GEOARCHAEOLOGY: ORIGIN OF ABANDONED MEANDER SEDIMENTS, RíO VERDE, OAXACA, MEXICO BY MINERALOGICAL ANALYSIS

The Río Verde is one of the largest rivers on Mexico’s west coast. Its upper drainage contains the important archaeological site of Monte Alban among many others. The lower portion of the drainage contains the similarly important Río Viejo site. Previous geoarchaeological research showed that the river shifted course and altered the environment over the last few millennia. The Río Verde has shifted from the west to its present location but doesn’t appear to have ever shifted to the east. At different periods in time the archaeology of the east side of the river has differed from the west side. One question is whether these temporal changes were influenced by changes in the position of the Río Verde. A recent rise in the watertable caused by a new irrigation project has revealed many buried meanders east of the river. Sourcing these abandoned meander sediments may provide insights on the archaeology of the region.

Sand samples were collected by bucket auger from the center and on either side of five meanders. Samples were also collected from modern depositional environments, including the Río Verde, Río San Francisco, and Río Yutanana. Mineralogical analysis of both grain shape and mineral species provides a means to determine the origin of the eastern meanders. Local streams drain only two main bedrock types, granite and grano-diorite with small areas of gneiss. On the other hand, the Río Verde drains multiple bedrock types, most notably sedimentary and high grade metamorphics. Observation and measurement of minerals provides answers to the origin of the meanders. Mineralogy of the sediments was determined by oil immersion microscopy.

Mineralogy of the buried meander sediments was primarily quartz, plagioclase, muscovite, and biotite. Accessory minerals include chlorite, apatite, and hornblende. Mineral assemblages observed within the meander sediments display similarities to those observed from local streams (e.g., modern Yutanana). The mineralogy of the sediments suggests that these meanders represent either former courses of local streams or an amalgamation of coastal plain fluvial systems. The sediments do not appear to be associated with the Río Verde.