OBSIDIAN BUTTE

IMPERIAL COUNTY,

SOUTHEASTERN CALIFORNIA

 

Obsidian Butte, (CA-IMP-245) in Imperial County, California, part of the Salton Butte complex has been extensively studied geologically, in part due to its location on the San Andreas fault, and partly due to the construction of the geothermal plant nearby (Robinson et al. 1976; Schmitt et al. 2013; Wright et al. 2015). This prehistorically popular, somewhat vitrophyric, obsidian source was originally covered in fine detail by Richard Hughes (1986), although it was not ground-truthed in the field by Hughes. We have a good source standard data library for this source at Albuquerque, and routinely analyze archaeological collections from San Diego, Imperial, and Riverside Counties. In December 2009 the samples originally collected in the 1980s were re-analyzed on the ThermoScientific Quant'X for both oxides and trace elements (Tables 1 and 2).  This mildly peralkaline obsidian exhibits a unique elemental composition in the region (Hughes and True 1985; Panich et al. 2017).

In October 2017, Obsidian Butte was revisited, re-sampled, and mapped.  Modern earth moving, possibly in association with geothermal investigations in the area, have extensively impacted the site.  There is no certain archaeological evidence at the locality, and there has been extensive recent collecting of the obsidian at each of the three vents or domes.  The purpose in 2017 was to sample each of the domes in order to obtain a more complete picture of the elemental variability of the source (see Tables 1 and 2 and Figures 1 through 3 below).

While all the Obsidian Butte domes produced vitrophyric obsidian with relatively sparse sanidine (alkali-feldspar) phenocrysts and some with larger spherulites, the vitrophyric quality was not enough to prevent the production of projectile points, some well executed including keyhole notching and serrations (McDonald 1997; Shackley 2017; see Figure 4).  As eluded to by True's early (1970) study in the northern Peninsular Ranges (Cuyamaca Rancho State Park) in eastern San Diego County, Obsidian Butte obsidian is quite common in the mountain sites, and was distributed throughout Kumeyaay territory by exchange that occurred during the fall acorn harvest and deer hunting (see Shackley 2004, 2017).  The obsidian is a brittle raw material that knaps quite easily, but my experience at the source suggests that some selection is necessary - some material is more brittle and/or vitrophyric than others. The obsidian at the smaller western butte (101917-3; see Figure 1), appears to be somewhat less vitrophyric, and could have been the dome where most prehistoric quarrying occurred.  It is impossible to see certain evidence of prehistoric quarrying today, and the elemental composition of the three buttes is statistically similar and cannot be discriminated by XRF (see Tables 1 and 2 and Figures 2 and 3). 

Early investigations suggested that the source was 30,000 years old (Robinson et al. 1976).  Recent research, and archaeologically important, indicates that the obsidian was actually erupted during the late Holocene about 2500 years ago, dated by U-Th-Pb, and (U-Th)/He zircon to 2.48 0.47 ka at 2 sigma, and since verified with Ar40/39 dates at 2.83 0.60 ka at 2 sigma (Schmitt et al. 2012, 2013; Wright et al. 2015).  This means that this source would not be present in regional archaeological contexts during much of the Archaic, and can stand as a terminal ante quem date archaeologically (see McDonald 1992 at Indian Hill; also Kyle 1996; Hughes and True 1985).  Late Archaic Elko-eared dart points were recovered from Indian Hill rockshelter in eastern San Diego County produced from Obsidian Butte, and have also been recovered in San Diego County sites (Hughes and True 1985; Shackley 2017; see Figure 4 here).  It appears that not long after eruption, Late Archaic knappers were using Obsidian Butte for tool production (Kyle 1996; Shackley 2017).  By the Late Prehistoric period Obsidian Butte became the dominant obsidian used for tool production in San Diego and Imperial Counties of southern California, and to a lesser extent in Orange, Riverside, and San Bernardino Counties, as well as far northern Baja California (Hughes and True 1985; Panich et al. 2017; Shackley 2017).

