BRISTOL MTS

Bristol Mountains Volcanic Field in background. Most of the features are rhyolite dome structures.

This source is located in the Bristol Mountains, San Bernardino County, California in Quaternary alluvium consisting predominately of rhyolite, tuff, and marekenites (Apache Tears). The primary context of this late Tertiary (probably Miocene) source appears to be located in rhyolite domes of the Bristol Mountains on the Siberia 7.5' Provisional Quad (1985) in the northeast quarter, particularly in Sections 7 and 18 T7N R11E, and Sections 12 and 13 T7N R10 E (see also Bagdad 15' 1956). The domes of this Tertiary source are so eroded that the obsidian zone has apparently completely moved into the alluvium, not unusual in the Southwest (Shackley 1988a, 1990). Secondary deposits continue down Siberia and unnamed washes to Lavic and Bristol dry lakes approximately 15 to 20 km south. This is an interior drainage basin and so further downstream deposition is impossible.

The density of nodules near the primary context is up to 10 per 2 m²with nodule sizes up to 72 mm, although most are 40 to 50 mm. Ten km downstream the nodule density declines to less than 1 per 500 m² and nodule sizes are generally near 20 mm or less. In the desert pavements the cortex is generally reddish-brown which erodes to black after entering the washes. The glass is typically translucent gray, sometimes banded, and an excellent medium for tool production. Few modified cores or bipolar flakes were noted in two days of survey, although these artifact forms have been recovered from archaeological contexts in the area (Jerry Schaefer: personal communication 1992).

This source was first located by Philip Wilke in the 1980s (Wilke:personal communication 1993). Wilke and others noticed the presence of nodules in the alluvium near Bagdad and the source was colloquially called Bagdad. It is more properly called Bristol Mountains obsidian because the glass is derived from the Miocene rhyolite in the mountains. There is no known geologic publication on this source other than Dibblee (1966) for the adjacent Broadwell Lake 15' Quad. This source was reported in the Journal of California and Great Basin Anthropology (Shackley 1994).

2010: In September 2010, Janet Harvey of the California Institute of Technology forwarded four marekanites found in-situ in perlite in the Bristol Mountains a few km north of the original study (samples with prefix 10 below). These are well within the range of the collections from the 1990s discussed above. Included is also a major oxide analysis of one sample and a revised data table.

Elemental concentrations for Bristol Mountain source standards. All measurements in parts per million.

Sample¹	Mn	Fe	Zn	Rb	Sr	Y	Zr	Nb	Ba	Pb	Th	La	Ce
BM1-1	378	9390		194	148	19	133	20	901	27		66	112
BM1-2	366	8956		180	137	22	129	23	899	29		68	116
BM1-3	385	8734		181	139	21	128	20	1096	28		84	137
BM1-4	400	9142		189	143	20	130	22	837	26		66	108
BM1-5	381	8516		175	133	21	120	18	1054	25		82	134
BM2-2	423	9341		193	146	20	126	22	845	27		65	108
BM2-3	433	9863		196	147	20	128	26	889	23		64	113
BM2-4	381	8770		184	138	23	124	23	956	27		76	122
BM2-5	346	8813		189	142	17	132	17	979	25		66	133
BM2-6	421	9497		194	144	18	128	20	832	25		62	108
BM3-2	382	9326		194	145	20	130	24	848	25		69	106
BM3-3	399	9435		192	143	21	133	27	834	28		62	108
BM3-4	426	9476		198	144	21	135	26	923	29		73	108
BM3-5	528	8486		167	129	22	125	21	856	27		65	114
BM3-6	390	9298		194	147	21	129	21	846	27		64	104
BM3-7	386	9083		184	141	19	126	20	890	28		65	114
BM3-8	424	9208		189	145	21	129	24	890	29		64	118
BM3-9	362	9073		186	135	19	128	23	832	28		64	102
BM3-10	397	9744		199	149	22	138	22	873	30		58	110
10-591-1	386	8454	48	183	140	22	136	24	910	26	24
10-591-2	320	6980	40	162	122	20	128	25	1191	25	25
10-591-3	372	7471	49	174	134	21	133	22	1040	22	22
10-591-4	400	8603	49	188	145	25	136	24	1127	28	24

¹ The BM prefix samples are from the Shackley (1994) analyses, and the 10 prefix samples from Harvey. Empty cells indicate elements not analyzed in that run.

Table 2. Oxide analysis of one Harvey source rock and RGM-1, USGS standard.

Sample	SiO2	Al2O3	CaO	Fe2O3	K2O	MgO	MnO	Na2O	TiO2
	%	%	%	%	%	%	%	%	%
10-591-1	76.288	11.939	1.037	1.163	5.927	0.001	0.073	3.238	0.133
RGM1-S5	74.509	12.239	1.502	2.214	5.228	<.001	0.056	3.766	0.295

Figure 3. Silica-alkali plot of one Harvey sample.