Differentiating paleowetland subenvironments using a multi-disciplinary approach: An example from the Morrison formation, South Central Wyoming, USA

Small lakes are excellent records of environmental changes and are sensitive to local climate fluctuations. Large lacustrine systems record larger-scale climate events. Wetland deposits also provide high-resolution paleoclimatic and paleoenvironmental data. Although wetland deposits are typically associated with wet climatic conditions, they actually occur in a wide variety of climates. Whereas lakes and wetlands are very sensitive to changes in climate, high-resolution data in these archives are lost unless it is possible to differentiate these deposits from other “floodplain” deposits. Synthesis of micromorphological and geochemical data with facies relationships is critical to accurate interpretations of ancient wetland–lacustrine deposits. This study illustrates how micromorphology and geochemistry, combined with facies relationships, provide a viable method for differentiating complex, ancient wetland–lacustrine subenvironments. A hydrogeomorphological classification is proposed to provide appropriate genetic terminology that is useful in the rock record. Eight Morrison Formation subenvironments within lacustrine depositional successions from north central Wyoming were delineated using this interdisciplinary approach. Subenvironments include shallow-water wetlands, marsh, lacustrine fringe, and fen basin and fringe deposits. Synthesized data indicate that an overall basinal infilling succession was punctuated by 3 major lake contraction events, which were followed by extended periods of landscape stability. Initially balance-filled conditions dominated the shallowing-upward lacustrine deposits such that sediment and water supply, and accommodation space were relatively equal. As the lake basin filled, shallower water, underfilled lake conditions prevailed. The distribution of wetland pedotypes indicates that evapotranspiration remained high, suggesting that a stable, semi-arid to arid climate dominated in this area throughout Morrison time. Also supporting a semi-arid to arid climate are more positive carbon isotopes compared to values for disseminated organic material in the surrounding area, abundant barite nodules, gypsum pseudomorphs, and zeolites. Although major fluctuations in lake level may have resulted from punctuated wetter episodes, synthesized data suggest that overall the climate remained relatively arid to semiarid and the lake basin remained closed throughout most of Morrison time. Fluctuations in lake level likely resulted from a signi