Geology of the Great Orme, Llandudno

Professor Cathy Hollis, Dr Alanna Juerges, University of Manchester

The Great Orme is of great interest to Earth Scientists because of its spectacular exposure of Lower Carboniferous Limestone. As well as forming a distinctive landscape, and supporting a unique ecosystem, limestone has significant socio-economic importance. It forms aquifers for the supply of freshwater and geothermal water, hosts mineralization that supplies copper, lead, barium and fluorine and reservoirs giant oil and gas reserves in some parts of the world. Limestone is often an important source of local building stone and aggregate, as well as supplying the raw materials for cement. Although not all of these resources are exploited on the Great Orme, the same rocks that outcrop here have been mined and quarried extensively across North Wales. In addition, the Great Orme provides an important record of continental break-up and mountain building during the Carboniferous period, which coincided with a major change in the Earth’s climate.

The Great Orme Limestone: an ancient carbonate platform

Carbonate platforms are accumulations of organisms and grains that are composed of calcium carbonate (CaCO3). In order for carbonate-secreting organisms to grow, it is usually necessary to have clear, warm shallow water – this occurs today largely in sub-tropical zones between the Tropic of Cancer and Tropic of Capricorn. It is at these latitudes that we find modern carbonate platforms, such as the Bahamas, British West Indies, Maldives and the Great Barrier Reef. Through geological time, however, the temperature of the World’s oceans has varied, and during warm periods it was possible for carbonate platforms to grow at higher latitudes.

Figure 1 Bedded limestone of the Great Orme Limestone Formation, looking north-west from Llandudno Pier

In the uppermost Lower Carboniferous (approximately 335 to 330 million years ago), the Earth’s climate was warm. It was on the brink of a major glaciation, however, that led to the formation of a large ice sheet in the southern hemisphere; the so-called Gondwana glaciation. The limestone that outcrops on the Great Orme is part of the North Wales Platform, which stretched from Anglesey to the Welsh Borders, and formed on the margin of a landmass, called the Wales-Brabant Massif. It was one of many carbonate platforms that formed in the shallowest parts of warm equatorial seas across an area covering modern day western Europe and north America, on the edge of the Euramerican continent.

The Great Orme is largely made up of the Great Orme Limestone (Figure 1), which has a diverse array of fauna such as crinoids (sea lilies), echinoids (sea urchins), corals, bryozoan, brachiopods and small planktonic organisms called foraminifera. This suggests that environmental conditions were good; the water was warm and clear with good light penetration. The presence of small coral – crinoid – bryozoan rich mounds on Little Orme and Great Orme, as well as preservation of ancient sedimentary structures that show evidence for wave activity, suggest that the margin of the North Wales carbonate platform occurred in the vicinity of the Great Orme. To the north, a deeper water ocean basin was connected to the Pennine Basin to the east.

Figure 2 Dark, bedded limestone of the Bishops Quarry Formation, with abundant Gigantoproductid brachiopods (notebook for scale, ~20cm long)

At the top of the Orme, in the Bishops Quarry and Summit area, the character of the limestone changes dramatically- it becomes darker and dominated by large brachiopods, called Gigantoproductids (Figure 2). This suggests that by around 330Ma, the North Wales Carbonate Platform had begun to decline, either because it was poisoned by suspended clay material derived from the landmass or because sea level had begun to rise.

The Great Orme as a record of sea level change

Across the Great Orme, the Great Orme Limestone shows a distinctive layering where beds of limestone form apparently regular layers (Figure 1). Some of these layers are capped by red and yellow coloured mudrocks, which are interpreted to be ancient soils (Figure 3). During the latest Lower Carboniferous, the formation of the Gondawanan ice sheet in the southern hemisphere led to a cooling of the Earth’s climate and a change in seawater chemistry. As the ice began to form, sea level was lowered, rising again during inter-glacials. It is possible that these changes are recorded by the ancient soils on the Great Orme; since limestone can only form beneath sea level, it will become altered by rainfall and plant colonisation when it is exposed above sea level. It is also possible, however, that there is no regularity in the pattern of soil formation on the Great Orme, and that soils simply formed locally as a result of subtle changes in topography on the platform top.

