Early Paleozoic Geologic Events

An Introduction and Continental Architecture

• Sequences are related to the geologic stratigraphy.

  • Sea transgression and regression

  • Layers of rock get eroded away and erased from the rock record (gaps in time)

• Ron Blakey - a retired geology professor, reconstruction of paleogeographic maps.

The Paleozoic

The Cambrian Explosion

• Precambrian → Simple Archean fossils, Stromatolites, Prokaryotes, Eukaryotes, and Metazoans.

• Base of Cambrian Period is defined by the trace fossil Treptichnus pedum (Trichophycus) which is a preserved worm burrow

• The Cambrian Explosion refers to seemingly instantaneous appearance of many varied animals in early-middle Cambrian

• New phyla appeared

Enter the Phanerozoic

• The Great Unconformity is a significant geological gap in the rock record where older rocks, dating back over a billion years, are directly overlain by much younger sedimentary layers, indicating a period of erosion or non-deposition.

  • Frenchman Mountain

  • Unconformity - missing geology

• 2 layers

• Tapeats sandstone, Vishnu Schist, and Granite

William Smith

• English engineer, made first geologic map.

• Deciphered England geology using relative age dating

• 5 of 6 Paleozoic Periods were described and named for rocks exposed in England.

  • Cambrian, Ordovician, Silurian, Devonian, and Carboniferous

  • The Permian comes from the Russian region of Perm in and around the Ural Mountains

The Cambrian Continents

• As Pannotia broke up, 5 major continents (and many small) formed.

  • Laurentia

  • Baltica

  • Siberia

  • China

  • Gondwana

Iapetus Ocean

• Opening of the Proto-Atlantic (Iapetus) ocean

  • Passive margin: the side of a continent where no real tectonic activity is occurring.

• Separated Laurentia from Pannotia

• Passive margin started along Eastern US

Epeiric Seas

• Epeiric seas: shallow seas

• Laurentia and other continents were composed of cratons (shields and platforms).

• Epeiric, epicontinental, or shallow seas, were common during Paleozoic, depositing sediment on basement.

  • Changes in climate allowed sea levels to rise and fall

  • Deposits included wave-washed sands, muds, and carbonates

  • Changes in sea level lead to sequence stratigraphy

    • A sequence of sea level rise and sea level fall producing a certain type of deposit as that process continues to occur.

Transgression

• Transgression: Sea level rise

• Coarse grains found at the bottom, finer grains at the top

  • Carbonates (top, fine)

  • Shale

  • Sand (bottom, coarse)

• Carbonates - limestone

Regression

• Regression: Sea level fall

• Finer grains at the bottom, coarser grain on top

  • Sands (top, coarse)

  • Shale

  • Carbonates (bottom, fine)

• Carbonates - limestone

Arches, Basins, and Faults

• Sedimentary layers deposited in these seas were warped.

• Throughout N. American craton, broad gentle warping of crust occurred in response to small changes of isostatic equilibrium due to density changes in lithosphere.

  • Isostatic equilibrium: When the Earth's crust balances out with the underlying mantle so that the crust stays at a stable level. If weight is added or removed, the crust adjusts to maintain balance.

• Raised areas are called arches

• Depressed areas are called basins

• Faults also present within cratons

• Sedimentation largely influenced by these structural modifications

• Not a lot of plate tectonic activity

Isostatic Rebound

• Asthenosphere is ductile and changes in density in the rock above can cause the lithosphere to push down into the asthenosphere causing warping or deformation.

• Isostatic rebound: Happens when the crust rises back up after the weight, such as ice, is removed, allowing it to return to its original position.

  • Taking weight off of a mattress, slow rise.

Sedimentation on Arches and Basins

• Basins allow for deposition and are therefore heavier; thus basins likely subside faster and allow for more continuous sedimentation.

• During the Cambrian, arches were sometimes shallow banks, experiencing little sedimentation and were at other times islands being erodes.

The Suak Sequence

Sequence Stratigraphy and The Suak

• N. America’s 1st sequence

• Late Proterozoic, sea level rising due to warming climatic conditions and melting of ice/glaciers

• Rising seas triggered sedimentation and the subsidence of passive margin wedges on the edge of continents.

