Sally D. Zellers
Mississippi State University
Interbedded
marine and glacial-marine deposits of the Yakataga Formation (late Miocene
to Recent), which underlie the northern Gulf of Alaska continental shelf,
are the focus of an integrated study to understand the depositional and
climatic history of the late Cenozoic Gulf of Alaska. Yakataga strata
accumulated on a tectonically active, glaciated continental margin; they
record initial late Cenozoic glaciation in the Gulf of Alaska (latest Miocene
to earliest Pliocene) and the onset of major northern hemisphere glaciation
during the late Pliocene. Foraminiferal sequence biostratigraphy,
seismic stratigraphy, and tectonic subsidence/uplift analyses define and
characterize eight seismic stratigraphic sequences in the Yakataga continental
shelf.
Foraminiferal
biostratigraphy establishes age, paleoenvironmental, and paleoclimate relationships.
These studies and seismic stratigraphy show that the base of the Yakataga
Formation is an unconformity offshore that developed as a result of a change
in Pacific Plate motion during the early Pliocene.Analyses
of benthic foraminifera demonstrate that paleobathymetry fluctuated from
inner neritic to upper bathyal.Pleistocene
paleoenvironments were deeper on average than those in the Pliocene.Many
paleobathymetry fluctuations are non-synchronous between well sections
and exceed 200 m; these fluctuations are mainly due to differential uplift
and subsidence on discrete structures.Isopach
and structure contour maps illustrate the geometry of sequences, and demonstrate
that depocenters shifted through time.These
shifts could be the result of northwest movement of the Pacific Plate and/or
changes in dominant ice streams along the margin.Tectonic
subsidence/uplift analyses quantify the controls on accommodation; these
results show that tectonics and sediment-loading subsidence are the primary
controls on the magnitude of accommodation ion the margin.
The
youngest (upper Pleistocene) sequence boundaries may have formed during
eustatic lowstands as glacial ice advanced and eroded parts of the margin
that had been tectonically uplifted.The
base of each of the oldest (Pliocene and lower Pleistocene) Yakataga sequences
is bounded by a tectonically formed or tectonically enhanced sequence boundary.Neither
the relationship to eustasy, nor the mode of formation of the oldest boundaries
can be determined with the available data.
Modern
seismic surveys and new drilling, accompanied by high-resolution biostratigraphic
analyses, are needed to further refine the stratigraphic relationships
of this margin.
