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Kogel 30 items. For the history of Hacking Ferry see Appendix, section 8. At this time in the Carboniferous period, relative sea level was influenced by at least three major factors, syndepositional faulting, thermal subsidence and the coming and going of polar ice caps. The faulting and thermal subsidence created the accommodation which is the available space for sediment to be deposited.
The total thickness of Carboniferous Strata is about metres, mostly deposited in quite shallow water, as deposition generally kept pace with subsidence. On a much smaller scale, the changing sea level due to the accumulation and melting of polar ice, altered the depositional environment from shallow to deeper water and vice versa. These were deposited in quite shallow water, so at times of high relative sea level limestone was deposited and at times of low relative sea level erosion took place and terrestrial sediments were depositied. Much of the Pendleian succession at Dinckley is the lateral equivalent of Yoredale sediments but the Pendleian sediments were deposited near the mouth of a major river system flowing from the north.
Sand was deposited when energy levels were high, while a much lower energy level was required for the deposition of mud. River deltas with distributaries which meander and change course provide many different depositional environments. Because of a lack of fossils such sand and mud rocks are difficult to date, but fortunately there were occasional periods when relative sea level was high and muds containing the remains of marine organisms were deposited. Goniatites, which are marine ammonoid molluscs, evolved rapidly over this period and the ornament on their shells changed over time.
The fossilised remains of these goniatites provide a relative time scale and the strata in which they are found are called marine bands. For an example of a goniatite see Figure History 6. The Pendle Grit was deposited in relatively long and narrow submarine channels, recognised mainly by basal erosional contacts.
In this environment small lobes of sand are located at the distal end of the channel with a short transition to basinal mud. The Pendle Grit is feldspathic, which means that the sediment has been transported from a granitic source area so rapidly that there has been insufficient time for the breakdown of feldspars by weathering.
Figure Schematic cross section showing total thickness of Namurian strata, based on a comprehensive section in British Regional Geology, The Pennines and Adjacent areas 7. The Askrigg block is to the left north side of the section. In the schematic cross section of Figure 12 the approximate location of the Dinckley Gorge sediments can be seen. The difference in thickness of Namurian sediment reflects both original topography and subsidence, which was much less in the area of the Askrigg Block, a massif underpinned by the Wensleydale Granite.
The variation in thickness shows how the depositional environment evolved during the Namurian. The Pendle Grit was deposited in the lower part when basinal topography inherited from the Dinantian was dominant; the Warley Wise Grit was deposited on top of the Pendle Grit but over a somewhat wider area. The Rough Rock, at the top of the Namurian, was deposited over the whole length of the section shown and provides evidence that by then the basin was substantially filled up with sediment and little submarine topography remained.
In Figure 12 three marine bands have been shown for illustrative purposes. Without marine bands, relative dating would not be possible. Marine bands are therefore of fundamental importance in Namurian basin analysis. The exposure is not continuous, although more might be visible at very low river levels. See Garstang Memoir 8 for stratigraphic details. The Copster Green sandstone is not easily differentiated. It consists of an interbedded pebbly sandstone.
Its facies is uncertain, whereas that of the WWG is described as fluviatile, consistent with the interpretation that it is delta top facies. The exposure is not continuous Figure 13 so interpretation is difficult. This requires some explanation. This led to the widely known description of the resulting deposits known as the Bouma sequence.
Today the Bouma model for turbidites is seen to be much too restrictive in that it fails to encompass a wide range of turbiditic deposits which are not event deposits. Some of that sand may have been reworked from the shelf as the river in flood eroded a deeper or wider channel. Sediments resulting from this process do not have the characteristics of the Bouma sequence. When the river level is very low the succession in the lowest part of the Sabden Shales can be examined. Riley 9 interpreted sedimentary structrures in the topmost WWG as rootlets, see Figure The facies is comparable with the Brennand Grit Formation, which consists of a wedge of delta top facies, at the top of which is a ganisteroid palaeosol from 0.
More recently Riley has revised his interpretation of the WWG and believes that the sedimentary structures were probably evidence of burrowing. Figure Note that genus and species names may have changed since this log was published. The base of the Sabden Shales contains marine fossils.
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Clearly relative sea level deepened between the deposition of the WWG and the Sabden Shales and the supply of coarse grained sediment was cut off. Preservation of fossils in marine bands Some of the black shales within this succession are laminated or at least very little disturbed by burrowing.
Mud provides nutrients, so the paucity of burrowing organisms needs to be explained. In addition the marine bands preserve goniatites which have been little damaged by sea floor biota.
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Fossilised plant fragments are also preserved in some horizons. One mechanism for preservation of goniatites and other organic matter is a low level of oxygen in the water adjacent to the sea floor. Another is early cementation with calcite. Both preservation mechanisms occurred In Silesian marine bands. Anoxic depositional environments If no free oxygen is present the environment is described as anoxic. For simplicity the word anoxic will be used here for both anoxic and depleted-oxygen environments.
The Black Sea, which is up to m deep and an example of a silled basin, is anoxic at the sediment-water interface.
A schematic of a fjord with anoxia at the sediment-water interface is shown in Figure Other modern examples of silled basins can be found in the Californian borderland. Figure Schematic by Smittenberg 10 representing a fjord with anoxia at the sediment-water interface. The interface between high and low density is also called the pycnocline. Note that high salinity water from the ocean is denser than fresh water, so the bottom water has high salinity and tends to be undisturbed, at least on a short term basis.
Anoxia is promoted by influx of organic matter, which uses oxygen in decay processes. Under anoxic conditions sulfate-reducing bacteria give rise to H2S, which is toxic to most organisms including us and is likely to lead to the formation of pyrite. The Baltic Sea has extensive areas of sea floor hypoxia oxygen levels which are sufficiently low that life forms are very severely restricted.
The Baltic hypoxic area increased from km2 in to km2 in and because of its impact on fishing etc. Anoxic conditions may be periodic, alternating with conditions where oxygen is present at the sediment-water interface. Data from Greenland ice cores suggest that anoxic cycles correlate well with rapid climate excursions which lead to changes in ocean current patterns, upwelling of nutrients and high organic productivity. Figure 16 is adapted from Leeder It illustrates the change in depositional fabric of sediments in the Santa Barbara basin related to oxygen levels at the sediment-water interface.
When oxygen concentration is normal the sediment is bioturbated. It should be noted that when anoxic conditions exist within the sediment and the water is oxic, burrowing organisms allow oxygen to permeate the upper levels of the sediment. For further information about anoxic events see Leeder Benthic means adjacent to the sea floor. The proportion of heavy oxygen isotopes in ice cores is a proxy for global temperature.
Figure 17 below is from a British Antarctic Survey report13 at the web address below:. Figure Temperature proxy versus time, over a 25, year period, derived from oxygen isotopes in a Greenland ice core. The graph also shows that warming is always much more rapid than cooling. Figure 18 below is based on a figure in Leeder For the late-Pleistocene to Holocene period it shows, on the left side, periodic warm interstadial excursions identified from Greenland ice cores. On the right side it shows the stratigraphic record of anoxic laminated sediments seen in cores from a Californian borderland silled basin.
The time of rapid increase in temperature and low bioturbation index are correlated.
The relationship may reflect changes in ocean currents and upwelling of nutrients. Note the asymmetry of the cycles, rapid warming and slow cooling.
Low bioturbation correlates with times of rapid increase in temperature. It is also possible to have anoxia in a non-saline lake environment.