Transgressions and regressions: the key to Earth history Geology 103

Consider a modern depositional environment – the barrier island complex near Toms River, New Jersey

The barrier island parallels the coast and separates the open ocean from the protected bay (lagoon) Barnegat Bay Atlantic Ocean

The exposed foreshore side of the barrier island is a high-energy environment

By contrast, the backshore area on the protected side of the island is lower energy

The lagoon contains relatively low-energy water; it is sub-tidal, meaning it is always water-filled

The tidal flats are inundated by water twice per day, but dry out: no plants

In contrast, the supra-tidal (“above tide line”) marsh supports much plant life

Completing the trip away from the shore, the high-energy fluvial (river) depositional environment

Other depositional environments (not found at Toms River) Delta Mangrove swamp Alluvial fan

Offshore environments: the continental shelf The water energy of the shelf environments lessens further from the shore, which allows finer grain size particles to settle. Thus the inner continental shelf has sands and silts; the mid-shelf has silts and clays and the outer shelf has clays and carbonate particles.

Uniformitarianism allows geologists to infer ancient depositional environments from rock observations The fossiliferous limestone that makes up the Guadalupe Mountains in west Texas was, during the Permian period, an active offshore reef built by calcareous sponges and algae

Thus, we can infer that oceans once existed where none do today Rock (principally marine limestones and shales) and fossil evidence indicates that the ocean occupied the interior of North America (as well as other continents) Conclusion: sea level was higher (change in the elevation of the continent also played a role) This is a sea level transgression

The existence of submarine canyons allows the inference that the oceans were once shallower; this is a sea level regression

Facies = a rock or rocks deposited in a particular environment Because the continental shelf environment water energy distribution is consistent from place to place, the rock (sediment) sequence is always the same Facies = a rock or rocks deposited in a particular environment A sandstone facies, followed by a siltstone or shale facies, followed by a limestone facies

Walther’s Law of Facies Johannes Walther (1894) stated “A vertical change in facies in a sequence is reflected in the horizontal change in facies along a surface.”

Facies change during a transgression As sea level increases in an area, the water energy decreases, allowing the deposition of finer-grained sediments in the same area This is an example of Walther’s Law

Facies change during regressions, too

So, at an outcrop, transgressive sequences are distinguishable from regressive sequences

Worldwide transgressive sequences are found in the rock record Indicates that sea level changes occur over the millions of years timescale, so correlated to plate tectonic changes and continent orientations (especially over polar regions) Why no regressive sequences recorded?

Cratonic transgressive sequences are related to orogenic events Transgressive sequences coincide with significant orogenies and glaciations The mountains, which may be long gone, were the source of sediment for the sequences

For instance, the Tippecanoe transgressive sequence is coincident with the Taconic orogeny Little of the Taconic mountains remain, but the sediments eroded from those mountains makes up the bedrock of Pennsylvania and New York.