THE DUNCAN SOURCE, EASTERN ARIZONA

 

In 2016 John Roney while directing an archaeological survey for the University of Texas, San Antonio field school north of Duncan, Arizona above the Gila River discovered a potential primary source of obsidian. The initial XRF analysis indicated an elemental composition similar to the Cow Canyon obsidian source located in the Blue Range to the north of the San Francisco and Gila Rivers in northern Greenlee County, Arizona approximately 80 linear km north of Duncan.  On 31 May 2018 a geoarchaeological examination and collection at what is now called the Duncan obsidian source indicated that the source is likely a Tertiary (Neogene) period coalesced rhyolite dome complex with scattered small marekanites at light density, most smaller than 30 mm in diameter, and not yet discovered in regional archaeological contexts. The similar elemental composition between Cow Canyon, a major Mogollon-Datil Volcanic Province source of archaeological obsidian in the Southwest, and the Duncan source requires careful analysis when using x-ray fluorescence spectrometry (XRF) in order to discriminate these two sources (Shackley 2005; Shackley et al. 2018).

Geoarchaeological Source Description

            The Duncan obsidian source is located in Greenlee County, Arizona at an elevation of 1150 m AMSL approximately 5 km northwest of Duncan, Arizona above Plio-Pleistocene alluvial terraces on the west side of the Gila River and directly above Burma Road (Figure 1).  The source is defined by two coalesced rhyolite domes that exhibit remnant obsidian marekanites (nodules) in highly eroded aphyric perlitic tuff and perlite, in a structure that is about 1500 m long oriented northwest/southeast (see Figure 1).  Typical of Tertiary (Neogene) rhyolite dome structures in the North American West, the overlying obsidian zone and perlite has generally eroded off the domes in this case into the saddle between the two domes, on the slopes of the domes, and likely eroding into the Gila River system (Fink and Manley 1987; Hughes and Smith 1993; Shackley 2005; see Figure 2 here).  No marekanites were discovered in the existing wash below and east of the domes. 

            There have been no specific geological studies of this structure.  The original geological mapping of Greenlee County, Arizona by Wilson and Moore (1958) shows the area here as "Tr -  rhyolite, includes tuff and agglomerate", but nothing specific at that scale.  More recently Richter et al.'s (1983) geologic map of the Guthrie 1 quadrangle including Graham and Greenlee Counties provides no specific data either.  Walker's 1990 Ph.D. dissertation is the best discussion of the Ash Peak area rhyolites, but again his study area was just north of the Duncan domes.  We can say that given the above discussion in the geological literature, and the eroded structure of the domes, that the source is likely Tertiary (Neogene) in origin, typical of rhyolite extrusions in the region (Shackley 2005; Shackley et al. 2018), but no specific chronology is available.  See discussion below.

Ash Peak and Mogollon-Datil Area Geology

            As noted above, Walker's Ph.D. dissertation study examined the Ash Peak volcanics, including what he petrochemically defined as biotite rhyolites, crystal-rich rhyolites, and crystal poor rhyolites, the latter best describes the rhyolite (obsidian) at the Duncan domes, however the composition of the biotite rhyolite with higher Sr and Ba is more similar to the Duncan obsidian composition (Walker 1990:178). No mention of obsidian was made specifically by Walker.  These rhyolites are dated to ~20 Ma typical of Neogene silicic events in this part of western North America including the adjacent Mogollon-Datil Volcanic Province (Richter et al. 1983; Shackley et al. 2018).  As in the Mogollon-Datil these rhyolites were likely produced by re-melting a granite basement (see Elston 2008; Shackley et al. 2018; Walker 1990).  This re-melting of a large plutonic rock body is likely responsible for the similar composition seen in both the Duncan and Cow Canyon obsidian as well as the similarities observed in Mogollon-Datil rhyolites (Shackley et al. 2018; see discussion below). 

            The marekanite density as seen in May 2018 was rather low, and probably no more than about five per 25 m2. The largest marekanite recovered in 2016 and 2018 was 44.9 mm in largest dimension, but most are half that size or smaller.  At least two bipolar flakes were present, so it had been discovered prehistorically.

