TOPAZ BASIN

Lying between the Colorado Plateau and Basin and Range Provinces, the Transition Zone is characterized by rugged relief, dissected drainage basins, and large early bedrock basins and plateau remnants (Menges and Pearthtree 1989). In Arizona the concept of “Transition Zone” moves slightly beyond the more traditional definition as a physiographic/geologic region. Since the Colorado Plateau is a result of largely continental tectonic movement and a true basin and range province to the south, it becomes a true structural transition between very large events, the Basin and Range mainly in the Tertiary, and the uplifted Colorado Plateau afterwards into the Quaternary, and is characterized as a a region of “complex crustal geochemistry and structure” (Moyer and Esperanca, 1989:7841; see also Bennett and DePaolo, 1987). Volcanism in the Transition Zone tends to be dominated by basalt and a lesser extent andesite. Rhyolite, particularly eruptive events that quenched to produce glass is very rare, unlike in volcanic regions of the Basin and Range and Colorado Plateau provinces. The only artifact quality obsidian located in the Transition Zone occurs at Burro Creek and Bull Creek in the Kaiser Spring Volcanic Field in western Arizona west of Prescott, Arizona (Figure 1) and the Topaz Basin domes discussed here (see Moyer, 1982, 1986; Moyer and Esperanca, 1989; Shackley, 2005a). Parenthetically, these three sources are all “minor” in that they rarely occur in the regional archaeological record, although do occur in minor proportions in assemblages within radii of perhaps 30-50 km around the sources. Additionally, while all of these sources have been redistributed through erosion into secondary deposits, none were transported great distances like Mule Creek, Cow Canyon in eastern Arizona and western New Mexico (>100 km from the primary deposits), or the northern New Mexico sources such as Mount Taylor and Cerro Toledo Rhyolite that were transported hundreds of kilometers south at least to Chihuahua (Church, 2000; Shackley, 1992, 1998a, 1998b, 2005a). This is potentially one of the reasons that these Transition Zone sources are not as common in the archaeological record as sources from other geological regions. While it could be argued that the nodule sizes are small, these sources exhibit marekanites equally as large as Mule Creek, a source that is distributed widely across the Southwestern archaeological landscape.

No intensive geological investigations have occurred in the specific area near Topaz Basin (see Arizona Bureau of Mines, 1958; Menges and Pearthtree 1989; Nealy and Sheridan 1989). There have been tectonic and geophysical studies to the east at Hackberry Mountain, and northwest in the Black Hills, but not along Cienega Creek or the Bald Hill area (Lewis 1983; Wittke 1984). The Topaz Basin appears to be on a plateau remnant composed of basalt flows and rhyolite domes in the Central Basin Section of the Transition Zone. The basin proper, while noted on the USGS Arnold Mesa 7.5’ topographic map, is not delimited in any way. In fact, the “basin” around the notation on the map is mainly rhyolite domes pertinent here. There are no radiometric dates for the basalt or rhyolite, but could be similar to the bimodal basalt to high silica rhyolite Transition Zone fields to the west. (Moyer, 1982, 1986; Moyer and Esperanca 1989; Neeley and Sheridan, 1989). In the Kaiser Spring Volcanic Field in the western portion of the Transition Zone, early tholeiitic basalt (ca. 12-15 ma) is followed by high silica rhyolites (ca. 11-12 ma) and final sporadic alkalic basalt. Without radiometric dating at Topaz Basin, it is impossible to know what the eruptive trajectory was, although my in-field assessment suggests that the rhyolite dome events are earlier and have been partly subsumed and underlie subsequent basalt flows.

The basin is tilted to the west and so all material is eroding into Cienega Creek (Figure 2), a tributary of Ash Creek which flows into the Agua Fria River and ultimately into the Gila River, rather than the Verde River to the east. The most recent geological investigations in the basin are by Ed Dewitt of the U.S. Geological Survey who has been working on a generalized map of the region. His mapping based on GIS satellite technology did not detect the rhyolite domes in the area, precisely because they are limited in distribution, and were only indicated by “ashy areas” thought to be “siliceous volcanic rocks”, which is indeed the case. The rhyolite dome complex that produced the obsidian is relatively small compared with the large expanse of basalt cinder cones and flows in the area, and this level of investigation can easily miss small dome structures – a common structural context for quenched rhyolite (see Fink and Anderson, 2000; Fink and Manley, 1987; Hughes and Smith, 1993; Shackley, 2005a.

