--> Abstract: Estuaries Without Incised Valleys - Example from the Tarbert Fm, Norwegian Block 30/9, by Tore M. Loeseth and Mike Young; #90078 (2008)

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Estuaries Without Incised Valleys - Example from the Tarbert Fm, Norwegian Block 30/9

Tore M. Loeseth and Mike Young
Exploration Technology, StatoilHydro, Bergen, Norway

The Bajocian Tarbert Formation in the Oseberg Sør Field (Norwegian Block 30/9) constitutes a paralic to shallow-marine regressive-to-transgressive shoreline wedge, deposited during a continuous rise of relative sea level.

The lower portion of the wedge contains both coarsening upward wave/storm dominated shoreface deposits as well as coal-bearing paralic deposits, and was deposited during “normal” (or ascending) regression. As the shoreline migrated seawards, the shoreline trajectory gradually became more aggradational, resulting in wetter conditions on the delta plain, reflected by an upward transition into deeper and wider embayments/lakes in the paralic deposits. In the most seaward area, these paralic deposits are replaced by a contemporaneous aggradational shoreface, demonstrating a seaward pinchout of the delta plain. The turnaround from regression to transgression was accompanied by the formation of an extensive barrier bar, with widespread mudstone deposited landward of the barrier. The upper, transgressive portion of the wedge is characterised by estuarine deposits, exhibiting an upward change from inner to central to outer estuary deposits. In the landward area, there is preferentially less of the outer estuary deposits, reflecting a temporal evolution from a highly to a less aggradational shoreline trajectory.

The study demonstrates that estuaries can develop without any fall of relative sea level and valley incision. Furthermore, we demonstrate that as the shoreline trajectory ascended and the depositional system became dominantly aggradational, the shoreline was very sensitive to small changes in subsidence (fault-controlled). Regions of greater subsidence experienced restricted transgression at the same time as regression occurred laterally in regions of lower subsidence.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas