RIGS of the month [March] - Middle Hope

RIGS of the Month - March
Middle Hope, Kewstoke, Somerset

Fig 1: Locality map. Blue arrow shows location of RIGS on OS map (original scale = 1:25,000).

Directions: After parking at the end of Beach Road, take the steps up to the top of the headland and keep right, away from the trig point. Go through the kissing gate in the wall and immediately turn left. Head down the bank and turn left again (heading east), keeping parallel with the coastline. Proceed for ~300m until you read a small pebbled beach.

SITE SPECIFIC INFORMATION 

Location: Roughly BS22 9UR (GR = ST 324661. Marked as "Swallow Cliff" on 1:50,000 OS Map)

Accessibility: Car park at the end of Beach Road. Footpath with steep steps.

Risks: Check tides before heading down onto the beach. Beware of falling rocks beneath cliff faces
Topography: Hilly, grassed footpaths
Restrictions: The site is a designated as a SSSI so please do not hammer or sample from the exposures

Fig. 2: View of exposures looking directly west from ST 32466. The oldest Black Rock Limestone unit lies to the north (out of the field of view - see fig. 3).

Introduction
The geology of Avon is dominated by sedimentary rocks, but the coastal exposure at Middle Hope offers the chance to see volcanic deposits intercalated with Carboniferous Limestone. The mix of sediments and volcanic deposits allows accurate reconstruction of the sequence of events that took place in this area some 350 million years ago (Ma). Ash layers within fossiliferous sediments record the onset of volcanic activity in a shallow sub-marine setting, before proceedings culminated in an extrusive pillow basalt (fig. 3). Such volcanism is typical of the early Carboniferous in the UK; most vents were small and short-lived, having only a localised impact on their surroundings.

Outcrop description
The younging direction is roughly north-south. Carboniferous Limestone is gradually disturbed and then overwhelmed by volcanic deposits before reappearing at the top of the sequence. The entire succession has been tilted due to the huge compressive forces associated with formation of Pangea during the Variscan Orogeny at the end of the Carboniferous period.

Fig.3: Schematic representation of Middle Hope geological setting. Outcrop is located below the storm wave base (SWB) on a carbonate platform. FWB = Fair-weather wave base. After Faulkner (1989).

Unit name colours correspond to labelling on fig.2

Limestone (pre-volcanic)
The oldest unit at this exposure dates from the early Carboniferous (~355 Ma), and forms part of the Black Rock Limestone group. Bedded at roughly 20cm intervals, it is fine-grained and highly fossiliferous, containing abundant crinoid ossicles, brachiopods and corals fragments (fig.4). In places the sediments have been bioturbated by ancient burrowing organisms and yield a variety of trace fossils. The preservation of these delicate structures indicates these sediments were deposited in a low energy sub-marine environment, below the storm wave base of an offshore carbonate ramp (fig. 3).

Fig. 4: The oldest limestone unit at Middle Hope dips 30º to the south and contains abundant fossils. Image on right shows close up of crinoid ossicles in long and cross section.

Interbedded tuffs and limestones 
About 10m to the south, more tilted beds appear out of the shingle. This part of the outcrop consists of interbedded limestones and tuffs, the latter heralding the onset of volcanism in the area. In general, the grain size of the tuffs increases up-section, corresponding to increasing intensity of volcanic activity. The first beds are red in colour and are rich in bioclastic material. The alternating thin-bedded limestones, some of which have ripples and cross-bedding, show that the volcanic outbursts were sporadic and of limited duration. Small lapilli-rich layers can be found in the tuff and conglomeritic deposits indicating there may have been pyroclastic currents or debris avalanches flowing down the carbonate ramp.

Fig. 5: Image on left shows interbedded limestone and tuff. Volcanic activity was impersistent, allowing reestablishment of the carbonate platform in between eruptive episodes. Image on right is a conglomeritic sub-layer, possibly resulting from pyroclastic or debris flows. 

Massive tuff (Middle Hope Volcanic Member)
Above the bedded units lies the Middle Hope Volcanic Member. Greeny-black in appearance, this massive unit varies between 5-10m in thickness. Lapilli-rich zones appear in places and calcite veining associated with late Carboniferous tectonics is prominent. Near to the basal contact with the interbedded tuff there is evidence for metasomatism (chemical alteration by hydrothermal fluid); here, brown patches contain highly altered fossil fragments which have a distinctive green colouration.
 

Fig. 4: Middle Hope Volcanic Member. Massive green-black tuff deposit with extensive calcite veining. Close up shows metasomatised basal contact with underlying limestone.

Pillow basalt
Immediately above the tuff is a fine-grained pillow basalt, the result of an extrusive sub-marine lava flow. It varies in thickness up to 4.5m and good pillow shapes can be made out towards the base. The upper surface of the basalt is full of voids (amygdales) up to 10cm in diameter; when the lava was molten, these would have been filled with gas. Some have been stretched parallel to the flow direction of the lava. Later alteration has infilled the amygdales with calcite.

Fig. 6: Pillow basalt. Outcrop in left image is about 3m in height. Close up on left shows amygdales (voids left by gas bubbles) elongated parallel to flow direction which have been infilled later by calcite.

  

Limestone (post-volcanic)

After the extrusion of the basalt, the intensity of volcanic activity receded and is again recorded in gradually fining tuff layers interbedded with fossilerous limestone. The cliff deposits then grade into Black Rock Limestone, and finally Gully Oolite (which can also be found in the Avon Gorge).


Charly Stamper

Quaternary geology

The capping deposits on the cliff are a Pleistocene aged fossil cliff and shore platform. The raised-beach deposits have been correlated with the last interglacial, and this is the only Pleistocene raised-beach site where such a sequence can be demonstrated and as such is of fundamental importance to quaternary geology.

 
Biological
Middle Hope is designated as an SSSI for both its geological and biological features. The headland supports a calcareous grassland community with a restricted British distribution (Festuca species and Dactylis glomerata).

References:

- Faulkner TJ (1989) The early Carboniferous (Courceyan) Middle Hope volcanics of Weston-super-Mare: development and demise of an offshore volcanic high. Proc. Geol. Ass., 100(1), 93-106.

- Volcanic rocks of the Bristol region, Speedyman DL. in Geological excursions in the Bristol District. Savage RJG (1977). University of Bristol.