Ultrapetrography of a miocene chalky limestone, St. Croix, U.S. Virgin Islands
Bakos, Nancy A.
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The Kingshill Marl, which crops out on the island of St. Croix in the U. S. Virgin Islands, is a sequence of interbedded chalks and turbidites. The chalks are composed almost entirely of the tests and fragments of tests of planktonic foraminifera and calcareous nannoplankton, and the interbedded turbidites consist of unsorted volcanogenic terrigenous debris and coral fragments, with a marly matrix. The Kingshill Marl was deposited during Miocene time in a shallow trough of fault origin, which trended northeast across the island. To the east and west of the trough, Cretaceous volcanogenic rocks formed highlands that supplied detrital material to the basin of deposition. A modern-day sedimentary analog to the chalky fraction of the Kingshill Marl exists on the island slope off of Canebay, on the northwest shore of St. Croix. The sediment here consists of the tests of planktonic foraminifera and calcareous nannoplankton, but also contains abundant aragonitic pteropods and sponge spicules of opaline silica. The absence of pteropods and sponge spicules in the Kingshill Marl suggests that some process must have acted to dissolve them, and suggests a shallow, warm water environment of deposition for the Kingshill Marl.