Correlation and Analysis of Volcanic Ash in Marine Sediments From the Peru Margin: Explosive Volcanic Cycles of the North-Central Andes

To decipher the episodicity of explosive volcanic activity in the North-Central Andes, we have measured the thickness and calculated the volume of ash layers from sites drilled along the Peru margin during Leg 201 of the Ocean Drilling Program (ODP). The geographic distribution of the sites (over 3 degrees of latitude and from 50 to 300 km offshore) and correlation of ash units between sites form the basis for minimal estimates of explosive volcanic activity in the region (only eruptions large enough to deposit ash in excess of 100 km from source are represented). Pouclet et al., (1990), estimated the minimum explosive activity along the Andean Arc from ash-bearing sediments and ash layers within cores from sites along the Peru margin collected during ODP Leg 112. As a result of higher recovery (as much as ten times more core recovery in many intervals) and decreased disturbance in cores recovered during Leg 201, our documentation of ash content in cores from Leg 201 has led to a more complete record of the explosive volcanic activity along the Andean Arc. For example, Pouclet, et al., (1990), reports four ash layers from Sites 680 and 684, whereas we have documented fourteen ash layers from cores recovered from the same locations (Sites 1228 and 1227, respectively). Our stratigraphic record agrees with Pouclet, et al., (1990), suggesting that explosive activity began in the early Eocene ( ∼35Ma) and continued with explosive pulses during the Miocene. The greatest explosive activity occurred within the past 5 million years, with peak activity in the late Pliocene to early Pleistocene. Based on petrographic and geochemical analysis, most of the volcanic ash within cores from Leg 201 was derived from the Andean volcanic arc. These plinian eruptions produced acidic glasses and ash layers with abundant feldspar, hornblende, and biotite. Pouclet, et al., (1990), reports a transition from andesitic volcanism in the Middle to Late Miocene to a more shoshonitic composition through the late Pliocene to early Pleistocene during peak volcanic activity. However, ash layers from at least one drilling location (Site 1228) bracketed by biostratigraphic and oxygen isotope dates may correlate with the ~254 kyr eruption of Taupo. Previous studies have recognized the presence of tephra from this eruption in cores as far away as 1100 km from the source, and suggest that ash may be found in sediments off South America (Froggatt, et al., 1986). REFERENCES Froggatt, P.C., Nelson, C.S., Carter, L., Griggs, G., Black, K.P., 1986. An exceptionally large late Quaternary eruption from New Zealand. Nature (London), 319; 6054: 578-582. Pouclet, A., Cambray, H., Cadet, J.P., Bourgois, J., De Wever, P., 1990. Volcanic ash from Leg 112 off Peru. Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 112: 465-480.