AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Importance of Dimorphosiphon and Other Skeleton-Building Green Algae for Facies and Biostratigraphic Studies of the Late Ordovician Red River Formation, Williston Basin
(1) Geology, St. Lawrence University, Canton, NY.
Paleozoic calcareous algae are potentially useful biostratigraphic proxy 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. It is the unusal abundance of green, skeleton-building algae within the B-interval of the subsurface upper Red River Formation, Williston Basin that is interesting, because it must reflect some important paleoenvironmental control as well as a major role in carbonate production during that stratigraphic inverval. 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 codiacean genus Dimorphosiphon is abundant in algal wacke-packstones of the B burrowed interval (shoal water). Other green algae were 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, bryzoan Stictopora, and fragments of other bryozoans, articulate brachiopods, crinoids, and rugose corals. Dimorphosiphon was studied in detail in more than 100 randomly oriented thin sections, and the following measurements were made: thallus diameter (1.5-2mm), longitudinal tube diameter (0.15-0.30mm, radial tube diameter (generally less than half the average longitudinal tube diameter), cortex thickness (~3 mm, varies from 0.1 mm to more than 5 mm). Number of longitudinal tubes varied with specimens and could not be determined with certainty in many due to randomness of sections.
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 base of the B laminated members. The previously described occurrence of conodont Aphelognathus divergens in association with Dimorphosiphon from Fort Gary Member, Manitoba yields a Mid-Late Richmondian age for the B-interval of the Red River Formation (Amorphognathus ordovicicus conodont zone).