EAS 209 - Lecture 22

Inga Lehmann

• Inga Lehmann (1888–1993) was a geophysicist who studied Earth's interior using seismic waves generated by earthquakes.

• Originally trained as a mathematician before moving into geophysics.

• Noticed that seismic wave intensity and waveforms differed depending on the location of seismic stations.

• Investigated these differences and developed a hypothesis about Earth's internal structure.

• Proposed the modern model of Earth consisting of:

  • Crust.

  • Mantle.

  • Liquid outer core.

  • Solid inner core.

• A discontinuity associated with the core is named the Lehmann Discontinuity in her honor.

• Lived to over 100 years old and was a highly influential scientist.

ANS: C

Winterburn Group Overview

• Follows deposition of:

  • Elk Point Group → when sea level was low + evaporites forming.

  • Beaverhill Lake Group → more carbonates.

  • Woodbend Group.

• Elk Point Group formed during low sea level conditions.

• Beaverhill Lake Group formed as sea level began rising and carbonate deposition increased.

• Woodbend Group represented ideal reef-building conditions.

• During Woodbend time:

  • Sea level was favorable.

  • Environmental conditions supported extensive reef growth.

  • Leduc reefs flourished.

• Winterburn Group marks the beginning of declining reef conditions.

• Sea level began falling during Winterburn time.

• Reef growth became less favorable as marine environments became shallower.

Frasnian–Famennian Boundary

• Located within the interval being discussed during Winterburn deposition.

• Corresponds to the Late Devonian mass extinction.

• One of the most significant extinction events in Earth's history.

• Many Winterburn-equivalent deposits are not preserved in the Cordillera.

• Mountain exposures preserve:

  • Woodbend equivalents.

  • Wabamun equivalents.

• Much of the Winterburn interval is missing in the mountains.

• Loss of preservation is unfortunate because the Frasnian–Famennian extinction occurred during this interval.

Sea-Level Fall During Winterburn Time

• Retreat of the seas shifted the platform margin back towards NW.

• This was in general a poor interval for reef growth, but much oil is trapped in porous Nisku Formation carbonates where they are draped over underlying Leduc reefs (e.g. Leduc No. 1); this is caused by differential compaction of the intervening ireton shales.

• Marine waters gradually retreated.

• Falling sea levels reduced ideal reef-building environments.

• Reefs were no longer growing under optimal conditions.

• Carbonate deposition still occurred despite declining reef growth.

• Nisku carbonates continued to form.

• Nisku carbonates later became important oil and gas reservoirs.

Differential Compaction and Hydrocarbon Traps

• Nisku carbonates were deposited above older Leduc reefs.

• Differential compaction created structural traps for hydrocarbons.

• Stratigraphic sequence:

  • Cooking Lake Platform.

  • Leduc reefs.

  • Duvernay Shale.

  • Ireton Formation.

  • Nisku Formation.

  • Wabamun Group.

• Ireton shale is clay-rich and highly compressible.

• Carbonates such as limestone are much less compressible.

Why Shales Compact More Than Carbonates

• Shales contain platy clay minerals.

• Clay particles are initially arranged randomly.

• Compression causes clay particles to align parallel to one another → can be compacted a lot.

• Alignment reduces pore space.

• Porosity decreases significantly during compaction.

• Carbonates generally retain their structure during burial.

• As a result:

  • Shales compact substantially.

  • Carbonates compact relatively little.

• Things like limestone doesn’t compress easily (unlike shale).

Differential Compaction:

• Shales → compacted.

• Where there is carbonates (more than shales) it is not compacted the same the shales are.

  • Remains a high.

• Structures in the subsurface are formed based on the compressibility based on the shales versus the reef complexes.

• Hydrocarbons tend to migrate to high points (same with water) → due to buoyancy factors.

  • As a result, hydrocarbons migrate and become trapped on the arch.

  • Arch is formed through sediment overlying it through time.

• Compaction is greater off reef.

Formation of Differential Compaction Structures

• Areas containing thick shale sequences compact more.

• Areas underlain by reefs or carbonates compact less.

• Carbonate-rich regions remain relatively elevated.

• Overlying layers drape over these highs.

• Creates subtle arch-like structures in the subsurface.

• These structures are not caused by tectonic deformation.

• They develop solely from differences in sediment compaction.

