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1 Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada, N6A 5B7; gplint{at}julian.uwo.ca
2 Department of Geology and Geophysics, and Geophysical Institute, University of Alaska, Fairbanks, Alaska, 99775-5780; mccarthy{at}gi.alaska.edu
3 Universidade do Vale do Rio dos Sinos-UNISINOS, Geología-Centro 7 Av Unisinos 950, São Leopoldo-RS, Brasil, 93022-000; ufaccini{at}euler.unisinos.tche.br
Guy Plint received a B.Sc. degree from the University of Reading in 1977 and a D.Phil. from the University of Oxford in 1981. After postdoctoral fellowships at the University of New Brunswick (1981-1984) and McMaster University (1984-1986), he joined the faculty at the University of Western Ontario. His current research interests include sedimentology and high-resolution stratigraphy in the Cretaceous of the Western Canada basin and in the Oriente Basin of Ecuador, in an effort to understand controls on cyclic sedimentation.Paul McCarthy received B.Sc. and M.Sc. degrees from the University of Western Ontario and a Ph.D. from the University of Guelph, Canada. From 1996 to 1999 he was a postdoctoral fellow at the University of Western Ontario. Since June 1999 he has been an assistant professor of geology at the University of Alaska, Fairbanks. His current research interests include paleopedology, fluvial sedimentology, and nonmarine sequence stratigraphy of Paleozoic and Mesozoic alluvial deposits in Alaska, western Canada, and Nova Scotia.
Ubiratan F. Faccini is currently an associate professor at Universidade do Vale do Rio dos Santos (UNISINOS), Brazil. He received his B.Sc. (1983) and M.Sc. (1990) degrees in geology from Universidade Federal do Rio Grande do Sul. Presently he is concluding his Ph.D. thesis at that university. His research focuses on sedimentology and application of sequence stratigraphic concepts to nonmarine deposits, with particular interest in the Permian and Triassic red beds of the Paraná basin, Brazil.
Ten allomembers, labeled J-A, have been mapped in the deltaic Dunvegan Formation using 2340 wells and 63 outcrop sections. Each allomember spans about 200 k.y., is bounded by regional transgressive surfaces, and consists of 3-10 component shingles, apparently of autogenic origin. Allomembers H-E are discussed in this article. Their internal stacking patterns and bounding surfaces permit interpretation as depositional sequences. Sequences H-F have a sigmoidal-prismatic geometry, downlap to the southeast, and thin northwest onto the coastal plain. No differential thickening exists toward the orogen in the southwest. In contrast, sequence E thickens markedly toward the northwest. The tabular geometry of sequences H-F suggests accommodation was controlled by sea level change, whereas the wedge geometry of sequence E suggests significant tectonic tilting. Subaerial unconformities that bound valleys and extend onto adjacent interfluve paleosols can be traced 120-330 km updip into outcrop. The base of each sequence is characterized by shoreline backstep of 80-150 km. Transgressive and highstand shingles in each sequence comprise marine deltaic deposits that pass landward into coastal plain strata. Falling-stage shingles offlap seaward with little or no nonmarine equivalents updip, whereas lowstand shingles show renewed coastal onlap. In outcrop, three nonmarine systems tracts are recognized: (1) A channel-dominated, low-accommodation systems tract fills valleys up to 35 m deep and is equivalent to the early transgressive systems tract. (2) A lacustrine-dominated, high-accommodation systems tract comprises mudstone enclosing ribbon channel sandstones, blankets interfluves, and forms the bulk of each sequence. It is equivalent to late transgressive and early highstand systems tracts. (3) A paleosol-dominated, low-accommodation systems tract occupies the upper part of each sequence and is capped by the sequence boundary. It is equivalent to the late highstand systems tract. Sequence-bounding interfluve paleosols represent tens of thousands of years of pedogenesis related to slow aggradation, followed by fluvial entrenchment and clastic bypass, and represent the late highstand, falling-stage, lowstand, and early transgressive systems tracts. Coastal plain deposits record only part (perhaps <50%) of the time represented by each sequence. Updip sediment bypass promoted falling-stage delta progradation, characterized by offlap. These observations imply that, in this example, the subaerial sequence boundary and fluvial valleys formed initially in updip areas and expanded progressively seaward.
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