Petrography of Prehistoric Moapa Valley Pottery, Clark County, Nevada (Tusayan Gray Ware – Virgin...

Petrography of Prehistoric Moapa Valley Pottery, Clark County, Nevada

Mary F. Ownby Submitted to: Margaret Lyneis University of Nevada at Las Vegas

Petrographic Report No. 2012-01 Desert Archaeology, Inc. 3975 N. Tucson Blvd., Tucson, AZ 85716 ● March 2012

TABLE OF CONTENTS

LIST OF FIGURES ............................................................................................................................... 3 LIST OF TABLES ................................................................................................................................. 3 INTRODUCTION ................................................................................................................................ 4 GEOLOGICAL SETTING ................................................................................................................... 4 PETROGRAPHIC ANALYSIS ........................................................................................................... 5 RESULTS ............................................................................................................................................... 6 Sample A202-578 ............................................................................................................................ 6 Sample A202-579 ............................................................................................................................ 6 Sample A202-580 ............................................................................................................................ 8 DISCUSSION ....................................................................................................................................... 8 CONCLUSION .................................................................................................................................. 10 REFERENCES CITED ....................................................................................................................... 11

LIST OF FIGURES

1. Photomicrographs: (a) cross polarized image of Sample No. A202-579 showing tuff rock fragments (black inclusions) and quartz grains (white inclusions), taken at 100x magnification; (b) plane polarized image of same view .............................. 9 2. Photomicrographs: (a) cross polarized image of Sample No. PMB1 (Logandale clay and gray volcanic clast gravel) showing tuff rock fragments (black inclusions) and quartz grains (white inclusions), taken at 100x magnification; (b) plane polarized image of same view ................................................................................................. 9

LIST OF TABLES 1. Sample inventory ....................................................................................................................... 5 2. Codes for grain size and grain frequency used in analysis of thin sections ...................... 6 3. Description of fired paste ......................................................................................................... 7 4. Types and amounts of rock fragments ................................................................................... 7 5. Types and amounts of monomineralic inclusions ................................................................ 7

PETROGRAPHY OF PREHISTORIC MOAPA VALLEY POTTERY, CLARK COUNTY, NEVADA INTRODUCTION Petrographic analysis of three pottery samples from the Yamashita sites in Moapa Valley aimed to confirm whether these vessels were made locally. The sand temper in the ceramic samples was compared to several thin sections of sand from an outcrop in the Moapa Valley that seemed similar to the sherd temper. A sample of local clay was also examined to assess whether it was used in making the pottery samples. Additionally, the three analyzed samples were compared to several existing thin sections of pottery from this area to assess their similarity in production materials. The results suggest that two samples are likely to have been made in the Moapa Valley, while one was either made in this area or possibly in the Virgin River Valley. GEOLOGICAL SETTING The Moapa Valley is located in southeastern Nevada (Clark County) and was created by the Muddy River that flows southward into Lake Mead. The mountain range to the west is the North Muddy Mountains, while to the east is Mormon Mesa. Geologically the Moapa Valley consists of sedimentary and volcanic outcrops. Mormon Mesa is predominantly composed of the Muddy Creek Formation, which comprises tuffaceous sedimentary rocks of Pliocene and Miocene date (Crafford 2007). These sedimentary rocks have a strong volcanic component. This unit is also found in the North Muddy Mountains, but additional formations are included in this range. The Overton Fanglomerate (now called the Overton Conglomerate Member of the Baseline Sandstone) is a Cretaceous layer comprised of conglomerate, limestone, siltstone, and shale. Below this formation is the Willow Tank Conglomerate Formation also of Cretaceous age that rests unconformly on the Aztec Sandstone of Jurassic date. Below is the Lower Jurassic to Lower Triassic Chinle and Moenkopi formations. This unit consists of claystone, mudstone, clayey sandstone, siltstone, sandstone, limestone, and conglomerate. Finally, remnants of a thrust plate are found comprising Middle to Lower Permian limestone, dolomite, sandstone, and shale. Thus, there is a clear division between the dominantly sedimentary rocks of the North Muddy Mountains, and the more volcanic rocks found in Mormon Mesa. The Muddy River has deposited three series of surfaces and three major gravel layers (Gardner 1968). The youngest surface is the Lost City, followed by the Overton surface, and the Mormon surface, the oldest. Below these are several layers of gravels. The first is the red carbonate clast gravel consisting of gray limestone clasts within a red sand. The angularity and sedimentary nature of the clasts indicate a local origin for this material. Next is the gray volcanic clast gravel comprising clasts of various tuffaceous volcanic rocks along with more andesitic rocks. Also present are gray limestone, quartzites, chert, and red sandstone clasts.


