The Jurassic/Cretaceous boundary interval is represented in Lincolnshire by the Spilsby Sandstone Formation, a shallow water marine deposit that spans the Volgian stage of the Jurassic to the Berriasian stage of the Cretaceous (Hopson et al. 2008). The ammonite faunas of this formation are of particular interest, exhibiting affinities with correlative forms in both Russia on the Siberian plain, as well as Greenland and Canada (for example, Casey, 1973; Mikhail Rogov, personal communication 2015).
Exposures of the Spilsby Sandstone Formation are currently almost absent. The last substantial exposure that allowed collecting was at Castle Top Quarry in Nettleton (TF1198), which, following its closure in 1998, was subsequently backfilled. The most complete sequence of the Spilsby Beds is present in south Lincolnshire (Casey, 1973) To the north of the county, at Nettleton, the beds have been eroded down to the level of the ammonite zone of Subcraspedites lamplughi of Lower Cretaceous (Berriasian) age. However, it is beyond the scope of this account to deal with the complex biostratigraphy of the Spilsby Formation as a whole, and the present account will focus on my observations at Castle Top Quarry.
The Spilsby Formation here is about 8.5m thick, consisting of the Basal Spilsby Nodule Bed at the base, succeeded by a largely undifferentiated, coarse, friable sandstone, with no obvious bedding or marker horizons. This sandstone is characterised by common belemnites such as Borioteuthis sp. (Simon F Mitchell, personal communication 2014) as well as rarer ammonites that suggest faunal links with the regions discussed above.
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