FLYNN, Brian C., GLEASON, Allison, SOJA, Constance M., and WHITE, Brian,
Geol. Dept., Colgate Univ., Hamilton, NY 13346, bflynn@center.colgate.edu

    In Glacier Bay National Park (GBNP), samples were collected from six stratigraphic sections on Drake Island to determine the paleoecology of Silurian molluscan assemblages. Each section contained several beds dominated by the megalodont bivalve, Pycinodesma, and by several gastropod taxa, including Euomphalopterus, forming as much as 50% of rock volume. Associated beds comprise extensive burrows and low-diversity suites of amphiporids, rugose corals, and massive stromatoporoids. Petrographic analysis reveals that peloidal wackestones/packstones predominate and are interbedded with rare grainstones and thrombolites. Moderate- to well-sorted "peloids" are the dominant grains in these micritic rocks but do not appear to be of fecal origin. Instead the "peloids" formed in a multi-step process beginning with the fragmentation of skeletal grains followed by their micritization, which was induced by endolithic (microbial) encrusters like Girvanella forming an oncolitic cortex and boring into the shell's exterior.

    Based on these characteristics, these sections of the Willoughby Formation are interpreted to represent a restricted, shallow-marine lagoon. Relatively low-energy subtidal conditions are implied by the abundant micrite, whole gastropods, in situ orientation of whole, articulated pelecypods, amphiporids with well-preserved delicate branches, and 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 grainstone 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 mollusks in GBNP to predominate because of their adaptations to a high-stressed environment and lack of competition from normal-marine biotas.

http://departments.colgate.edu/geology/
Copyright 1997 © Colgate University.