Lecture 12: Late Paleozoic Geology

The Paleozoic Era

The Carboniferous

• Carboniferous used throughout the world

  • Coal beds in north-Central England

  • Subdivided by English/European geologists: lower and upper

• U.S. found a comparable system

  • Mississippian and Pennsylvanian

  • Distinguish coal-bearing layers of the Pennsylvanian from the mostly limestone Mississippian.

  • The Carboniferous Period was dominated by coal.

Late Paleozoic Geologic Events

• Similar to early Paleozoic, sedimentary strata were formed in “packages” due to rising and falling seas.

  • With falling seas comes unconformities

  • Kaskaskia and Absaroka sequences

• For mountain building, a clash of continents was occurring on the east coast, with some smaller events happening elsewhere

  • Pangea forming

  • Acadian (east), Alleghenian (east), Antler (west), and Hercynian (Europe).

The Devonian World

• Fourth Period of the Paleozoic

Sequences and Europe Orogenies

West and East Coast Orogenies

Kaskaskia Sequence

The End of the Silurian

• Marks end of Tippecanoe sequence and beginning of the Kaskaskia sequence.

• A major continent wide unconformity is found at the end of Early Devonian.

Late Silurian

• Regression of the Tippecanoe sequence (sea level fall)

Kaskaskia

• Throughout the Devonian and into the Mississippian, sea level rises.

• Sandstone at the bottom, then shale, then limestone.

Oriskany Sandstone

• Oriskany Sandstone: Initial deposit of the Kaskaskia sequence as sea level begins to rise.

• Consists of very mature and clean quartz sands that eroded from Taconic mountains in the east.

Marcellus/Chattanooga Shale

• During Kaskaskia sequence, mountains uplift in east during Acadian Orogeny.

• Eroded material carried west by streams to form a massive sedimentary deposit called Catskill clastic wedge.

• Inside wedge, we find a widespread deposit of black muds that eventually formed the Chattanooga Shale.

  • Widespread, easy to recognize → important marker for correlation.

• Darkness of deposit suggest water was anoxic (lacked free oxygen).

  • Possible source for natural gas (hydrofracking).

Carbonates

• Sea level continues to rise.

• Mountains that provide mud to the Chattanooga Shale have reduced (less clast sediments); eroded material away.

• Oceans became more oxygenated and carbonates became the dominant deposit (with lots of fossils).

End of the Kaskaskia Sequence

• Marine regression at the end of Mississippian drained continent.

• Results in major continental unconformity between the Late Mississippian and Early Pennsylvanian.

  • Similar to Sauk-Tippecanoe and Tippecanoe-Kaskaskia.

• New sequence begins → Absaroka

What About Western N. America?

• Western part of craton was too far from eastern mountains to receive many clastic sediments.

• As a result, Middle and Upper (Late) Devonian strata are largely limestone (extensive reefs).

• We can find these Devonian and Mississippian Limestone in the Frenchman Mountains, Red Rock, and Mount Charleston.

Possible Alamo Impact

• Late Devonian, possible bolide (meteorite) impacted southwest NV.

• Due to mountain building processes through time, we can’t see impact crater.

• A thick bed of carbonate breccia found throughout state.

Absaroka Sequence

Craton-Wide Unconformity

• End of Mississippian, exposed terrain eroded away to form widespread regional unconformities in the world.

• The resulting unconformity separates strata into Mississippian and Pennsylvanian.

• The Absaroka cratonic sequence includes strata ranging from Pennsylvanian through Triassic/Early Jurassic.

• Pennsylvanian strata near eastern highland are thicker consisting of sandstones, shales, and coal beds.

• Moving west, this thins out and is dominated by half marine and half non-marine rocks.

Cyclicity

• Cyclicity: a pattern where events or conditions repeat in a regular; a cycle.

• During the Pennsylvanian we are generally seeing an overall rise in sea level (Absaroka)

  • Steady sequence

• However, strata show a marked cyclicity (or pulses) of changing sea level also occurring.

  • Smaller and shorter time period.

• Repetitive pattern seen on other continents.

Cyclothems

• Cyclothems: A cyclic repetition of sequences of marine and non-marine strata.

  • Sea level rise produces marine strata.

  • Sea level fall produces non-marine strata (clasts).

