The only bad news to report from the Peach Spring Tuff field trip to northwestern Arizona in October, 2016 was that a couple of side-wall punctures killed two tires. One, at least, happened on the first afternoon during a scenic excursion down Sawmill Canyon in Kingman. Here, some of the participants rightly complained that the rocks are too deeply weathered to appreciate petrographically. Adding to this frustration, the final stop of the day was at a City park where hammers are not allowed. Because of this, no one got a chance to collect decent samples of the main body of the Peach Spring Tuff. Realizing this need, two alternative stops have been added to the guide. The first is at the mouth of Sawmill Canyon where a quarry wall exposes fresh ignimbrite of that zone, and the second is the classic, monster road cut that looms over the west-bound lane of I-40 at Cook Canyon near old downtown Kingman. Good samples for teaching and/or research collections can be obtained at both of these stations, and by not taking the risky, shrapnel-ridden scenic drive down Sawmill Canyon, groups can save at least an hour on future trips.
Directions for the scenic drive are still in the guide where it is identified as a specialty excursion for those keen to familiarize themselves with the ignimbrite’s weathering profile and geomorphic expression.
For the rest of the trip, five significant questions / dissenting opinions were specifically addressed while completing the guide. The guide now includes a regional geologic map and cross-section. Special thanks to University of Arizona economic/bedrock geologists Max Larkin, Roy Grieg, Eric Seedorff, and Jon Spencer, Prescott professional geologist Don White, and Cal Tech geologist Joann Stock for their challenging and constructive input.
At “early bird” optional stop O-2 Paleoproterozoic gneiss in contact with Kessler’s (1976) Mesoproterozoic (1335 +/- 35 Ma Rb/Sr) coarse-grained K-feldspar megacrystic Holy Moses Granite produced an interesting discussion and well-founded dissenting opinion. As it turns out, even though much of the contact shows clear evidence of metasomatic overprint, it is also clear that the granite intrudes the gneiss.An interesting dissenting opinion regarding the origin of the Oatman epithermal gold district was offered by Eric Seedorff on the morning of the second day just prior to examining the classic Gold Road vein along old route US 66 just east of Oatman (Stop 10). Gold mineralization at Oatman clearly post-dates two phases of caldera-related alteration that are, by themselves, devoid of gold mineralization: first, a pervasive propylitic alteration of pre-caldera, dacitic/lacustrine rocks, and second, an advanced argillic alteration associated with a post-caldera, bi-modal granitic-monzonitic hypabyssal intrusive suite. Optimistically, one might suppose that the older alterations served to prepare the host rock for gold mineralization during emplacement of a suite of rhyolitic dikes and stocks ~ 1 million years after caldera formation. Eric suggested that gold mineralization might be totally unrelated to the caldera. A quick and dirty review of the literature inspired by a dissenting opinion to Eric’s suggestion (sent by to me after the trip by Don White) suggests that there is in fact a close, if not always direct, association between silicic calderas and epithermal gold mineralization world-wide. Certainly, there are places where low-sulfidation quartz-calcite-adularia +/- fluorite like the ones at Oatman have no connection with a caldera. How many of those however, might be like the mid-Cenozoic Akarca district in western Turkey where veins (Figure 3) remarkably similar to the Oatman veins, are hosted in a volcaniclastic conglomerate many 10s of km away from any similarly aged volcanic field? Since the veins at Akarca are now known to be truncated and carried in the hangingwall of a major detachment fault (Ferguson, Polat, and Akbayram, in progress), it is entirely possible that they may have been peripheral to a silicic caldera during mineralization.
The third significant question to be raised was again by Eric Seedroff who suggested that the Peach Spring Tuff’s absence in the volcanic pile around Oatman was the result of post-caldera uplift and erosion. The main reason it took so long to recognize that Thorson’s (1971) Alcyone caldera was source of the Peach Spring Tuff (as was astutely suggested by Thorson in the appendix of his dissertation) is that the ignimbrite is absent from the volcanic pile immediately east of the caldera. The guide suggests that pre-caldera uplift created a highland east of the caldera where the ignimbrite was never deposited, and to expand and explain this interpretation, the outflow sheet’s zero isopach line, based on mapping of the Mt Nutt quadrangle (Ferguson and Cook, 2017), is shown on Figure 2a of the guide. The ignimbrite near the zero edge consists of a condensed section of the uppermost zone(s) of the ignimbrite encased within a monotonous section of dacitic lava. Conglomerates that overlie the ignimbrite farther east contain sparse to zero clasts of the ignimbrite.
