Sea Level Fluctuations as the Primary Control on the Emplacement of a Deltaic Clinoform Package in an Active Rift Setting

Abstract

Sedimentary systems on exposed basement highs bordering an active rift basin are rarely preserved in the geological record. We present a case study of a proximal syn-rift sedimentary system that was preserved due to early transgression of the footwall high.

At the onset of Late Jurassic rifting the Frøya High emerged as a prominent N-S trending, 25 km wide basement high located on the mid-Norwegian margin. The flat-topped high became tilted towards the east in response to several kilometers of fault displacement along the high’s western flank. Consequently, the Frøya High became a semi-isolated promontory that shed sediment to marine basins that existed to the west and to the east.

Using well-calibrated 3D seismic reflection data we observe a conspicuous series of Upper Jurassic wedges along the eastern flank of Frøya High. Internally, these wedges show clinoform geometries with top- and foresets. The clinoform package reaches a maximum height of 200 meters; the foresets are dipping c. 10 degrees. We interpret these wedges to represent a series of eastward outbuilding deltas. Updip of these depositional features E-W-trending canyons and valleys are observed. These are interpreted to represent the river catchments of the clinoform package.

The studied clinoform package formed towards the end of the rift climax stage, when the eastern part of the footwall crest was transgressed in a westerly direction. This transgression occurred over a period that lasted several million years. The modeled sediment flux as constrained by the size and characteristics of the mapped catchment area suggests that the total sediment volume contained in the clinoform package may have formed over a relatively short timespan; perhaps only a few hundreds of thousands of years. We use numerical modeling to test the hypothesis that the clinoform package may have formed over a short period of time in response to a period of sea level still stand. We use size and characteristics of the sediment source area as well as the mapped volume of sediment stored in the clinoform package to constrain and calibrate our model.

This study contributes to a better understanding of the relative importance of sea level fluctuations and tectonics as controls on the evolution of sedimentary systems on basement highs in active rifts. These proximal systems control the delivery of sediment to the down-faulted basin and therefore influence the distribution of reservoir facies in syn-rift basin-fills.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018