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Overview of Sedimentary Rocks
I. Basic Definitions
A. Denudation of the Earth's surface
- Dynamic interaction between uplift of Earth's crust and erosion of materials
B. Weathering - disintegration and decomposition of rock at or near the surface of the earth, fragmenting rock into particles
- Physical Weathering - physical breakdown
- Chemical Weathering - chemical weathering and dissolution
C. Sediment - fragments of rocks and/or minerals that are produced from the weathering of pre-existing rock
- Examples: clay, silt, sand, gravel, boulders
D. Erosion - incorporation and transportation of sediment by a mobile agent
- Agents of Transportation a. wind (e.g. sand dunes, atmosperic dust) b. water (rivers, coastal areas) c. ice (glaciers) d. gravity
E. Mass wasting - transfer of rock material downslope under the influence of gravity
II. Weathering - surface processes operating on rock from the earth's crust
A. Driving mechanism of rock weathering
- Igneous and metamorphic rocks a. Form at High temperature, High pressure
- Rock exposure at Earth's surface a. uplift and denudation
- Surface: low temperature / low pressure environment a. Water-rich environment
- High temp / high pressure rocks are unstable in water-rich, low temp/low press. environment of Earth's surface a. Weathering drives system to new equilibrium state
B. Physical or Mechanical Weathering
- defined: physical diminution or fragmentation of the rock a. sediment = fragmented rock particles b. sediment composition is controlled by the mineralogy of the parent rock that was subject to weathering
(1) e.g. weathering of granite produces sediment rich in feldspar and quartz
- Frost Wedging - process of alternate freezing and thawing of water/moisture contained in cracks and fractures of rock.
a. liquid water to ice: ~9% volume expansion b. "wedging" apart pieces of rock to form sediment
(1) e.g. TALUS SLOPES - found at base of cliffs or mountain fronts
- Unloading or release of overburden pressure - particularly common in weathering of granite bodies, e.g. Yosemite National Park. a. rock expansion as it is exposed and "unroofed" at the Earth's surface via denudation
b. Sheeting - rock breaking into concentric, onion-like slabs
- Thermal Expansion - rock heating / differential expansion a. Hot desert regions (e.g. Death Valley) b. Forest Fire
- Organic/Biologic Activity - activities of plants, animals, and humans can act as a weathering agent. a. root wedging b. burrowing organsims
C. Chemical Weathering
- Defined - complex chemical processes at the surface of the earth that alter the internal atomic structure of minerals by removing and/or adding elements.
a. Chemical agents of weathering:
(1) Water - H 2 O - universal solvent (a) dipolar molecule
(2) Carbon dioxide - CO 2 - Carbon dioxide dissolved in water (H 2 0), produces carbonic acid (H 2 CO 3 ),
(a) hydrogen in the acid readily displaces any metal ions in minerals and results in producing clay minerals (hydrous aluminum silicates).
b. Clay Minerals - the stable end product of chemically weathering silicate minerals ("igneous minerals")
(1) "Clay" - two uses of the term (a) a grain size term (very fine mud particles) (b) a mineral family i) hydrous layered minerals ii) stable minerals at Earths' surface
- Stable vs. Unstable Minerals
a. the higher the temperature of mineral formation, the less stable the mineral is chemically at the Earth's surface b. vice - versa, lower-termp. minerals are more stable
(1) e.g. quartz is very stable at Earth's surface (a) commonly forms sand at the beach
D. Rates of Weathering (How fast, over time, rocks weather)
a. compaction - as sediments accumulate and become buried with time, the weight of overburden compact the sediment
(1) clay minerals are cohesive (sticky) and serve as a binding agent for sediment (2) compaction also involves dehydration and hardening of sediment into rock
b. cementation - solutions carry ions into pours between sediments, with time ions may be precipitated as cements under appropriate chemical condition.
(1) Common cements include calcite, silica, and iron oxide.
C. Sedimentary Rock Types/Classification
Detrital vs. Chemical Sedimentary Rocks.
- Detrital Sedimentary Rocks: "Detrital" = Fragmented Origin.
a. Composition: sediments composed of quartz, clay, feldspars, and associated array of just about any other mineral in lesser proportions (e.g. amphibole, or any of silicate/igneous minerals, as well as recycled sedimentary rocks).
