GRANTS RIDGE

AND

HORACE/LA JARA MESA

MOUNT TAYLOR VOLCANIC FIELD

NORTHWESTERN NEW MEXICO

 

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 was initially sampled and analyzed in 1997 (Shackley 1998). The Horace/La Jara Mesa source, part of the Grants Ridge Rhyolite was while discovered by Shackley in 1997, was mentioned to me previously by John Roney, then of the BLM.  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 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 8.5 cm in diameter are aphyric and are a better medium for tool production. The chemistry differs in a number of incompatibles, but is derived from the same magma source of high silica rhyolite, a late Tertiary and early eruptive phase in the Mount Floyd field (see updates below). 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 structural mainly due to a normal fault that separates the two, but this research and that of Goff et. al (2008, 2019) indicates that the ash flow on Horace and La Jara mesas are a single unit, now dated to 3.26 ± 0.07 Ma by Ar40/39  (Goff et al. 2019).  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.

Updated September 2021: A two day field visit and collection at a number of localities on Horace and La Jara Mesa with Fraser Goff increased the sample size from these sources (see table below).  Collection locality S091521-1 was the ignimbrite locality sampled by Goff and an obsidian sample dated by 40Ar/39Ar yielded a date of 3.26 ± 0.07 Ma (see Goff et al. 2019).  The Mount Taylor volcanic field has been re-mapped by Goff and others (Goff et al. 2019).  This sampling has not significantly changed the overall understanding of the composition, with the exception of a few elemental outliers (see plots below), possibly due to wall rock reaction, but the outlier sample was collected in a ancient fluvial surface on Horace Mesa (Qtvs - volcaniclastic sedimentary rocks).  While the samples from the formation on Horace Mesa are technically secondarily deposited, the composition is indistinguishable from the in-situ samples in ignimbrite on La Jara Mesa.  The Horace and La Jara Mesa tuffs (ignimbrite) are all from the same event called by Goff Grants Ridge rhyolite (Goff et al. 2019:17-19).  It is obvious that Horace and La Jara Mesa obsidian is derived from the same magma source and for archaeological purposes can be considered a single source.

Collection localities at Horace and La Jara Mesas September 2021 (see also Goff et al. 2019 and 1998 map below).

 

Selected topographical and geological map of study area from the 1998 study. 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). See Goff et al. (2019) for an accurate geological map.

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)

