Fluvial processes and vertebrate taphonomy: the upper cretaceous Judith River formation, South-Central dinosaur Provincial Park, Alberta, Canada
Abstract Superb three-dimensional exposures of the Upper Cretaceous Judith River Formtion in the badlands of Dinosaur Provincial Park, Alberta offer excellent opportunities for the integration of sedimentological and architectural analysis of an ancient coastal plain sequence with taphonomic investigations of rich vertebrate assemblages. Based upon two detailed lithofacies studies, a fluvial, meandering-channel depositional model has been developed for the upper 60 m of the Judith River sequence in the south-central area of the Park. Using this model, major taphonomic modes of occurrence of vertebrate remains found within the study sequences are readily explicable in terms of the sedimentological context of their host lithofacies. The sequences studied are composed of four main lithofacies: (1) trough cross-bedded sandstone; (2) large-scale inclined heterolithic strata (IHS); (3) sandstone with large-scale inclined bedding (IBS); and (4) shale. IHS and IBS units are interpreted as products of point-bar lateral accretion within large, meandering, freshwater-dominated, coastal plain rivers. Trough cross-bedded sandstone sequences are recognized as being genetically related to overlying IHS (or IBS) units, and formed as contemporaneous channel bottom and lower point-bar deposits. Shale sequences represent a variety of floodplain deposits including immature paleosols. Within the detailed study areas vertebrate fossils occur as articulated skeletons, bonebeds and isolated skeletal elements. All of these three major taphonomic occurrence modes are dominantly associated with lower levels of trough cross-bedded sandstone sequences indicating preferential preservation of vertebrate material at deeper levels within palaeochannels. Three categories of bonebeds have been recognized within the sequences studied: (1) high diversity, and (2) low diversity, macrovertebrate accumulations; and (3) microvertebrate sites. High diversity bonebeds occur at or near the bases of trough cross-bedded sandstone sequences and formed as channel lag deposits. Two low diversity bonebeds, one dominated by the disarticulated remains of Styracosaurus, the other by Centrosaurus, are associated with pointbar deposits. Both are believed to represent the end products of mass deaths of herding ceratopsians, perhaps by drowning as animals attempted to cross flooded rivers. Microvertebrate sites, generally associated with trough crossbedded sandstone sequences within the areas studied, represent mechanical accumulations of the most resistant skeletal components. The presence of both “fresh” and abraded vertebrate elements within most of the bone-bearing lithofacies units reflects variations in bone sources and transportation histories. Highly abraded “bone pebbles” represent skeletal elements that underwent several cycles of river transport, deposition and exhumation. From observations made outside the areas studied in detail, the integrated sedimentologic-taphonomic model proposed herein appears to be valid for correlative strata elsewhere within Dinosaur PPark. Thus the model may serve as a useful aid for future taphonomic and palaeoecological research at this famous dinosaur site.