A Superchron appears to be a Geomagnetic Reversal that lasts 10 million years or more with one polarity.
Superchrons
A superchron is a polarity interval lasting at least 10 million years. There are two well-established superchrons, the Cretaceous Normal and the Kiaman. A third candidate, the Moyero, is more controversial. The Jurassic Quiet Zone in ocean magnetic anomalies was once thought to represent a superchron, but is now attributed to other causes.The Cretaceous Normal (also called the Cretaceous Superchron or C34) lasted for almost 40 million years, from about 120 to 83 million years ago, including stages of the Cretaceous period from the Aptian through the Santonian. The frequency of magnetic reversals steadily decreased prior to the period, reaching its low point (no reversals) during the period. Between the Cretaceous Normal and the present, the frequency has generally increased slowly.[9]
The Kiaman Reverse Superchron lasted from approximately the late Carboniferous to the late Permian, or for more than 50 million years, from around 312 to 262 million years ago.[9] The magnetic field had reversed polarity. The name "Kiaman" derives from the Australian village of Kiama, where some of the first geological evidence of the superchron was found in 1925.[10]
The Ordovician is suspected to host another superchron, called the Moyero Reverse Superchron, lasting more than 20 million years (485 to 463 million years ago). But until now this possible superchron has only been found in the Moyero river section north of the polar circle in Siberia.[11] Moreover, the best data from elsewhere in the world do not show evidence for this superchron.[12]
Certain regions of ocean floor, older than 160 Ma, have low-amplitude magnetic anomalies that are hard to interpret. They are found off the east coast of North America, the northwest coast of Africa, and the western Pacific. They were once thought to represent a superchron called the Jurassic Quiet Zone, but magnetic anomalies are found on land during this period. The geomagnetic field is known to have low intensity between about 130 Ma and 170 Ma, and these sections of ocean floor are especially deep, so the signal is attenuated between the floor and the surface.[12]
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