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

Sample Locality Mesa Loc. Ti Mn Fe Zn Rb Sr Y Zr Nb Ba Pb Th
S090321-1-1 S090321-1 Horace Mesa 711 623 10352 198 534 11 92 146 245 4 64 33
S090321-1-2 S090321-1 Horace Mesa 670 594 10429 207 547 15 89 145 241 34 61 33
S090321-1-3 S090321-1 Horace Mesa 640 542 9524 178 488 10 83 136 221 7 50 30
S090321-1-4 S090321-1 Horace Mesa 682 537 9451 184 492 16 88 133 231 27 51 29
S090321-1-5 S090321-1 Horace Mesa 711 495 8923 158 468 9 82 128 221 12 49 31
S090321-1-6 S090321-1 Horace Mesa 674 489 9022 166 468 10 86 136 225 11 50 26
S090321-1-7 S090321-1 Horace Mesa 724 616 10525 218 543 15 94 144 233 0 66 43
S090321-1-8 S090321-1 Horace Mesa 733 551 9831 177 516 11 89 134 231 19 60 29
S090321-1-9 S090321-1 Horace Mesa 696 527 9550 179 502 13 88 136 228 9 54 37
S090321-1-10 S090321-1 Horace Mesa 672 576 9833 187 527 11 85 137 234 22 55 24
S090321-1-11 S090321-1 Horace Mesa 1010 896 9831 154 462 12 90 139 217 8 51 30
S090321-1-12 S090321-1 Horace Mesa 670 583 9675 190 506 9 89 141 237 40 53 27
S090321-1-13 S090321-1 Horace Mesa 700 587 9925 197 520 11 87 144 232 0 53 36
S090321-1-14 S090321-1 Horace Mesa 719 670 10819 226 558 12 97 149 244 12 61 37
S090321-1-15 S090321-1 Horace Mesa 744 545 9630 182 505 11 83 138 223 12 53 31
S090321-1-16 S090321-1 Horace Mesa 733 560 9506 169 494 13 51 126 215 23 43 33
S090321-1-17 S090321-1 Horace Mesa 764 644 10436 197 509 15 93 152 234 59 52 35
S090321-1-18 S090321-1 Horace Mesa 683 556 9766 194 511 13 86 136 236 34 53 32
S090321-1-19 S090321-1 Horace Mesa 660 543 9477 187 499 11 83 141 223 21 53 31
S091997-1-1 S091997-1 Horace Mesa   641 9529 186 542 2 91 151 244 17   40
-2 S091997-1 Horace Mesa   692 10248 201 557 4 91 149 247 18   46
-3 S091997-1 Horace Mesa   658 9271 170 499 3 82 136 232 16   35
-4 S091997-1 Horace Mesa   654 9553 176 535 3 89 145 240 20   42
-5 S091997-1 Horace Mesa   615 9378 172 535 6 89 142 242 15   44
-6 S091997-1 Horace Mesa   691 9848 178 543 3 92 146 238 16   41
-8 S091997-1 Horace Mesa   580 9170 179 514 4 87 143 228 17   38
-9 S091997-1 Horace Mesa   672 9573 174 519 2 85 139 231 16   41
-10 S091997-1 Horace Mesa   668 9607 187 542 3 85 142 241 19   40
-11 S091997-1 Horace Mesa   660 9296 164 519 5 84 136 227 19   38
-12 S091997-1 Horace Mesa   667 9740 182 538 4 89 146 236 19   42
S091421-2-1 S091421-2 La Jara Mesa 678 551 9544 184 517 9 88 144 235 0 55 27
S091421-2-2 S091421-2 La Jara Mesa 721 524 9557 193 492 14 88 138 225 45 54 28
S091421-2-3 S091421-2 La Jara Mesa 689 533 9702 182 503 11 86 139 229 2 56 29
S091421-2-4 S091421-2 La Jara Mesa 724 550 9946 205 502 11 90 137 239 0 54 30
S091421-3-1 S091421-3 La Jara Mesa 1082 825 11430 189 513 18 91 143 233 49 63 27
S091421-3-2 S091421-3 La Jara Mesa 803 540 10047 192 490 12 86 145 226 0 58 15
S091421-3-3 S091421-3 La Jara Mesa 735 512 9411 185 472 13 84 137 228 0 50 25
S091421-3-4 S091421-3 La Jara Mesa 784 807 12317 256 616 17 94 153 255 13 76 45
S091421-3-5 S091421-3 La Jara Mesa 1565 768 13051 173 472 16 89 138 237 10 49 27
S091421-3-6 S091421-3 La Jara Mesa 708 511 9405 187 502 14 87 138 231 0 55 35
S091421-3-7 S091421-3 La Jara Mesa 719 515 9324 196 474 11 86 130 220 34 51 29
S091421-3-8 S091421-3 La Jara Mesa 714 554 9581 190 509 13 81 139 222 0 56 37
S091421-3-9 S091421-3 La Jara Mesa 737 536 9662 209 494 12 87 141 223 10 58 19
S091421-3-10 S091421-3 La Jara Mesa 1092 539 11377 181 463 16 83 136 221 29 48 36
S091421-4-1 S091421-4 La Jara Mesa 671 506 9108 169 481 13 89 138 232 0 51 27
S091421-4-2 S091421-4 La Jara Mesa 718 566 9798 200 507 12 84 133 228 18 51 38
S091421-4-3 S091421-4 La Jara Mesa 938 576 11392 247 486 12 83 132 216 7 52 28
S091421-4-4 S091421-4 La Jara Mesa 681 504 9349 187 484 10 86 133 219 15 50 38
S091521-1-1 S091521-1 La Jara Mesa 701 539 9607 194 503 12 85 144 236 36 56 23
S091521-1-2 S091521-1 La Jara Mesa 695 526 9386 186 489 11 86 138 224 36 52 33
S091521-1-4 S091521-1 La Jara Mesa 707 519 9441 203 482 15 84 131 219 43 52 23
S091521-1-5 S091521-1 La Jara Mesa 652 577 9580 184 515 12 82 144 221 35 50 36
