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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Unconformities occur in three different parts of the depositional environment--on the shelf, the basin-margin coastal plain, and within the basin. Those on the shelf, typified by the Pennsylvanian-Permian of the Mid-Continent, are regional disconformities occurring above and below coal cyclothems. Sand-filled channels are commonly present above these surfaces, and they can form long, narrow, oil and gas traps.
Regional low-angle unconformities characterize the coastal plain as exemplified by the Cretaceous of southern Arkansas and east Texas. They are angular unconformities only from the regional viewpoint, for the structural difference between stratigraphic units is generally less than ½°. Porous belts are commonly truncated and overlapped by impermeable layers, producing large-scale stratigraphic traps concealed in a confusing array of overlapping and offlapping sequences.
Erosion occurred at places deep in the depositional basin on relatively local anticlines. Such folds may be part of a mid-basin arch or may simply be local tectonic features. Salt or shale domes and igneous intrusions produce similar effects. Porous formations are sharply truncated by unconformities; locally the difference in dip between units above and below may be as much as 90°. Traps formed under these conditions are narrow and commonly short, but the oil or gas column may be high.
Of the various kinds of traps associated with unconformities, those which form in the area of gentle and repeated tilting and warping on the basin margin are the largest and most copious. Search for them involves problems in stratigraphy and geometry, but ultimately may prove to be vastly rewarding.
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