Earth Geology Geologic Time Scale Cambrian Marine Deposition

Cambrian Period

Marine Deposition

544 to 505 million years ago

The Cambrian Period is the first time period of the Paleozoic Era. During the first part of the Cambrian, the Ozarks area remained above sea level and subject to continued erosion. Toward the end of the Cambrian, the sea level rose or the land sank and most of the Ozarks, including the Bryant area, became a shallow seabed. The peaks of the St Francis Mountains of southeast Missouri remained exposed as islands in this late Cambrian sea. The first sediment layer in this Cambrian sea was a sandstone bed, which we now know as the LaMotte Sandstone. This sandstone was deposited directly on the eroded Precambrian granites and volcanic rocks. Continued subsidence or sinking of the land produced a deeper sea further from a source of sediments. The environment was right for the formation of carbonate sediments, and a thick sequence of limestone (later altered to dolomite) was deposited throughout the region. This limestone or dolomite sequence in the Bryant area is about 1,500 feet thick and all in the subsurface. This Cambrian sea was regional in extent, stretching from what is now Mexico into the New York area.

Carbonate sedimentation occurs mainly in tropical waters and paleogeographers have determined that the Ozarks area was very near the equator during Cambrian time. A modern day equivalent would be the area around the Bahama Islands or the Persian Gulf.

The rocks of the Cambrian worldwide contain an abundance of fossils. Fossils were so much more common compared to the Precambrian that it is often referred to as the Cambrian explosion. Life forms multiplied dramatically in quantity and variety in the Cambrian seas. There are more fossils in the Cambrian rocks for two reasons. First, there were no doubt more marine animals living in the Cambrian seas; and second, the animals of the Cambrian had hard shells or skeletons made of calcium carbonate and these hard shells were far more likely to be preserved as fossils after the animal died.

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