Page 431 - Carbonate Facies in Geologic History

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Plate V. Mound Bamestone and Whole Fossil Wackestones

(A) Baffiestone, standard microfacies 7. Tubular-cylindrical shaped sponges and
sponge-like organisms occur in micritic mounds in the Cambro-Ordovician strata
in North America. Such mounds also may have stromatolitic structures. Follow-
ing Embry and Klovan's (1971) interpretation, the trapping and baffiing action of
such a loose framework results in accumulation oflime mud and mound buildups
like those seen around dendroid corals in the Devonian and Mesozoic. Fenestrate
bryozoans and crinoids serve this function in the Middle Paleozoic. This sample
and photograph, courtesy of D.F. Toomey, University of Texas Permian Basin, is
from the Lower Ordovician Arbuckle group, Arbuckle Mountains, on U.S. High-
way 77 between Anadarko and Davis in southern Oklahoma. Thin section, x 3

(B) Whole fossil wackestone, standard microfacies 8. Sample is from a bio-
strome of Monopleura in the middle Glen Rose formation (Albian, Cretaceous) at
the upper end of Medina Lake, Medina County, Texas. The bed is a light-colored
argillaceous micrite and contains abundant whole monopleurid rudists which
have been somewhat crushed by compaction. Note the preservation of the un-
compacted pelloidal fabric within the protection of the shells compared with the
micrite texture between the fossils. The Monopleura also formed a baffiestone
texture, being always in small mounds or biostromes. The occurrence of well-
preserved whole fossils in micrite matrix indicates generally in situ burial and a
very quiet water environment. Thin section, x 10