Carboniferous Fossil Plants and Soils of an Early Tundra Ecosystem

Abstract

Siltstones with fossil root traces and surface mats of fossil plants in the mid-Carboniferous (Westphalian or 310-312 Ma) Seaham Formation near Lochinvar, New South Wales, represent ancient soils of tundra ecosystems. Formerly well-drained paleosols show deeply-penetrating root traces, ferruginous nodules, freeze-thaw banding, and earth hummocks (thufur), but little evidence of mineral weathering. These paleosols also preserve leaf fragments of the seed fern Botrychiopsis plantiana. Formerly poorly-drained paleosols show tabular root systems, partly decomposed mats of the horsetail Dichophyllites peruvianus, and parent material of varved shale. Comparable soils are widespread in tundra regions of the northern hemisphere today. The paleosols show greater organic matter content than polar desert soils and much less weathering than soils of polar woodland. Shrubs or prostrate woody plants are represented by slender compressed fossil logs, but these are a less reliable guide to ancient ecosystem type than the paleosols because scattered small trees are found throughout the modern tundra belt. These fossil soils and plants are the most geologically ancient record of tundra vegetation known.