HOW ABUNDANT WAS VEGETATION ON THE WESTERN MARGIN OF PANGEA? ABUNDANCE, DIVERSITY AND TAPHONOMY FOR THE LOWER PERMIAN HALGAITO FORMATION OF THE PARADOX BASIN, UTAH

During the Early Permian, the plains of western Pangea included eolian dunes and fine-grained sheets (loess), suggesting an arid landscape and minimal vegetation cover. To assess the importance of vegetation in the landscape, we examined the upper Halgaito Formation of earliest Permian age, widely exposed in the Valley of the Gods region of the Paradox Basin.

Stem fragments and comminuted plant debris were discovered in only a few channel bodies, in basal plane-bedded sandstone and climbing ripples that indicate rapid accumulation of water-saturated sediment under seasonal flow conditions. Rare macrofossil foliage includes impression remains of the conifer Walchia, tree fern Pecopteris and sphenopsid Annularia. More common are impressions of stems of tree ferns and calamitaleans, and rarely lycopsids (Sigillaria).

In contrast to the scarcity of above-ground material, regularly spaced large root traces are abundant. The root traces penetrate up to 3 m within the upper parts of channel bodies, are prominent in near-structureless to stratified beds of eolian fines and, where adequate clays were present, in soils with vertic features and locally with carbonate nodules. Exposed surfaces of sandstone show abundant stem bases with radiating, shallowly disposed roots and carbonate rhizoconcretions, as well as smaller root systems.

Vegetation evidently grew at many times and in most places throughout the area, where roots penetrated deeply to access groundwater. The high ratio of root traces to preserved plant material implies a taphonomic megabias that may operate in most such seasonally dry settings. The Paradox Basin patterns suggest 1) that vegetation was prominent at times across these Early Permian landscapes, with a probable range of plant sizes, but 2) that the scarcity of shallow groundwater and standing-water bodies limited the preservation of plant litter to active stream deposits during times of widespread vegetation.