The gravel varies in its consolidation and associated sand and in general is rounded. In some places this gravel is exposed without the overlying red carbonate clast gravel capping. Its composition suggests a mix of local material and sediments from the White River and Meadow Valley wash that feed in to the Muddy River. The outcrops along these two tributaries are dominantly volcanic. This gravel, comprising a mix of volcanic and sedimentary clasts, is believed to be the source of the sand temper within the North Creek Gray and North Creek Corrugated wares found at the Yamashita sites. However, a final gravel layer, the tan quartzite-granite clast gravel, is found comprising mostly quartzites and limestone along with lesser amounts of granite, gneiss, schist, sandstone, chert, and clay-silt mudballs. Rarely volcanic rock fragments are found in the gravel. The gravel fragments are well rounded and contain various amounts of sandy matrix material along with lenses of fine silt and clay. Significantly, this gravel is not found in the Moapa Valley, but is present beneath the Mormon Mesa along the Virgin River Valley located to the east of the Moapa Valley. The Virgin River cuts through the Muddy Creek Formation of tuffaceous sedimentary rocks, but also receives input from the Early Proterozoic formations consisting of gneiss and schist near its confluence with the Muddy River. PETROGRAPHIC ANALYSIS In order to determine if the gray volcanic clast gravel located near the Yamashita sites was the source of sand temper in the North Creek Gray and North Creek Corrugated sherds petrographic analysis was performed. Three ceramic samples were examined and compared to examples of this sand along with a local clay and several ceramics previously analyzed (Table 1). Thin sections of the three new samples were made parallel to the vessel wall to provide a larger view of the paste, and stained for feldspar identification. A qualitative description was made for each including clay matrix color in plane and cross polarized light, optical activity (to assess firing temperature), temper type, estimated percentage of inclusions, sorting, and size and shape range (following Whitbread 1989). The minerals and rock fragments were identified and a general estimate of their frequency made (Table 2). Table 1. Sample Inventory.

Sample No. Sample Type Ceramic Types A202-578 Sherd (new thin section) Tusayan Gray Ware – Virgin Series A202-579 Sherd (new thin section) Tusayan Gray Ware – Virgin Series A202-580 Sherd (new thin section) Tusayan Gray Ware – Virgin Series A202-261 Gray sand (existing thin section) NA A202-569 Gray sand (existing thin section) NA A202-571 Gray sand (existing thin section) NA PMB1 Logandale clay and gray sand

(existing thin section) NA

A202-547 Sherd (existing thin section) Tusayan Gray Ware – Virgin Series A202-549 Sherd (existing thin section) Tusayan Gray Ware – Virgin Series A202-551 Sherd (existing thin section) Tusayan Gray Ware – Virgin Series A202-553 Sherd (existing thin section) Tusayan Gray Ware – Virgin Series A202-555 Sherd (existing thin section) Tusayan Gray Ware – Virgin Series A202-560 Sherd (existing thin section) Tusayan Gray Ware – Virgin Series A202-562 Sherd (existing thin section) Tusayan Gray Ware – Virgin Series


Table 2. Codes for grain size and grain frequency used in analysis of thin sections.

