sedimentary rocks
formed by weathering and erosion of igneous rocks
record depositional environments over time
only rocks with fossils
*
tell history
*
formed by the burial, compaction, and cementation of sediment, made of the erosional debris of other rocks
loose unconsol
rock type
rapidly moving streams move larger cobbles and gravel, compared to slow-moving streams which can only move sand and finer particles
conglomerates, made of pebbles and cobbles, are deposited near a mountain front, where stream velocity slows down going int
conglomerates
formed at alluvial fan deposits from alluvium
made of pebbles and cobbles
are deposited near a mountain front where stream velocity slows down going into the plains (*
this forms sandbars
*)
*
always indicate a mountain range that has eroded away leaving
stream gradient
controls what material can be transported
the steeper the gradient, the bigger the material that can be moved and faster the current
alluvial fans
fan-shaped wedges of cobbles and pebbles deposited around the mouth of a canyon, where braided streams deposit coarse material because the stream gradient and velocity decrease in going from the mountains to the plains
are built up as the mountains erode
stratigraphy
in the stratigraphic record, it records changing environments and life forms over time
the sequence of sedimentary units in a given area
formations
layers in the stratigraphy that represent a unique depositional environment
sill
tabular igneous intrusion parallel to structure
by fossils present
2 billion year old limestones in Glacier National Park have stromatolite fossils
if we go to a contemporary shallow marine environment in Australia, we can see that modern-day stromatolites are created by blue-green algae (cyanobacteria)
algal mats secret
by sedimentary structure
a desert sand dune moves as wind pushes sand up the gentle down-wind face of the dune and over the top of the dune
it then cascades down the steep up-wind slip face
this creates an internal structure of layers within the dune which make an angle with the
4 steps in making a sedimentary rock
1. weathering to produce sediment
2. transportation of sediment (by water, wind, ice, and gravity)
3. deposition of sediment (in streams, deserts, lakes, oceans, swamps, etc)
4. the conversion of sediment into sedimentary rock (diagenesis): burial, compac
weathering
begins with weathering of igneous rocks to create sediments
generation of sediment
mechanical weathering, joints, frost wedging, spheroidal weathering
mechanical weathering
the physical breaking down of rocks exposed at the surface into smaller fragments which increases the surface area for subsequent chemical weathering
principal effect is to increase surface area available for the reactions of chemical weathering
mostly oc
joints
fractures in rocks
parallel to bedding
when granite is exposed, it expands and has concentric joints called exfoliation sheets
frost wedging
dominant mechanism that breaks mountains apart
water enters fractures in rocks and then freezes and expands, breaking off the rock
(plants can also break rocks)
spheroidal weathering
more active chemical weathering at the corners of cubes (created by three intersecting joint sets) where there is the most exposed surface area
angular blocks become rounded because corners expose most of surface area
chemical weathering
minerals in igneous and metamorphic rocks are formed at high T and/or high P
these minerals are stable at high T and P, and when they are exposed by erosion to low T and P at the earth's surface, they are no longer stabe
they undergo chemical reactions to
hydrolysis
reactions of minerals with acidic ground water
as the tiny H+ replaces much larger K+, Na+, Ca2+, Mg2+, and Fe2+ ions in a mineral's structure, it causes the minerals to break down
pH, acidic groundwater, feldspars, quartz, mafic minerals
high T, high P m
pH
a measure of the [H+] concentration (hydrogen ion)
pH = -log[H+]
a neutral pH is 7 (distilled water)
a pH of < 7 is acidic
a pH of > 7 is basic
acidic groundwater
CO2 dissolves in water to produce carbonic acid
CO2 + H2O = H2CO3 = H+ + HCO3- (rain water)
decay of organic matter produces humic acids
acid rain - burning of fossil fuels in power plants and cars releases SO2 and NO2, which form sulfuric and nitric acid
feldspars
feldspars + H+ + H2O = clay + K+, Na+ and Ca2+ ions + silica (SiO2) in solution
2KAlSi3O8 + 2H+ + H2O = Al2Si2O5(OH)4 + 2K+ + 4SiO2
mafic minerals
mafics + H+ + H2O = clay + Fe2+ and Mg2+ ions + silica (SiO2) in solution
quartz
stable at earth's surface
just breaks down mechanically to sand
unique in that it does not chemically weather
oxidation
Fe2+ + O2 = Fe3+
mafic minerals --> hematite (Fe2O3) gives red tint
pyrite (FeS2) ubiquitous in ore bodies (always associated with pyrite)
pyrite + H2O + O2 = limonite (FeOOH) + sulfuric acid (H2SO4) [acid run-off from mines]
yellow soils around mines
dissolution
limestone, a rock made of calcite (CaCO3), dissolves in acidic groundwater (fizzing)
