Deep sea seismic stratigraphy

Biart, Brian N. M. (1986). Deep sea seismic stratigraphy. PhD thesis The Open University.



Horizons responsible for the reflection of seismic waves within deep-sea sediments are shown to be less reliable for the purposes of correlation than their counter-parts in shallow margin sequences. Similar surfaces, such as abrupt lithological changes and unconformities, in the two different realms are not neccessarily produced by the same processes. It is the nature of these processes which control the chronostratigraphic significance of a reflector.

Thus reflectors may be correlated with reference to their genetic process. Horizons caused by time-restricted physical processes have enhanced chronostratigraphic significance.

In the deep-sea, layers in which the physical properties change slowly with depth (transition layers) are also important for reflector formation. In as much as these transitions can be affected by temperature, pressure and sediment geochemistry, as well as time, the equation of an horizon at two different localities does not neccessarily imply correlation in time (i.e. the horizon is not neccessarily a chronostratigraphic time line).

The two most important factors affecting impedance are the primary sedimentary geochemical composition and the nature of the grain to grain contacts within the sediment. Impedance increases with increasing grain density and increased rigidity of the sedimentary frame.

The inter-dependance of all sediment physical properties greatly complicates the study of the relationships between them. Modelling can be used to demonstrate the affects of variation of individual properties. Synthetic seismograms can be generated using either physical properties data measured from discrete samples or from wire-line data. While quality is a limiting factor to the performance of .-. physical properties modelling, the latter is of value in that it enables modelling at many more localities than is possible with wire-line techniques alone.

Abrupt impedance contrasts that produce reflectors important in deep-sea seismic stratigraphy may be grouped into a) Compaction horizons produced by gradual increase in over-burden pressure, b) Cementation horizons produced by variation in diagenesis with depth c) Calcite compensation depth (CCD) controlled horizons characterised by marked variation in primary sedimentary content and d) Unconformities produced by bottom current action.

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