EROSION AND DEPOSITION BY CROSS-SHELF GLACIAL ADVANCE AS A MECHANISM FOR CHANNEL INCEPTION IN THE SURVEYOR FAN, GULF OF ALASKA

The collision of the Yakutat Block with the North American plate formed the St. Elias Mts., the highest relief coastal mountains in the world. Uplift of these mountains coupled with glacial erosion since the Miocene has distributed an immense sediment load into the Gulf of Alaska, leading to the formation of the deepwater Surveyor Fan. Due to shelf erosion during local glacial maxima, the only complete record of tectonic/climatic interaction in southern Alaska may exist in the Fan. The primary sequence boundary within the Surveyor Fan correlates to the base of the Surveyor Channel and divides the Fan into upper and lower sequences. The lower sequence is chaotic, containing discontinuous reflectors and mass-transport deposits. The upper sequence has laminated, continuous reflectors and no evident chaotic facies. The head of each channel in the Surveyor system geographically correlates to sea valleys on the shelf, from the buried sea valley at Icy Bay in the west to the Yakobi Valley at Cross Sound in the east. Previous studies postulate that these valleys were carved when glaciers traveled to the shelf edge during the mid-Pleistocene transition, a glacial intensification ~1Ma. Others studies state that glaciers first reached the shelf edge during the onset of Northern Hemisphere glaciation ~2.5Ma. We suggest that whichever climate event spurred the first glacial shelf crossing also initiated the formation of the Surveyor Channel system in the Surveyor Fan due to the formation of the shelf sea valleys. Sediments delivered to the shelf edge through this and subsequent glacial occupations of these shelf valleys exhibit a different seismic facies, a distinct distribution pattern, and greater accumulation rates than in the lower Surveyor Fan sequence. Since the triggering climatic event, terrigenous sediment sourced from the Icy Bay area in the west to the Cross Sound area in the east, including the entire Malaspina Glacier system, is represented by the upper sequence in the Surveyor Fan. However, glacial advances of the Bering Glacier system across the shelf within the Kayak and Bering Troughs appear to be isolated from the upper sequence by a deep slope-toe channel that is bathymetrically continuous with the Aleutian Trench. Thus a substantial part of the St. Elias sediment record is likely not contained within the Fan.