Site Formation Processes Since the Early Holocene in a Planosol Sequence from the Laguna Negra Basin

Abstract

The low lands in southeastern Uruguay have witnessed the beginning of the traditions of mound builders on this side of the world. However, evidence of the first settlers in the area is scarce. In this paper, a site record is presented which dates to the early Holocene in the region, which could significantly change our perspective on the settlement process and further cultural developments. Based on the granulometric, color, carbonate, and organic matter values for the sediments in the soil stratigraphy, it is observed that the formation processes occurred toward the end of the loess sedimentation, which is typical of the late Pleistocene in the Pampas region. The unit corresponding to the early Holocene is in the contact layer between the Pampas silts and the overlying sands, creating a pale gray, sandy-silt lens with a high density of archaeological materials. It is concluded that, until the late Pleistocene, the landscape consisted mainly of silty soils of aeolian origin, followed by a sudden, net, nonerosive change which can be observed if compared to the later units, characterized by a sequence of upward-fining sand layers. The deposit process occurred along with the sedimentation of a granulometric fraction which coexists with the main sandy fraction throughout the profile, together with an abundant cultural record. The contrast of hydraulic transmissivity between the loess and bimodal sands has contributed to the development of an eluvial horizon and consequently, of a typical planosol. The underlying unit contains abundant lithic archaeological materials, which represent the first settlements in the current Atlantic plains of Uruguay during the late Pleistocene sedimentation process and prior to the paleo-environmental change, which resulted from an increase in humidity and temperature in the early Holocene.