Fossils of
Avon RIGS region
'Lithostrotion'
Name: Siphonodendron (formerly Lithostrotion) martini
Phylum: Cnidaria
Order: Rugosa
Horizon: Carboniferous
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Photo credit - T. Bolton
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Source rock: Clifton Down Limestone Formation
Age: Tournasian stage of the Lower Carboniferous Period (345 - 359Ma)
Locations: Avon George, Broadfield Down, Mendip Hills. Tickenham Ridge
Note: The underlined area is the type location for this
Formation
Description of fossil
Siphonodendron
martini corals were colonial, cylindrical and showed
growth increment bands. Increment bands arise as a result of daily changes in
light (which would affect the algae living in the outer cells of the organism)
or temperature as well as monthly changes, associated with the lunar
cycle. In the cool season or at night the corals secrete less calcite and so
monthly and daily bands are visible on the surface of the corallites. Devonian
corals studied at a different location show 400 daily bands in a year while
these in the Lower Carboniferous have 391. This would indicate that the earth
is slowing down on its rotational axis with tidal friction thought to be a
major causal factor. The decrease in speed of the Earth's rotation is shown to
accelerate in the Lower Carboniferous due to widespread occurrence of shallow
shelf-seas.
Siphonodendron martini has a solid rod columella, major and minor septa (28 septa
in each order) and dissepiments. In some areas many corallites did not reach
maturity, only growing to 3mm. in diameter. Three ecotypes of the fossil are
identified probably representing growth stages in the coral. The third ecotype
is characterised by small radius, comprising only twenty septa and a single row
of dissephnents. Some samples showed rejuvenescence i.e. the corallites reached
adult dimensions then stopped growing only to recommence with smaller radius
and more juvenile features. Measured growth was reduced from 4 mm a month in
the limestone down to 2 mm a month in the calcareous shales where stunted
growth is also observed to affect the diameter of the corallites. In the shale,
rejuvenescence took place and there was little evidence of asexual breeding or
monthly bands as the increase in nutrients from high levels of sedimentation is
likely to have restricted breeding. Alternatively, a decrease in light
levels light may have affected the algae living in the outer cells of the
organism, have reduced breeding and thus eliminated the monthly banding.
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Photo credit - T. Bolton
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Description
of source rock ( Clifton Down Limestone )
Paleoenvironment
During the Lower
Carboniferous the British Isles were just south of the equator
and under water. As the Laurasian continent drifted north a large delta
carrying eroded sediments from the north deposited its load in the sea,
eventually killing the coral reefs and starting the Millstone Grit beds –
forming the Quartzitic Sandstone Formation in the Avon RIGS area.
Lithology
The lithology is dominated by calcite mudstones with a locally abundant but low diversity fossil assemblage. At Burrington Combe the formation is about 170 m thick, and three subdivisions can generally be recognised across the Mendip area. The lowest unit comprises a mixture of calcite mudstones, white oolitic limestones and dark splintery limestones. This interval is relatively expanded in the Cheddar area, where a 38 m thick dark limestone ('Cheddar Limestone Member') is overlain by a 58 m thick white oolitic limestone ('Cheddar Oolite Member'). The middle part of the succession is dominated by fine-grained, grey-black limestone with nodules and bands of chert and abundant remains of the coral Siphonodendron['Lithostrotion'] martini ('Lithostrotion Limestone'). Porcellaneous calcitic mudstones dominate the highest part of the formation, including locally developed algal mudstones and stromatolites, indicating deposition in a very shallow-water, near-shore or lagoonal environment.
The lithology is dominated by calcite mudstones with a locally abundant but low diversity fossil assemblage. At Burrington Combe the formation is about 170 m thick, and three subdivisions can generally be recognised across the Mendip area. The lowest unit comprises a mixture of calcite mudstones, white oolitic limestones and dark splintery limestones. This interval is relatively expanded in the Cheddar area, where a 38 m thick dark limestone ('Cheddar Limestone Member') is overlain by a 58 m thick white oolitic limestone ('Cheddar Oolite Member'). The middle part of the succession is dominated by fine-grained, grey-black limestone with nodules and bands of chert and abundant remains of the coral Siphonodendron['Lithostrotion'] martini ('Lithostrotion Limestone'). Porcellaneous calcitic mudstones dominate the highest part of the formation, including locally developed algal mudstones and stromatolites, indicating deposition in a very shallow-water, near-shore or lagoonal environment.
Richard Kefford
References
- British geological Survey –
Lexicon of rock units
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