            Thin flakes appear smoky gray and translucent, and seem to be a good media for tool production based on bipolar reduction for XRF analysis (c.f. Shackley 1990, 2005).  The megascopic character is different from the Cow Canyon marekanites that are uniformly nearly transparent (Shackley 1995, 2005). While it is likely that the Duncan marekanites have been eroding into the Gila River system since the Tertiary, it does not appear in the archaeological record even in sites in the immediate area (Shackley 2016, 2017).  Archaeological sites in the region exhibit a small proportion of Cow Canyon, but the majority are from one of the three major source localities of the Mule Creek Obsidian Complex (Shackley 2005; Shackley et al. 2018). Both Cow Canyon and the Mule Creek Complex source groups erode into the San Francisco and Gila River systems (Shackley 1992, 2005, 2016, 2017; Shackley et al. 2018).  While it could certainly just be sampling error (none yet recovered), the Duncan obsidian source is likely such a numerically small source, it probably was not encountered often in prehistory.  In Gila River Quaternary alluvium secondarily deposited Cow Canyon and the Mule Creek complex sources are often found in higher densities and larger nodule sizes than at the primary Duncan source.  This could also be a factor explaining why Duncan obsidian is not recovered in regional archaeological contexts.  Finally, without acquiring Ba when analyzing archaeological obsidian with XRF, it is certainly possible that any Duncan obsidian recovered archaeologically might be mistaken for Cow Canyon, as discussed below.  This means that assemblages analyzed by XRF instruments that cannot acquire the higher Z elements could mistakenly assign Duncan obsidian to Cow Canyon.  Indeed, all of the analyses completed by the laboratory with the benchtop Quant'X XRF that did not acquire Ba could be in that category, although the low density and small nodule sizes at Duncan and the results in local archaeological contexts suggest that Cow Canyon would dominate in any regional archaeological assemblage.  Additionally, as mentioned above, Cow Canyon obsidian is uniformly nearly transparent in flakes and small bifaces and the Duncan marekanites are smoky brown and translucent.  While always hazardous, the megascopic character could be used as a first approximation when assigning to source and selecting the non-transparent artifacts for further analysis (see Shackley 2011).

The Mogollon-Datil Volcanic Complex and Cow Canyon Versus Duncan Sources

            Both Cow Canyon and the Duncan sources are on the western margin of the Mogollon-Datil Volcanic Complex, a large volcanic field that includes some of the most frequently used obsidian sources in the North American Southwest (Elston 2008; Mills et al. 2013; Shackley 2005; Shackley et al. 2018).  The Antelope Creek locality at Mule Creek and Cow Canyon are Neogene Period signature sources that were used from Paleoindian through historic periods (i.e Apache) for tool production, and have been integral in understanding exchange, territory, migration and social networks during the Late Classic of the Southwest (Hamilton et al. 2013; Mills et al. 2013; Shackley 2005, 2007; Shackley et al. 2018).

           

ACKNOWLEDGEMENTS

 

Special thanks to John Roney for alerting me to the Duncan source, and taking me up to the domes, and to Bob Hard (UTSA) for letting me run with this data. The fieldwork and laboratory analysis of the Duncan source was undertaken as part of the UTSA project and U.S. Bureau Land Management permitting.

 

REFERENCES

Elston, W.E.

  2008  When Batholiths Exploded: the Mogollon-Datil Volcanic Field, Southwestern New Mexico. In Geology of the Gila Wilderness-Silver City Area, edited by G. Mack, J. Witcher, and V.W. Lueth, pp. 117-128. New Mexico Geological Society Guidebook, 59th Field Conference, Socorro, New Mexico.

 

Fink, J.H., and C.R. Manley

  1987  Origin of Pumiceous and Glassy Textures in Rhyolite Flows and Domes.  In The Emplacement of Silicic Domes and Lava Flows, edited by J.H. Fink, pp. 77-88. Geological Society of America Special Paper 212, Boulder, Colorado.

