PALEOZOIC

Overviews

• Plate Tectonic Overview

  • Supercontinent of Pannotia began breaking up around 550 Ma

  • North America (part of Laurentia, which included Greenland and parts of NW Europe) probably split off from what was to become Gondwana (a supercontinent composed of Africa, South America, Australia, India, and Antarctica).

  • Middle Ordovician (ca 450 Ma): Gondwana had rotated counterclockwise, and it and Laurentia converged, creating the Taconic orogenic belt.

  • Devonian (ca 370 Ma): Collision between Laurentia and Baltica possibly associated with an oblique collision of Laurentia with Andean margin of South America. These collisions resulted in the Acadian/Caledonian orogenic belt.

  • End of Devonian (360 Ma): Laurentia and Baltica were sutured together to form Laurasia.

  • Mississippian (ca 320 Ma): Collision between plates containing Laurasia and Gondwana created a subduction zone and the Ouachita orogenic belt.

  • Pennsylvanian (ca 300 Ma): Collision between Gondwana and Laurasia produced the Alleghenian orogenic belt and sutured these lands together as part of the assembling of the supercontinent of Pangea.

  • Permian (ca 260 Ma): The assembly of Pangea is complete.

• Geology Overview

  • Epeiric seas: Shallow seas that periodically flooded much of the North American continent.

  • Arches, domes, and basins: These are flexures of the craton, probably in response to tectonic events on the edge of the continent, that produced topographic highs and lows.

  • Arches and domes: Topographic highs covered with sedimentary rocks in an anticline-like pattern. However, this pattern is not due to folding but to the topographic high. Sedimentary beds are thinner toward the center (unlike anticlines) and thinner than they are in basins, and there are many unconformities.

  • Basins: Topographic lows filled with sedimentary rocks in a syncline-like pattern. Like arches, this pattern is due to the topographic low, not folding. Sedimentary beds are thicker toward the center (unlike synclines) and thicker than they are on arches, and there are fewer unconformities.

  • Mobile belts: These are linear regions around the edge of continents where mountain building (orogenies), due to subduction and collision events, have occured. They are populated by highly folded, faulted, and often metamorphosed rocks.

  • Cratonic sequence: This is a large stratigraphic rock unit (that is, a "stack" of rocks consisting of many sedimentary beds) that marks a major transgression-regression cycle. It is bounded above and below by major, craton-wide unconformities. These unconformities are often worldwide in extent with variations from continent to continent. The rocks in a cratonic sequence typically consist of quartz sandstones that were originally deposited near shore, shales consisting of fine sediment that was deposited in quiet water, and limestones that accumulated far from land.

  • Clastic wedge: A body of sediments, wedge-shaped in cross-section, that forms on the flanks of a mountain chain as the mountains are uplifted and eroded. The sediments are typically coarser and thicker near the source of the sediment and become thinner and finer away from it.

Cambrian Period (542 - 505 Ma)

• Overview

  • The Cambrian has recently been defined to begin at the appearance of a worldwide decrease in the carbon-13 isotope in sediments. This coincides with a disappearance of distinctive Precambrian fossils. An ash layer in Oman that occurs at the same time as the isotope change has been dated to 542 Ma.

  • The continents were dispersed in tropical areas. There was no land mass in polar areas to support glaciation.

  • At the beginning of the Cambrian, there were passive continental margins all around Laurentia with the sea confined to the continental edges.

  • Laurentia was on the equator and no land plants yet existed, resulting in rapid erosion of exposed rock. Also strong tides occurred due to the closeness of the moon. The continent began to be worn down to wave level.

• Sauk Cratonic Sequence (Cambrian through early Ordovician)

  • The transgression of the Sauk Sea occurred in a series of more minor transgressions and regressions.

  • It is represented stratigraphically by sandstones overlain and interbedded with shales, which in turn were overlain and interbedded with limestones. A classic example is the (bottom to top) Tapeats Sandstone, Bright Angel Shale, and Muav Limestone at the bottom of the Grand Canyon.

  • By the end of the Cambrian the sea covered all of North America except parts of the Transcontinental Arch (a linear topographical high stretching from southwest to northeast across central North America) and the Canadian Shield. Therefore, no Cambrian rocks are found where this arch existed, from New Mexico to the Northern Plains.

  • The sandstones deposited were of mature quartz grains (up to 99% quartz and very well-rounded). This was due to lack of vegetation, wind, and strong tidal action.

  • There was little land left by the end of the Cambrian and deposition of carbonate rocks was widespread.

