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Willwood Formation, Lower Eocene, Northwest Wyoming, U.S.A.) University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USGS Staff -- Published Research US Geological Survey 1993 Time-Stratigraphic Reconstruction and Integration of Paleopedologic, Sedimentologic, and Biotic Events (Willwood Formation, Lower Eocene, Northwest Wyoming, U.S.A.) Thomas M. Bown U.S. Geological Survey Mary J. Kraus University of Colorado Follow this and additional works at: https://digitalcommons.unl.edu/usgsstaffpub Part of the Earth Sciences Commons Bown, Thomas M. and Kraus, Mary J., "Time-Stratigraphic Reconstruction and Integration of Paleopedologic, Sedimentologic, and Biotic Events (Willwood Formation, Lower Eocene, Northwest Wyoming, U.S.A.)" (1993). USGS Staff -- Published Research. 206. https://digitalcommons.unl.edu/usgsstaffpub/206 This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. PALAIOS, Vol. 8, No. 1 (Feb., 1993), pp. 68-80 68 RESEARCHREPORTS Time-StratigraphicReconstruction and Integration of Paleopedologic, Sedimentologic,and Biotic Events (Willwood Formation, Lower Eocene, Northwest Wyoming, U.S.A.) THOMAS M. BOWN U.S. Geological Survey, Denver, CO 80225 MARY J. KRAUS Department of Geological Sciences, University of Colorado, Boulder, CO 80309 PALAIOS, 1993, V. 8, p. 68-80 imentologic and biologic events may reflect tectonic puls- es, periods when the rate of basin subsidence increased Relative paleosol maturities are inversely proportional to more rapidly. the accumulation rates of the sediment upon which they formed, and are therefore excellent relative indicators of how much geologic time elapsed between any two hori- INTRODUCTION zons. An empirically-based model is advanced using pa- leosol maturities to estimate the relative geologic time Stratigraphic sequences reflect a complex interplay separating any stratigraphic levels within the lower Eo- among sedimentation, erosion, and non-deposition. In al- cene Willwood Formation. The revised Willwood time luvial settings, overbank floods are generally of short du- stratigraphy from this analysis helps evaluate the nature, ration and recur every year or two. During the hiatuses tempo, and possible causes of three major episodes of between floods, pedogenesis modifies the alluvium. In fact, mammalian appearance and disappearance. These fau- the proportion of geologic time represented by soil for- nal events are directly correlated with certain aspects of mation (as opposed to deposition or erosion) in a sequence paleosol evolution in the Willwood Formation. That evo- of alluvial rocks is very large (Kraus and Bown, 1986). lution is tied directly to climatic changes and to varying Because pedogenic modification was penecontempora- sediment accumulation rates in response to tectonism. neous with deposition of the parent material, the maturity The first faunal turnover occurs at the base of the Will- of alluvial paleosols was partly controlled by the rate at wood Formation. It coincides with a major increase in which alluvium accumulated (e.g., Leeder, 1975; Retallack, pedogenic maturity, reflecting a major decrease in sedi- 1986; Bown and Kraus, 1987). In aggrading alluvial se- ment accumulation rate, and accompanying general cli- quences, the faster the rate of sediment accumulation, the matic warming at about the time of the Paleocene-Eocene less mature are the paleosols. boundary. Throughout the remainder of Willwood time, Both modern (e.g., Bridges, 1973) and ancient studies there was a gradual, yet continual, decrease in paleosol (e.g., Bown and Kraus, 1987) show that, in aggradational maturity and degree of hydromorphy, probably related alluvial systems, increasingly mature paleosols occur with to the progressive structural elevation of the Owl Creek increasing distance from a channel belt as a result of slower antiform bounding the south and southeast margins of sediment accumulation rates. Rates decrease because new the Bighorn Basin. This gradual decrease was punctuated alluvium from a single flood typically thins away from a by two intervals of more significant decline in paleosol channel and areas farther from the channel are flooded maturity and in the incidence of hydromorphic soils. Both less frequently than areas close to the channel (e.g., Bridge intervals are also marked by faunal turnovers. These sed- and Leeder, 1979). Furthermore, several sequential pro- Copyright? 1993, SEPM (Society for SedimentaryGeology) 0883-1351/93/0008-0068/$3.00 TIMERECONSTRUCTION AND BIOTICEVENTS, WILL WOOD FORMATION 69 Levee to place critical geologic (and paleobiologic) events in their proper temporal perspective. Lack of definitive control becomes an especially obdurate problem in cases in which sediment accumulation rates changed considerably through time. In addition, resolution of the temporal contributions of periods of erosion and non-deposition, largely ignored in such studies, has been a formidable obstacle to progress Sandstone .:.