Sedimentological and geochemical study of the late Eocene to early Oligocene Yumaque formation, east Pisco Basin, Peru

Abstract

Yumaque formation biosiliceous sediments resemble modern and Neogene sediments deposited under coastal upwelling conditions. High biogenic silica accumulation rates (up to 69 g$\cdot$cm$\sp{-2}\cdot$yr$\sp{-3}$), preservation of fine sedimentary features, and high original organic carbon, resulted from favorable basin geometries and upwelling of nutrient-rich water. Light biosiliceous-rich and dark detrital-rich alternations occur at scales ranging from millimeters to meters. Mm-scale laminae couplets are interpreted as varves. Spectral analysis of laminae thickness using a varve-calibrated time scale reveals significant variance at periods of 5-6 and 8 years, within the El Nino (ENSO) frequency band, and at 11 years, possibly linked to the 10-12 year solar cycle. Centimeter to meter-scale cyclicity representing between a few hundred to a few thousand years correlates to solar and geomagnetic phenomenon. The Yumaque formation and similar biosiliceous deposits along the eastern Pacific margin may be indicative of widespread late Eocene biosiliceous sedimentation around the Pacific.