Revised 8 / 06 (Monroe 6th ed.)
Including...
Origin of Sedimentary Materials
Classification of sedimentary rocks
Click here for online mineral and rock ID charts
We've studied igneous rocks & the minerals of which they are composed
Basement rocks
Most are covered by a thin veneer of debris
Consolidated into a "rock" through slow-acting processes
Usually involving pressure and fluid penetration
Relatively simple to understand
Relatively near-surface processes
As opposed to igneous & metamorphics, which usually occur at depth
Secondary (or derived) rocks
Several main categories
Clastic sedimentary rocks - The classic sedimentary rock
Accumulations of debris derived from the disintegration of pre-existing rocks
DIGRESS TO: Terrigenous sediments
Chemical sedimentary rocks - Chemical precipitates
Usually as the result of the evaporation of water
Ex. Salt (NaCl); Gypsum (CaSO4 · 2H2O)
Organic sedimentary rocks
All hydrocarbons
Coal, peat, oil, etc.
The distinction between these three categories can get pretty fuzzy at times
Ex. Limestone, chert
Hard rocks vs. Soft rocks
DIGRESS TO: Physical vs. Chemical weathering
Click here for additional information on water, weathering, and erosion (RCC)
Click here for additional information on surface processes (GPHS)
Clasts - derived from physical (and chemical) weathering processes
Smaller solid particles
Derived directly from the source area
Reflect lithology of the source area
Wide range of sizes, from silt to boulders
Chemical processes can result in the relative enrichment of more resistant (or inert) minerals
Clay minerals
I'm not a clay kind of guy
Extremely complex mineralogy
My understanding is minimal
Easy to get confused by the term
The term "clay" refers to both a size and a mineral family
A clast can be clay size without being clay
DIGRESS TO: "clay the size" vs. "clay the mineral"
Clay formation forms small, sheet-like minerals (look like the micas)
Lots of different clay minerals
Which mineral is formed reflects primary lithology and environment
Can change to a different mineral if moved to a different environment
Downslope? Downstream?
Near-surface, low temperature environments
Hot and humid works best
Water is a universal solvent (H·OH)
Tends to work parallel to Bowen's Reaction Series
The higher temperature minerals are more susceptible to chemical weathering
Therefore, especially hard on the mafics and feldspar
To repeat what was mentioned above
Chemical processes can result in the relative enrichment of more resistant (or inert) minerals
Describe "decomposed granite"
Ions
Chemical weathering also results in "ions" which are "held in solution"
The solution is usually water
Remember: Water is a universal solvent (H·OH) and will play merry hell with anything "over the course of geologic time!"
Some elements will dissolve and be held in solution
Ex. salt, sugar
DIGRESS TO: Solution (ions) vs. Suspension (clays)
Both make fundamentally different types of sed. rocks
Common ions include: Ca+2, Na+, CO3-2, Cl-
DIGRESS TO: What do the superscripts mean?
Atomic structure & the role of the electron
These ions are responsible for the "mineral taste" in some water
Therefore, we can tell that iron and sulfur must also be common
If the amount of ions increases relative to the amount of water, minerals can precipitate
Ex. salt (Na+ + Cl- -> NaCl)
Saturation is the key
An undersaturated solution can become oversaturated in 2 ways
Increase the dissolved ions
Decrease the solvent (water)
This is more common (probably)
Can be initiated by the evaporation process
Organisms can also extract the ions directly from the water
Use them to build shell material
Ex.: Ca+2 + CO3-2 -> CaCO3
Can result in extensive deposition of calcium or silica sediments
(Monroe; fig. 7-3, pg. 202)
Water plays an important role in most aspects of sedimentary rocks
From weathering and erosion to transportation and deposition
DIGRESS TO: V=Q/A
Deposition occurs in a wide variety of locations
Basically, any low spot is a potential depositional environment
On both regional and local level - expand
Three major divisions - Continental deposition, marine deposition, and transitional (inter-tidal)
Infinite possible combinations of environments and materials
Results in infinite possible sedimentary rocks
Fortunately, most fall into one of several common environments
And as we already know from our study of igneous rocks, most of the rocks start with a similar chemistry
It can still be tough to recognize the depositional environment
DIGRESS TO: This is the ultimate goal of the study of sedimentary rocks
The names are important, but only insofar as they provide clues to how they got there
The interpretation of earth's history is the purpose of any geological examination
In any event, this will usually take lots of field work
And the examination of lots of different rocks
As well as copious amounts of lubricant to make sense of the data!
