Correlation and Analysis of Volcanic Ash in Marine Sediments From the Peru Margin

While land studies have identified the major volcanic centers of historic eruptions and active to recent volcanism within the Central Volcanic Zone (CVZ) of the Central Andes, the tephrachronologic records are disturbed by the high erosion rates of this arid region. However, volcanic material frequently occurs in marine sediment as discrete ash-fall layers and, or disseminated ash accumulations. Cores from three Peru Margin sites sites(1227, 1228, and 1229) drilled during Ocean Drilling Program (ODP) Leg 201 have been studied to determine the occurrence of volcanic ash layers and ash accumulations within marine sediments along the Peru shelf. The thickness of each ash layer and accumulations has been measured and the volumes calculated in order to decipher the episodicity of explosive volcanic activity in the North-Central Andes recorded in the off shore sediments. 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 better 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 684, 680, and 681, whereas forty ash layers have been documented from cores recovered from the same locations (Sites 1227, 1228, and 1229 respectively). Our stratigraphic record agrees with Pouclet, et al., (1990), suggesting that explosive activity began in the early Eocene (35 Ma) 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 and quartz, and minor amounts of 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. 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, New Zealand. 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 from this volcano may occur 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.