FORAMINIFERAL AND SEDIMENTOLOGIC CHARACTERIZATION OF SEISMIC FACIES SEAWARD OF THE BERING GLACIER, GULF OF ALASKA

Cores at sites U1420 and U1421 drilled by Integrated Ocean Drilling Program Expedition 341 in the Gulf of Alaska and seismic data document at least eight advances of the Bering Glacier since the end of mid-Pleistocene transition (1.2-0.7 Ma). We previously recognized a number of seismic sequences, grouped into two megasequences: one tectonically controlled and formed by sediment bypassing; and a younger one with a trough mouth fan composed of multiple glacigenic debris flows. Five distinct seismic facies were also defined. Here we use quantitative analyses and shipboard lithologic descriptions to enhance our seismic facies interpretations.

Cluster and non-multidimensional scaling analyses of benthic foraminiferal data define three biofacies: I) Eubulimina exilis, Bolivina spissa, and Epistominella pacifica; II) E. pacifica and Uvigerina peregrina; and III) species of Buccella, Elphidium, Islandiella, Cibicides, and Quinqueloculina. Biofacies I represents middle bathyal, hypoxic conditions, while biofacies II represents middle to upper bathyal, and Biofacies III represents a neritic environment.

Seismic facies, recognized in other glacial settings, are described as: chaotic, internally transparent (C); mounded and hummocky (M); high-amplitude, continuous (S); semi-continuous, gently mounded, onlapping (O); and stacked, transparent lenses bounded by continuous reflections (L).

Seismic facies are further described and interpreted as follows:

C - poorly-recovered, massive clast-rich diamict with reworked taxa from III; subglacial till,

M - extremely poor recovery (<10%) with washed pebbles and drilled rock, few to no foraminifera; recessional morainal banks formed during stillstands or glacier retreat,

S - biosiliceous oozes that contain abundant well-preserved planktics and benthics from I and II; ice-distal, open marine conditions with increased productivity and reduced oxygen during interglacials or interstadials,

O - mud with/without lonestones, clast poor/clast-rich diamicts dominated by II and III; turbidity currents, ice-rafting, and settling from meltwater plumes,

L - clast-rich to clast-poor diamicts with moderate- to poorly-preserved taxa from III, bounded by sediments similar to facies S; formed by submarine sliding and slope failures.