Lecture 9: Surface Processes: chemical and physical weathering and sedimentary rocks

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Lecture 9 Surface Processes chemical and
physical weathering and sedimentary rocks

• Questions
• What is the rock cycle? How do rocks get
  destroyed and recycled at the surface of the
  Earth?
• At the other end of the transport system, how do
  weathered and eroded materials end up making the
  various kinds of sedimentary rocks?
• What can observations of the sedimentary record
  reveal about the tectonics, petrology, and
  climate of both depositional environments and
  upstream source environments?
• Reading
• Grotzinger and Jordan, Chapters 5, 16, 18, 19

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Weathering and Sedimentation in the Rock Cycle

• Our geology so far has focused on
  internally-driven processes plate tectonics,
  magmatism, metamorphism, orogeny.

• The rest of geology is driven by surface
  processes the hydrologic cycle (rainfall,
  streams, ice), gravity, aqueous chemistry.
• Weathering and erosion are the processes that
  form and transport form sediment.
• Sedimentation, burial and lithification are the
  processes that transform weathering products into
  sedimentary rocks.

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Weathering and Sedimentation in the Rock Cycle

• A more detailed view of the surface-driven parts
  of the rock cycle shows the various steps between
  source rock and sedimentary product

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Weathering decomposition of rocks

• There is a distinction between weathering and
  erosion
• Weathering converts exposed rock to soil in place
• Erosion transports dissolved or fragmented
  material from the source area where weathering is
  occurring to a depositional environment .
• Most of the earths surface is covered by
  exposure of sediment or sedimentary rock, by
  area.
• But the sediment layer is thin in most places,
  with respect to overall crustal thickness, so
  sedimentary rock is a minor volume fraction of
  the crust (in part by definition once buried to
  the mid-crust, sediments get cooked to
  metasediments).

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Weathering chemical and physical

• The destruction of rocks at the Earths surface
  by weathering has two fundamental modes of
  operation
• Chemical weathering is dissolution or alteration
  of the original minerals, usually by reactions
  with aqueous solutions
• Chemical weathering puts ions from the source
  minerals into solution for subsequent erosion by
  transport in flowing water as dissolved load.
• Physical weathering is fragmentation into
  progressively smaller particles, from intact
  outcrop to boulders and on down to mineral
  fragments and sand grains.
• Physical weathering makes loose pieces of rock
  available for downslope movement by mass wasting
  or transport in flowing water as suspended or bed
  load.

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Chemical Weathering

• Chemical weathering is driven by thermodynamic
  energy minimization, just like chemical reactions
  at high temperature.
• The system seeks the most stable assemblage of
  phases.
• The differences are that (1) kinetics are slow
  and metastability is common (2) the stable
  minerals under wet, ambient conditions are
  different from those at high T and P (3)
  solubility in water and its dependence on water
  chemistry (notably pH) are major determinants in
  the stability of minerals in weathering.
• A fresh rock made of olivine and pyroxenes will
  end up as clays and iron oxides, with other
  elements in solution
• A fresh rock made of feldspars and quartz will
  end up as clays, hydroxides, and quartz in most
  waters.

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Chemical Weathering
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Chemical Weathering

• The most common alteration product of feldspars
  is kaolinite, Al2Si2O5(OH)4, which serves as a
  model for the formation of clays by weathering
  generally.
• The reactions of feldspars to kaolinite
  illustrate some of the basic trends
• K, Na, Ca are highly soluble and readily leached
  by chemical weathering.
• Excess Si can be removed as silicic acid although
  quartz is relatively insoluble.
• Al is extremely insoluble, and is essentially
  conserved as source rock is converted to clays.
• Weathering is a hydration process, leaving H2O
  bound in the altered minerals.
• 2 KAlSi3O8 9 H2O 2 H -gt Al2Si2O5(OH)4 2 K
  4 H4SiO4
• Note the H on the left-hand sideonly acidic
  water can drive this reaction

• Natural waters are acidic due to equilibrium of
  carbonic acid with CO2 in the atmosphere
• CO2 (g) H2O H2CO3
• 2 KAlSi3O8 9 H2O 2 H2CO3 -gt
  Al2Si2O5(OH)4 2 K 4 H4SiO4 2HCO3
• Alteration of rock transforms acidic rainwater
  into neutral surface or ground water, with
  bicarbonate the dominant species (relative to CO2
  and CO32).
• Mg and Fe2 are also readily leached, but Fe3 is
  very insolublethe ultimate residue of alteration
  of mafic rocks is hematite.