(1) E.g. Granite is weathered - produces quartz, and feldspars, plus mica: quartz is most resistant mineral and as a result is most common remnant product.
(2) the composition of the original rock (ig., sed., met.) that is weathered will have a direct influence on the composition of the sediment/sed. rock that results.
b. Texture of Sediment - size, shape and arrangement of sediment grains in the rock
(1) Sediment Size Classification (i.e. Average Diameters of Particles) Sediment Size Resulting Rock
Clay = < 1/512 mm = < 0.002 mm Shale or Mudstone Silt = 1/512 - 1/16 mm = 0.002 - 0.0625 mm Siltstone Sand = 1/16 - 2 mm = 0.0625 -2.0 mm Sandstone Gravel = >2.0 mm Conglomerate / Breccia Pebbles= 2.0 - 64 mm Cobbles = 64 - 256 mm Boulders = >256 mm
(2) Grain Shape
(a) Angular vs. Rounded Grains i) Rounded Grains = more transport / tumbling (b) Spherical vs. Non-spherical Grains
(3) Grain Sorting
(a) Degree to which grains are same size
i) poorly sorted sediment - different sizes ii) highly sorted sediment - same size
c. Detrital Rock Types (shale, sandstone, conglomerate)
(1) Shale - detrital sed. rock consisting of lithified clay and silt sized particles, very small particles < 1/16 mm (must use microscope to see particles),
(2) Sandstone - detrital sed. rock made up of sand sized grains.
(3) Conglomerate - lithified gravels (boulders, to pea sized sediment), often poorly sorted with finer sediment between gravel.
- Chemical / Biochemical Sedimentary Rocks -
a. Processes (1) Chemical sediments may be directly precipitated under high concentrations of ions in water (e.g. halite/rock salt), or (2) ions may be "fixed" by organisms living in the water in shells, accumulation of shells may then provide material for chemical sedimentary rock.
b. Texture of Chemical Sedimentary Rocks (1) Crystalline Texture (a) sugary appearance, crystals visible to eye (b) interlocking mineral crystals (2) Microcrystalline Texture (a) crystalls too small to see, need microscrope
c. Limestone - composed of a mosaic of the mineral calcite (CaCO3) and forms by either chemical precipitation or biochemical processes. Biochemical processes account for 90% of the limestones.
d. Dolomite - similar to limestone, but has Mg incorporated into CaCO3, CaMgCO3.
e. Chert - SiO2, microscrystalline silica deposited from solution in open ocean.
f. Evaporites - chemical sed. rock that result from precipitation of minerals via evaporation of water.
(1) E.g. halite/rock salt (NaCl) and Gypsum (CaSO4). Commonly associated with shallow seas and brine lakes (e.g. Salt Lake).
g. Ironstone - composed of iron-bearing minerals (hematite, limonite)
h. Coal - carbon-based rock derived from plant material (1) Bituminous coal - "soft coal", black (2) Anthracite coal - "hard coal", brownish (3) Lignite - "brown" immature coal (4) Peat - brown / compressed plant debris
- Other Terms
a. Clastic vs. nonclastic -
(2) Offshore (a) e.g. deep ocean b. Nonmarine - terrestrial (1) e.g. Rivers, Lakes, Glaciers
- Sedimentary environments influence style of process, which in turn influences the style of sedimentary deposit
- Sedimentary Facies a. Physical, chemical, and biological characteristics of sedimentary rocks b. Sedimentary facies are the "footprint" left by the sedimentary environment (1) Reconstruction of ancient sedimentary environments (2) "paleogeography" - reconstructing ancient surface systems of the the Earth.
C. Sedimentary Structures
- Depositional forms in sediment that are preserved in the rock record a. e.g. ripple marks, or worm burrows
- Sedimentary Processes Produce Sedimentary Structures / Sedimentary Structures are used to Reconstruct Sedimentary Process
D. Stratigraphy
- Study of the spatial and temporal relationships in sedimentary layers or strata
a. 3-D spatial geometry (1) Vertical vs. Horizontal Stratigraphic Relationships
b. Geologic time perspective: when? how long ago?
(1) Ordering of geologic events through time
V. Basics of Sediment Transportation
A. Energy vs. Mass of Sediment
- The greater the mass (e.g. grain diameter) of sediment, the greater the energy needed to transport
a. e.g. question: could wind transport a boulder the size of this room?
