THE OCCURRENCE OF VERTEBRATE AND INVERTEBRATE FOSSILS IN A SEQUENCE STRATIGRAPHIC CONTEXT: THE JURASSIC SUNDANCE FORMATION, BIGHORN BASIN, WYOMING, USA
The Sundance Formation contains three depositional sequences. The lowest sequence consists of three parasequences, each consisting of a marine mudstone that grades upward into tidal-flat and ooid-shoal carbonates. The third of these parasequences contains abundant Gryphaea. The base of the second sequence is marked by a tan sandstone overlain by a bioclastic grainstone, or by several meter-scale parasequences capped by a bivalve-crinoid grainstone. These bioclastic units are overlain by a thick belemnite-rich gray-green shale, which contains oyster-rich beds near its top. Sandstone concretions occur above and below the oyster-rich beds, but only the lower concretions are fossiliferous, with a rich assemblage of ammonites, bivalves, belemnites, orthocerids, plesiosaurs, and ichthyosaurs. Much of the vertebrate material is articulated and well-preserved. Vertebrates are also found in the shale directly below the oyster beds, but this material is typically disarticulated and poorly preserved. Above the oyster shoals are mudstone beds that pass upwards into shoreface wave-rippled sandstone. The final sequence of the Sundance is an incised valley-fill with coarse-grained, cross-bedded sand and shell units, commonly with a basal lag containing abraded vertebrates, belemnites, iron nodules, and mudstone rip-up clasts. The terrestrial Morrison Formation unconformably overlies this incised valley fill.
Invertebrate and vertebrate fossils occur in predictable sequence stratigraphic positions in the Sundance, such as in shallow-water facies that cap parasequences, concretions that represent sediment starvation near the maximum flooding surface, and as lag deposits at the base of incised valley fill deposits. Many of these occurrences match previous studies on invertebrates as well as a study of Eocene whales by Peters et al. (2009).