Category Code Definition Size Very fine 0.0625-0.125 mm Fine 0.125-0.25 mm Medium 0.25-0.5 mm Coarse 0.5 – 1 mm Very Coarse 1 – 2 mm Frequency 1 Very rare (1-5 grains) 2 Rare (c. 10%) 3 Sparse (c. 10-25%) 4 Frequent (c. 25-50%) 5 Abundant (c. 50-75%) 6 Highly Abundant (c. >75%)

RESULTS Sample A202-578 Petrographic analysis of sample A202-578 revealed that the paste1 was composed of a brown clay with the addition of around 30 percent fairly sorted sand temper2

(Table 3). Small silt-sized grains of quartz, opaques, iron oxides, biotite, and muscovite were present in the clay. The sand temper was fine to coarse in size and from subangular to rounded in shape. It was dominated by volcanic rock fragments including mostly welded, devitrified tuffs and andesite, but lesser amounts of basalt with various textures (Table 4). A single fragment of possible diabase was also noted. Sedimentary rock fragments, encompassing about 20 percent of the rock fragments, were present as well comprising mostly quartzite, siltstone, and chert, which in some cases had chalcedony intergrowths. Fairly large pieces of limestone were seen that probably derived from the sand temper. The majority of the monomineralic grains derived from these rock fragments (Table 5). Most common were quartz, potassium feldspar, plagioclase, and opaques. Less frequent were inclusions of microcline, pyroxene, olivine, and iron oxides. The overall characteristics of the sand temper suggest it could derive from the gray volcanic clast gravel.

Sample A202-579 Analysis of the thin section from sample A202-579 revealed its clay was reddish tan and very active suggesting a low firing temperature (see Table 3). It had a clean appearance with only a few silt-sized grains of muscovite, biotite, quartz, and calcite. The sand temper of subangular to rounded grains ranging in size from fine to very coarse included predominantly quartz and various volcanic rock fragments (see Table 4). Other monomineralic grains were plagioclase, potassium feldspar, microcline, pyroxene, olivine, opaques, iron oxides, and calcite grains (see Table 5). Most of these appear to derive from the 1 Paste is defined as the pre-fired mixture of clay and inclusions. 2 Temper is defined as intentional non-plastic materials added to the clay, features such as bimodal grain sizes, roundness of grains and geological differences between inclusions and clay are criteria for indicating temper.

Table 3. Description of fired paste.

Sample No. Color PPL1 Color XPL2 Optical Activity Temper3 Type Percentage of Inclusions Sorting Size Range Shape Range

A202-578

Brown Grayish brown

Slight Sand 30 Fair Fine to coarse Subangular to rounded

A202-579 Reddish tan Reddish tan Active Sand 25 Fair Fine to very coarse

Subangular to rounded

A202-580 Tan Tan Active Sand 20 Fair Fine to very coarse

Subrounded to well rounded

1 PPL: plane polarized light 2 XPL: cross polarized light Table 4. Types and amounts of rock fragments.

Sample No. LVF1 LVFB LVI LVM LVV LVH LMM LMF LMA LMT LMTP LMVF LMSS LSS LSA LSCH LSCA A202-578 3 2 3 2 2 0 0 2 0 0 0 0 0 1 0 2 2 A202-579 3 2 3 2 2 0 0 2 0 0 0 0 0 2 0 2 3 A202-580 2 1 2 0 1 0 0 3 1 0 0 0 0 2 1 3 3

1 LVF=felsic volcanic (e.g. rhyolite), LVFB=biotite-bearing felsic volcanic, LVI=intermediate volcanic (e.g. andesite), LVM=mafic volcanic (e.g. basalt), LVV=vitric volcanic, LVH=hypabyssal volcanics, LMM=microgranular quartz aggregate, LMF=foliated quartz aggregate (e.g. quartzite), LMA=quartz-feldspar (mica) aggregate, LMT=quartz-feldspar-mica tectonite (e.g. schist), LMTP=fine-grained quartz-feldspar-mica-tectonite (e.g. phyllite), LMVF=metamorphosed felsic volcanic, LMSS=metamorphosed sedimentary, LSS=granular aggregates of equant subangular to rounded grains (e.g. siltstones), LSA=very fine- grained, semi-opaque sedimentary (e.g. shale), LSCH=microcrystalline aggregates of silica (e.g. chert), LSCA=very fine calcite crystals (e.g. carbonate). Table 5. Types and amounts of monomineralic inclusions.

