G.A. Belousov, A.B. Gorodkov, A.V. Gvozdeva, Eh.A. Taratyn, B.K. Frolov, T.Yu. Shishkina
Delineation of unconformities and stratigraphic limits in carbonate organogenic sections on the basis of analysis of layer thickness variations
Key words: carbonate organogenic deposits; unconformities; section breakdown; layer thickness; hiatus; erosion; step; point of inflection.
For citation: Belousov G.A., Gorodkov A.B. , Gvozdeva A.V., Taratyn Eh.A., Frolov B.K., Shishkina T.Yu. Delineation of unconformities and stratigraphic limits in carbonate organogenic sections on the basis of analysis of layer thickness variations. Geologiya nefti i gaza. 2021;(2):15–25. DOI: 10.31087/0016-7894-2021-2-15-25. In Russ.
The authors propose to improve the ways of unconformities and stratigraphic limits delineation in carbonate organogenic sections on the basis of analysis of layer thickness variations. Materials of sedimentological analysis of natural outcrop cross-sections characterised in detail by lithological, geochemical, and paleontological data are used in this work. The cross-sections used are unique as they characterise the carbonate sequence structure within three different depositional zones of the basin, they are comprehensive, and based on layer-by-layer analysis at an interval of one metre. Process of the section formation is simulated using a diagram of layer thickness changes, which shows the layer-by-layer accumulation of deposits and takes into account their post-depositional transformation. Diagrams of thickness changes are composed of segments having different lengths and slope angles. The segments correspond to the layers; position of the layers is specified by the depth of their beginning and end (layer Top and Bottom). Change in thickness of overlaying bed is shown in the diagram by the change in the slope angle of a segment corresponding to this layer. In the case the overlaying layer is significantly thicker than the underlying layer, steps of varying heights are formed; their presence is indicative of unconformity, and the steeper the step, the more reason to classify such an unconformity as stratigraphic. In addition to steps, inflection points are identified, which divide the diagram of thickness change into areas that differ in thickness change trends. Unlike the steps, there are no drastic changes in layer thickness above and below an inflection point; gradual thickness changes are typical there. Unconformities can be reliably delineated in the section based on the presence of steps and inflection points on the mentioned diagrams.