Nonmarine sequence stratigraphy: Updip expression of sequence boundaries and systems tracts in a high-resolution framework, Cenomanian Dunvegan Formation, Alberta foreland basin, Canada

Abstract

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. (Begin page 1968) (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