Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

178 PATA Days 2024 M E T H O D S The excavation of 5 pits and the study of surface geomorphological landforms have led to the recognition and interpretation of five representative sedimentary facies reflecting the conventional sedimentary functioning of this system, characteristic of a coastal lagoon-wetland environment. The sedimentation of mud and peat predominates under low-energy and low-sedimentation-rate conditions, but they occur stratigraphically truncated by successive high-energy marine flooding facies deposited by tsunami waves. The peat formation occurs in the more protected areas of the lagoon, while sandy mud deposits with rhizoliths correspond to wetland facies. In higher topographic zones, the stressed wetland is preserved, disconnected from the groundwater table and featuring vegetation adapted to saline soils. Similarly, alluvial deposits have been identified intercalated among these facies, indicating disruptions in the conventional functioning of the system (Figure 2). R E S U LT S Up to 13 distinctive layers interpreted as tsunamigenic events have been identified within the stratigraphic succession as medium to fine sand layers, with moderate sorting, unimodal distribution, fine skewness, and leptokurtic to very leptokurtic grain size distributions. In all the cases an erosional surface was observed at the base of the tsunami layer. The deposits are normally graded or massive. Furthermore, they are characterized by the presence of marine microfauna, including benthic foraminifera, marine ostracods, and sponge spicules. Additionally, brackish and freshwater specimens of gyrogonite charophyte algae, the ostracod Cyprideis and gastropods of the genus Heleobia are also identified int this facies. The tsunami layers are notable for their abundant specimens of all these taxa, reflecting the mixing of populations from different environments during the marine inundation of the creek. The tsunami facies are enriched in Si, with respect to the adjacent layers, derived from the abundance of quartz and feldspars, and characterized by a relatively low content of K and Al, resulting from the relative low percentages of phyllosilicates. Carbon-14 dating of peat levels, allowed to constrain the age of the paleotsunami events interpreted from the distinctive layers. Through this method, 12 events (E) were determined: E13: 1922 AD. E12: After 1302–1362 cal AD, before 1922 AD. E11: After 1211–1276 cal AD, before 1302–1362 cal E A 1 D 0: After 1269–1285 cal AD, before 1211–1276 cal AD. E9: ca. 944 ± 31 cal AD. E8: ca. 944 ± 31 cal AD. E7: After 748–772 cal AD, before 772–896 cal AD. E6: After 525–566 cal AD, before 648–668 cal AD. E5: After 21–61 cal AD, before 59–129 cal AD. E4: ca. 70 ± 49 cal AD. E3: ca. 120 ± 61 cal BC. E2: ca. 120 ± 61 cal BC. E1: ca. 426 ± 65 cal BC. C O N C L U S I O N S The coastal wetland-lagoon system of Carrizal Bajo represents a key area for the recognition of tsunami deposits of different magnitudes in the southern Atacama Desert. Through geomorphological analysis together with the study of pits, we found 13 distinctive layers in between the wetland and alluvial succession, interpreted as paleotsunami deposits, encompassing roughly the last 2500 years. This suggests a mean recurrence of about 190 years for the impact of large tsunamis in this area. The findings presented in this study enhance our understanding on the record of tsunami deposits in arid wetlands, a crucial topic for advancing the assessment of tsunami hazard in coastal areas such as the Atacama region in northern Chile. A C K N O W L E D G E M E N T S This research was supported by FONDECYT project 1201387, titled “Unveiling Multimillennial Cycles and Super-Cycles of Tsunamigenic Megathrust Earthquakes in theMajorNorthernChile SeismicGap”. Additional support was provided by the TRAMPA Project, “Multi-Proxy Analysis of Holocene Tsunamis on Arid Coasts: Identification of the Camouflaged Record of Large (Paleo)Events in the Atacama Desert”.