• End of the Cambrian, ¾ of N. American lowlands flooded by shallow sea

• Rise was ~10 miles every million years

Where Were We?

• Suak Transgression, N. America at or close to equation.

• The continent was rotated 90°

• Late Cambrian, sands dominated shoreline for many km behind advancing sea due to the upland areas (such as the Transcontinental Arch) eroding.

Mature Quartz Sandstones

• These sands formed the Cambrian quartz sandstones and are considered the most mature in the world.

• Widespread, sheet-like distribution of these sandstones implies tectonic stability.

• Wind is much more effective at rounding quartz grains than moving water.

• Thus grains must have been moved to some degree by wind and deposited along the shoreline.

• No plants in the Cambrian, so barren wind-swept surfaces.

Tapeats Sandstone

• Grand Canyon Tapeats Sandstone

  • Cross-bedding

  • Numerous fossilized worm trails, burrows, and grazing patterns

  • Marine beach environment

• Can be found in Frenchman Mountain

Shale

• Shale: a fine-grained sedimentary rock formed from compacted clay and silt, often characterized by its ability to split into thin layers.

• Marine coast, sea level rises and produces shale.

• Shale is relatively rare during the Cambrian.

• Clays which would have formed from the weathering of igneous and metamorphic Precambrian rocks blew out to sea.

• There were important clay/mud deposits during this time.

  • Bright Angel Shale: a Cambrian sedimentary rock formation, characterized by its gray and greenish-gray shale and siltstone layers.

Carbonates

• Beginning of Ordovician, most of the Cambrian sand had been deposited (Tapeats Sandstone).

• As sea level continues to rise, no new clastic material was weathered or available to the depositional system.

• Through the Cambrian, carbonates deposited overlaying the clastics.

The Carbonate Factory

• Carbonates eventually took over the craton.

• Originally deposited as limestone (Muav Limestone in Grand Canyon and Frenchman Mountain) and subsequently altered to dolostone.

  • Dolostone is limestone that was later hydrologically altered by some magnesium substitution for calcium.

• Many of these limestones are composed of shells and other skeletal debris.

• Some have cross-strata and other sedimentary structures (this are clastic limestones).

Depth of the Suak Sea

• Signs of agitation in quartz and carbonate sediments we can surmise sea was shallow.

• Using modern continental shelves as an analog, its possible the Suak Sea was less than 200 m deep.

• The presence of common stromatolites in the carbonate system give further evidence as these are formed in the photic zone.

Flat-Pebble Conglomerates

• First evidence of epeiric seas (shallow sea)

• Fine grained dolomitic and lime-mud sediments (carbonate material) show signs of having been ripped up and remobilized during storms, creating flat-pebble carbonate conglomerates.

Stromatolites

• Stromatolites: Layered sedimentary structures created by the growth of cyanobacteria (photosynthetic microorganisms) in shallow water; often found in fossil form.

• Stromatolites need energy from the sun, so they must been in shallow water.

• Photic zone: depth of water which light reaches; good for photosynthesis.

• Clearest waters, max depth ~150-200 m

• Modern stromatolites attacked by snails (snails evolve in the Cambrian).

• Modern stromatolites only found in shallow swash zone where snails cannot pursue due to agitated sea water.

The Importance of Oolites

• Fragmented fossils, scattered quartz grains, carbonate pebbles, and ripples in Early Ordovician carbonates point to a setting of intense agitation.

• Abiotic round carbonate concretion were formed called ooids → as agitated sea water (back/forth wave action) interacted with air, ooids precipitated around a nucleus or central core.

• Found in Great Salt Lake or Bahamas

• Hurricanes strongly modify sediments

• The appearance of flat-pebble conglomerates and layers of conglomerates interbedded with quartz sandstone in the Cambrian-Ordovician tells us that major storms have been influencing sediment in N. America for millions of years.

The Suak Transgression

• Flat-pebble conglomerates, stromatolites, and oolites indicate Suak sea was shallow.

• Most of our interpretations of the Cambrian-Ordovician craton are based upon observations of modern systems and from experimental studies.

