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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 56 (1972)

Issue: 3. (March)

First Page: 610

Last Page: 610

Title: New Component of Braided River Model: ABSTRACT

Author(s): W. R. Costello, R. G. Walker

Article Type: Meeting abstract


During the late Wisconsin glacial retreat in southern Ontario, extensive sheets of braided gravelly and sandy outwash were deposited. In the Credit River valley, near Toronto, there are 2 overall fining-upward sequences of braided deposits, with gravels passing upward into sands. The gravels contain large-scale (up to 3 m) crossbedding and abundant channeling, suggesting deposition from migrating mid-channel braid bars. The sands contain some tabular crossbedding (sets up to 1 m), and also a distinctive, newly defined coarsening-upward lithofacies sequence.

The sequence begins with a thin (3-6 cm) layer of clay, which overlies abruptly on rippled sand. The clay grades upward into silty clay with faint wavy laminations, and some cross-lamination with preserved stoss sides. The silty clay grades upward into cross-laminated silt, and then ripple-drift cross-laminated sand with eroded stoss sides. The rippled sand is followed erosionally by trough crossbedded sands (sets up to 40 cm) resting in channels up to 1 m deep and 3 m wide. In places, these channels cut down to the basal clay layer. The uppermost part of the sequence consists of tabular and trough crossbedded coarse sands and gravels.

The coarsening-upward sequence is interpreted as the fill of an abandoned channel. The clay represents fines washed over a levee from an active channel in flood. As more material was washed in, gentle flow began and ripples were formed in silt and fine sand. Levee breaching introduced bed load material into the new channel, forming the crossbedded gravels and sands.

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