Hydrocarbon Accumulation

• Oil and gas migrate upward due to buoyancy.

• Hydrocarbons move toward structural highs.

• Differential compaction arches provide trapping geometries.

• Hydrocarbons accumulate beneath impermeable seals.

• Many economically important reservoirs formed this way.

Nisku Formation Lithology - Amphipora

• Amphipora resembles coral but is sponge-like in nature.

• Often occurs within mud-supported deposits.

• Deposits are classified as floatstones because:

  • Mud supports the framework.

  • Amphipora fragments are abundant.

Stylolites

• Stylolites appear as irregular black jagged lines in carbonate rocks.

• Represent surfaces where dissolution occurred.

• Despite limestone being generally resistant, with enough pressure and high temperature the limestone can actually dissolve.

  • As a result, the carbonate dissolves away. Clays and heavy minerals (that survive the pressure and temperature) remain.

• Carbonate material dissolves and is removed by fluids.

• Insoluble residues remain behind.

• Residues commonly include:

  • Clay minerals.

  • Heavy minerals.

• Amount of dissolved rock is difficult to determine.

• Presence of stylolites indicates loss of part of the rock record.

• Important sedimentary contacts may be removed by dissolution.

Nisku Dolomitization

• Some Nisku deposits are heavily dolomitized.

• Dolomitized rocks are typically fine-grained.

• May contain wisps of preserved mud.

• Generally display limited visible porosity in hand sample.

Nisku Paleogeography

• Nisku deposition occurred across much of Alberta.

• Depositional environments included:

  • Carbonates.

  • Dolomites.

  • Clastics (green).

  • Mixed carbonate-evaporite systems.

• Evaporites became more common toward the Williston Basin.

• Indicates increasingly restricted marine conditions.

• Reflects continued sea-level decline.

• A lot of the Nisku have been dolomitized.

Zeta Lake Member

• Consists of isolated pinnacle reefs.

• Occurs within shale basins.

• Commonly associated with the Wild River Subbasin.

• Pinnacle reefs are surrounded by deeper-water basin sediments.

• Many Zeta Lake reefs became dolomitized.

• Represent isolated reef growth centers within basinal settings.

Nisku Shelf and Basin Relationships

• At the western edge of the Nisku Formation platform: isolated pinnacle reefs at West Pembina.

• Nisku platform occupied shallower marine areas.

• Shale basins developed in deeper water.

• Zeta Lake pinnacle reefs formed within shale basins.

• Nisku deposits overlie older Leduc reef complexes.

• Cooking Lake Platform remains beneath both systems.

Oil and Gas in Pinnacle Reefs

• Pinnacle reefs commonly contain oil and gas accumulations.

  • Green is oil and red is gas.

• Reservoir quality results from:

  • High porosity.

  • High permeability.

• Reefs occur near shelf margins.

• Preferred growth location was the medial ramp.

• Medial ramp provided:

  • Adequate sunlight.

  • Appropriate water depth.

• Hydrocarbon pools commonly align with these reef trends.

Vertical Exaggeration in Cross Sections

• Many geological cross sections use vertical exaggeration. They are not 1:1 measurements.

• Vertical scale is larger than horizontal scale.

• Makes structures appear more pronounced.

• Pinnacle reefs often look like tall towers in exaggerated sections.

• In reality, reefs were much broader and less steep.

• Without vertical exaggeration, reefs appear as modest topographic highs.

• Not ein passing that “pinnacle reefs” are in fact tablet/Aspirin-shaped.

Late Winterburn Conditions

• Sea level continued falling.

• Reef development declined further.

• Evaporites and clastic sediments became increasingly common.

• Blue Ridge Member records these more restricted conditions.

• Peace River Arch remained exposed as an island, but mantled by clastics.

• Clastic sediments continued to be shed from the arch.

• The Winterburn Group ended with deposition of evaporative clastics.

Kalmar Formation

• Overlies the Nisku Formation → sharp contact.

• Characterized by calcareous shale.

• Commonly has a greenish coloration.

• Similar to the Duvernay in carbonate content.

• Acts as a seal over Nisku reservoirs.

• Contact with the Nisku is sharp and distinct.

Frasnian–Famennian Mass Extinction

• Occurred near the end of Winterburn deposition.

• One of the Big Five mass extinctions.

