Date and Time
Sandy Lane, Aust, South Gloucestershire, South West England, England, BS35
Field Trip Description
The late Triassic rocks at Manor Farm indicate an arid or semi-arid coastal sabkha (salt-flat) environment (the Mercia Mudstone Group) followed by a shallow, marine and lagoonal facies teeming with life (Rhaetian Penarth Group). Exposed at the top of the sequence are pale, grey-brown bioclastic limestones and calcareous mudstones of the Blue Lias (Pre-planorbis Beds). They contain abundant marine benthic fauna but lack ammonites.
The section represents the highest part of the famous Aust Cliff section. This is internationally renowned for its mixed terrestrial-aquatic vertebrate fauna of the lenticular Ceratodus Bone Bed which marks the transgressive base of the Westbury Formation. Much of the Aust section is inaccessible so the new sections at Manor Farm are very valuable indeed. The Ceratodus Bone Bed can be seen as discontinuous lenses occupying shallow depressions in the eroded top of the Blue Anchor Formation.
A small section has been excavated on a long face in the Westbury Formation at Manor Farm to reveal derived clasts of grey-green Blue Anchor Formation mudstone, occasional quartz pebbles and incomplete to complete bones, teeth, fish scales and coprolites in a well-cemented matrix of comminuted bone and shelly quartz sand. Ichthyosaur vertebrae, parts of a plesiosaur, possible theropod dinosaur vertebrae and shark fin spines have all been found in this horizon. It is an unusual mixture of terrestrial and aquatic life.
One explanation is that the area was a shallow marine shelf containing terrestrial debris - there was a shoreward storm surge which concentrated the skeletal debris and rip-up clasts of Blue Anchor Formation mudstone. It was therefore a rapidly deposited ‘tempestite’. There is other evidence to support this idea of rapid deposition. Details of the many fossils found at this site are given in the article.
Another face exposes pale-coloured marls of the Cotham Member with some beds of chalky, micritic, tabular to nodular limestone. Ichthyosaur and plesiosaur vertebrae, a coprolite and possibly part of a theropod have been found here.
A small excavation has been dug along the northern face of the exposure. Here you can find the stromatolitic ‘landscape marble’ - the highest level of the Cotham Member. ‘Crazy’ Cotham Marble can also be found here. It probably represents channel-fills between the algal ‘heads’.
The landscape marble is overlain by the tough, flaggy limestones and weathered brown mudstones of the Pre-planorbis beds. The limestones are full of disarticulated bivalves. Minute echinoid spines stud the surfaces of some slabs and an ichthyosaur vertebra has been recovered.
Meet at 10.30 at GR ST 574896. We shall investigate the site in the morning and it is hoped that in the afternoon some conservation will be undertaken. Please bring spades etc. Hard hats essential.
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