arizona with evidences of shallow seas in Paleozoic time: Am. Geologist, vol. 27, p. 166, 1901. es Lee, W. T., Underground waters of the Salt River valley, Ariz. U. S. Geol. Survey Water-Supply Paper 136, p. 97, figs. 15, 16, 1905. hole in the pass on the Maricopa-Covered Wells road, 18 miles from Maricopa. The beds dip 14° SW. and strike N. 42° W. The conglomerate is interbedded with basalt flows, the lowest of which rests on granite on the north side of the pass. The conglomerate, however, has very few basalt pebbles but is made up of granitic material. Some of the boulders are 6 to 8 feet in diameter. Conglomerate at Totobit Tanks.-The Totobit Tanks lie along the main drain of Vekol Valley, 5 miles southwest of the Vekol mine. They consist of a series of plunge pools and potholes in conglomerate and lava. The conglomerate has a tuffaceous matrix, and the pebbles are rounded boulders from 1 inch to 2 feet in diameter, mostly of basalt. As the conglomerate is interbedded with basalt it probably represents stream action during the period of Tertiary volcanism. Conglomerate at Sand Tanks.-At Sand Tanks, 23 miles southeast of Gila Bend, tuffaceous conglomerate crops out in a narrow canyon in the dissected pediment of the Sand Tank Mountains. The tanks consist of plunge pools eroded in the conglomerate, which dips 20° N. and strikes N. 80° W. The following is a section of the beds exposed: Section of Tertiary rocks at Sand Tanks Brown tuffaceous conglomerate with large granite pebbles and Brown sandy tuff with small pebbles of various rocks, lower Green sandy tuff___ White tuff, laminated and with occasional pebbles; thickness Brown pebbly tuff. Normal fault with 6 feet throw to west 15 Granite gneiss, much fractured, on which the conglomerate It is evident from the section that these rocks were formed after volcanism had begun in the neighborhood. The pre-Tertiary rocks were exposed, and at this place streams deposited poorly assorted débris derived from the erosion of the pre-Tertiary volcanic rocks, mixed with the materials of ash showers. |