CHRONOSOMES AND QUANTITATIVE STRATIGRAPHY

The three-dimensional mapping of time-bounded bodies of rock (chronosomes, after Schultz, 1982) forms the basis of quantitative prediction of both rock properties and biostratigraphy in the subsurface. High resolution seismic data can provide mappable reflection-bounded units (RBU's) with a vertical resolution of 10 to 20 ms two-way-time (approximately 10-20m in Tertiary sediments) . Lateral resolution varies from 10m in a 3D survey, to 500 m plus in the case of a regional 2D survey. We can begin with the working hypothesis that seismic reflectors are isochrons and map RBU's to chromosomes. Each chronosome has a unique combination of geological time interval and paleogeographical location. They are composed, generally, of lesser chronosomes below the current observational resolution, and can be combined to form higher-order chronosomes. The chronosome concept is therefore both time, and scale independent. Seismic stratigraphy forms a genetic framework in which the vertical development of chronosomes at any given location can be related to regional and/or global processes. Current seismic stratigraphic concepts and terminologies are essentially two-dimensional, yet sediment distribution throughout a basin is, of course, three-dimensional. Only by mapping chronosome development in three-dimensions can one find the predictive relationships between shape, location and content that are needed both for practical application of quantitative stratigraphy and in the theoretical analysis of causative processes within the basin.