Abstract

Limestone beds containing rhythmic parallel laminations occur within a channel-fill sequence exposed locally in the Salem Limestone (Mississippian) of Indiana. The laminations consist of carbonate grains and micrite in layers up to 2 mm thick capped by thin (0.1-0.3 mm) drapes of organic matter. Each couplet was generated by mixed or semidiurnal tidal events, during which carbonate sediment was mobilized and deposited by the dominant tide and organic material settled from suspension during slackwater conditions. Laminae thicken and thin systematically in a vertical sequence as a result of neap-spring tidal current fluctuations that occur during a lunar month. Laminations are well preserved in certain beds ranging from 20 to 40 cm thick through much of the channel-fill sequence. Other beds are extensively bioturbated and contain only a partial record of the laminations. The sequence of bioturbated and unbioturbated units defines a rhythmic pattern, suggesting periods of seasonal environmental restriction that precluded burrowing organisms. Estimation of sedimentation rates based on the neap-spring tidal cycles indicates that deposition within the channel occurred very rapidly (averaging 35 cm/year), a rate four orders of magnitude faster than calculated long-term depositional rates.

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