-;;I:'-...--' in temporal sequencing within formation-rank alluvial FIGURE 1 -Schematic diagram showing pedofacies relations. Num- units. When it is recognized that nearly all alluvial units bers refer to paleosol stages. Plus or minus signs indicate a greater contain paleosols and that the temporal nature of paleosols or lesser degree of maturity than is typical for a particular stage. A is, by definition, hiatal, the significance of paleosol studies major crevasse-splay sandstone typically forms the base to a pedo- to elucidating sediment accumulation rates is clearly crit- facies package. Not to scale. ical. In developing the paleosol maturation sequence and pe- dofacies model for of the Willwood Formation files form to the channel the same paleosols may proximal during Kraus and Bown and time that a in the distal (Bown, 1985; Bown, 1986; Kraus, period single profile develops it became clear that anal- where sediment influxes are both thinner and 1987; Kraus, 1987), pedofacies floodplain, for a less Bown and Kraus used the term yses possess great potential establishing technique frequent. (1987) pe- fluvial suites dofacies to describe the lateral in whereby containing numerous, superposed changes paleosol type could be in a relative to a channel A is a paleosols, temporally sequenced relative, yet sandbody. pedofacies package based manner without recourse to either bio- of with ver- empirically prism essentially contemporaneous paleosols or determinations. What had stacked and close to a stratigraphy geochronologic tically poorly developed paleosols been until was a determinative basis for channel which into coeval flood- lacking recently sandbody, grade laterally the relative of time with fewer but more assessing proportion represented by plain deposits strongly developed of different maturities. A mechanism of Pedofacies are both to and paleosols propor- paleosols (Fig. 1). responsive tional of fluvial rocks has obvious indicators of: relative short-term net sediment temporal sequencing good 1) for and in accumulation and the distribution utility assessing geologic paleobiologic tempo rates; 2) proportionate event and and has of time in alluvial rock To the extent stratigraphy biostratigraphy, unique geologic sequences. for rocks for which this cannot be as- that tectonism influences sediment accumulation applications tempo regional sessed more conventional means. We stress a record of tectonic by that, given rates, pedofacies provide good activity the coarseness of current for fluvial as well and Formation- strategies determining (Kraus Bown, 1986; Kraus, 1987). for more scale vertical of also a record tempo, any empirically-based technique precisely sequences pedofacies provide and events in time is of soil for considerable of time. apportioning geologic paleobiologic paleoenvironments periods a substantial Pedofacies were first worked out in the improvement. relationships Wheeler first introduced the of con- lower Eocene Willwood Formation of northwest (1958) possibility Wyoming, or "chrono- an alluvial unit about 700 m thick with numerous stacked structing time-stratigraphic cross-sections, stratigraphic charts" in sequence stratigraphic parlance pedofacies (Bown and Kraus, 1987; Kraus, This 1987). (e.g., Vail et al., 1984). Embroidering on concepts devel- paper expands upon those earlier studies and examines oped partly by Blackwelder (1909), Wheeler (1958, p. 1050) the relationship of Willwood pedofacies to fluvial sedi- conceived of subjective time-stratigraphic units as mentation, geologic time, and paleobiologic events. The first part of the paper focuses on temporally reconstructing "... all three-dimensional entities (material, non-ma- the Willwood section based on pedofacies. We then use terial, or combinations of both) ... which are defined this temporal framework to document significant episodes in a framework consisting of two lateral space dimen- of faunal turnover. Lastly, the faunal events are evaluated sions and a vertical time dimension." These units "... in light of changes in paleosol maturity and hydromorphy must delineate all interpreted deposition combination through Willwood time. of these may be useful in the interpretation and vi- sualization of geologic history." TIME STRATIGRAPHIC RECONSTRUCTION Wheeler introduced the "holostrome", which is a time- stratigraphic unit representing the temporal record of both For
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