Due to filling of the Salton Basin at least six times during the Holocene creating Lake Cahuilla, Obsidian Butte would be unavailable during the Late Prehistoric period in the region for varying periods of time (Schmitt et al. 2013; Waters 1983; Wright et al. 2015).  The filling of Lake Cahuilla and occupation of the lake shore by groups moving west from Arizona was a prime stimulus for the Patayan occupation of what is now San Diego and Imperial Counties, and northern Baja California (Shackley 1981, 1984, 2004).  The Kumeyaay, a Yuman speaking group, moved west from their ancestral home in western Arizona first to sites along the Lake Cahuilla shoreline, and finally west to the coast of San Diego County, and northwestern Baja California by about A.D. 1100-1200 (Cooley 1998; Shackley 1981, 1984, 2004; Waters 1982).  Obsidian Butte, then, became a part of Kumeyaay geography and territory and the major obsidian toolstone for the group, and likely exchanged with Takic speaking groups to the north in what is now Riverside, and San Bernardino Counties (Cooley 1998; Hughes and True 1985; McGuire and Schiffer 1982; Panich et al. 2017; Shackley 1981, 1984, 1998, 2004; Waters 1982).

 

Figure 1. Obsidian Butte (Google mapping from 2016)

                   

Figure 2.  Detail of obsidian at the East Butte (left), and detail of less vitrophyric boulder at the West Butte (right).

Table 1. Raw elemental concentrations for Obsidian Butte source standards. All measurements in parts per million (ppm). Sample prefixes designate collections from the individual domes as in Figure 1.

 

Sample Locality Ti Mn Fe Zn Rb Sr Y Zr Nb Ba Pb Th
101917-1-1 East Dome 1221 358 15910 72 138 33 110 332 33 679 11 17
101917-1-2 East Dome 1201 332 15290 70 133 34 108 327 27 666 12 17
101917-1-3 East Dome 1319 365 17726 87 142 35 113 338 34 687 10 32
101917-1-4 East Dome 1292 431 17920 87 143 31 119 356 36 573 12 25
101917-1-5 East Dome 1853 426 17950 83 140 67 116 334 34 794 12 27
101917-1-7 East Dome 1277 386 17422 74 141 34 119 353 31 639 14 35
101917-1-9 East Dome 1279 361 17134 74 133 38 116 344 30 699 12 28
101917-1-10 East Dome 1234 385 17052 77 137 34 113 340 28 639 15 25
101917-1-11 East Dome 1318 355 15559 67 125 36 111 335 30 660 14 23
101917-1-12 East Dome 1356 368 16463 72 133 35 113 332 35 630 12 31
101917-1-13 East Dome 1568 362 16595 77 134 67 111 344 32 785 17 32
101917-1-14 East Dome 1236 338 16002 84 158 30 112 342 33 665 9 33
101917-2-1 Middle Dome 1455 418 18126 94 145 39 115 330 28 646 18 21
101917-2-2 Middle Dome 1411 388 18126 84 146 32 119 353 38 677 14 25
101917-2-3 Middle Dome 1195 358 15255 68 126 32 107 323 29 643 8 23
101917-2-4 Middle Dome 1359 384 17892 77 140 43 111 359 32 705 15 24
101917-2-5 Middle Dome 1304 399 17120 73 138 42 110 353 26 721 8 17
101917-2-6 Middle Dome 1352 376 16619 79 136 32 114 345 33 647 11 18
101917-2-7 Middle Dome 1327 388 17528 71 133 39 108 351 33 720 18 21
101917-3-1 West Dome 1424 397 17985 81 142 33 111 355 29 634 14 31
101917-3-2 West Dome 1563 447 20466 93 156 33 132 357 30 661 16 28
101917-3-3 West Dome 1248 348 16975 68 137 31 109 335 25 590 7 23
101917-3-4 West Dome 1273 386 15833 72 152 35 104 327 23 659 14 27
101917-3-5 West Dome 1317 368 15854 79 122 26 112 308 33 641 15 24
101917-3-6 West Dome 1438 399 17954 90 149 36 119 334 35 691 19 24
101917-3-7 West Dome 1338 379 18626 84 152 37 117 344 30 701 12 26
101917-3-8 West Dome 1477 388 19171 81 144 37 116 353 36 670 15 29
101917-3-9 West Dome 1223 376 16260 75 129 32 113 313 31 639 11 21
101917-3-10 West Dome 1410 414 18974 96 152 30 129 341 32 629 14 41
101917-3-11 West Dome 1538 440 19505 86 139 38 116 361 31 728 13 27
OB-1 (1980s) East Dome 1183 403 17247 78 134 26 116 331 31 641 18 23
OB-2 East Dome 1664 438 21732 76 125 53 98 422 30 727 24 18
OB-3 East Dome 1640 449 21495 76 132 52 100 435 27 767 19 18
OB-4 East Dome 1435 410 19807 78 137 28 125 369 35 594 19 20
OB-5 East Dome 1366 406 18883 73 126 45 105 377 26 702 20 17
OB-6 East Dome 964 365 14559 72 131 20 119 303 33 552 20 26
OB-7 East Dome 1464 424 19694 71 122 46 94 419 22 730 18 20
OB-8 East Dome 969 381 16358 80 148 20 133 313 35 625 21 20
OB-9 East Dome 1598 422 19922 75 121 47 98 415 24 684 19 15
OB-10 East Dome 1467 401 19067 68 121 48 94 405 21 701 19 23
OB-11 East Dome 1358 387 17890 75 133 41 113 355 29 685 19 24
OB-12 East Dome 1758 462 22674 81 129 51 100 437 27 804 17 24

