LECTURE 19

THE PALEOZOIC

Review of Precambrian

Near the end of the Precambrian, it seems the first supercontinent formed. This is not so well documented as later events but the idea appears substantial. Sometime, probably several tens of millions of years before the end of the Proterozoic (but late in the Proterozoic) this supercontinent broke up into smaller continents which drifted apart by plate tectonics.

Meanwhile, it should be added, in reviewing the Precambrian, how normal and similar to present times it was. We know this mostly for three reasons: 1) normal (albeit early they are immature) marine sediments occur throughout the Archeozoic and Proterozoic, This tells us that water wasn't boiled away or all frozen. 2) Evaporite deposits (sedimentary rocks precipitated because evaporation was more severe than usual and so the concentration of some ions exceeded their solubility) are evenly distributed through the whole Precambrian suggesting an evenness to the climate. Finally -- 3) life persisted throughout the Precambrian and all life is unable to stand extreme cold or boiling water. This, again, suggests an evenness and hospitality to the environment.

Some things did come up. For example, there are two times of glaciation in the Precambrian -- one in the early Proterozoic (2 - 2.5 BYA) that has been found in North America, Australia, and South Africa and one in the late Proterozoic. These glaciations represent several things. One is that there is a change in environment so that glaciations are possible in the Proterozoic as compared with the Archeozoic. This probably is related eventually to the decrease in CO₂ so that the greenhouse effect is not as strong by the first glacial times in the Proterozoic. Another remark one can make is this shows an event like our recent one and therefore, emphasizes how normal Precambrian times were.

The second glacial time was in the late Proterozoic (between 1 and .6 BYA) and was more severe than the first because evidence of it has been found in every continent except Antarctica and it has been found in much lower latitudes (recent article in Nature) than our recent Pleistocene glaciation.. In a way, this still emphasizes the loss of CO₂ and the normalness of events. Other debates may arise over the years to come.

Life was mostly Prokaryotic (bacteria), Eucaryotes (organisms with organized nuclei) appeared in the Proterozoic, about 1.5 BYA.. Ediacaran times(about 680 MYA -- million years ago) include the first metazoans (multi celled animals with the cells doing specialized things). Acritarchs appear about a billion years ago. These are odd shape small things that may represent planktonic (floating on or near the ocean's surface organisms).

The end of the Precambrian is marked by a profound unconformity at most localities. Through much of North America, for example, one finds a late Cambrian unit lying unconformably on the Precambrian. The late Cambrian has a basal conglomerate followed by coarse and then fairly fine clastics and eventually carbonate sediments on top. This emphasizes the profoundness of the unconformity.

THE PALEOZOIC

We can look at the entire Paleozoic sedimentary history of the Paleozoic (see diagram in illustrations - history showing grain size through time). In that history, time is listed as C (with a horizontal line) for Cambrian, O for Ordovician, S for Silurian, D for Devonian, M for Mississippian, P (with a double vertical line) for Pennsylvanian, and P for Permian. Note that the Mississippian and Pennsylvanian are, in European etc. usage and sometimes in American habits called the Carboniferous. Note also, that at Split Rock you saw the unconformity between the Mississippian and Pennsylvanian that led American geologists to name the periods. Where you saw the unconformity, the Pennsylvanian lied upon the Ordovician. This does not represent the actual break. Before the Pennsylvanian was deposited, the Mississippian, Devonian, Silurian, and part of the Ordovician were eroded away.

Anyway, The history shows a simple pattern. Its coarse and getting finer to the end of the Cambrian. The Ordovician has the reverse pattern; that is it is fine grained (mostly limestones) at the beginning of the Ordovician and gets coarse grained at the end. The Silurian, in a way, is just like the Cambrian -- coarse grained at the beginning and fine grained at the end. The Devonian reminds one of the Ordovician, fine grained at the beginning and coarse grained at the end. The Mississippian starts coarse and gets finer (limestones as always). Late in the Mississippian the strata start to vary frequently between clastics and limestone. The Pennsylvanian and Permian continue the variation. In these times, it has been shown that the variation is around the deposition of coals. Here the idea is that transgressing seas advanced through coal (at about sea level when coal deposited) into marine shales and then limestones. Then the seas regressed through clastics into terrestrial sediments and then transgression through coal etc. continued (see sedimentary rocks for transgression--regression description and see illustrations). These cycles of deposition (starting before the Pennsylvanian coals and lasting into the Permian) are called cyclothems..

The Paleozoic sedimentary history has explanations. The continents were separate at the end of the Precambrian and the Cambrian facies map (see illustrations) shows coarsening toward the center of the continent. This means the sediments were derived from the main body of the continent (not unlike today's situation) and their were some hills there supplying the sediment. Cambrian sediments are quite mature which suggests they were well transported and unlikely came from active mountain making. Cambrian sediments became finer (especially limestones) with time which suggests (along with the dating) that the seas were transgressing through Cambrian times.

