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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 54 (1970)

Issue: 5. (May)

First Page: 836

Last Page: 836

Title: Recent Braided-Stream Sedimentation, South-Central, Alaska: ABSTRACT

Author(s): Jon C. Boothroyd

Article Type: Meeting abstract


The Scott Glacier outwash fan, in the Copper River delta area, Alaska, shows a progressive decrease in grain-size and slope away from the glacier terminus. Bar and channel morphology change systematically from proximal to distal areas.

Detailed study along a 12-km tract shows that average clast size on bar surfaces decreases downfan from 30 to 3 cm, and slope decreases from a maximum of 15.5 m/km to 2.0 m/km. Upstream, bars are present between channels in sheetlike deposits; downstream, bars are present largely in mid-channel. Bars are composed of poorly sorted, well-imbricated coarse to fine gravel and plane-bedded, better sorted sand. Plane-bedded sand is more abundant in distal areas and may be capped by small-scale cross-bedding.

Cobble and pebble imbrications are the most persistent and reliable indicators of flow direction. Modal dip direction is upstream with long axes transverse to current flow. Other directional features measured included lineations, ripples, bar slipfaces, and logs. Linear stripes of large pebbles, which are oriented transverse to flow direction (determined from pebble imbrication) and occur in groups on bar surfaces, are interpreted as relict antidune bedforms.

Most deposition, which occurs during high-water stages, takes place on bar surfaces under upper flow-regime conditions. Flow separation along bar edges allows avalanche slipfaces to form. During low-water stages, when flow is mostly confined to channels, lateral erosion removes bar deposits and some channel deposition occurs under lower flow-regime conditions.

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Copyright 1997 American Association of Petroleum Geologists