Quaternary seismic stratigraphic investigations, Yakutat Bay region, Gulf of Alaska : subglacial drainage mechanics and glacial expanse

Date
2008
Authors
Elmore, Christopher Ryan, 1984-
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Abstract

Quaternary tectonic and climate interactions have and continue to contribute to a substantial stratigraphic record that preserves the key regional events that result from the interplay of these dynamic processes. Near Yakutat Bay, Alaska, temperate glacial cover continuously manipulates and shapes the southern Alaskan margin and delivers substantial amounts of sediment annually. During cooler climatic periods, these glaciers, like most glaciers in Alaska, experience increases in volume and advance across the shelf. Using an integrated seismic data set, we investigate stratigraphic signatures buried along the shelf, between the present Yakutat and Alsek Sea Valleys, in order to understand the mechanics governing the regional glacial system during advance phases. Our investigations reveal four glacial unconformities, of which, two show preserved retreat sediment packages. The events are indicative of ice advance phases during the Little Ice Age (LIA), the Last Glacial Maxima (LGM), and two pre-LGM events. The events show dominance by ice expanse from the Malaspina and Alsek River districts. The LGM event transgresses the shelf and concentrates erosion in highly overdeepened troughs, somewhat analogous to ice stream behavior. In addition, associated with these regional events, we find a suite of large-scale channel forms, exhibiting widths between 0.6-4.2 km and depths between 49-353 m. The channels are highly eroded vertically relative to lateral extent; widths to thickness ratios of the buried channel deposits are 15.7 on average. The channels show good correlation with the interpreted erosional events and other stratigraphic signatures indicative of ice occupation, i.e. moraines, grounding lines, etc. The channels exhibit distinct facies and facies assemblages of variable thicknesses and occurrence and show common morphologic characteristics with one another, i.e. cross-cuts. The facies and assemblages appear dominated by glacio-proximal, glacio-fluvial and glacio-lacustrine/marine physical processes. The characteristic morphology of the channels implies they are subglacially derived, and facies suggest subsequent fill by subaerial/submarine processes after ice retreat. Based on morphologic interpretations of the channels and their associations with glacial stratigraphy and erosional events, we interpret them as tunnel valleys. Expanding on their predictable relationship with specific glacial stratigraphy, we conclude that the tunnel valleys are indicators of retreat following glacial advance periods, when significant amounts of ice and melt are available to initiate and generate large tunnel valley deposits. As such, we conclude that the tunnel valleys are useful paleoclimate indicators

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