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Chemical Weathering

• Knowing the chemistry of reaction of minerals to
  kaolinite, it is possible to reconstruct from the
  dissolved ions in stream water the amount of each
  source mineral that reacted with the water.

• Questions How do you do the correction for
  atmospheric input? Do the source minerals in the
  Sierra Nevada all weather at equal rates?

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Chemical Weathering

• Some minerals are congruently soluble in acidic
  water, leaving no residue
• The most abundant is calcite CaCO3 H2CO3
  Ca2 2HCO3 (the Tums reaction)
• Effects of dissolution (and precipitation) of
  calcite can be dramatic, to say the least.

Sinkhole
Speleothems
Karst terrain
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Rates of Chemical Weathering

• Many factors affect the rate at which a rock will
  weather, as summarized here.

• Some of these variables are local (e.g., source
  rock), some are global. These include temperature
  and pCO2, leading to the CO2-weathering feedback
  cycle.

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Physical Weathering

• Anything that promotes disaggregration of a rock
  so that pieces can form soil or be eroded away by
  wind, water, or gravity transport is physical
  weathering.
• The distinction between physical weathering and
  erosion is subtle, but think of physical
  weathering as fragmenting the rock and erosion as
  carrying the fragments away at times these may
  be the same event, of course.
• Rocks that are jointed or faulted or have
  pre-existing weak zones are most easily
  weathered.
• Few of the stresses associated with physical
  weathering are significant compared to the
  tensile strength of intact rocks something, has
  to start the process, either initial cracks and
  weaknesses or chemical attack on mineral
  cohesion.
• Organisms, especially plants (think tree roots),
  are fond of breaking up rocks.
• Freeze-thaw, frost wedging, frost heavethe
  volume change between ice and water is effective
  in widening cracks in rock in suitable climates.
• Physical abrasion by flowing air or water, or
  more often by rock particles already mobilized by
  water or wind (think Fossil Falls).
• Tectonicsrocks caught in a fault zone are
  definitely undergoing physical weathering.
• Etc.

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Weathering feedbacks chemical and physical

• Physical weathering and chemical weathering
  generally proceed in parallel in most
  environments.
• Physical and chemical weathering promote one
  another
• Formation of cracks by physical weathering
  increases reactive surface area, promoting
  chemical weathering.
• Chemical weathering replaces intact interlocking
  minerals with weak clays or void space, making
  the rock easier to physically disaggregate,
  promoting physical weathering

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Weathering feedbacks more generally

• Weathering of both kinds plays key roles in
  several feedbacks.
• Tectonics affects weathering through slopes and
  elevations, climate affects weathering through
  temperatures (via chemical kinetics and
  freeze-thaw), rainfall, pCO2, etc.
• Conversely, weathering and erosion affect
  tectonics and climate
• Denudation by erosion must be isostatically
  compensated and so affect vertical motions of the
  crust
• Weathering controls water chemistry, courses of
  streams and groundwater, removes CO2 from the
  atmosphere, etc.

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Soil formation

• Chemically and physically weathered rock that is
  not eroded or transported but remains in place
  becomes soil.

• A weathered surface develops a stratified
  structure, with intact rock at the bottom (or
  inside) and maximum weathering at the top .
• Leachable ions are transported downwards by
  groundwater flow, possibly redeposited as water
  chemistry adjusts towards equilibrium with the
  developing soil profile.

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Soil formation

• The mineralogy and thickness of soil layers
  depends on source rock, climate (temperature and
  rainfall), and age.
• Which of these soil types would you rather farm?

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Erosion and Transport

• Between weathering and sedimentation, matter must
  be transported from source to destination. This
  is erosion.
• We dealt with the landforms generated by erosion
  in the geomorphology lecture here our concern is
  with the effects of transport on sedimentary
  rocks.
• Modes of transport
• Gravity (short distances and steep slopes)
• Wind (small particles only)
• Glaciers
• Water
• Surface runoff carries dissolved, suspended, and
  bed loads
• Groundwater flow only carries dissolved load
• All these mechanisms carry products of physical
  weathering and insoluble residues of chemical
  weathering.
• Only water transport carries away leached soluble
  products of chemical weathering.

