LITHOFACIES AND SEISMIC-REFLECTION INTERPRETATION OF TEMPERATE GLACIMARINE SEDIMENTATION IN TARR INLET, GLACIER BAY, ALASKA

High-resolution seismic-reflection profiles of sediment fill within Ta rr Inlet of Glacier Bay, Alaska, show seismic facies changes with incr easing distance from the glacial termini. Five types of seismic facies are recognized from analysis of Huntec and minisparker records, and s even lithofacies are determined from detailed sedimentologic study of gravity-, vibro- and box-cores, and bottom grab samples. Lithofacies a nd seismic facies associations, and fjord-floor morphology allow us to divide the fjord into three sedimentary environments: ice-proximal, i ceberg-zone and ice-distal. The ice-proximal environment, characterize d by a morainal-bank depositional system, can be subdivided into bank- back, bank-core and bank-front subenvironments, each of which is chara cterized by a different depositional subsystem. A bank-back subsystem shows chaotic seismic facies with a mounded surface, which we infer co nsists mainly of unsorted diamicton and poorly sorted coarse-grained s ediments. A bank-core depositional subsystem is a mixture of diamicton , rubble, gravel, sand and mud. Seismic-reflection records of this sub system are characterized by chaotic seismic facies with abundant hyper bolic diffractions and a hummocky surface. A bank-front depositional s ubsystem consists of mainly stratified and massive sand, and is charac terized by internal hummocky facies on seismic-reflection records with significant surface relief and sediment gravity flow channels. The de positional system formed in the iceberg-zone environment consists of r hythmically laminated mud interbedded with thin beds of weakly stratif ied diamicton and stratified or massive sand and silt. On seismic-refl ection profiles, this depositional system is characterized by disconti nuously stratified facies with multiple channels on the surface in the proximal zone and a single channel on the largely flat sediment surfa ce in the distal zone. The depositional system formed in the ice-dista l environment consists of interbedded homogeneous or laminated mud and massive or stratified sand and coarse silt. This depositional system shows continuously stratified seismic facies with smooth and flat surf aces on minisparker records, and continuously stratified seismic facie s which are interlayered with thin weakly stratified facies on Huntec records. (C) 1997 Elsevier Science B.V.