Volcanoes of the CVZ of the Central Andes are responsible for producing some of the largest volcanic deposits within the Andean chain (e.g., Lindsay et al., 2001; de Silva and Francis, 1991; Francis and de Silva, 1989; Sparks et al., 1985; Hall and Calle, 1982; Lahesen, 1982; Baker, 1981; Tosdal et al., 1981; Baker and Francis, 1978; Noble et al., 1974). The large ignimbrite deposits of this region are of the most significance for this study because they indicate the likelihood of airborne ash (Baker and Francis, 1978) and large plinian-style eruptions, which can produce widespread air fall ash (Fisher and Schmincke, 1984), such as that deposited off the Peru coast and present in cores from all three Leg 201 sites studied.
From the detailed examination of Leg 201 cores drilled off the coast of Peru, we have documented ash layers in Miocene, Pliocene, Pleistocene, and Holocene sediments that amplify the signal of eruptive activity from the Andes with nearly 25 times more primary ash layers reported than in previous marine ash layer studies. We also note an even greater thickness of diagenetically altered ash deposits that are likely representative of major explosive volcanic events owing to the distance offshore of our study area.
The comparison of Leg 201 ash layers with land and marine studies is based on both whole-rock and glass geochemistry. These comparisons can suggest which volcanic zone(s) of the Andes provide the provenance for the ash layers but cannot determine the individual volcano from which these layers were erupted. We surmise that the majority of the ash layers we analyzed have affinities with compositions from the CVZ and are not likely derived from the NVZ or SVZ.
The explosive cycles of Andean volcanoes are recorded in the marine sediments of all Leg 201 sites studied, as well as within the voluminous deposits of ignimbrites on land. Each of these regions has been studied and documents concurrent cycles of explosive activity. In general, the land records of Baker and Francis (1978) and Francis and Hawkesworth (1994) suggest that the greatest volume of eruptions was experienced during the Miocene (1300 km3), with a significant decrease in activity during the Pliocene (650 km3), and even less volcanic ash was deposited during Pleistocene and Holocene (275 km3) within the northern area of their study regions. Whereas our Type 1 ash record records only a single event in the Miocene with significant ash deposited, there are at least three more layers of diagenetically altered ash in this section. Type 1 ash layers are most common our Pliocene record, and there is a lower number of ash layers and cumulative ash layer thickness in Pleistocene–Holocene sediments. Additionally, three altered ash layers (containing between 15% and 60% biogenic material) are present in Holocene sediments at our the southern sites, indicating the possible occurrence of three large-magnitude eruptions not preserved within the land record for this time period.