Santa Elena Canyon, Big Bend National ParkIn this site the Rio Grande River has carved through the r

Santa Elena Canyon, Big Bend National ParkIn this site the Rio Grande River has carved through the remnants of an ancient ocean. During the Cretaceous Period, sea levels around the world rose, reaching such a level that the interior of North America hosted a vast seaway. The area east of the Rocky Mountains that today is open and filled with grasses and farmland was submerged beneath hundreds of meters of water.The lowermost unit is the Glenrose formation, a mixture of limestones, chalky layers, and siltstones deposited when the water was at its deepest and only fine-grained sediments could be deposited. In some places in Texas this unit can be almost 500 meters thick.Atop this unit is a thin sandstone, only several meters thick, called the Maxon formation. Since sandstone contains more coarse-grained sediment than found deep offshore, we can infer that the sea level decreased and perhaps even a beach migrated through this area as that unit formed.On top of the Maxon formation sits two units showing gradually deeper water, the Telephone Canyon Formation and the Sue Peaks Formation. Those units go back to being finer-grained, deeper water, limestone and siltstone rich rocks, with silts more abundant in the Sue Peaks. The package here therefore reflects a cycle of sea level starting off deep, dropping, and then rising again. The top of the Sue Peaks formation can be recognized as the major break in slope in the Canyon Walls.Finally, the uppermost unit in this canyon is the Santa Elena Limestone. It forms massive cliffs and represents a coral reef that grew in this area. Coral reefs grow in water of moderate depth that is also clear and sediment free. The water depths are difficult to pin down, but they’re probably somewhat shallow and whatever silt supply was reaching this area disappeared, allowing clear water where a thick reef could be deposited. After this thick reef formed, rocks representing swamps and tidal flats appear, suggesting the end of the Cretaceous Seaway.-JBBImage credit: NPShttp://1.usa.gov/1Egaqh0References:http://bit.ly/1IVfE7hhttp://bit.ly/1IyYx7rhttp://maroon.com/bigbend/time/index.htmlhttp://3dparks.wr.usgs.gov/bibe/html/bb353.htmhttp://bit.ly/1TeKRnk -- source link

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