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Erosion and Transport

• Certain modes of transport physically modify and
  physically and chemically sort particles en
  route.
• Size sorting by surface water runoff flow

Current of a given velocity can generally carry
all noncohesive particles smaller than a critical
size since current velocity drops with
decreasing slopes from mountains to lowlands, it
follows that sediments evolve from poorly sorted
and coarse-grained near source to well-sorted and
finer grained with increasing transport distance.
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Erosion and Transport

• Chemical sorting with increasing transport
  distance is like a continuation of chemical
  weathering most stable minerals are transported
  the farthest.
• Textures of particles are modified by abrasion
  during wind or water transport. Close to source
  particles are angular far from source particles
  are rounded.

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Sedimentation

• Eventually transported particles and dissolved
  ions reach a place where they can be permanently
  deposited and accumulated. This is sedimentation.
• The sedimentary rocks that result from this
  accumulation are controlled by and record the
  sedimentary environment where they were
  deposited.
• We interpret ancient sedimentary rocks by
  comparison to modern environments where we can
  observe ongoing sedimentary processes and relate
  them to the composition, texture, and structure
  of the resulting rocks.

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Sedimentation

• Sediments and the environments in which they form
  are fundamentally divided into clastic and
  chemical
• Clastic sediments are made of physically
  transported and deposited particles (they may
  later gain chemically grown cement during
  diagenesis)
• Chemical sediments are grown from solution,
  organically or inorganically biochemical
  sediment more specifically refers to minerals
  grown from solution by organisms
• In some cases the relationship between the
  environment and the character of the sediment is
  absolute and obvious (carbonate in reefs,
  boulder-strewn till in periglacial deposit,
  etc.) other cases are more subtle.

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Diagenesis

• The process of modification of newly deposited
  sediments into sedimentary rocks is diagenesis or
  lithification.
• Processes include
• physical compaction by the pressure of
  overburden, accompanied by expulsion of pore
  waters
• Growth of new diagenetic minerals and continued
  growth of chemical sediments from pore waters.
• Dissolution of soluble elements of clastic rocks.
• Recrystallization and remineralization as water
  chemistry, pressure, and temperature evolve.
• At the high-T and P end, diagenesis merges
  smoothly into the low-T and P end of
  metamorphism. The distinction is arbitrary.

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Sedimentary Rocks

• The preserved end-result of weathering, erosion,
  transport, sedimentation, and diagenesis is
  sedimentary rocks.
• Like sediments and sedimentary environments, the
  resulting rocks are divided into clastic (or
  siliciclastic or volcaniclastic, etc.) and
  chemical (or biochemical).
• Clastic rocks are classified by particle size
  (and sorting) and composition.

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Sedimentary Rocks

• Chemical sediments are primarily classified, of
  course, by mineralogical composition.

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Sedimentary rocks and environmental information

• How do sedimentary rocks preserve information
  about their depositional environments?
• By composition, mineralogy and grain size,
  obviously, but also through sedimentary structure
• Elements of sedimentary structure
• Bedding
• Bed thickness, from finely laminated to massive

Burgess Shale fine
Vasquez formation massive
30 m
30 cm
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Sedimentary structure

• Character of bedding, from simple horizontal
  laminae to cross-bedding, ripples, soft-sediment
  deformation, or bioturbated.

• Cross-bedding indicates high and unidirectional
  current velocity, often winds in terrestrial
  settings, forming sand dune lee-slopes.

• Ripple marks record back-and-forth action by
  waves in shallow water.

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Sedimentary Structure

• Mud cracks demonstrate drying-out of a thin layer
  of sediment fine enough to have significant
  cohesion. Definite proof of terrestrial setting
  or very shallow water marginal marine.

MODERN
ANCIENT

• What about this structure? (Hint it is not the
  surface of the Moon)

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Sedimentary Structure

• Soft-sediment deformation indicates slumping or
  compression of layers before complete
  lithification.

• Bioturbation is the vertical mixing of
  sedimentary layers by burrowing organisms.
  Evidence of such activity can be preserved on
  bedding surfaces as trace fossils. Indicative of
  water depth, availability of nutrients and
  oxygen, etc.

