2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 1
Presentation Time: 8:00 AM-6:00 PM

A Climate-Induced Interruption in the Prehistoric Occupation of a Site in Coastal Ecuador

WESOLOWSKI, James H., Department of Geosciences, Indiana University Northwest, 3400 Broadway, Gary, IN 46408 and ARGYILAN, Erin P., Dept. of Geosciences, Indiana University Northwest, 3400 W. Broadway, Gary, IN 46408, jwesolow@iun.edu

An alluvial terrace in the Salango River valley of southern Manabí Province in coastal Ecuador provides evidence of a series of distinct cultural occupations. Excavations have revealed a set of two occupational layers, likely from the Guangala culture (~2.5-1.2 ka), between which a gravel deposit is found. The gravel layer suggests that occupation of the terrace was interrupted by a climate-driven event, and the gravel has been attributed alternatively to two sources. The first hypothesis describes mass wasting from the adjacent slopes of the Chongón-Colonche Cordillera. The second attributes the sediment to stream flood deposits. Interestingly, similar gravel deposits appear to be absent since the upper Guangala settlement, possibly indicating a major climatic anomaly during the period of Guangala occupation of the terrace.

This project evaluates sediment characteristics and maps sediment distribution across the terrace to better determine the source and extent of the gravel deposit. 73 sediment samples were obtained from ten locations laid out in a grid on the terrace, both near and away from the archaeological excavation. The samples were collected using a soil auger between the modern surface and a depth of 1.5 m, and represent the distinct sediment types observed in each location. Laboratory analyses characterize the sediments on the basis of grain size distributions and mineral/rock composition. The extent, depth, and thickness of each stratum was mapped and correlated to determine consistency with a sediment source on the adjacent hill or upstream.

Finally, the deposition event was examined for its impact on the settlement of the terrace and its relationship to the regional climate, including the El Niño Southern Oscillation. This allows a better understanding of the sensitivity of the landscape and human settlements to climatic variability as well as the role of climate in driving changes in human occupation.