Rifgronden Member SLCFR

 

Premise The Rifgronden Member, defined by Van Adrichem Boogaert & Kouwe (1993), is part of the Friese Front Formation.
Derivatio nominis Named after the Borkummer Rifgronden, an area with a stony sea-bottom in the North Sea, south of the German Wadden island of Borkum.
Type section Location map See figure (pdf)
  Well F17-04 (pdf)
  Location N 54°03’40.0
E 04°26’13.2
  Depth 2497 to 2572 m
  Length 75 m along hole
Additional section Location map See figure (pdf)
  Well L05-04 (pdf)
  Location N 53°49’06.7
E 04°24’13.3
  Depth 2817 to 2825 m
  Length 8 m along hole
Definition The member comprises dark-grey, carbonaceous, locally silty to sandy claystone, with thin intercalated beds  of  well-sorted, very fine to fine-grained sandstone, dolomite and coal. An up to 25 m thick interval with several thicker sandstone and coal beds is included in the member. It can be found some 20-40 m below the top of the formation in most sections, but is not encountered in reference well L05-04. Cored sections show bioturbated wavy and flaser bedding. The sandstone beds are cross-bedded. Internal coarsening upward trends are frequently observed (cores, funnel-shaped log patterns). This member is age equivalent of the Middle Graben Formation and the upper- most part of the Lower Graben Formation to the north.
Upper Boundary The upper boundary, with the main Friese Front (informal) Member, is placed at the base of the second thick sandstone interval in the type well, often marked by a resistivity-log peak (e.g. Van Adrichem Boogaert & Kouwe, 1993; Annex G-8). Resistivity and sonic log readings above this boundary seem to shift and change slightly, but not uniformly. Upwards, the grain size of sandstone increases slightly, and the marine character disappears.
 
Lower Boundary The lower boundary, with the Altena Group (dark-coloured marine shales), is unconformable and can be seen as a shift in sonic log patterns and as a seismic reflector.
Distribution The member is restricted to the southern Dutch Central Graben, Blocks F17-L05 (see pdf).
 
Age Sequence 1 sensu Abbink et al. (2006): latest Callovian - Early Oxfordian age. Characteristic dinocysts are Durotrigia filapicata (LOD: E. Oxfordian), Liesbergia scarburghensis, Rigaudella aemula and Wanaea (LOD: E. Oxfordian). Towards the top Gonyaulacysta jurassica and Rhynchodiniopsis cladophora become typical. Extreme abundance of the agglutinated foraminifer Ammodiscus sp. is a marked feature of this member.
Depositional Setting Lagoonal.
References See References Upper Jurassic/ Lower Cretaceous

D.K. Munsterman, R.M.C.H. Verreussel, H.F. Mijnlieff, N. Witmans, S. Kerstholt-Boegehold & O.A, Abbink (2012), Revision and update of the Callovian-Ryazanian Stratigraphic Nomenclature in the northern Dutch Offshore, i.e. Central Graben Subgroup and Scruff Group. Netherlands Journal of Geosciences-Geologie en Mijnbouw, 91 (4):555-590.

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