Sedimentary Rocks Flashcards
Geology Exam 2
Formed at Earth’s surface by cementing together weathered fragments of preexisting rock, fragments of shells, accumulation of organic matter, or precipitation of minerals dissolved in water.
Sedimentary Rocks
What rocks often preserve evidence of their mode of origin in the nature of the sediment grains that comprise the rock and the cements that bind those grains together?
Sedimentary Rocks
Basement Rocks
Metamorphic & Igneous
underneath Sedimentary Rocks
Sedimentary rocks occur only in the ________ part of the crust?
Uppermost
What provide the raw materials (particles and dissolved ions) for all sedimentary rocks?
Physical and chemical weathering
Physical Weathering
breaking rocks into smaller pieces via physical processes
Wedging (or Frost Wedging)
Water expands when it freezes, exacerbating pre-exisiting cracks
Biological Wedging
Roots can cause cracks to grow bigger
Salt Wedging
Growth of salt crystals creates bigger fractures
Chemical Weathering
The change of the chemical structure of the mineral(s)
Dissolution
Water molecules remove ions from grain surfaces
Four sedimentary rock classes
- Biochemical
- Clastic
- Chemical
- Organic
Clastic (or detrital) sedimentary rock
Consist of mineral grains, rock fragments, and cementing material.
Erosion
removal of grains from parent rock
Transportation
dispersal of solid particles and ions by gravity, wind, water and ice.
or
ions dissolved in groundwater.
Deposition
settling out of the transporting fluid
Lithification
Final stage in transformation sediments into solid rock.
Minerals (often quartz or calcite) precipitate from groundwater into pore spaces. This _______the loose sediments together.
cement glues
Sedimentary rocks are classified on the basis of
texture and composition
Clast size
A measure of the size of fragments or grains. Size ranges from very coarse to very fine (gravel, sand, silt, and clay). As transport distance increases, grain size decreases.
Clast composition
Refers to the mineral makeup of sediment grains.
Mineral composition yields clues about the original source rock. A variety of different clast compositions (or a lack thereof) hints at source area and transport processes
Angularity
the degree of edge or corner smoothness
indicator of the amount of grain abrasion during transport
Fresh detritus is usually angular and nonspherical
Sphericity
the degree to which the shape of a clast approaches that of a sphere.
Indicators of the amount of grain abrasion during transport.
Grain roundness and sphericity increase with transport.
Sorting
Is a measure of the uniformity of grain sizes in a sediment population. Degree of sorting increases with transport distance.
The most common cements?
Quartz and calcite
Cements are derived from
Dissolved ions in groundwater that move through the pores present in the original sediment.
Breccia
Coarse, angular rock fragments.
Angularity indicates the absence of rounding by transport, hence, deposited relatively close to clast source
Conglomerate
Rounded rock clasts.
Clasts rounded as flowing water wears off corners and edges. Deposited farther from the source than breccia
Arkose
Sand and gravel with abundant feldspar. Commonly deposited in alluvial fans. Feldspar indicates short transport and arid conditions
Sandstone
clastic rock made of sand-sized particles. Quartz is, by far, the most common mineral in sandstones
Silt-sized sediments are lithified to form ______.
siltstone
Clay-sized particles form ________.
mudstone or shale
Where are fine clastics found?
floodplains, lagoons, mudflats, deltas, deep-water basins.
Biochemical sedimentary rocks
Made of sediments derived from the shells of once living organisms. Hard mineral skeletons accumulate after the death of the organisms.
Limestone
A sedimentary rock made almost entirely of calcite or aragonite.
These minerals are the most common materials used by organisms that make seashells
Often preserves the shells of fossil organisms, sometimes in great abundance.
Depositional environment of Biochemical Limestone.
Warm, tropical, shallow, clear, O2-rich marine water.
Chert
Rock made of cryptocrystalline quartz derived from opalline silica (SiO2) from the skeletons of some marine plankton.
