INAA analysis to differentiate and source chert and other cryptocrystalline silicates has been conducted on assemblages in the Great Lakes and adjacent regions (Luedtke 1978, 1979; Hoard et al. 1993; Jarvis 1990; Julig et al. 1987, 1988, 1989, 1991a,b, 1992; Goatley 1994), however, the routine sourcing of chert by INAA is not yet too common. Luedtke (1978; 1979) chemically characterized a large suite of Great Lakes region cherts, and demonstrated that chemical differences were generally greater between chert bearing geological formations then within them. The more recent work of Julig et al (cited above) and Hoard et al. (1993) have supported this general principle. We have had reasonable success in chemically "fingerprinting" and sourcing of visually similar materials, from widely separate sources, rather than the differentiation of sources close in space and/or time (outcrops from the same geological formation), which are often chemically more similar. However a study of Onondaga chert by Jarvis (1990) found strong chemical separation between chert from different outcrops of the same formation (Devonian), and attributed the differential elemental concentrations to facies shift. However, in that study the discriminate analysis classification for the geological samples did improve with distance between sources.
With further testing and characterization by INAA of chert proceeds, we now have the beginnings of a chemical data base to assist in the resolution of provenance problems of certain visually similar materials (Goatley 1994, Julig et.al. 1992); as will be discussed later.
The characterization of the geological sources normally involves testing a minimum of 30 samples ( except in some small pilot studies), these being selected from various parts of the formation. Ideally they should include the range of variability within the formation, with respect to the knappable chert. This part of the research is obviously critical, yet very difficult to conduct unless there is good access to the geological exposures and the primary and secondary deposits are well mapped. In the Great Lakes region the chert deposits are often covered by glacial deposits as well as being distributed by Pleistocene glaciation. Some sources, such as Hudson Bay Lowland chert (HBL) have primary sources in remote regions, and secondary sources widely dispersed by the Pleistocene glaciation (Julig et al. 1992); this makes the geological sampling and chemical characterization difficult.
Geological samples in the range of 200 to 600 mg were used for INAA, while archaeological samples varied in size from 50 to 10,000 mg (in the case of some whole artifacts). The geological samples are obtained by flaking chert cobbles with a moose antler billet and then snapping off appropriate sized flake fragments, washing them in distilled water, air drying and then sealing in polyethylene vials for irradiation after being weighed. It is important to avoid contamination by flaking with metal implements in the sub-sampling procedure.
The samples were analyzed at the SLOWPOKE Reactor Facility of the University of Toronto. The individual samples in sealed vials were irradiated for five minutes at a neutron flux of 1.0 x 1011n.cm-2.s-1. After a delay time of 15 to 20 minutes to allow the 28Al to decay to analytically acceptable levels, the samples were assayed for 5 minutes using Ge detector gamma ray spectrometers. This allowed the measurement of fifteen elements including U, Dy, Ba, Ti, Sr, Br, Mg, Si, Na, V, K, Al, Mn, Cl, and Ca, the majority of which were present in measurable concentrations in most samples. The analytical precision ranged from <1% to detection limits.
Some clean chert and quartz samples that were analyzed (as part of other sourcing studies, e.g. Julig 1987) were irradiated as much as five times as much as normal to produce enough radioactivity to allow the measurement of their lower concentrations of trace elements. Elemental concentrations were calculated using the comparator method (Hancock 1978). This analytical procedure produced chemical data that allowed for the discrimination of different chert sources.
The research discussed here used only the short-lived radioisotopes noted above, and although valuable chemical information may be obtained in the long-lived radioisotopes, this information was not required for this research. An additional advantage of only analyzing for the short-lived radioisotopes is that small and valuable whole artifacts may be analyzed and then returned to the collections from where they were borrowed. Larger size vials were used in this procedure, as reported in Julig et.al.(1992).
