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Complex Geological Problems: Integration of Physical, Chemical and Biological Factors. Continued from last class: . Sediment becomes lithified (hardened) by: compaction 2. cementation (sediment particles “glued ” together by minerals precipitated from sediment porewaters).

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Complex Geological Problems:

Integration of Physical, Chemical and Biological Factors


Continued from last class:

  • Sediment becomes lithified (hardened) by:
  • compaction
  • 2. cementation (sediment particles “glued ” together by minerals precipitated from sediment porewaters)

An Example of Using Integrated Approach

to Interpreting Geological Events:

Concretion-Hosted Shell Clusters in

The Upper Cretaceous Bearpaw Formation

of southern Alberta

Concretions (hard, cemented bodies of sediment) common

In many mud-dominated successions.

Concretions are formed after sediment is deposited


First attempt at explanation:

Clumped appearance of shells due to preferential

preservation within concretions

In other words, the original shell concentration may

have comprised a continuous bed, but were not preserved

outside the concretions because they were dissolved


Problems with dissolution idea:

  • Shells can be found (in lower abundances) in the
  • host sediment of the “event” horizons.
  • 2. In some cases, shell concentrations extend outside
  • of concretions then pinch out
  • 3. In some cases, shell concentrations pinch out within
  • concretions

Verdict: Cam’s a dumbass

The clustering habit of the shells is real, and

was produced before the concretions formed


Basic Aspects of Sedimentary Environment:

  • Succession dominated by claystone (mud) indicating generally low energy conditions (i.e. water calm enough to permit settling of mud)
  • Sediment type appears to be deep subtidal, mudstones merge laterally into sandier sediments toward the west where sedimentary characteristics indicate nearshore (beach/delta) to onshore (river deposits) envrionment
  • 3. Mud-dominated succession generally has low diversity shelly assemblages, forms typical of normal-salinity.
  • 4. Thick intervals with few or no shelly faunal elements, and trace fossils indicating low oxygen
  • So, in general, the depositional environment was probably characterized by:
  • Relatively deep water
  • Low energy conditions with little water circulation
  • Dysoxic bottom waters

Additional Observations:

Concretion-hosted shell clusters commonly occur in horizons indicating higher energy sedimentary processes

Beds and lenses of silt/sand preserving:

A) hummocky cross-stratification / B) rhythmite lamination / C) ripples/ D) load casts


Tentative (Second) Interpretation:

The depositional environment was generally

quiet, with low oxygen conditions on seafloor

Storm waves / currents occasionally influenced

the sea floor, concentrating shells and other

coarser-grained sedimentary particles and

washing away the mud

Patchy distribution of coarser grained particles

(silt, sand and shells) due to low availability of


In other words, coarser grained sediments

(silt, sand and shells) formed isolated patches

Rather than continuous beds during storms


Another problem:

Organisms found in shell clusters often completely

different from those found in mud

Complete shells in the clusters dominated by

those of suspension feeders

Possible solution:

Isolated silty/sandy bedforms (ripples) were sites of

preferred colonization by suspension feeders


Model for second attempt at explanation

Isolated patches of sediment formed during storms

Extension of idea:

Isolated silty/sandy bedforms (ripples) were sites of

preferred colonization by suspension feeders


Problems with bedform idea:

1. Some of the clusters are nearly globular in shape,

and obviously do not conform to the cross sectional

shape of a bedform.

2. Bioturbation probably not capable of redistributing shells

to form this shape because burrowing tends to disperse

particles rather than concentrate them.

Verdict: Cam’s a dumbass

The clustering must reflect something else


Critical Observation:

In examples where bioturbation is low

(i.e. where primary sedimentary structures

are not obscured by churning of burrowers)

sharp contact is preserved under shell


Shell concentrations are therefore scour infills

Note: scour in C is associated with a burrow


So… another idea

Clustering is due to infilling of storm-produced


Scours preferentially sited in areas of surface

Roughness on the seafloor (due to generation

of turbulence)

Could be negative elements (e.g. burrows) or

positive elements (individual shells or clumps

of shells)


clumping is common among suspension-feeding



Incorporate observations on faunal assemblages

  • Main Observations:
  • Clustered assemblages occur in distinct horizons marking storm events
  • Individual clusters often comprise shells of single species not generally found in muds
  • Shells in least disturbed clusters tend to be those derived from suspension feeders.

Another approach:

How do assemblages of clusters differ from those of the more “typical” muddy sediments ?

