Sedimentological nature and tectonic implications of marine flooding surfaces and their basinward equivalents: examples from Middle Jurassic, South Viking Graben, North Sea and Middle Ordovician, Williston Basin, western North America

Abstract

Sequence stratigraphic analysis of Middle Jurassic (Bathonian to mid-Oxfordian) strata in the South Viking Graben results in a consistent basinwide framework for better understanding sedimentary facies distribution. Five sequences are defined on the basis of the depositional systems they each contain and the surfaces which bound them. The ages of the boundaries of the sequences are taken from an extensive palynostratigraphic database. We believe the boundaries represent surfaces of initial marine flooding and basinward sediment starvation and mark the bases of transgressive-regressive episodes principally controlled by tectonically induced subsidence. Sedimentological changes observed at any point along these marine flooding surfaces (and their basinward equivalents) vary depending on the proximal to distal (subaerial to basinal) location of the depositional sites above and below the flooding surface. This concept is illustrated at the marine flooding surface separating the Middle and Upper Callovian sequences. At five positions along a proximal to distal transect from Sleipner Terrace (Norway Blocks 15/12 and 15/9) to Beinn-East Miller Shelf (UK Blocks 16/8a, 16/7a) the following facies juxtapositions are observed: o (1) Cross-bedded and burrowed delta-front sandstone overlying delta-plain coal (southern and eastern Sleipner Terrace); (2) Cross-bedded and burrowed delta-front/upper shoreface sandstone overlying thin delta-plain coal and sandstone (northern and central Sleipner Terrace); (3) Burrowed fine sandstone and shale of lower shoreface origin erosionally overlying burrowed and cross-bedded sandstone of delta-front or upper shoreface origin (northern Sleipner Terrace); (4) Outer shelf open marine shale and siltstone overlying shelf-ridge sandstone (UK Block 16/13); (5) Condensed sections of marine shale overlying siltstone or submarine channel sandstone (eastern and central UK Block 16/8). Based on palynostratigraphic data, the Middle/Late Callovian flooding surface and its basinward equivalent can be demonstrated to be generally synchronous. The sedimentological changes observed at this sequence bounding surface are abrupt, especially in the more proximal parts of the transect. For these reasons, we believe that this boundary and its associated facies juxtapositions are the result of relative sea level changes and sedimentation rate variations related to tectonically induced subsidence during the syn-rift phase of South Viking Graben development. The recognition of sedimentological changes above and below marine flooding surfaces (and their basinward equivalents), where sequences have been defined without adequate biostratigraphic control, is problematic since basinwide sequence and bounding surface correlation cannot always be clearly demonstrated. A Middle Ordovician example from the Winnipeg Group of the Williston Basin (western North America) illustrates such difficulties.