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The origin and meaning of blackened limestone grains and lithoclasts that occur throughout the geologic record have long been a mystery. The Pleistocene-Holocene unconformity and those within the Pleistocene throughout the Caribbean are often characterized by the presence of blackened limestone lithoclasts. Thoroughly blackened fragments may consist of laminated soilstone crusts (i.e., caliche or calcrete), coral, or oolitic, pelletal and skeletal grainstone derived from the underlying limestone. Blackened fragments occur sporadically or in pockets comingled with nonblackened but otherwise identical fragments. Simple cooking experiments with typical Pleistocene and Holocene limestone fragments showed that only laminated soilstone crusts, poorly cemented pelletal and oolit c grainstone, and aragonitic coral fragments are selectively blackened, whereas well-cemented, nonaragonitic fragments retained their light color. Blackening is caused by charring of organic matter within the rock. Heat from forest fires and smoldering humus accumulations is interpreted to cause the naturally occurring blackened lithoclasts.
Fire-blackened limestone lithoclasts differ from the more well-known salt-and-pepper sands, which typically result from selective blackening of individual Foraminifera, mollusk fragments and other fossils under subtidal conditions. Subtidal blackened grains are associated usually with unconformities and tidal channel deposits where they become mixed with unstained grains. Correct identification of the 2 differing types, when detected in ancient limestone, offers important environmental information, not only to distinguish marine and subaerial unconformities, but for clues to paleoclimate, vegetation, and soil development.
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