Multiple Working Hypotheses
Need to keep an open mind
Several working together with different ideas can be good
As can a "Devil's Advocate" to keep the group from getting cocky.
It's far too easy to only see those units and/or features which support the currently favorable model
Important factors include:
Sorting - key to interpreting the depositional environment
"The degree in similarity in particle size in a sediment"
Important in the clastic sediments
Particle size
Important in the clastic sediments
Particle composition
Important in chemical and organic sediments
Sediments trapped on land
Lots of different environments
Usually can be viewed directly, so relatively easy to understand
Rivers (click here for additional information on streams from RCC and Secondary level)
Riverbed - size directly related to energy of the stream
Can be poorly sorted (high energy) or well sorted (low energy)
Floodplain - Flat surfaces adjacent to a river
Represents sediments deposited during flooding
All different scales
Major floodplains - Nile, Amazon, Mississippi, etc.
Minor floodplains - localized along selected stretches of most streams and rivers
Sorting varies in response to local conditions
Usually well-sorted, but not always
Sorting can be a VERY local phenomena
Both laterally and longitudinally
Draw a X-section of a stream bed at a meander
Before, during, and after flood stage
Glaciers (click here for additional information on glaciers from RCC and GPHS)
Non-turbulent flow (unlike rivers)
Can and will carry all sizes of material
Commonly poorly sorted, but not always!
Alpine vs. continental glaciation
Distinctive types of deposits
Lakebeds
By nature a temporary feature
A sure trap for sediments - Q=AV
Will certainly fill in "over the course..."
Usually well-sorted "locally"
Variations in grain size related to distance from inflow
Diagram: long section
Example: mouth of Carberry Creek during January 1997 flood
Evaporites - common to arid regions
Ex.: Bonneville Salt Flats
Also can be marine in origin
Alluvial Fans
Generally arid and semi-arid climates
Q=AV
Generally poorly sorted
Deltas
Essentially an underwater alluvial fan
Again, Q=AV
React to the base level of a given river
Lakes are temporary base levels
The ocean is the "ultimate base level"
Eolian Deposition
Wind can also play a role in the erosion, transportation, and deposition of sediments
Can affect wide areas
Not confined to a defined channel like a river is
Can move vast quantities of material
Always well sorted (unless contaminated by other processes)
Small stuff only - no boulders!
Sand dunes
Combination of:
Lots of sand sized pieces (and smaller)
Little or no vegetation to hold material in place
Strong winds
Loess deposits - fine dust and silt
Common along margins of continental glaciers
The seafloor is the final resting place for the majority of weathered rock materials
Factors affecting deposition include: (Monroe; fig. 7-12, pg. 211)
Distance from shore
Depth of the water
Physical & chemical properties of the water
Variety of plant and animal life
These result in 4 major zones of deposition
Relatively good sorting within each zone
DIAGRAM: X-section of seafloor (should be a review from G-101)
Shore Zone
The shore acts like a channel and restricts the "flow" of the ocean
High energy zone
Coarse sand and gravel are deposited here
Smaller material stays in suspension/solution and moves offshore
Continental Shelf
Much broader than the shore zone
Most terrigenous sediments end up here (sooner or later)
Mostly silt & clay
Some coarser material
Has to be related to times of higher energy
Storms? Seismic disturbances?