 

Hamilton, M.J., B. Buchanan, B.B. Huckell, V.T. Holliday, M.S. Shackley, and M. E. Hill

  2013  Clovis Paleoecology and Lithic Technology in the Central Rio Grande Rift Region, New Mexico.  American Antiquity 78:248-265.

 

Hughes, Richard E., and Robert L. Smith

  1993  Archaeology, Geology, and Geochemistry in Obsidian Provenance Studies.  In Scale on Archaeological and Geoscientific Perspectives, edited by J.K. Stein and A.R. Linse,  pp. 79-91.  Geological Society of America Special Paper 283.

 

Le Maitre, R.; Bateman, P.; Dudek, A.; Keller, J.; Lameyre, J.; Le Bas, M.; Sabine, P.; Schmid, R.; Sorensen, H.; Streckeisen, A.; Woolley, A.; Zanettin, B.

  1989 A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, edited by Le Maitre, R.W. Blackwell, Oxford.

 

Mahood, G., and J.A. Stimac

  1990  Trace-Element Partitioning in Pantellerites and Trachytes.  Geochimica et Cosmochimica Acta 54:2257-2276.

 

Mills, B.J., J.J. Clark, M.A. Peeples, W.R. Haas, Jr., J.M. Roberts, Jr., J.B. Hill, D.L. Huntley, L. Borck, R.L. Breiger, A. Clauset, and M.S. Shackley

  2013  Transformation of Social Networks in the Late Pre-Hispanic US Southwest.  Proceedings of the National Academy of Science 110:5785-5790.

 

Richter, D.H., B.B. Houser, and P.E. Damon

  1983 Geologic map of the Gutherie quadrangle, Graham and Greenlee Counties, Arizona: U.S. Geological Survey Miscellaneous Investigation Map 1455.

 

Shackley, M. Steven

  1990  Early Hunter-Gatherer Procurement Ranges in the Southwest: Evidence from Obsidian Geochemistry and Lithic Technology.  Ph.D. dissertation, Department of Anthropology, Arizona State University, Tempe.

 

  1992  The Upper Gila River Gravels as an Archaeological Obsidian Source Region: Implications for Models of Exchange and Interaction.  Geoarchaeology 7:315-326.

 

  1995  Sources of Archaeological Obsidian in the Greater American Southwest: An Update and Quantitative Analysis. American Antiquity 60(3):531-551.

 

  2005  Obsidian: Geology and Archaeology in the North American Southwest.  University of Arizona Press, Tucson.

 

  2007  Sources of Obsidian at the Murray Springs Clovis Site: A Semi-Quantitative Analysis.  In Murray Springs: A Clovis Site with Multiple Activity Areas in the San Pedro Valley, Arizona, edited by C. Vance Haynes, Jr., and Bruce B. Huckell, pp. 250-254. Anthropological Papers of the University of Arizona 71. University of Arizona Press, Tucson.

 

  2011  An Introduction to X-Ray Fluorescence (XRF) Analysis in Archaeology. In X-Ray Fluorescence Spectrometry (XRF) in Geoarchaeology, edited by M.S. Shackley, pp. 7-44. Springer, New York.

 

  2016  Source Provenance of Obsidian Artifacts from Seven Archaeological Sites in Eastern Arizona.  Report prepared for Robert Hard, Department of Anthropology, University of Texas, San Antonio.

 

  2017  Source Provenance of Obsidian Artifacts from Three Excavated Sites from the University of Texas, San Antonio 2016 Field School Project in Eastern Arizona. Report prepared for Robert Hard, Department of Anthropology, University of Texas, San Antonio.

 

Shackley, M.S., F. Goff, and S.G. Dolan

  2016  Geologic Origin of the Source of Bearhead Rhyolite (Paliza Canyon) Obsidian, Jemez Mountains, Northern New Mexico.  New Mexico Geology 38:52-62.

