Shallow water to lagoonal carbonates

 

Lord’s Wood Quarry is located a few hundred metres south of the Little Doward car park. The quarry is owned by the Herefordshire Wildlife Trust and should not be entered without permission. Using the information in this section you can gain an insight into the palaeo-environmental clues provided by the rocks in this quarry.

The lower beds in the quarry are formed of the Lower Carboniferous Gully Oolite Formation, the upper beds of Lower Carboniferous Llanelly Formation – see the lower image. Within the Wye Valley and Forest of Dean areas the Gully Oolite Formation was previously known as the Crease Limestone and the Llanelly Formation as the Whitehead Limestone. Many books on the geology of the area are likely to use these previous names.

 

The diagram below highlights key features to be found in the rocks at Lord’s Wood Quarry. These features are clues to the ancient (palaeo) environments that were present here when the rocks were forming.

In this aerial view of Lord’s Wood Quarry sections of the face have been highlighted which relate to the  videos you can select below.

Videos in this section were produced by Attwood Media deploying a drone equipped with 4K HD camera.

Video A
A vertical traverse down the buttress in the NW corner of the quarry.
Display the video full screen for maximum information.

  • Find the division between the Gully Oolite and Llanelly Formations.
  • What is the distinctive whitish layer associated with?
  • What are the greenish grey pathches within the whitish layer?
  • Locate the stromatolites.
  • Can you see any palaeokarst?
  • Can you see any becciated / conglomertic layers?

 

Video B

Traverses the main face of the quarry from east to west, focussing on the Llanelly Formation.  You have a clear view of the stromatolites (the whitish thin horizontal streaks), above the thin distinctive bed marking the palaeokarst. Below the palaeokarst is the breccia/conglomerate layer of channel infills, cut into the massive carbonate below.

The stromatolites occur within the lower part of a massive carbonate bed, above which is a breccia/conglomerate layer extending to the top of the face.


Video C

Focus on the eastern side of the quarry, which is divided into the lower face and an upper terrace visible at times through the trees. We begin with a view of the lower beds of Llanelly Formation. Many interesting features are to be found here – see photos below.

What can you say about the magnitude and direction of dip of the strata here?

Note the upper terrace (0:16). As the video pans around towards the main face of the quarry (0:17) the Gully Oolite Formation comes into view beneath the sharply defined eroded surface between the Gully Oolite and the Llanelly.

 

The video below provides an aerial overview of the whole quarry and surrounds.
The small face in the SW corner of the quarry (top of frame at 0:14) has interesting paleokarst features which feature in the photos below.
With the quarry as orientated in the opening frame of the video, north is to the top.
In which rock is the lowest part of the quarry excavated?

The Deep Time setting for the rocks at Lord’s Wood Quarry

This part of the crust (known as Avalonia) was located around 14 degrees south of the equator during the Visean stage of the Lower Carboniferous ( 347Ma to 331Ma) when the rocks here were first forming. The palaeoenvironment was one of shallow warm carbonate rich seas, becoming predominantly lagoonal with barrier islands by the time of the Llanelly Formation. The overall tectonic trend was the progressive closure of the Rheic Ocean to the south (see diagrams below), leading to the terrestrial conditions of the Upper Carboniferous and ultimately desert environment by end Carboniferous – early Permian, with the formation of the Pangaea super-continent.

Diagrams like the one below, represent an attempt based on the accumulated evidence from extensive field work over decades, to reconstruct what possible overall distribution of environments was likely during a period often of many millions of years. Over such an expanse of time coast lines will migrate with sea level changes, erosion and tectonic activity. Such changes are evident in the rock beds of Lord’s Wood Quarry, with thick limestone formed in a shallow sea, to karst surfaces implying times when the area was exposed as dry land, to stromatolites formed in shallow inter-tidal waters.

Reconstruction of the likely palaeoenvironement looking towards the north when the Gully Oolite was forming. Key features are the oolite dunes and strong currents off a coastline to the north. The water would have been warm, the air hot. Similar conditions are found today in the Bahamas where oolites are forming and being swept into underwater dunes where strong currents are present. (Oolites are millimeter sized concretions of calcium carbonate).

