Glacial Lithofacies of Neogene Yakataga Formation, Gulf of Alaska: ABSTRACT

Abstract

Lithofacies, biofacies, and chronostratigraphic analyses indicate three relatively warm and three relatively cool paleoclimatic intervals within the 5,000-m thick strata of the lower Miocene through Holocene Yakataga Formation of the Robinson Mountains, eastern Gulf of Alaska. Glacial periods, recognized by the predominance of glacial lithofacies associated with populations of Neoglobigerina pachyderma s.l., are interpreted for the cool intervals. In the sections studied, the interglacial intervals have little or no glacial deposits. Yakataga Formation strata of glacial origin can be classified as having formed in either ice-contact or glacial-aqueous depositional environments. Ice-contact deposits consist of stratified or chaotic sedimentary materials, but erratics are rare in the Yakataga Formation. Glacial-aqueous deposits are of three principal types: glaciolacustrine, glaciofluvial, and glaciomarine. Glaciolacustrine deposits have not been identified in the Yakataga Formation. Glaciofluvial deposits, characterized by interbedded poorly sorted lenticular sand and gravel, occur in geographically limited and stratigraphically restricted intervals of the Yakataga Formation. Glaciomarine silts and clays, containing floating pebbles and cobbles and in-situ marine fossils, are the dominant glacial aqueous deposits i the Yakataga Formation. Detailed lithofacies associations and geometries define open-shelf and fjord glaciomarine subenvironments. Lithofacies sequences within these subenvironments suggest deposition by advancing and retreating ice masses. Mapping of the relative percentages of lithofacies types within the late Pliocene-Pleistocene glacial sequence defines the late Neogene paleogeography of the Yakataga district. This mapping suggests that the late Pliocene and Pleistocene distribution of alpine glaciers, coastal-outwash and alluvial flood plains, and fjords, and the associated rates of sedimentation, are analogous to the depositional pattern during the Holocene. End_of_Article - Last_Page 891------------