Columbia Mountains Research Articles

Glacial Marine Sediments from the Northeast Pacific

ABSTRACT Pleistocene sediments of glacial marine origin have recently been cored within Cascadia Basin in the northeast Pacific Ocean as far south as 44°10N. A uniform distribution of sand grains and pebbles in a clayey silt matrix indicates that ice rafting was the depositional mechanism. The mineralogy of the clay and sand fractions and the petrology of the pebbles strongly suggest a source area in the Puget Lowland and the adjacent mountains of Washington and British Columbia. These areas were covered by the Cordilleran glacier complex in late Pleistocene time. Calving of the ice front produced bergs laden with debris scoured from the continent. The distribution of the glacial marine deposits indicates that Pleistocene circulation m st have been very similar to that of today. Icebergs could have easily been carried south by the portion of the West Wind Drift which intrudes into the area of Vancouver Island and then moves southward along the coast. As the ice melted, it left a trail of debris 400 km south of the previously believed boundary for these deposits.

Late Pleistocene and Recent Sedimentation in Strait of Juan de Fuca: ABSTRACT

During part of the late Pleistocene (at least 14,000 years B.P.), the Strait of Juan de Fuca was occupied by a lobe of the continental ice sheet that extended from the Cascade Mountains of Washington and British Columbia on the east, to the nearshore waters of the Pacific Ocean on the west. As the front retreated, marine waters were able to re-invade the lower region, changing the type and character of sedimentation, and permitting benthic Foraminifera to migrate into the strait. The sediments of this time appear to be glacio-marine, and indicate that the fauna was able to adapt to a sedimentary environment of cold brackish waters containing vast amounts of sediment, with a few coarser sediments and cobbles supplied by berg or shelf ice. As the ice front retreated farther toward the east and north, the Strait of Juan de Fuca waters became progressively less brackish and supported a more marine fauna. Later, almost catastrophically, the environment changed from one of primary deposition to one of non-deposition or erosion. The upper part of the section and the surface sediments today are primarily coarse sand and gravel. This upper sand layer appears to lie disconformably on the underlying glacio-marine section. Measurements of the currents in the strait suggest that it is presently an area of non-deposition for fine sediments. Whether the upper sand layer in the strait represents a lag deposit after the removal of the finer fraction, or sediments added from coastal erosion, has not been determined. There is some seismic evidence, however, that Recent sediments are being shed into the strait from both sides. End_of_Article - Last_Page 452------------

Late Pleistocene Stratigraphy and Chronology in Southwestern British Columbia and Northwestern Washington

Six geologic-climate units are proposed for the late Pleistocene sequence in southwestern British Columbia and northwestern Washington. They include two major units, the Olympia Interglaciation and the Fraser Glaciation, and four subdivisions of the latter—the Evans Creek, Vashon, and Sumas Stades, and the Everson Interstade. The Olympia Interglaciation is a nonglacial episode that started at least 36,000 years B.P. and continued until the advance of Cordilleran glacier ice during the Fraser Glaciation. During the Evans Creek Stade, alpine glaciers formed in the mountains of western Washington and British Columbia while nonglacial sediments were still being deposited in the southern Puget Lowland. Further growth of glaciers in British Columbia resulted in the formation of the Cordilleran ice sheet. This ice entered the northern end of the area after 25,000 years B.P. but did not reach the southern end until after 15,000 years B.P. The Vashon Stade of the Fraser Glaciation began with this advance of Cordilleran ice into the lowlands. It ended with the beginning of marine and glaciomarine conditions there, which commenced in the southern Puget Lowland about 13,500 years B.P. and in the Strait of Georgia about 13,000 years B.P. The episode represented by the marine conditions is called the Everson Interstade and lasted about 2000 years, during which the sea contained much floating ice. The Interstade ended when the land rose with respect to the sea level forcing withdrawal of the sea and the disappearance of floating ice in most of northwestern Washington and southwestern British Columbia; in the eastern part of the Fraser Lowland this event coincided with the advance of a valley glacier during the Sumas Stade.