Abstract

Braided fluvial sandstones of the Pennsylvanian lower Port Hood Formation (Cumberland Group) contain extensive soft-sediment deformation in the form of convolute bedding and load casts. Also present are sedimentary structures resembling, in cross section, trough-shaped depressions that contain angular to typically well rounded, pebble- to boulder-size bodies of mudstone (claystone, silt-stone, or interlaminated siltstone and very fine-grained sandstone). At specific sites, several potential interpretations are available for these structures: (a) bedload transport and deposition within a fluvial scour channel, (b) a slump-scour deposit, (c) soft-sediment deformation and brecciation of an abandoned and plugged fluvial channel, and (d) the oblique intrusion of "diapiric melange". Our previous work has adopted (a) as the best interpretation for Site 1a, but for the other sites (Sites 1b, 2, and 3), soft-sediment deformational mechanisms most likely caused the formation of these clast-like (clastiform) mudstone bodies. This interpretation is based on the internally deformed and/or intricately fissured nature of the clastiforms and their close association with indisputable soft-sediment deformation structures. Accordingly, likely more than one mechanism has produced morphologically similar structures within the same formation.

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