The unconformity
The spectacular angular unconformity seen in the deep valleys and quarries west of Frome was first described by
Conybeare & Phillips (1822), but it was Sir Henry de la Beche, the first Director-General of the British Geological
Survey, who noted in 1846 that the unconformity surface was ‘drilled into holes by lithidomous animals, which must
have existed in the seas at the commencement of the inferior oolite’. De la Beche referred to the detailed
observations made by Professor John Phillips in 1829, for it was he who recognised that the holes were of two
kinds; one long and slender, the other short and squat. The remarkably perceptive illustrations of the unconformity
in Vallis Vale that appear in De la Beche’s monumental work (1846, Figures 43 & 44) are reproduced here as
Figure 16.
Figure 17 (upper). Block diagram showing the unconformity
The steeply dipping Carboniferous Limestone is truncated by a near-horizontal unconformity that resulted from multiple phases
of erosion. The surface is encrusted by oysters (in brown) and bored by worms and lithophagid bivalves. Vertical scale is about
10cm.
The most numerous ichnogenus at Tedbury Camp is the worm tube Trypanites, a simple, unbranched, cylindrical boring
that penetrates more or less vertically into a solid substrate from a single aperture. Two species are distinguished on the
basis of size; T. weisei has an average diameter of 1.4mm (Figure 18),while the much rarer T. fosteryeomani has a
diameter of about 4.5mm. The latter has not been identified with certainty at Tedbury Camp.
Figure 17 (lower). Section through the unconformity showing multiple phases
of colonisation by fossils
Large, worn oyster valves (in brown) rest on the unconformity surface, post-dating several phases of boring into the lithified
Carboniferous Limestone. The overlying Inferior Oolite is itself bored and contains more oyster shells on a hardground surface a
few centimetres above the unconformity. The numbers indicate a chronology based on cross-cutting relationships. Vertical scale is
about 5cm.
Figure 16. De La Beche’s (1846) sketches of the unconformity surface at
Vallis Vale, 1km east of Tedbury Camp Quarry
The upper sketch shows the two types of boring, whilst the lower sketch shows a bivalve in its life position, boring a
short distance into the lithified sea-floor.
Figure 18. Trace fossils
Pale grey-yellow Inferior Oolite infilling borings in dark grey Clifton Down Limestone. The slender borings (Trypanites
weisei) are circular in cross-section, while Gastrochaenolites is larger and more elliptical. Sample from the northeastern
edge of the quarry floor, at the base of the thinly bedded unit, see Figure 5.
T. weisei locally riddles the unconformity surface with thousands of pin-prick apertures per square metre. Many of the
tubes are oblique rather than vertical and cross-cutting relationships indicate multiple phases of colonization. Unusually
short specimens represent the ends of borings from early colonization phases that have been truncated by erosion
(Figure 19).
Figure 19 Trypanites weisei from Tedbury Camp
Cross-cutting worm tubes infilled with pale Inferior Oolite. Specimen from the eastern edge of the quarry floor, see Figure
5, Location 2; see also Location 2 for an annotated version.
Figure 20. Detail of the unconformity surface
Pale grey-yellow Inferior Oolite overlying dark grey calcite mudstone (Clifton Down Limestone) in a hand specimen of the
unconformity. Vertical and oblique Trypanites weisei borings show cross-cutting relationships and some are truncated by the
unconformity itself. A large elliptical crypt, Gastrochaenolites, contains shell fragments (arrowed). Specimen from the eastern
edge of the quarry floor, see Figure 5, Location 2.
Figure 21. Ridges on the unconformity surface produced by Tertiary faulting
A small amount of normal, dip-slip movement between adjacent limestone beds accounts for the ridges, and the fact that
the slip plane is not bored indicates that the displacement post-dates the development of the unconformity. As the Inferior
Oolite is also faulted along the same trend, the displacement is post-Bajocian, most likely caused by Tertiary extension.
