Hasiotis, Stephen T.¹, Mary J. Kraus²
(1) University of Kansas, Lawrence, KS
(2) University of Colorado, Boulder, CO

ABSTRACT: Examples of how Ancient Soil Biota Mediated Features in Paleosols: Lower Eocene Willwood Formation, Wyoming

Paleopedogenic features of Lower Eocene Willwood paleosols in the Bighorn Basin, Wyoming, include carbonate nodules and complicated paleosol color patterns. These features are intimately related to ancient, above- and below-ground soil biota activities. Previous studies attributed roots and root channels to these features. Willwood paleosols, however, also show that animal burrows were important mediators of carbonate nodules and paleosol color patterns.
Carbonate nodules precipitated in and around burrows and roots. Elliptical to spherical carbonate nodules, 2 to 30 mm in diameter, are composed of solid to boxwork carbonate surrounded by gray zones depleted of iron oxides. Distinct burrows and roots cemented by carbonate are also present. These features grade into segments of burrows and roots offset from the main trace by paleosol slickenside shear planes. In turn, these grade into carbonate nodules with features attributed to burrows and roots but are dispersed through the paleosol. Original shape and surficial morphology of burrow and root pieces are gone, probably due to continued pedogenesis.
Red, yellow-brown, and purple mottles are common and attributed to roots as well as activities of such soil organisms as bacteria, annelids, insects, and other arthropods. Mottles range in size from 0.1 to tens of mm and in shape from crescents and spheres to ellipses that overlap and crosscut one another. Immature paleosols have discrete burrows with distinct mottle patterns. More mature paleosols have complex mottles that, on close examination and by comparison to mottles in immature paleosols, are amalgamations of different shapes and sizes of burrow remnants.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.