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Sedimentary Structure

• Graded Bedding sorting of particle sizes within
  beds indicates time dependence and hence process
  of deposition
• An environment in which a episodes of high-energy
  transport give way to periods of low-energy
  transport gives normal graded beds

• Alluvial settings, with wandering channels that
  fill up and become overbank deposits
• Continental slopes with turbidity currents

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Carbonate Rocks

• Most carbonate rocks are entirely biochemical
  sediment, made up of the body parts of calcite or
  aragonite-precipitating organisms
• Deep-sea carbonate ooze is made of foram shells
• Reef carbonates are made of coral reefs (usually)
• Stromatolites are formed by carbonate
  precipitation by microorganisms

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Tour of sedimentary environments

• Let us go through each of the major categories of
  sedimentary environment, keeping in mind the
  relationship between observable processes in
  modern settings and the preserved features in
  ancient examples, and the ways in which
  observation of a sedimentary rock formation can
  be used to infer the type of setting and detailed
  information about it.

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Sedimentary environments Terrestrial

• I. Fluvial (rivers and streams of all kinds and
  sizes)
• a. Alluvial Fans
• We saw alluvial fans on the field trip. They
  form where drainages exit mountain fronts onto
  surrounding lowlands.
• Individual fans may merge to form a piedmont
  slope (like Pasadena).

In arid regions like California, sediment
transport on alluvial fans is dominated by debris
flows like mudslides and landslides, and by
periodic stream flows that divide the fan into
channel and overbank deposits. Sorting is poor,
but increases downstream grain size decreases
downstream sediments are often oxidized and poor
in fossils or organic matter.
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Sedimentary environments Terrestrial

• I. Fluvial

b. River systems Rivers are classified into
meandering or braided, most often. Braiding is
favored by high sediment load, steep gradients,
variable stream flow, and unstable poorly
vegetated banks. Meandering is favored by the
opposite.
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Sedimentary environments Terrestrial

• I. Fluvial

b. River systems Meandering rivers develop in a
fairly regular pattern by channel migration,
leaving a predictable sequence of cyclic,
fining-upward sedimentary deposits. Braided
river deposits are more chaotic leave somewhat
random deposits, since channels wander randomly
across the floodplain.
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Sedimentary environments Terrestrial

• II. Desert environment
• Deserts basins are basically alluvial fans,
  playas, and sand dunes. They may be dominated by
  wind transport or by fluvial transport restricted
  to rare, seasonal storms and floods
• Alluvial fans are debris flow and stream flow
  deposits (as above).

• Playas are dry or seasonal lake beds dominated by
  evaporites or fine-grained and finely laminated
  mudstones and siltstones.
• Sand dunes leave fascinating cross-bedded to
  massive sandstone deposits.

• Sustained deposition of wind-blown dust makes
  thick deposits of loess.

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Sedimentary environments Terrestrial

• III. Lacustrine (i.e., lakes)
• Lakes are special, compared to rivers and oceans,
  in several ways
• Small size (no large waves), absence of tides,
  and low currents makes lakes very low-energy
  sedimentary environments. Coarse sediments are
  limited to their margins.
• Lakes generally keep all sediment that arrives
  from a large drainage area, so sedimentation
  rates are high, often ten times higher than
  marine settings.
• Open lakes (with inlet and outlet streams) are
  usually fresh-water and generate only clastic
  sediments. Closed basin lakes become saline and
  lead to chemical-dominated sedimentation. Many
  lake deposits show cyclic alternations between
  closed and open conditions.

Annual variations in sediment supply (especially
if the lake freezes over each winter) are often
preserved in low-energy lacustrine depositional
environments as countable annual layers or varves.
Varves
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Sedimentary environments Terrestrial

• IV. Glacial and peri-glacial
• We saw some of the typical valley glacier
  deposits on the field trip. But there is more to
  the glacial environment than moraines and tills.
• Glaciers generate characteristic river deposits
  (frequently braided) and lake deposits
  (frequently varved) when they terminate on land,
  and characteristic marine deposits when they
  terminate in the ocean (dropstones). They move
  large boulders, but they also generate huge
  amounts of very fine rock flour that ends up as
  mud or loess.