Sample No. QTZ1 KSPAR MICR PLAG MUSC BIOT CHLOR PX AMPH OLIV OPAQ FOX GAR EPID SPH CACO A202-578 4 3 1 3 1 2 0 2 0 1 3 2 0 0 0 0 A202-579 4 1 1 2 1 1 0 1 0 1 2 2 0 0 0 2 A202-580 4 2 1 2 1 1 0 1 0 0 2 1 0 0 0 2

1 qtz=quartz, kspar=potassium feldspar, micr=microcline, plag=plagioclase, mus=muscovite, bio=biotite, chlor=chlorite, px=pyroxene, amph=amphibole, oliv=olivine, opaq=opaques, fox=iron oxides, gar=garnet, epid=epidote, sph=sphene, caco=calcite.

Petrography of Prehistoric Moapa V

alley Pottery, Clark County, Nevada


volcanic rock fragments of welded, devitrifed tuff and andesite, with less frequent basalt. Sedimentary fragments comprising chert, quartzite, limestone, and sandstone were present at about 40 percent of the rock fragments. These inclusions are similar to the previous sample and also suggest they may originate from the gray volcanic clast gravel. Sample A202-580 Sample A202-580 was made with clay of a similar appearance to that used for Sample A202-579, however, the temper inclusions were different (see Table 3). They ranged from fine to very coarse in size and were subrounded to rounded in shape. Sedimentary rock fragments were the dominant inclusion type consisting of quartzite, chert, and limestone, with less common sandstone, shale, and chalcedony (see Table 4). Volcanic rock fragments were also present but were only about 20 percent of the rock fragments. These were mostly devitrified tuffs with lesser andesite. Some granite was present comprising quartz, potassium feldspar, and plagioclase. A few fragments of metaquartzite were also identified. As with the previous samples, the monomineralic grains appeared to derive mostly from the rock fragments and included quartz, potassium feldspar, plagioclase, opaques, and calcite, while muscovite, bitotite, pyroxene, and iron oxides were rarer (see Table 5). The prevalence of sedimentary grains, granite, and its derived minerals, suggests the source of this sand may have been the tan quartzite-granite clast gravel. DISCUSSION Petrographic analysis of the three samples indicated their clay and sand temper shared some common features. The sand temper is similar between samples A202-578 and A202-579; however, the clay has a different appearance. This is more likely due to a different clay source than firing as the temperature for the samples appears to have been low due to their optical activity. Sample A202-580 has a sand temper dominated by granite and sedimentary rocks with only minor volcanic rock fragments in contrast to the prevalence of volcanic rocks and minor sedimentary rock fragments in the other two samples. Interestingly, the clay in Sample A202-580 and Sample A202-579 is similar. Comparison of a sample of the so-called Logandale clay with added temper from the gray volcanic clast gravel, Sample PMB1, revealed a close similarity to Sample A202-579 (Figures 1 and 2). The clay was also similar to that in Sample A202-580 even though the inclusions were different. Sample A202-578 had a clay with a different appearance. Three samples of the gray volcanic clast gravel were also examined petrographically. Sample A202-261 contained a variety of volcanic rock fragments dominated by welded, devitrified tuff with lesser quantities of andesite, basalt, chert, sandstone, quartzite, and limestone. Its overall characteristics were similar to samples A202-578 and A202-579. A second sand sample, A202-569, also contained the same volcanic and sedimentary rock fragments, although the quantity of limestone, quartzite, and chert was higher. This sample contained more potassium feldspar as well that was probably derived from granite. Its appearance was more comparable to the sand temper in sherd A202-580, although characteristics were also similar to the sand temper in A202-578 and A202-579. The final sand


sample, A202-571, resembled the sand A202-261 and therefore was more analogous to the sand temper in A202-578 and A202-579. The three newly analyzed samples were also compared to existing thin sections of seven sherds from the Yamashita sites. Most of the samples had a similar sand temper composed predominantly of volcanic rock fragments including welded, devitrified tuff, andesite, and basalt, along with sedimentary rock fragments of limestone, quartzite, and chert. However, the amounts of these inclusions did vary along with slightly different appearances of the clay. Sample A202-560 although having poor thin sections to analyze, did appear to have a clay and sand slightly similar to the recently analyzed Sample A202-578. Several existing thin sections had sand temper and clay similar to Sample A202-579, these included Samples A202-549 (had less tuff, basalt, and andesite), A202-551 (small amount of temper), and A202-562 (some sandstone and granite). Three existing samples were similar to Sample A202-580.

