SAUCEDA MTS

SOUTH SAUCEDA MOUNTAINS

WESTERN ARIZONA

An artist's rendition of Casa Grande (Compound A) during the Classic Period. Sauceda Mountains obsidian was a dominant raw material choice during this period.

(Rebecca Leer, National Park Service drawing)

Sections 20,21,22,27 through 35 R3W, T9S; Section 36 R4W, T9S; and adjacent area (no sections delimited) in R3W, T10S USGS Tom Thumb 7.5' Quad, Barry M. Goldwater Range, southwest Maricopa County, Arizona.

This is a previously unknown marekenite source first surveyed in September 1986. At the south end of the Sauceda Mountains, as part of the Tertiary Sauceda Volcanic Field, are a series of north to south arcing rhyolite domes (Wilson et al. 1957). Similar to Vulture, many of the rhyolite units exhibit devitrified and somewhat vitrophyric glass (vitrophyre) and perlite toward the outer margins due to more rapid cooling. No nodules were found in situ within the perlite or vitrophyre, again like Vulture. This suggests that many of these Tertiary marekenites were pyroclasts in the tuff eruptions rather than remnants in the perlite as at Superior and Los Vidrios.

The source is chemically bimodal, particularly in strontium and to a lesser extent zirconium (see figure below). The bimodality may be due to magmatic differentiation and/or significant time depth between eruption. The bimodality is nearly 50/50 and nodules of both modes are intermixed within a conglomerate and developing pavement consisting of rhyolite, perlite, vitrophyre, basalt, and obsidian very similar to Vulture. It is impossible, at this point, to determine which rhyolite structure(s) contribute to Sauceda modes A or B. Sauceda is more precisely termed a chemical group, since there is an unusual level of variability among some elements. This is not a significant problem archaeologically since nodules of both modes are mixed within the conglomerate of the source area, and selection by prehistoric knappers corresponded to the quality of the material which has no relation to the source chemistry in this case. Secondary deposition follows the course of Sauceda Wash for at least 20 km north and perhaps as far as Gila Bend 35 km north. Nodule density ranges up to 30 per 5m² probably the prehistoric density at Vulture and other marekenite localities before they became popular rockhound collection points---not possible until recently on Luke Air Force Range. The largest nodule recovered measures 11 cm in diameter. Nodules 5 cm are quite common and a few 8 to 9 cm were recovered. The range of nodule sizes at Sauceda is probably similar to the pre-Anglo Southwestern marekanite sources before collection of "Apache Tears" became popular, although the bipolar reduced nodules at Vulture and Tank Mountains suggest that large nodules were relatively rare at these sources. Nodule size, as discussed later, may have been one of the factors influencing selection by prehistoric knappers.

The color and opacity is as variable as Vulture if not more so. All the glass is aphyric and the most common color is a translucent green brown, but black opaque, green and black banded, and even some light gray/black banded nodules are represented. Color or opacity does not correlate with the bimodal chemistry. Cortex varies from a waterworn gray in the washes to a velvet-like surface on the conglomerate. This obsidian makes excellent knapping raw material, equal to any in the Southwest.

The prehistoric procurement pattern is somewhat unusual here. Rejected flakes and bipolar cores occur throughout the conglomerate and in the braided Sauceda Wash system, but at very low densities. However, near the rhyolite domes, flake and core fragment densities approach 50 per m² in some areas. Perhaps more interesting, virtually no unreduced nodules remain near the rhyolite/perlite domes. It appears that there was some preference for nodules close to the perlite rather than in the alluvium, possibly due to higher density and size. This pattern was not present at any other source in the Southwest, and was particularly noticeable in Section 36 R3W, T9S. There are no published sources on this locality other than the county geological map (Wilson et al. 1957).

In May 1996, samples submitted by Bruce Masse and Adrienne Rankin of the Department of the Air Force, Luke Air Force Range from west of the Sauceda Mountains further defined the chemical differentiation within the Sauceda magma chamber. This is evident in the matrix plot below and suggests the possibility of separate temporal events and/or vents located west and south of the Sauceda Mountains.

Matrix plot of samples from west and south sections of the Sauceda Mountains indicating the chemical differentiation.

Elemental concentrations for Sauceda Mountains source standards. Samples with a "W" prefix from west of the Sauceda Mountains, all others from the southern end of the range. All measurements in parts per million (ppm).

SAMPLE	Ti	Mn	Fe	Rb	Sr	Y	Zr	Nb	Ba
1	1828	280	11487	163	105	28	189	20	891
2	1562	306	11915	179	109	29	198	22	882
3	1325	254	11245	165	106	24	191	18	917
4	1451	288	11412	171	106	30	190	22	959
5	1501	293	11530	174	110	26	198	23	903
6	1427	259	11200	169	105	28	189	19	902
7	1634	283	11748	178	112	28	196	23	924
A1	1559	285	12420	179	114	30	199	21	981
A2	1564	287	12083	173	110	32	196	22	889
A3	1517	286	11658	171	111	28	191	24	906
A4	1525	274	11757	175	113	30	194	20	906
A5	1508	279	11416	166	112	25	190	21	858
B1	1517	361	11381	173	78	32	213	21	986
B2	1580	273	11533	171	111	28	197	23	933
B3	1736	390	12575	192	87	41	227	27	953
B4	1418	258	10606	161	101	23	184	20	880
B5	1625	379	11692	180	81	32	224	25	1026
WS1	1458	454	10839	170	71	31	211	23
WS2	1549	425	10399	169	67	36	205	26
WS3	1578	388	10549	175	70	32	209	21
WS4	1278	352	9641	145	61	24	178	21
WS5	1125	297	9234	148	62	28	189	20
WS6	1274	373	9998	158	69	31	197	23
WS7	1218	408	9770	124	51	22	153	22
WS8	1333	314	9979	159	63	28	190	24
WS9	1474	377	10250	166	70	31	198	19