GRANTS RIDGE

AND

HORACE/LA JARA MESA

MOUNT TAYLOR VOLCANIC FIELD

NORTHWESTERN NEW MEXICO

 

mttaylor.jpg (46496 bytes)

 

 

Landsat photo of the Mount Taylor Volcanic Field.  East Grants Ridge is in the lower left with the late mafic table land known as Horace and La Jara Mesa to the right of East Grants Ridge.

 

 

 

 

 

 

grntsrdg.JPG (90724 bytes)

East Grants Ridge coalesced domes with Horace Mesa in left rear background from La Jara Mesa.

 

The "Grants Ridge" source of archaeological obsidian in the Mount Taylor Volcanic Field in northwestern New Mexico has just been systematically sampled and analyzed this year (1997). Previous chemical analyses by Baugh and Nelson (1987) and others have generally been based on grab samples from the East Grants Ridge area. Our lab's recent analysis of archaeological obsidian from the Zuni and Hopi areas suggested that, unlike the somewhat vitrophyric glass from Grants Ridge, prehistoric knappers preferred an aphyric glass that while chemically similar, does not elementally covary with samples from Grants Ridge. Systematic survey and sampling this summer resulted in the discovery of another source on Horace Mesa to the east of East Grants Ridge. These nodules up to 8 cm in diameter are aphyric and are a better medium for tool production. The chemistry differs in a number of incompatibles, but appears to be derived from the same magma source of high silica rhyolite, a late Tertiary and early eruptive phase in the Mount Floyd field. A complete major, minor, and trace analysis has been completed using the Philips PW2400 WXRF in the lab published in Shackley (1998).  The Mount Taylor obsidian appears to be yet another example of chemical gradients in silicic melts that have archaeological relevance (see Shackley, 1995, 2005).

 

Updated June and September, 2013: La Jara Mesa was sampled on the mesa top and in the road cut and ash flow where Hwy 547 cuts through La Jara Mesa (see image below).  The distinction between the two mesas is mainly due to a normal fault that separates the two, but this research and that of Goff et. al (2008) indicates that the ash flow on Horace and La Jara mesas are a single unit, now dated to 3.26 0.04 Ma by Ar40/39  (Goff et al. 2008).  Lipman and Mehnert (1979) dated the East Grants Ridge glass at an unknown locality by K/Ar at 3.34 0.16, potentially older, but statistically similar, given the vagaries of early K/Ar dating. The analysis below and plot of Nb/Y/Rb indicates this relationship and the distinction between Horace/La Jara mesa and East Grants Ridge obsidian.  Again, the obsidian from both Horace and La Jara mesas is generally aphyric as opposed to vitrophyric fabric at East Grants Ridge.  The Grants Ridge obsidian, however, is an adequate media for tool production and formal tools including projectile points were produced from this obsidian in prehistory.

Figure 1. Selected topographical and geological map of study area. Contours are approximate, but accurate at points noted. All measurements in meters. Tr = Plio-Pleistocene rhyolite; Qb=Quaternary basalt; Qta=Quaternary andesite. Collection localities noted by date/sample number (i.e. 081897-1). Fr=Forest Road. Map based on Dillinger (1990), Hunt (1938), and Lipman and Moench (1972).

 

Rhyolite tuff of Grants Ridge (Tgrt) with sparse marekanites of the Horace/La Jara Mesa chemical group at a normal fault at the junction of Horace and La Jara Mesas cut by Lobo Canyon Road (Hwy 547 on map above) dated by Ar39/40 to 3.26 0.04 Ma (Goff et al. 2008).

Raw elemental concentrations for East Grants Ridge, and Horace/La Jara Mesa source standards. All measurements in parts per million (ppm).

SAMPLE Mn Fe Zn Rb Sr Y Zr Nb Ba La Ce Pb Th
HORACE MESA 641 9529 186 542 2 91 151 244 17 7 25   40
   
081997-1-1   
-2 692 10248 201 557 4 91 149 247 18 10 23   46
-3 658 9271 170 499 3 82 136 232 16 9 20   35
-4 654 9553 176 535 3 89 145 240 20 10 25   42
-5 615 9378 172 535 6 89 142 242 15 9 28   44
-6 691 9848 178 543 3 92 146 238 16 13 23   41
-8 580 9170 179 514 4 87 143 228 17 6 24   38
-9 672 9573 174 519 2 85 139 231 16 6 25   41
-10 668 9607 187 542 3 85 142 241 19 11 20   40
-11 660 9296 164 519 5 84 136 227 19 10 23   38
-12 667 9740 182 538 4 89 146 236 19 9 25   42
LA JARA MESA                          
062813-1 542 9422 190 486 12 90 131 224 0     52 29
062813-2 487 9174 186 474 10 83 129 219 0   50 24
062813-3 528 9539 190 496 12 90 135 227 0   55 26
062813-4 546 9373 177 490 11 86 127 228 0     54 26
062813-5 595 10233 212 517 11 91 138 232 22   62 28
062813-6 523 9183 177 481 10 87 134 230 11   54 20
062813-7 488 8792 157 448 11 81 124 218 0     53 26
062813-8 509 9227 176 481 13 88 131 226 8     52 26
062813-9 508 9064 169 467 11 85 130 221 0     53 24
GRANTS RIDGE 804 8093 148 548 3 75 118 197 13        
         
