Sedimentology and taphonomy of a Syracosaurus bonebed in the late cretaceous Judith River Formation, Dinosaur Provincial Park, Alberta

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Within the circa 90 m thick Judith River Formation (Campanian) sequence exposed in Dinosaur Provincial Park, bonebeds represent the most common mode of occurrence of vertebrate fossils. Bonebed 42 is located in the south-central portion of the Park. It encompasses an area of 550 m2 and has a thickness ranging from a few centimetres to approximately 20 cm. A total of 373 vertebrate elements was excavated from 46 m2 of Bonebed 42, with an average density of eight bones per square metre. Most (57%) of this vertebrate material is ceratopsian, of which nearly a quarter has been identified as Styracosaurus. A minimum count of ten adult Styracosaurus individuals has been established, based on nasal horncores collected from Bonebed 42. These animals may have been killed by a catastrophic event such as drowning while attempting to cross a river in flood. The sedimentary sequence above and below Bonebed 42, which has a total thickness of 27.5 m, comprises three dominant lithofacies: 1) erosionally based, trough cross-bedded, fine-grain ed sandstone, 2) inclined heterolithic strata consisting of alternating thin beds of fine-grained sandstone and mudstone with original dips of up to 29°, and 3) massive mud s tone. These facies are interpreted as components of a fluvially-dominated, meandering channel system and represent channel bottom, point bar and overbank deposits, respectively. Mud-filled channels associated with steeply dipping mud-dominated inclined heterolithic strata are probably the product of meander loop cutoff. Channel depths ranged from 5 m to 23.5 m with widths of 30 m to 165 m at meander bends. Mean paleoflow was to the southeast (155°). Bonebed 42 occurs within a set of inclined heterolithic strata and is interpreted to be the result of rapid deposition upon a point bar surface during a major flood. The bones lie in an intraformational conglomerate consisting of small to very large mudstone pebbles with a matrix of fine-grained sandstone. Vertebrate material and mudstone pebbles were hydraulically equivalent and comprised a bedload traction carpet. Sand was transported by saltation forming a hydrodynamically distinct particle population. The deposit is poorly sorted, especially on upper point bar levels where both pebbles and bones are largest. All of the vertebrate material at Bonebed 42 is disarticulated and limb bones (Voorhies Group 2) are under-represented relative to vertebrae and ribs (Voorhies Group 1). Elongate bones display preferred orientations with their long axes either parallel or perpendicular to paleocurrent direction. Varying degrees of bone breakage and rounding are indicative of multiple source areas and/or reworking from older deposits.
Bibliography: p. 144-150.
Visser, J. (1986). Sedimentology and taphonomy of a Syracosaurus bonebed in the late cretaceous Judith River Formation, Dinosaur Provincial Park, Alberta (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from doi:10.11575/PRISM/11601