CENTROCLINAL CROSS STRATA – ORIGIN, MORPHOLOGY, AND IMPLICATIONS FOR UNDERSTANDING ANCIENT TERRESTRIAL ECOSYSTEMS

Vegetation contributes to local sediment accumulation by interacting with sediment-laden floodwaters in channels and overbank areas. Although sedimentary structures formed from plant-sediment interaction are common in modern depositional systems, descriptions of these structures from the ancient record are surprisingly rare. We add to this modicum of data by describing eighteen examples of centroclinal cross strata formed around standing vegetation in the Joggins and Sydney Mines Formations (Pennsylvanian) of Nova Scotia.

Centroclinal cross sets in these formations formed around plants where symmetric, bowl-shaped scours excavated by erosive floodwaters were infilled by sandy sediment during waning flow. The resulting fill is typically organized into concentric, form-concordant laminae which dip towards the plant at between 13° and 46°. These structures vary in diameter from 20 cm to 3.5 m, in thickness from 5 cm to 2.1 m, and contain as much as 7.2 m³ of sandstone. The examples described in this study are present within sediments deposited in both poorly drained floodplain (PDF) and well-drained floodplain (WDF) environments. Obstacle-forming vegetation was preserved in nine of the twelve PDF examples and comprises eight lycopsids (10-54 cm diameter) and one calamite (8.6 cm diameter). Standing trees were not preserved within the six WDF examples, but stigmarian roots extended away from the base of three of them.

Variations in fill morphology include downstream tapering forms, “uniclinal” fills where laminae dip only in a downstream direction, and complex PDF examples wherein erosion surfaces separate discordant packages of concentric laminae. Centroclinal cross strata formed in WDF environments were frequently preserved despite decay of the obstacle-forming vegetation.

We draw upon recent studies of bridge scour and use the geometry of centroclinal cross strata to help constrain flow depths of the ancient floods which formed them. Recognition of these structures in Mississippian and Devonian rocks is particularly important because they provide valuable information about the distribution of early vascular plants in oxidizing terrestrial environments where organic material was not preserved.