Under the term paleogeomorphology are grouped all geomorphological phenomena which are recognizable in the subsurface. Buried relief features with a marked three-dimensional geometry are of importance to the petroleum geologist whenever they lead to the trapping of hydrocarbons. Paleogeomorphic traps form a third and distinct group which ranks in importance with stratigraphic and structural traps as a major mechanism for localizing hydrocarbon occurrences. They are not simply another type of stratigraphic trap, but form by themselves a category of considerable economic significance. Paleogeomorphic traps can not be analyzed, nor can their occurrence be predicted, by stratigraphic or structural methods of study, but must be treated as a geomorphological problem. Of the man types of buried relief features, some are of greater interest to the petroleum geologist than others. In this paper, the morphology of buried erosional landscapes is discussed in greater detail. Other types of buried relief features are fossil reefs, barrier beaches, and submarine canyons.

Hydrocarbons may be trapped either directly or indirectly as a result of paleogeomorphological processes. In either case, the traps may occur below or above (against) the morphological surface. Numerous examples of the effects of erosion are known from the Paleozoic and Mesozoic buried landscapes of western Canada and the Mid-Continent area of the United States. Hydrocarbon traps occur below the highs on such erosional surfaces as well as in sandstone bodies deposited in the lows on these surfaces. The rules which govern the formation of ancient landscape forms are worked out in detail, with particular emphasis on the application of quantitative geomorphology to the pattern of the ancient drainage system and on such features as summit levels and the influence of geological factors (er sion-resistant levels, influence of faults and fractures, etc.). Weathering and underground solution also play a role in providing both reservoirs and buried traps for oil and gas.

The method of analysis must take into account (1) the structural attitude of the strata below and above the erosional surface, (2) the lithology of the formation overlying the unconformity (if used for isopachous mapping), (3, 4) synsedimentary structural movements and the compaction factor in the formation overlying the unconformity, (5) problems of correlation, (6) reservoir development, (7) presence of a "seat seal," and (8) other factors.

Paleogeomorphology provides an interesting evaluation of "modern" geomorphological thinking. The fossil relief forms "frozen" by the transgression of younger rocks neither disprove the validity of the classical peneplain concept, nor do they support fully the newer ideas regarding slope retreat. The actual conditions found are described by a re-definition of the old term paleoplain. The summit level is an intrinsic part of this feature, and not a dissected earlier peneplain. A new geomorphological concept introduced here concerns the alternation of obsequent and resequent interfluve spurs.

Buried landscapes should provide a high percentage of the future oil and gas fields yet to be discovered in North America. The most important stratigraphic levels at which buried landscapes occur are those that formed after major periods of orogenesis. Their geographical locus corresponds to the broad belts of subsequent transgressions.