Thursday, April 13, 2006

Cartography: Paleogeogrpahic Maps of North America

Blogged at the CCA weblog on Tuesday is a link to a fantastic project undertaken by Dr. Ron Blakey.The maps created by Dr. Blakey represent time slices of the geologic transformations, via plate tectonics, to the core of ancient North America, "Laurentia". The time slice maps are taken at 5-10 million year intervals over the past 550 million years. (PerryGeo has created a fantastic animated gif of these maps!)

The projects website contains 40 of the most representative time slices. From these maps you can witness how the physiography of Laurentia has changed through geologic history. Mountains rise, rifts open, deserts form, and lakes deepen as the earth's tectonic plates subside and become reborn.

Of particular interest to me is the time slice of Late Triassic (210Ma).

Eastern Pennsylvania sits contently just above the Triassic equator. Africa has recently slammed into the East Coast forming the super continent of Pangaea. This collision, the Appalachian orogeny, is reminiscent of the modern collision of India and Asia. The mountains formed on the East Coast may have been as grand as the Himalayas. But by 250 Ma, Africa and Laurentia have been sent on their way. A rift basin develops as the two continents drift in opposing directions.

Along the Eastern Seaboard of the
US, this Triassic basin (geologic history) widened and deepened as huge blocks of the earth dropped due to tensional stress. Lakes formed within many of these half graben basins. Some of these lakes would rival the Great Lakes and Caspian Sea.

Eastern Pennsylvania, erosion from the massive mountains to the West was transported in to this basin. Large chunks of rock and sandy sediment were moved by water and gravity to form mud chocked braided streams and gravely sand bars on the margin of the newly formed lake. As the basin deepens, the lake goes through cycles of drainage and enclosure. These cycles change the depth and depositional regime of the basin lake. Some times shallow, the lake deposits are sandy and exposed to air and the critters that roam the area. At other times, and in other areas of the lake, it is incredibly deep and deprived of oxygen. Thin chemical laminea are deposited in these areas. As the mountains eroded and the subsiding resigned the basin in-filled and shallowed again. In the hot and dry region above the equator, the deposits on the lake margin formed a playa like environment. The muddy shoreline cracked and split as it was wetted and then baked by the sun.

For the next few million years, the lake shallowed and the mountains eroded. As pressure was release, the Earth thrust magma into near surface chambers and lava outflows. Into the Cretaceous and Tertiary periods of the past 150 million years, the regal mountains which once supplied the Triassic lake with sediment are eroded to reflections of their former selves. The sediments of the
Piedmont and Coastal Plain are now the home of the highest peaks.

Since I am no geologist, understand that this description may be a bit fictitious, but it is the way I like to see it.

Head on over to the CAA blog to read some more, or to read another rendition of the geologic history of Laurentia, based on these maps, check out the BLDGBLOG.

(Pictures are credited to Dr. Ron Blakey. Fantastic work!)

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