S091521-1-6 S091521-1 La Jara Mesa 682 512 9128 181 483 9 84 132 214 3 53 28
S091521-1-7 S091521-1 La Jara Mesa 746 606 10519 198 540 13 100 149 244 12 58 34
S091521-1-8 S091521-1 La Jara Mesa 660 555 9533 185 507 12 87 136 226 32 57 36
S091521-1-9 S091521-1 La Jara Mesa 700 503 9094 174 476 9 79 131 225 28 54 27
S091521-1-10 S091521-1 La Jara Mesa 678 588 9770 208 505 12 93 140 227 60 58 25
S091521-1-11 S091521-1 La Jara Mesa 702 535 9524 176 501 14 88 139 229 16 54 21
S091521-1-12 S091521-1 La Jara Mesa 821 653 11257 223 549 11 87 151 244 27 67 39
S091521-1-13 S091521-1 La Jara Mesa 748 563 9739 193 501 11 83 138 220 10 53 31
S091521-1-14 S091521-1 La Jara Mesa 751 589 9846 198 504 11 89 137 235 32 57 26
S091521-2-1 S091521-2 La Jara Mesa 698 622 10674 213 557 14 95 145 239 17 63 38
S091521-2-2 S091521-2 La Jara Mesa 759 578 9896 186 533 13 89 140 239 14 60 41
S091521-2-3 S091521-2 La Jara Mesa 733 598 10348 204 534 12 93 145 238 0 59 41
S091521-2-4 S091521-2 La Jara Mesa 680 423 8164 134 429 10 78 133 228 16 44 31
S091521-2-5 S091521-2 La Jara Mesa 673 625 10522 208 538 13 94 144 247 39 58 41
S091521-2-6 S091521-2 La Jara Mesa 702 564 10050 196 530 12 90 141 242 15 53 40
S091521-2-7 S091521-2 La Jara Mesa 705 584 10004 187 508 12 91 142 240 23 59 38
S091521-2-8 S091521-2 La Jara Mesa 770 568 9772 195 510 15 85 141 237 49 58 31
S091521-2-9 S091521-2 La Jara Mesa 646 517 9455 182 500 9 89 141 223 23 50 31
S091521-2-10 S091521-2 La Jara Mesa 727 518 9227 178 480 13 91 134 221 27 48 38
S091521-2-11 S091521-2 La Jara Mesa 732 608 10595 225 527 14 88 141 226 6 57 40
S091521-2-12 S091521-2 La Jara Mesa 700 569 9900 193 518 10 84 144 237 9 51 25
S091521-2-13 S091521-2 La Jara Mesa 692 572 9799 190 509 9 86 140 233 7 58 30
S091521-2-14 S091521-2 La Jara Mesa 743 570 9902 196 513 11 90 140 236 0 56 41
S091521-2-15 S091521-2 La Jara Mesa 713 532 9629 177 496 13 88 136 240 11 53 22
S091521-2-16 S091521-2 La Jara Mesa 727 598 9797 209 506 11 88 137 232 0 57 36
S091521-2-17 S091521-2 La Jara Mesa 753 529 9442 183 484 9 90 140 235 14 54 36
S091521-3-1 S091521-3 La Jara Mesa 690 550 9462 180 500 19 87 137 229 24 58 25
S091521-3-2 S091521-3 La Jara Mesa 719 505 9283 170 484 12 80 130 221 16 50 30
S091521-3-3 S091521-3 La Jara Mesa 732 552 9437 173 497 11 85 140 235 17 49 33
S091521-3-4 S091521-3 La Jara Mesa 805 567 9817 181 502 9 89 139 230 50 49 28
S091521-3-5 S091521-3 La Jara Mesa 735 557 9810 197 516 12 82 139 238 15 57 21
S081897-2-1  081897-2 Grants Ridge   804 8093 148 548 3 75 118 197 13    
-2 081897-2 Grants Ridge   807 8181 152 566 4 77 117 197 14   36
-3 081897-2 Grants Ridge   773 7907 143 540 5 74 116 192 15   38
-4 081897-2 Grants Ridge   923 8928 165 603 5 79 123 199 14   34
-5 081897-2 Grants Ridge   1026 9114 180 626 4 78 121 210 11   30
-6 081897-2 Grants Ridge   817 8166 144 561 4 77 124 201 16   33
-7 081897-2 Grants Ridge   768 7655 130 514 5 72 115 186 14   26
-8 081897-2 Grants Ridge   825 8045 146 534 4 72 116 192 15   41
-9 081897-2 Grants Ridge   839 7992 149 562 4 76 116 196 11    
-10 081897-2 Grants Ridge   845 8292 156 568 3 75 115 194 12    
S081897-3-1  081897-3 Grants Ridge   845 8390 153 580 4 75 126 195 12    
-2 081897-3 Grants Ridge   879 8493 160 587 1 77 120 197 12    
-3 081897-3 Grants Ridge   878 8461 160 590 7 80 118 204 13    
-4 081897-3 Grants Ridge   826 8177 161 571 4 78 116 195 13    
-5 081897-3 Grants Ridge   886 8651 164 601 3 79 122 208 12    
-6 081897-3 Grants Ridge 691 741 9081 175 564 10 73 115 180 41 55 33
-7 081897-3 Grants Ridge 696 719 8905 170 568 11 76 118 201 25 62 35
-8 081897-3 Grants Ridge 665 713 8656 162 546 12 73 121 191 15 57 24
-9 081897-3 Grants Ridge 693 648 8441 156 523 10 73 112 194 0 52 30
-10 081897-3 Grants Ridge 692 653 8368 145 515 9 77 106 187 34 60 27
-11 081897-3 Grants Ridge 665 704 8694 161 536 13 75 116 195 25 53 28
-12 081897-3 Grants Ridge 692 737 8753 170 534 15 79 116 188 8 57 27
-13 081897-3 Grants Ridge 700 796 9373 183 579 13 81 125 202 9 69 35
-14 081897-3 Grants Ridge 689 639 8545 156 538 11 74 112 190 0 56 32
-15 081897-3 Grants Ridge 676 738 8864 166 560 9 74 117 190 23 63 28

