The Middle Miocene Grand Bay Formation is exposed on the eastern half of the island of Carriacou, Grenada Grenadines. The formation was deposited in water depths of greater than 150 m and is mainly composed of a sequence of bioclastic and volcanogenic turbidites. The dominant rock type in the Grand Bay Formation is fine- to coarse-grained sandstones. The volcaniclastic sandstones are poorly sorted and immature, and contain volcanic clasts, clinopyroxene, amphibole, plagioclase feldspar and opaque crystals in an argillaceous matrix. Within the Grand Bay Formation are beds of subaequeous lapilli and ash tuff representing pyroclastic fall deposits that have a mineralogy that is similar to the heavy minerals in the turbidites. The mineral assemblage of the lapilli tuffs suggests that these eruptions were basaltic andesite or andesite in composition. It is postulated that the volcanogenic turbidites in the Grand Bay Formation formed as a consequence of volcanic eruptions along the southern Lesser Antilles arc during the Early to Middle Miocene. The lithic volcanic fragments and mineral composition of the volcaniclastic turbidites indicate a monomagmatic source in which reworked pyroclastic deposits were temporarily stored on a shallow shelf prior to deposition downslope, in a deep water basin.
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