Sedimentology and subaerial exposure surfaces of the Middle Devonian Snipe Lake reef complex, Swan Hills Formation, west-central Alberta

The Snipe Lake reef complex in the Swan Hills Formation (earliest Frasnian) is a well-preserved, hydrocarbon-bearing reef buildup in west-central Alberta. It consists of five platform megacycles and five reef megacycles which are arranged in an overall backstepping pattern. This study uses an integrated approach that combines core examination, detailed petrography and geochemistry to evaluate the depositional and diagenetic history of the reef complex. This study focuses on the P4 and P5 platform successions and the R0.5 reefal succession and, in particular, the subaerial exposure features in these successions.; Nine lithofacies have been recognized in the P4 to R0.5 successions that range from tidal flat lithofacies in the platform-reef interior to reef foreslope lithofacies at the reef margin. Lithofacies can be grouped into lagoonal lithofacies associations and reef margin lithofacies associations. Cyclical deposition in the Snipe Lake reef complex was recorded by the stacking patterns of third-order reefal cycles and fourth-order lagoonal cycles. The P4 and P5 platform successions are composed of a series of backstepping shallowing-upward lagoonal cycles, whereas the R0.5 reefal succession is composed of a progradational reefal cycle at the reef margin and a series of lagoonal cycles in the reef interior.; Subaerial exposure surfaces have been recognized at the P4 and R0.5 cycle contacts. Detailed petrographic and isotope geochemistry analysis has identified several phases of diagenesis below these emergent surfaces, including early marine diagenesis, meteoric diagenesis during subaerial exposure, a subsequent phase of marine diagenesis as relative sea level rose and a final phase of burial diagenesis as sediments were progressively buried.; A variety of pedogenic diagenetic features are observed below the P4 and R0.5 subaerial exposure surfaces which include, from top to bottom, green shale horizons, rhizoliths, fitted-fabric lithoclasts, glaebules, peds, laminar calcretes, green shale in cracks and vugs, marine pendant cements developed in vugs and meteoric pendant cements. Four types of paleosol profiles (A, B, C and D) have been recognized that characterize these features below the subaerial exposure surfaces. The P4 paleosol ranges from 0.75 to 2.1 m thick and is composed predominantly of the type A paleosol and rarely the type B paleosols. The R0.5 paleosol ranges from 0.12 to 1.3 m thick and is composed of types B, C and D paleosols. These subaerial exposure surfaces are interpreted to be the result of relative sea level falls which are estimated to be a minimum of 2.1 m and 1.3 m at the top of the P4 and R0.5 successions, respectively. The main factors that influenced the development of paleosols include climate, topography, vegetation and organisms, and length of exposure. In addition, erosion during the ensuing transgression significantly affected the preservation of paleosol profiles.