Stratigraphic Relationships and Geologic History Depicted by Computer Mapping

Abstract

The construction of detailed contour maps showing forms and relationships of stratigraphic horizons has become increasingly important in exploration and production. A complete depiction of geology must include known geometric relationships (onlap, offlap, truncation, etc) among the various stratigraphic horizons, as well as depths to horizons and thickness of intervals. Pre-orogenic and pre-erosional forms should be restored as part of the geologic history. A large geographic area that includes many stratigraphic horizons with several periods of diastrophism and erosion usually involves so many data that use of the computer is necessary. Unfortunately, many computer-generated maps and cross sections do not include such relationships and restorations. Most computer mapping programs generate grids of interpolated data that are used for drawing contour maps. Procedures to generate grids that show the configurations of horizons at various stages of geologic development, as well as providing estimates of the pre-erosional forms, are straightforward, and are not significantly different from manual methods. The geologic section is divided into sequences of conformable horizons. The horizons in a sequence are estimated by adding or subtracting isopach grids from a well-established control horizon. Younger sequences are then related to those below by onlapping, offlapping, or truncating relationships. Pre-erosional surfaces can be restored by estimating the amount of missing material. This is then incorporated into a grid, conformable hori ons are generated, and stratigraphic relationships are introduced. The final grids can be used to draw maps showing present structure, including depiction of nondeposition or erosion of the horizon along with subcrop lines. Paleostructure maps may similarly show these features, but as referenced to some past geologic age. Isopach maps also reflect historical events. Cross sections, referenced to some past or present datum, show stratigraphic relationships.