Oyster Ground Claystone Member SGSKO


Premise The Oyster Ground Claystone Member is defined by Van Adrichem Boogaert & Kouwe (1993). It includes deposits from the southern Dutch Central Graben as well as the Terschelling Basin. Since the member does not always consist of claystones, the lithological affix has been dropped here. In the southern part of the Terschelling Basin (M4 and M7 blocks) and also in several wells in the southeastern area of the Dutch Central Graben (blocks L02 and L03 p.p.) a sandy subfacies is described. The member is transferred from  the Friese Front Formation to the new Skylge Formation, because of the restricted marine character. The Skylge Formation is part of the marine Scruff Group.  
Derivatio nominis Named after the Oyster Ground, a fishing ground situated at approx. N 54°, E 05° in the Netherlands offshore, north of the Dutch Wadden island of Terschelling.  
Type section Location map See figure (pdf)  
  Well L06-02 (pdf)  
  Location N 53°48’53.7
E 04°59’19.97
  Depth 2500 to 2565 m  
  Length 65 m along hole  
Additional section Location map See figure (pdf)  
  Well F18-02 (pdf)  
  Location N 54°04’25.3
E 04°43’48.7
  Depth 2440 to 2512 m  
  Length 72 m along hole  
Definition Medium to dark brown-grey claystones, non- to slightly silty, non-calcareous,  lignitic  and  fossiliferous  (thick  pelecypod shell fragments, ostracods). Some limestone beds have been encountered. Thick sand beds, characteristic for the main Friese Front Formation, are absent. Along the basin margins the lithology may change into a more sandy subfacies, like in the southern Terschelling Basin. The member can be separated from the Kimmeridge Clay Formation by its marginal marine depositional environment and geographical setting.  
Upper Boundary The top is generally marked by a conformable contact with massive glauconitic sands of the overlying Noordvaarder Member in blocks F15 and F18 or the Terschelling Sandstone Member in blocks L03 and L06. This contact may locally become unconformable in the southern Central Graben (blocks L02-L05) where it is covered by the Stortemelk Member (e.g. L05-01). In parts of blocks L03 and L09 the Oyster Ground Member interfingers with the coarse-grained coastal clastics of the Terschelling Sandstone Member.  
Lower Boundary The base of the Oyster Ground Member in the southern Dutch Central Graben and the Terschelling Basin is the conformable contact with the informal main Friese Front Member (boundary: top of the first variegated or grey, non-marine clay-/siltstones or thick sand bed). In the Terschelling Basin the Oyster Ground Member may interfinger with the Friese Front Formation.  
Distribution The member is restricted to the southern Dutch Central Graben, blocks F15, F17, L02 and the Terschelling Basin, blocks F18, L03, L06, M01, M04 and M07 (see Fig.). In the southwest, the member’s lateral equivalent is the main Friese Front Member. The Kimmeridge Clay Formation in the northern Dutch Central Graben is the open marine equivalent of the restricted marine Oyster Ground Member (see pdf)  
Age Sequence 2 sensu Abbink et al. (2006): Late Kimmeridgian – Early Volgian. Age-diagnostic dinoflagellate cysts are present but rare, like Dichadogonyaulax pannaea (FOD: Late Kimmeridgian), Senoniasphaera jurassica and Gochteodinia mutabilis (FOD: E. Volgian). Time significant sporomorphs are Kraeuselisporites huntii (FOD: Late Kimmeridgian) and Couperisporites jurassicus. The frequent occurrence of ostracods such as Macrodentina rugulata, M. transiens, Cytheropteron purum, Paranotacythere pustulata, Klieana calyptroides and Exophtalmocythere? gigantean verifies the interpretation.  
Depositional Setting Lithology, fossils, lignite and regional palaeogeography suggest that the Oyster Ground Member was deposited in restricted lagoon-like conditions with washover deposits. The monotypical thin-walled shell assemblages confirm a restricted marine setting.  
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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