Gulf of Mexico Analogue

• Gulf shelf < 200 m depth

• Moderate currents and varied sediment sources, but bulk of sediment coming from Mississippi River

• Southeast of Mississippi Delta we encounter Florida peninsula → Almost entirely composed of Cenozoic carbonates and no clastic dirt in the way.

Grand Canyon Cross Section

• From top to bottom

  • Dolostone

  • Muav Limestone

  • Bright Angel Shale

  • Tapeats Sandstone

  • Vishnu Schist

The Tippecanoe Sequence

Tippecanoe Sequence

• Regression occurred in the Ordovician (part of the Suak) leading to a period of erosion (Knox Unconformity).

Deposits of the Tippecanoe

• As seas were rising (again), sediments were deposited in a similar way to that of the transgressing Sauk.

• Some Tippecanoe sandstones were more mature (supermature) than those deposited during the Sauk.

• Clays followed by shale (only in a few places).

• Then followed by enormous volumes of carbonate deposition (large reefs); some of which chemically altered into dolostone.

• As the Tippecanoe Sea gradually regressed during the Late Silurian, large evaporite deposits (salts) precipitated.

Marine Evaporites

• Evaporites are important chemical sediments, which are commonly associated w/ carbonates.

• Require evaporation of large volumes of water to concentrate brines to the point of mineral precipitation.

• Known from the Proterozoic to the present, the Silurian records the earliest extensive example in North America.

• Two examples: Restricted-Basin Evaporites and Supratidal Sabhka Evaporites

Restricted-Basin Evaporites

• Restricted-Basin Evaporites: Sea water is replenished to a physically restricted basin at a rate which cannot keep up with evaporation.

• Brines at bottom of water body begin to precipitate evaporites.

• Leaves behind salts and other dissolved substances.

• Modern day examples: Tunisia and Kara-Bogaz-Gol

Supratidal Sabhka Evaporites

• Supratidal: slightly above high-tide line

• Sabhkas: flats in Arabic

• Supratidal Sabhka Evaporites: Salt and mineral deposits that form in coastal areas above the high tide line, where seawater occasionally floods the area and then evaporates.

• Seawater fills pores during high tide and becomes abnormally saline.

• Evaporation draws sea water through the pores and to the surface.

• Modern day example: Abu Dhabi

The Michigan Basin

• Silurian deposits that include reefs and evaporites.

• New York, Ohio, Michigan, and western Canada.

• > 750 meters of salt and gypsum

• An immense volume of water is needed to precipitate that much evaporite.

• Likely the Michigan Basin was a restricted basin like the Kara-Bogaz Gol, with constant replenishment.

• Massive reef complexes likely aided in the restriction of water circulation.

The Taconic Orogeny

The Cambrian

• Just after Paleozoic began, Pannotia fragmented into 5 smaller continents:

  • Laurentia, Baltica, Siberia, China, Gondwana

• Laurentia was relatively stable.

• The Suak Sea was rising forming epeiric seas depositing different strata on top of one another with the lowermost dominated by sands, then clays, then calcium carbonate, and then magnesium-calcium carbonate.

• Western N. America was a passive margin.

Evidence of Ordovician Tectonic

• Ordovician rocks in Appalachian region (Newfoundland, New York, Pennsylvania, Carolinas) provided evidence of intense folding and deformation.

• Helter-skelter fragment deposits (e.g., breccia) believed to have come from submarine avalanches.

• Volcanic pyroclastic material and interbedded lava flows reaching far out onto the craton.

Taconic Orogeny

• 1st Paleozoic mountain building event.

• Although plates were moving, this orogeny began Mid-Ordovician.

• Uplift, erosion, intrusion of batholiths along the eastern margin of North America.

Mid-Cambrian to Mid-Ordovician

• A. Quiet depositions during Cambrian and into Early Ordovician

• B. Continued closing; convergence; partial closure of Iapetus ocean; subduction; ophiolites.

• C. Iapetus ocean completely closes; major thrust faulting, metamorphism, volcanism.

Ophiolites

• Ophiolites: Distinctive rock assemblage.

• Remnants of oceanic plate scraped off and added to an accretionary prism.

• Mark zone of contact between continental and oceanic plates.

The Queenston Clastic Wedge

Effects of the Taconic Orogeny

• Converging lithospheric plates closing in on another.