Periglacial deposits, like most sedimentary
sequences, have several facies a basal till
deposited in front of the glacier is overlain by
moraines, lake sediments, glacio-fluvial
deposits, and finally loess.
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Sedimentary environments Marginal Marine

• I. Deltaic environment Deltas form wherever
  rivers empty into oceans or lakes. Much of the
  clastic load carried to the mouth of the river is
  deposited in a restricted area at or near the
  coast, forming a delta.
• Because deltas prograde outwards, they build
  deposits with reverse grading, coarsening upwards
  as the delta moves past a given location.
• The forces affecting sedimentation in a delta are
  fluvial, tidal, and waves, and different deltas
  display effects of dominance by different forces.

The Mississippi delta is fluvial-dominated Both
tides and waves are weak in the Gulf of Mexico,
so distribution of sediment is dominated by the
river itself, which forms long, relatively stable
channels (life span 1000 years) with levees
each channel narrows upwards until it pinches off.
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Sedimentary environments Marginal Marine

• I. Deltaic environment
• Flow at the mouth of a fluvial-dominated delta is
  controlled by the relative density of river
  outflow and ambient sea-water. Depending on river
  sediment load and temperature (and on ocean
  salinity and temperature), the flow may be
  hyperpycnal (river outflow denser), or hypopycnal
  (river outflow less dense).
• Hyperpycnal flow leads to turbidite deposits from
  sediment-rich flows along the bottom. Hypopycnal
  flow leads to uniform, well-sorted sediments
  since in this case settling is controlled by
  flocculation of fine particles.

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Sedimentary environments Marginal Marine

• The Ganges-Brahmaputra delta is tide-dominated
• Although the river outflow is higher and more
  sediment-laden than the Mississippi, the tidal
  range is large (about 4 meters). This type of
  delta breaks up into sand bars and channels
  oriented parallel to the tidal inflow-outflow
  direction. There is a large, intermittently
  exposed, tidal flat.

• The Sao Francisco river in Brazil is
  wave-dominated
• Wave-energy here is 100 times that at the
  Mississippi. Sediments reaching the mouth of the
  river are rapidly reworked and redistributed by
  longshore currents to build beaches, barriers,
  and lagoons, similar to stretches of coast where
  no river is present.

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Sedimentary environments Marginal Marine

• II. Beach-barrier environment
• Any continental margin where there is not a river
  mouth is likely to form a beach with a single
  shoreface or a beach-barrier island-lagoon system
  

• A beach produces a distinctively ordered set of
  recognizable facies, from dune sands through the
  surf zone, breaker zone and into deeper water.
• A barrier complex has a lagoon and often a swamp
  deposit behind the barrier.

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Sedimentary environments Marginal Marine

• II. Beach-barrier environment
• If a simple beach is prograding, i.e. building
  out to sea and depositing near-shore facies on
  top of distal facies, it might produce a
  stratigraphic column like this, coarsening
  upwards and hence clearly distinct from any river
  floodplain or continental slope deposit.
• Keep in mind the relationship between the lateral
  succession of environments at any constant time
  across a beach and the vertical succession of
  sediments shown in a column like this one.

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Sedimentary environments Marginal Marine

• III. Estuarine environment
• An estuary is a partly enclosed body of water at
  the mouth of a river. It may be part of a delta
  it may be the lagoon behind a barrier-island.
  Generally, estuaries must have a connection to
  the open ocean at least at high tide. They are
  environments of mixing between seawater and
  freshwater. Example San Francisco Bay

• IV. Tidal flats
• A wide, flat area of land between low-tide level
  and high-tide level is a tidal flat. These are
  common environments for deposition of carbonates
  and evaporites. They may be associated with
  deltas, beaches, or estuaries

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Sedimentary environments Marine

• I. Neritic environments
• This term refers to depths below wave-base and
  low tide, and above the shelf-slope break.
• At times of sea level highstand, when shallow
  seas cover the continental platforms, the neritic
  environment may encompass a significant fraction
  of the earths area.
• The neritic environment is where carbonate reefs
  are built.

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Sedimentary environments Marine

• II. Oceanic environments
• Continental slope deposits are characterized by
  turbidites, cyclic fining-upward sedimentary
  sequences that form by turbidity flows of
  suspended sediment down the moderately steep
  slopes of the continental slope.
• Deep sea (abyssal) deposits

There is a clear regional pattern with areas
dominated by chemical sediment (carbonate ooze or
siliceous ooze) or by a very slow accumulation of
fine clastic particles (pelagic clay). We will
develop the ocean chemistry and geology to
understand this pattern...