Thursday, 21 October 2010

Blockley Fossils

The final field trip to Blockley Quarry was very successful - mentioned in the post of 13th October. Lots and lots of fossils were found, some of which are illustrated. Thanks to Richard for the photos; he says the ammonite looks identical to the one on page 102 of 'British Mesozoic Fossils', i.e. Lipoceras cheltiense, but the bivalves are harder to identify. Can anyone help?


Peter said...

The ammonite is almost certainly Liparoceras cheltiense (Murchison) from the Tragophylloceras Ibex Zone, Lower Pleinsbachian (Carixian) Stage of the Lower Lias. (see Spath Liparoceratid Ammonites Plate ii. fig. 7, 8. description p.46to 49.) The small lamellibranchs may well be Protocardia truncata which is from the Middle Lias Upper Pliensbachian (Domerian). The larger lamellibranchs appear to be closest to Pseudomytiloides dubius an Upper Lias fossil which is abundant in the Toarcian, Harpoceras falciferum Zone. What is the range of Liassic deposits at Blockley Quarry?

Richard said...

Stratigraphy of Blockley Pit, with great acknowledgements to John Willmouth who provided plenty of hand-out material on the visit.

Thicknesses in metres at end of each para.

?davoei Zone, maculatum subzone

8 Clay, weathered seen 1-2
7 Siltstone, buff,lenticular,bioturbated, sparsly fossiliferous, ammonites 0.0-0.2

ibex Zone, luridum Subzone
6 Clays, grey and brown, limonitic nodules abundant; sparsly fossiliferous 6.0
5 Clays, as above, with scattered small concretions and crushed shells 4.0
4 Mudstone, impersistant, light brown or layer of calcareous concretions; Fauna IV. 0.0-0.15
3 Clay, grey, fairly common ammonites 0.3
2 Pecten Bed. Mudstone, grey to light brown, fossils, many well preserved; highly diverse bivalve fauna, belemnites; Fauna III 0.15-0.45
1 Clay, grey very fossiliferous throughout, ammonite fauna; Fauna II 8.5
Z Crinoid-Belemnite Bed coarse calcite shell and ossicles of Belanocrinus forming hard quarry floor; Fauna I

Richard said...

Thanks Peter for you post re Blockley.

The BMNH book says Middle Lias only for the range of P. truncata, but it does look just like that fossil.

Further to the Blockley table, the bivalves were liklely collected from the Pecten bed. There were also plenty of mussel-like (Modiolus?) bivalves.

The Lipoceras cheltiense were mostly in a bucket saved by the brickmakers (who hate anything hard in their lovely soft clay). However a few L.cheltiense were in situ and one was seen c. 3 metres above the pit's lowest (current) horizon. John managed to dislodge it with a digging tool, broke up during its descent into a muddy pool, and he only found part of it after fishing.

The only other ammonites I have seen this solid and bronzy-coloured were Reineckia in the Callovian/Oxfordian vicinity, Cotswold Water Park.


Alan said...

The ammonites there are in fact an earlier form, L. pseudostriatum (Trueman) - this was established
by Phipps (1982) and later confirmed by John Callomon, part of his "Fauna II". There are no Middle or Upper Lias at Blockley; the section concludes just below the onset of the U.Pliensbachian (Lower Lias) Davoei Zone. However, the bivalves are long-ranging species and I have no problem with
the determinations. You probably found Pseudopecten acuticosta and
belemnites there as well.

Alan, again said...

Further to my earlier comment, I would prefer to place the Inoceramid bivalve as Pseudomytiloides (I) pinnaeformis, Dunker or ventricosus, J de C Sowerby (hard to tell as it is incomplete at the perimeter) - the form dubius comed higher in the succession - and the cardioids as Mactromya (Unicardium) cardioides (Phillips).

Richard said...

Thansk to Alan for several posts re Blockley.

We did find a few Pecten or Pseudopecten. (like Shell petrol scallops!) The several cardioid bivalves I collected had a notable offset to the two umbones. This seems to be 'real' not an offset casues by disarticulation after death.