Cratonic sequence

The sea moves over the land.

Sometimes, the water rises up high. It covers much of the land. This is called a sea cycle.

Then, the water goes away. It moves back out to the ocean. The land is dry again.

These changes leave layers of sand and mud. These layers turn into rock. You can see them in the Grand Canyon.

Rocks tell us how the world changed. They show us where the water once was.

Earth has a way of changing its sea levels.

Sometimes, the ocean moves onto the land. This is called transgression. Then, the water moves back out to the sea. This is called regression. These steps make a cratonic sequence. A cratonic sequence is a large set of rock layers. These layers show one full cycle of rising and falling seas.

These cycles leave layers of sand and mud. Over time, these layers turn into rock. You can see these layers in the Grand Canyon. Laurence L. Sloss first proposed this idea in 1963.

Why does the sea level change? One cause is the mid-ocean ridges. These are long lines of mountains on the ocean floor. When these ridges spread fast, they grow longer. Heat makes the floor rise up. This pushes the water onto the land. When the ridges spread slowly, they sink. The water then drains away from the land.

North America has seen six of these sequences. They are the Sauk, Tippecanoe, Kaskaskia, Absaroka, Zuñi, and Tejas sequences. Each one marks a time when seas covered the land.

A cratonic sequence is a huge pattern in the Earth. It shows how sea levels rise and fall over time. These cycles happen on a craton. A craton is a large block of continental crust. Scientists also call these patterns megasequences or Sloss sequences. They help us see the history of our planet. Each sequence marks a time when seas covered the land.

This way of working happens in two main steps. First, the sea moves onto the land. This step is called transgression. Next, the sea moves back out to the ocean. This step is called regression. As the water moves, it leaves layers of sediment behind. These layers of sand and mud turn into sedimentary rock. You can see these layers in the Grand Canyon.

Laurence L. Sloss first shared this idea in 1963. He proposed that these layers show complete cycles. Each sequence has a top and a bottom edge. These edges are called unconformities. An unconformity is a gap in the rock record. It shows when the sea receded. During these times, sediment was washed away instead of being left behind.

Many things can cause these sea levels to change. One cause is the mid-ocean ridges. These are long lines on the ocean floor. When ridges spread fast, they grow longer. Heat makes the floor rise up. This pushes seawater onto the continents. When spreading slows down, the ridges sink. Then the seas drain away from the land.

North America has had six of these sequences. They started during the Cambrian Period. The oldest one is the Sauk sequence. Then came the Tippecanoe and Kaskaskia. After those were the Absaroka and Zuñi. The youngest is the Tejas sequence. These names help us track deep time. They show how our continent has changed.

A cratonic sequence is a massive pattern found in the Earth's rock record. Geologists also call these patterns megasequences, Sloss sequences, or supersequences. They represent a complete cycle of sea level changes on a craton. A craton is a large block of continental crust. These sequences show how oceans once moved across the land. They leave behind vast layers of sedimentary rock. These rocks serve as evidence of ancient environments.

The process begins with a marine transgression. This is when the sea level rises and moves onto the land. As the water moves inland, it carries sediment like sand and mud. These materials settle on the craton and form layers. Later, a marine regression occurs. This is when the sea level falls and moves back toward the ocean. This cycle of rising and falling water creates the distinct layers we see today. The Grand Canyon provides a famous visual example of these changing layers.

Each cratonic sequence is bounded by specific markers. The top and bottom edges are called craton-wide unconformities. An unconformity is a gap in the geological time record. These gaps happen when the seas recede from the land. Instead of depositing new sediment, the receding water causes erosion. This erosion washes away older material. Consequently, the rock record shows a break where time is missing. These boundaries help scientists define where one sequence ends and another begins.

Laurence L. Sloss first proposed this concept in 1963. His work helped geologists understand how to organize large-scale rock layers. Before this, the relationship between sea level and continental layers was less clear. Sloss identified that these sequences represent entire cycles of change. His ideas changed how we view the history of the Earth's crust. Today, his name is attached to these patterns through the term "Sloss sequence."

Scientists study several causes for these massive sea level shifts. One major cause involves changes in mid-ocean ridge volume. This is linked to the rates of seafloor spreading. When ridges spread rapidly, they become longer than usual. The high heat from this process elevates the lithosphere, which is the Earth's outer layer. This elevated floor displaces seawater onto the continents. Conversely, when spreading rates decline, the ridges subside or sink. This causes the seas to drain away from the cratons.

Other complex mechanisms may also influence these cycles. Dynamic topography can play a role through mantle mass anomalies. This involves changes in the Earth's mantle that affect the crust. Additionally, intraplate stress can cause significant tectonic movement. This includes episodes of contractional and extensional tectonics. These forces can cause the craton to undergo uplift or subsidence. Such movements change how much land is covered by the ocean.

Since the beginning of the Cambrian Period, North America has experienced six distinct cratonic sequences. The oldest sequence is known as the Sauk. It is followed by the Tippecanoe and the Kaskaskia sequences. The next two are the Absaroka and the Zuñi sequences. The youngest is the Tejas sequence. The Zuñi sequence is specifically linked to the Western Interior Seaway. This seaway existed about 95 million years ago during the mid-Cretaceous period.

Understanding these sequences helps us connect local rocks to global systems. While some changes are eustatic, meaning they are global sea level changes, they are often studied as relative changes on a continent. Scientists have tried to find matching sequences on other continents. However, they have met with only limited success. This suggests that global sea level change is not the only reason these cycles happen. Instead, a mix of tectonic and oceanic factors shapes the history of our continents.