Geology Reference
In-Depth Information
Fig. 1.5.
East-west cross section across the structure in Fig. 1.1 produced by taking a vertical slice through
the 3-D model. Line of section shown on Fig. 1.3
horizons. In this topic cross sections will be assumed to be vertical unless it is stated
otherwise. A cross section through multiple surfaces illustrates their individual geom-
etries and defines the relationships between the surfaces (Fig. 1.5). The geometry of
each surface provides constraints on the geometry of the adjacent surfaces. The rela-
tionships between surfaces forms the foundation for many of the techniques of struc-
tural interpretation that will be discussed.
1.3
Map Units and Contact Types
The primary concern of this topic is the mapping and map interpretation of geologic
contacts and geologic units. A contact is the surface where two different kinds of rocks
come together. A unit is a closed volume between two or more contacts. The geometry of
a structure is represented by the shape of the contacts between adjacent units. Dips or
layering within a unit, such as in a crossbedded sandstone, are not necessarily parallel to
the contacts between map units. Geological maps are made for a variety of purposes and
the purpose typically dictates the nature of the map units. It is important to consider the
nature of the units and the contact types in order to distinguish between geometries
produced by deposition and those produced by deformation. Units may be either right
side up or overturned. A stratigraphic horizon is said to face in the direction toward which
the beds get younger. If possible, the contacts to be used for structural interpretation should
be parallel and have a known paleogeographic shape. This will allow the use of a number
of powerful rules in the construction and validation of map surfaces, in the construction
of cross sections, and should result in geometries that can be restored to their original
shapes as part of the structural validation process. Contacts that were originally horizon-
tal are preferred. Even if a restoration is not actually done, the concept that the map units
were originally horizontal is implicit in many structural interpretations.