 

Shackley, M.S., L.E. Morgan, and D. Pyle

  2018  Elemental, Isotopic, and Geochronological Variability in Mogollon-Datil Volcanic Province Archaeological Obsidian, Southwestern USA: Solving Issues Of Inter-Source Discrimination.  Geoarchaeology 33:486-497.

 

Walker, Robert J.

  1990  Magmatism and Mineralization of the Ash Peak Area, Arizona: Petrochemical Interpretations. Ph.D. dissertation, Department of Geosciences, Oregon State University, Corvallis.

 

Wilson, E.D., and R.T. Moore

  1958  Geologic Map of Graham and Greenlee Counties, Arizona.  Arizona Bureau of Mines, University of Arizona, Tucson.

 

Zielinski, R.A., P.W. Lipman, and H.T. Millard

  1977  Minor-Element Abundances in Obsidian, Perlite, and Felsite of Calc-Alkalic Rhyolites.  American Mineralogist 62:426-437.


 

  Table 1.  Trace element concentrations for the samples, and USGS RGM-1 rhyolite standard. All measurements in parts per million (ppm).

 

Sample1

Locality

Zn

Rb

Sr

Y

Zr

Nb

Ba

Pb

Th

Rock Type

IO280-1

Duncan, AZ

47

140

70

29

129

26

573

18

10

obsidian

IO283-1

Duncan, AZ

55

163

74

31

137

23

659

22

21

obsidian

IO275-1

Duncan, AZ

46

155

72

36

124

18

579

21

18

obsidian

IO296-1

Duncan, AZ

50

155

71

31

129

27

610

24

16

obsidian

IO274-1

Duncan, AZ

42

146

68

31

125

20

676

18

19

obsidian

IO270-1

Duncan, AZ

43

151

74

33

127

20

579

22

23

obsidian

IO269-1

Duncan, AZ

61

157

69

29

137

17

609

24

25

obsidian

IO276-1

Duncan, AZ

52

154

75

32

135

19

606

24

22

obsidian

IO276-2

Duncan, AZ

46

151

65

30

126

18

567

22

29

obsidian

IO276-3

Duncan, AZ

56

160

71

27

136

30

584

23

22

obsidian

IO276-4

Duncan, AZ

49

160

73

31

131

20

602

23

25

obsidian

IO276-5

Duncan, AZ

59

155

68

31

137

17

593

26

10

obsidian

IO276-6

Duncan, AZ

46

163

71

28

128

22

635

23

16

obsidian

IO276-7

Duncan, AZ

55

153

71

32

129

19

546

19

26

obsidian

IO276-8

Duncan, AZ

54

155

73

30

129

21

604

20

19

obsidian

IO276-9

Duncan, AZ

58

147

68

28

132

23

587

24

26

obsidian

IO276-10

Duncan, AZ

60

161

73

28

136

25

558

23

20

obsidian

IO276-11

Duncan, AZ

59

161

80

27

130

22

558

22

12

obsidian

IO276-12

Duncan, AZ

88

150

71

31

123

18

642

22

22

obsidian

IO276-13

Duncan, AZ

76

145

67

27

129

23

571

20

10

obsidian

IO276-14

Duncan, AZ

93

154

68

29

127

12

581

21

28

obsidian

IO276-15

Duncan, AZ

68

160

73

29

132

20

574

22

24

obsidian

IO276-16

Duncan, AZ

100

149

70

27

124

28

552

20

15

obsidian

IO276-17

Duncan, AZ

102

140

67

27

125

22

558

23

25

obsidian

IO276-18

Duncan, AZ

89

147

66

28

111

24

557

17

15

obsidian

IO276-19

Duncan, AZ

84

149

68

31

131

19

667

22

29

obsidian

IO276-20

Duncan, AZ

75

148

64

27

125

21

561

21

8

obsidian

IO276-21

Duncan, AZ

138

141

70

28

121

20

584

20

20

obsidian

IO276-22

Duncan, AZ

126

137

63

26

118

20

549

24

27

obsidian

IO276-23

Duncan, AZ

134

125

60

27

110

18

482

20

12

obsidian

IO276-24

Duncan, AZ

152

133

61

24

117

