Marine Transgressive Overlap - Fining Upward Sequence
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.
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.
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.
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
Regressive Overlap - Coarsening Upward Sequence
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.
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.
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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