Can occur in beds or as nodules. It has all of the properties of quartz, including hardness and conchoidal fracture.
Chert
Organic sedimentary rocks
Made of organic carbon, the soft tissues of living things.
A combustible organic sedimentary rock, made from the altered (compressed, heated) accumulated remains of fossil land plants.
Coal
Chemical sedimentary rocks
Comprised of minerals precipitated from water solution. They have a crystalline (interlocking) texture developed from initial crystal growth from solution (which may be recrystallized during burial).
Classes of chemical sedimentary rocks
evaporites
travertine
dolostone
replacement chert
Evaporites
Are derived from evaporation of large volumes of sea or lake water.
Minerals include halite (rock salt) and gypsum.
Travertine
Calcium carbonate (CaCO3) precipitated from ground water where it reaches the surface. This process occurs in thermal (hot) springs and in caves.
Dolostone
Limestone that has been altered by Mg-rich fluids. Dolostone looks like limestone, except it has a sugary texture, and often weathers to a buff, tan color.
Replacement chert
Differs from biogenic chert in that it does not originate from living organisms.
Varieties of replacement chert
Flint is colored black or gray from organic matter.
Petrified wood has original wood grain preserved by silica. Agate forms concentrically layered rings.
Sedimentary structures
Features imparted to sediments at (or near) the time of deposition.
The boundary between two beds
Bedding plane
_____ have a definable thickness that can be uniform or that can pinch and swell or taper to a zero edge.
Beds
Formations
Distinct rock units that are so unique that they can be recognized—and mapped—over large regions.
Geologic maps display the distribution of formations.
Cm-scale ridges and troughs that develop perpendicular to flow in sandy sediments.
Ripple Marks
Larger scale (50 cm to over 100 m) versions of ripple marks.
Dunes
Dunes occur in
Streams and in desert or beach regions from water- or wind-transported sand.
Dunes often preserve large internal ______
Cross beds
Cross beds
Sand moves up the gentle side and piles up at the crest. Then, it slips down the steep face. The slip face moves downcurrent and is buried by the next avalanche of sand. The slip faces are preserved as cross beds
Turbidity currents
Formed in deep basins that receive periodic pulses of turbid water. Such pulses might result from an earthquake shock loosening sediment on a slope.
This process forms graded beds (coarse to fine upward).
Mudcracks
polygonal desiccation features in wet mud. They indicate alternate wet and dry terrestrial conditions.
Evidence of past life
Fossil footprints
Depositional environments
Locations where sediment accumulates
Environments range from terrestrial to coastal to marine.
Terrestrial environments
Are those where sediment is deposited above sea level
Glacial environments (terrestrial)
Sediments are created, transported, and deposited by the actions of moving glacial ice. Ice carries and dumps every grain size.
Glacial till
A common feature of Glacial environments, a poorly sorted mixture of all grain sizes, gravel, sand, silt, and clay.
Mountain stream environments (terrestrial)
Water carries large clasts during floods. During low-flow conditions, cobbles and boulders are immobile. Coarse conglomerate is a characteristic of this setting.
Alluvial fans (terrestrial)
Cone-shaped wedges of sediments that pile up where a rapid drop in stream velocity occurs at a mountain front. Sediments drop out rapidly with a change in stream gradient.
Sand-dune (terrestrial)
Environments develop where there is an abundance of wind-blown, well-sorted sand. Dunes move according to the prevailing winds and result in uniform sandstones with gigantic cross beds.
River environments (terrestrial)
preserve evidence of channelized sediment transport. Sand and gravel fill concave-up channels that often scour into previously deposited floodplain fines.
Lake environments (terrestrial)
Result from large ponded bodies of fresh water. Gravels and sands are trapped near shore. Well-sorted muds are deposited in deeper water.
Sediment piles up in a _____ where a river enters a lake
Delta
Marine environments
Those where sediment is deposited at or below sea level
In a ________ environment, sediment accumulates where river velocity drops upon entering the sea.
