570 million to 345 million years before present: Cambrian to Devonian
    The Paleozoic Era consists of the Cambrian thru the Permian Periods, about 325
million years. Compared to other eras, it was a geologically quiet time, with large
continental areas experiencing relatively mild changes in elevation and sea level,
thus producing widespread layered formations. Throughout the Paleozoic, Laurasia and
Gondwanaland lay nearly side by side, with this area near the Tropic of Capricorn.
    The Cambrian Period began about 570 million years ago. The Archaeocyatha (a sponge-
like organism), bivalves, gastropods, brachiopods, echinoderms, foraminifera, and
ostracods evolved early in the Cambrian. Therefore, the ability to produce, support,
and protect one's self with layers of carbonate then existed. 
    Chordate evolution may have begun as early as 550 million BP, and the trilobite 
explosion was well underway by then. Middle Cambrian evolution also produced echinoids, 
blastoids,and homalozoans. The Chengjiang, China, faunal assemblage lived around 
530 million BP, and the Sirius Passet, Greenland fauna lived around 525 million BP.
    Late Cambrian evolution produced crinoids, nautiloids, and fish (which introduced
the vertebrates). The Burgess Shale faunas, from British Columbia, include five distinct
communities buried around 515 million BP. Other life forms appearing sometime during the 
Cambrian include priapulid worms, graptolites, conodonts, carpoids, edrioasters, sea 
cucumbers, radiolaria, ctenophores, dinoflagellates, red algae, bryozoa, and many 
strange beasts less identifiable. Though light sensitive patches had been around 
since the Precambrian, true eyes developed in the Middle or Late Cambrian. The 
sense of touch expanded greatly during the Cambrian, and the chemical senses of 
taste and smell probably made their beginnings. The taking of prey above the 
microscopic level, introduced early in the Cambrian, induced the production of 
many protective devices such as spines and armor plates. And the increased agility 
inherent to legs and fins also served to separate the quick and the dead. 
    Throughout the early and middle Cambrian, this area experienced an arid climate
above sea level with likely much wind erosion. That produced a deeply cut granitic
terrain showing few water effects. Desert sands may have collected in the hollows
and may be responsible for the fine-grained basal sandstone of the LaMotte formation.

Then around 520 million years ago, oceanic waters rolled in. Called the Lamotte Sea

(Sauk sequence), it laid from 0-600 feet of upper Cambrian deposits dependant largely

on the roughness of the Precambrian base. The rise in sea level is difficult to ascribe

to polar melting since polar Cambrian deposits indicate there were no ice caps

throughout and worldwide temperatures were high since the late Precambrian. So either

a large mass rose somewhere displacing the water, or much of the continent lowered.

    The late Cambrian Ozarkian series of sediments begins with the LaMotte sandstone.

It is the oldest rock of known age in this area as the age of the granite beneath is

more difficult to ascertain. The Lamotte consists of white to pink quartz sand with

a fine-grained zone at the bottom. A small amount of granitic debris is typically

mixed into the base. Other large and small grained zones exist higher up. Though

usually not found at all in this area (it is more entrenched to the north and east),

a few deep wells have shown it to be up to 70 feet thick in spots. No fossils are

known to have come from it.

    Above the LaMotte sandstone lie a gray series of beds variously called the

Elvins Group/Bonneterre-Davis-Derby/Doerun formations. They are silty to sandy, and

apparently also only fill clefts in the granite base in this area. They can reach

a thickness of 380 feet. Pellets of pyrite are often found near the base, and

hexactinellid sponge spines and spicules were reported from an Ottawa Co. well.

    Above the Bonneterre formation, when present, lies the Potosi dolomite formation.

It can also occur, as can later Cambrian and Ordovician formations, directly on the

granite basement, suggesting a leveling of the deeply cut Precambrian terrain in

some localities. Above the Potosi, when present, occurs the Eminence dolomite

formation, which, together with the Potosi has left up to 372 feet of coarsely

crystalline white to light gray dolomite. No fossils are known to have been extracted

from them. Some geologists place the Proctor formation above the Eminence formation

in this area but little has been reported concerning it.

    That concludes the upper Cambrian deposits in this area and an obvious unconformity

exists at the upper Cambrian/lower Ordovician interface, indicating a short erosional

episode. The Cambrian Period closed around 500 million years ago, leaving this area

south of the equator, and experiencing a 22 hour day.

    A note should be made of the Arbuckle Formation, or Arbuckle limestone, because it

is a widely used term in the literature of this area. It refers to all the late

Cambrian and lower Ordovician deposits. In effect, to all the deposits produced by the

LaMotte Sea. Another geological occurrence spread widely over time is the chertification

which begins in the leveled upper Cambrian deposits and continues through the middle

Mississippian deposits.

    When the Lamotte Sea returned to this area, it began depositing the Canadian or

Beekmantownian series of sedimentary formations. These were produced during the lower

Ordovician Period, lie either on the Eminence Formation or Precambrian granite high

spots, and here can reach a thickness of 900 feet. They begin with the Gasconade

Formation, which has been called the Van Buren-Gasconade Formation. And the oldest

or lowest portion of the Gasconade is the Gunter member, unless Precambrian granite

highs prevented sedimentation till later.

