CALCAREOUS ALGAE OF THE LATE ORDOVICIAN RED RIVER FORMATION AND THEIR BIOSTRATIGRAPHIC SIGNIFICANCE

Paleozoic calcareous algae are potentially useful biostratigraphic tools but remain under-researched in the United States, likely due to presence of graptolites, conodonts, brachiopods and other fossils which are commonly used in high-resolution biostratigraphy. The subsurface Upper Red River Formation of the Williston Basin provides an opportunity to study the calcareous algae of the Late Ordovician. The Lower Red River Formation (Tyndall Stone) is found exposed in Canadian outcrops, but the entire formation is deeply buried in the United States, and representative cores are very rare. The Upper Red River is composed of three shallowing upwards sequences (C, B, and A intervals, respectively); each sequence is composed of burrowed subtidal limestones, laminated carbonates and capped by anhydrites. The anhydrite is developed only locally in A interval.

For this study 218 thin sections from 18 cores were analyzed for calcareous algae. Dasyclad and codiacean algae were identified in 83 thin sections. The Dasyclad Vermiporella is found throughout the C burrowed member in skeletal wackestones (shallow subtidal). The codiacean genus Dimorphosiphon is abundant in algal wacke-packstones of the B burrowed interval (shoal water). Other green algae are present in both B and C intervals but are difficult to identify due to recrystallization as a result of deep burial diagenesis; these likely include members of the genus Hedstroemia. Other common microfossils include chaetetid sponge Solenopora, coral Tetradium, poorly preserved Girvanella? tubules, bryozoan Stictopora, and fragments of other bryozoans, articulate brachiopods, crinoids, and rugose corals.

The abundance of Dimorphosiphon and its exclusive occurrence within the B interval enables establishment of the Dimorphosiphon Taxon-range Zone. The zone is defined based on the first and last occurrence of Dimorphosiphon, which corresponds to the base of the B burrowed and the top of the B laminated members. Members of the genus Dimorphosiphon are rarely found as single organisms and are more typically found as skeletal accumulations commonly in association with fragmented brachiopods. This suggests that Dimorphosiphon had a major role in carbonate production, similar to its relative Halimeda today.