Insights into syndepositional fault movement in a foreland basin; trends in seismites of the Upper Cretaceous,Wahweap Formation, Kaiparowits Basin, Utah, USA Hannah L. Hilbert-Wolf, n ,1 Edward L. Simpson, w Wendy S. Simpson, n Sarah E. Tindall w and Michael C. Wizevichz n Parkland High School, Allentown, PA, USA wPhysical Sciences, Kutztown University of Pennsylvania, Kutztown, PA, USA zDepartment of Physics and Earth Sciences, Central Connecticut State University, New Britain, CT, USA ABSTRACT The Upper Cretaceous Wahweap Formation accumulated in the active Cordilleran foreland basin of Utah. Soft-sediment deformation structures are abundant in the capping sandstone member of the Wahweap Formation. By comparing with well-established criteria, a seismogenic origin was determined for the majority of structures, which places these soft-sediment deformation features in a class of sedimentary features referred to as seismites. A systematic study of the seismite trends included their vertical and horizontal distribution and a semi-quantitative intensity analysis using a scale from 1 to 5 that is based on magnitude, sedimentary structure type, and the predominance of inferred process of hydroplastic deformation, liquefaction or £uidization. In addition, orientations of soft-sediment fold axes were recorded. Construction of a northwest-to-southeast stratigraphic and seismite intensity cross-section demonstrates: (1) reduction in stratigraphic thickness and percentage of conglomerates to the southeast, (2) the presence of lower seismite, middle nonseismite, and upper seismite zones within the capping sandstone (permitting subdivision of the capping sandstone member), and (3) elimination of the nonseismite zone and amalgamation of the lower and upper seismite zones to the southeast. Regional isoseismal contour maps generated from the semi- quantitative analysis indicate a decrease in overall intensity from northwest to southeast in the upper and lower seismic zones and in sandstone within 5 m stratigraphically of the contact between the upper and capping sandstone members. In addition, cumulative seismite fold orientations support a west^northwest direction towards regional epicentres. Isoseismal maps are used to distinguish the e¡ects of intrabasinal normal faulting from those of regional orogenic thrusting.Thus, this study demonstrates the utility of mapping seismites to separate the importance of regional vs. local tectonic activity in£uencing foreland basin sedimentation by identifying patterns that delineate palaeoepicentres associated with speci¢c local intrabasinal normal faults vs. regional trends in soft- sediment deformation related to Sevier belt earthquakes. INTRODUCTION Seismites, earthquake-generated secondary sedimentary structures ( sensu Seilacher, 1969), have recently received serious study because of their ability to (1) ascertain seis- micity in areas where previously unrecognized (Atwater, 1994; McCalpin, 1996; Obermeier, 1996; Ettensohn et al., 2002a) and (2) clarify the in£uence of tectonics in regional studies (Pope et al., 1997; Jewell & Ettensohn, 2004; Takada & Atwater, 2004; Foix et al., 2008). Distinctive criteria have been established that permit recognition of seismites from nonseismic-induced sedimentary structures (Sims, 1975 and many others). Furthermore, seismicity spatial trends can be identi¢ed and linked to probable fault generators (Leeder, 1987; Ringrose, 1988; Guiraud & Plaziat, 1993; Jewell & Ettensohn, 2004). Stratigraphic distribution of re- peated seismite deformation zones has been attributed to the earthquake recurrence interval (Guiraud & Plaziat, 1993; Pratt, 1994). Systematic examination of orientations EAGE 1 Present address: Carleton College, North¢eld, MN 55057, USA. Correspondence: Edward L. Simpson, Physical Sciences, Kutz- town University of Pennsylvania, 424 Boehm, Kutztown, PA 19530, USA. E-mail: simpson@kutztown.edu Basin Research (2009) 21, 856–871, doi: 10.1111/j.1365-2117.2009.00398.x r 2009 The Authors Journal Compilation r Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 856