MGA field trip to Castleshaw Moor and Readycon Dean, July 2008

Deep Water! Abundant Life or Lifeless Mud?

On Saturday 12 July Fred Broadhurst, Joe Macquaker and Paul Aplin treated us to an exceptional visit to Namurian, Upper Carboniferous, sedimentary sequences at Castleshaw Moor and Readycon Dean, near Denshaw. This field trip was exceptional for several reasons:

First there was the evolutionary sequence of the goniatites (marine, coiled shells, ancestral to the ammonites) from Reticuloceras reticulatum through Bilinguites gracilis and Bilinguites bilinguis to Bilinguites superbilingue. The earliest, Reticuloceras reticulatum, has a criss-cross pattern of spiral and radial ribs. The four species shown in the diagram represent stages in evolution, during which the spiral ornament progressively reduced. These shells are abundant in thin bands of clayrock, known as marine bands, each representing a rise in sea level to flood a delta top, implying that the sediments between the marine bands are non-marine, ie fresh or brackish water in type. At Castleshaw Moor we examined the Bilinguites bilinguis marine band. It was pointed out that recent research in this area, by Kevin Riley (an MGA member who sadly died a few years ago) established the presence of goniatites in the sediments between the gracilis and bilinguis marine bands, indicating that this succession, at least, is all marine.. There is no evidence to suggest the reasons for the gradual change in ornamentation.

Using hammers and chisels to split the clayrock along the bedding, a multitude of fine samples of the various goniatites were collected by the enthusiastic participants, among them two very keen Rockwatchers!

Bilinguites superbilingue
Bilinguites bilinguis
Bilinguites gracilis
Bilinguites reticulatum
Second, came the controversy about the depositional process by which the ‘marine bands’ formed. Fred gave the conventional explanation of the past 150 yrs saying that the regional environment was a southerly prograding delta top, periodically colonized by vegetation and swamps – the source of the coal seams. Occasional rises in sea level deposited marine sediments over the delta top, sometimes in shallow, sometimes in deeper water and forming the marine bands characterized by the goniatites. The conventional view held that deeper water sediments were considered to be anoxic and lifeless (but with the remains of swimmers, like the goniatites from higher levels in the water column). However, Joe explained that recent electron microscope examination of 20 micron thin sections of the clayrock, showed that it had been full of life! In fact it is has been shown that there is a continuous succession of goniatites throughout the clayrock and not just in the marine bands. There is also a myriad of microscopic coccoliths, forams, faecal pellets, burrows and fragments of vegetation. So, far from being anoxic there was a rich supply of oxygen in the water as well as abundant organic matter for food for bacteria and marine animals. The clayrock, which has a particle size (by definition) of less than 62.5 microns, also displays a range of sedimentary structures like ripples, slumps and turbidites as well as evidence of storms. This combination indicates a shallow water, shelf delta environment subject to variable sediment input, wave action and sea level fluctuations.

Part of the succession above the Kinderscout Grit
The tougher marine bands stand out from the clayrock, because the particles are cemented by calcium carbonate – demonstrated with hydrochloric acid. Joe explained that this indicated a break in sediment deposition to give time for a complex series of chemical reactions promoted by bacterial respiration (oxygen to CO2) and digestion of organic matter to form methane. These reactions produce the bicarbonate ions which combine with calcium to form the calcium carbonate cement.

The tougher marine bands stand out from the clayrock; there were about ten cyclic units

The clayrock, being rich in organic matter which decomposes to methane, is a potential source of natural gas. Watch this space for those gas rigs! The exposure shows brown staining indicating the presence of ferric oxide produced by weathering (oxidation) of the pyrites in the clayrock. This process also forms sulphuric acid which reacts with calcium carbonate to form gypsum, small white patches of which could be seen on exposed, weathered surfaces.

Fred conjectured about the reason for the ‘marine incursions’. He said there were about ten cyclic units, each about 1 m thick, spanning a period of, say, 2 million years giving a periodicity of 200,000 years, which fits neatly with the frequencies of Milankowich Cycles. Joe accepted this possibility but added that the rate of deposition could not have been constant because the marine bands indicate periods of no deposition.

Third, having collected many excellent, but very fragile, goniatite samples we enjoyed a hearty lunch at the Junction Inn in Denshaw. Joe then did a laptop presentation displaying the remarkable sedimentary features observed in the electron microscope images of the clayrock described above. Minerals present include muscovite, quartz, pyrite and kaolinite – clay formed by weathering of feldspars. In my experience this is the first time we have had a lunchtime lecture on a field trip!

After lunch we went to Readycon Dean where a coal seam above the Kinderscout Grit was exposed. Fred explained that ‘cleats’ in coal seams have a NW-SE orientation, which is pervasive throughout Britain, and are indicative of tension during its formation. Rootlets were clearly seen in the white seat-earth beneath the coal. At this locality many fine examples of Dunbarella speciosus were unearthed from the clayrock of the Bilinguites gracilis marine band.

Looking for cleats and Dunbarella    Dunbarella speciosus

This exceptional day was brought to a close by Jane thanking our three leaders for their expertise and time and by presenting Joe with a farewell card from MGA members, expressing their thanks for his role as our President and their best wishes for his future in Newfoundland….a likely venue for another exceptional field trip!

Fred Owen 27 August 2008