MUDSTONE-CLASTIFORM CONGLOMERATES AND TROUGH-SHAPED DEPRESSIONS FROM THE PENNSYLVANIAN LOWER PORT HOOD FORMATION OF EASTERN CANADA - OCCURRENCES DUE TO SOFT-SEDIMENT DEFORMATION

Braided fluvial sandstones of the Pennsylvanian lower Port Hood Format ion (Cumberland Group) contain extensive soft-sediment deformation in the form of convolute bedding and load fasts. Also present are sedimen tary structures resembling, in cross section, trough shaped depression s that contain angular to typically wed rounded, pebble- to boulder-si ze bodies of mudstone (claystone, siltstone, or interlaminated siltsto ne and very fine grained sandstone). At specific sites, several potent ial interpretations are available for these structures: (a) bedload tr ansport and deposition within a fluvial scour channel, (b) a slump-sco ur deposit, (c) soft-sediment deformation and brecciation of an abando ned and plugged fluvial channel, and (d) the oblique intrusion of ''di apiric melange''. Our previous work has adopted (a) as the best interp retation for Site la, but for the other sites (Sites Ib, 2, and 3), so ft-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 me chanism has produced morphologically similar structures within the sam e formation.