The primary source appears to be a southeast-northwest trending highly eroded dome complex that apparently extends discontinuously from the eastern end of Topaz Basin to nearly Interstate 17 on the west (Figure 2). Artifact quality marekanites are eroding out of the margins of remnant domes particularly around the “basin” proper. The upper reaches of Cienega Creek appear to have eroded between what apparently are dome remnants north and south of the Cienega Creek wash. These were conveniently called “north” or “south” domes, but could have been one large continuous dome complex in the Tertiary. Perlite with embedded remnant marekanites is visible on the domes when exposed. One probable historic perlite prospect in the “northern domes” exhibits marekanite remnants within perlite in relative high densities (> 100/m²), and on the surface just north of the prospect the surface density is ≥ 400 marekanites/m²(Figure 3). Remnant marekanites in a devitrified glass (perlite) lava is typical in Tertiary sources worldwide, and particularly in the North American Southwest (Shackley 2005a).

The nodule sizes vary from pea gravel to over 57 mm in largest dimension. The domes to the south of the perlite prospect appear to exhibit somewhat larger mean sizes (over 30 mm), but this could be that most modern collections could have occurred nearer the perlite prospect. The cortex on the marekanites varies from perlite coverings for those recently eroded, to the “velvet like” chemically weathered cortex common on marekanites in arid climates (Shackley, 2005)

What is most interesting about these obsidian marekanites is how nearly transparent they are. Similar to the Superior (Picketpost Mountain) source east of Phoenix, thin flakes of this obsidian appear transparent and slightly smoky with rare banding or clouding (Shackley, 1988, 2005a). Most of the recently eroded marekanites such as those excavated from the perlite, are nearly transparent in transmitted light. Holding small nodules up to the sun they could easily be mistaken for topaz nodules. Topaz an orthorhombic aluminum-silicate mineral is precipitated from siliceous igneous rocks such as rhyolite, so the environmental context is approximate (see Burt et al., 1981). They can occur as prismatic crystals or rounded waterworn pebbles, and look very much like transparent obsidian nodules. No topaz proper was located during the survey. It’s perfectly possible that these transparent nodules were mistaken for topaz in the late 19^th or early 20^th century, and might have led to the prospecting evident today. Topaz Basin then becomes an appropriate name for this obsidian source.

The secondary distribution of a stone source is just as important to understand as the primary source, since aboriginal knappers care little about where they get quality toolstone (Church, 2000; Shackley, 1992, 1998b, 2005a). As mentioned above, the Topaz Basin is tilted slightly to the west. The Tertiary Topaz Basin marekanites have probably been eroding west through the upper Cienega Creek and southwest potentially into the Agua Fria River for millions of years, although I have never detected obsidian in the Agua Fria River alluvium, nor has it appeared in archaeological context in the Phoenix Basin (see Shackley, 1990). Given the shallow hillslope gradient, at least in the Holocene, and probable bedrock weathering mechanics, it was not surprising that Topaz Basin marekanites were rare downstream. Two nodules, the largest only about 20 mm in diameter were located in the Cienega Creek alluvium approximately six stream kilometers west (see Figure 2). The analysis of these two specimens was well within the elemental concentrations from the primary source (Table 1). Given that marekanites are up to 50 mm in diameter at the primary domes, stream competence of first order drainages within the basin,at least in the Holocene, has been generally low, and/or the primary material has not been released in any quantity into the Cienega Creek system. As noted above, this is probably one reason that this source has not appeared commonly in archaeological context – it is a true point source with little secondary distribution.

Approximately 500 samples were collected at the source from pea size to over 50 mm in diameter. A judgmental sample of 45 (» 9 %) was selected from the northern and southern domes from pea size to that largest nodule from the southern domes for EDXRF analysis.

