Making a mountain out of an anthill: Ancient Hymenopteran influence on calcrete outcrop expression, Neogene Ogallala Formation, Western Kansas, USA

The Neogene Ogallala Formation consists of discontinuous beds of conglomerate, sandstone, and siltstone composed of sediment eroded from the Rocky Mountains and deposited as a broad, eastward-thinning clastic wedge in the High Plains region of the United States.  Outcrops of the Ogallala Formation in western Kansas have long been recognized for resistant carbonate rocks that weather with a distinctive honeycomb or boxwork pattern; these have been relatively recently recognized as calcretes associated with paleosols.  Our research in Ellis, Logan, and Scott Counties, Kansas has revealed that much of the Ogallala in these areas consists of stacked paleosols and calcretes developed on mostly friable, silty and sandy alluvial parent material.  Paleosols are typically reddish in color with rare redoximorphic features and soil structure ranges from structureless to angular blocky to prismatic.  Tubular to bulbous, well indurated carbonate accumulations in paleosols are abundant and we interpret many of these as carbonate-filled macropores that represent root channels and burrows excavated by soil macrofauna, i.e., trace fossils.  Our goals were to describe and interpret trace fossils, describe calcrete morphologies, and evaluate the distributions of trace fossils and calcretes.  We recognize a range of trace fossils, including rhizoliths, mammal burrow networks, and two very distinct and abundant forms: 1) carbonate-filled, horizontal, flattened, pancake to kidney-shaped chambers connected by vertical straight to helical tubes, and 2) flask-shaped to ovoid carbonate casts of voids, some with clay linings, that are found individually with random orientations and in clusters radiating outward from vertical to subvertical shafts.  The former are interpreted as fossil ant-nest casts, nearly identical to modern harvester ant nests, and the latter are interpreted as bee brooding cells.  The horizontal chambers and vertical shafts of the ant nests are responsible for much of the honeycomb calcrete pattern.  Prominent vertical calcrete blades appear to be carbonate-filled interped voids between prismatic peds and also contribute to honeycomb calcrete outcrop expression.