Figure 3 Red-coloured palaeosol (ancient soil) on Marine Drive (East of Happy Valley) providing evidence that the Great Orme Limestone became exposed above sea level and colonised by plants during the Carboniferous.

The Great Orme as a record of mountain-building processes

As well as changes in the Earth’s climate in the Lower Carboniferous, major tectonic events were leading to the collision of plates to form the super-continent Pangea. After carbonate deposition stopped, around 330 million years ago, the North Wales Platform was buried to a depth of around 1km. The basin was inundated with sediment supplied by large river systems flowing southwards from the Euramerican continent, forming the swampy conditions that led to deposition of the Upper Carboniferous Coal Measures. In the latest Carboniferous and early Permian, closure of the Rheic Ocean to the south of the North Wales Platform led to collision of the Euramerican plate with Gondwana, to the south. The impact of this collision led to folding and faulting across North Wales and Northern England. On the Great Orme, the effect of this collision is seen in a number of large faults that cross cut the outcrop (Figure 4).

Figure 4 Fault zone on western Orme, near Tollhouse on Abbey Road / Marine Drive

Unravelling chemical processes in carbonate rocks

One of the characteristic features of the faults that cut the Great Orme Limestone is that they exhibit alteration of the limestone to a magnesium-rich carbonate rock called dolostone. Dolostone and the mineral that it comprises, dolomite (CaMg.(CO3)2), has intrigued geologists for centuries. Although it is abundant in the Earth’s crust it does not form in modern sedimentary systems, even though seawater is supersaturated with respect to magnesium.

Figure 5: contact between dolostone (brown) and limestone (white) within the Pier Dolomite, on the beach near Llandudno Pier (left) and close-up (right) of dolostone-limestone contact, an ancient reaction front

On the Great Orme, there is excellent exposure of dolostone in a number of locations. It can be identified by it’s characteristic dark brown colour, which contrasts with the white colour of the Great Orme Limestone, and its crystalline texture (Figure 5). It is particularly well exposed on the beach near the Pier (the so-called ‘Pier Dolomite’). Here, the Pier Dolomite shows a spectacular example of the termination of a dolostone body – an ancient reaction front formed when replacement of the limestone by dolomite stopped (Figure 5). Geologists still don’t fully understand what controls these reactions and their termination, which could be related to changes in fluid chemistry, temperature or hydrology. At other locations on the Orme, dolostone is usually restricted to faults, forming a zone – or halo- of alteration around the fault. This alteration zone ranges in thickness from a few centimetres to several hundred metres, depending on the size of the fault.

Figure 6 Clasts of brown dolostone cemented by white dolomite cement within a fault zone on the western side of the Great Orme (camera lens for scale, ~ 5cm)

The occurrence of dolostone along faults suggests that the fluids needed to form dolostone were supplied along the faults which can act as very effective fluid conduits when they are activated by plate tectonic events. Within a fault on the western side of the Orme, clasts of dolostone have been broken up and healed by dolomite cement to form a breccia (Figure 6). Around some faults, and within the Pier Dolomite, classic examples of so-called ‘zebra dolomite’ are observed. These enigmatic features give dolostone a stripy, zebra-like, effect (Figure 7). Although their origin is not fully understood, they are thought to form when stress is applied to rock prior to faulting.

Figure 7 ‘Zebra dolomite’ fabrics within the Pier Dolomite record stresses imposed on the dolostone during plate collision and faulting in the Upper Carboniferous (pencil ~15 cm, camera lens cap ~ 5cm diameter)

The Great Orme as a mineral deposit

The Great Orme is well-renowned for its copper deposits, which have been exploited since the

Bronze Age. The primary ore is chalcopyrite (CuFeS2), which has locally been oxidized to malachite

(Cu2CO3(OH)2). It is often observed within veins (fractures) in the rock, which cross-cut the dolostone (Figure 8). This means that it must have formed after the dolostone, and microscopic examination of the rocks suggests that it was one of the last geological events to occur on the Great Orme. Current estimates suggest that copper ores were formed from fluids expelled into the Great Orme limestone during the Alpine Orogeny in the late Cretaceous.

Figure 8 Zebra fabrics in Pier Dolomite cross cut by black veins (fractures) containing malachite