MORPHOLOGY AND PALEOENVIRONMENTAL IMPLICATIONS OF ADHESIVE MENISCATE BURROWS (AMB), PALEOGENE WILLWOOD FORMATION AND OTHER CONTINENTAL DEPOSITS

Adhesive meniscate burrows (AMB) are abundant in alluvial paleosols of the Paleogene Willwood Formation, Bighorn Basin, Wyoming. AMB are sinuous, variably oriented, unbranched, unlined, backfilled burrows with circular to elliptical cross sections from 0.7 to 14.0 mm in diameter. The burrows are composed of a nested series of distinct, half-ellipsoid-shaped packets containing thin meniscate laminae with each packet outlined by a wall thicker than the laminae it contains. Internal laminae are concentric and subparallel to the bounding packet. The burrows are termed adhesive because they do not weather differentially from the surrounding matrix and cannot be removed easily as individual specimens.

In the Willwood Formation, AMB are present in groups of tens to thousands of individuals in weak, moderate, and well-developed paleosols. AMB are especially abundant in strongly developed paleosols with rhizoliths, often to the exclusion of other trace fossils. Identical burrows are reported from paleosols in the Upper Triassic Chinle Formation, the Upper Jurassic Morrison Formation, and Miocene alluvial deposits in Montserrat, Spain.

Based on burrow morphology and comparison to similar structures in modern soils, AMB were constructed most likely by burrower bugs (Hemiptera: Cydnidae), cicada nymphs (Hemiptera: Cicadae), and less likely by scarabaeid (Coleoptera: Scarabaeidae) or carabaeid beetles (Coleoptera: Carabidae). Extant cydnids excavate backfilled burrows in well-rooted soils with 7-37% moisture content. Cicada nymphs and adult and larval scarabaeid beetles also excavate backfilled burrows with elliptical chambers, likely with similar soil moisture constraints.

AMB have been assigned to such other backfilled ichnogenera as Beaconites, Taenidium, Scoyenia and Ancorichnus. This study, however, demonstrates that AMB are distinct morphologically from these previously described ichnofossils. In addition, while backfilled burrows are reported mostly from marine strata, AMB appear exclusively in pedogenically modified continental deposits. Thus, AMB in outcrop and core can be used to differentiate alluvial paleoenvironments from marine and lacustrine paleoenvironments, as well as indicate periods of subaerial exposure of strata deposited in aquatic settings.