Abstract

Photogeology is literally the interpretation of aerial photography for geological purposes. In the strictest sense, photogeology includes geomorphology. Most domestic photogeology, until the mid-1950's, was done in the Rocky Mountain area where bedrock exposures were relatively easily mapped, structurally and stratigraphically. By the mid-50's oil companies began to search for surface data which might give clues to subsurface structural features not directly expressed at the surface. The hope was to acquire more economical surface data on which to base future geophysical work, and with which to re-evaluate existing records. The two activities have been combined with good results.

In the Gulf Coast and Mississippi Embayment country from Southwest Texas to Florida, geomorphology should be the primary tool of surface mapping for oil and gas exploration. Geomorphology, as applied in the oil industry, is a science, and should not carry the old connotation and stigma of "creek-ology". The logical application of geomorphic principles is proving to be a valuable exploration tool.

Basic concepts which are well documented in the published literature are necessary to the comprehensive application of geomorphology, and are discussed.

Geomorphic analysis is concerned primarily with the determining of the degree of influence which the structure and lithology of.the surface rocks have had on the morphological development of the area. The four basic categories of this approach, generally in order of their importance, are:

1. Drainage analysis

2. Land form analysis

3. Fracture pattern analysis, and

4. Tonal characteristic analysis

Although more stress may be placed on one of the above categories in a given area, a comprehensive analysis includes consideration of all of them. Drainage analysis is usually the most important, and drainage terminology carries a structural connotation. The terms consequent, resequent, subsequent, and obsequent can be used in a structural sense. All these streams can be related to the original consequent surface, which is the initial slope of the land surface upon regression of a sea. All streams are lazy and take the direction of least resistance; thus, all streams controlled by folding definitely migrate down dip and all streams controlled by faulting are linear in nature. The greatest deterent to the structural analysis of drainage is the homogeneity of surface rocks.

Photogeology and geomorphology are not the panacea for all exploration activity. They are tools which will be of value to each geologist only if he coordinates his interpretations of subsurface and geophysical data with them.