Marine delta
Coastal beach (Marine)
Sands are constantly being processed by wave attack, which often produces well-rounded medium sand. Oblique wave attack results in transport of sand along the coastline.
Beach sands may preserve ______ripples.
Oscillation
Shallow marine clastic deposits (Marine)
Are composed of fine sands and silts that accumulate in quieter waters offshore. The sea-floor in these settings supports active biotic communities.
Shallow water carbonate environments (Marine)
Develop in tropical, warm, clear, shallow, marine water, relatively free of clastic sediments. Protected lagoons accumulate mud. Wave-tossed reefs are made of coral and reef debris.
Deep-marine deposits (Marine)
Accumulate fines that settle out far from land. The skeletons of planktonic organisms make chalk or chert; fine silt and clay lithifies into shale.
Metamorphic rocks
Those that have undergone solid-state alteration of preexisting rocks. Meta = change. Morphe = form.
Metamorphism changes
mineralogy & texture
Texture changes during metamorphism create____
intergrown and interlocking grains
Planar fabric that cuts through the rock.
Foliation
Foliation
Alignment of platy minerals (i.e., micas), or creation of alternating light/dark bands.
Recrystallization
Minerals change size and shape by dissolution and growth of crystals. The identity of the mineral doesn’t change
EX: limestone → marble
Phase change
New minerals form that have the same chemical formula but a different crystal structure.
EX: : andalusite → kyanite → sillimanite
Neocrystallization
New minerals form from old. Protolith minerals become unstable and undergo chemical reactions that recycle elements to form a new mineral assemblage.
EX: : shale → garnet mica schist
Pressure solution
Mineral grains partially dissolve where their surfaces press together. Dissolved ions migrate in a thin water film and reprecipitate. This process requires small amounts of water.
Plastic deformation
Mineral grains soften and deform when rock is squeezed or sheared at elevated temperature and pressure. The minerals change shape without breaking, like a plastic.
Metamorphic sources of heat
the geothermal gradient,
magmatic intrusions, and
tectonic compression.
Compression
is stress that is greater in one direction. It differs from pressure (P), because pressure is of equal magnitude in all directions. Common result of tectonic forces. Mountain building creates horizontal compression.
Shear
Acts parallel to a surface. Stress moves part of a material sideways causing it to be smeared out. It is like sliding out a deck of cards
Elongate
(cigar-shaped): one dimension is longer than the other two (staurolite).
Compression and shear combine with elevated T and P to cause minerals and rocks to ____________
Change shape without breaking
Hydrothermal fluid
(hot water with dissolved ions and volatiles) accelerates metamorphism.
Hydrothermal alteration is called ______________
metasomatism
Nonfoliated metamorphic rocks, like this_________ marble, have no planar fabric evident because they lack and/or they recrystallized without differential stress.
inequant minerals
Slate
A fine-grained, low-grade metamorphic shale. It has a distinct foliation called slaty cleavage.
Slate breaks along foliation, making perfect roofing tiles.
_______is a fine-grained mica-rich rock that forms by the metamorphic alteration of slate. With further alteration, _______turns into schist.
phyllite
Schist
is a fine to coarsely crystalline rock with larger micas indicating medium- to high-grade metamorphism. It has a distinct foliation from large micas called schistosity.
_______has distinct compositional bands, composed of light bands of felsic minerals (quartz and feldspars) alternating with dark bands of mafic minerals (biotite or amphibole).
Gneiss
Metaconglomerate
A metamorphosed conglomerate.
Pebble clasts are flattened by pressure solution and plastic deformation. Foliation is defined by the flattened inequant clasts.
Compositional banding can develop by
metamorphic differentiation
A migmatite is a partially melted _______
gneiss
Nonfoliated rocks lack ________
a planar fabric.
