OBSIDIAN BUTTE

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 Ar^40/39 dates at 2.83 ± 0.60 ka at 2 sigma as well as IRL recently (Schmitt et al. 2012, 2013, 2019; 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; Schmitt et al. 2019). 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 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 numbers with prefix "OB" from the early 1980s collection.

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 and minor 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	73.85	12.49	1.16	2.95	4.36	<.001	0.07	4.40	0.24
RGM-1	75.68	12.48	1.30	1.81	4.55	<.001	0.04	3.8	0.19

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, southeastern San Diego County (bottom; no site number), and Late Prehistoric Desert Side-Notched points from the Scissors Crossing site (CA-SDI-857) northeastern San Diego County, from Shackley (2017) top; 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).

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