A dissenting view regarding the location of the cryptic, Peach Spring Tuff hiatus near the Gold Road vein (Stop 10) was brought up during a reprise of the field trip to a group from Cal Tech lead by Joann Stock. The best known exposure of the caldera margin along Times Gulch was visited at a Stop (11) that was skipped during the original trip because it would have required a short but strenuous hike. The skipped Stop 11 visits exposures of a sharp contact between the intracaldera Peach Spring Tuff and the Oatman andesite, the unit that supposedly underlies the Peach Spring Tuff-aged hiatus at the Gold Road vein stop (10). The contact at Stop 11, like the semi exposure of contact that was visited along the road during the original field trip farther down the canyon, is interpreted to be a northwest-facing buttress unconformity. Suppose the contact at Stop 11 is, instead, an intrusion of hypabyssal Oatman andesite into the intracaldera ignimbrite. This, I reason in an expanded discussion inserted into the revised guidebook, is possible but, based on stratigraphic relationships established in the Boundary Cone quad (Spencer et al. (2006) just to the south of the Oatman area, highly unlikely. Still, it is possible and the implications are intriguing since placing the “hiatus” at a lower interval might explain why the Oatman andesite is not present in the pre-Peach Spring Tuff “Snagletooth Volcanics” of the Sacramento Mts. The expanded discussion is also useful in that it emphasizes a fundamental difference between the caldera margins on either side of the Colorado River. In California, the caldera margin is a southeast-facing trap door mantled by a thick outflow of Peach Spring Tuff. In Arizona the margin is a scoured, highland rim devoid of outflow, bounded by a precipitous, west-facing escarpment.
The outline of a Pease et al.’s (1999) composite granodiorite-granite pluton of identical (18.8 Ma) age to the Peach Spring Tuff in the footwall of the Sacramento Mountains detachment (SMD) 30 km south-southwest of the main caldera fragment in Arizona is shown on Figure 2a of the new guide. Its location northeast of the tectonically dismembered segments of the caldera margin in the Sacramento Mts, and the lack of any known significant mid-Tertiary plutons in basement rocks to the southwest, constitute the two principal justifications for how the regional geologic cross-section shown in Figure 2b of the new guide was drawn. The cross-section, which had been rejected by three of the four reviewers of the original Ferguson et al. (2013) Geology manuscript, is regurgitated principally to fulfill the request of Jon Spencer, who asked to see an alternative to the rolling hinge model. The cross-section shows the pluton of Pease et al. (1999) as part of a sub-caldera mesozonal pluton. Jon remarked that the exposure of the low-angle fault visited at Stop 15, a scant 5 km south of the limit of his mapping in 1983, is one the best examples of a detachment fault that he has ever seen. Sue Beard, who wrote the one positive (and highly constructive) review of the aforementioned rejected manuscript, reproduced the 215 deg. slickenline lineation (she measured 216 deg) quoted for the fault at Stop 15 by Simpson et al. (1991).
Ferguson C. A., McIntosh, W. C., and Miller, C. F., Silver Creek caldera, 2013, The tectonically dismembered source of the Peach Spring Tuff: Geology, v. 41, p. 3-6.
Ferguson, C. A., and Cook, J. P., 2017, Geologic map of the Mt Nutt 7.5’ quadrangle, Mohave County, Arizona: Arizona Geological Survey Digital Geologic Map DGM-118, 1 sheet, 24,000 scale.
Kessler, E. J., 1976, Rubidium-strontium geochronolocy and trace element geochemistry of Precambrian rocks in the northern Hualapai Mountains, Mohave County, Arizona: Tucson, University of Arizona, M.S. thesis, 73 pp.
Pease, V., Foster, D., Wooden, J., Olivine’Sullivan, P., Argent, J., and Fanning, C., 1999, The northern Sacramento Mountains, southwestern United States: Part II, Exhumation history and detachment faulting, in Mac Niocaill, C., and Ryan, P. D., eds., Continental Tectonics: Geological Society of London Special Publication, v. 164, p. 199-237, doi:10.1144/GSL.SP.1999.164.01.11.
Simpson, C., Schweitzer, J., and Howard, K. A., 1991, A reinterpretation of the timing, position, and significance of part of the Sacramento Mountains detachment fault, southeastern California: GSA Bulletin, v. 103, p. 751-761.
Spencer, J. E., Ferguson, C. A., Pearthree, P. A., and Richard, S. M., 2006, Geologic map of the Boundary Cone 7.5’ quadrangle, Mohave County, Arizona: Arizona Geological Survey Map DGM-54, 2 sheets, 24,000 scale, 10 p. text.
Thorson, J.P., 1971, Igneous petrology of the Oatman district, Mohave County, Arizona (Ph.D. thesis): University of California–Santa Barbara, 189 pp., 2 plates, 1:24,000 scale.