Whilst this interpretation satisfies the large-scale observations, it is also apparent that at a much smaller scale there are
differential weathering features caused by the more resistant nodules and beds of chert standing proud of the surrounding
limestone. The overlying Inferior Oolite is banked up against these micro-topographic features, indicating that the
weathering process was pre-Bajocian in age.
As an added point of interest, the largest borings on the unconformity are not caused by Jurassic animals, but by man! A
series of drill holes are aligned along the strike of a prominent bed in the middle of the quarry floor (Figure 5) and they
penetrate a centimetre or two into the limestone. They were probably made when trial boreholes were drilled through the
Jurassic overburden in order to ascertain how thick it was, prior to its removal. A worn (now 40mm) rotary drill bit
discovered in the quarry fits the holes perfectly (Figure 22).
More detailed work undertaken in the 1980s by Copp (pers. comm.) and Cole & Palmer (1999) forms the basis
for our current understanding of the complex sequence of events that fashioned this surface. As the sea
advanced and retreated around the Mendip islands during Early and Middle Jurassic times, the sea floor was
probably buried many times by sediment and then re-exposed by erosion. Whenever conditions were
appropriate, the sublittoral sea floor would have been colonised by rock boring and encrusting animals. Such
surfaces are called ‘rockgrounds’ and at Tedbury Camp it consists of highly indurated Carboniferous Limestone
that was deposited more than 170 million years before being overlain by the Inferior Oolite. The rock-borers
included worms and lithophagid bivalves, whilst the encrusters were mostly oysters and serpulid worms (Figure
17). At the present day the most diverse communities of boring and encrusting organisms occur on calcareous
rocks in shallow tropical seas.
Larger, club-shaped (clavate) or flask-shaped borings are thought to be produced by lithophagid bivalves that
excavated the substrate by mechanical and/or chemical means, as do modern forms such as Pholas and Hiatella.
These relatively large (5-15mm), well-defined cavities (crypts) are referred to the ichnogenus Gastrochaenolites.
They occasionally contain the bivalve shell still in its life position (Figure 20), but are more commonly infilled with
cemented sediment (crypt casts).
The Tedbury Camp rockground is assumed to result from multiple marine transgressions during the 40 million year
period between the Rhaetian and Bajocian. During that time an extensive wave cut platform developed and thin
deposits of sublittoral conglomerates, gravels, limestones and marls were deposited on it. Most of these deposits
were removed during intervening regressive periods that presumably reworked the unconformity surface as well.
The result is that the Inferior Oolite typically rests directly on the Carboniferous rockground, but locally there are
patches of thin, condensed pre-Bajocian sediments preserved between them (Wall & Jenkyns, 2004).
Widespread exposures of the unconformity in east Mendip demonstrate that the rockground is remarkably planar
and extensive, although small cliff-like features and upstanding ‘reefs’ provide local relief. Flat erosion surfaces of
this magnitude are difficult to explain through mechanical or subaerial processes, so some form of submarine
biochemical erosion is envisaged to render the original wave cut platform so flat. This process may occur at the
microscopic scale, particularly on carbonate substrates, as a result of cyanobacteria and algae (Bromley, 1994).
Surface ridges. When looked at in detail, the ‘flat’ unconformity surface is slightly ridged. This feature has been
noted by Duff et al. (1985) and Wilson (1994) and attributed to differential weathering of juxtaposed chert and
limestone beds. An alternative interpretation is offered here, based on the following observations:
•
Most ridges do not obviously correlate with resistant chert beds, but are caused by juxtaposed limestone
beds, one of which is displaced by a small (10-30cm) dip-slip movement, to create a monoclinal profile
(Figure 21; see also Figure 5 & 6).
•
The ridges are terminated abruptly, or displaced, by cross-cutting faults, most obviously the N-S tear fault
(F-F on Figure 5) and smaller conjugate faults trending 310°.
•
The ridges broadly align with near-vertical shatter zones in the Inferior Oolite.
•
The unconformity surface on either side of the ridge is riddled with Trypanites borings, but the inclined slip
plane is not.
Figure 22. Drill bit and hole indicating former quarrying operations
