CONTROLS ON SEQUENCE DEVELOPMENT OF THE OFFSHORE YAKATAGA FM., GULF OF ALASKA

Marine and glacimarine deposits of the Yakataga Formation (late Miocene to Recent) on the Gulf of Alaska continental shelf are the focus of a study to understand the depositional and climatic history of the late Cenozoic Gulf of Alaska.

Isopach and structural contour maps illustrate that stratal geometries of Yakataga sequences are a function of syndepositional folding, position of depocenters, magnitude of accomodation, and location of sediment sources.

Sequence boundaries on the upper Pleistocene sequences are angular unconformities that correspond with eustatic falls; the relationships of older sequences to eustatic cycles are equivocal due to limited chronostratigraphic control. Magnitude of paleobathymetric change on this margin is greater than the range of eustatic sea level change. Backstripping suggests a strong tectonic component to the paleobathmetric changes corresponding to sequence boundaries.

Seismic stratigraphy, paleoenvironmental analyses, and backstripping analyses illustrate that depocenters and sediment sources shifted through time; these shifts may be due to differential uplift and subsidence, shifting of sediment-producing tidewater-glacial margins or river systems, and/or northwest motion of the Pacific Plate. The differential movements were a function of the complex tectonic setting, a region in the juncture between subduction to the west and strike-slip faulting to the east. Backstripping analyses show that sediment-loading subsidence and R1 subsidence were the major controls on the magnitude of accomodation.

Two, and possibly more, of eight identifed sequences are tectonically-enhanced, glacially-eroded sequences formed during lowstands. At least one sequence (upper Miocene to lower Pliocene) is a tectonosequence, formed during a eustatic highstand when climate was relatively warmer. The younger sequences would be analogous to Exxon sequences, the difference being that they are glacially, not fluvially eroded, and that tectonic uplift instead of eustasy is a major control on accommodation. These significant results are suggest that it may be possible to recognize eustatically-related sequences in active margins. If the Pleistocene boundaries accurately correlate with eustatic lowstands, then Gulf of Alaska tidewater glacial advances may have been in phase with continental glaciation.