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Archeological Obsidian and Secondary Depositional UC Berkeley Archaeological X-ray Fluorescence Reports Title Archaeological Obsidian and Secondary Depositional Effects in The Jemez Mountains and The Sierra De Los Valles, Northern New Mexico Permalink https://escholarship.org/uc/item/6tq782ss Author Shackley, M. Steven Publication Date 2002-09-01 Supplemental Material https://escholarship.org/uc/item/6tq782ss#supplemental eScholarship.org Powered by the California Digital Library University of California ARCHAEOLOGICAL OBSIDIAN AND SECONDARY DEPOSITIONAL EFFECTS IN THE JEMEZ MOUNTAINS AND THE SIERRA DE LOS VALLES, NORTHERN NEW MEXICO M. Steven Shackley, Ph.D. Archaeological XRF Laboratory University of California, Berkeley Report prepared for The Ecology Group Los Alamos National Laboratory Los Alamos, New Mexico September 2002 INTRODUCTION Distributed in archaeological contexts over as great a distance as Government Mountain in the San Francisco Volcanic Field in northern Arizona, the Quaternary sources in the Jemez Mountains, most associated with the collapse of the Valles Caldera, are distributed at least as far south as Chihuahua through secondary deposition in the Rio Grande, and east to the Oklahoma and Texas Panhandles through exchange. And like the sources in northern Arizona, the nodule sizes are up to 10-20 cm in diameter; El Rechuelos, Cerro Toledo Rhyolite, and Valle Grande glass sources are as good a media for tool production as anywhere. While there has been an effort to collect and record primary source obsidian, the focus here has been to understand the secondary distribution of the Jemez Mountains sources. Until the recent land exchange of the Baca Ranch properties, the Valle Grande primary domes (i.e. Cerro del Medio) have been off- limits to most research. The discussion of this source group here is based on collections by Dan Wolfman and others, facilitated by Los Alamos National Laboratory, and the Museum of New Mexico (see Broxton et al. 1995; Wolfman 1994). Due to its proximity and relationship to the Rio Grande Rift System, potential uranium ore, geothermal possibilities, an active magma chamber, and a number of other geological issues, the Jemez Mountains and the Toledo and Valles Calderas particularly have been the subject of intensive structural and petrological study particularly since the 1970s (Bailey et al. 1969; Gardner et al. 1986; Heiken et al. 1986; Ross et al. 1961; Self et al. 1986; Smith et al. 1970; Figures 1 and 2 here). Half of the 1986 Journal of Geophysical Research, volume 91, was devoted to the then current research on the Jemez Mountains. More accessible for archaeologists, the geology of which is mainly derived from the above, is Baugh and Nelson’s (1987) article on the relationship between northern New Mexico archaeological obsidian sources and procurement on the southern Plains, and Glascock et al’s (1999) more intensive analysis of these sources including the No Agua Peak source in the Taos Plateau Volcanic Field at the Colorado/New Mexico border. This study is focused on the analysis of obsidian and rock samples submitted by Los Alamos National Laboratory (LLNL), and the report of the long-term secondary depositional study by this laboratory, in part funded by LLNL. The secondary depositional study is geared toward an understanding of the probable patterns of prehistoric procurement of artifact quality obsidian from sources in the Jemez Mountains. 2 Figure 1. Topographical rendering of a portion of the Jemez Mountains, Valles Caldera, and relevant features (from Smith et al. 1970; formation explanations in Smith et al. 1970). 3 BEDROCK AND ALLUVIAL DEPOSITION OF THE SIERRA DE LOS VALLES Due to continuing tectonic stress along the Rio Grande, a lineament down into the mantle has produced a great amount of mafic volcanism during the last 13 million years (Self et al. 1986). Similar to the Mount Taylor field to the west, earlier eruptive events during the Tertiary more likely related to the complex interaction of the Basin and Range and Colorado Plateau provinces produced bimodal andesite-rhyolite fields, of which the Paliza Canyon (Keres Group) and probably the Polvadera Group (El Rechuelos) is a part (Broxton et al. 1995; Shackley 1998a; Smith et al. 1970; Figure 1 here). While both these appear to have produced artifact quality obsidian, the nodule sizes are relatively small due to hydration and devitrification over time (see Hughes and Smith 1993; Shackley 1990, 1995). Later, during rifting along the lineament and other processes not well understood, first the Toledo Caldera (ca. 1.45 Ma) and then the Valles Caldera (1.12 Ma) collapsed causing the ring eruptive events that were dominated by crustally derived silicic volcanism and dome formation (Self et al. 1986). The later eruptive sequence of the Valle Grande Member is significant for the prehistoric procurement of the obsidian as discussed below. The Cerro Toledo Rhyolite and Valle Grande Member obsidians are grouped within the Tewa Group due to their similar magmatic origins. The slight difference in trace element chemistry is probably due to evolution of the magma through time from the Cerro Toledo event to the Valle Grande events (see Hildreth 1981; Mahood and Stimac 1990; Shackley 1998a, 1998b). Given the relatively recent events in the Tewa Group, nodule size is large and hydration and devitrification minimal, yielding the best natural glass media for tool production in the Jemez Mountains. 