calcite reacts with acid to create carbon dioxide, which produces the fizzing
limestone is a rock that can dissolve creating a unique terrain with caverns
products of weathering
raw materials for sedimentary rocks
rock fragments and feldspar --> conglomerate and arkose [mechanical weathering product - can be transported] [bed load]
quartz --> sandstone [mechanical weathering product - can be transported] [bed load]
clay --> silts
transport of sediments
principally by water from highland to lowland
by gravity, by ice and glaciers, by water, by wind
transportation by gravity
moving things down slope to stream
talus
landslides
debris flows
talus
collection of large angular rock fragments at the base of a cliff
frost wedged off of jointed rocks
landslides
movements of dry rock and soil down a slope
chunk of rock fails
down slope motion
debris flows
movements of mud and rock down a slope facilitated by water
mud flow
created by movement of water-saturated soil and rock
transportation by ice and glaciers
great thickness of ice that can deform plastically and flow
moraines
glacier picks up fragments of rock on bottom and sides
moraines
deposits of material embedded in glacial ice left behind as the glacier melts and retreats
piles of rock transported by glacier and is then transported further by water
transportation by water
hydrologic cycle
bed load
rolling and saltation
suspended load
dissolved load
stream gradient
ripples
erosion agents
bed load
for cobbles, boulders, pebbles, and sand
are moved along the bed by rolling and saltation (hopping)
cobbles are deposited by fast-moving mountain streams and these deposits become conglomerates
slow moving rivers deposit sand, which becomes sandstone
suspended load
for silt and clay
fine clays and silt suspended in the water column
fine particles are carried in suspension, making water turbid
these particles only settle out in still water to form siltstone and shale
dissolved load
for ions and silica
soluble ions carried in solution
erosion agent
streams are very effective agents of erosion
the abrasive material in the bed load acts as a saw to wear rock away
the Grand Canyon has been excavated in the last 6 my
ripples
ripples are small-scale dunes and are similarly cross-bedded
asymmetric ripples [tidal flat, stream]
symmetric ripples [shallow marine environment]
can be used to tell the original "up" direction, since the troughs of ripples (down part) are rounded and t
asymmetric ripples
current ripples
formed by water or wind moving in a consistent direction and have a gentle up current slope and a steeper down current slope
can be used to determine the paleo-current direction
symmetric ripples
oscillatory ripples
formed by water moving back and forth and have symmetric up-current and down-current faces
typical in shallow marine settings
transportation by wind
less competent than water, so it only moves smaller particles
typically in deserts
cannot move large rock particles
suspension - silt and clay
saltation as bed load - sand
deflation and desert pavement - larger particles cannot be moved by wind and are le
desert pavement
armor for land
land is stripped of fine particles so what is left is the desert pavement
dunes
produced when wind encounters a barrier (such as a mountain range or a plant) that sand cannot be lifted over and sand builds up
dunes move as sand is pushed up the gentle up-wind face and cascades down the steeper slip-face
sand cascading down the slip-f
cross-beds of a dune
internal structure: concavity of the cross-bed tells you the wind direction
indicate which way is up: they are concave up, they are truncated (cut off) at the top by erosion, they come in smoothly (tangentially) at the lower bed as sand cascades down the
sedimentary rock classification
sedimentary rocks are made up of transported weathering products
there are two kinds of sedimentary rocks: clastic (detrital) and chemically precipitated
clastic sedimentary rocks
made up of particles of rock, quartz, and clay carried as the bed or suspended load (deposited debris - mechanically transported)
conglomerate, arkose, sandstone, siltstone, and shale
classified by grain size [*
grain size correlates with a depositional e
clast composition and maturity
minerals break down during weathering in the order of their melting points
minerals with higher melting points weather chemically first
rocks deposited closer to their erosional source have feldspar and mica, in addition to quartz; such sediments are imma
classification by grain size
coarse grained: conglomerate (rounded particles, poorly sorted, deposited in alluvial fans; makes up gravel), breccia (angular particles, not transported at all or very little, most common are from limestone; makes up gravel); anything with more than 2 cm
chemically precipitated sedimentary rocks
formed by the precipitation of ions from solution
classified by chemical composition
limestone and calcite cement, chert and silica cement, hematite and limonite cement
saturation - the maximum amount of a material that can be dissolved in water
evaporati
