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10.17: Paleozoic and Mesozoic Seas

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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. Surf's up!

Marine Transgressions and Regressions

Sea level rises and falls. During the Paleozoic and Mesozoic, the seas rose over the continents. This is called a marine transgression. During a marine regression, the sea level retreats. The Paleozoic had four complete cycles of marine transgressions and regressions. The Mesozoic had two (Figure below).

Six marine transgressions and regressions have occurred during the Phanerozoic.

What makes sea level rise? There are two possibilities: Either the land sinks or the water level rises. Tectonic processes can cause the land to sink. Melting glaciers can cause the water level to rise. Sea level rises if seafloor spreading goes faster. The hotter crust is buoyed up and displaces the water. This causes the ocean basin to become smaller. What could cause sea level to fall in a marine regression? Land rises up. Glaciers grow. Seafloor spreading slows down. Just the opposite of what makes sea level rise!

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.

Rock Facies

Geologists know about marine transgressions and regressions from sedimentary rocks. These events leave characteristic rock layers known as sedimentary facies. Sand and other coarse grained rock fragments are deposited on the beach where the wave energy is high. Offshore in lower energy environments, fine-grained silt is deposited. Farther offshore in lower energy waters, carbonate mud that later hardens is deposited. The rocks that result are sandstone, shale and limestone. As the sea moves on or offshore, one rock layer is on top of the other.


  • facies: Characteristic sedimentary rock layers. They indicate the processes and environments in which they were formed.
  • marine regression: The falling of sea level so that seas no longer cover the continents.
  • marine transgression: The rising of sea level over the continents.


  • Sea level depends on the relationship of the water level to the land.
  • Sea level will rise when glaciers melt. Sea level will fall when glaciers grow.
  • Facies are characteristic rock layers, such as sandstone, shale, limestone for a marine transgression. The reverse indicates a marine regression.


Use this resource to answer the questions that follow.


  1. What are the paleontologists searching for in Utah?
  2. How many years ago was there a sea in Utah?
  3. How far did the sea extend?
  4. What does tropic shale contain?
  5. How do the scientists date the shale?
  6. What do they learn from the fossils?


  1. How do scientists use sedimentary facies to recognize a marine transgression or regression?
  2. What are the possible causes of a marine transgression?
  3. What are the possible causes of a marine regression?
  4. How do geologists know that there were two transgressions and regressions in the Mesozoic?




Characteristic sedimentary rock layers; they indicate the processes and environments in which they were formed.
marine regression

marine regression

Falling of sea level so that seas no longer cover the continents.
marine transgression

marine transgression

Rising of sea level over the continents.

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