The interaction of aeolian and fluvial processes during deposition of the Upper Cretaceous capping sandstone member, Wahweap Formation, Kaiparowits Basin, Utah, U.S.A. E.L. Simpson a, ⁎, H.L. Hilbert-Wolf b , W.S. Simpson b , S.E. Tindall a , J.J. Bernard a , T.A. Jenesky a , M.C. Wizevich c a Department of Physical Sciences, Kutztown University of Pennsylvania, Kutztown, PA 19530, United States b Parkland High School, 2700 North Cedar Crest Blvd., Allentown, PA 18104, United States c Central Connecticut State University, Department of Physics and Earth Sciences, New Britain, CT 06050, United States abstract article info Article history: Received 14 January 2008 Received in revised form 8 August 2008 Accepted 12 August 2008 Keywords: Aeolian deposits Cretaceous Wahweap Formation Braided rivers Utah Detailed examination of the Upper Cretaceous capping sandstone member of the Wahweap Formation in the Kaiparowits Basin, Utah reveals the presence of aeolian stratification. Deposition by aeolian processes is recognized and distinguished from subaqueous deposition by the presence of centimeter-scale, inversely graded wind-ripple stratification. The aeolian stratification in the Wahweap Formation is the first occurrence reported from Upper Cretaceous strata on the Colorado Plateau of the western U.S.A. Aeolian stratification occurs as thin wind-reworked caps of fluvial-bar sandstones deposited within low-sinuosity braided streams and as more extensive deposits of small-scale dunes that developed in geographically restricted dune fields. Aeolian reworking of bars took place during low-stage flow and was possibly controlled by intermittent (seasonal) discharge variations. Prolonged aridity led to increased sand supply entering the aeolian system, dunes nucleated and grew rapidly between the braided stream systems. These small-scale dunes rarely developed extensive or tall slip faces as evidenced by the rare preservation of grain flow strata and the dominance of wind-ripple stratification. The dune field was characterized by sinuous-crested dunes, probably barchans, amid and linked to the extensive braided stream systems. Based on modern analogs, the resulting change in sand storage/supply probably is related to a short-term shift to a more semi-arid/arid climate, possibly seasonally. However, a longer-term climate change to semi-arid/arid may be indicated for the dune complex near the boundary with the overlying Kaiparowits Formation. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Systematic study of the generation of small-scale aeolian stratifi- cation has led to recognition of unique physical processes that produce inversely graded ripple stratification, the distinctive product of wind-ripple migration (Hunter, 1977, 1981). These inversely graded, wind-ripple strata provide a criterion that allows separation of subaerial from subaqueous depositional processes at a centimeter scale (Hunter, 1977, 1981; Kocurek and Dott, 1981). Although aeolian stratification is widespread throughout the Pennsylvanian to Jurassic age strata of the Colorado Plateau (Blakey et al., 1988; Marzolf, 1988; Peterson, 1988), this study is the first reported occurrence of aeolian stratification from Upper Cretaceous sediments of the Plateau. The Upper Cretaceous capping sandstone member of the Wahweap Formation in the Kaiparowits Basin, Utah preserves relatively thin units of aeolian stratification within largely fluvial deposits. Two distinct geometries of the aeolian stratification units suggest two scales of aeolian processes: fluvial bar-top reworking and small dune development in geographically restricted dune fields. This study of the Upper Cretaceous capping sandstone member: 1) describes the preserved inversely graded wind-ripple stratification, including its associations with encompassing fluvial deposits; 2) inter- prets the depositional processes and geomorphic systems; and 3) exam- ines possible climatic factors that controlled the interaction between aeolian and fluvial systems. 2. Geologic setting Within the Kaiparowits Basin, the Upper Cretaceous Wahweap Formation conformably overlies the Straight Cliffs Formation and is in turn overlain by the Kaiparowits Formation (Fig. 1; Peterson, 1969; Eaton, 1991; Lawton et al., 2003). The Wahweap Formation is informally subdivided into, from oldest to youngest, lower, middle, upper and capping sandstone members (Eaton, 1991). In the study area, the upper and capping sandstone members are distinguished. Palaeogeography, Palaeoclimatology, Palaeoecology 270 (2008) 19–28 ⁎ Corresponding author. Tel.: +1 610 683 4445; fax: +1 610 683 1352. E-mail address: simpson@kutztown.edu (E.L. Simpson). 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.08.009 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo