The photographs below are thin section photographs of samples taken from towards the top of the Millepore Bed of the Lebberston Member, Cloughton Formation within the Ravenscar Group, Middle Jurassic at Yons Nab (Grid Ref. TA083 843) about 4km south east of Scarborough. The photo's in the left column are in plane polarised light and those on the right in crossed polars. All sections are to the same scale. The bioclast in the top photographs is 3mm in length. The Millepore Bed was probably laid down in a low energy shallow marine tide dominated environment. The Lebberston Member was the product of the three marine transgressions during the progradation of deltaic (or coastal plain) deposits of the Middle Jurassic.

 The top of the Millepore Bed is a bioclastic limestone with scattered quartz grains, the latter showing up as pale yellow, grey and white interference colours  under crossed polars. The bioclasts are fragments of bryozoans, e.g. Haploecia straminea (Millepora) from which the bed derived its name as well as shell and crinoidal debris. The rounded fragments are ooliths (or ooids). These are formed by concentric layers of calcium carbonate being precipitated around a nucleus, e.g. a quartz grain, while being rolled around by the action of wave and/or tidal currents. Some ooids have a radial as well as a concentric structure due to subsequent internal recrystallisation.

The photo' to the left is a polished section of the Millepore Bed. The hexagonal bioclast fragment, just below centre right (?crinoid ossicle) is about 3mm wide.


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