Do you like the beach? If so, the Paleozoic may be for you!
If we were living right now at the time of a marine transgression, there would be a lot more beach. Of course, it would be hard to find land for all of the people to live on or for all the crops to grow.
Marine Transgressions and Regressions
Some of the most important events of the Paleozoic and Mesozoic were the rising and falling of sea level over the continents. Sea level rises over the land during a marine transgression. During a marine regression, sea level retreats. During the Paleozoic there were four complete cycles of marine transgressions and regressions. There were two additional cycles during the Mesozoic (Figure below).
Six marine transgressions and regressions have occurred during the Phanerozoic.
One of two things must happen for sea level to change in a marine transgression: either the land must sink or the water level must rise. What could cause sea level to rise? When little or no fresh water is tied up in glaciers and ice caps, sea level is high. Sea level also appears to rise if land is down dropped. Sea level rises if an increase in seafloor spreading rate buoys up the ocean crust, causing the ocean basin to become smaller.
What could cause sea level to fall in a marine regression?
Geologists think that the Paleozoic marine transgressions and regressions were the result of the decrease and increase in the size of glaciers covering the lands.
Geologists know about marine transgressions and regressions from the sedimentary rock record. These events leave characteristic rock layers known as sedimentary facies. On a shoreline, sand and other coarse grained rock fragments are commonly found on the beach where the wave energy is high. Away from the shore in lower energy environments, fine-grained silt that later creates shale is deposited. In deeper, low-energy waters, carbonate mud that later hardens into limestone is deposited.
The Paleozoic sedimentary rocks of the Grand Canyon contain evidence of marine transgressions and regressions, but even there the rock record is not complete. Look at the sequence in the Figure below and see if you can determine whether the sea was transgressing or regressing. At the bottom, the Tonto Group represents a marine transgression: sandstone (11), shale (10), and limestone (9) laid down during 30 million years of the Cambrian Period. The Ordovician and Silurian are unknown because of an unconformity. Above that is freshwater limestone (8), which is overlain by limestone (7) and then shale (6), indicating that the sea was regressing. After another unconformity, the rocks of the Supai Group (5) include limestone, siltstone, and sandstone indicative of a regressing sea. Above those rocks are shale (4), sandstone (3), a limestone and sandstone mix (2) showing that the sea regressed and transgressed and finally limestone (1) indicating that the sea had come back in.
The Paleozoic sedimentary rocks of the Grand Canyon were deposited during marine transgressions and regressions.
- Sea level depends on the amount of water tied up in ice and, locally, the position of the land.
- Facies are characteristic rock layers, such as sandstone, shale, limestone for a marine transgression and the reverse for a regression.
- The Grand Canyon has an incomplete record of the four marine transgressions and regression of the Paleozoic.
- How do scientists use sedimentary facies to recognize a marine transgression or regression?
- What are the possible causes of a marine transgression? What are the possible causes of a marine regression?
- What must geologists have found to know that there were two more transgressions and regressions in the Mesozoic?