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
We will concentrate on this type
Chemical sedimentary rocks - Chemical precipitates
Usually as the result of the evaporation of water - Ex. Salt (NaCl)
Organic sedimentary rocks
Limestone, all hydrocarbons - Coal, peat, oil, etc.
DIGRESS TO: Physical vs. Chemical weathering
Clasts - derived from physical (and chemical) weathering processes
Smaller solid particles
Wide range of sizes, from silt to boulders
Clay minerals
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
Clay formation forms small, sheet-like minerals (look like the micas)
Near-surface, low temperature environments
Hot and humid works best - chemical weathering!!
Note: chemical weathering also results in "ions" which are "held in solution"
Can result in chemical sedimentary rocks
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 deposition of organic sediments
Water plays an important role in most aspects of sedimentary rocks
From weathering and erosion to transportation and deposition
DIGRESS TO: Q=AV
Deposition occurs in a wide variety of locations
Basically, any low spot is a potential depositional environment
Two major divisions - Continental and marine
Also there are inter-tidal (transitional) environmnets
Important factors include:
Sorting - 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
Rivers and streams
Riverbed - size directly related to energy of the stream
Can be poorly sorted (all different sizes) or well sorted (all the same size)
Floodplain - Flat surfaces adjacent to a river
Represents sediments deposited during flooding
Usually well sorted
Glaciers
Non-turbulent flow (unlike rivers)
Can and will carry all sizes of material
Commonly poorly sorted, but not always!
Lakebeds
By nature a temporary feature
A sure trap for sediments (because Q=AV)
Evaporites - common to arid regions with seasonal lakes (playas)
Ex.: Bonneville Salt Flats
Alluvial Fans
Generally arid and semi-arid climates
Deltas
Essentially an underwater alluvial fan
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
Always well sorted (unless contaminated by other processes)
Small stuff only - no boulders!
Sand dunes
The seafloor is the final resting place for the majority of weathered rock materials
Please refer to Strickler's 3rd Law of GeoFantasy
Remember - "The earth breaks what it makes and puts it in the ocean"
Factors affecting deposition include:
Distance from shore
Related to energy
Depth of the water
These result in 3 broad zones of deposition
Relatively good sorting within each zone
In general, the shore and shelf contain the majority of "terrigenous" sediments
Gravel ---> Sand ---> Silt ---> Clay ---> Carbonate Ooze
The 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
The Continental Shelf
Much broader then the shore zone
Most terrigenous sediments end up here (sooner or later)
Mostly silt & clay
Locally coarser material related to times of higher energy
Carbonate deposits also common
Inorganic and organic deposits of CaCO3 - Limestone
Common to "shallow, warm water"
The Abyss - much of this ends up being subducted
Mostly very fine grain sediments
Water depth important in which is deposited
Calcareous to siliceous to terrestrial clay ooze
As depth increases and/or temperature decreases
Stratification - the most common and distinctive
Most sedimentary rocks are composed of particles which settle through water (or air)
Generally quiet water deposition results in nearly horizontal layers
Differences through time result in visible layering
Variation in clast size
Variation in clast composition/mineralization
Special enhancements to visible layering
Graded Bedding
Cross Bedding
Size and Roundness of the clasts
Usually reflects transport distance and/or time in transit
Long distance = smaller and rounder clasts
Color
Most igneous rocks are some shade of gray
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
Dark to black color commonly the result of organic material
EXAMPLE: Black shale
Fossils - the classic sedimentary feature
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)
But any evidence is considered a fossil
Soft body molds
Footprints
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
Lithification - "the process of converting soft, unconsolidated sediments into hard rock"
Two major factors contribute to the lithification process
Remember: we are usually starting with a loose pile of debris, which is saturated with water
Compaction
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)
"Lateral change in the basic properties of a sedimentary horizon"
DIGRESS TO: Time-Stratigraphic Horizons
EXAMPLE: Conglomerate into sandstone into siltstone into shale
Reflect local variations in the depositional environment
DIAGRAM: on board
Transgression / Regression
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
Three major types of unconformities
Angular Unconformity
Easiest to recognize - describe
Non-parallel beds above and below
Represents: deposition, uplift, deformation, erosion, subsidence, and new deposition
Disconformity
Parallel beds above and below
Can be real tough to recognize
Nonconformity
Sedimentary beds overlying igneous or metamorphic rocks
Represent immense time periods
As we said, there are 3 general categories
Clastic/fragmental; Chemical precipitates; and Organic
Distinction between different types often fuzzy in reality
Clastic Sedimentary Rocks - true secondary rocks
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 and 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 results in greater surface area for chemical attack
Classification generally based on the size of the clasts
Conglomerate - cemented gravel
Usually poorly sorted, calcium or silica cement
Sandstone - Sand-sized clasts
Often interbedded with shale or conglomerate (facies changes)
Indicate near shore marine - your basic beach
Calcium or silica cement
Which one is present determines hardness
Friable - breaks up easily due to weak cement
Compositional differences
Classic sandstone is generally quartz - final weathered product
Graywacke - "dirty sandstone"
Generally dark in color
Quartz, feldspar, mafics, lithic fragments all present
Indicates very short distance of transport
Silt & clay sized clasts
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
Chemical sedimentary rocks
Evaporites
Result from the evaporation of water
Halite (salt), Gypsum (sheetrock)
Carbonates
Limestone - calcite (CaCO3)
Travertine
Hot springs deposits
Organic sedimentary rocks
Hydrocarbons
Coal - lithified plant and animal remains
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
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° of the equator
Silica based rocks
Chert - "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?)
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