Nonfoliated Rocks
Hornfelds
Quartzite
Marble
Metamorphic grade
A measure of the intensity of T and P conditions that lead to alteration. Different T and P conditions occur in different geologic settings.
A _________ is a set of mineral assemblages that indicate a certain range of P and T conditions.
metamorphic facies
Index minerals
indicate the metamorphic grade of a rock and make useful maps that define metamorphic zones.
types of metamorphism
Thermal—heating by a plutonic intrusion
Burial—deep burial in a basin
Dynamic—shearing in a fault zone.
Regional—P and T change due to orogenesis.
Hydrothermal—alteration by hot water leaching
Subduction—high-P and low-T alteration
Shock—extreme high P from a bolide impact.
__________ involves breakage of rock by shearing within a fault zone.
Dynamic metamorphism
hydrothermal metamorphism happens at _______
Mid-ocean ridges
subduction metamorphism
Trenches and accretionary prisms have a low geothermal gradient.
blueschist
How do metamorphic rocks return to the surface?
Exhumation
Exhumation is due to uplift induced softening of the tectonically thickened crust, which leads to eventual collapse and thinning. Erosion takes over and removes the upper material exposing deeper rock below. Many metamorphic rocks are dry, which prevents retrograde reactions.
shields
Large regions of ancient high-grade metamorphic rocks are exposed in continental interiors. are the eroded remnants of orogenic belts. Shield rocks form the basement under sedimentary cover over much of the world.
channels of water that drain the landscape
Streams
Stream runoff
water in motion over the land surface, is crucial for humans.
Streamflow is used for
for drinking water, transportation, waste disposal, recreation, commerce, irrigation, and energy generation.
Flowing water
erodes, transports, and deposits sediments and sculpts landscapes.
The hydrologic cycle
Evaporation Transpiration Precipitation Infiltration Runoff
Water is added to a stream channel by rainmelt, and groundwater discharge.
rain, snowmelt, and groundwater discharge.
Some ________ is temporarily stored in lakes and wetlands. Some infiltrates to become groundwater
streamflow
Streamflow begins as moving
sheetwash
sheetwash
A thin surface layer of water that moves down the steepest slope. Sheetwash erodes the substrate and creates tiny rill channels that coalesce, deepen, and downcut, eventually concentrating flow in a single channel.
Headward erosion
occurs via intense scouring where sheetflow enters the uppermost part of a channel. Headward erosion is the means by which stream channels, and the drainage networks they are a part of, expand to consume the landscape
Smaller _________ join a larger trunk stream.
tributaries
Drainage networks
often form geometric patterns that reflect underlying geology.
Dendritic
drainage forms a pattern like a branching tree, a result of erosion of material that is relatively
________ drainage develops from a point uplift (a mesa, volcano, etc.)
Radial
drainage basin (catchment or watershed)
A land area that drains into a specific trunk stream.
A drainage divide separates adjacent drainage basins. A raindrop flows to one basin or the other, depending on which side of the divide it falls.
________ streams are defined by water flowing all year. These streams are common where there is abundant rainfall, groundwater discharge, and low rates of evaporation.
Permanent
________streams do not flow all year. They are common in places with low annual rainfall, a low water table, and high rates of evaporation.
Ephemeral
Discharge
measures the volume of water passing a point per unit of time. It is determined by measuring the cross-sectional area of the channel multiplied by the flow velocity. varies seasonally due to changes in precipitation and runoff.
In curved channels
The maximum velocity is swept to the outside curve, which is preferentially scoured and deepened. Outside of curve has higher velocity and goes faster. Deposition happens occurs in the middle of the stream. Curved streams don’t have the same velocity throughout.
Streamflow is
turbulent. It is chaotic and erratic, with abundant mixing via eddies, which scour sediment and bedrock.
Erosional processes: how do streams erode?
Streamflow does work by converting potential energy into kinetic energy. The energy imparted to streamflow is derived from gravity, which acts upon water that was transported by the action of the Sun.
