A palynological investigation of the Upper Pliensbachian and Lower Toarcian strata at Brown's Hill Quarry, Holwell, Leicestershire

The fourteen samples from Brown’s Hill Quarry produced palynofloras of variable productivity. The lowermost sample from the uppermost Dyrham Formation produced an extremely sparse palynoflora, which is consistent with a late Pliensbachian age but is unable to refine this assessment. This sandstone unit appears to be primarily organic-lean and/or deeply weathered. Five samples from the overlying Marlstone Rock Formation also yielded sparse palynofloras of low diversity. These are dominated by pollen and spores, but also include some marine microplankton and forms such as Halosphaeropsis liassica are consistent with the late Pliensbachian-early Toarcian interval. The range bases of the dinoflagellate cysts ?Luehndea spinosa, ?Mancodinium semitabulatum, Nannoceratopsis deflandrei and Scriniocassis weberi are also consistent with the Pliensbachian-Toarcian transition. The occurrence of ?Luehndea spinosa in sample 4 indicates that it is probably no younger than the Dactylioceras tenuicostatum standard (ammonite) zone. The palynomorphs from the Marlstone Rock Formation at this locality are less abundant than those from coeval strata at Tilton Railway Cutting. It is possible that the organic material at Holwell was winnowed away in a high-energy depositional setting. The eight samples from the Whitby Mudstone Formation all produced abundant palynofloras of moderate diversity; amorphous organic material is also present in relatively high proportions. The palynofloras are all dominated by Halosphaeropsis liassica and other miscellaneous microplankton and dinoflagellate cysts are more diverse and prominent. These floras are entirely consistent with the early Toarcian. High levels of Halosphaeropsis liassica, other prasinophytes, rare dinoflagellate cysts and abundant amorphous organic material is indicative of the most intense part of the early Toarcian anoxic event, in the Harpoceras falciferum standard (ammonite) zone. Also, the co-occurrences of the dinoflagellate cysts Mancodinium semitabulatum, Nannoceratopsis deflandrei subsp. deflandrei, Nannoceratopsis deflandrei subsp. senex and Nannoceratopsis gracilis are characteristic of the earliest Toarcian. For example, the range bases of the dinoflagellate cyst Nannoceratopsis gracilis and the pteridophytic spore Ischyosporites variegatus are interpreted to be early Toarcian markers.
The foraminifera and ostracods from this succession include several key late Pliensbachian and early Toarcian marker forms.