Stratigraphic Evidence for Major Shifts in Meander Bend Planform Morphology: Examples From Late Cretaceous Meander Belt Deposits of Southern Alberta

Abstract

Point-bar deposits are composite elements often characterized by complex scroll-bar patterns formed in response to processes of expansion, rotation, and translation of a meander bend. Intra-point-bar erosion is commonly observed on the floodplain of modern river systems, yet its product is rarely described from the ancient record. Late Cretaceous units comprised of point-bar, counter-point-bar, and abandoned channel fill deposits crop out along the Red Deer River valley in south central Alberta, Canada. The strata are characterized by widespread evidence for intra-point bar deposit erosion and punctuated rotation, which provides unique insights into possible stage changes, flood events or channel cut-offs and avulsions. Two study areas are investigated: the Horseshoe Canyon Formation at Willow Creek and the Dinosaur Park Formation at Dinosaur Provincial Park. Detailed sedimentologic characteristics were compiled from 75 measured sections, and paleocurrent measurements from trough-cross stratification. Individual lateral accretion and erosional surfaces were surveyed using a high resolution (10 cm) differential GPS unit. Accretion and erosion surface data were projected in 3-D, and dip angle and azimuth calculated. In the stratigraphic record, intra-point bar deposit erosion and punctuated rotation is expressed by steeply dipping discordant surfaces (up to 20°), which truncate previously deposited lateral-accretion packages. Across these erosion surfaces the direction of bar migration often rotates significantly, up to 50°, and can result in significant facies shifts. Subsequent accretion surfaces onlap intra-point-bar-deposit erosion surfaces. The analysis reveals that point-bar surfaces are formed through a complex interplay of erosional and depositional processes along their length, resulting in stratigraphic surfaces that may be highly composite. This study departs from more simplistic process models of point bars that consider deposits to be the product of continuous lateral accretion, and as such has important implications for paleoenvironmental reconstructions and subsurface reservoir characterization. Specifically, recognition of counter-point bar deposits greatly informs the distribution of reservoir (sandstone) verses non-reservoir (siltstone), while regular truncation of lateral-accretion deposits may increase connectivity by reducing the length scale of fine-grained baffles and barriers to fluid flow.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016