Resumo

SOLIS-CASTILLO, Berenice; ORTIZ-PEREZ, Mario Arturo  e  SOLLEIRO-REBOLLEDO, Elizabeth. Geomorphology and pedostratigraphy in the Mayan Lowlands, Tabasco-Chiapas: an approach to the alluvial history of the Usumacinta river. Bol. Soc. Geol. Mex [online]. 2014, vol.66, n.2, pp.279-290. ISSN 1405-3322.

This work presents the reconstruction of the fluvial processes that have modified the alluvial landscape in the Maya Lowlands, based on the identification of geomorphologic-environmental units, and their temporal relation from the Pliocene to the late Holocene. The unit identification has been made according to the following criteria: a) genesis of the geoforms; b) configuration and arrangement of the geoforms, c) processes of the terrain modelling, and d) the temporality (ages of the structures and forms), using a pedostratigraphic analysis. Eight units are proposed, labeled from 1 to 8, four of them (1, 2, 3, 5) are present in the south area, in the Sierra of Chiapas. These units are the oldest in the study area and are controlled by: erosion, following the original tectonic structures (unit 1); dissolution of calcareous rocks (karstic - unit 2); erosion of the sedimentary clastic rocks (unit 3); and colluvial accumulation in the foothills (unit 5). The soils characterizing these units are variable, from poorly developed at higher elevations (rendzic Leptosols), to highly developed in the valley bottoms (Luvisols). On the other hand, the units in the alluvial plain (4, 6, 7, 8) are younger (Pleistocene-late Holocene) and have developed a terrace system. The soils of the Pleistocenic terraces (TP3, TP2, TP1, from the oldest to the youngest), from unit 4, show strong weathering. Soils with gleyic properties are found especially in TP1. Optical luminescence (OSL) dating of material from the base of the terrace reveals an age of 122000 years, which constrains its temporal development. Unit 6 corresponds to the depressions and wetlands in the alluvial plain. Unit 8 is restricted to the Chacamax River, which has an autochthonous sediment source. Unit 7 presents a series of terraces developed during the Holocene (TH2, TH1). An age date of 9000 years in sediment from terrace TH2 gives evidence of environmental changes in the Usumacinta system. The pedogenesis of soils in terrace TH1 is characterized by vertic features and processes of accumulation of organic matter and carbonates, which are related to drier conditions. The development of these soils occurs during the middle Holocene, a period when regional climatic changes are documented. The soils of the late Holocene, also present in TH1, are less developed, and evidence of impact by human activities in the area is high since the Early Formative.

Palavras-chave : Geomorphologic-environmental units; fluvial processes; Usumacinta; pedostratigraphy; terraces.

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