One of the major discoveries of Leg 165 drilling in the western Caribbean was the great abundance of volcanic ash layers interbedded with dominantly pelagic sediments (Sigurdsson, Leckie, Acton, et al., 1997). In total, more than 2000 ash layers from Sites 998, 999, 1000, and 1001 were identified during shipboard core description. Site 999 contained the most complete record of tephra deposition with excellent recovery of sediments spanning ~70 m.y. Most of the layers were produced by the fallout of ash from the atmosphere during large-scale explosive eruptions. They consist of abundant clear glass shards, crystals, lithics, and alteration products, such as smectitic clay. Most layers are several to tens of centimeters thick with sharp bases and bioturbated tops. However, some were deposited from sediment gravity flows, implying a more proximal source of material.
The presence of ash or tephra fall layers define two major peaks of explosive volcanism during the early to mid-Miocene and the mid-to late Eocene (Sigurdsson, Leckie, Acton, et al., 1997). It has been proposed that the source area for the Miocene tephra layers was an extensive Tertiary volcanic province that extended through Central America and produced large volumes of explosive volcanism, now preserved as voluminous ignimbrite deposits. The cumulative thickness of Miocene tephra layers in the western Caribbean sites also suggests that the main dispersal axis of tephra fallout was located somewhere near Site 999 in the Colombian Basin.
In general, the grain size and thickness of tephra fall layers are determined by a combination of prevailing wind strength, initial size range of tephra, and eruption column height (Carey, 1996). Thus, if the atmospheric circulation in an area is known, grain-size and thickness parameters can be used to infer the location and characteristics of the source eruptions. In this paper we present a study of the grain size of tephra layers from the major Miocene peak in explosive volcanism preserved at Sites 998, 999, and 1000. The work focuses on using crystal size as a parameter for evaluating the dispersal characteristics of individual tephra layers. Results of the work are integrated with existing information about the atmospheric circulation in the study area to evaluate the potential source areas of the layers and to make inferences about the nature of the source eruptions.