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Title: Structure, evolution and geophysical expression of mud volcano systems from the South Caspian Basin
Author: Evans, Robert John
ISNI:       0000 0001 2430 5093
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2007
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Abstract:
This thesis uses a combination of industrially acquired seismic reflection data, well data, topographic data and satellite imagery to investigate the structure, evolution and geophysical expression of extrusive constructions found within large (-500 m diameter) mud volcano systems from the South Caspian Basin. The principal aim is to gain a better understanding the structural architecture of mud volcano systems and the ways in which they are constructed. To this end this thesis includes three core research chapters which present investigations into the internal structure and eruptive history of a large mud volcano system, the geophysical response of seismic data to gassy seabed conditions and the structure and formation of mud volcanic subsidence craters. In the first core chapter the edifice of the giant C'hirag mud volcano system was investigated using three-dimensional (3D) seismic data. Internally, this feature consists of a number of discrete seismic facies units interpreted to represent either wedge-shaped units of erupted mud volcanic sediment or sheet-like units of non-eruptive sediment. Unit stacking patterns indicate the importance of pulsed mud volcanic activity as a control on the internal architecture of large mud volcano edifices and suggest it to be an important mechanism of basinal sediment and fluid expulsion. Analysis of the geometrical relationships of the internal sediment units to an underlying collapse caldera allows for a reconstruction of the system's history of collapse. Together, the details of internal unit type, stacking and relative edifice collapse timing constitute a detailed reconstruction of the volcano system's eruptive history and a record of the structural evolution of a large focussed fluid flow system. At the seabed a number of volcano systems within the South Caspian Sea study area were found to be imaged by areas of phase-reversed seabed reflection in seismic data. These "seabed phase reversals" are useful for better delimiting and understanding the structure of mud volcano source points and the extent of recent eruptive deposits. At one example kilometre-scale lobate mudflows are seen emerging from two seabed mud pool (salses) emphasizing the importance of both features in shaping the volcano's seabed morphology. Testing the hypothesis that seabed phase reversals are the result of gas within the seabed sediment took the form of a one-dimensional geophysical model of the study area seabed constructed using data from a borehole together with other published data. The results revealed that a phase reversal of the seabed reflection is a geophysical possibility under conditions typical of gassy seabed sediment. There is therefore a high likelihood that seabed phase reversals are the result of gas within seabed sediment. It is therefore suggested that seabed phase reversals can be used alongside other acoustic phenomena that indicate the presence of gas in a sedimentary section. In the final core chapter circular craters found at the upper terminations of onshore and offshore mud volcano systems are investigated. Using field maps. seismic lines, topographic data and satellite images it has been possible to describe these craters in detail for the first time and compile a generalized model for their structure. This includes a crater rim, an inward dipping crater margin fault, a moat and raised crater pedestal of freshly extruded mud volcanic sediment. The characteristic "moat and pedestal" morphology features at most of the craters featured here as well as at a number of others from elsewhere. Whilst the precise mechanism of crater formation is unclear it is strongly suspected that they form as a result of subsurface evacuation and collapse. They are thus similar to other sedimentary and igneous collapse features for which they may be useful analogues.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.584126  DOI: Not available
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