Sedimentology and paleontology of a Carboniferous log jam

Abstract

A localized sandstone split in the Mary Lee coal (Early Pennsylvanian, Langsettian) of the Black Warrior Basin, Alabama, has a channel-form geometry and preserves a concentrated log-and-gravel (pebbles, cobbles, and boulders) assemblage at the top of the fill sequence. Gravel lithotypes within and amongst rooting structures of lycopsid, cordaitean, and calamitean trees are indicative of an Appalachian orogenic provenance, and support an allochthonous origin for some of the logs. In addition, a Skolithos ichnological assemblage within the channel is indicative of opportunist colonization during channel fill. A low sinuosity geometry characterizes the overall channel-form belt, exhibiting a general northwesterly trend. Paleocurrent measurements from the cross-bedded sandstone at the margin of the channel belt indicate flow was to the northwest. Log orientations at these sites are subperpendicular to perpendicular to the overall sediment transport direction and are interpreted to represent an ancient log jam. The genesis of the fluvial channel, the introduction and emplacement of the concentrated log assemblage, the effects of this floating log accumulation on sedimentation within the channel, and the relationship between paleocurrent and log orientation are discussed. The Red River, located in the southern United States, is used as a modern analog for comparative purposes.