1. Transgressions and regressions: the key to Earth history
Geology 103

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

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

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

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

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

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

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

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

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

12. 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.

13. 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

14. 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

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

16. 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

17. 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.”

18. 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

19. Facies change during regressions, too

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

21. 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?

22. 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

23. 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.