FLYNN, Brian C ('98) (Advisor: Connie Soja)
In Glacier Bay National Park (GBNP), samples were collected along three stratigraphic sections to determine the paleoecology of the Silurian clam Pycinodesma. Each section contained several beds dominated by Pycinodesma, which forms up to 50% of the rock volume. Associated beds comprise extensive burrows and low diversity suites of gastropods, rugose corals, massive stromatoporoids, and Amphipora. Petrographic analysis reveals that peloidal wackestones/packstones predominate and are interbedded with rare grainstones and thrombolites. Moderate- to well-sorted "peloids" are the dominate grains in these micritic rocks but do not appear to be of fecal origin. Instead, the "peloids" formed in a multi-step process involving fragmentation and micritization of skeletal grains. This mechanism, induced by endolithic (microbial) encrusters like Girvanella, led to the formation of an oncolitic cortex and boring into the shell's exterior prior to the partial or complete micritization of the grain.
Based on these characteristics, the sections of the Willoughby Formation are interpreted to represent a restricted lagoon on a shallow-marine, rimmed platform. Relatively low-energy subtidal conditions are implied by the abundant micrite, whole gastropods, in situ orientation of whole, articulated pelecypods, Amphipora with well-preserved delicate branches, and an overall lack of intertidal indicators. The low-diversity faunas, which generally lack stenohaline brachiopods, crinoids, bryozoans, and trilobites, suggest that the restricted lagoon was inhospitable to many normal marine organisms because of fluctuations in temperature, salinity, and (or) oxygen concentrations. Higher energy events involving strong waves and currents possibly associated with periodic storms are reflected in the high degree of skeletal fragmentation (micritized grains), rare grainstones deposits, and symmetrical oncoids. These assemblages are unlike most other shallow marine (B.A. 2-3) Silurian communities, suggesting that unusual environmental conditions enabled Silurian bivalves in Glacier Bay to dominate. Their adaptations to this high-stressed environment as well as the lack of competition from normal marine biotas allowed them to flourish.