• Took place approximately 360 million years ago.

• Consisted of two major extinction pulses.

• Particularly devastated tropical marine ecosystems.

End of the Winterburn:

• End of the Winterburn deposition was also coincident with worldwide extinction of the Devonian reef community.

• Consequently, for the balance of the Kaskaskia Sequence, there were no more reefs (thus, none in the Wabamun Group, next).

• Drastic difference from the Winterburn to the Wabamun. The Winterburn has corals, stromatoporids, etc. The Wabamun has mostly calcium carbonate mud.

• Unlike the Cretaceous mass extinction, there is no crater (like an easy depiction of a “smoking gun”). It’s unclear what caused it.

Devonian Extinction - 1st Pulse:

• Second of the “Big Five” mass extinction.

• 2 extinction pulses caused by cooling events.

• Both cooling events are marked by positive O and C isotope excursions.

• 1st pulse hit tropical taxa: reef builders, ammonoids, brachiopods, and placoderms).

• Coral-Strome reefs never fully recovered.

• Cold water inhabitants were not heavily affected (consistent with cooling as the main cause).

• Spread of cold-adapted silica sponges from deep to shallow seas also supports cooling as the cause.

Evidence for Cooling

• Positive oxygen isotope excursions indicate cooling.

• Positive carbon isotope excursions also occur.

• Cold-water organisms survived relatively well.

• Silica sponges expanded into shallow marine environments.

• Extensive glaciation developed.

Second Extinction Pulse

• Second pulse of extinctions: less severe…

• Placoderms went extinct and planktonic acritarchs suffered major extinctions.

• Second pulse of cooling: most severe than the first one.

• Glacial deposits spread to regions 30 degrees south of the equator indicating widespread glaciation.

• The second pulse of extinctions would’ve been more devastating if the initial cooling event hadn’t already wiped out warm-adapted species.

• Associated with even stronger cooling.

• Glaciers expanded to regions near 30° south latitude.

• Warm-adapted species suffered major losses.

• Placoderm fishes became extinct.

Oceanic Anoxia

• But cooling wasn’t the only cause…

• From mid to late Devonian, oceanic bottom waters became anoxic.

• Oxygen requiring benthic organisms died, tropical reef communities were hit hard.

• There were frequent sea-level changes, with one sea-level rise associated with the onset anoxic deposits.

• Each sea level rise was followed by glaciation and sea-level fall. The most severe during the second pulse of extinctions.

• Bottom waters became oxygen depleted.

• Oxygen-requiring benthic organisms died.

• Reef ecosystems were especially vulnerable.

• Anoxia coincided with major ecological disruption.

Role of Land Plants

• Vascular plants spread across continents during the Devonian.

• Plants evolved:

  • Deep root systems.

  • Greater height (from 30cm to 30m) as a result of root systems.

  • Evolution of seeds permitted plant growth in drier (previously uninhabited) areas.

• Roots accelerated weathering of bedrock and formed deep layers of soil.

• Thick soils developed for the first time.

Weathering and Nutrient Input

• Soil promotes weathering, which transports nutrients to the ocean causing eutrophication and anoxia.

• Increased weathering transported nutrients to the oceans.

• Key nutrients included:

  • Phosphorus.

  • Nitrogen.

• Nutrient influx triggered algal blooms.

• Decomposition of algae consumed dissolved oxygen.

• Resulted in widespread oceanic anoxia.

Cooling and Glaciation

• Forest expansion removed CO₂ from the atmosphere.

• Weathering also consumed atmospheric CO₂.

  • Ultimate result of both of these causing cooling.

• Lower CO₂ levels produced global cooling.

• Glaciation expanded.

• Glaciation and subsequent sea level fall following the mild climate of the Devonian also caused extinctions.

• Dropdown of C eventually pulled Earth out of greenhouse that into icehouse that continued through the Carboniferous and Permian.

• Warm-adapted species experienced extensive extinction.

Additional Extinction Hypotheses

• Nearby supernova may have damaged the ozone layer.

• Increased UV radiation could have harmed organisms.

• Evidence includes UV-damaged fossil spores and pollen.

• Volcanism may also have contributed.

• Large flood basalt eruptions occurred in Siberia.

• Sulfur dioxide emissions may have increased planetary reflectivity.

• Increased reflectivity could have promoted cooling.

