FORAMINIFERAL BIOFACIES AND PALEOENVIRONMENTS IN A PLIOCENE MEGACHANNEL OF THE GLACIOMARINE YAKATAGA FORMATION, GULF OF ALASKA
Published:May 11, 1994
M.B. Lagoe, C.H. Eyles, N. Eyles, 1994. "FORAMINIFERAL BIOFACIES AND PALEOENVIRONMENTS IN A PLIOCENE MEGACHANNEL OF THE GLACIOMARINE YAKATAGA FORMATION, GULF OF ALASKA", Late Cenozoic Benthic Foraminifera: Taxonomy, Ecology and Stratigraphy. In honour of Rolf W. Feyling-Hanssen on his 75th Birthday. July 24, 1993, Hans Petter Sejrup, Karen Luise Knudsen
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Foraminifera are common in the glaciomarine Yakataga Formation of the eastern Gulf of Alaska and can provide key insights into the depositional history of Late Cenozoic glaciomarine paleoenvironments in the northeast Pacific Ocean. Lithologic evidence of tidewater glaciation consists of two main intervals of diamictites and sediments containing ice-rafted debris. The first is in the basal Yakataga Formation and is of latest Miocene age, while the second consists of 2-4 km of late Pliocene-early Pleistocene glaciomarine sediment in the upper Yakataga Formation. A distinctive feature of this latter interval is megachannels up to 400 meters deep and several kilometers wide. Megachannels cut into, and are filled with, a variety of lithofacies, including massive and stratified diamictites, thinly interbedded turbidite sandstones and mudstones, massive to laminated mudstones and crudely stratified conglomerates. These megachannels have been identified as possible paleofjords by some investigators but may also represent glacially influenced sea valleys, similar to the modern Yakutat Sea Valley or Bering Trough. A study of foraminiferal biofacies and sediments provide a paleoenvironmental framework for evaluation of the origin of the megachannels. Channel-fill successions begin with conglomerates overlain by fine-grained turbidites and mudstones. The turbidites pass upwards into massive and stratified diamictites deposited predominantly by sediment gravity flow processes. The turbidites and mudstones contain faunas characterized by Epistominella pacifica, agglutinated taxa and contain planktic foraminifera. These faunas represent upper bathyal water depths (150-500 m). Some diamictites contain sparse faunas dominated by Elphidium excavatum clavatum and represent neritic water depths. Near the margins of megachannels, vigorous downslope gravity processes are reflected by gravel beds and upper bathyal biofacies containing greater numbers of displaced shallow water taxa (particularly Elphidium excavatum clavatum), including rare innermost shelf taxa (e.g., Elphidiella oregonense). The distribution of foraminiferal biofacies suggests water depths for channel-fills consistent with the amount of channel incision (100’s of m) and does not suggest restricted or silled conditions as seen in many modern fjords. Thus, the megachannels most closely resemble the glacially-influenced sea valleys found on the modern Gulf of Alaska continental margin.