Excursion  Saturday 12th August 2006

Excursion to Wick Quarry

led by

John Toller and Mark Michelmore

Most of the following material comes from Elizabeth Devon and John Parkin.

Background Geology of Wick Quarry

The Wick Inlier consists of a ‘window’ showing older (Palaeozoic) rocks through a covering of younger (Mesozoic) rocks. The Palaeozoic rocks within the inlier are mainly Carboniferous limestones and sandstones, occurring in beds which dip steeply towards the west; i.e. they are tipped up at an angle of up to 60°, sloping down to the west. This is because they lie on the eastern of a major syncline in the Palaeozoic rocks, the axis of which occurs several kilometres to the west of Wick. The younger Mesozoic rocks around the Wick inlier consist of Triassic and Jurassic rocks, made up of layers of mudstone, conglomerate and limestone. The beds in these younger rocks are nearly flat, sloping very gently by only a few degrees towards the east. This means there is an unconformity between the steeply dipping Palaeozoic rocks in the inlier, and the gently dipping Mesozoic rocks around it. The younger Mesozoic rocks would have covered the much older, more deeply buried, Palaeozoic rocks entirely, but erosion has worn them away exposing the older rocks in an inlier.

This map is of unknown provenance. We would appreciate if the copyright holders could contact us so that we can acknowledge the source.

The Carboniferous rocks at Wick Quarry were formed about 360 million years ago in a warm, shallow sea that sometimes became very shallow. They are mostly limestones with some sandstones, mudstones and shales. They contain the fossils of corals and shellfish that lived at that time. There are also the trails of animals that walked across the sea floor.

The rocks in the inlier are much deformed, mostly by the Variscan mountain building episode. Although they have a general dip towards the west, they show complex folds and faults. The latter were sometimes filled by hot fluids circulating from nearby sedimentary basins and these have left a wide variety of minerals including iron, manganese, lead and zinc. Veins of calcite are common in some areas, often with bands of other minerals in them.

Fig. 9 (amended) from the Bristol and Gloucester regional guide (BGS)

Wick is uniquely rich in four areas:-

Stratigraphy: At the time of deposition, the area was greatly affected by fluctuating sea levels. To the north, in the Forest of Dean area, was dry land. The Edgehill Sandstone Quarry contains shoreline facies. To the south, in the area of the Mendip Hills, the shelf sea was sufficiently deep for limestone to be laid down throughout the period. The walk into Wick Quarry goes from limestone (shelf sea), into lagoonal and subaerial conditions with dessication cracks and possibly fossil soil. Ripple beds are present.

Fossils: The beds are rich in fossils – see the ‘Fauna’ column in the succession diagram, taken from one of the memoirs. There are also trace fossils e.g. Gyrochorte (Journal Bath Geological Society, No. 17 Jan. 1988, p.43).

Structure: Wick lies within the Variscan Foreland ‘Thrust Belt’ , a crumple zone. The Clifton Down Mudstone has been extensively and spectacularly folded by the Wick Thrust Fault, the shale beds providing the necessary lubrication.

Mineralisation: The Cromhall Sandstone is mineralised with galena and copper. Also present are haematite, limonite, goethite, marcasite, sphalerite, smithsonite (calamine), manganese, calcite and quartz. In the early days, the quarrymen only expected to find calcite at Wick and when they found similar but much harder minerals (quartz), they called them Bristle (Bristol) diamonds. Neptunian dykes also penetrate this bed.

Description and interpretation of the succession at Wick

The succession extends from the Gully Oolite to the Hotwells Limestone. The total thickness is 325 m. This succession is a version taken from some unknown memoir.

In the Fauna column F = foraminiferids; C = calcispheres; Co = corals; Cr = crinoids; B = brachiopods

To facilitate interpretation of the above succession here is a system of carbonate classification taken from http://csmres.jmu.edu/geollab/fichter/SedRx/Carbonate.html

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Carbonate rock names (limestones and dolomites) consist of a conjunction of two names, one describing the ALLOCHEMS, the large pieces, the other describing the INTERSTITIAL MATERIAL. Allochems are equivalent to gravel, sand, lithics or feldspars in the siliciclastics. Interstitial material is equivalent to clay or cements in clastics. There are four kinds of allochems:

Micrite and Sparite Interstitial Material
     Micrite is "lime mud", the dense, dull-looking sediment made of clay sized crystals of CaCO₃. Much micrite today forms from the breakdown of calcareous algae skeletons. It is not clear if all ancient micrites formed in the same way. Many carbonates are composed of nearly 100% micrite. Such rocks are simply called micrites.
      With carbonates containing allochems the question is whether micrite is present or absent as an interstitial material, and if present by how much. If micrite is present during deposition then it fills the spaces between the allochems and the rock will be given a name which describes the allochems in a micrite matrix. For example, a rock with fossil fragments embedded in micrite is called a "biomicrite". Biomicrite is analogous to a siliciclastic wacke, sand imbedded in a lot of matrix.
      If, on the other hand, the depositional environment has strong currents, only allochems may be deposited. If we could see the sediment during deposition all the allochems would be loose, like a pure sand or gravel. This is analogous to a 100% siliciclastic sand on a beach with no silt or clay. (With carbonates, though, such "clean" sands are not confined to beaches.) Micrite in these cases, being clay sized, has been washed away. The rock formed is then composed only of allochems, held together by clear to translucent calcite crystals with rhombohedral cleavage (called SPAR or SPARITE) acting as a cement. The spar is precipitated from fresh or marine water percolating through the sediment after deposition, but before final cementation. This oosparite shows well the spar cement.

Classification of Carbonates
     The classification of carbonates using the allochem/interstitial material system (the Folk System) is very systematic and straight forward. The allochem name is combined with the interstitial name (micrite or spar). The table below shows the major categories of carbonate rocks based on their allochems and interstitial material.

     But what happens if there is more than one allochem in the rock, or there is a mixture of micrite and spar? This classification system has great flexibility and creativity. You can easily build your own descriptive rock names. The name is built up by stringing together all the allochem names in order from least to most abundant, and then adding the interstitial material name ("matrix" below for short). For example, a rock like this:

Oolites + Fossils + Spar matrix = Oo bio sparite
   The name is written as one word, Oobiosparite.

     Another example (again allochems from least to most abundant):

Pellets + Oolites + Fossils + Micrite matrix = pel oo bio micrite
   The name is written as one word, Peloobiomicrite.

     But what if there is both micrite and spar matrix? The system is the same; just list them from least to most abundant.

Fossils + Spar matrix + Micrite matrix = bio spar micrite

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