EVALUATION OF CONDITIONS ALONG THE GROUNDING LINE OF TEMPERATE MARINEGLACIERS - AN EXAMPLE FROM MUIR INLET, GLACIER BAY, ALASKA

In the marine environment, stability of the glacier terminus and the l ocation of subglacial streams are the dominant controls on the distrib ution of grounding-line deposits within morainal banks. A morainal ban k complex in Muir Inlet, Glacier Bay, SE Alaska, is used to develop a model of terminus stability and location of subglacial streams along t he grounding line of temperate marine glaciers. This model can be used to interpret former grounding-line conditions in other glacimarine se ttings from the facies architecture within morainal bank deposits. The Muir Inlet morainal bank complex was deposited between 1860 A.D. and 1899 A.D., and historical observations provide a record of terminus po sitional glacial retreat rates and sedimentary sources. These data are used to reconstruct the depositional environment and to develop a cor relation between sedimentary facies and conditions along the grounding line. Four seismic facies identified on the high-resolution seismic-r eflection profiles are used to interpret sedimentary facies within the morainal bank complex. Terminus stability is interpreted from the dis tribution of sedimentary facies within three distinct submarine geomor phic features, a grounding-line fan, stratified ridges, and a field of push ridges. The grounding-line fan was deposited along a stable term inus and is represented on seismic-reflection profiles by two distinct seismic facies, a proximal and a distal fan facies. The proximal fan facies was deposited at the efflux of subglacial streams and indicates the location of former glacifluvial discharges into the sea. Stratifi ed ridges formed as a result of the influence of a quasi-stable termin us on the distribution of sedimentary facies along the grounding line. A field of push ridges formed along the grounding line of an unstable terminus that completely reworked the grounding-line deposits through glacitectonic deformation. Between 1860 A.D. and 1899 A.D. (39 years) , 8.96 x 10(8) m(3) of sediment were deposited within the Muir Inlet m orainal bank complex at an average annual sediment accumulation rate o f 2.3 x 10(7) m(3)/a. This rate represents the annual sediment product ion capacity of the glacier when the Muir Inlet drainage basin is fill ed with glacial ice. (C) 1997 Elsevier Science B.V.