Eleven elements of the 16 acquired by EDXRF are reported here. As noted above, and discovered by Phil Geib during the project, the elemental composition of the Topaz Basin sources does not match any known from the North American Southwest; it is compositionally unique (Table 1; see Shackley 1995, 2005a). Most of the transition metals exhibit a relatively tight distribution except for Rb and to a certain extent Ba. Iron that does not always operate as an incompatible in silicic volcanics is more variable (Table 1).

Table 1. Elemental concentrations for the Topaz Basin obsidian source standards, Yavapai County, Arizona. All measurements in parts per million (ppm). Measured on the Quant’X EDXRF spectrometer at Berkeley (see Shackley 2005a or http://www.swxrflab.net/anlysis.htm for instrument settings and protocols).

Sample	Ti	Mn	Fe	Zn	Rb	Sr	Y	Zr	Nb	Ba	Th
BH-1	499	658	5207	45	116	26	24	37	60	106	20
BH-2	614	752	5760	44	122	28	19	41	60	106	16
BH-3	572	709	5623	47	126	30	21	43	65	120	15
BH-4	632	626	5434	50	111	25	22	39	60	111	10
BH-5	510	754	5784	54	134	26	21	47	70	136	23
BH-6	792	688	6548	55	117	29	18	41	57	121	15
BH-7	748	682	6562	52	118	26	17	37	56	98	14
BH-8	531	731	5535	79	122	23	20	38	64	121	17
BH-9	1100	742	8734	53	121	31	18	44	59	100	15
BH-10	527	701	5566	59	116	26	21	41	61	128	13
BH-11	669	622	6175	57	118	25	14	40	60	130	18
BH-12	616	578	5551	50	118	27	21	41	62	120	20
BH-13	622	727	6084	59	160	31	26	63	86	134	15
BH-14	949	602	8477	44	121	34	21	46	69	133	23
BH-15	1231	1069	10371	66	137	32	19	47	69	124	23
060708-1-1	727	635	5679	57	132	29	22	56	66	133	14
-2	722	586	5317	53	126	27	22	53	65	145	13
-3	716	613	5614	59	125	27	17	59	66	160	17
-4	691	616	5566	60	128	27	19	60	68	157	16
-5	688	555	5259	51	123	27	18	57	64	160	11
-6	683	569	5351	61	123	27	23	53	63	158	19
-7	718	606	5513	55	122	28	18	56	64	140	23
-8	710	568	5433	60	125	28	18	55	62	133	16
-9	739	530	5051	94	108	22	17	51	57	148	9
-10	873	672	6898	56	135	28	16	60	70	136	13
-11	677	530	5338	55	118	26	18	59	62	143	14
-12	747	642	5983	57	130	28	18	58	69	147	16
-13	720	577	5713	56	123	27	18	57	63	160	12
-14	741	559	5623	62	118	25	18	56	63	140	19
-15	762	636	5833	57	126	28	20	58	64	144	13
-16	1101	572	7782	70	111	27	21	57	62	145	15
-17	1007	643	7557	55	128	32	22	59	72	167	16
-18	1501	678	10214	61	130	30	19	63	70	159	20
-19	725	628	5619	54	125	28	18	58	70	176	16
060708-1-20	1194	632	8200	59	121	29	22	61	63	132	16
-21	992	621	7219	57	121	29	21	57	66	137	21
-22	1097	656	7656	59	129	28	19	59	64	119	19
-23	828	568	5899	52	123	26	18	59	68	156	21
-24	729	574	5802	54	122	24	15	56	65	159	13
-25	836	604	6196	54	126	26	16	57	69	140	15
-26	928	612	7138	59	124	30	20	59	66	143	14
-27	724	593	5639	55	120	26	17	58	64	133	17
-28	837	589	6296	60	123	24	18	60	61	153	14
-29	891	573	6954	57	127	25	18	57	65	174	11
-30	1172	606	8201	57	128	29	21	54	63	132	16