DATING OF A HIGH-ELEVATION PLEISTOCENE MARINE SURFACE IN NORTHERN CHILE – IMPLICATIONS FOR THE AGE AND ORIGIN OF THE CENTRAL ANDEAN RASA

The central Andean rasa, a prominent coastal landform in northern Chile and southern Peru, is a wide, seaward-sloping erosional surface reaching elevations of ~90 to ~130 masl at its inner edge. The rasa is commonly backed by a coastal cliff that in places exceeds a kilometer in height. This landform is a potential indicator of uplift magnitudes and rates, but its inner edge has proven difficult to date. Different assumptions about its age have led to different interpretations of its genesis and uplift history.

We report ages from solitary corals (Sphenotrochus sp.) and molluscs (Leukoma sp.) in marine terrace sediment on a portion of the rasa near its southernmost extent at Morro de Copiapó (27°10’S) using multiple geochronometers. The terrace, with an inner edge at ~125-130 masl, contains warm-water faunal elements (Marquardt et al., 2004, Tectonophysics 394, 193) that have been interpreted to indicate an MIS 11 (~425-375 ka) age for the formation of the uppermost portion of the rasa. However, a combination of coral uranium-series ages, coral and mollusc Sr-isotope values, and nearly racemic mollusc amino acid D/L ratios indicates an older age, most likely 1.0 Ma, and possibly as old as 1.2-1.4 Ma .

These ages suggest that rasa formation was initiated during the early Pleistocene, and that long-term uplift rates have been slower by a factor of about two than those envisioned by earlier workers (e.g., Regard et al., 2010, EPSL 297, 199). The extensive rasa may have resulted from multiple reoccupations of the surface by successive interglacial high sea-level stands of the early Pleistocene, or the early part of the “Mid-Pleistocene Transition”, when glacial-interglacial cycles were modulated by ~41 kyr orbital forcing. At low uplift rates, the minimal amount of uplift during a 41 kyr interval between interglaciations may have led to repeated reoccupations of the surface, resulting in its broad extent, as proposed by Melnick (2016, NatGeo 9, 401).