081897-2-1  7 19   36
-2 807 8181 152 566 4 77 117 197 14 7 18   36
-3 773 7907 143 540 5 74 116 192 15 9 16   38
-4 923 8928 165 603 5 79 123 199 14 7 16   34
-5 1026 9114 180 626 4 78 121 210 11 4 18   30
-6 817 8166 144 561 4 77 124 201 16 6 18   33
-7 768 7655 130 514 5 72 115 186 14 7 19   26
-8 825 8045 146 534 4 72 116 192 15 4 18   41
-9 839 7992 149 562 4 76 116 196 11 5      
-10 845 8292 156 568 3 75 115 194 12 8      
081897-3-1  845 8390 153 580 4 75 126 195 12 4      
-2 879 8493 160 587 1 77 120 197 12        
-3 878 8461 160 590 7 80 118 204 13        
-4 826 8177 161 571 4 78 116 195 13        
-5 886 8651 164 601 3 79 122 208 12        

 

Selected WXRF oxide values (wt.%) for two Mount Taylor field source standardsa, the Baker & Ridley (1970) "average" analysis of rhyolite pumice and glassy flow from Mount Taylorb, followed by analysis of RGM-1, USGS rhyolite rockc, and recommended values in Govindaraju (1994).

 

SAMPLE SiO2 Al2O3 CaO Fe2O3T K2O MgO MnO Na2O TiO2
081897-2-11a

75.90

13.20

0.42

0.84

4.18

0.00

0.12

4.80

0.06

081997-1-7 a

76.00

13.20

0.34

1.01

4.14

0.00

0.09

5.00

0.06

Baker & Ridley. (1970)b

74.09

13.58

0.30

0.86

4.46

0.37

0.06

4.92

0.07

RGM-1 (this study)c

73.18

13.70

1.23

1.85

4.32

0.280

0.034

4.16

0.29

RGM-1 (Govindaraju, 1994)

73.45

13.72

1.15

1.86

4.30

0.280

0.040

4.07

0.27

a 081897-2-11 is from the Grants Ridge locality, and 081997-1-7 is from the Horace Mesa locality noted on Figure 1. Both samples analyzed by WXRF. Analytical methods and instrument settings in Shackley 1998.

b This is an "average" analysis of an unknown number of samples from rhyolite pumice and "glassy flow", presumably from the Mount Taylor stratovolcano (Baker & Ridley 1970:109).

c This is a single analysis by WXRF of a fused disk standard. Analytical methods and instrument settings in future publication.

 

 

 

Zr, Y, Nb three dimensional plot of Horace Mesa and Grants Ridge source standards.

 

REFERENCES

Baker, I., and Ridley, W.I. (1970). Field evidence and K, Rb, Sr data bearing on the origin of the Mt. Taylor volcanic field, New Mexico, U.S.A. Earth and Planetary Science Letters 10, 106-114.

Baugh, T.G., and Nelson, F.W., Jr. (1987). New Mexico obsidian sources and exchange on the southern Plains. Journal of Field Archaeology 14, 313-329.

Dillinger, J.K. (1990). Geological map of the Grants 30' X 60' quadrangle, west-central New Mexico. Map C-118-A, Denver, U.S. Geological Survey.

Goff, F., S.A. Kelley, K. Zeigler, P. Drakos, and C.J. Goff (2008). Preliminary geologic map of the Lobo Springs quadrangle, Cibola County, New Mexico.  New Mexico Bureau of Geology and Mineral Resources Open File Geologic Map Series OF-GM-181.

Govindaraju, K. (1994). 1994 compilation of working values and sample descriptions for 383 geostandards. Geostandards Newsletter 18.

Hunt, C.B. (1938). Igneous geology and structure of the Mount Taylor Volcanic Field, New Mexico. Washington, DC, U.S. Geological Survey Professional Paper 189B.

Lipman, P.W., and Moench, R.H. (1972). Basalts of the Mount Taylor Volcanic Field, New Mexico. Geological Society of American Bulletin 83, 1335-1344.

Lipman, P.W., and Mehnert, H.H. (1979). Potassium-argon ages from the Mount Taylor volcanic field, New Mexico.  U.S. Geological Survey Professional Paper  1124-B, 8 pp.

Shackley, M. S. (1995). Sources of archaeological obsidian in the greater American Southwest: An update and quantitative analysis. American Antiquity 60, 531-551.

Shackley, M.S. (1998).   Geochemical differentiation and prehistoric procurement of obsidian in the Mount Taylor Volcanic Field, northwest New Mexico.  Journal of Archaeological Science 25:1073-1082. 

Shackley, M.S. (2005). Obsidian: geology and archaeology in the North American Southwest.  Tucson: University of Arizona Press.

This page maintained by Steve Shackley (shackley@berkeley.edu).
Copyright 2015 M. Steven Shackley. All rights reserved.
Revised: Wednesday, 02 December 2015 07:33:09 AM -0800

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