Figure 1. Photomicrographs: (a) cross polarized image of Sample No. A202-579 showing tuff rock fragments (black inclusions) and quartz grains (white inclusions), taken at 100x magnification; (b) plane polarized image of same view.

Figure 2. Photomicrographs: (a) cross polarized image of Sample No. PMB1 (Logandale clay and gray volcanic clast gravel) showing tuff rock fragments (black inclusions) and quartz grains (white inclusions), taken at 100x magnification; (b) plane polarized image of same view.


Sample A202-547 had similar round and large grains but with more welded, devitrified tuff than Sample A202-580. The percentages of volcanic and sedimentary rocks were more comparable between samples A202-553 and A202-555, and A202-580. In all cases, the clay was also analogous and confirmed the similarity between the Logandale clay and A202-579 and A202-580, while the clay in Sample A202-578 is somewhat different. It appears the Logandale clay was used more frequently and its temper inclusions often resembled the sand samples of the gray volcanic clast gravel. This temper was also noted for the other clay utilized for several existing samples and one newly analyzed sherd. Examination of the existing samples provided further information on the variability within the gray volcanic clast gravel. Thus, there is the possibility that the more granitic temper in Sample A202-580 and some of the existing samples still derived from the gray volcanic clast gravel, especially as in Sample A202-547 a small amount of granite was seen with more volcanic rock fragments. Due to the deposition of the gray volcanic clast gravel along a river system, it is natural for there to be variability seasonally and long term in the types of rock fragments deposited in these gravels. CONCLUSION The petrographic analysis of three ceramic samples from the Yamashita sites aimed to assess if the vessels were locally produced. This was accomplished through the comparison of these samples to examples of a local sand, the gray volcanic clast gravel, and a sample of local clay with this sand added as temper. The petrographic results suggest it is likely that Samples A202-578 and A202-579 were made with this sand resource, although only for Sample A202-579 was the clay similar to the local Logandale clay. Sample A202-580 was also produced with a clay similar to the Logandale clay, but the sand temper was more granitic and contained a greater amount of sedimentary rock fragments than the sand from the gray volcanic clast gravel. From the description of the tan quartzite-granite clast gravel, the sand temper in A202-580 appears more related to this resource. However, this gravel is not found in the Moapa Valley, but occurs in the Virgin River Valley. There is the possibility the vessel was made in this area utilizing a clay similar to the Logandale clay. Only further analysis of the distribution of this clay and the variability within the gray volcanic clast gravel will be able to determine if Sample A202-580 was made in the Virgin River Valley or the Moapa Valley. One consideration is that at the Main Ridge site, located five miles southeast of the Yamashita sites where the Moapa and Virgin valleys meet, some of the tan quartzite-granite clast gravel has entered the Moapa Valley. This area may therefore be the source of the clay and sand found in Sample A202-580. Based on the results of the current analysis, Sample A202-580 could still have been locally produced in the Moapa Valley due to the similarity to the Logandale clay and the range of sand temper seen in the existing thin sections.

REFERENCES CITED

Crafford, A. Elizabeth Jones 2007 Geologic Map of Nevada: U.S. Geological Survey Data Series 249, 1 CD-ROM, 46 p., 1

plate. Gardner, Leonard Robert 1968 The Quaternary Geology of the Moapa Valley, Clark County, Nevada. Unpublished Ph.D.

Dissertation, Department of Geology and Geophysics, the Pennsylvania State University. University Microfilms, Ann Arbor.

Whitbread, Ian 1989 A Proposal for the Systematic Description of Thin Sections towards the Study of

Ancient Ceramic Technology. In Archaeometry: Proceedings of the 25th International Symposium, edited by Y. Maniatis, pp. 127-138. Elsevier, New York.

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