95 Archaeological Geology (Posters)

Monday, 10 October 2011
9:00 AM - 6:00 PM
Hall C

ORIGINS OF VOLCANIC BALLAST STONES – CARA (QUEDAGH) MERCHANT SHIPWRECK 1699 ISLA CATALINA, DOMINICAN REPUBLIC

Paper number: 95-10
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Geochemistry and petrology of ballast stones excavated from the 1699 wreck site of the Cara Merchant, and the beach zone behind the wreck, were analyzed for evidence of the ship’s origin. Fixed ballast has the potential to be preserved with other shipwreck remains along with the secondary, or temporary, ballast. Following the route of the Cara Merchant from its construction in Surat, India near the mouth of the Tapi River, a literature compilation of possible source materials for the volcanic ballast stones was developed for geochemical comparison with representative ballast types excavated from the shipwreck. Upstream from Surat, volcanic gravels weathered from the Deccan Traps are found along with sand-sized particles. Hydrological and fluvial studies indicate that monsoon rains are adept at moving large quantities of gravel along the upper and middle Tapi River drainage, though the development of hydroelectric facilities have altered the transport in recent decades.

Based on a the compositional fields of the SiO2 vs. Na2O + K2O discrimination diagram, 88.0 % of the 55 volcanic samples excavated, plot in dacite to rhyolite composition fields with 10.9 % plotting in the andesite field. One sample plots on the boundary between the basalt-picrabasalt fields. The remaining 22 ballast stones excavated represent sedimentary and metamorphic lithologies and coral gravels. Volcanic ballast samples were analyzed on a WinFloyd1 diagram and Harker plots against data from the Deccan and Madagascar rift volcanics, both of Cretaceous age, and with data from the volcanic rocks of the Greater and Lesser Antilles. These data included the Cretaceous volcanic arc rocks from eastern Hispaniola, and the arc volcanics of Martinique. The evaluation of the data suggests at least one of the ballast stones excavated from the shipwreck is of Deccan composition. Many of the remaining volcanic ballast stones, approximately 64.3%, appear to be sourced more locally from the Cretaceous age island-arc volcanics of the Cordillera Oriental, Dominican Republic.