14

390

18

22

obsidian

053118-1-1

Duncan, AZ

55

159

74

32

138

24

693

26

19

obsidian

-2

Duncan, AZ

47

165

76

30

140

21

616

25

20

obsidian

-3

Duncan, AZ

74

172

75

36

133

20

589

29

29

obsidian

-4

Duncan, AZ

60

162

70

28

136

24

595

24

14

obsidian

-5

Duncan, AZ

64

171

78

28

135

21

589

27

21

obsidian

-6

Duncan, AZ

54

175

81

33

135

23

565

21

19

obsidian

-7

Duncan, AZ

49

153

72

27

132

26

591

22

23

obsidian

-8

Duncan, AZ

77

168

77

29

130

24

584

26

23

obsidian

-9

Duncan, AZ

58

161

75

33

133

25

618

22

18

obsidian

-10

Duncan, AZ

46

160

66

27

125

22

558

22

18

obsidian

-11

Duncan, AZ

64

162

78

28

130

27

582

19

16

obsidian

-12

Duncan, AZ

52

151

69

31

128

17

572

27

11

obsidian

-13

Duncan, AZ

73

158

71

35

130

22

596

25

23

obsidian

-14

Duncan, AZ

85

152

72

29

122

20

553

30

18

obsidian

-15

Duncan, AZ

75

164

74

30

132

20

592

26

26

obsidian

-16

Duncan, AZ

57

156

71

26

131

23

620

23

24

obsidian

-17

Duncan, AZ

58

163

71

32

131

20

593

26

20

obsidian

-18

Duncan, AZ

59

159

73

28

131

15

583

26

26

obsidian

-19

Duncan, AZ

63

154

69

25

126

21

623

24

20

perlite

-20

Duncan, AZ

53

153

72

31

124

22

605

27

19

obsidian

-21

Duncan, AZ

69

156

72

32

132

19

581

26

16

obsidian

-22

Duncan, AZ

69

151

69

32

132

22

595

21

12

obsidian

-23

Duncan, AZ

65

156

74

28

129

21

569

24

30

obsidian

-24

Duncan, AZ

74

156

73

30

125

21

555

22

17

obsidian

-25

Duncan, AZ

96

151

71

29

125

23

607

26

12

obsidian

-26

Duncan, AZ

48

154

79

32

132

19

568

31

30

tuff

-27

Duncan, AZ

56

145

71

26

125

21

499

24

27

perlite

RGM1-S4

 

42

147

104

25

219

7

806

24

19

standard

 

1 Those samples with a prefix of "IO" are from the 2016 UTSA collection, and those with the "053118" prefix are from the 2018 collection.

 

 

Table 2.  Major and minor oxides for a single Duncan obsidian sample and the USGS RGM-1 source standard.  All measurements in weight percent (%).

 

Sample

Na2O

MgO

Al2O3

SiO2

K2O

CaO

TiO2

MnO

Fe2O3

Ʃ

 

%

%

%

%

%

%

%

%

%

 

053118-1-2

2.989

0

12.931

73.049

6.671

1.261

0.311

0.079

2.386

99.677

RGM1-S4

4.045

0

12.918

73.857

4.933

1.433

0.292

0.05

2.228

99.756

USGS RGM-1 recommended

4.07

0.28

13.7

73.4

4.30

1.15

0.27

0.036

1.86

99.076

 

 

Table 3.  Mean and central tendency for Duncan obsidian trace elements (ppm).  Only obsidian included in this data table.

 

 

 

 

 


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1. Duncan obsidian source locality and geological features.


 

 

 

 

Figure 2.  Zr versus Rb (a) and Ba versus Sr (b) bivariate plots of the Duncan obsidian source standards, perlite and tuff samples from the Duncan obsidian dome complex.  Confidence ellipses at 95%.

Figure 4. TAS plot of Duncan obsidian sample 053118-1-2 and USGS RGM-1 rhyolite standard (Le Maitre et al. 1989). 

 

 

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Revised: 03 November 2018