Micropaleontological insights into Late Pleistocene coastal lagoon and tsunami deposits at Copiapó mouth river (Southern Atacama Desert)

Deposits associated with MIS 3 have been recently described and dated along the coastal margin of the Southern Atacama Desert. These deposits generally exhibit a wide heterogeneity of littoral facies, including beaches, deltaic fans, dunes, and coastal lagoons. In some of these marine terraces, coastal boulder deposits interpreted as tsunami-related have been identified, extending the chronology of major earthquakes and tsunamis in northern Chile and expanding the current geodynamic scenario of a tectonically active coastline back to the terminal Pleistocene.

This work describes deposits from a coastal lagoon exposed in an abandoned quarry at the mouth of the Copiapó River (northern Chile). The stratigraphic succession reaches approximately 22 m in thickness and has been dated at its base and top, yielding ages of 42,431 ± 1,891 yr and 35,984 ± 277 yr, respectively. The lower and middle sections consist of centimeter-thick layers of gypsum and gray argillites, occasionally containing solenoid bivalves and the abundant benthic foraminifera Ammonia confertitesta, with less frequent Buliminella sp., Bolivina sp., and some planktonic forms as Orbulina universa. Interbedded within these deposits are thicker beds (25–165 cm) of gray arkosic sands, slightly micaceous, showing normal grading and horizontal planar lamination.  These levels exhibit tabular geometry and slightly erosive base, dominating the middle and upper parts of the section and defining a coarsening-upward sequence. Additionally, near the base, two layers of fine yellowish sands (<20 cm thick) with gravels and highly erosive surfaces have been identified, containing abundant bioclastic remains, echinoderm spines, siliceous sponge spicules, plant debris and large fragments of Late Miocene calcarenites eroded from surrounding outcrops. The foraminiferal specimens in these layers are relatively scarce, although they exhibit a similar assemblage characterized also by the occurrence of numerous individuals of Cibicides spp. Their tests are commonly broken and/or abraded, which strongly suggests the simultaneous presence of allochthonous marine taxa together with autochthonous groups, providing robust evidence of a high-energy marine inundation of the coastal lagoon.

This stratigraphic succession records the progressive and increasingly frequent arrival of sheet floods into a coastal lagoon from alluvial fans which are likely located at the inland reliefs. The lagoon was connected to the sea and intermittently isolated from the marine basin by a littoral barrier that has not been preserved in the outcrops. During its early stages of evolution at least two high-energy episodes are recorded, in which marine flooding transported sediments from the shallow marine zone, littoral barrier and nearby cliffs into this area. In MIS 3 deposits, that are exceptionally exposed in the Southern Atacama, these findings extend the chronology of major earthquakes and tsunamis and underscore the value of foraminiferal as proxies for coastal dynamics, salinity variability, and high‑energy marine events in the recent Quaternary geological record.

The authors thank project PID2021-127268NB-I00 funded by MCIN/AEI /10.13039/501100011033 and by FEDER/UE