 

Now the view to the south, towards open and deepening sea. This would be the shelf sea of the Rheic Ocean to the south. Some of the marine creatures of these times are shown, although there is no fossil evidence of their existence in these waters.

 

The diagram below portrays the conditions a few millions years later at the time the rocks of the Llanelly Formation were developing. The rocks of the Lord’s Wood Quarry and wider area of the Forest of Dean, were formed in a back lagoon environment sheltered most of the time from the sea by barrier islands. Remember at this time the area was close to the equator, so in the intense heat the lagoon waters would readily evaporate, concentrating the dissolved minerals to the point where they would readily precipitate from solution, the first to do this would be calcium carbonate (oolite formation). The stromatolites are formed by mats of blue-green algae or bacteria, the mats trap particles of sediment which leads to the accretion of layered structures. These may take a largely sheet like form,  as displayed in the rocks of this quarry, or produce domed structures. Shark Bay on the western shore of Australia is a place where stromatolites are forming now.

Reconstruction of the view southward over a lagoon at the time the Llanelly Formation was forming. The sea invaded this area at some time in the future and led to the deposition of the massive carbonate bed that lies above the stromatolite layers.

 

The following images (click on them to view full size) show specific features from various locations within Lord’s Wood Quarry. The first group of images illustrates some key events which led to the creation of what we see in quarry face today. Starting with a section of the main quarry face as it is now, an overlay is provided as the next image showing the location of the two formations and their junction. Some time after the formation of the marine Gully Oolite limestone, the upper surface became land and was subject to weathering in which water (rainwater is naturally slightly acidic) dissolved the limestone producing a karst landscape. At some stage marine conditions returned (this marks the beginning of the Llanelly Formation). The karst surface hollows became infilled with debris made up of angular pieces of Gully Oolite and a pale greenish colour mud, which is a residue palaeosoil of the former land surface. The marine environment of the Llanelly Formation time with its saline lagoons favoured the development of stromatolites.

Karren is a general term for describing the small scale solution features of limestone landscapes, in particular of limestone pavements (an area of present day limestone pavement formed in the Gully Oolite is visited in the Wye Valley Voyage). Kluftkarren is where the dissolved hollows (called grikes) are deep. In contrast if the grikes are only shallow the term rundkarren is used.

The numerous rubble filled channels in the Llanelly Formation are interpreted as storm channels cut through the sediments of the lagoon – see reconstruction illustration above with the reference to ‘pebble filled sub-tidal channels’.

 

The following images are from various locations on the rock faces at the quarry. Spot and describe the possible origin of as many features as you can.

 

Rocks of the quarry up close

The first image shows a typical piece of Gully Oolite. The ooids that make up the oolitic limestone are clearly visible.  Note, not all specimens of Gully Oolite Formation will display ooids. The ooids are formed by the precipitation of calcium carbonate around a nucleus. The spherical form is due to a consistent rate of precipitation in all directions around the nucleus as a result of the rolling back and forth of the ooids by wave action.

The ruler is displaying centimetres with millimetre subdivisions. In the case of this specimen the ooids are 0.5 millimetre or less in diameter. The ooids are cemented together by calcium carbonate formed during the lithification of the deposits of ooids.

A typical sample of Llanelly Formation limestone. In general the rock displays no internal structures, for example, ooids. The rock has a porcelain appearance and will fracture with sharp edges. The absence of internal structures in a limestone is indicative of bioturbation, whereby the calciite or dolomite mud (the forerunner of the lithified rock) has been well mixed by burrowing creatures seeking nutrients in the sediment.

 

From a bed showing karst features (a palaeokarst) in the Lllanelly Formation. Many of the eroded spaces (where calcium carbonate has been dissolved away) display the dark green/grey clay which is interpreted as the remnants of an ancient soil (paleosol). During periods when the limestone surfaces were exposed, plant growth may have been present with the associated formation of soil. Acids produced by the plants would have accelerated dissolution of the limestone.