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Title: An investigation of the sediment dispersal operating to control lithofacies variability and organic carbon preservation in an ancient mud-dominated succession : a case study of the Lower Jurassic mudstone dominated succession exposed in the Cleveland Basin (North Yorkshire)
Author: Ghadeer, Samer
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2011
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Abstract:
In this study the Cleveland Ironstone and Whitby Mudstone Formations have been investigated to characterise: a) the evolving mudstone facies present in a basin that is gradually deepening and developing bottom water anoxia over time and b) what the fundamental geological controls were on this variability. Using detailed facies descriptions obtained from analyses of approximately 151 samples obtained from combined optical, electron optical and geochemical methods 6 lithofacies have been identified. These include: 1) sand and clay-bearing, silt-rich mudstones, 2) siltbearing, clay-rich mudstones, 3) clay-rich mudstones, 4) clay, calcareous nannoplankton-, and organic carbon-bearing mudstones, 5) fine-grained muddy sandstones, and 6) cement rich mudstones. Individually, the samples are highly heterogeneous and typically organised into thin beds (<10 mm thick). These beds contain varying proportions of materials derived from inputs to the basin, primary production within the basin and the effects of diagenesis. In addition, they are microtexturally diverse and preserve primary depositional textures such as: sharp bases, normal graded bedding, starved ripple laminae, triplet fabrics, tempestites, pelleted laminae as well as a variety of burrowing fabrics. With these data three main questions were addressed (each of these questions forms the basis of a paper). These include (1) Identifying the main processes responsible for sediment dispersal in this succession, (2) Discussing the mechanisms of organic carbon preservation when bottom water anoxia was not as prevalent as most authors have assumed and (3) Determining if this succession can be interpreted within a sequence stratigraphic framework. The presence of ripples and triplet fabrics throughout this succession indicate that mud deposition in this succession was much more dynamic than most researchers had assumed. Moreover there was not that much difference between the processes operating to deliver and disperse sediment in the coarser parts of the succession compared with those operating in the finer grained, more production parts of the succession. The large volumes of organic carbon preserved here indicate that the role of bottom water anoxia as a pre-requisite for enhanced organic carbon preservation in sediments has been overstated. Moreover, the presence of marine snow suggests that much of the organic carbon was delivered episodically to the sea floor following phytoplankton blooms. Finally, as a significant fraction of the sediment is being dispersed by advective processes operating to infill available accommodation the building of sequences can be recognised (namely beds, parasequences, and systems tracts at stratal surfaces) this type of succession is reasonably interpreted using sequence stratigraphic principles. This study demonstrates that it is possible to directly link up-dip lithofacies variability in proximal sandy-mudstone deposits with coeval variations down-dip in distal basinal deposits. There is no reason why the processes occurring in basinal settings should be disconnected from those occurring up-dip in the lower shore face and offshore transition environments as their deposition represents a continuum of processes.
Supervisor: Hughes, Colin ; Macquaker, James Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.542755  DOI: Not available
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