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Growth faults and associated folds are the dominant structural features along many passive margins, including the U. S. Gulf Coast. Most of the exploration effort in these areas is directed toward prediction of fold geometry and location of fold crests. Previous attempts to explain geometry have focused on only one folding mechanism per fold. A new concept presented in this paper is that multiple folding mechanisms are operative and variously contribute to the geometry of each fold. Recognition of the dominant fold mechanism enhances prediction of fold geometry.
Folding caused by growth faulting, or tectonic folding, causes fold crests to shift basinward with depth. Drape compaction causes vertical alignment of fold crests, and differential compaction puts fold crests in the area of greatest sandstone percentage. Folds dominated by tectonic folding lie within the concave trace of growth faults. Tectonic folds bulge upward above regional dip, a phenomenon here termed "upfolding." An isopach of an interval above an upfold reveals thickening in all directions away from the fold crest. If upfolds have sufficient relief, shallower layers may align vertically due to the dominance of drape compaction. Subsurface studies of gas fields along the central Texas Gulf Coast provide examples of the contribution of various mechanisms to a single fold. Separ ting the contribution of fold mechanisms aids in prediction of fold geometry and has important implications for hydrocarbon exploration.
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