Carbonate deposits also common
Inorganic and organic deposits of CaCO3
Limestone
Common to "shallow, warm water"
Topographic highs on shelf (banks, seamounts)
Minimal contamination by terrigenous sediments
Continental Slope
Debris collects in canyons traversing the slope
"Perched pre-turbidites"
Can be quite poorly sorted
Abyssal Plains
Coarse sediments common at base of slope
Turbidites
Very poorly sorted
Set in motion by storms and quakes
Mostly very fine grain sediments
Calcareous and siliceous oozes
Water depth and temperature generally determine which is deposited
Calcareous to siliceous to terrestrial clay ooze
Basically the inter-tidal zone
Fluctuates between marine and continental deposition
Lithification - "the process of converting soft, unconsolidated sediments into hard rock"
(Monroe; fig. 7-4, pg. 203)
DIGRESS TO: Hard rock vs. soft rock geology
Two major factors contribute to the lithification process
Remember: we are usually starting with a loose pile of debris, which is saturated with water
Pressure
Weight of overlying sediments results in compaction
Reduction in pore space
Interstitial fluids (water) may be removed
Cementation - "The most significant process"
"The deposition from solution of a soluble substance"
Fills the interstitial pore spaces
Cements the grains together
Three common types of cement
Calcium
Probably the most common
Easily dissolved in groundwater
H20 + CO2 = H2CO3 (Carbonic Acid): will dissolve calcium and put it into solution
Silica - less soluble than calcite
Will form a much harder and stronger cement
Iron Oxide (Fe2O3)
Ferrocrete
Stratification - the most common and distinctive (Monroe; fig. 7-13, pg. 212)
Most sedimentary rocks are composed of particles which settle through water (or air)
Generally quiet water deposition results in nearly horizontal layers
Layers are called strata (stratum: singular) (like data and datum)
Greater than 1cm thick: beds
Less than 1cm thick: laminae
Differences through time result in visible layering
Variation in clast size
Direct result of fluctuating energy within the system
Variation in clast composition/mineralization
Affects color, size, amount
EXAMPLE: Limestone/Shale
Shale during storms, seismic events, turbidites, etc.
EXAMPLE: Flood into a lake
Coarser during flood (high energy)
Example: mouth of Carberry Creek during January 1997 flood
EXAMPLE: Discontinuous or intermittent deposition
Climatic or tectonic changes
Graded Bedding (Monroe; fig. 7-15, pg. 214)
Larger pieces on bottom, finer at top
Common when a poorly sorted debris is dumped into quiet water
EXAMPLES: Storms into a lake, turbidites
Cross Bedding (Monroe; fig. 7-14, pg. 213) (Monroe; fig. 7-19, pg. 218)
Non-horizontal bedding
Moderate to steeply-dipping layers
Common in arid environments - sand dunes
Wind moves grains up the windward slope, where they fall off the edge
"Slip-face" - the steep downwind side
At the Angle of Repose
"The maximum slope at which grains will remain stationary without sliding down the slope"
Fluctuating wind direction can result in thick sequences with complex, changing patterns
Also in deltaic environments
Topset, bottomset, and foreset beds (at the angle of repose)
Cross-bedding can also be the result of uplift, tilting, erosion, subsidence, and additional horizontal deposition
Roundness of the clasts (Monroe; fig. 7-5, pg. 205)
Usually reflects transport distance and/or time in transit
Long distance = rounder clasts
Obviously, composition of the material will affect this to some degree
Color
Most igneous rocks are some variation of grey
Sedimentary rocks can be quite colorful
Different pigments can fill the void spaces between the clasts
Iron - very common
Results in shades of red, brown, pink, or yellow
Also purple, green, or black
Dark to black color commonly the result of organic material
EXAMPLE: Black shale
Also, very fine grained pyrite (with or without gold) can cause the rock to appear black
Origin of color often uncertain
Can be carried to depositional site with sediments
Or added later (exotic)
Or produced chemically in place
Mud cracks (Monroe; fig. 7-18, pg. 215)
Form in wet clays and muds which are dehydrated
Loss of water causes the silt to contract and crack
Similar to columnar jointing in thick basalt flows
Basalt - from cooling
Mud - from dehydration
Well developed mud cracks indicate repeated wet/dry cycles
Common in shallow, seasonal lake beds
NOT common in tidal flats
Drying period not long enough
Ripple marks (Monroe; fig. 7-16, pg. 214)
Develop perpendicular to direction of current
Commonly asymmetrical in cross-section
