The sedimentary intertidal surfaces (tidal flats from here on) in coastal wetlands are globally recognized by their biological diversity and available ecosystem services. In Chile, these environments are mainly distributed between 30 and 41°S and associated with micro-tidal estuaries. The estuarine Cruces River Wetland (CRW) was formed by tectonic subsidence caused by the giant 9.5 MW Valdivia earthquake in May 1960. The CRW is characterized by relatively uniform water quality conditions and tidal ranges and a significant increase of discharge and elevation of the water table concurrent with intense winter precipitations. The present study focuses on the presence and distribution of tidal flats in the CRW. It evaluates the spatio-temporal variability (April vs. December 2016) of texture and geochemical parameters (pH and ORP (oxidation/reduction potential, ORP)) of surface sediments in six particular tidal flats located along the main channel of the Cruces River. Most of the tidal flats are located in the northern and southern sectors of the CRW. Those located in the middle and southern sectors of the study area exhibit denser coverage with vestigial tree trunks that evidence the former presence of marsh forests along the flooded shores. The texture of the studied sediments is dominated by sand (0.063-2 mm), followed by mud grain size fractions (0.063 mm). The mud fraction is the only grain size showing significant differences between the studied tidal flats. Both the pH and ORP values vary significantly between sites and sampling periods. In general, ORP values were lower or negative in samples with higher mud and total organic matter contents collected mainly during both sampling times. These results set the first baseline for textural and basic geochemical characteristics of surface sediments from tidal flats in the Cruces River wetland. The database provides a primary tool for evaluating potential effects of possible alterations along the margins of the wetland, which is located in an area of the rapid development of forestry and industrial activities as well as urban expansion.
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