4 Figure 2. Generalized stratigraphic relations of the major volcanic and alluvial units in the Jemez Mountains (from Gardner et al. 1986). Note the near overlapping events at this scale for the Cerro Toledo and Valles Rhyolite members, and the position of Cerro Toledo Rhyolite at the upper termination of the Puye Formation. Some of the potentially minor sources of archaeological obsidian from the Jemez Mountain area such as the glass from the Bland Canyon area, appear to be better artifact quality obsidian than previously reported. My recent survey in Bland Canyon (August 1999) below Bearhead Rhyolite yielded a sample of over 100 marekanites of which a sample of 15 analyzed matched the Cerro Toledo Rhyolite signature, not the “Bland Canyon/Apache Tears” signature reported by Glascock et al. (1999:Table 1). The exact sampling location for the Glascock et al. (1999) samples is apparently unknown (see also Wolfman 1994). The Bland Canyon data 5 reported could be rare nodules from an earlier eruptive event contemporaneous with the pre- caldera Polvadera Group, since obliterated by subsequent volcanism and thus making the nodules rare. While this discrepancy could be sampling error in this study, it certainly suggests by this research that the eruptive history and trace element chemistry of artifact quality obsidian from the Jemez Mountains is somewhat more complex than originally described and warrants more intensive geoprospection, a major stimulus for the LLNL project here. SECONDARY DEPOSITION AND PREHISTORIC PROCUREMENT IN NORTHERN NEW MEXICO Recent research by this lab investigating the secondary depositional regime from the Jemez Mountains (Sierra de los Valles), indicates that: 1) Valle Grande Member rhyolite and obsidian in the Jemez Mountains, the result of the most recent eruptive event that produced glass in the caldera, does not erode out of the caldera in nodules of any workable size; 2) During the Pleistocene, Cerro Toledo Rhyolite and glass, mainly the result of the Rabbit Mountain ash flow eruption deposited vast quantities of ash and quenched rhyolite through erosion in the Rio Grande River basin as discussed above (Shackley 1998a, 2000). While Cerro Toledo Rhyolite obsidian is found in secondary contexts in the Puye Formation along the northeastern margin of the caldera (see Figures 3 and 4), the greatest quantity of obsidian found today in the Rio Grande River alluvium most likely came from the Rabbit Mountain ash flow event. 6 Cerro Toledo Rhy. Valles Rhyolite Rabbit Mtn Figure 3. Generalized large scale view of major obsidian source areas and relevant secondary depositional features in north-central New Mexico (adapted from Heiken et al. 1986). 7 Figure 4. Distribution of tuffs and epiclastic sediments derived from Toledo Embayment and Rabbit Mountain eruptions (from Heicken et al. 1986). There were six pyroclastic eruptive events associated with the Cerro Toledo Rhyolite: All tuff sequences from Toledo intracaldera activity are separated by epiclastic sedimentary rocks that represent periods of erosion and deposition in channels. All consist of rhyolitic tephra and most contain Plinian pumice falls and thin beds of very fine grained ash of phreatomagmatic origin. Most Toledo deposits are thickest in paleocanyons cut into lower Bandelier Tuff and older rocks [as with the Rabbit Mountain ash flow]. Some of the phreatomagmatic tephra flowed down canyons from the caldera as base surges (Heiken et al. 1986:1802). Two major ash flows or ignimbrites are relevant here. One derived from the Toledo embayment on the northeast side of the caldera is a 20 km wide band that trends to the northeast and is now highly eroded and interbedded in places with the earlier Puye Formation from around Guaje Mountain north to Santa Fe Forest Road 144. This area has 8 eroded rapidly and obsidian from this tuff is now an integral part of the Rio Grande alluvium north of Santa Fe. The other major ash flow is derived from the Rabbit Mountain eruption and is comprised of a southeast trending 4 km wide and 7 km long “tuff blanket” interbedded with a rhyolite breccia three to six meters thick that contains abundant obsidian (Heiken et al. 1986; see also Broxton et al. 1995). All of this is still eroding into the southeast trending canyons toward the Rio Grande. The surge deposits immediately south of Rabbit Mountain contain abundant obsidian chemically identical to the samples from the ridges farther south and in the Rio Grande alluvium. Heiken et al. NAA analysis of Rabbit Mountain lavas is very similar to those from this study (1986:1810; Table 1 here).
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