Shingled, sharp-based shoreface sandstones: depositional response to stepwise forced regression in a shallow basin, Upper Triassic Gassum Formation, Denmark

Abstract Sharp-based marine shoreface sandstones interpreted as forced regressive deposits are a characteristic feature of the Gassum Formation in the intracratonic Danish Basin. Detailed process-based sedimentological and a high-resolution, sequence-stratigraphic interpretation of cores from closely-spaced wells has led to improved understanding of the erosional and depositional processes active during the formation of the sharp-based sandstones. Each sandstone shows an internal stacking of forced regressive shoreface units separated by thin muddy offshore facies. This stacked pattern records low-amplitude but widespread changes in relative sea-level during the overall progradation due to low depositional gradients. Laterally, the stacked forced regressive shoreface deposits show a seaward-dipping, shingled geometry indicating seaward displacement of the shoreline through stepwise, forced regressions during overall fourth-order relative sea-level fall. Thereby each sharp-based shoreface sandstone records deposition resulting from interaction of from two scales of superimposed relative sea-level fluctuations: a lower fourth-order fall responsible for the overall seaward shoreface displacement, and a higher fifth-order oscillation that resulted in repeated forced regression within the lower-order sequences. Although these stepwise, forced regressive deposits dynamically resemble ‘stranded’ parasequences, they differ from the conceptualized picture of ‘stranded’ parasequences as simple downstepping of forced regressive deposits, because of their gently dipping shingled geometry and distinctive deposition component resulting from intervening, high-order drowning. For both the fifth-order forced regressive units and the lower-order forced regressive sharp-based sandstones it is possible to differentiate between: (1) deposits formed during falling sea level as part of the forced regressive systems tract and (2) the last, forced regressive to progradational part formed at sea-level lowstand representing the lowstand systems tract. Accordingly, the sequence boundary, whether of high- or low-order, is placed below the last, forced regressive deposits and associated lowstand progradational deposits, but above the deposits formed during falling relative sea-level. Thus the sequence boundary is placed at the surface of subaerial exposure passing seaward into a marine regressive surface of erosion reflecting maximum regression. The basal, regressive surface of erosion below the fourth-order forced regressive systems tract is demonstrated to consist of coalesced fifth-order forced regressive surfaces. Therefore, the fourth-order regressive surface is a composite surface reflecting a series of forced regressions and intervening drowning and as such is diachronous. The basinwide dominance of sharp-based, forced regressive shoreface deposits in Upper Triassic of the Danish Basin is interpreted to reflect the interaction between low gradient and shallow palaeobathymetry, sediment supply and low-amplitude relative sea-level changes.