A Pennsylvanian-age terrestrial storm deposit: using plant fossils to characterize the history and process of sediment accumulation

A thin black shale overlying the B-coal underclay (in the Middle Pennsylvanian post-Pottsville strata of the Bernice Basin) contains a compression flora composed of large, prostrate, unidirectionally oriented lycopod trunks and randomly oriented pteridosperm stems. Analyses of modern log accumulations indicate that unidirectional trunk orientations can be produced by riverflood currents, volcanic blasts, and most high-energy windstorms. Since there are neither fluvial sediments nor ash deposits associated with the Bernice assemblage, this deposit is believed to have been formed by high-energy winds. Furthermore, this deposit is interpreted to be in situ because storm winds (and volcanic blasts) rarely have sufficient energy for the physical transport of large, intact tree trunks. The sedimentary history of the B-coal underclay can be determined from the successional changes in the species and plant part compositions (leaves, seeds, branches, trunks, etc.) of the preserved plant material. The underclay is an accretionary floodplain soil which accumulated as discrete increments during episodic floods. The sediments deposited with each flood incorporated the litter layer of the lycopod-pteridosperm forest which occupied this site. Ordinarily, the flood water would recede, and renewed root growth would destroy the primary sedimentary structures and the newly incorporated organic material. Because the bedding and forest litter are preserved in the top 5 cm of the underclay, root growth and silt deposition must have been terminated by the last flooding event. The site eventually became permanently inundated, and an organic-rich mud began to accumulate in the resulting floodplain lake. The lycopod-pteridosperm forest drowned and, at some later time, was blown down into the lake. The trunks are preserved on a single bedding plane in a 2-cm-thick, organic-rich lacustrine black shale. Continued organic accumulation in the lake resulted in the accumulation of a hypautochthonous peat which eventually was colonized by a peat-forming flora.