entirely under water. Not all of the condensed beds are of this type and others may contain abundant nodules formed around individual ammonites. The Lias Junction Bed which is exposed between Seatown and Burton Bradstock on the Dorset coast provides an excellent example of such a condensed limestone.
Fig. 69 Ironstone Community a Pentacrinus (Echinodermata: Crinozoa) b belemnite (Mollusca: Cephalopoda: Coleoidea) c Asteroceras (Mollusca: Cephalopoda:
Ammonoidea) d rhynchonellids (Brachiopoda: Articulata:
Rhynchonellida) e Gryphaea (Mollusca: Bivalvia: Pterioida-oyster)
g Pholadomya (Mollusca: Bivalvia: Anomalodesmata)
h Cardinia (Mollusca: Bivalvia: Veneroida)
i Rhizocorallium (trace-fossil — crustacean)
j Pseudopecten (Mollusca: Bivalvia: Pterioida — pectinid)
k Diplocraterion (trace-fossil — annelid or crustacean)
1 terebellid (Annelida) m Chondrites (trace-fossil — annelid)
Many marine ironstones had a rich fauna dominated by suspension feeders. The rocks may contain abundant sphaeroidal structures (ooids) composed of a variety of iron materials (chamosite, limon-ite, siderite). The ooids were probably formed on a turbulent sea floor but the iron minerals probably resulted from the alteration after burial of an originally iron-rich sediment. Like condensed limestones, ironstones often accumulated on swells.
The dominant species were suspension-feeding bivalves, but many different types were present. Infaunal suspension feeders included Pholadomya, Pleuromya and Hippopodium and, in some beds, Astarte and Cardinia; the large semi-infaunal Pinna sometimes occurred, and the epifaunal bivalves included oysters (Gryphaea, Liostrea), thick-shelled pectinids (Pseudopecten, Lima) and many others.
Brachiopods (all of them suspension feeders) occurred commonly in 'nests'. Each 'nest' consisted of numerous individuals of one species (monospecific clusters) though within one bed many different brachiopods (rhynchonellids, terebratulids and spiriferi-nids) may be present. Many individuals appear to have preferred shells of their own species as an anchorage. In general, Jurassic brachiopods preferred beds without much sand or mud, so they are more common in ironstones and certain limestones rather than in clastic sediments.
The gastropods included scavengers (procerithids) and grazers (Amberleya and Pleurotomaria); the grazers probably fed on algae on the floor of the shallow and therefore well lit sea. There is little good evidence of algae, but some beds contain large ooids (lcm) which may have been produced by algal growths.
The trace fossils also reflect shallow turbulent conditions on the sea floor; they included Diplocraterion, Rhizo cor allium and tere-bellid worm tubes. The terebellid worms lined their burrows with mucus to which shell fragments became attached; the fossil burrow is thus recognized by a cylindrical arrangement of shell fragments surrounding material that is finer grained than the host sediment.
Echinoderms were represented by both crinoids and echinoids; the former were fixed on the sea floor and disintegrated after death; the latter often burrowed and are sometimes well preserved.
Ammonites and belemnites are usually common fossils in ironstones. In some beds ammonites of several zones are present within a single band a few metres thick. These ironstones must have accumulated very slowly (Hallam, 1963, 1966). They developed most easily in shallow 'swell' or shoal areas away from land, where turbulence was coupled with a lack of terrestrial detritus (i.e. sand and clay). The reason why the iron minerals were precipitated is still not understood. If physical conditions were paramount, chamosite and siderite would be more stable in sea water with low oxygen concentrations. But we know from the fossils that the composition of the sea was normal, and from the cross-bedding that the sea floor was often subject to turbulent conditions. It may be that iron concentration occurred organically; both algae and bacteria are known to be able to precipitate iron minerals.
Examples of oolitic ironstones may be seen in the Upper Lias of Raasay, Scotland; the Middle Lias Marlstone of Dorset, central England and Yorkshire and in the Lower Lias at Frodingham in Lincolnshire. Some of the Middle Lias ironstones are capped by basal Upper Lias condensed limestones.
70 Calcarenite Community
Calcarenites are limestones made up of sand-sized fragments; they occurred in shallow turbulent marine areas surrounding low islands, or on 'swells'.
The dominant faunas of calcarenites were suspension-feeding bivalves. Most of the epifaunal bivalves had thick shells that had been adapted specially to the turbulent conditions (Liostrea, Gryphaea and some pectinids), but there were also some byssally-fixed streamlined forms (Gervillella, Modiolus). The epifauna also included large brachiopods (rhynchonellids and terebratulids), preserved as they lived (in 'nests'), or drifted with other shells into shell banks. The mollusca were also represented by thick-shelled grazing gastropods. Crinoid and echinoid debris was present, though complete specimens from either group are rare as fossils; the fragments include spines from cidarids and carnivorous diademoids. Many of the shells and fragments were bored by algae, sponges and bryozoans.