Extraterrestrial Cause…?

• Some attribute the extinctions to UV damage.

• UV damage to fossil pollen and spores during this event suggests long-term destruction of ozone.

• It has been hypothesized that a nearby supernova damaged ozone leading to massive UV damage to life, triggering mass extinctions.

• However, destruciton of the ozone would’ve required sustaining global warming (rather than cooling…)

Volcanic Activity…?

• Volcanism has also been suggested as a cause of these extinctions.

• Ar dating confirmed correlation between eruption of the Viluy traps on the Siberian Craton and the first pulse of extinctions.

• SO2 ejected into the atmosphere form the Viluy volcanism could’ve formed high albedo sulfuric acid aerosols causing rapid global cooling.

Overall Cause of the Devonian Extinction

• Likely resulted from multiple interacting factors:

  • Oceanic anoxia.

  • Cooling.

  • Glaciation.

  • Sea-level fluctuations.

  • Volcanism.

  • Potential UV damage.

• No single universally accepted cause exists.

Wabamun Group

• Deposited after the Winterburn Group.

• Equivalent to the Palliser Formation in the mountains.

• Sea level continued falling.

• Carbonate deposition became increasingly restricted.

• Large limestone bodies transitioned basinward into shale.

Palliser Formation

• The Palliser formation is one of the spectacular cliff-formers of the Front Ranges of the Rocky Mountains.

• There was continued regression of sea level to the NW, with limestones again shaling out to NW (for both the Palliser and the Wabamun).

• Forms prominent cliffs throughout the Rocky Mountains.

• Common around:

  • Canmore.

  • Jasper.

  • Grassi Lakes.

• Extremely thick carbonate succession.

• Dominated by calcareous mudstones.

• Contains very few fossils.

• Represents post-extinction conditions.

Evidence for Mass Extinction in the Rock Record

• Winterburn rocks contain:

  • Corals.

  • Stromatoporoids.

  • Diverse fossils.

• Wabamun/Palliser rocks contain:

  • Very few fossils.

  • Mostly carbonate mud.

• Abrupt disappearance of reef communities supports occurrence of a major extinction event.

Wabamun Lithology

• Commonly displays pinprick porosity.

• Little evidence of bioturbation.

• Fossils are rare or absent.

• Breccias occur locally.

• Stylolites may form after deposition through dissolution.

Exshaw Formation

• Marks the end of the Devonian sequence.

• This period was closed out by deposition of the Exshaw formation of Bakken formation that spans the Devonian/Mississippian boundary.

• Represents a major marine transgression.

• Sea level rose again after prolonged regression.

• Organic-rich shale deposited across the region.

• Shale blanketed older formations.

Characteristics of the Exshaw Formation

• Dark organic-rich shale.

• Contains evidence of life.

• Bioturbation is present.

• Burrows such as Chondrites occur.

• Organisms mined sediment for organic matter.

Bakken Formation

• Equivalent to the Exshaw Formation in Saskatchewan and the United States.

• In the Bakken formation, a number of siltstone units have become prominent hydrocarbon reservoirs.

  • Has a different name because it has a different location.

• Major unconventional oil and gas resource.

• Organic-rich shale generates hydrocarbons.

• Technological advances such as hydraulic fracturing made production economically viable.

Cordilleran Equivalents:

• No middle Devonian.

• Flume Formation = Beaverhill Lake Group.

• Exposure of grey Flume Formation carbonates along Yellowhead Highway.

• Fairholme Group between Flume Formation and Wabamun Group; significant unconformity at its top.

• Exposure of Perdrix Formation shales (= Ireton/Duvernay) and older Flume Formation carbonates Yellowhead Highway.

Mountain Formation Equivalents

• Flume Formation = Beaverhill Lake Group equivalent.

• Palliser Formation = Wabamun Group equivalent.

• Exshaw Formation = Bakken equivalent.

• Different names are used because the rocks occur in different geographic settings despite being time-equivalent.

Recognizing Devonian Rocks in the Mountains

• Fossil-rich units generally belong to older Devonian intervals.

• Common fossils include:

  • Stromatoporoids.

  • Corals.

  • Stachyodes.

• Many middle Devonian deposits are absent due to non-preservation.

• Palliser Formation is typically fossil-poor and dominated by carbonate mudstones.

ANS: A