Helder Member KNNSH


Premise Member defined by van Adrichem Boogaert & Kouwe (1993).
Derivatio nominis .Named after the Helder Oil Field, Block Q01, Dutch off-shore sector.
Type section Location map See figure (pdf)
  Well Q01-Helder-A-01 (originally Q01-07) (pdf)
  Location N 52°55’17.2
E 04°05’54.3
  Depth 1412 to 1547 m
  Length 135 m along hole
  Reference .
Additional section Location map See figure (pdf)
  Well Q01-03 (pdf)
  Location N 52°52’00.2
E 04°08’33.4
  Depth 1275 to 1347 m
  Length 72 m along hole
  Reference .
Definition Clear, quartz-rich sandstones with minor marine and lagoonal claystone intercalations. The fine to medium, well-rounded sand grains are unconsolidated to moder-ately cemented. Lithofacies can be differentiated into: - fine-grained sands characterised by bioturbation and parallel bedding, - slightly coarser-grained sands, massive, or with multidirectional trough and tabular crossbedding, - parallel-laminated to cross-bedded sands with rootlet development. The Helder Member can contain up to four stacked sandstone bodies. A single sandstone body, unconformably overlying Lower-Middle Jurassic or Triassic sediments in L16/L17/Q01/Q02 is informally referred to as Logger Sandstone member (KNNSO).
Upper Boundary In general, the Helder Member is overlain by the Vlieland Claystone Formation. In well Q01-03 biostratigraphical data indicate a minor unconformity at the top of the Helder Member
Lower Boundary In the centre of the Broad Fourteens Basin the Helder Member stratigraphically rests on the Helm Member, which can also be located in lateral positions in more south-eastward localities. The Helder Member unconformably covers older Mesozoic to Palaeozoic strata at the basin margins (e.g. L16, informal Logger Sandstone member).
Distribution The occurrence of the Helder Member is restricted to the Broad Fourteens Basin and is well-developed along the northern margin of this basin. Thick develoments are found in P03 and Q01, thinning into L16 and K18. Basinward (to the north/northwest) the litto-ral sands of the Helder Member grade into the open ma-rine claystone of the Vlieland Claystone Formation.
Age A Valanginian age is inferred from biostratigraphical data (mainly palynomorphs). The range top of Cantulodinium speciosum, in combination with the range bases of Chlamydophorella membranoidea, C. nyei, Surculosphaeridium sp. II of Davey, Kleithriasphaeridium simplicispinum, Oligosphaeridium asterigerum, Wallodinium krutzschii and Apteodinium spp. mark the base of the Helder Member and correspond to the Egmontodinium pharo dinoflagellate subzone. The abundance bases of Surculosphaeridium sp II Davey and of Oligosphaeridium spp., and the range top of Muderongia extensiva mark the upper boundary of the Helder Member The upper part of the member oversteps the basin fringe, and is also known as Logger Sandstone member (informal). It is latest Valanginian - earliest Hauterivian in age. This period marks a next stage of marine basin expansion.
Depositional Setting The Helder Member was deposited in a shallow-marine, littoral facies. A strand-plain of amalgamated beach bars was formed in a wave-dominated environment. The three lithofacies mentioned above represent lower-shoreface, upper-shoreface and fore-/backshore respectively. The Helder Member can be subdivided in up to three transgressive-regressive cycles. The transgressive base of each cycle is often marked by a thin, open-marine claystone. In a typical cycle, stacked progradational shoreface sandstones are overlain by foreshore and backshore sands (with rootlet horizons), and lagoonal claystones which mark the base of a cycle.
Sequence Stratigraphy Sequences LZB 2.1 (late) and LZB 2.2. At least three subsequences can be discerned in LZB 2.2. The last subsequence coincides with a period of marine basin expansion, flooding the thusfar elevated basin margins. This gave rise to the formation of the so-called Logger Sand-stone member.
References See References Upper Jurassic/ Lower Cretaceous

Van Adrichem Boogaert, H.A. & Kouwe, W.F.P., 1993-1997. [Stratigraphic unit]. In: Stratigraphic Nomenclature of the Netherlands.
Retrieved [Datum] from [url].