Allison Gleason '98

    Analysis of Silurian paleoenvironmental conditions at Glacier Bay National Park contributes to a greater understanding of the tectonic setting of the Alexander terrane (southeast Alaska). Three sites were studied on Drake Island, where isostatic rebound and Willoughby limestone outcrop surfaces smoothed by recent glaciation have created pristine rocks ideal for study. Through quadrate analysis and field observations, the dominance of a relatively homogeneous assemblage of large gastropods, including the previously described Euomphalopterus, was established in several well defined beds of the two eastern sites. These localities also contained communities of amphiporids, fragmented bivalves, and solitary corals. Alternate locations on the island contained strata dominated by bivalves instead of gastropods. An additional lens of small brachiopods was noted as unique to the western site. Grain analysis on the PetroScope revealed that wackestones are the dominant rock type, although both mudstones and grainstones were also observed. Peloids of skeletal origin were the dominant grains, composing over 30% of the rock on average; these skeletal fragments were found to be encrusted by micritized rind created in part by the endolithic borer, Girvanella. Oncoids, derived from whole mollusks, corals, and indistinguishable clumps, were coated to varying degrees of thickness and symmetry by similar rinds.

    This evidence led to the interpretation that Drake Island lithology formed in a shallow-marine lagoon. The absence of several key taxa was noted at all three sites, as no stenohaline bryozoans, trilobites, crinoids, or colonial corals that are typical of normal marine Silurian environments were found. The small percentage of brachiopods found at the western DC-97 site place it closer to the shelf slope at the deep end of the lagoon. Low faunal diversity can be attributed to a fluctuation in temperature, salinity, and/or oxygen levels from normal shallow-marine conditions (B.A. 2-3). Gradational bedding seen within the sections can be attributed to interspersed periods of high-energy storm events, which also caused skeletal fragmentation and symmetrically coated fossils. The resulting abundance of the large gastropod taxa that were the focus of this study was brought about by their ability to resist these high stress periods, as well as a lack of competition from other species in this restricted lagoon.