Classify into “background” and “event” assemblages


Background community: dominated by

Deposit feeding clams, predators/scavengers,

And specialized suspension feeders adapted to

Living on/in soupy mud


Event community – dominated by suspension feeders (in this case, bivalves), generally adapted to more stable substrata


Eureka !:

To establish event community, must have:

1. Relatively stable substrate (“primed” by storms)

2. Elevation in oxygen

3. High larval production to permit

establishment of high-density communities

  • …and clustering is due to interaction of
  • waves/currents during storms with presence of
  • living things on seafloor due to turbulence
  • generated around areas of surface roughness
  • on sea floor
  • e.g. burrows, shells, naturally patchy clumps of
  • organisms

General Model for Shell Clusters

Storm brings in sand/silt

New substrate during high

Larval productivity allows

Establishment of

suspension-feeding community

During next storm,

shells locally generate

turbulence to produce

Scour (shells themselves

act as tools in the scouring


Sand/rains out as storm


Primary sed. Structures

generally erased due to

bioturbation after storm


How are multiple events preserved in the

Geological record ?

Consider a slot machine


So where do concretions fit in ?

These concretions are cemented mostly by

calcite (calcium carbonate)

In anoxic part of the sediment column, a few cm

below sediment-water interface, sulphate reducing

and methane oxidizing bacteria eat up organic matter

in sediment, releasing bicarbonate and sulphide.

If porewaters become saturated with bicarbonate, the

abundant calcium ions combine with bicarbonate and

calcite is precipitated.

Verdict: Cam’s still a dumbass, but managed

to figured this one out.


The more technical version:

Sulphate Reduction

2CH2O + SO42- 2HCO3- + H2S

Anaerobic Methane Oxidation

CH4 + SO42- HCO3- + HS- + H2O

Calcite Precipitation

Ca 2+ + 2HCO3- CaCO3 + H20 + CO2


Concretion Seeding

  • A basic principle: in a supersaturated solution, minerals
  • can precipitate more readily if crystals of that mineral
  • are present in the environment (.e.g the effect of dropping
  • a grain of salt in a glass of salt water)
  • Clusters of shells made of calcium carbonate acted as
  • seeds for the precipitation of calcium carbonate cements
  • from sediment porewaters.
  • Thus the association of shell clusters and calcite-cemented
  • concretions makes complete sense.
  • So…concretion-hosted shell clusters formed due
  • a coincidence of factors:
  • Establishment of suspension-feeding community due to
  • Priming of sediment by storm
  • Event of high larval production
  • 2. Formation of scours during later storm due to surface
  • roughness provided by organisms as depressions or
  • areas of positive relief and infill of scours by skeletal remains
  • 3. Seeding of concretions by virtue of calcium carbonate
  • Composition of skeletal remains in clusters

Single or Multiple Causes ?

Any other applications of this approach ?

Plenty !

Example: Mass Extinctions


K-T boundary: General Trends

See detail (next slide)


Detail at K-T Boundary

(detail of previous slide)


Mass Extinctions: Summary of “The Big Five”

5. Cretaceous-Tertiary extinction (65 Ma)

-bolide impact

-flood volcanism (Deccan Traps in India)

-cooling, rapid sea level fall ?

victims: 47 % marine genera

4. Triassic-Jurassic extinction (199 - 214 Ma)

- flood basalt volcanism (central Atlantic)

victims: 52 % of marine genera.

1. Permian-Triassic extinction (251 Ma)

-bolide impact ?

-flood basalt volcanism in Siberia ?

-assembly of Pangaea (continents interconnected)

-global cooling, major sea level fall

victims: 84 % marine genera;

95 % all species on Earth !

3. Late Devonian extinction (364 Ma)

- global cooling (note: coincident with expansion of land plants) ?

victims: 57 % of marine genera.

2. Ordovician-Silurian extinction (439 Ma)

- global cooling, then rapid warming

- rapid sea level fall followed by rapid sea level rise

victims: 60 % percent of marine genera.

Different mass extinctions, different causes ?


K-T boundary debate: oh, the fights

The Fights

Luis Alvarez (to Dewey McLean – opponent of Impact Hypothesis)

“If the president of the college had asked me what I thought about Dewey McLean, I’d say he’s a weak sister. I thought he’d been knocked out of the ball game and had just disappeared,

because nobody invites him to conferences anymore.”

Walter Alvarez (to Dewey McLean)

“Dewey, count them, 24 are with us. You are all alone. If you continue to oppose us, you will wind up being the most isolated scientist on this planet.”

Dewey McLean (to David Raup, proponent of Impact Hypothesis)

“Now, for your “Just So” comments in a Mosaic article (1981, v. 12) that you “know nothing about.” The one that got asteroid opponents lumped into an insulting blue–outlined box titled “Just So,” with the note that “some of the proponents abide in lofty isolation.” David, I believe you introduced the “Just So” epithet into the K–T.”

David Raup to Cam Tsujita (2000)

“In his recent paper, Tsujita makes parallels between a multicausal model of shell beds and mass extinction. Nice touch. Tsujita does, however, remain an outspoken opponent to the consensus that asteroid impact is the one and only possible cause of the end-Cretaceous mass extinction. His attempts to construct a “just so” story are poignant.”