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Development of a shelf margin delta due to uplift of Munkagrunnur Ridge at the margin of Faroe–Shetland Basin: a seismic sequence stratigraphic study

ABSTRACT During the last decade several 3D digital reflection seismic datasets have been acquired in the Faroese sector of the Faroe–Shetland Basin which allow detailed seismic interpretation and mapping of parts of the area. This study presents mapping and seismic sequence stratigraphic interpretation of parts of a c. 450 m thick, mid-Eocene delta extending over some 3500 km2 in the Faroe–Shetland Basin. The delta built out into the basin from the Munkagrunnur Ridge and has been divided into nine seismic units. Isochore maps of these seismic units reveal a NW–SE depositional maximum trend, parallel to the slope front of the delta in most units. The dip of the slope front of these units is about 2.3–3.8°. The lowest unit has the outer form of a thin sheet, whereas the upper unit, a basinward divergent wedge, is mostly constrained to the shelf area. Using the outer form of the seismic units and reflector terminations against the bounding horizons, the surfaces separating the nine mapped units are interpreted as either maximum regression surfaces (MRS) or surfaces representing the start of base level fall (BSFR) or a combination of these two types of surface. Surfaces of regression and erosion are observed within several units and, with additional information regarding the seismic character and nature of internal unconformities and systems tracts, the nine seismic units are divided into systems tracts deposited during eight cycles of relative sea level change. Within the study area falling stage and lowstand systems tracts (FSST and LSST) are generally better preserved than transgressive and highstand systems tracts (TSST and HSST). Maximum flooding surfaces separating a TSST from the overlying HSST are not recognized, and in several sequences the TSST and HSST are either absent or so thin that they cannot be resolved in the seismic data. The slope front of the delta was first developed in the lower, aggradational part of the delta and retained during the upper, progradational part, where the delta evolved as a small shelf margin delta. Adding realistic thermal subsidence to the global sea level curve indicates that the Faroe–Shetland Basin was characterized by almost continuous relative sea level rise in the mid Eocene. Multiple stages of uplift of the Munkagrunnur Ridge during the mid-Eocene are considered to be the most likely explanation for the multiple cycles of sea level change inferred in the delta.