Active emergence, chronology, and limestone facies in southeastern windward Barbados

Abstract

ABSTRACT Barbados is actively rising in the latest phase of a long history of emergence that began as far back as 15 Ma. The current phase began at or before ca. 700 ka, is highly nonuniform, and at least locally, has been nonsteady. The uplift rate field in SE Barbados ranges between near-zero and 0.47 m/k.y. and is harmonic to active structures of NNW-SSE contraction. Emergence markers include limestone strata, coral, and shoreline angles, but we used only shoreline angles in calculations. We divided the capping limestone of windward Barbados into 10 units using physical criteria and dated them with over 40 230Th ages as oxygen isotope stages 5a, 5e, late 7 and early 7, and old (older than 300 ka). The oldest unit is a relic of an earlier phase of emergence. Younger units, probably as old as 700 ka, downlap the eroded flank of the oldest unit and sublimestone foundation. Younger units comprise landward clastic facies deposited on abrasion platforms during eustatic highstand and seaward-coalescent fringe reef blankets deposited on preexisting slopes, mainly in transgression. Earlier models of ridged reefs of catch-up growth origin are not supported in windward Barbados. Shoreline angles, the updip tips of terrace floors and of younger limestone units, are isochronous markers of maximum highstand levels. Despite the lack of direct determination of their ages, shoreline angles provide the truest measures and highest values of emergence. Coral thought to indicate highstand growth gives moderately lower uplift rates due to depths of growth and collapse. Coral grown during transgression gives a marked error in emergence.