Corals (which are very rare in clays and sands) were abundant locally (Oppelismilia, Styllophyllopsis and Isastrea), but they never formed substantial reefs. Like the other epifauna, they took advantage of the clear water with a rich supply of suspended food, and the temporary hard substrates of drifted shells provided suitable attachment. Many colonial corals were overturned, either by large animals or, more probably, by storm action. The corals were often bored and encrusted and while some of the boring may have taken place while the coral was alive, the encrusting serpulids, oysters and bryozoans probably grew on dead and drifted corals.
The infauna included suspension-feeding bivalves (Pholadomya, Pleuromya, Cardinia, Astarte and Pinna). Infaunal crustaceans and worms left a variety of trace fossils including Diplocraterion, Rhizo cor allium and Thalassinoides.
Ammonites and belemnites are sometimes abundant as fossils;
Fig. 70 Calcarenite Community a pseudodiadematid (Ectinodermata: Echinozoa — sea urchins)
b Pentacrinus (Echinodermata: Crinozoa)
c Thalassinoides (trace-fossil — crustacean)
d Oppelismilia (Coelenterata: Anthozoa: Scleractinia)
e rhynchonellids (Brachiopoda: Articulata: Rhynchonellida)
f Isastrea (Coelenterata: Anthozoa: Scleractinia)
g Lithophaga (Mollusca: Bivalvia: Mytiloida)
h Pleurotomaria (Mollusca: Gastropoda: Archaeogastropoda)
i Liostrea (Mollusca: Bivalvia: Pterioida — oysters)
j Modiolus (Mollusca: Bivalvia: Mytiloida) k Pholadomya (Mollusca: Bivalvia:
Anomalodesmata) 1 Astarte (Mollusca: Bivalvia: Veneroida) m serpulid (Annelida — polychaete) n Uptonia (Mollusca: Cephalopoda:
Ammonoidea) o belemnite (Mollusca: Cephalopoda:
Coleoidea) p cement of calcite (in inset)
like the other shells, they were often encrusted by serpulids after their death.
Some hardgrounds may exist within the calcarenites; the com-munites on these are described below. In the Lower Jurassic of Britain, calcarenites are found in the littoral areas of Glamorgan and the Mendips, and on local temporary 'swells', as in Dorset. In the littoral areas they often contained pebbles of older rocks upon which the Lower Jurassic rests unconformably. Splendid examples can be seen in Normandy where the Lower Jurassic overlaps the Armorican Massif.
Ancient cemented sea floors can be recognized by the presence of borings and encrustations. They often occur in beds that show other signs of slow depositions, and the hardgrounds themselves marked non-depositional or even erosive phases. They can occur above beds of variable lithology and also at unconformities above older rocks. Although they were more common in the limestones of the British Middle and Upper Jurassic, they could also occur in mudstones, sandstones and limestones of the Lower Jurassic.
In calcareous or sandy sediments, large areas of the sea floor may have been hardened, especially in calcarenites at the top of coarsening upward sequences. If deposition was very slow, the dissolution of aragonite shells on or just below the sea floor was followed by precipitation of the calcium carbonate within the sediment; this then acted as a cement for the hardground (Purser, 1969). The Liassic hardgrounds provide no evidence of exposure to the air.
The fauna on hardgrounds was dominated by suspension feeders. Encrusting epifaunas consisted of bivalves (especially cemented oysters), serpulids and bryozoans, while byssal bivalves (pectinids and pterioids), crinoids and corals also attached themselves to the hard substrates.
If the hardground surface was irregular, small pockets could develop and these were sometimes filled with shell debris including eroded and abraded ammonites, belemnites and gastropods. Often these shell particles were overgrown and encrusted with serpulids and bryozoans. Pebbles of the hardground material are sometimes found near the base of the overlying bed; these too, may be encrusted and bored. The size of the pebbles (up to 20cm) indicates the strength of the currents that flowed over hardgrounds and it seems likely that their formation was favoured by shallow water conditions.
Fig. 71 Hardground Community a Pentacrinus (Echinodermata Crinozoa)
Liostrea (Mollusca: Bivalvia: Pterioida — oyster)
Montlivaltia (Coelenterata: Anthozoa: Scleractinia) echinoids (Echinodermata: Echinozoa — sea urchin) serpulids (Annelida — polychaete)
boring worms (Annelida) Lithophaga (Mollusca: Bivalvia: Mytiloida) bryozoan (Bryozoa: Ectoprocta) patellid (Mollusca: Gastropod: Archaeogastropoda) encrusting foraminiferida (Sarcodina) Amaltheus (Mollusca: Cephalopod: Ammonoidea) belemnite (Mollusca: Cephalopod: Coleoidea) m Pseudopecten (Mollusca:
Bivalvia: Pterioida — pectinic n Tetrarhynchia (Brachiopoda: Articulata: Rhynchonellida) x bedrock (pre-Jurassic) y hardened Jurassic sediment k
Was this article helpful?