MICROSTRATIGRAPHY AND MODES OF OCCURRENCE OF DIPLOPORITAN ECHINODERMS IN THE NAPOLEON QUARRY LAGERSTÄTTE (MIDDLE SILURIAN MASSIE FORMATION, SOUTHEASTERN INDIANA)

The Napoleon quarry Lagerstätte, within the lower mudstone lithofacies of the middle Silurian Massie Formation at Napoleon, Indiana, is marked by exceptional abundance, diversity, and preservational quality of diploporitans. Specimens are preserved primarily as intact thecae; however, most display some degree of post-mortem damage and/or encrustation on at least one side. Detailed microstratigraphic measurement, documentation of in situ specimens and their sedimentary contexts, and focused collection from specific horizons has provided new insights into the complicated taphonomic history of this assemblage. Thecae are found in two primary modes: (1) the greatest abundance of specimens occurs in dense, texturally complex wackestones composed of clastic clay, micrite, and coarse shell material that are interbedded with more pure clastic mudstones; (2) a smaller number of specimens are present in overlying softer, texturally simple, slightly calcareous clastic mudstones containing fine shell debris. Thecae in the first setting are more poorly preserved, with elevated proportions of encrustation, plate jumbling/plate loss, and compaction, whereas specimens in the second setting are more commonly preserved as uncompacted, complete, and un-encrusted thecae. The more texturally complex carbonate beds are interpreted as representing an extended, polyphase formational history associated with periods of slow net sedimentation during which initially buried diploporitan thecae with early diagenetically cemented infill could be exhumed, overturned, encrusted, and/or damaged. This also explains the elevated abundance of specimens in these beds, as the diploporitans represent a time-averaged parautochthonous assemblage. In contrast, the texturally simple clastic beds represent the product of simple episodic obrution events, accounting for the better preservation and lower abundance of specimens.