Marine Transgressive Overlap - Fining Upward Sequence
A sedimentary structure is generally initiated by the accumulation of sediment on an old erosion surface. Deposition does not occur everywhere at once. Sediment will be deposited gradually at a location close to the source of sediments and will be spread outward from the source. This process is called overlap since the new sediments gradually overlap onto the older sediments.

Two kinds of overlap are recognized. Transgressive overlap occurs when the sea is advancing, or transgressing, upon a low, relatively flat land mass. In this case the land mass is subsiding relative to sea level. At the shore line, sand and gravel accumulate. Away from the land, beyond the beach sands, mud is laid down, and beyond that there may be organic oozes deposited on the sea floor.


Transgressive overlap - fining upward sequence (top)
          Regressive overlap - coarsening upward sequence (bottom)

The sand, mud, and lime phases extend along the coast in three roughly parallel belts. As the sea encroaches on the land, the three types of deposits remain in the same relative position to each other, but they shift landward. The beach sand and pebbles are laid down on the old erosional surface. The mud from this phase overlies the sand deposited earlier, and the ooze overlies the older mud layer. Thus mud overlaps the sand and ooze overlaps the mud. This is marine transgressive overlap, and grain size becomes finer in the upward direction.

Thus, a vertical section through the formation shows finer grained rocks lying over coarser rocks, or deep water sediments over shallow water sediments. These sands are described as "fining upward". The gamma ray and SP curve shapes are shown below , and are called bell shaped curves, due to the visual effect generated by a pair of regular and mirror image curves. Typical transgressive sedimentary structures are alluvial point bars in meandering rivers and tidal channels. Delta distributary channel fill can also show transgressive curve shapes, but more show regressive or high energy shapes.


Fining upward, coarsening upward, and cylindrical curve shapes

A serrated or saw tooth curve shape may be superimposed on a bell shaped curve; this is caused by interbeds of shale between the sand layers. Such interbeds represent short periods of deeper water deposition, possibly caused by floods or erratic transport of sediments.
 

Marine Regressive Overlap - Coarsening Upward Sequence
Regressive overlap is produced when the sea recedes from the land, as when the land mass is uplifted or the sea bed subsides. By this process, the beach zone migrates out over earlier offshore muds. See bottom half of previous illustration. At the seaward end, mud beds overlap the older ooze. A vertical section shows coarser material overlying finer material, or shallow water sediments overlying deep water sediments. These sands are described as "coarsening upward" and have a gamma ray or SP curve shape as in the center of the illustration shown above. These are called funnel shaped curves and would be indicative of barrier bars, or the edge of delta fronts. These curve shapes may also be serrated or saw tooth shaped.

Sea level dropping relative to the land is not necessary for marine regressive overlap. The same regressive sequence is produced if the sea level is stationary and there is sufficient supply of sediments, or even if the sea level is rising, provided the rate of supply exceeds the rate of rise of the ocean.

Different types of overlap often combine in the same stratigraphic section. Thus the sea may advance on the land and then recede, so that regressive overlap is found above transgression. This alternation may be repeated many times.


High Energy Marine Deposition - Cylindrical Sequence
Distributary channels in delta sequences represent high energy deposition. At flood stage, these deposits will fill old channels while new ones are formed. The sand bodies will be quite uniform, with large grain size and little visible bedding. These form cylindrical curve shapes, similar to those in the bottom illustrations. If flooding is erratic, the curve shape may be serrated.

Similar curve shapes can be formed in deeper water as sand slides, or fans, formed at the end of submerged canyons. Turbidite deposits, formed in deep water by slumping of unconsolidated slopes or by rapid movement of heavy, silt laden water, form serrated cylindrical curve shapes, often covering hundreds or thousands of feet of vertical section.


Curve Shape Patterns in Continental Sequences
Bell shaped curves are found in alluvial point bar sands, in meandering streams, and in drape inside a channel fill. Funnel shaped curves occur in foreset and crossbeds in channels. Cylindrical patterns are restricted to sand dunes and scree slopes, the latter being often serrated.

There are obvious ambiguities between marine and continental curve shapes, so curve shape alone will not suffice to uniquely determine environment. With all four sources of environmental data, namely curve shape, dip angle and spread, bedding type, and shale volume, it is usually possible to assess the environment and hence the sedimentary structure quite precisely. If one source is missing, especially the dip data, life becomes more difficult, but regional knowledge will usually fill in the gaps.

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