Slip-face at the Angle of Repose (like sand dunes and drumlins)
Symmetrical ripples are called "Oscillation Ripples"
Sharper crests with gently rounded troughs
Indicate alternating directions
EXAMPLE: Surface waves along a beach (marine or non-marine)
Can provide clues to ancient paleo-currents
Originally assumed to be restricted to shallow water
Recent studies on the seafloor have identified them at depths of several thousand meters
Fossils - the classic sedimentary feature (Monroe; pgs. 216-217)
Evidence of once-living organisms
Characteristic of many sedimentary rocks
Not igneous or metamorphic
Most relate to remains of "hard body parts" (bones, shells, teeth)
Any evidence is considered a fossil
Soft body molds
Footprints
Petrification
Coprolites
Some amazing parts have been preserved
Jellyfish, compound eye parts, dragonfly wings
Clues to depositional environments
EXAMPLE: Clam fossils pretty much indicate marine deposition, etc.
Used to establish the Relative Time Scale (Monroe; Fig. 1-16, pg. 24)
Concretions
Nearly spherical solid bodies found in sedimentary rocks
"Composed of material that solidified around a small, hard nucleus after the sediment was deposited"
Any small, hard particle will work
CaCO3 common
As we learned last term, sedimentary rocks are usually deposited in horizontal layers
Also learned that tectonic forces can significantly change the orientation of rocks at and near the surface of the earth
In a situation where we are trying to unravel the tectonic history of an area...
It can be pretty tough to come up with definitive clues
Igneous rocks provide few, if any
Except for flows, no original preferred orientation
Metamorphics are even worse
The very process which forms them will usually complicate (or destroy) any primary orientation features
Sedimentary beds are our only real method of figuring out the degree of tectonic deformation in a regional or local area
Most studies are based on the premise of original horizontality
How far has a sedimentary unit been rotated from the horizontal
In extreme cases, the tilting can be of such an extent that the beds are completely overturned
Even back to a horizontal position!
It can be very important to determine tops from bottoms of sedimentary beds
Can be nearly impossible to determine, and is most often inconclusive without extensive field mapping and laboratory work
Many methods can be used as appropriate to the situation
Graded bedding
Cross-bedding (concave up)
Scour & Fill
Baked zones (inter-layered flows)
Ripple marks
"Lateral change in the basic properties of a sedimentary horizon"
(Monroe; fig. 7-12, pg. 211)
DIGRESS TO: Time-Stratigraphic Horizons
EXAMPLE: Conglomerate into sandstone into shale
Reflect local variations in the depositional environment
DIAGRAM: on board
Transgression / Regression (Monroe; fig. 7-12, pg. 211)
The sedimentary record is not complete
Long term gaps in the sedimentary record indicate periods of non-deposition and/or erosion
We actually can see only a small part of the earth's history in sedimentary rocks
The gaps clearly represent more time than do the beds themselves
Angular Unconformity (Monroe; fig. 9-9, pg. 272)
Easiest to recognize - describe
Non-parallel beds above and below
Represents: deposition, uplift, deformation, erosion, subsidence, and new deposition
Disconformity (Monroe; fig. 9-8, pg. 271)
Parallel beds above and below
Can be real tough to recognize
Nonconformity (Monroe; fig. 9-10, pg. 273)
Sedimentary beds overlying igneous or metamorphic rocks
Represent immense time periods (EXPLAIN)
As we said, there are 3 general categories (Monroe; Table 7-1, pg. 207)
Clastic/fragmental; Chemical precipitates; and Organic
Distinction between different types often fuzzy in reality
Click here for online mineral and rock ID charts
Derived from the breakdown of pre-existing rock at the surface of the crust
Most sedimentary rocks are clastics
Quick review:
Surface weathering produces small clasts (physical / chemical processes)
As soon as a clast (at whatever size) is broken from bedrock, it is involved in the erosion and transport process
Gravity is the ultimate driving force here
Clasts moved downslope to creek/river systems
Carried downstream to a suitable depositional environment
Weathering can continue during transport
Both physical and chemical
Its reasonable to assume that physical weathering dominates in the headwaters at higher elevations
Chemical weathering takes on a more active role at lower elevations
Smaller clast size = greater surface area for chemical attack
Erosion or deposition is controlled by the energy of the system
F=MA and Q=AV: review these?