Potholes are formed by
The sand and gravel swirled by turbulent eddies. This abrasive material drills holes in the bedrock
Dissolved load consists of
ions from mineral weathering.
___________ is made of fine particles (silt and clay) entrained in the flow.
Suspended load
Bed load is composed of
The larger particles that roll, slide, and bounce along the bed of the channel. Movement is called saltation.
Sediment transport
the material moved by streams is the sediment load.
Sediment deposition: how do streams deposit sediment?
Gravel settles in the highest velocity setting (channels), sands are deposited in slightly lower velocity settings (point bars and levees), and the muds (silts and clays) settle in slack water.
Most common for ______ is V shaped. Glaciers are more U shaped
rivers
Valleys
gently sloping stream troughs with sidewalls that define a V-shape.
Canyons
steep-walled stream incisions that form in resistant rock
_______ rocks produce vertical cliffs, while _______ rocks produce sloped walls.
Stronger, Weaker
Braided streams
Form where channels are choked by sediment. Flow is forced around sediment obstructions, and the diverging and converging flow creates sand and gravel bars. Bars are unstable, rapidly forming and being eroded away, and flow occupies multiple channels across a valley. More sediment than water. Coarse grain sizes. Gravel usually. In stream prob sand. Found in mountain range.
Meandering Streams
Channels form intricately looping meanders along the lower gradient portion of the longitudinal profile. Think Nebraska. Flat (low gradient environment) Need to curve to make momentum. Steam straightens its self out when there’s rainfall/ flood
________evolve over time, becoming more sinuous (by cut bank erosion and point bar growth) before eventually being chopped off.
Meanders
The highest velocity water erodes the outside of a bend, which is called the ________. The inside of the bend (the _________) is the site of sediment deposition.
cut bank, point bar
Erosion accentuates the cut bank.
High velocity flow scours the base of the cut bank, which collapses into the channel. Fallen cut bank material is transported away by flow.
Deposition builds the point bar. Slowe
Slower current velocity causes sediment to accumulate inside the meander bend. The point bar grows toward the channel.
Meander cutoffs occur when
cut banks converge and a meander neck thins. During floods, high-velocity flow jumps out of the channel and erodes the meander neck away.
The meander cutoff forms an ________, which eventually fills with sediment. Old _______ are visible as arcuate scars on the floodplain.
oxbow lake, oxbow lakes
Delta
Consist of sediment deposited at the mouth of a stream. When a stream enters standing water, the current slows, loses competence, and sediments drop out.
Distributaries. Split apart to make a fan shape and distributes water back to the ocean.
On top of a delta, the stream divides into a fan of ______________.
Distributary channels
Abandoned delta lobes
degrade over time. Abandoned lobes no longer receive sediment from distributary channels. Compaction causes subsidence, and storm waves attack the delta front.
Raging Waters : floods
Discharge and velocity increase and flow spills out of the stream channel, immersing adjacent land. Water scours floodplains, altering the landscape and destroying structures.
Floods occur when:
- abrupt, heavy rains dump large volumes of water quickly.
- long continuous rains have saturated soil pores.
- abrupt warm weather rapidly melts winter snow.
- a natural or artificial dam breaks catastrophically releasing water.
Seasonal floods
take time—hours or days—allowing for evacuation. However, so many people live in floodplain and delta-plain settings that losses are still gigantic.
__________occur from unusually intense rainfall, a dam collapse, or a levee failure
Flash floods
People living in _________face hard choices: move or realize eventual catastrophic loss
floodplains
This modifies streamflow by dramatically increasing the amount and rate of runoff delivery to streams.
Urban development covers the land with impervious surfaces (concrete, asphalt, buildings, etc.).
Sinkhole
Groundwater slowly dissolved the limestone bedrock, forming a cavern that collapsed inward.