  • Clastic limestones (limestones formed of carbonate fragments) and oolites (small, spherical grains of calcite) indicate the sea was very agitated.

  • The Sauk was a shallow sea as indicated by oolites, stromatolites, and mudcracks.

Ordovician (505 - 438 Ma)

• Overview

  • Gondwana drifted such that North Africa was at the south pole, resulting in glaciation of that area.

  • The Sauk Sea regressed from North America during the early Ordovician, creating a continent-wide unconformity.

  • By the Middle Ordovician a new epeiric sea, the Tippecanoe Sea transgressed onto the continent, beginning the Tippecanoe sequence.

  • Also beginning in the Middle Ordovician was the Taconic orogeny, which affected the eastern margin of Laurentia.

  • Some minor subduction occurred along the western margin of the continent, producing some volcanic action.

• Tippecanoe Sequence

  • The advancing Tippecanoe Sea reworked already mature Cambrian quartz sandstones into "supermature", nearly pure quartz, sandstones (such as the St. Peter Sandstone used to make glass). (The continents were still barren of vegetation and the tides were nearly as strong as those of the Cambrian.)

  • A new feature of this sea were reefs made by corals and stromatolites. A burst of reef building occurred in the late Ordovician, which continued into the Devonian.

• Taconic Orogeny

  • Convergent plate motion (involving Laurentia, Gondwana, and parts of Europe) led to the development of a subduction zone along the eastern margin of North America.

  • The orogeny began as an island arc, then evolved into a range of mountains that existed where the continental shelf is now.

  • Uplift and erosion of the Taconic highlands led to deposition of sediment to the west in the Tippecanoe Sea, creating the Queenston clastic wedge.

  • This clastic wedge was characterized by marine sandstones, black shales (often with abundant graptolite fossils), alluvial sandstones and shales, and volcanics.

Silurian (438 - 408 Ma)

• Overview

  • The Tippecanoe Sea retreated in the Late Silurian and was confined to the continental margins by the Early Devonian.

  • Subduction and continental collision along the eastern margin, beginning in the Late Silurian, led to the Acadian/Caledonian orogeny and the creation of Laurasia in the Devonian.

  • The Michigan Basin began to deepen as the crust began to sink in that area.

  • The western margin of the continent was quiet with epeiric sea deposits.

• Michigan Basin

  • As the basin deepened the production of carbonates by organisms kept up with the rate of sinking, filling the basin with thick limestones.

  • Massive carbonate reefs surrounded the basin and cut off the basin from the sea from time to time, creating extensive evaporites, such that as you drill into the basin rocks you see alternating carbonates and evaporites.

  • The Michigan Basin is an important oil and salt producing area.

Devonian (408 - 360 Ma)

• Overview

  • The Kaskaskia Sea transgressed over the continent by the Middle Devonian, beginning the Kaskaskia sequence.

  • The Acadian/Caledonian orogeny was active as subduction and eventual collision with Baltica occurred.

  • The Williston and Michigan Basins were major sites of sediment accumulation.

  • On the western margin the Antler orogeny began.

• Kaskaskia Sequence

  • A continent-wide unconformity in the Early Devonian marks the gap between the Tippecanoe and Kaskaskia sequences.

  • The first sediments deposited by the transgressing Kaskaskia sea were clean, well-sorted sandstones.

  • Later widespread carbonates were deposited.

  • As sediments from the Acadian orogeny spread westward, carbonate deposition was replaced by these sediments.

  • Toward the end of the Devonian black shales, such as the Chattanooga Shale became widespread over the interior of the continent, as far west as Montana and southern New Mexico. The dark color is from carbon compounds, indicating a lack of oxygen on the sea floor. It could be that decaying organic matter depleted the sea's oxygen.

• Williston Basin

  • Was associated with a large barrier reef system extending into Canada which acted much like the Michigan basin in producing limestones and evaporites. Potash, oil, and gas are important products of this area.

• Acadian Orogeny

  • Began as a continental crust-oceanic crust collision and progressed to a continental collision between Laurentia and Baltica forming Laurasia.

  • Was at its peak from the Late Devonian through Early Mississippian.

  • It was much more intense than the Taconic orogeny, involving volcanism, folding, thrust faulting, metamorphism, and the raising of the Acadian highlands.

  • The Catskill clastic wedge produced by the orogeny was three times the size of the Queenston clastic wedge of the Taconian orogeny.

  • A famous rock unit found both in North America and Europe that dates to this orogeny is continental red sandstone (Catskill redbeds in North America and the Old Red Sandstone in Europe).

