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Deltas generally are formed at river mouths during stillstands of sea level under conditions of either cyclic transgression or regression. Consequently, they generally are not isolated phenomena, but rather occur in multiples in a predictable fashion. Reservoir facies consist of both continuous and discontinuous bifurcating channel sandstones, which thicken downward at the expense of the underlying prodelta clays.
All the lithologic components of a deltaic complex are related and are collectively referred to as type of "genetic increment of strata" (G.I.S.). The G.I.S. is a sequence of strata in which each lithologic component is genetically related to all the others. It is defined at the top by a marker bed (such as a thin limestone or bentonite) and at the base by either a marker bed or an unconformity. It generally consists of the total of all marginal marine sediments deposited during a stage of either cyclic subsidence or emergence. An isopach map of a G.I.S. clearly shows the bifurcating trends of the individual distributaries and the shape of the delta, regardless of the varied lithology of the channel fills.
A "genetic sequence of strata" (G.S.S.) consists of 2 or more G.I.S. and, on an isopach map, the shelf, hinge line, and less-stable parts of a depositional basin are clearly defined. An isopach map of the McAlester Formation in the Arkoma basin is a good example of a G.S.S. The oil-productive Booch Sandstone is a good example of a deltaic complex occurring within a G.I.S. of this G.S.S. The upper Tonkawa, Endicott, and Red Fork Sandstones of the Anadarko basin are identified as deltaic accumulations within different G.I.S.'s.
A hypothetical model serves as a basis for establishing the criteria for (1) recognizing successive stillstand positions of a shoreline, (2) predicting paleodrainage courses, (3) predicting positions of a series of deltaic reservoirs, (4) locating isolated channel sandstone reservoirs, and (5) tracing related beach-sandstone reservoirs.
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