• Common in Pennsylvanian, not Mississippian.

• Valuable because they contain coal beds and can be correlated over great distances.

• Results from repetitive step-wise advances and retreats of epicontinental seas.

Late Paleozoic Repetition

• About over 100 cyclothems in N. America

• Occur on a much finer scale than the major sea level sequences.

• Potential causes:

  • Worldwide rise (melt) and fall (cooling) of sea levels due to glaciers.

  • Spasmodic tectonic up and down motions of the continent.

  • Fluctuations of cyclic climatic change influencing sediment supply to deltas.

Coal Swamps

• Sandstones of the cyclothems represent river/delta deposits.

• Coal bearing shales represent vast coal swamps (jungle-like vegetation).

• Immense areas must have been densely vegetated.

• Coal forms by the accumulation and compaction of vegetation into peat.

• Peat: lowest grade of coal we can access.

• As vegetative matter accumulated it became peat.

• Accumulating economically viable coal requires an immense volume of original vegetation.

• Further compaction, increases in economic grade.

Coal Formation

• Increasing time, pressure, and heat results in high grade

• Peat is low grade

• Brown coal is intermediate grade

• Black coal is high grade

• Bituminous coal burns hotter and cleaner than lower-grade coals.

• Coal is a dying source.

Permian Sediments

• Permian conditions east of Antler uplift were quieter than those to the west.

• Region occupied by a shallow sea.

• Formed Kaibab Limestone (Grand Canyon, Frenchman Mt., Blue Diamond Hill).

• While Kaibab Limestone deposited, a relatively deep marine basin developed in what is now WY, MT, and ID

  • Chert, sandstone, mudstone, and limestone, in addition to dark layers of phosphorous rich rocks.

  • Phosphoria Formation

  • Important resource of phosphorus today.

• Towards late Permian, sea level is a bit lower

• Western orogeny’s shaping Cordillera.

• Eastern orogeny’s shaping Appalachians

• Most Permian rocks are deposited in the Great Permian Basin.

  • Restricted basin that alternates between organic rich mudstone deposits and halite deposits that aid in trapping oil.

  • This is a major oil producing region

Acadian Orogeny

Acadian Orogenic Event

• Paleozoic, continents coming together to form supercontinent Pangea (last supercontinent).

• Mid- to Late Devonian, a microcontinent named Avalon (Avalonia) terrane collided with the northern margin of the N. American craton.

Middle-Late Devonian Geology

• The Acadian mountain belt extended from Newfoundland to West Virginia

• Thick, folded sequences of basin sediments (that were metamorphosed).

• Granitic intrusions (dimension stone)

Dimension Stone

• Dimension stone: Granitic intrusions; a resource for decorative rock.

Sediments Galore

• Catskill Wedge consisting of sandstones to shales over short distances.

• Deeply buried shale, known as Marcellus Shale (Chattanooga Shale is an extension of the Marcellus), is one of the largest known reservoirs of natural gas.

Chattanooga/Marcellus Shale

• Most Natural Gas Reservoirs on the Eastern US are from the Marcellus / Chattanooga Shale.

Alleghenian Orogeny

Alleghenian Orogeny

• Also known as the Appalachian Orogeny

• Collision of Gondwana w/ N. America beginning in the Mississippian and into the Permian.

• Connecting North America and northwestern Africa.

• Long orogenic belt extending laterally 1600 km from southern New York to central Alabama producing extensive thrust faulting, uplift, and metamorphism.

Formation of Supercontinent Pangea

• Baltica (Europe) joined together with North America during Caledonian orogeny (late Ord-Sil-Mid Dev).

• Formation of Northern Appalachian Mountains; Acadian Orogeny (middle Dev)

• Formation of Southern Appalachian Mountains: Alleghenian Orogeny (Permian) - Connecting North America and northwestern Africa.

Europe

Caledonian Orogeny

• Recognized primarily in western Europe.

• Slightly predates the Acadian Orogeny in North America.

• Continental warpings, sediment distribution and organisms are similar across North America and Europe during this time.

• Catskill Orogenic wedge is similar in age to the European Old Red Sandstone wedge.

• The Caledonian and Acadian Orogenies were likely opposite sides of the same coin.

Hercynian Orogeny

• Central Europe was the site of great mountain building.