classification by chemical composition
chemically-precipitated sedimentary rocks are named based on what chemical compound they are made of
limestone - CaCO3
flint and chert - SiO2
banded iron formation
evaporites
coal
limestone
CaCO3
most common chemically precipitated rock
1) when sea water becomes saturated in CaCO3, it precipitates. because CaCO3 is unusual in that it is more soluble in cold water, limestone forms in warm tropical seas where CaCO3 has the lowest solubility (b
flint and chert
SiO2
quartz
flint - SiO2 can become saturated in sea water along with CaCO3 and can co-precipitate with it to form nodules of flint in limestone
chert - radiolaria are tiny deep sea zooplankton which secrete a silica shell. as they die, these shells rain
banded iron formation
formed when hematite and silica co-precipitate. most BIF's formed between 3 and 1 mya, as blue-green algae (cyanobacteria) produced O2 in sea water by photosynthesis, which oxidized the soluble Fe2+ in sea water to Fe3+, which precipitated as hematite
evaporites
1) calcium sulfate and sodium chloride, unlike calcium carbonate, are very soluble in warm water. when 80% of saline water evaporates, CaSO4, 2H2O (gypsum) precipitates and when 90% evaporates, NaCl (halite) precipitates
2) gypsum - CaSO4, 2H2O
3) halite
coal
formed from plant material buried in sediment in an anaerobic (without oxygen) swamp environment
diagenesis
the processes by which sediment is turned into sedimentary rock
compaction
cementation
compaction
as sediments are buried under more sediment, the pressure causes the grains to compact, reducing the pore spaces between them
burial --> pressure causes compaction --> less pore space
cementation
chemicals that become saturated in the water in the pore spaces of sedimentary rock precipitate, forming cement. this cement fills the pore spaces, gluing the grains together
the two most common cements are silica (SiO2) and calcite (CaCO3)
hematite (Fe2O
sedimentary structures
bedding
dunes and cross-beds
ripples
mud cracks
trace fossils
fossils
bedding
sediment is laid down in layers, which is reflected in the bedding of a sedimentary rock
generally, coarse-grained sediment, such as conglomerate, has thick beds, and fine-grained sediment, such as shale, has thin beds
mud cracks
they form when mud dries out (dessicates) and contracts to form polygons (usually 4,5,6,or 7-sided)
indicate an alternately wet and dry environment
trace fossils
tracks and trails left by animals
tracks (dinosaur tracks)
bioturbation - burrowing organisms can disturb layers of sediment and their burrows can show up in rocks
fossils
bones, shells, or molds of animals and plants in a rock
stratigraphy
records changing earth conditions at that place over time
changes from one place to another
each rock layer is a formation
depositional environments
glaciers and moraines
alluvial fans and bajadas
deserts and playas
streams, rivers, and flood plains
lakes
swamps
beaches and tidal flats
reefs
deep marine
bajada
a continuous apron of alluvium along a mountain front, caused by overlapping alluvial fans
coarse grained, immature, and poorly sorted rocks are in alluvial fans
desert
has playas
when it rains, water carries fine sediments (silt and clay) and dissolved ions to the playa, which fills with water
during arid conditions, the water evaporates, and minerals such as gypsum and halite precipitate as the dissolved ions come out
playas
an ephemeral lake in a desert setting
filled when it rains and dries up in between rains
have interbedded siltstone/shale (from the rainy season) and evaporites (from the dry season)
streams
braided streams form near the mountains and are choked with sediment and have multiple channels
braided stream deposits are gravels (conglomerates) and sands (sandstones)
show ripple marks, cross-bedding and contain channel fill (a trough-shaped body of c
lakes
lake sediments are fine-grained siltstones and shales
swamps
swamps have lush and dense vegetation, as here in a cypress swamp in SW Florida
the water in swamps is relatively still (it does not overturn) and is rich in organic matter
these waters are therefore anaerobic (without oxygen)
when trees and other plant m
beaches and tidal flats
beach sandstones are well-rounded, well-sorted, and pure quartz (mature)
tidal flats are flooded at high tide and drain at low tide
they can have finer sediments than the sand of a beach and often have current ripples and sometimes mud cracks if they dry
deep marine
there are 4 basic deep marine deposits: 1) coarse-grained turbidites at the base of the continental shelf, 2) fine abyssal clay (shale) which forms from the settling of air-borne dust, 3) calcareous oozes formed from the accumulation of the calcite skelet
oozes
siliceous (chert)
calcareous (chalk)
turbidites
form from dense slurries of sediment and water (called turbidity currents) that race down submarine canyons
carry coarse-grained, continental sediment from the continental shelf to the abyssal plain
turbidity currents form graded beds in turbidites (grade