Table 2. Mean and central tendency for the Obsidian Butte data shown in the Table 1.

 Table 3. Selected major oxides for sample Obsidian Butte-9 (OB-9, East Butte)

Sample

SiO2

Al2O3

CaO

Fe2O3

K2O

MgO

MnO

Na2O

TiO2

Obsidian Butte, CA

 

 

 

 

 

 

 

 

 

OB-9

73.846

12.485

1.163

2.951

4.358

<.001

0.067

4.402

0.239

RGM1-S4

75.680

12.477

1.3024

1.806

4.550

<.001

0.0379

3.77

0.196

 

Figure 2. Matrix plot of six elements from the Obsidian Butte analysis above. Confidence ellipses at 95%

 

 

Figure 3.  Bivariate plot of four elements from the three buttes.  Note that as evident in the matrix plot that the East Dome is the most variable, but all the locality's elemental concentrations overlap at 95% confidence (see analysis in Hughes 1986).

 

 

 

Figure 4. Late Archaic and Late Prehistoric projectile points produced from Obsidian Butte obsidian. Left image: Late Archaic Elko-Eared dart point from Jacumba Valley (bottom; no site number), and Late Prehistoric Desert Side-Notched points from the Scissors Crossing site (CA-SDI-857), from Shackley (2017) top, eastern San Diego County California; right image: Desert Side-Notched points from Arrowmakers Ridge (CA-SDI-913, Cuyamaca Rancho State Park, in the Peninsular Ranges, east San Diego County, California (Shackley 2000; see also True 1970).

 

References

Cooley, T.G. (1998) Observations on settlement and subsistence during the Late La Jolla Complex: preceramic interface as evidenced at site CA-SDI-11,787, lower San Diego River Valley, San Diego County, California.  Proceedings of the Society for California Archaeology 11:1-6.

Hughes, R.E. (1986). Trace element composition of Obsidian Butte, Imperial County, California. Southern California Academy of Science Bulletin 85:35-45.

Hughes, R.E., and D.L. True (1985) Perspectives on the distribution of obsidians in San Diego County, California.  North American Archaeologist 6:325-339.

