SOME CONCEPTS OF SEISMIC STRATIGRAPHY WITH APPLICATION TO THE GIPPSLAND BASIN

Abstract

One approach to the problem of extracting stratigraphic information from seismic data is through the concept of sequences which are defined as time-stratigraphic units that are bounded by unconformities or their correlative conformities. These sequences are distinct depositional units that may be thought of as the building blocks of sedimentary basins. Sequence boundaries can be recognized on good quality seismic data by offlapping, onlapping and truncated patterns of cycle terminations. These sequence boundaries can be mapped to determine the configuration and lateral extent of each sequence and to provide a time-stratigraphic framework for analysing a basin. In addition, the logical lateral variations of environment and lithofacies that may be expected within each sequence provide a basis for rational extrapolation of lithological trends beyond the well control.

Seismic sequences can also be used to recognize and measure relative sea level fluctuations. The amount of coastal onlap shown by sequences can be used to measure the magnitude and timing of relative sea level rises, and similarly, basinward shifts in deposition across sequence boundaries can be used to document sea level falls. Using these principles, graphs can be constructed that show detailed relative sea level fluctuations within a basin as a function of geological time.

A relative sea level chart has been constructed for the Gippsland Basin by integrating seismic sequences with palaeontological data. When the Gippsland chart is compared with high resolution charts derived from seismic sequences in other areas it is found that many synchronous sea level fluctuations occur in widely spaced basins. These synchronous fluctuations are considered to be due to eustatic variations which contrast with other fluctuations that result from local tectonic movements.

As detailed relative sea level charts from many areas become available a more complete picture of ecstatic variations should emerge. Such a eustatic framework should assist in inter-regional correlations and also allow the geologist to distinguish between the effects of local tectonism and world-wide sea level changes.