Life Beneath the Surfaces of Active Jurassic Dunes: Burrows from the Entrada Sandstone of South-Central Utah

Abstract

Within the Middle Jurassic Entrada Sandstone of south-central Utah, cylindrical burrows, 15–95 mm diameter, are abundant in large-scale, eolian cross-strata. Burrows are oriented at a high angle to stratification and commonly extend more than 30 cm below their surface termini. They are rarely inclined more steeply than 22°. Many are sinuous, and they sometimes branch (∼120°) at bends. Burrows terminate upward against flat-topped cones of structureless sandstone that are up to 15 cm deep and present at numerous, closely spaced stratigraphic horizons. Entrada eolian dune deposits also host abundant burrows likely produced by small insects. Both large and small burrows are most numerous in the uppermost parts of very thick (up to 35 m) compound sets of cross-strata generated by superimposed dunes migrating along the lee slopes of giant dune ridges. The size and morphology of the large burrows and the nature of their fills suggest that they were excavated by vertebrates, possibly insectivores, but the possibility that scorpions or spiders dug the burrows cannot be ruled out. In modern dunes, the top 20 cm of rain-moistened sand dries quickly, but underlying sediment can remain moist for long periods. Conical pits formed on the dry surface of Jurassic dunes at the tops of burrows that were primarily excavated in underlying moist sand. Cones composed of structureless sandstone are active fills produced when burrowers pushed moist sand to the surface, forming spoil piles. Most cylindrical portions of the burrows were also actively backfilled; remaining parts were passively filled when burrow walls collapsed. Cones at burrow tops now delineate thin (∼5–10-cm-thick) packages of cross-strata that record slow (seasonal?) dune migration. Rainfall on dune surfaces allowed scattered plants, insects, and insectivorous vertebrates to inhabit the Entrada sand sea. Burrows provided animals with refuge from the hot, desiccating surface conditions.