• Antler Orogeny

  • Located in the Nevada region.

  • Evidence is that this orogeny was caused by subduction and the collision of the western margin of the continent with an island arc.

  • It was marked by thrust faulting and some metamorphism on its eastern side and volcanism on its western side.

  • It produced large amounts of sediment that was deposited on the craton to the east.

Mississippian (360 - 325 Ma)

• Overview

  • The Mississippian and Pennsylvanian Periods are collectively known as the Carboniferous.

  • A large unconformity separates the Mississippian from the Pennsylvanian in North America, hence the division into two periods.

  • The Kaskaskia Sea retreated and was replaced in the Late Mississippian by the Absaroka Sea, which initiated the last cratonic sequence of the Paleozoic, the Absaroka sequence.

  • Erosion of the Antler and Acadian highlands occurred.

  • The southern margin of North America became an active continental margin as Gondwana approached.

• End of Kaskaskia Sequence

  • After the black shales ceased to be deposited (as the Acadian orogeny wound down), carbonate deposition became dominant again. Especially noteworthy are the crinoid limestones (also called "clastic limestones") made of crinoid pieces. (Crinoids looked something like sea lilies.) These limestones have sedimentary structures like sandstones, including ripple marks and cross-bedding.

• Absaroka Sequence

  • The Absaroka epeiric sea invaded North America from the late Mississippian through the early Jurassic (in the Mesozoic).

• Ouachita Mobile Belt

  • As Gondwana approached, land began to rise along the southern margin of North America, greatly increasing the rate of sedimentation on the continental shelf there.

Pennsylvanian (325 - 286 Ma)

• Overview

  • Parts of Gondwana were at the south pole, resulting in extensive glaciation.

  • The final assembly of the supercontinent Pangea commenced, resulting in the Ouachita and Allegheny orogenies.

  • Sea level fluctuated due to glacial-interglacial cycles, resulting in sedimentary sequences called cylothems, in which much coal was formed.

  • A gradual change from marine deposition to continental deposition occurred.

• Cyclothems

  • Cyclothems are sedimentary sequences bounded above and below by unconformities and containing both marine and non-marine sediments.

  • They record transgressions and regressions of the Absaroka Sea.

  • As you go from east to west (toward the deeper areas of the Absaroka Sea) the marine sediments in the cyclothems get thicker and the non-marine thinner.

  • Vast swampy forests existed on the plains to the west of the Acadian highlands in which dead vegetation accumulated. Burial by sediments of the transgressing sea preserved them and subsequent pressure due to overlying sedimentation compressed them into coal.

  • Cyclothems are thought to reflect the rise and fall of sea level due to glacial-interglacial cycles occurring in Gondwana.

• Allegheny Orogeny

  • Marked by intense folding, thrust faulting, and metamorphism.

  • Caused by a continent-continent collision suturing Laurasia and Gondwana.

• Ouachita Orogeny

  • Caused by a subduction zone where ocean crust associated with the plate that included North America was subducted beneath what is now the western part of South America.

  • A thick (up to 50,000 ft) clastic wedge was formed from erosion of the mountains.

  • The folded and thrust-faulted rocks of this orogeny are exposed in the Ouachita Mountains of Arkansas and Oklahoma and the Marathon Mountains of Texas.

• Ancestral Rockies

  • Formed along nearly vertical faults from Texas through Wyoming.

  • Probably formed in response to stresses in the craton caused by the Ouachita orogeny.

Permian (286 - 248 Ma)

• Overview

  • There was a transition in climate from wet to dry tropical. (North America was on the equator throughout the Paleozoic.)

  • The Absaroka Sea, in a major regression, retreated to West Texas and New Mexico, resulting in continental deposition where marine deposition occurred earlier.

• Continental Deposits

  • Widespread redbeds (sandstones cemented by oxidized iron minerals, mainly hematite), were deposited on the continent.

• Marine Deposits

  • These were largely confined to the southwest and west.

  • Sediment rich in phosphate was deposited in the northwest, probably due to the upwelling of deep water coupled with biological action.

  • A large coral reef complex, the Capitan Reef, grew around the Delaware Basin in southwest Texas.

  • When the sea finally withdrew, the reefs died and the basins were filled with evaporites.

Microplates

• Complications to the Larger Plate Tectonic Picture

  • The existence of numerous small tectonic plates (microplates) complicates the interpretation of the assembly (and subsequent breakup) of Pangea.

  • For example, a microplate of Gondwana became Florida and parts of the southeast U.S. after Gondwana broke away from Laurasia.

  • Numerous other microplates have been identified.