There are two potential sources of Miocene to Holocene volcanic ash in the western Caribbean: (1) Central America and southern Mexico, and (2) the Lesser Antilles volcanic arc to the east. In Central America there are two major Tertiary ignimbrite formations that extend from the Mexican border in the north through Guatemala, El Salvador, and Honduras, and into Nicaragua over a region that is more than 800 km in length and up to 300 km wide. The ignimbrite formations underlie upper Tertiary and Quaternary andesitic and basaltic andesite stratocones of the active Central American volcanic arc (Reynolds, 1980). However, the mid-Tertiary ignimbrite-bearing formations generally extend far east of the active arc. The major formations include silicic welded tuffs of the Chalatenango Formation in south-central Guatemala and El Salvador (Reynolds, 1987; Wiesemann, 1975), thick rhyolitic ignimbrites of the Padre Miguel Group in southeast Guatemala and Honduras (Reynolds, 1980), and the thick siliceous ignimbrites of the Coyol Group in Nicaragua (Ehrenborg, 1996). Reliable 40Ar/39Ar single crystal biotite ages for ignimbrites within the Coyol Group cluster from 12.3 to 18.4 Ma, defining an early-middle Miocene volcanic episode. Many of the formations are linked to caldera structures and have aggregate thicknesses of several hundred meters. Some of the co-ignimbrite tephra from eruptions of this ignimbrite province was transported into the Pacific where it was recovered off Guatemala during Deep Sea Drilling Project (DSDP) Leg 67 (Cadet et al., 1982a), and near the Middle America Trench off Mexico during DSDP Leg 66 (Cadet et al., 1982b).
The Lesser Antilles arc has been the site of explosive volcanism since at least the Eocene with the formation of tephra layers in marine sediments of the western equatorial Atlantic and eastern Caribbean (Natland, 1984; Sigurdsson et al., 1980; Sigurdsson and Carey, 1981). Drilling during Leg 78A of the DSDP in the Lesser Antilles forearc recovered siliceous volcanic ash layers with peak occurrences in the lower to middle Miocene, lower Pliocene, and upper Pleistocene (Natland, 1984). However, the main dispersal direction for the majority of fall layers is to the east into the Atlantic (e.g., Carey and Sigurdsson, 1980). This pattern appears to be a direct result of the importance of high-level transport where the strong winds blow from west to east. Minor transport of tephra has occurred to the east from the Lesser Antilles, but the amount has been insufficient to form discrete layers in the sediments beyond 100 km from the arc. The distribution of Quaternary tephra layers in the Lesser Antilles region thus provides an important analog for atmospheric transport processes occurring in the western Caribbean.
In view of the eastward dispersal of tephra from the Lesser Antilles and its great distance from the Leg 165 sites (>1800 km) we can rule out this area as the source of numerous tephra fall layers at Sites 998, 999, and 1000. It is, on the other hand, most likely that the source area of these layers is the widespread Tertiary volcanic province of Central America. This is supported by (1) the prevailing upper wind patterns and (2) the great abundance of Miocene-age ignimbrite deposits. Derivation of some layers from the Mexican ignimbrite province of the Sierra Madre Occidental is a possibility. However, the age of voluminous ignimbrite volcanism in the area (27-34 Ma; McDowell and Clabaugh, 1979) appears to be too old to explain the majority of Miocene layers. Further support for the Central American source area can be found by considering the measured feldspar sizes in relation to the likely distances from source eruptions.