2.4.12 Special procedures for units on the continental shelf
A lithostratigraphical unit on the continental shelf, which it is desired to erect as a formal unit, should preferably be defined on the basis of one or more cores that constitute a type section (stratotype) for the unit, a boundary stratotype or perhaps a composite stratotype (see Section 2.4.5 for definition of stratotype). Core material provides the most direct expression of the lithology of a unit that is not exposed on land. It should therefore be given priority as the basis for defining the unit. Such cores should be specially protected under storage, and preferably be easily accessible.
A unit that has been proved by well logs and seismic profiling will often not be documented by drill cores. Such a unit can be defined on the basis of one or more well logs, but when it is subsequently core drilled, representative drill cores should be given the status of stratotypes or reference sections for the unit.
It is essential that structural form elements and planar structural units (faults, fracture zones, etc.) on the continental shelf are formally defined and named on the basis of seismic profiles, perhaps in combination with well data and drill cores (see Sections 2.4.6, 2.4.7 and 2.4.8).
It should be noted that the other requirements stated in Section 2.4 also apply when formal geological units are being erected on the continental shelf. Before a lithostratigraphical unit is formally defined and named, considerable emphasis should be placed on obtaining a reasonable regional knowledge of its extent and stratigraphical development (see Sections 2.4.3 and 2.4.8). If a unit is only known from one well, or from several closely spaced wells and a few seismic profiles, it should be given a provisional informal designation until adequate regional data are available as a basis for formal naming (see Figs. 2, 3 and 4).
To ensure that formal stratigraphical division of the succession on the continental shelf is as permanent and useful as possible, it is important that geologists working on a relevant portion of the shelf are in broad agreement about ways of dividing the stratigraphy and the basis for defining new units.
