Spilsby Sandstone Formation

updated to follow: Stratigraphic Guide to the Cromer Knoll, Shetland and Chalk Groups of the North Sea and Norwegian Sea. Felix M. Gradstein & Colin C. Waters (editors), Mike Charnock, Dirk Munsterman,  Michelle Hollerbach, Harald Brunstad, Øyvind Hammer & Luis Vergara (contributors). Newsletter on Stratigraphy, vol 49/1 pp71-280, 2016

Cromer Knoll Group

Introduction

The term Spilsby Sandstone Formation was introduced by Rhys (1974) for a unit of fine grained marine sandstones that lay between the Speeton Clay Formation (Cretaceous: now renamed Valhall Formation) calcareous mudstones above and Kimmeridge Clay Formation (Jurassic) variably calcareous mudstones below.

Name

From the village of Spilsby in Lincolnshire (Rhys 1974).

Lithology

The Spilsby Sandstone Formation consists of grey to white, poorly to well-cemented, very fine to medium grained sandstone, with thin interbeds of green to blue-green, firm to fissile mudstone. Glauconitic grains and phosphatic nodules commonly occur (Lott & Knox, 1994) and calcareous concretions occur at certain levels (Cameron et al., 1992).

Thickness

The formation is generally thin (<50 m) over most of its subcrop area of the Southern North Sea but thickens substantially along the western margin of the Sole Pit Inversion (Dowsing Fault zone) to >250 m (e.g.  53/2-5, Lott & Knox, 1994). Onshore, the formation has a maximum thickness of 200 m on the East Midlands Shelf and onto the Anglo-Brabant Massif (Cameron et al., 1992).

Geographical distribution

The Spilsby Sandstone Formation is largely restricted to the southwestern margins of the Southern North Sea basin over the East Midlands Shelf and onto the northern flank of the London-Brabant Massif. Isolated occurrences of basal Cretaceous sandstones have been proved further eastwards in wells 50/16-1 and 54/1-2 (Lott & Knox, 1994).

Type well

Well name: 48/22-2

WGS84 coordinates: Lat. 53º 15’ 39.5”N      Long. 01º 22’35”E (Rhys, 1974, pp. 7­9, fig. 7, table 5)
UTM coordinates:
UTM zone: 31
Drilling operator name: BP Exploration Operating Company Limited
Completion date: 28.01.1968
Status: P & A
Interval of type section & thickness in type well: 417-435 m (1368­1427ft- amended depths by Lott & Knox, 1994)

UK Reference wells

Southern North Sea (Lott & Knox, 1994)
14/17a-2: 661-792 m (2170­2600ft)
48/23-1: 769-905.5 m (2523­2971ft)
53/2-5: 836-1104 m (2744­3622ft)		 Lat. 53º 24’ 35.90”N
Lat. 53º 12’ 57”N
Lat. 52º 52’ 58.193”N		 Long. 01º 22’31.19”E
Long. 01º 3’ 03”E
Long. 02º 14’ 27.706”E

Upper and lower boundaries

Upper Boundary

The upper boundary of the Spilsby Sandstone Formation is defined by a marked downward change from calcareous mudstone of the Valhall Formation to sandstone lithologies (Lott & Knox, 1994).

Lower Boundary

The lower boundary of the Spilsby Sandstone Formation is defined by a downward change from variably cemented sandstones to dark, variably calcareous mudstone lithologies of the Kimmeridge Clay Formation (Lott & Knox, 1994).

Well log characteristics

The upper boundary of the Spilsby Sandstone Formation corresponds with a reduction in the gamma-ray values and generally higher, irregular sonic log profile, the latter related to the amount of carbonate cement present (Lott & Knox, 1994). Prominent gamma-ray spikes within and at the base of some sequences represent pebbly phosphatic horizons (e.g.  48/23-1 Lott & Knox, 1994). There is a corresponding marked increase in gamma values and the velocity becomes generally more regular at the lower boundary (Lott & Knox, 1994).

Biostratigraphy

Biostratigraphic data suggests that the lower part of the formation may span the Jurassic/Cretaceous boundary, as occurs in the equivalent onshore sandstone sequences of eastern England (Lott & Knox, 1994). The Spilsby Sandstone Formation yields variably abundant dinoflagellate cyst assemblages. The Rotosphaeropsis thula biomarker occurs within the formation (Lott & Knox, 1994).

Age

Tithonian (Volgian/Portlandian) to Valanginian

Correlation

The Spilsby Sandstone Formation equates, at least in part, with the sandy Spilsby and Sandringham formations of eastern England (Rawson et al. 1978) which range from Tithonian (Ryazanian) to Valanginian in age. In the UK Sector of the Central North Sea area partly equivalent basal Cretaceous sandstone developments are termed the Devil's Hole Sandstone Member of the Valhall Formation (Johnson & Lott 1993).

Depositional environment

The Spilsby Sandstone Formation was deposited on the southern fringes of a well-oxygenated shallow marine shelf (Cameron et al., 1992; Lott & Knox, 1994).

Subdivisions

An informal subdivision into two units is possible with the lower unit (SP1) containing more mudstone interbeds than the upper, massive sandstone interval (SP2) (e.g.  48/17a-2, Lott & Knox, 1994).