Platform-Margin Deltas in the Early-Middle Triassic in the Norwegian Barents Sea
Evy Glorstad-Clark1, Ellen Birkeland2, and Johan Petter Nystuen1
1Department of Geoscience, University of Oslo, Oslo, Norway.
2Exploration, Wintershall Norge ASA, Stavanger, Norway.
The Early to Middle Triassic in the Barents Sea was dominated by transgressive-regressive clinoform sequences prograding out from the Fennoscandian Shield in the south and the Uralian Mountains in the east and southeast observed on 2D seismic data. These systems were deposited in a large (several hundred kilometers wide), relatively shallow epicontinental sea, where modest variations in relative sea-level relocated the shoreline significantly over large areas. Time-thickness and seismic facies maps were the fundamental parts for reconstructing the paleogeography for each third-order sequence and for interpreting the distribution of reservoir, seal and source rocks.
In this study we use the term platform to denote the top-sets of clinoform packages, representing the shallow marine platform located around the margin of a deeper basin within the epicontinental seaway. These seismic scale platform-margin clinoforms should not be confused with deltaic clinoforms as they are a magnitude larger, hundreds of meters vs. tens of meters high, respectively. Different types of clinoform geometries were analyzed to understand the position of the shoreline at the time of deposition as well as sand delivery potential.
This study shows the development of strike elongated wedges that thickens just outboard of the platform-margin. Seismic facies mapping and time-thickness maps show the position and development of platform margin delta complexes within the sequences. Seismic clinoforms and trajectory analysis show significant lateral variation from the axis of the delta complex to areas adjacent to the main deposition. Frequent toplap geometries were observed in proximity of coarse grained deposits, and aggradation of low-angle seismic clinoforms lateral to the main delivery system. A complex shift in depocenters were observed based on large-scale compensational stacking and the creation of relict shelf breaks due to sediment partitioning prior to the next phase of progradation into the basin. This study shows that the shoreline often was positioned landward of the seismic offlap break, but that careful mapping of clinoform geometries, clinoform angles and trajectory analysis can reveal areas where the shoreline was most likely coinciding with the seismic offlap break. Sometimes this was caused by locally increased sediment supply during slightly rising relative sealevel, and occasionally caused by a regional drop in relative sea level with significant shelf bypass.
AAPG Search and Discover Article #90096©2009 AAPG 3-P Arctic Conference and Exhibition, Moscow, Russia