Quaternary shorelines in southern Peru: A record of global sea-level fluctuations and tectonic uplift in Chala Bay

Abstract

The coastal area bordering the bay of Chala (14°50′S., 74°15′W.), southern Peru, contains one of the most complete sequences of Quaternary shorelines in South America. Remnants of about 27 high seastands have been preserved between present mean sea-level and +275 m. Most remnants consist of staircased marine terraces and associated deposits which are partly covered by alluvial fan and colluvial units deposited during intervening periods of lower sea-levels. No geochronological data are yet available; a tentative chronostratigraphy of the terrace sequence is based on the geometric and stratigraphic relationships between successive landforms, and deposits. We group most marine terraces into 15 ‘major morphostratigraphic units’ (MMUs). Some of these major units seem to correlate with interglaciations, for example Isotopic Stage 5, or with interstadials (Stage 3). Higher in the Chala sequence, major morphostratigraphic units mapped during field and air-photo studies may represent only parts of Middle Pleistocene interglaciations. We infer that shorelines located at +68, +121, +168, and +184 (or +200) m correlate with the highest seastands of Isotopic Stages 5, 7, 9, and 11, respectively. The proposed chronostratigraphy suggests an average uplift rate of ca. 460 mm/kyr for the Chala Bay area during the last 500 ka. This rate is slightly higher than in the surrounding coastal areas (ca. 270–350 mm/kyr), but significantly lower than rates in the area where the Nazca Ridge is being subducted below the South American Plate (maximum rate ca. 740 mm/kyr). Deformed shorelines evidence several fault displacements within the Chala basin, but such deformation does not seriously disort the record of former sea-levels in the basin. The Chala terrace sequence is the first reliable record of Middle and Late Quaternary sea-level fluctuations described from Peru.