Abstract

The interval between the Lower and Upper Block Coal Members of the Brazil Formation (Lower Pennsylvanian, Indiana) records a transgression of brackish water tidal deposits over a coastal domed(?)-peat field, followed by a regression, and then re-establishment of the peat-forming environment. Within the study area, the interval averages 8 m in thickness and consists of a basal sequence of laminated mudstone that coarsens upwards to thinly interlayered sandstone and mudstone (lenticular, wavy, and flaser bedding). This is capped by an upward-fining sequence into a rooted mudstone. Sedimentation rates can be estimated by derailed thickness measurements of vertically stacked mudstone laminae and sandstone layers that thicken and thin rhythmically. This rhythmic thickness variation can be related to neap-spring tidal-current cyclicity and indicates that sedimentation varied from 1 cm/yr to 1 m/yr. Locally, as much as 8 m of accommodation space for the sediments appears to have been generated and mostly filled in a matter of decades to a few centuries. Such rapid rates of filling indicate the possibility that the accommodation space was generated by compaction of the transgressed peat and mud-rich sediments.

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