Abstract

Large, well-preserved carpofloras from Miocene and Pliocene lignite-bearing sequences of the Lower Rhine Basin in Germany occur in a variety of fluvial and lacustrine facies, including channel, point-bar, cut-off channel (oxbow-lake), crevasse-splay, channel-bar (sand-gravel flat), and lake-delta deposits. Despite their occurrence in a wide spectrum of depositional facies, the accumulations share a suite of distinctive characteristics. They appear as a jumble of woody material dominated by abraded pieces of wood, including conifer cones, woody fruits, seeds, small branches, and charcoal in an unconsolidated matrix of medium- or coarse-grained sand. The key characteristic of the fossil plant remains is woodiness. Close association between the lignified material and these grain sizes indicates that these woody fruits are hydrodynamically equivalent to medium- to coarse-sized sand, and were transported as bedload during flood events. Transport by bedload also is evidenced by the high degree of roundness in associated wood clasts, by abrasion in some diaspores, and by the lack of non-resistant plant parts (e.g., leaves, flowers, fleshy or delicate tissues) in the accumulations. Thus, the lignified plant remains behave as sedimentary clasts in the water column, but only after they have been fully saturated with water. Mass carpological accumulations in very coarse-grained sand on active point bars in the Sieg River near Bonn and their formation during events of extreme discharge serve as a modern analog for the fossil carpofloras. New terminology and concepts pertaining to these mass carpological deposits, or “bedload carpodeposits,” also include the terms “bedload carpobiofacies,” and “bedload carpolithofacies.” A basic depositional model for the genesis of bedload carpodeposits is presented.

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