Lecture 1 - Sediment and Sedimentary Rocks Flashcards
what rocks are best for learning history
sedimentary rocks
what rocks are best for dating
igneous
metamorphic to extrusive igneous rocks
melting, magma, lava, consolidation
metamorphic to intrusive igneous rocks
melting, magma, crystallization
rocks to sediments
uplift and exposure, weathering, transportation, deposition
sediments to sedimentary rocks
lithification
ANY rock to metamorphic rock
metamorphism
what are sediments
loose solid particles formed by weathering or erosion of pre-existing rocks on the Earth’s surface, or by chemical precipitation from solution through organic or inorganic environments
unconsolidated sediment
sediment that is loosely arranged or unstratified and whose particles are not cemented together
3 types of particles
fragments (clasts) (eroded from pre-existing debris), skeletal debris (produced by organisms), crystals (precipitated by solution)
lithification process
compaction and cementation. the process that converts loose sediment into sedimentary rock
compaction
decrease in rock volume due to weight of overlying sediment
cementation
bind grains together with cement
common cementation molecules
carbonate (CaCO3) and silica (SiO2)
diagenesis
process of changing sedimentary rocks after lithification is termed
how can limestone go through diagenesis
the movement of MG-rich fluids through the rock. Mg substitutes for Ca ions in the rock to produce a carbonate rock called dolostone. The decreased rock volume forms vugs.
CaCO3 + Mg –> CaMg(CO3)2
vugs
voids of spaces in a rock
clastic sedimentary rocks
made from fragments of pre-existing rocks or organic particles such as shells and skeletal fragments (bioclastic)
siliciclastic (terrigenous clastic)
made from fragments of pre-existing rocks
what are chemical/biochemical and carbonaceous sediments made of
made from organic particles such as shells and skeletal fragments
what percentage of the earths surface is covered by sediments and sedimentary rocks
70%
what percentage of the volume of the earths crust is sediments and sedimentary rocks
5%
what are some interesting parts about sedimentary rocks
- contain most of the worlds energy resources (fossil fuels)
- hold most of the worlds subsurface aquifers
- contain fossils that documents the history of the development of the earth
how are sediments and sedimentary rocks classified
clastic vs non-clastic
how are clastic sedimentary rocks classified
grain size, grain size distribution, grain shape
grain size
classification according to the grain size of the fragments they contain using a standardized scale like the Wentworth scale
what does the sediment gravel become
conglomerate
what does the sediment sand become
sandstone
what does the sediment silt become
siltstone
what does the sediment clay become
shale
chemical (non-clastic) sedimentary rocks
formed by direct precipitation of minerals from solution. commonly form in arid, tropical environments. “inorganic” limestones and cherts evaporite deposits.
ex. halite, gypsum
grain size distribution
sorting - organization according to grain size.
what can grain size distribution tell you
tell us the degree of transport and reworking such as wave action
very well-sorted will be far from the source with reworking and poorly sorted is close to the source with little reworking
grain shape
angularity/roundness of a rock
what can grain shape tell you
tell you about transport mechanisms. better rounding indicated more transport and reworking. ranges from very angular to well-rounded
how does sediment move
transported by water, wind, or ice as suspended load, bedload, or dissolved load
bedload
grains move in continuous or intermittent contact with the bed. rolling or jumping. coarse grained sediment
traction
rolling or dragging of grains in a bedload.
saltation
bounding or jumping of grains in a bedload. repeatedly picked up and dropped.
suspended load
sediment carried in fluid without coming in contact with the bed. fine-grained sediment. deposited under low energy conditions
coarse-grained sediment
gravel and sand
fine-grained sediment
silt and clay
bedforms
topographic features on the bed. different bedforms develop as current velocities change, but can also depend on grain size. can be conserved in the rock record.
very low flow velocity bedform
plane bed
low flow velocity bedform
ripples
moderate flow velocity bedform
dunes
high flow velocity bedform
plane bed
what are flat bed bedforms preserved as
horizontal lamination
what are ripple bedforms preserved as
ripple X-lamination
what are dune bedforms preserved as
cross bedding
what does oscillating current result in
symmetrical ripples, with equal slopes on either side
what does unidirectional current result in
asymmetrical ripples, with a gentler slope on thewindward and steeper slopes on the leeward face
windward face angle
10-12 degrees
leeward face angle
33-34 degrees
wave base
depth at which water movement is negligible. is 1/2 wavelength. water molecules have circular orbits below waves that get smaller with depth
fairweather wave base
depth beneath average daily waves
storm wave base
depth beneath storm waves
hummocky and swaley cross-stratified sands
formed under storm waves with water depths of 10-30m. indicate storm activity
sediment gravity flows
density-driven currents of sediment downward that are triggered by slumping or failure of a slope
grain size distribution of debris flows
poorly sorted
sediment gravity flow of turbidites
turbidities become graded beds with more fine sediment upwards. this can be seen by looking at the x-axis of logs to see the decreasing pattern in grain size with decreasing depth. as size increases you see how the bigger grains settle to the bottom.
