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Using Tentaculitid Fossils for Environmental and Diagenetic Interpretations (Horn River Formation, Northeastern British Columbia, Canada): Many Questions and a Few Preliminary Answers

Abstract

Tentaculitids are common fossils in carbonate-rich beds within basinal Devonian rocks. Whereas other fossils are problematic in their use for geological study – size, abundance, preservation, etc. – tentaculitids are large enough to be studied with a handlens, are well preserved, and are often sufficiently abundant to be of use. Here, we present preliminary results of our exploration of tentaculitids as indicators of paleoenvironmental and diagenetic parameters in cores from the Middle to Upper Devonian Horn River shale sequence, British Columbia. Although the life habit of tentaculitids is uncertain (i.e. benthic or pelagic), they remain useful in environmental interpretation. Cross-bedded and/or imbricated deposits of tentaculitid fossils indicate concentration by water currents during reworking, transport, and/or deposition, whereas individual, sporadic fossils are likely those that were either in-situ (if benthic) or settled from the water column (if pelagic). Non-imbricated concentrations of tentaculitids, in the absence of fabrics indicative of transport, may have resulted from “blooms” of these tiny organisms or a decrease in sedimentation rate. Processes that effected the preservation of tentaculitid shells are also clues to environmental and diagenetic interpretations. The degree of abrasion and fragmentation is a potential indicator for the intensity of currents and/or relative distance of transport. Dissolved shells were a potential source for carbonate cement, and provide information about the variability of primary allochem dissolution through a stratigraphic section. A bed of uncompacted specimens may indicate early cementation within that bed; ovate to flattened specimens suggest relative degrees of compaction, on small scales, with a stratigraphic interval. However, care must be taken when considering taphonomy in environmental and diagenetic interpretations; recrystallized and highly compacted specimens can easily be misinterpreted for dissolution and fragmentation. Other phenomena such as the density of fossils, size ranges, taxonomic composition, and shell plasticity are also under investigation, but without preliminary conclusions at the time of writing. Furthermore, biostratigraphy of Western Canadian tentaculitids is in its infancy; future work will build a biostratigraphic framework for these important basinal fossils.