Preliminary Paleoenvironmental Analysis of the Drexel Road
Reach of the Santa Cruz River, Pima County, Arizona

Owen Davis
David Dettman
Chris Eastoe

November, 2001

Our radiocarbon, isotopic and microfossil analyses of Tucson Basin arroyos, supported by a UofAz Small Grant (acct. 210212), have demonstrated the potential success of this kind of collaborative research, and they have shown that some of the goals in our research proposal will be difficult to achieve. The large channel we studied at Drexel Road filled in less than 200 years (6 m from 890±135 - 725± 85 14C years). Therefore, a chronological sequence spanning the last few millenia in the Tucson Basin must be obtained by studying several dated alluvial profiles. Our research therefore shifted to establishing the age of (and obtaining samples from) several areas in the Tucson Basin. We were successful at finding older sedimentary packages that will become the focus of future studies. However, as with earlier investigations, only late-Holocene dates were found for the Santa Cruz.

Our initial results indicate that samples from Drexel Road dating to the Medieval Warm Period have more positive d18O values than modern (-6.8 ‰ vs. -7.2 ‰) possibly indicating greater summer rainfall.

Palynological study of the Drexel Road site indicates only minor change of the upland vegetation. However, these analyses have provided an important basis for evaluating palynological samples from archeological sites in the Tucson Basin. The percentages of streamside plants (> 40%) are many times greater than from archeological sites on floodplain of the Tucson Basin Rivers. Furthermore, corn (Zea) pollen is frequent in the filled channel, indicating either that corn was being growing around the infilling channel, or that corn was so abundant that its pollen cannot be use as an indication of local fields.

Comparison of previous dates (histogram, data from Haynes & Huckell, 1986)
for the Santa Cruz River with dates obtained in this study (red "+'s").
The 3 older dates are from Pantano and Alamo Wash, not included in this report.
Haynes, C.V., Jr. and Huckell, B.B. 1986. Sedimentary successions of the
prehistoric Santa Cruz River, Tucson, Arizona. Arizona Geological Survey
Open-File Report 86-15.

Percentage pollen diagram for selected taxa.

Location of pollen sampling profiles (vertical red lines), radiocarbon dates (red "+'s"),
and outline of channel sampled (blue lines).