The initial INAA analysis of chert artifacts involved a problem of sourcing dark to medium brown chert and/or agate artifacts from the Cummins Palaeoindian site near Thunder Bay Ontario. These visually similar artifacts consisted of small biface thinning and scraper resharping flakes, and small scrapers (Julig 1988, Julig et.al. 1988, 1989, 1991a). Three possible geological sources that were visually very similar were tested by INAA including Knife River Flint (KRF) from western North Dakota, Hudson Bay Lowland chert (HBL) from the Hudson Bay Lowlands of northern Ontario, and agates from the Osler Formation in the northwestern Lake Superior region of Ontario. The three prospective geological sources were tested by INAA for 15 short half-life producing elements; and based on the elemental concentrations of Al, Si, U, Dy, and Cl (plotted on a ternary diagram) it was shown that geological KRF was separate from the HBL and agates. The KRF was chemically different from the other sources based on higher normalized and standardized U and Dy values, and generally lower Al/Si ratios. It was not possible to chemically separate the HBL and agate sources with the short half-life suite of elements. In testing the artifacts KRF was found to be only sparsely represented at Cummins site and that most of the brown artifacts were HBL and/or agates.
Other researchers (Clark 1984, Wright 1972) had previously reported KRF in western Great Lakes regional Archaic and Woodland assemblages. These reports had been based primarily on visual identifications, and Clark (1984) proposed several trade mechanisms to account for the reported distribution of KRF in both utilitarian and ritual (mortuary) contexts. INAA was conducted on 60 brown chert artifacts from five regional sites ranging in age from Archaic to late Woodland (Julig et.al. 1991b, 1992). Both KRF and HBL were represented in the utilitarian and ritual (mortuary) assemblages, but the number of KRF artifacts (most distant source) was less than 10%. Our research confirmed that some KRF was present, but questioned the previous visual identification of large numbers of KRF artifacts in regional assemblages (Clark 1984), and the trade mechanisms proposed for their (observed) distribution. However, this is clearly an area that requires further investigation.
Another related provenance problem addressed concerning the identification and distribution of HBL chert in secondary deposits in southern Lake Superior and northern Lake Michigan (Julig et.al. 1992). HBL is widely distributed on the Canadian Shield and in the upper Great Lakes as a result of Pleistocene glaciation. Brown, grey and tan coloured chert pebble are common in upper Great Lakes beach deposits and were used by the prehistoric native peoples (Binford and Quimby 1972). The question concerns the archaeological identification of some of these cherts as HBL, and if they were chemically similar to HBL from the source region. INAA data from a total of six secondary sources (35 samples) indicated that there was close chemical similarity between the HBL from the geological source region and the samples identified as HBL from secondary sources. Furthermore, this chert is chemically distinct from other regional (e.g. Gunflint chert from the Gunflint Formation north of Lake Superior) and distant chert look-alikes (e.g. KRF).
In south central Michigan dark and medium brown cherts of unknown provenance have been recovered in small quantities in Late Woodland assemblages (Goatley 1994). These recoveries of small brown flakes, normally less than 1% of the total lithic assemblage, compare visually with brown HBL or KRF samples. This material is reported to increase in frequency from south to north, and is not one of the southern Great Lakes brown cherts such as Flint Ridge or Plum Run (Goatley 1994, B. Luedtke pers. comm.). INAA analysis of eight samples of this unknown material indicated chemical similarity with HBL, based on the elements U, Dy, Al, Si, Cl, and that it was chemically distinct from KRF (Julig, in Goatley 1994:118).
In addition, INAA of geological samples of other Upper Lake Michigan and Lake Huron basin cherts (namely Fossil Hill Formation chert, Cordell Formation chert, and Norwood chert of the Middle Devonian Petosky Formation, and three samples of till chert) indicated these lithic materials are also chemically distinct from HBL, based on the elements Ba, Br and Cl (Julig et.al. 1992). The reasons for testing these materials is the close visual similarity of the unknown brown chert and some samples of Cordell chert. The unknown brown chert from Late Woodland sites in southwestern Michigan thus compares most closely with HBL and not with any of the other local or distant visual look-alikes. It can now be stated with confidence that they are northern imports, probably from secondary deposits of HBL on the Canadian Shield.
In summary, INAA has proven to be a useful analytical technique for the sourcing of chert artifacts as to their geological sources. This method with the SLOWPOKE reactor permits INAA of the short-lived isotopes on a large range of sample sizes and to retain the artifact in its original form, so it can be returned to the curator undamaged. This research has also demonstrated the potential for the separation of various regional chert sources based on the short-lived isotopes, and this has proven too be a very cost effective and rapid analytical method to solve some chert provenance problems.
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