DEFINITION OF TRANSGRESSION

A rise or landward migration of the shoreline

caused by either a global sea-level rise, a fall in the land’s

surface, or a supply of sediment that is less than the space

created for the sediment by subsidence. Transgressions may

also be marked by a replacement of shallow water deposition

to deep-water facies or a general landward shift in marine

facies. Global sea-level rises and falls on different timescales

depending on the cause. Changes in ridge volume or mantle

plume activity cause slow changes to the ocean ridge volumes

and slow rises or falls in sea level, whereas changes in the volume

of continental glaciers may cause faster changes in the

volume of water in the ocean. All of these may be related to

the supercontinent cycle and cause sea-level regressions or

transgressions. Local tectonic activity may cause the land surface

to rise or fall relative to a stable global sea level, causing

local regressions or transgressions. For instance, rapid subsidence

caused by tectonic thinning or loading of the crust may

cause the shoreline to migrate landward in a local transgression.

To interpret patterns of global sea-level rise and fall it is

necessary to isolate the effects of local tectonic subsidence or

uplift, and sediment supply issues, from the global sea-level

signature. This can be difficult and requires precise dating

and correlation of events along different shorelines, plus a

detailed understanding of the local tectonic and sedimentation

history. When the local effects are isolated they can be

subtracted from the global sea-level curve, and the causes of

global sea-level changes can be investigated.

Transgressive marine sequences record major marine

advances over the land at several times in the Phanerozoic.

Most transgressive sequences are preceded by an erosional

unconformity and show a progressive landward shift in sedimentary

facies that, according to Walther’s Law, is also

recorded in the vertical sequence. The base of transgressive

sequences is typically marked by a beach sandstone or conglomerate

unit, followed upward by an offshore muddy

facies, then typically a deeper water limestone facies.

In the 1950s, using index fossils and isotopic dates of key

horizons, an effort pioneered by Laurence L. Sloss and

coworkers in the petroleum industry, correlated many transgressive

sequences across North America and the world.

Many large, laterally extensive rock units that are bounded by

unconformities of regional or global significance were recognized

and precisely dated in many places. Some of these

unconformity-bounded sequences are so significant that they

are found in almost all shallow water deposits of that age in

the world. These sequences always occur where sea level has

dropped from high to low, and the overlying sequence is

transgressive. Index fossils were used to show that these

unconformities have the same age on all continents and are

clearly related to changes in sea level. Sea level has fluctuated

by as much as 1,150 feet (350 m) higher than the present level

and 655 feet (200 m) below the present level. Using these correlations,

six major transgressive sequences have been recognized

in the stratigraphic record of the continents. These

transgressive sequences include the Eocambrian-Cambrian

(600–500 Ma) Sauk Sequence, the Middle Ordovician–Lower

Devonian (470–410 Ma) Tippecanoe Sequence, the Middle

Devonian–Upper Mississippian (410–320 Ma) Kaskaskia

Sequence, the Lower Pennsylvania–Lower Jurassic (320–185

Ma) Absaroka Sequence, the Middle Jurassic–Upper Cretaceous

(185–30 Ma) Zuni Sequence, and the Tertiary-Recent

(30–0 Ma) Tejas Sequence.

See also REGRESSION; SEQUENCE STRATIGRAPHY.

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