|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)|
|Depth||2497 to 2572 m|
|Length||75 m along hole|
|Additional section||Location map||See figure (pdf)|
|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.|
|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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