For the last decade, the sequence-stratigraphic methods have become a widely-discussed issue in the geology worldwide. Qualitative and quantitative estimation of parameters influencing the sedimentation process is an important task for the sequence-stratigraphic analysis. Sea level changes can be the effect of ocean level changes and tectonic movements.
A grid of regional seismic lines and their seismic-stratigraphic interpretation are the input for constructing the curves of relative sea level changes. The seismic lines must be acquired over the area including the shelf edge and near-coast downlap. High-quality seismic data with sufficient well-log control are recommended. Besides, the interval in study must have a quite simple structure. Presently, a method developed by Vail's group at EXXON is mostly used for the definition of sequences. The determinations of the coast accretion and of the vertical component of the near-coast base downlap are method's key points. It is well known that such a method for constructing the curves of relative sea level changes is only applicable for the data with near-coast downlap recorded. However, more often we deal with the data with the prograding accumulated shelf accretion.
In order to build the curve, we step by step, from top to bottom, flatten the section on each seismic horizon. We also account for the erosion in particular time intervals. This approach enables to compensate for post-sedimentation tectonic movement in the basin. Thus, we exclude their impact on the variation of the accommodation space.
As in Vail's method, it is necessary to analyze the sedimentary complexes composing the clinoforms and to pick the main reflectors.
The next step is to identify the characteristic points of clinoforms: A, B, C, and D. Their meaning is as follows. Point A is the shelf point, from which parallel layers go towards the coast. Point B marks the shelf bend point. Point C marks the minimum sea level in the low-stand systems tracts. Point D marks the start of the parallel-layered depression.
In order to build the curve of relative sea level changes, Vail and his team use the point of near-coast downlap. As I mentioned before, this part of sedimentary basin sections is mostly absent. That is why we use the point A located at the same hypsometric level as the point of near-coast downlap according to Vail. In the present research we have obtained this curve for the Caspian basin. The method should be improved and applied for various geological objects.