Geology of Geikie Inlet area, Glacier Bay, Alaska

Seitz, James

The Geikie Inlet area is in the Glacier Bay region of southeastern Alaska, about 100 mi. NW. of the city of Juneau. The area is mountainous, with relief of slightly more than 5,000 ft., and the coastline is deeply indented by fjords and inlets. Most of the western half of the area is covered by glaciers. Beds of metamorphosed marine sedimentary and volcanic rock constitute about one-third of the bedrock exposed in the area and total 23,000 ft. and possibly more of stratigraphic thickness. Included in this is one sequence 8,000 ft. thick composed mostly of limestone beds with some intercalated beds of shale and volcanic rock in the upper part. The lower part is of relatively pure, gray limestone and contains fossils that have been tentatively identified as belonging somewhere in a Silurian to Permian age range. The bedded rocks also include a sequence, possibly 15,000 ft. thick, composed largely of metamorphosed shale and graywacke beds, but containing large amounts of interbedded limestone and volcanic rock. The shale, which includes calcareous and dolomitic shale, and the graywacke have been metamorphosed to hornfels and granulite and have predominantly granoblastic textures. The most common minerals in these metamorphosed rocks are hornblende and oligoclase or andesine, with the addition of diopside where the composition of the original sediment favored its formation. Two other bedded rock units, having a thickness of a few hundred feet at the most, are exposed at only two localities; they are a bed of conglomerate and a sequence of argillite and graywacke. The rock in both units is relatively unmetamorphosed, which indicates that the two units probably are unrelated to the other bedded sequences in the area. Limestone of the Willoughby limestone formation (probably of Late Silurian age) forms one isolated outcrop within the area, but its relationship to other bedded rock in the area has not been determined. In adjacent areas the Willoughby limestone forms large and conspicuous outcrops that reveal a stratigraphic thickness of more than 5,000 ft. Bodies of diorite ranging from 25 ft. to 12 mi. in length are distributed through the area; they total about one half of the bedrock exposed. The largest of these, in the western part of the area, is a fairly well-defined elongate northward-trending body. The rest of the diorite is distributed in many smaller and less well-defined masses through parts of the eastern third of the area. A notable characteristic of the diorite is the heteorogeneity of its appearance. This heteorogeneity is due to the pronounced irregularity in texture, variety, and abundance of inclusions and the scattered migmatitic zones in the rock. The irregularity of texture is due to variations in the size of constituent mineral grains and the irregular outlines of the larger crystals, which are typically porphyroblastic. This textural irregularity is most pronounced near contacts between diorite and bedded rock where broad irregular zones of gradational types of rock have formed. In many of these places migmatitic rock has formed; the migmatitic rock, which is composed largely of porphyroblastic plagioclase and hornblend is termed hornblende-plagioclase rock in this report. Another characteristic of the diorite is planar fabric that is reflected by gneissic banding and oriented inclusions of both bedded rock and fine-grained diorite. The orientation of this fabric follows a generally northward strike and is parallel to the prevailing strike of the bedded rocks in the area. This parallelism is maintained across contacts between diorite and bedded rock regardless of whether the contact is parallel to the bedding or at right angles to it. These gross features of the diorite, and other smaller scale ones, indicate that the diorite is of metamorphic rather than igneous origin.

Geology of Geikie Inlet area, Glacier Bay, Alaska

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