Stratigraphic Distribution of Fossils In a Tropical Carbonate Succession: Ordovician Bighorn Dolomite, Wyoming, USA

Previous models of the stratigraphic distribution of fossils were based on models of siliciclastic deposition, which also work well for mixed carbonate-siliciclastic settings. Pure tropical carbonate successions differ in that they may keep up with most rates of sea level rise, resulting in aggradational stacking, weakly developed flooding surfaces, and symmetrical sequences. As a result, fossil occurrences in tropical carbonate settings may not show strong clustering of first and last occurrences, even though facies control on faunas may be as strong as in siliciclastic and mixed systems. Stratigraphic sections from the Upper Ordovician Bighorn Dolomite of Wyoming confirm this pattern. The Bighorn consists of three depositional sequences, roughly corresponding to the Steamboat Point, Leigh, and Horseshoe Mountain Members of Goodwin (1964) and tentatively assigned to the C2, C3, and C5 sequences. Only the Steamboat Point and Horseshoe Mountain contain fossiliferous subtidal facies. The Steamboat Point lacks well-developed flooding surfaces and displays a gradual deepening-upward transgressive systems tract (TST) and gradual shallowing-upward highstand systems tract (HST). First and last occurrences are staggered around the maximum flooding zone in the middle of the sequence. The Horseshoe Mountain contains shallow subtidal to peritidal parasequences in upward-thickening (TST) and upward-thinning (HST and falling-stage systems tract: FSST) patterns. A thin tongue of deep subtidal facies occurs at the maximum flooding surface (mfs; Hunt Mountain beds of Macomber, 1970). Owing to nearly aggradational stacking and the presence of unfossiliferous peritidal facies, neither sequence displays a cluster of first or last occurrences at the sequence boundary, but the Horseshoe Mountain displays a cluster of first occurrences at the mfs near the center of the sequence.