Paleogeologic and Onlap Maps for Seven Major Unconformities of North America

Donald E. Owen

Maps are presented of most of North America (scale of 1:5,000,000) showing (a) subcrops below the Cambrian, Middle Ordovician, Lower to Upper Devonian, latest Devonian to Mississippian, Pennsylvanian, Middle Jurassic, and Cretaceous and (b) onlap maps which show the various units which lie directly upon these old erosion surfaces.

Where an unconformity coincides with a systemic boundary, these maps are similar to previously published regional paleogeologic maps; elsewhere, there are significant differences. The paleotectonic patterns of units below an erosion surface are generally different from those shown by the units which progressively buried that surface.

The paleogeologic and onlap maps viewed in conjunction show the location of the various rocks exposed to erosion at the same time that successively younger sediments were being deposited. On a regional basis, each pair of maps outlines general areas of known and potential subcrop oil traps as well as showing the overlying units. The latter may be vitally important as source beds and seals.

Such a "big picture" regional compilation represents the input of a large number of individual geologic studies. As most of the mapping concerns the subsurface geology, revisions and additions will be required as new information becomes available and more exact stratigraphic relationships are determined. The representations provide an aid to the portrayal, identification, and understanding of some of the stratigraphic complexities in North America.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.