Abstract

Glaciation has been the major process that produced sediments and modified morphology in the northeastern Gulf of Alaska region since at least middle Miocene time. However, many unanswered questions remain about the extent, timing, and sequence of events that comprise the region’s glacial history. This paper attempts to summarize pertinent information in order to construct a synthesis of the major glacial events of the past 6+ million years.

Additionally, by using the few available dates and other facts known about the sequence of late Pleistocene and Holocene events, a model and working scenario are presented to explain the observed glacially influenced coastal morphology of the Gulf of Alaska region, This paper concentrates on the area west of the Alsek River, where as many as 20 distinct parallel coastal plain ridges exist. All are less than 3,000 years old, as determined by radiocarbon dating.

Approximately 4,000 years ago, as eustatic sea level reached present level, the Pacific Ocean covered a deep, glacially scoured bedrock shelf and reached directly to the foot of the coastal mountains. No coastal plain existed at that time. Rivers draining onto the continental shelf had to fill fjords and shelf basins before a sediment wedge could develop on the continental shelf. Only in the past 2,000-3,000 years has the coastal plain sediment wedge projected above sea level. As this occurred, a combination of glacial sedimentation, seismicity, and tectonic uplift began the ridge building process. Locally, the influence of isostatic processes resulting from glacier advance and retreat also affected the coastal plain. This combination of processes has produced an extremely young, very dynamic, changing coastline and coastal plain, most features having formed during the past millenium.