    The Gunter member consists of sandy dolomite or sandstone, which is common after

a prolonged erosional episode. Its thickness varies from none to over 50 feet, and

grades into the main body of the Gasconade formation - first as a cherty dolomite,

then on up into a noncherty coarsely crystalline dolomite. The Gasconade can be

over 475 feet thick in this area. Next lies the Roubidoux Formation, a

sandstone-dolomite mix, up to 200 feet thick. It's porosity produces the Roubidoux

aquifer, from which many municipalities obtain their water. Above that is the Jefferson

City Formation, up to 250 feet of cherty, dense dolomite. Occasional silicified

masses of the sponge, Ozarkocoelia, have been reported in it. Also small gastropods,

sponge spicules, and echinoderm fragments were reported from zones near the top

and bottom. It is capped by a cherty breccia zone which separates it from the

Cotter Formation in southwest Missouri.

    In this area, the top of the Canadian Series is at either the Cotter or the

Powell formations, depending on later erosion. These formations together can be

up to 700 feet thick in parts of this area. The Cotter is built of many small layers

representing various environments. It is composed of dolomite, sandstone, shale,

chert, and quartz. It contains the Swan Creek sandstone member almost 100 feet

above the base, and frequent echinoderm columnals have been found above and below

this member. In parts of northwest Arkansas, a drusy quartz (Black Ledge member)

separates the Cotter from the above lying Powell Formation. Cephalopod and gastropod

quartz replacement fossils have been found in this member. The Powell possesses a

thick zone of shale at its base and was probably deposited around 475 million

years ago and represents a late stage of the Lamotte Sea.

    Erosion occurred until another sea rolled in 5-10 million years later (Tippecanoe

transgression), as attested to by the St. Peter Sandstone to the north and east of us,

but erosion after it withdrew near the close of the period removed all or most traces

in this area. Evidence of a massive ice cap over the south polar region may account

for some of the fluctuating sea levels of late Ordovician times. The concurrent drop

in global temperatures to about 12 degrees C., typical of ice ages, probably had little

effect here within the tropics as this area still held an arid and warm climate.

    The Ordovician Period may be characterized as equatorial and mostly underwater, with

shallow, changing depths in a dry, uniform climate. The seas withdrew twice, and a major

uplift occurred near its close. Sea salinity has varied little since its beginning.

Evolution produced starfish, stromatoporoids, cystoids, paracrinoids, ostracoderms,

anthozoans, asterozoans, scaphopods, ophiocystoids, cyclocystoids, cystoids,

edrioblastoids, parablastoids, brittle stars, sea urchins, sea scorpions, jawed fish,

and the rugose and tabulate corals. Chitinozoans have been found in Oklahoma

formations. It closed with a mass extinction, in which an estimated 75% of animal

species, and 25% of existing animal families perished.

    The Silurian Period began around 440 million years ago. At that time this area was

above sea level, in an arid climate near the Tropic of Capricorn, but although the sea rolled

in again and out again later in the period, erosion has wiped out all traces of this period

here. Global temperatures rose again to pre-ice age normals from early Silurian through

middle Devonian times. At the beginning of this period is when it appears that true land

plants arrived. First were the rhyniophytes, followed by bryophytes and lycopods. Plant

vascular tissue was unknown before then as were spores bearing trilete sutures. Vegetation

greatly enhanced the hospitality of the barren earth, and arthropods quickly took advantage.

Their exoskeletons probably aided against the harsher conditions outside the sea. Evolution

produced millipedes, centipedes, spiders, and scorpions above ground, and tintinnids,

coccoliths, tusk shells, and barnacles underwater. The order - Tintinnia, first appeared

during this period. Atmospheric oxygen at the close of this period was similar to today's.

    About 400 million years ago, the Devonian Period began. Early in this period, this

area was probably below the Tropic of Capricorn, in a warm temperate climatic zone, and the

sea rolled in again. Called the Kaskaskia Transgression, it left the 6 ft. thick Fortune

Formation beneath parts of southwest Missouri. It backed out again shortly, but returned

in the late Devonian. It then left the Sylamore Formation, a 4 ft. thick layer of sandstone,

and above that, the Chattanooga Shale. Found only in local pockets and lenses in the Tri-

State District, the Chattanooga is a 25 ft. thick layer of black shale further south.

It is 15-20% organic matter, and has a relatively high uranium content. Fossils are

rare, but occasional Tasmanites species of planktonic green algae cysts have been found

in it. It is the earliest easily available formation in the area, found at the base of road

cuts on Mo. Hwy. 59 and U.S. 71 in McDonald County, Mo., and quarries in that area, and

along Indian Creek in Lanagan. It was formed during the Late Devonian and Early

Mississippian periods.

    Early Devonian evolution had lobe-finned fish walking on land and spiders spinning

silk threads. By 370 million years ago, amphibians existed in the form of tetrapods - 3 ft.

long four legged creatures. They've been found at Pennsylvania, Scotland, Ireland, and

Greenland, which shared landmass at that time. Around then another mass extinction

occurred - 19% of families became extinct and 70% of marine animal species perished.

The northern Appalachian mountains also formed around then, as Pangea began to form.

    During the Devonian, giant predatory armored fish and pre-sharks stalked the seas,

along with early ray-finned fish, hagfish, and ammonoids. Extinction struck the

ophiocystoids, cyclocystoids, and cystoids. Springtails are added to the list of

insects swarming amongst the early plant life. The first megaspores are seen, and early

seed plants arrive in the form of seed-ferns. Archaeopteris, an early forest tree, and

tree-sized club moss dominate the heights. By the end of the Devonian, primal shrubs and

large trees exist, and some insects became airborne.






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