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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 57 (1973)

Issue: 4. (April)

First Page: 810

Last Page: 810

Title: Transport Processes for Lower Paleozoic Resedimented Conglomerates of Appalachians: ABSTRACT

Author(s): Roger G. Walker, Ian C. Davies

Article Type: Meeting abstract


Three fining-upward sequences of resedimented conglomerate, totaling about 40 m in thickness, are present in the lower Paleozoic Cap Enrage Formation of Gaspe, Quebec. Boulders, dominantly of shelf-type carbonate rocks, are up to 3 m in diameter, but average about 10-50 cm. Individual beds are more commonly massive than normally graded, although inverse grading occurs at the base of many beds. Stratification, delineated by alternations of grain size, is common.

At the base of each fining-upward sequence, beds tend to be poorly sorted, and contain very large boulders. At the top of the sequences, well sorted conglomerates, with pebbles normally less than 5 cm, are interbedded with sandstones. Deep scouring is uncommon.

The best clue to the transport process which resedimented the conglomerates into the Appalachian geosyncline is the well-defined pebble fabric. Most beds show a strong preferred southwest orientation of a-axes in plan view. In vertical cross section, most beds show a well-developed pebble imbrication. The a-axes of the pebbles dip upstream (rather than lying horizontal, transverse to flow) and indicate flow toward the southwest, parallel with the present tectonic strike. This type of fabric is very rare in deposits where pebbles rolled as bed load, because such movement develops a transverse a-axis fabric. Bed load transport was minimal to nonexistent for these conglomerates. Alternative mechanisms include some form of mass flow for each bed, or movement of pebbles in suspension. Mass flow is unlikely because of the well-developed imbrication and stratification, and hence it is suggested that the pebbles fell onto the bed out of fluid suspension, and were not subsequently moved. The outstanding problem is how the boulders were maintained in fluid suspension.

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Copyright 1997 American Association of Petroleum Geologists