- Examples of Sedimentary Processes and Energy Source
a. Flowing Water / Rivers ---- gravity + climate b. Flowing Glacial Ice ------ gravity + climate c. Landslide / Rock Fall ------ gravity d. Wind Blown Sediment ---------climate / air flow
- Agents of Transportation: gravity, wind, water, ice
a. Energy relates directly to velocity of motion (1) stream velocity, wind velocity, etc.
B. Methods of Transporting Sediment
- Suspension - sediment is suspended in the body of the transporting medium a. e.g. blowing dust in the atmosphere b. e.g. brown clay and silt in the river after a rain storm
- Bedload / Traction - sediment is rolled and tumbled at the base of the transporting medium a. e.g. a cobble rolling along the bottom of a stream
b. Saltation: bouncing of particles via upcurrents, and trajectory fall under force of gravity.
- Dissolved Load - dissolved ions transported in a fluid medium a. e.g. dissolved salt in a river
VI. STRATIFICATION A. Stratification = Horizontal layering of sediment under gravity
- Strata = multiple layers of sediment (plural),
- Beds: Strata greater than 1 cm in thickness,
- Laminae: Strata thinner than 1 cm in thickness
B. Internal Stratification
- Massive - no evidence of internal layering within a sedimentary rock bed
VII. Sedimentary Structures
A. Sedimentary Structures primarily result of physical transportation processes, or biologic processes, or post-depositional processes or chemical processes.
- Paleocurrents - flowing fluids at the time of deposition mold sediment into characteristic shapes a. shapes can provide evidence of the direction of current flow at the time of deposition (maybe millions of years ago!!!).
B. Basic Examples of Sedimentary Structures
- Asymmetric Current Ripples - flowing water and currents move sediment in small-scale forms know as "ripples" of sediment
- Symmetric or Oscillation Ripples: symmetrical in cross-sectional form, developed under shallow water conditions within oscillating wave base (lakes or ocean beaches)
- Sole Marks: sedimentary structures found on the bottom or "soles" of beds, essentially comprised of positive relief casts derived from underlying sediment surface.
a. Flute Casts: current-formed erosion structure, bulbous cast formed by scouring of
sediment interface, bulbous end generally points up-current.
b. Load Casts: irregular knobs found on sandstones overlying shale beds. c. Tool Marks: groove casts; infilling of mold formed by dragging object across sediment interface
- Trace Fossils: i.e. Ichnology:
a. Tracks, trails, burrows b. Bioturbation: general mixing of sediment by dirt eaters.
- Imbricate Structure (Pebbles): under high energy flow conditions pebbles may take on inclined imbricate orientation, with inclination pointing upstream.
- Mudcracks and Raindrop Imprints: evidence of subaerial exposure of semi-consolidated sediment surface.
a. Clay shrinks as it dries, results in polygon fractures
VIII. Sedimentary Environments and Facies Analysis
A. Sedimentary Environments
- Defined by the complex interaction of physical, chemical and biological conditions under which sediment accumulates.
a. Characteristics of a given sedimentary environment yield a specific product of sediment and/or sedimentary rock.
B. Sedimentary Products
- "Facies": a body of sediment or sedimentary rock that display characteristic or distinctive textural, structural and compositional properties.
a. Facies by definition: readily detectable and discernable characteristics (chemical, physical or biologic).
**A sedimentary facies is the product of the sedimentary environment and its processes
on landward side of island
c. Estuarine/Lagoonal: similar to lacustrine, only in marginal marine setting, water chemistry is saline, quiet water sedimentation with bio-critter processes abundant.
d. Tidal Flat: low-lying coastal areas heavily influenced by tidal rise/fall of water, tides rework sediment, bio-critter processes abundant.
- Open Marine a. shallow ocean setting
(1) Shelf: encompasses sand-dominated shelf or carbonate-dominated shelf accumulations
(2) Organic Reef: biochemical build-up of carbonate structure comprised of living marine organisms, dominated by corals and algae + other critters
b. Oceanic: i.e. deeper ocean setting
(1) Slope: steep-gradient slope transitional to shallow-water shelf and deep ocean floor (a) Submarine canyons/submarine fan systems
(2) Deep-ocean Floor (a) Abyssal plain, quiet water mud accumulation