Classification generally based on the size of the clasts
Also important as modifiers of the general size designation:
Sorting
Composition
Degree of rounding
Large clasts
Conglomerate - cemented gravel (Monroe; fig. 7-5a, pg. 205)
Usually poorly sorted, calcium or silica cement
Well rounded
Common to upper portions of rivers and high energy shorelines
Breccia (Monroe; fig. 7-5b, pg. 205)
Same idea as conglomerate except angular fragments
Indicated deposition close to source area
Fault breccia - explain
Sand-sized clasts
Sandstone (Monroe; fig. 7-6, pg. 206)
Often inter-bedded with shale or conglomerate
Review facies changes
Indicate near shore marine - your basic beach
Often well rounded - long transport distance
These represent the final product of the weathering process
Mafics & feldspars gone, only quartz remains
Can also occur as arid eolian deposits
Commonly angular
Both types generally well sorted (especially the eolian deposits)
Calcium or silica cement
Which one is present determines hardness (induration)
Friable - breaks up easily due to weak cement
Compositional differences
Arkosic sandstone
Quartz and feldspar
Shorter distance of transport?
Graywacke - "dirty sandstone"
Generally dark in color
Quartz, feldspar, mafics, lithic fragments all present
Indicates very short distance of transport
Deposition adjacent to source area
No time for chemical weathering
Commonly very poorly sorted
Common to flanks of island arc systems
Common filling of trenches
A real mess when exposed to the surface
All the unaltered materials rapidly alter to clays
Silt & clay sized clasts (Monroe; fig. 7-7, pg. 207)
Lots of names based on size of clasts
Siltstone, claystone, mudstone
Shale works as a general descriptive name for most of them
Usually impossible to determine composition of clasts due to small clast size
Shale is composed primarily of clay minerals
With clay sized clasts of quartz, feldspar, and other minerals
Commonly exhibits fissility
The ability to split along closely spaced sub-parallel bedding planes
A result of the platy nature of the clay minerals
Mud & siltstone may not have fissility
Due to lack of clay minerals
Tend to break into chunks (like most rocks)
Evaporites (Monroe; fig. 7-9, pg. 209)
Result from the evaporation of water
Dissolved solids (ions) precipitate as minerals as the fluids reach saturation
EXAMPLE: Halite (NaCl)
Evaporation of sea water, inland seas
Repeated flooding can produce thick deposits
Also saline lakes
EXAMPLE: Great Salt Lake, Bonneville Salt Flats
Any lake with no outlet will become saline over time
DIGRESS TO: Shattered fence posts at Bonneville
Acquisition and control of salt deposits has historically been of great importance
Empires have risen and fallen due to control of salt!
"Worth his salt" and "Salary" both indicate importance of this substance
Currently in use as storage areas for sensitive materials
WHY? (No moisture and relative tectonic stability)
Gypsum (CaSO4 · 2H2O): The Hidden Hydrosphere!!
