Abstract

The lower Tindir Group in east-central Alaska is composed of six formations with a cumulative thickness of as much as 2,000 m. These rocks are mainly carbonates and quartz sandstones of shallow-marine origin; a black shale unit is a deeper water deposit. They are similar to the lower part of the Mackenzie Mountains supergroup in the northern Canadian Cordillera. An unconformity between the two parts of the Tindir succession is indicated by erosion of the lower Tindir during deposition of the upper.

The upper Tindir has a maximum thickness of ∼ 2,000 m; it can be subdivided into five formations which thicken to the west. Paleocurrent data suggest westerly transportation. Tholeiitic mafic pillow lavas and volcaniclastic rocks make up unit 1. Unit 2, an assemblage of purple hematitic mud-stones, includes turbidites and purple diamictite beds; the latter are coarser and thicker near the top. The stratified diamictites are interpreted as mass-flow deposits, but striations on some lonestones indicate a glacial provenance for some of the clasts. Unit 3 comprises crudely stratified purple diamictites of glaciomarine origin. This facies is only locally preserved and is considered to be allochthonous, having slid westward in response to contemporaneous fault activity. The iron-rich nature of the two glacially influenced formations, units 2 and 3, and local development of iron-formation in unit 3 are interpreted as being due to hydrothermal activity at a volcanic center somewhere to the west.

Overlying units display rapid lateral facies variations, and so both local and regional correlations are tentative. In western parts of the study area, unit 4 consists mostly of shales and turbidites; to the east, it is represented by thin platformal dolo-stones. It includes some thin diamictites with granitic and other exotic clasts. Unit 5 consists mainly of basinal shales, resedimented carbonates, and siliclastic turbidites which pass rapidly eastward into rocks of shallow-water aspect. These include pebble and cobble conglomerates with carbonate, quartzite, and chert clasts probably derived from the lower Tindir, and shallow-water sandstones and shales. A break in sedimentation is proposed between unit 5 and overlying Cambrian carbonates. Rocks of the upper Tindir resemble those of the Rapitan Group and succeeding upper Proterozoic rocks of the Mackenzie Mountains area and may be the northwestern termination of a late Proterozoic-early Phanerozoic continental margin.

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