IN THE EARLY JURASSIC ERGS OF SOUTHERN UTAH, ABUNDANT SANDSTONE CASTS SHOW THAT TREE-SIZED HORSETAILS SURVIVED MULTIPLE EPISODES OF SAND ACCUMULATION

Widely scattered exposures of eolian strata in the Navajo Ss and Wingate Fm contain bioturbated sandstone casts of rhizomes and tree trunks (up to 30 cm diameter). These resemble casts of Neocalamites (an extinct sphenophyte, a large group now represented only by Equisetum). Vertical casts with root flares are closely spaced; vertical casts connect to branching rhizomes that also have central voids. Remnant crossbeds with large-scale soft-sediment deformation are present at all the Navajo sites, indicating the plants occupied a dune-covered landscape underlain by a shallow water table. During one seismic, a buoyant trunk or rhizome dragged scores of small-diameter rhizomes upward through the dense liquefied sand. Rock masses containing these rhizomes and stems extend over as much as 1 km² and are up to 6m thick.

Plants with rhizomes and stolons dominate the floras of modern coastal dunes that undergo frequent burial episodes. Taphonomic studies of coal-bearing Carboniferous strata have shown that if buried upright by sediment, the stems of Calamites (like modern Equisetum) produced rhizomes and continued growth. Subsurface energy storage in the large Navajo rhizomes (connected to all upright stems) allowed rapid growth through the sand to the new land surface. At one Navajo site, branching networks of rhizomes are tiered or “stacked”, with each tier recording the biological response to an episode of sand accumulation and stem burial. The cast-bearing rocks (relative to surrounding crossbeds) are resistant to weathering. Quartz overgrowths are present on a large percentage of their detrital grains; opaline phytoliths were the likely source of SiO₂ (Equisetum is a well-known silica accumulator). Vertebrate bones and silicified wood are very rare in eolian sandstones of the Colorado Plateau, but trackways are abundant; horsetail casts in the Navajo now bolster the ranks of the primary producers.