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THE PRECAMBRIAN ERA
(from 4.6 billion to 570 million years ago)
From the formation of its crust over 4.5 billion years ago, the Earth was covered by ocean. Within a billion years, simple one-celled bacterial organisms had appeared. The formation of Gondwanaland during the Late Precambrian (uniting the modern continents of the Southern Hemisphere) provided warm, shallow, off-shore environments where complex living organisms first appeared.
Precambrian Texas
Geologically speaking, Texas began to form about one billion years ago. Large amounts of coarse to fine sediment were deposited in an adjacent sea. An episode of mountain building buried and metamorphosed these sediments into schists and gneisses, and igneous processes produced magmas that cooled to form granites. By the end of the Precambrian, the mountain system produced by this tectonic event had eroded to near sea level.
Precambrian rocks can be seen today in the area commonly referred to as the Llano Uplift and in the Van Horn and El Paso areas of far west Texas. As the rocks are primarily igneous, fossils are rare to non-existent in the Precambrian rocks of Texas.
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| Llano Uplift showing fault-lines |
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| Llano Uplif |
are held up by the more resistant Paleozoic cover sequence. The exposure below overlooks Lake LBJ near Marble Falls, Texas, and is used as a type locality for several members of the Cambrian Riley and Wilberns Formations. In the photograph below the distinctive glauconitic Lion Mt. member of the Upper Cambrian Riley Formation is exposed along the roadside, and is overlain by the Welge Sandstone and basal portion of the Morgan Creek members, both being stratigraphic components of the Wilberns Formation. Mapping of these stratigraphic units is a major component of the projects conducted in Texas.
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| Red Rock Ranch, Van Horn, TX |
Red RockRanch has one of largest natural Precambrian sandstone exposures in North America, according to their website. The Ranch is located outside of Van Horn.
Exposures of Precambrian metamorphic rocks occur in the Van Horn Mountains of west Texas. These exposures are part of the igneous-metamorphic platform on which Paleozoic and younger sedimentary rocks were deposited in the southeastern United States. Knowledge about their history comes basically from these isolated exposures and from samples taken from wells drilled through the sedimentary cover. The youngest of these rocks appear to be about 1 billion years old based on radiometric dates. The oldest Cambrian sedimentary rocks are only 570 million years old. This suggests that a long period of weathering and erosion strongly modified the Precambrian surface after emplacement of the intrusive igneous rocks and eruption of volcanic rocks. A substantial amount of rock and historical record was probably removed before the first sediments were deposited in Cambrian seas.
Precambrian Animal Life
It is not known when life originated, but carbon in 3800 million year old rocks from islands off western Greenland may be of organic origin. Well-preserved bacteria older than 3460 million years have been found in Western Australia. Probable fossils 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder of the Precambrian.
Excepting a few contested reports of much older forms from Texas and India, the first complex multicelled life forms seem to have appeared roughly 600 Ma. A quite diverse collection of soft-bodied forms is known from a variety of locations worldwide between 542 and 600 Ma. These are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that timespan. So, very little evidence of life did exist in present day Texas.
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