INTEGRATING SEDIMENTOLOGY, ICHNOLOGY, AND PALEOPEDOLOGY TO RECONSTRUCT LANDSCAPES AND ECOSYSTEMS OF THE UPPER EOCENE PIPESTONE SPRINGS STRATA, WESTERN MONTANA

The ichnofossil–paleosol patterns exhibited in the Pipestone Springs vertisol, alfisol, and inceptisol paleosols record periods of landscape stability within a loessic depositional environment where pedogenesis was the most prevalent process that occurred between sedimentation episodes. Late Eocene (Priabonian) strata of the Pipestone Springs are part of the informal Sequence 2 of Hanneman and Wideman (1991, 2006) and a lithostratigraphic proxy, the Renova Formation, of Kuenzi and Fields (1971). The recognition of loessites comprising these strata is a new depositional interpretation of Pipestone Springs strata, making these loessites some of the oldest known aeolian Eocene strata in the Great Plains–Rocky Mountains regionCollectively, the Pipestone Springs paleosols are tentatively interpreted as including compound, composite, and cumulative paleosols because of their stacking pattern within the pedogenic profiles with respect to the parent material, formation on pre-existing pedogenic horizons, and/or overthickening of horizons. Vertisol paleosols exhibit both compound and composite profiles depending on whether the profile reflects rapid sedimentation and insignificant erosion (compound paleosol) or the rate of pedogenesis was greater than sedimentation (composite paleosol). The alfisol paleosols typically developed composite profiles because the rate of pedogenesis was greater than that of sedimentation. Nonsteady and steady sedimentation produced inceptisols with reworked volcanic and non-volcanic parent material, thus generating both compound and cumulative paleosols. Nearly all paleosols are extensively bioturbated, containing complex trace fossils that weather into individual components resembling Rebuffoichnus, Feoichnus, Eatonichnus, Fictovichnus, and Coprinisphaera. During the interval of soil formation, the Pipestone Springs landscape was covered with ample vegetation that fed dung-producing herbivores. This dung was most likely used by adult dung beetles to construct shallow nests in which their eggs were laid. These eggs later hatched into larvae that fed on the provisions. The combination of biotic and abiotic features in the vertisols, alfisols, and inceptisols indicate that they all represent well-drained paleosol profiles in which vadose zone conditions dominated. The ichnofossil-bearing soils were each subsequently buried when the accumulation of loess resumed.