Groundwater
the liquid water that resides in sediment or rock under the surface of the Earth, is a major component of the hydrologic cycle. An important resource that is used extensively by humans as a source of water for drinking, for agricultural irrigation, and for industrial processing.
Groundwater flows slowly _________, eventually resurfacing after months to thousands of years to rejoin the hydrologic cycle.
underground
Porosity
Groundwater resides in subsurface pore spaces, the open spaces within any sediment or rock. The total volume of open space.
Primary porosity
Is open space within rock that originally formed with the material. It includes voids in sediment, vesicles in basalt, and open cavities in reef limestone.
Cementation has an effect on porosity. How much space you have
decreases with burial compaction and cementation. Crystalline rocks have very little primary porosity.
Secondary porosity
is new pore space created after the rock was first formed. Examples include fractures, fault breccia, and solution cavities.
Permeability
is the ease of water flow due to pore interconnectedness. High-permeability material allows water to flow readily. Water flows slowly through low-permeability material.
Where does groundwater reside?
Aquifers and aquitards.
An aquifer is
a high-porosity, high-permeability rock that transmits water easily. more likely to be sedimaentary.
An aquitard is
lower-permeability rock that hinders water flow. Aquitard are more clay like like shale. stops water flow from top to bottom
Aquifers and aquitards are commonly ___________
interlayered
unconfined aquifer
lies at the surface. Because it is in contact with human activities, it is easily contaminated.
confined aquifer
lies beneath an aquitard. Being isolated from the surface, it is less susceptible to pollution.
The _______ is the top of the groundwater reservoir in the subsurface.
water table
The _______ separates the unsaturated zone from the saturated zone. The capillary fringe forms at the boundary.
water table
In humid regions,
the water table lies close to the surface. Ponds and streams are connected to the water table and remain filled.
Topography can be used to
estimate groundwater flow direction.
A perched water table occurs where
a mound of groundwater becomes trapped above a discontinuous aquitard that lies above the regional water table.
Hydraulic head
the potential energy driving flow, is due to elevation above sea level and the pressure exerted by the weight of overlying water. A piezometer is a small well used to measure hydraulic head.
Groundwater infiltrates at _________, which are at higher elevations. Groundwater exits the subsurface at _________, which occur at lower elevations.
recharge areas, discharge areas
If groundwater is extracted faster than it can be replaced, a ________ develops around the well.
cone of depression
Artesian wells
tap confined, tilted aquifers that are pressurized by upland recharge.
Water rises in artesian wells to the __________, which is an analogue of the water table for a confined aquifer. A well casing below this surface will flow without pumping.
potentiometric surface
A spring develops
where the water table intersects the surface, most often in a valley. Springs are marked by wetland vegetation, perennial wetlands, saturated soils, nonfreezing ground, and streamflow. Anywhere the water can meet the surface springs can appear (perched water table or fault) Springs develop where there are strong permeability contrasts in layered sedimentary materials.
A network of interconnected fractures facilitates
spring flow.
Hot springs develop in two settings
where deep groundwater discharges along faults and in geothermal regions.
Boiling water and steam erupts cyclically from
geysers
Hot springs lose __________ upon cooling, crystallizing as deposits of travertine.
dissolved minerals
Groundwater is an important natural resource. It accounts for __% of all the liquid freshwater on Earth.
95%
Ground water is __% of fresh water we have
30%
The natural hydraulic gradient can be reversed by
groundwater withdrawal.
A large irrigation well creates a large _________that reverses the hydraulic gradient and causes septic contamination of the home well.
cone of depression
land subsidence
When groundwater is removed, sediment grains compress and the pores collapse. This causes the land surface to crack and subside irreversibly.
Groundwater is often of high quality because of
the filtering effect of porous media and clay absorption.
contaminant plume
Groundwater transports pollutants away from a source of input. plumes have high concentrations near the pollutant source. Concentrations decrease with distance.
Bioremediation is
a technique that utilizes bacteria to clean groundwater.