 

Mean and central tendency for Grants Ridge source standards

Element N Minimum Maximum Mean Std. Deviation
Ti 10 665 700 686.0 12.6
Mn 25 639 1026 793.2 91.0
Fe 25 7655 9373 8489.1 418.1
Zn 25 130 183 158.3 12.3
Rb 25 514 626 560.5 28.3
Sr 25 1 15 6.9 4.0
Y 25 72 81 76.0 2.5
Zr 25 106 126 117.7 4.4
Nb 25 180 210 195.2 6.9
Ba 25 0 41 15.1 8.9
Pb 10 52 69 58.5 5.1
Th 17 24 41 31.5 4.8

 

Mean and central tendency for Horace/La Jara Mesa source standards

Element N Minimum Maximum Mean Std. Deviation
Ti 72 640 1565 742.1 128.7
Zn 84 134 256 188.7 18.8
Rb 84 429 616 508.8 27.6
Sr 84 2 19 10.9 3.7
Y 84 51 100 86.8 5.7
Zr 84 118 153 139.3 6.0
Nb 84 197 255 231.0 9.3
Ba 84 0 60 18.5 14.4
Pb 72 43 76 54.7 5.3
Th 83 15 46 32.7 6.6

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. 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" (Baker & Ridley 1970:109).

c This is a single analysis by WXRF of a fused disk standard. Analytical methods and instrument settings in Shackley 1998.

 

 

Zr/Rb and Nb/Y bivariate plots of the Grants Ridge and Horace/La Jara Mesa source standards from the table above (confidence ellipses at 90%).

 

 

 

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.

Goff, F., S.A. Kelley, C.J. Goff, D.J. McCraw, G.R. Osburn, J.R. Lawrence, P.G. Drakos, and S.J. Skotnicki. (2019). Geologic map or the Mount Taylor volcano area, New Mexico. New Mexico Bureau of Geology and Mineral Resources Open File Geologic Map 80.

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.

Shackley, M.S. (2021). Distribution and sources of secondary deposit archaeological obsidian in Rio Grande alluvium New Mexico, USA.  Geoarchaeology 36:808-825.

This page maintained by Steve Shackley ([email protected]).
Copyright © 2021 M. Steven Shackley. All rights reserved.
Revised: Tuesday, 14 December 2021 12:40:40 PM -0700

  Back to SW Obsidian Sources page

To the XRF Lab Homepage