ABSTRACT

Glaciation has been an active sediment-producing process in the northern Gulf of Alaska area since the Miocene. No consensus exists for the date of the onset of glaciation, but the range of published ages is from 5.1 to 30 m.y. B.P. Areally, the influence of glaciation has included the entire Gulf of Alaska continental shelf and much of the seafloor to the south.

Today, the Gulf of Alaska continental shelf is largely covered with glacially derived Pleistocene and Holocene materials. Bottom sediments from Miocene time until the end of last glacial coverage (12,000-14,000 years ago) are predominantly dense and hard, reflecting glacial ice loading. In contrast, most modern sediments are extremely weak and soft. In most places rapid sedimentation has left extensive areas of underconsolidated material.

The engineering properties of the seafloor of the northern Gulf of Alaska are directly related to the geologic and glacial history of the region. Engineering classification, consolidation-compression and strength properties for representative deposits of units found throughout the northern Gulf of Alaska are presented to define this relationship.

INTRODUCTION

The purpose of this study is to describe the areal and temporal distribution of glacially derived Gulf of Alaska sediment and to characterize the engineering properties of this material. Results of a suite of engineering tests including Atterberg limits grain size analyses, unit weight (bulk density), consolidation-compression characteristics, and triaxial strength tests are presented to describe the engineering characteristics of the glacial sediment.

The continental shelf and coastal region of the from Gulf of Alaska (Fig. 1) has received glacially derived sediment since Miocene time. Today, the shelf is covered by glacial material deposited as till and moraines, deposited by ice' rafting, and deposited from settling of suspended fines through the water column. Each type of material differs from the others in its engineering characteristics. An understanding of the Gulf of Alaska's depositional regime and engineering characteristics is important in design of offshore pipelines, platforms and other structures. The wide variety of sediment types and engineering characteristics of the Gulf of Alaska continental shelf complicates engineering designs. As examples, sediment runs the range from underconsolidated to highly overconsolidated while water contents range from more than 100% of dry weight to near zero.

In 1977, to characterize the engineering properties of the sediment in the Gulf of Alaska, the U.S. Geological Survey?s R/v SEA SOUNDER collected over 40 cores, which were analyzed for their engineering characteristics. These analyses plus the analyses of sediment collected by R/V SEA SOUNDER in 1976 and the NOAA SHIP DISCOVERER in 1975 form the data base for the formulation of the engineering characteristics of Gulf of Alaska sediment presented in this paper.

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