Paleopedogenesis, sedimentation and holocenic geomorphologic evolution in the fluvial system of Arroyo Tinajas, Sonora

Abstract

In this work an integral analysis of a pedo-sedimentary sequence is presented, found in the lower fluvial terrace of the arroyo Tinajas, Sonora. The objective of this research is to establish the mechanism of the sequence formation in order to reconstruct the paleoenvironmental conditions. The sequence consists on a modern soil and eleven paleosols interbedded with fluvial sediments. The physical (color, grain size distribution, rock magnetic properties), chemical (pH, electric conductivity, total calcium content), and micromorphological properties are evaluated. The clay mineralogy determined by X-ray diffraction is also presented. Practically all the paleosols are truncated. Due to the lack of A horizons, the chronological frame of the sequence was obtained by using a small amount of organic matter found in the paleosol 8. To concentrate the organic matter, an extraction by demineralization with hydrofluric acid was used. This concentrated organic matter is dated by ¹⁴C accelerator mass spectrometry (AMS), giving an age of 8415 ± 90 years cal BP. The results show that all paleosols are weakly developed. However, it is possible to establish the main pedogenetic tendencies: the lower part of the sequence is finer, with strong reductomorphic features and vertic properties. In contrast, the upper paleosols are coarser in texture, more porous and have clay coatings. From paleosol 7, a change in the relief energy is detected due to the sedimentological variations (inverse gradation, increase in grain size), which is more evident in the paleosols 2, 3, 4, due to the high amount of gravels, bad sorting and angular shapes, related to fluvio-colluvial processes. We suppose that this change in the pedogenetic trend and the sedimentological variations is related to regional climatic oscillations in the middle Holocene, because no recent tectonic activity has been documented in the area.