LATE PLEISTOCENE AND HOLOCENE ARROYO STRATIGRAPHY OF THE MIXTECA ALTA, OAXACA, MEXICO

We examine arroyo stratigraphy of the Yanhuitlan Basin in the Mixteca Alta, Oaxaca, Mexico. Arroyo cut-and-fills in this region have been dated back to the late Pleistocene, and currently, some channels are incised up to 37 meters in places. After thorough reconnaissance in the Río Culebra watershed, nine profiles were selected for more in-depth description and sampling. Arroyo cut-and-fill features and well-represented paleosol-sedimentary sequences were selected for close examination (including alluvial and/or colluvial sediments). For all profiles, multiple samples of charcoal and bulk organic rich A horizons were used for ¹⁴C dating.

Radiocarbon dates indicate that the stratigraphy in the Río Culebra spans the late Pleistocene to the present, with some major and minor gaps in chronology. The oldest stratigraphy was mainly observed along the mainstem of the Río Culebra and its higher tributaries. Based on a combination of lithology, chronology, and location in the Río Culebra watershed, the alluvium and paleosols were divided up into three groups, described as follows: Group I corresponds to the late and terminal Pleistocene; Group II relates to the early and middle Holocene; and Group III concerns the late Holocene. During the late Holocene, people constructed lama-bordo terrace and check-dam walls in an attempt to conserve soil and water. These walls were first built around 3500 cal BP (Leigh et al 2013), and are thought to have their widest impact during the Natividad phase, ~1000-500 years ago, when the population was relatively large. In order to better understand the alluvial history of the entire region, the profile data from the Río Culebra basin was compared to the stratigraphy of Río Verde watershed, located in nearby Nochixtlan (Mueller et al. 2012). It was found that times of alluviation and paleosol formation overlapped between the two basins, though cumulic and non-cumulic soils varied through time and space. A comparison of the periods of alluviation and paleosol characteristics including the Pleistocene, early to middle Holocene, and late Holocene fails to suggest that human activities in the late Holocene had a greater impact on the arroyo cutting and filling than natural factors in earlier periods.