Several hundred thousand cubic miles of Gulf Coast Tertiary sediments have been deposited in a deltaic setting. Published literature attributes sedimentary sequences ranging from a few thousand feet to only several feet in thickness to delta sedimentation. Are such highly variable sequences the result of delta deposition and, if so, what factors control such spatial and temporal scales? Basin tectonics, climatic changes, long-term relative sea-level patterns, eustatic sea-level changes, regional and local variations in subsidence rates, changes in basin drainage areas, variations in basinal marine climates, and numerous other similar factors all play a major role in controlling the spatial magnitude of deposits interpreted to be deltaic in nature. In modern or relatively young deposits, recognition of these process controls is sometimes possible, but in older (Tertiary) sequences such factors as lack of precise chronostratigraphic control, poor knowledge of quantitative faunal and sedimentary characteristics of a large number of deltaic environments, relatively meager understanding of past environmental conditions and regional depositional patterns, etc., add extreme complexity to basic recognition of variable delta cycles.

One of the major characteristics of deltaic sedimentation is cyclicity displayed at various temporal and spatial scales. Periodicity of the delta cycles recorded in the sediments at any one vertical site ranges from only a few years to several million years, with corresponding thicknesses from a few tens of feet to several thousand feet. Such nested frequencies and lack of criteria to recognize them make it extremely difficult to develop consistent terminology. A 2000-foot, well-developed coarsening upward sequence commencing with marine shales and capped by in situ terrestrial organic deposits can be described as a delta cycle beginning with deposition of prodelta shales, grading upward into alternating sandstones and siltstones of the delta front, to sandstones of the distributary mouth bar, and capped by delta plain deposits. In other instances, a 20-foot cored section containing a marine shale at the base, coarsening upward to a sandstone, and capped by in situ organic deposits has had similar terminology applied to the various facies. Both sequences may be the direct result of sediment being deposited directly from a river system, but the temporal and spatial scales of the sedimentary units, lateral continuity of units, and processes of deposition were probably quite different.

Relatively thick, coarsening upward sequences can result from long-term changes in basin tectonics, regional climatic changes and relative sea level changes, whereas delta cycles on the order of 100 feet thick often result from high frequency eustatic sea level changes, variations in regional rates of subsidence, and alongshore and on/offshore shifting sites of deposition. Very thin delta cycles (a few feet thick) result from short-term hydraulic variations, catastrophic events and localized sediment loading patterns. Some of the basic processes that control such variations in delta cycles and some of the resulting characteristics of the deposits will be discussed and illustrated.

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