Kyle, C. E. (1996) A 2,000 year old milling tool kit from CA-SDI-10148, San Diego, California. Pacific Coast Archaeological Society Quarterly 32:76–87.

McDonald, A.M. (1992) Indian Hill Rockshelter and Aboriginal Cultural Adaptation in Anza-Borrego Desert State Park, Southeastern California.  Ph.D. dissertation, Department of Anthropology, University of California, Riverside.

Panich, L.M., M.S. Shackley, and A. Porcayo Michelini (2017) A reassessment of archaeological obsidian from southern Alta California, and northern Baja California. California Archaeology 9:53-77.

Robinson, P.T., W.A. Elders, and L.J.P. Muffler (1976) Quaternary volcanism in the Salton Sea geothermal field, Imperial Valley, California. Geological Society of America Bulletin 87:347-360.

Schmitt, A. K., A. Martin, D. F. Stockli, K. A. Farley, and O. M. Lovera (2012), (U-Th)/He zircon and archaeological ages for a late prehistoric eruption in the Salton Trough (California, USA), Geology 41:7–10

Schmitt, A.K., A. Martin, B. Weber, DF. Stockli, H. Zou, and C. Shen (2013) Oceanic magmatism in sedimentary basins of the northern Gulf of California rift.  GSA Bulletin 125:1833-1850.

Shackley, M.S. (1981) Late Prehistoric Exchange Network Analysis in Carrizo Gorge and the Far Southwest.  Master's thesis, Department of Anthropology, San Diego State University.

Shackley, M.S. (1984) Archaeological Investigations in the Western Colorado Desert: A Socioecological Approach (3 volumes).  Volume 1 published by Coyote Press, Salinas, California.

Shackley, M.S., (1998) Patayan Culture Area.  In Archaeology of Prehistoric North America: An Encyclopedia, edited by G. Gibbon, pp. 629-632.  Garland Publishing Inc., New York.

Shackley, M.S. (2000) Late prehistoric obsidian projectile point source provenance and technology from three late prehistoric contexts in San Diego County, California: The Rose Canyon Site, Arrowmaker’s Ridge, and W-256 (Ramona). Report prepared for the San Diego Museum of Man, San Diego, California.

Shackley, M.S., Ed. (2004) The Early Ethnography of the Kumeyaay, with reprints by T.T. Waterman, L. Spier, and E.W. Gifford.  Classics in California Anthropology, Phoebe Hearst Museum of Anthropology, University of California Press, Berkeley.

Shackley, M.S. (2017) The Shackley San Diego County archaeological collection. Manuscript submitted to the Southern California Information Center, San Diego State University.

True, D.L. (1970) Investigation of a Late Prehistoric Complex in Cuyamaca Rancho State Park, San Diego County, California.  Archaeological Survey Monograph, Institute of Archaeology, University of California, Los Angeles.

Waters, M.R. (1982) The Lowland Patayan ceramic tradition.  In R.H. McGuire, and M.B. Schiffer, Eds. Hohokam and Patayan: Prehistory of Southwestern Arizona, pp 275-299.  New York: Academic Press.

Waters, M. R (1983) Late Holocene lacustrine chronology and archaeology of ancient Lake Cahuilla, California. Quaternary Research 19: 373-387.

Wright, H.M., J.A. Vasquez, D.E. Champion, A. T Calvert, M.T. Mangan, M. Stelten, K.M. Cooper, C. Herzig, and A. Schreiner, Jr. (2015) Episodic Holocene eruption of the Salton Buttes rhyolites, California from paleomagnetic, U-Th, and Ar/Ar dating.  Geochemistry, Geophysics, Geosystems 16:1198-1210.

 

This page maintained by Steve Shackley (shackley@berkeley.edu).
Copyright 2017 M. Steven Shackley. All rights reserved.
Revised: 27 November 2017

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