Tectonic Evolution of the Bristol Channel Borderlands Chapter 6

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CHAPTER SIX

Tectonic Evolution of the Bristol Channel borderlands

SEISMIC INTERPRETATION, BRISTOL CHANNEL

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Brooks et al (1988) presented seismic sections that provided evidence for the structure and

normal reactivation of the Bristol Channel Thrust. It was proposed that the Bristol ChannelThrust juxtaposed the Devonian of North Devon and the Carboniferous Limestone of South

Wales (Fig. 6.3). In this chapter seismic evidence for another thrust in the east, the Gravel

Margin Thrust, will demonstrate that the onshore strata were juxtaposed by at least two

offshore thrusts whilst in the west there is evidence for only the Bristol Channel Thrust.

Geological surveys carried out in August 1990 along the coastal section from Lynmouth to

Foreland Point (Fig. 6.1) revealed evidence for a further thrust offshore, the North Devon

Coastal Thrust. A large variety of thrusts provide structural analogues for the offshore

thrusting and are used to postulate a thrust origin for major offshore reflectors and, secondly,

to assist speculation on the structural history including kinematics and amounts of

displacement along offshore thrusts.

Brooks et al (1993) give evidence for a major geological boundary at shallow depth in the

hangingwall of the Bristol Channel Thrust which, depending on the interpretation of age of

the boundary, constrains the combined displacement across the Bristol Channel and Gravel

Margin Thrusts. A comparison of estimates of displacement based on onshore geology and

seismic data is given.

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The upward continuation of the Bristol Channel Thrust is a normal fault, the Bristol Channel

Fault, intersecting the sea bed in the southern part of the Central Bristol Channel Fault Zone.

The fault shows a normal southerly downthrow juxtaposing Upper Jurassic in the footwall andhangingwall across the Inner Bristol Channel (BGS 1:250 000 sheet 51N - 4W Bristol

Channel).


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THE GRAVEL MARGIN THRUST

At a depth of 2km north of Foreland Point, the Gravel Margin Thrust (Fig. 6.4) occurs 3-4km

north of the Bristol Channel Thrust in its footwall . It is traceable, offshore, only from north of

Ilfracombe to north of Porlock. It has a moderate SSW dip of 26-40 (Fig. 6.6) and shows a

thrust-flat geometry in relation to the moderately SSW dipping reflectors of the South

Scarweather package defined in Fig. 6.9. The Gravel Margin Thrust continues upwards from a

depth of 6km, at which depth Precambrian crystalline basement is predicted from seismic

refraction studies (Brooks et al, 1983). The footwall of the Gravel Margin Thrust contains a

laterally restricted but prominent reflector.

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The position of the reflector accords with the above basement depth. On a southern strike

section the Gravel Margin Thrust rises gradually in structural level towards the east. The fault

intersects the sea bed as the Gravel Margin Fault, 2km to the north of the Bristol Channel

Fault within the northern section of the Central Bristol Channel Fault Zone.

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The Gravel Margin Fault downthrows Upper Jurassic in the hangingwall against Lower

Jurassic in the footwall in the eastern part of the Inner Bristol Channel (BGS 1:250 000 sheet

51N - 4W Bristol Channel).

THE NORTH LEDGE REFLECTOR

Seismic sections from the eastern part of the Inner Bristol Channel only to the south of the

Gravel Margin Thrust show a reflection event in the hangingwall of the Bristol Channel

Thrust. The event has a gentle dip of 12S to 9N and a strike which varies from WNW about

8km north of Woody Bay to ESE 4km north of Foreland Point (Figure 6.7). The reflector

occurs at about 1.0s Two Way Travel Time, equivalent to a depth of about 2.5km.


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The Palaeozoic can be subdivided into two groups here named the Southern and Northern

sequences (Fig. 6.9). The two sequences, described below, contain all recognisable seismic

packages which are separated by the Variscan thrusts defined earlier. The following packages

are of particular interest in defining the structural boundary of the Upper Palaeozoic sequences

observed onshore, to the north and south of the Bristol Channel: the Foreland Ledge package

within the Southern sequence and the South Scarweather package within the Northern

sequence.

FORELAND LEDGE PACKAGE (composite package)

The upper part of the Foreland Ledge package (thickness approx. 2km) is provisionally

correlated with the Devonian strata along the North Devon coast due to its seismic signature,

velocity from refraction surveys of 5.5kms-1, and structural position south of the Bristol

Channel Thrust (Fig. 6.9) The Foreland Ledge package also appears to continue into the

Devonian outcrop of North Devon.

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Beneath the western part of the Inner Bristol Channel, the Foreland Ledge package contains

prominent, north dipping, closely spaced reflectors which accord with the NNE dipping strata

in the northern limb of the North Devon Anticline. The lower part of the Foreland Ledge

package lying beneath the North Ledge reflector displays a different seismic character which

consists of short discontinuous undulose reflectors extending down to 1.4s Two Way Travel

Time (about 3-4km depth). This may represent a separate package within the southern

sequence equivalent to allochthonous Precambrian basement, sub-Silurian or Siluro-Devonian

strata.

SOUTH SCARWEATHER PACKAGE

The prominent sequence of moderately southward dipping reflectors (Fig. 6.9, Fig. 6.10)

constituting the South Scarweather package (750m) can be initially correlated with the

Carboniferous Limestone after Brooks et al (1988).

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Comparison of the offshore seismic sections with seismic reflection sections from the Vale of

Glamorgan, which transect Carboniferous Limestone inliers, suggests that the reflective upper

boundary of the South Scarweather package represents a horizon within middle-upper sections

of the Chadian Gully Oolite of Waters & Lawrence (1987).


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Tectonic Evolution of the Bristol Channel Borderlands Chapter 6

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