Before going on one should note that we are describing transgression and regression over epeiric seas. These seas cover continents and therefore are generally shallow. Objectively, for the past, we can recognize several depths of water. These are (1) the shoreline or the intertidal zone. These contain, characteristically, beach sands, evidence of tidal comings and goings such as skolithos escape tubes (seen by you at split rock and Buffalo Rock State Park). Then there is (2) wave base (1/2 wave length) which might be 5 to 20 feet down. Below wave base the fossils will tend to be better preserved than above. Then there is storm wave base that is 20 to 40 or more feet down. This is a sporadically disturbed layer with undisturbed beds above and below. Finally there is the base of the photic zone which would include animals (such as most corals) that depend on light and plants. This zone is 600 feet down in truly clear water but 2 or 300 feet is more often the base of the zone. Most epeiric sea deposits fit into one of these categories. Some small percentage is deeper. Just for emphasis, recall that every Great Lake except Erie gets below sea level and consequently, (according to which lake) is 500 -- 700 feet above sea level at its surface.

Ordovician, just after the Cambrian, is just like the late Ordovician to start. That is it is mostly limestones at the start and getting coarse grained at the end. Oddly, about a third of the way through the Paleozoic there is a sheet of extremely mature sandstone called the St. Peter Sandstone. This has continental sources (sea transgressing during its deposition) similar to the Cambrian clastics except more mature. This continental transgression is pretty much nation wide. For example, the Harding sandstone of Colorado and vicinity represents the same transgression.

 The clastics at the end of the Ordovician are an entirely new phenomenon. They coarsen and become less sorted toward the continental margin. Actually, lots of these clastics there are formed into terrestrial deposits, the Queenston delta. the event causing these deposits is the intensification of the subduction along the East coast, called the Taconic orogeny and is represented by deformation and intrusion, in addition to the clastics, in those areas.

The Silurian, continuing from the Ordovician, is coarse grained at first and gets fine grained showing the diminishing Taconic orogeny (subduction generated). The Devonian (really a semi-repeat of the Ordovician) gets coarser and coarser until the end. Late in the Devonian we have the deposition of the Catskill delta. This is a partly red bed huge deltaic deposit. No vertebrates have, so far been found, cut it does contain one unit with plants called the Gilboa forest. This has few modern plants (some mosses) but it is a multistoried forest.

The orogeny causing the Catskill deposits is the Acadian Orogeny and represents the collision of western Europe with North America. This means that the Iapetus ocean (or the Proto Atlantic) finally disappeared at the end of the Devonian. This, unlike that during the Ordovician, is a collision caused orogeny.

The Mississippian starts coarse grained (continuity with the Devonian) and the sediments get fine grained later. Toward the end of the Mississippian. Then cyclothems start (see above). These cyclical pulses have either a climatic or an orogenic (mountain making) explanation and one has never become clear. Anyway, the clastic content gets larger and larger through the Pennsylvanian into the lower Permian when it all ends because Africa collides with the U.S.A. from Pennsylvania to Georgia and causes the Appalachian Mountain deformation (mostly folds to the North and thrust faults and folds to the south) This gets rid of the southern parts of the Proto Atlantic ocean. At about this time Pangea is forming and so this is also a Hercynian event

Meanwhile in the southern U.S., one notes that the sediments (southern Oklahoma and adjoining parts of Arkansas) thicken and coarsen toward the south. This reflects developing subduction there. At the end of the Pennsylvanian, South America collided with north America and crushed the rocks. This orogeny is called the Ouachita orogeny and is also a Pangea forming Hercynian event (albeit a little early).

The west coast of the U.S. has a continuing subduction history that starts either in the Cambrian or Ordovician. Whenever subduction gets serious or suspect terrain of sufficient size collide, then an orogeny occurs. The Antler orogeny of late Mississippian times is one of those orogenies. Whenever an orogeny occurs, the deposition of clastic sediments increases. In the Jurassic, massive amounts of granite were intruded or partly metamorphosed into existence in the west coast. Presumably, this is all subduction induced.

The cordilleran orogeny, which pretty much ended the compressional history of the west coast occurred at the end of the Mesozoic and deformed, especially, the Rocky Mountains. At about this time the westward movement of North America over rode a Pacific spreading ridge and caused (transform faults) some strike slip faulting, especially in California. The most famous of these now land based transform faults is the San Andreas fault.

To the north of North America is the Franklin orogenic belt. Rocks in this belt thicken and coarsen to the north, suggesting subduction there. These rocks were finally deformed in the Mesozoic. It is not a well understood orogenic belt, but much oil comes from those rocks.