Also common due to evaporation of seas and saline lakes
Anhydrite (CaSO4) - Gypsum without the water
Lots of other evaporites - these are only the most common
Formation of evaporites
Different minerals precipitate in a specific order
Somewhat analogous to Bowen's Reaction Series
Controlled by concentration/saturation
Gypsum & Anhydrite form first
After approx. 80% of the water has evaporated
Halite after 90%
Sodium, magnesium, and potassium salts after that
Thick deposits are really hard to explain!
1000 feet of water will produce
0.3' gypsum; 11.5' halite; 3' other salts
How do we get 1000' thick gypsum deposits?
Repeated flooding with partial evaporation?
Secondary remobilization (after deposition)?
A full understanding of evaporites will take more work!
Carbonates (Monroe; figs. 7-8, pg. 208)
Calcite (CaCO3) makes Limestone
Aragonite - an "unstable" form of CaCO3
Dolomite (CaMg(CO3)) makes dolomite (dolostone)
Seawater is generally concentrated with CaCO3
Minor changes in temperature or composition can lead to precipitation
There is a continuing argument concerning the importance of inorganic processes to the formation of the world's great limestone deposits
Most agree that organic processes are most important - see below
Travertine
Hot springs deposits (Monroe; fig. 16-24, pg. 522)
Beautiful building and decorative stone
Tufa (or calcareous tufa)
Springs and lime-saturated lakes
May be related to lime-secreting algae
Oolites (Monroe; fig. 7-8, pg. 208)
Small spherical grains of CaCO3
Called ooliths (From the Greek: oo=egg; lith=stone)
Form around a nucleus - like a pearl
Grains of sand washing around in a bottom current
Tidal areas of warm, shallow seas
The origin of dolomite
A problem - we don't see it forming anywhere at this time
It is common in the geologic past
Primary precipitation vs. derived from limestone or aragonite
By the percolation of magnesium rich solutions through limestone
Many dolomites obviously Calcium replacement by Magnesium
Others well layered with limestone
Would require selective replacement of individual beds
Most favor replacement
Hydrocarbons
Oil and Gas (Monroe; fig. 7-20, pg. 220)
Commonly occur in sedimentary rocks
Coal - lithified plant and animal remains (Monroe; fig. 7-11, pg. 210)
Compacted swamps, etc.
Convert to coal in an anaerobic environment
Calcium based rocks
Limestone the most common
Most limestone is organic as opposed to chemical in origin
Foraminifera
Microscopic plants & animals extract CaCO3 from seawater and use it to build shells
These will settle to the seafloor and accumulate into Limestone deposits
Larger organisms also extract CaCO3 for shells which can accumulate on seafloor
Coquina - lithified shell debris
Can be reworked in the sea currents - broken and moved around
Are these then clastic sedimentary deposits?
Reefs (Monroe, pg. 380)
Made largely of corals and carbonate secreting algae
Like shallow, warm waters which are agitated by wave action
High in nutrients (for food)
Environment essentially free of terrigenous sediments
Can result in extremely pure limestone deposits
Commonly +/- 30 deg. of the equator
Complex structures
The reef itself - form by organisms building upward
Seaward - clastic carbonate debris
Broken off the reef by wave action
Landward - Lagoon with carbonate & clastic muds
Silica based rocks (Monroe; figs. 7-10, pg. 209)
Chert - the "general name used to cover many types of dense, hard, non-clastic, microcrystalline siliceous rocks"
Flint - dark color from included organic remains
Uniform texture - conchoidal fracture
Jasper - reddish flint
Sinter - hot springs (like travertine)
Thick beds of chert are found throughout the geologic record
Some may result from direct chemical precipitation
White smokers at spreading axes
Most are thought to be organic (like the carbonates)
Microscopic plants & animals extract silica from seawater and use it to build shells
These will settle to the seafloor and accumulate into chert deposits
Larger organisms do not use silica to build shells
WHY? (Not as much in the seawater? Less soluble so harder to extract?)
Banded Iron Formation (Monroe; figs. 7-22, pg. 221)
Much more on these in G-103 when we look at the evolution of earth's atmosphere
Click here for online mineral and rock ID charts
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