• Sediments were crushed, metamorphosed, and pushed northwestward along a thrust fault.

• Masses of granite typical of convergent plate boundaries.

• The uplifted rock during this orogeny began to weather and just like anything that sticks up in the air, it will break down chemically and physically; this creates sediment.

• A wedge of rust-red terrestrial clastics (Queenston clastic wedge) became increasingly coarser and thicker toward mountainous source area as streams along the mountains deposited this material.

• A thick sequence of sedimentary rocks, primarily composed of sandstones, shales, and conglomerates, that was deposited during the Late Ordovician period in the Appalachian Basin due to erosion of the rising Taconic Mountains.

Ordovician Paleogeographic Map

• Transgression of the Tippecanoe sequence occurs in mid- to late-Ordovician.

The Silurian

• During the Silurian, orogenic activity shifted northeastward and erosion of the Taconic highlands continued.

• Early Silurian sediments derived from the core of the Taconic orogeny found in New York.

• Silurian quartz sandstones are common in the Appalachian region.

• Result from erosion of massive source area of Taconic rocks.

Silurian Iron-Rich Deposits

• Rare, found in S. Appalachians

• Imply intense tropical weathering in Taconic mountains.

• Iron was transported by river to restricted marine basins north and south of the orogeny.

• Shells and oolites replaced and cemented with hematite.

• Provided ore used in the steel industry

The Caledonian Orogenic Belt

• Caledonia is the ancient Latin name for Scotland.

• An orogenic belt along NW border of Europe.

• Similar history to that of Appalachian belts.

• Occurred during Silurian and Devonian.

• Most intense in Norway with rocks compressed, folded, and thrust faulted above a subduction zone.

The Cordillera

Out West

• The Cordillera begins to take form.

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

• From Rocky Mountains to the Pacific Coast

  • Rockies, Sierra Nevada’s, and Coast Ranges

• During most of the Early Paleozoic (Cambrian-Silurian), this region was nearly flat, laying near or below sea level.

Orogeny Begins

• Then the Pacific plate moving against Laurentia switched to a subduction zone with a volcanic chain.

Review Questions

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

  • Suak Sequence during Late Precambrian to Early Ordovician; evidence includes the Grand Canyon's Tapeats Sandstone, Bright Angel Shale, and Muav Limestone are classic examples.

  • Tippecanoe Sequence during Mid-Ordovician to Early Devonian; evidence includes evaporates and the Michigan Basin.

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

  • Taconic Orogeny during the Late Ordovician to Early Devonian

    • Evidence:

      • Ordovician rocks that provided evidence of intense folding and deformation.

      • Helter-skelter fragment deposits (e.g., breccia) believed to have come from submarine avalanches.

      • Volcanic pyroclastic material and interbedded lava flows reaching far out onto the craton.

  • Caledonian Orogeny during the Late Ordovician to Early Devonian.

    • Evidence:

      • Extensive folding, faulting, and metamorphism of rocks.

• What sedimentary rocks formed during this time (particularly during the Sauk)?

  • Tapeat sandstones, Bright Angel Shale, Muav Limestone, Dolostone

• What are Oolites? What do they tell us about the environment in which they formed?

  • Oolites are tiny, round grains made of layered calcium carbonate. They form in warm, shallow, and turbulent marine waters

  • They indicate: Warm, tropical seas with strong wave action.

• How do evaporites form and how did they form during in Laurentia?

  • Evaporites form when water in a closed basin (like a lake or sea) evaporates, leaving behind salts and minerals.

  • In Laurentia: During ancient times, evaporites formed in basins where seawater evaporated quickly, concentrating salts like gypsum and halite.

• What are Ophiolites? What do they tell us about the environment in which they formed?

  • Ophiolites are slices of oceanic crust and upper mantle that have been pushed up onto land. They include rocks like basalt, gabbro, and peridotite.

  • Ophiolites reveal that these rocks originally formed under the ocean, often at mid-ocean ridges, where new oceanic crust is created.

• What was happening in western Laurentia at this time?

  • Western Laurentia was covered by shallow seas, where sedimentary rocks like sandstones and shales formed. The region experienced tectonic movements that began shaping the Cordilleran mountain range and some volcanic activity.