Investigation into the Origin of Buried Channels along the Glacially Impacted Southern Alaskan Shelf: A Case for Tunnel Valley Formation

Ryan Elmore

The interaction between tectonic exhumation and climatic events along the southern coast of Alaska provides a unique setting in which the interplay of these significant processes can be evaluated. High precipitation rates and topographic relief in conjunction with the cool, temperate nature of the Alaskan coast produce large-scale glaciations across the margin. As such, glacial erosion processes control much of the sediment yield to the continental shelf during advance and retreat stages that have persisted throughout the last ~ 3 myr. Numerous large-scale channel structures preserved within continental shelf sediments may serve as markers of glacial advances or retreats and may illustrate significant glacial-marine processes. We have undertaken an integrated study into the origin of these structures using 1979 industry seismic data provided courtesy of the USGS and NSF-funded, high-resolution seismic data (Gulick et al., 2007). Channel geometries range in scale from 513 m wide by 60 m deep, up to 4700 m wide and 355 m deep. Differentiation of channels from surrounding sediments is done so through facies analysis; within most of the channels strata display a hummocky, unstratified seismic character overlain by stratified deposits, which suggests a multi-phase filling during glacial retreat. Along the flanks of major sea valleys carved into the shelf during times of sea level low-stand, there is an observable increase in channel density, suggesting that the valleys may facilitate channel formation during times of ice occupation. Several fundamental mechanisms for channel formation along glacially impacted margins are known, i.e. sub-ice movement of water, catastrophic release of sub-glacially stored water, glacial scour, and amalgamated networks of outwash deposits. Previously proposed hypotheses particular to the southern Alaskan shelf include slump-generated mega-channels and paleo-fjord deposits; however, facies analysis of the channels and their relationship with major erosional events within the framework of the stratigraphic record suggest instead a sub-glacial origin, likely, networks of tunnel valleys. Currently the exact mechanism by which tunnel valley formation occurs is not known, but previous work on tunnel valleys suggest a combination of three end member possibilities: i) steady state flow of sub-glacial water, ii) major, sometimes catastrophic release of sub-glacially dammed water and, iii) direct glacial scour (Huuse and Lykke-Anderson, 2000). Using a glacial stratigraphic model put forth by Cooper and Powell (2006) and the relationship of channel positions relative to major unconformable surfaces, we demonstrate the validity of the tunnel valley hypothesis concurrently with the idea that they are markers in time for glacial advance, i.e. sea level lowstands. Implications are that these channels present new insights into the timing and extent of major glacial advances and contribute a key component for understanding the dynamic history of the margin.