Areas of Jurassic shelf often show sequences commencing with mud and passing upward through sandy mud and muddy sand and finishing with sand (Communities 64 to 68). If the water became very shallow at a distance from land (i.e upon a shoal or 'swell') the currents carrying sediments were usually diverted to deeper parts of the sea. Little material was deposited on the sea floor in these shallow 'swell' areas except the remains of the animals and plants living there. Condensed limestones are therefore frequently found above sandy beds and the areas of slow subsidence (swells) can be related to the underlying structure in the pre-Jurassic rocks (Sellwood and Jenkyns, 1975). The main addition to the fragmented shell material which formed the sediments in these environments is carbonate mud, sometimes trapped by blue-green algae which lived on the sea floor. These algae were covered by mucus to which the mud became attached in fine stromatolitic laminae with the laminations picked out by limonite (the rusty hydrated oxide of iron) and by manganese minerals which gave a brown or pink colour to the stromatolites.
The Middle-Upper Lias Junction Bed of Dorset is an excellent example of a condensed limestone. In this bed ammonites and belemnites are abundant. Some of the ammonites are covered by laminations of limonite-rich limestone (oncolites), which are also attributable to algae. The presence of stromatolites and oncolites indicates a clear well illuminated sea floor. Shallow, turbulent conditions are reflected in the numerous erosion surfaces and by the intraformational conglomerates (fragments of condensed limestone incorporated with the same sequence).
In some condensed sequences the benthos was very diverse, especially as regards the suspension feeders. The bivalves included thick-shelled forms (Liostrea and Plicatula), and the byssally attached Oxytoma and Inoceramus; the swimmer Bositra [Posidon-iaj is also present. Large numbers of brachiopods (mostly rhynch-onellids) were unusual in Lias clays and sands, but the clear water environments with no land-derived detritus, as in these condensed carbonates, proved more favourable to them.
Gastropods were also relatively common in condensed limestones, particularly the thicker-shelled genera (Amberleya, Pleuro-tamaria, Coelodiscus and Lewisella). Modern gastropods are scavengers, carnivores or grazers. The grazers today rasp the substrate for algal and bacterial material and it is probable that many of those in some condensed limestones fed on the algae.
Many modern algal flats are intertidal, and show shrinkage cracks and other signs of exposure at low tide. None of these signs of desiccation are seen in the British Lower Jurassic, and we must conclude that the stromatolitic limestones here accumulated
Fig. 68 Condensed Limestone Community a Dactylioceras (Mollusca:
Cephalopoda: Ammonoidea] c algae (Prokaryota-oncolites) d ammonite covered with algae as in c e belemnite (Mollusca:
Cephalopoda: Coleoidea) f Bositra (Mollusca: Bivalvia:
Pterioida) g Amberleya (Mollusca:
Gastropoda: Mesogastropoda h serpulids (Annelida — polychaete)
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