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!
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.
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!