Hemingway, John Edwin (1933) Whitby jet and its relation to upper lias sedimentation in the Yorkshire basin. PhD thesis, University of Leeds.
The Upper Lias of Yorkshire as represented to the north and west of the Peak Fault, is a phase of aeposition of argillaceous sediments under varying physical conditions. The Grey Shales are shallow water deposits which pass gradually but rapidly into the finely laminated, bituminous shales of the Jet Rock, which were laid down under 'Black Sea conditions', but in water much shallower than the Black Sea. As the basin was gradually filled up during Bituminous Shale and Hard Shale times, its floor rose into the zone of aerated water, when the poorly bedded Alum Shales were deposited. Thes sedimentational conditions were interrupted by the deposition of an argillaceous limestone at the end of Jet Rock-times and by several beds and rows of masses of siderite mudstone, at varying horizons during subsequent zone-moments. The best 'hard' jet is found only in the Jet Rock, and is entirely composed of collapsed and compressed wood structures, soaked in humic substances resulting from the decomposition of the original wood. Annual rings, medullary rays, tracheids, bordered pits, stem bases and bark are all recognised in the jet. Some specimens of jet are silicified along a central zone, and here the wood structure is well preserved in an uncrushed state. Within the jet are frequent rows of included quartz grains and other minerals, which had become wedged into cracks in the wood before sedimentation. Whitby jet is the altered ermains of coniferous wood washed from the Liassic land surface into the Yorkshire Basin, where it became waterlogged and sank. The dominating factors in the subsequent jetonisation were the conditions of reduction and stagnation in the deeper parts of the basin during Jet Rock times. These conditions caused the formation of a fine, black viscous mud which, together with the absence of oxygen due to the same reason, controlled the decomposition of the wood and limited it to a unique and prolonged breakdown of organic constituents. As a result the wood which was in the form of stems and trunks, was reduced to a pulpy condition. The constant slow deposition of muds caused the stems to be much flattened and to assume the form in which they are now found. The inferior varieties of jet are due to formation in more oxygenated conditions of sedimentation, and probably to a greater degree of aerobic decomposition before incorporation in the argillaceous sediments. The increased amount of oxygen would cause decomposition to follow a more normal course.
|Item Type:||Thesis (PhD)|
|Department:||The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds)|
|Deposited By:||Ethos Import|
|Deposited On:||30 Apr 2012 16:04|
|Last Modified:||30 Apr 2012 16:04|
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