Jurassic depositional systems of the horda platform, Norwegian north sea: practical consequences of applying sequence stratigraphic models

Abstract

During the Jurassic period the Horda Platform, on the eastern margin of the North Viking Graben received a steady supply of clastic sediments which have left a fairly complete stratigraphic record of cyclical shore line transgression and regression. The extensive seismic coverage and excellent sedimentological and biostratigraphic data from more than 30 wells makes the Horda platform a key calibration area for the sequence stratigraphy of the late-Middle to Upper Jurassic (Fensfjord-Sognefjord Formations) of the North Sea. The late-Middle to Upper Jurassic of the Horda Platform comprise very fine grained, highly micaceous to coarse grained sands deposited in shallow marine shelf to shore-face environments. No fluvial or coastal plain facies have been identified. High energy shore-face sands were deposited in north-south oriented linear trends aligned parallel with the structural grain of the Viking Graben. The sands are organised dominantly into 10-30 m thick coarsening-upwards units separated by condensed horizons. The condensed sections are interpreted as maximum flooding surfaces which have been used as the basic correlation framework in which to further investigate sequence stratigraphie relationships. The coarsening-upwards units are interpreted as 3rd order genetic stratigraphie sequences. Although some candidate sequence boundaries have been identified within the coarsening-upwards units significant truncation below the boundaries has not been established and no evidence of fluvial incision or exposure of the shelf has yet been observed. Stacking patterns of the coarsening-upwards units define lower order fining- and coarsening-upwards trends of 2nd order scale. Three 2nd order sequences, displaying characteristic cycle stacking patterns, with candidate sequence boundaries in the Bathonian, Early Oxfordian and Kimmeridgian have been designated. Three 2nd order condensed intervals can be recognised in the Bathonian to Kimmeridgian. The low frequency 3rd order sequences comprise thinner, higher frequency 3rd order sequences. The favoured mechanism of deposition of the 3rd order cycles is small scale fluctuations in the rate of relative sea level rise. Allocyclic and autocyclic mechanisms cannot be differentiated. The lower fine grained part of the 3rd order cycle represents deposition during periods when accommodation space was being produced rapidly. The upper part of the cycle, above the rapid transition into coarser grained lithofacies was probably deposited during periods of reduced accommodation space. They could be termed the highstand and lowstand systems tracks respectively although no erosional sequence boundary was produced because the magnitude of sea level fluctuations were small. Transgressive deposits seem to be poorly represented. Although pebble lags associated with ravinement surfaces are present above most of the coarsening-upwards units, extensive transgressive erosion of cycle tops has probably not occurred. This is based on the observation that fining-upwards profiles of transgressive sediments lying between the base of the transgressive surface and 3rd order maximum flooding surface are thin (generally less than 1 m). Thus, it is suggested that accommodation space created during relative sea level rise was large and the ensuing transgression rapid. This was followed by a period of condensed sedimentation when glauconite and faecal pellets accumulated. The overall geometry of the Sognefjord Formation sands appears to be controlled by local tectonic elements. Faulting was dominantly north-south and individual fault blocks on the Horda Platform were tilted to the east. Sediment appears to have been preferentially transported along the axis of these depressions as exemplified by southward prograding clinoforms and the concentration of high energy sands on the footwall highs. This is probably the major contributor to the development of a lensoid geometry to the Sognefjord Formation sands. Because the Horda Platform was a wide shallow platform with high sediment influx small reductions in accommodation space seem to have produced rapid progradational episodes without accompanying lowstand subaerial erosion. Thus, the 3rd order cycles are believed to be the result of lowstand deposition of the forced regression type.