Abstract

Unconformities, at one time regarded as representing records of simultaneous world-wide events, and thereby forming the "ultimate basis of correlation," are now known to merge laterally along strike and to disappear down dip. A continuous sequence of types occurs, from high angle unconformities to paraconformities. Most significant to the oil geologist are the low angular unconformities, found commonly on the shelf and geosynclinal margin. These characteristically occur in repetition as the basin margin is continually flexing during deposition. Analysis involves interpretation of combined outcrop-subcrop-worm's eye maps to the end of reconstructing geologic history. Given an onlapping sequence of beds, thinning shoreward, one of four possible results may accrue: tilt may take place directly down towards the sea, at an angle to the shoreline, warping into a syncline, or upbending into an anticline. Secondary and subsequent sea advances may or may not conform to the earlier structure. Erosion produces bands of outcrop and facies either parallel to one another and to the unconformity, or outcrops diverging, crossing facies, and intersecting unconformities at an angle. Pattern depends on the type of fold or tilt; angles upon amount.

Southwest Arkansas is presented as an example of an area where repeated straight tilts have produced five major unconformities. Northeast Texas is a region of repeated synclinal downwarp; four major unconformities are shown. The Hunton anticline of central Oklahoma is an area of repeated upwarp; four unconformities are present.