• Caused by collision of Northern Africa with Europe.

• Silurian-Devonian-Carboniferous

• Interconnected supercontinent → Pangea

Cordillera Continued

Out West

• The Cordillera refers to the western system of mountain ranges in North America.

• From Rocky Mountains to the Pacific Coast and from Mexico to Alaska.

  • Rockies, Sierra Nevada’s, Coast Ranges

• During the most Early Paleozoic, this region was nearly flat, laying near/below sea level.

Early Split in the Neoproterozoic

• Eastern Antarctica or Australia rifted off Laurentia at Late Neoproterozoic.

• Passive Margin developed in Western US/Canada (Laurentia) until Devonian.

Orogeny Begins

• With subduction

• Passive margin until Ordovician/Silurian

• A Panthalassic Plate (Proto-Pacific) is moving against Laurentia

• Subduction zone with volcanic chain

Antler Orogeny

• Late Devonian through Mississippian-Pennsylvanian

• First orogeny in the west coast.

• Island arc accretion western margin of North America

• The convergence was accompanied by massive thrust-faulting.

  • Roberts Mountains in central NV

Foreland Basin

• Foreland basins form when lithosphere flexes downward in front of mountain belt in response to added load of thickened crust that results from collision of two plates.

  • Basin → like a bowl

• Sediments eroded from mountain belt accumulate in foreland basin, causing it to further subside and make room for additional sediments.

  • Add weight means more depression

• The Antler Mountains produced large source of coarse clastics that was over 5000 meters deep.

• Such a thickness indicates basins were subsiding as they were being filled.

Sonoma Orogeny

• Another orogeny occurring on west coast.

• Occurred at the end of the Permian into the Triassic.

• Another collision of an eastward-moving island arc.

Review Questions

• What/when were the major sequences? What is the evidence?

  • Kaskaskia Sequence (385 to 320 million years ago): Characterized by widespread marine transgressions and regressions, evidenced by carbonate deposits and marine fossils like brachiopods and corals.

  • Absaroka Sequence (320 to 260 million years ago): Marked by frequent sea-level fluctuations with alternating marine and non-marine sediments, evidenced by coal beds, sandstones, shales, and fossils of both marine and terrestrial organisms.

• What/when were the major orogenies? What is the evidence?

  • Caledonian Orogeny (around 490 to 400 million years ago): Formed the Caledonian Mountains in Scotland and Scandinavia due to the collision of Laurentia and Avalonia, evidenced by folded and metamorphosed rocks.

  • Acadian Orogeny (375 to 325 million years ago): Created the northern Appalachian Mountains from the collision of Avalonia with North America, evidenced by folded rock layers and extensive Devonian sediments.

  • Hercynian (Variscan) Orogeny (350 to 290 million years ago): Resulted from the collision between Laurussia and Gondwana, forming the Appalachian and Variscan Mountains, evidenced by folded and metamorphosed rock layers.

  • Alleghanian Orogeny (325 to 250 million years ago): Produced the Appalachian Mountains and other ranges through the collision of North America and Africa during Pangaea's formation, evidenced by extensive folding and faulting.

  • Antler Orogeny (around 380 to 250 million years ago): Affected the western margin of North America, creating the Antler Mountains and evidenced by sedimentary rock layers with associated folds and faults.

• What sedimentary rocks formed during this time?

  • Sandstone, shale, limestone, coal, and conglomerates

• How is coal formed? What are cyclothems? Why did they occur?

  • How is coal formed?: Coal forms from the accumulation and compression of plant material in swampy, oxygen-poor environments over millions of years.

  • What are cyclothems?: Cyclothems are repeating sequences of sedimentary rock layers, typically including coal, sandstone, shale, and limestone, that reflect cyclic changes in sea level and sedimentation.

  • Why did they occur?: Cyclothems occurred due to fluctuating sea levels and climate conditions that alternated between marine transgressions and regressions, causing periodic changes in sediment deposition.

• What was happening in western Laurentia at this time?

  • Early stages of what would become the Cordilleran Mountain Range began forming due to tectonic forces and volcanic activity.

  • Varied sea levels led to the deposition of different sedimentary rocks, including sandstones, shales, and limestones.

  • Large sedimentary basins, like the Great Basin, developed as a result of tectonic activity and sediment deposition.