graded beds
change in grain or clast size from bottom to top of the bed
sediment gravity flow example
grand banks NFL earthquake and turbidity currents. the current could be recorded by when what cables broke on the sea floor
turbidity current deposits
Normally graded beds are coarse at the base and fine at the top. Bouma sequence predicts distribution, there are sole marks on base, and the change in character is from proximal to distal. only happens in subaqueous settings
Bouma sequence
A division - massive
B division - laminated
C division - rippled
D division - laminated
E division - massive silt/clay
facies
a body of sediment or rock with a particular characteristic that distinguishes it from other rock bodies. it is descriptive and not interpretive, and the result gives the process of the event.
how is paleoenvironmental information obtained from sediments and sedimentary rocks
interpretation of facies types according to depositional process to tell you about depositional environment
depositional process
looking at individual facies. ex. sediment gravity flow, traction current, etc.
depositional environment of facies
place and climate. looking at genetically related groups of facies. ex. coastline, lagoon, river, glacier, reef
facies associations
groups of facies genetically related to one another. have some environmental significance. ex. shoreface sandstones and lagoonal deposits
what is needed to determine a depositional environment
facies associations
walthers law
facies sequences observed vertically are also found laterally. we see rocks in vertical sequence that were deposited beside each other at the same time
“Only those facies and facies areas can be superimposed primarily which can be observed beside each other at the present time” explains how the vertical stacking of facies relates to environments that were laterally connected in the past. environments change through the horizontal transition. there is a coarsening upward sequence, where deposits at the bottom are muddy bottom sets that transition to silty/sandy forests into delta front sands. it shows a sequence of migration of shoreline - shallowing upwards in a transition from one to the next.
what is the vertical stacking of facies associated with
sea level change
transgression
shoreline moves landward as relative sea levels rise.
regression
shoreline moves seaward as relative sea levels lower. erosion of exposed surfaces can come with this.
how does glaciation affect sea level change
global sea levels lower during glacial events. known as glacio-eustatic sea level lowering. ice leads to lower sea levels
how does changes in sea floor spreading rates affect sea level
slow-spreading has a cool crust and low elevation, so no rises in sea level
fast spreading has a hot crust, high elevation, and displaces water onto land - transgression.
tectonic movements that lead to mountain building can also shift water nearby to the lower elevation area.
how are carbonate sediments created
biological activity. carbonates are born not made, and the sediment producers change over time
what percent of the sedimentary rock record is carbonate sediment
20-25%
what percent of the worlds hydrocarbons are in carbonate sediments
50%
what percent of N.A’s hydrocarbons are in carbonate sediments
20%
what hydrocarbon is often used in construction
Limestone CaCO3
what percent is limestone
> 50% calcite/aragonite
what percent is dolostone
> 50% dolomite
what is chalk composed of
it is veryfine grained and mostly composed of skeletal fragments of CaCO3 organisms. the grains are bioclasts and may be surrounded by mud matrix. it is a kind of mudstone
how is paleontology used for carbonate sediments
it is important in classifying the carbonate depositional systems to understand the conditions in which organisms live
what are limestones largely precipitated by
organisms like shells or skeletons
how is carbonate sand and mud produced
disaggregated skeletons
allochems
recognizable grains
why are lime muds not usually shales
they don’t yield clay minerals
examples of carbonate allochems
- skeletal particles
- ooids (coated grains)
- stromatolites (coated grains)
- peloids
- intraclasts
what rock do ooids make
oolite
what are ooids
spherical coated grains <2mm in diameter
how are stromatolites formed
formed by the photosynthetic cyanobacteria algae. the lamination formed by the vertical growth of bacterial filaments in daylight traps grains and horizontal growth at night that binds layers of sediment
what are peloids
silt to sand-sized mudballs, round to pellet-shaped, mostly originate as fecal pellets from shrimp (450/day) and other lifeforms
what are intraclasts
ripped-up clast that is typically mud
how are carbonates classified
based on matrix content
- contains mud or lacks mud
- mud-supported or grain-supported
what carbonate is identified as grains supporting one another and contains no mud
grainstone
what carbonate is identified as grains supporting one another and contains mud
packstone
what carbonate is identified as mud supported with less than 10 percent grains
mudstone
what carbonate is identified as mud supported with more than 10 percent grains
wackestone
what is the current big producer of carbonate
modern reefs. it is most productive in the reef and shallow platform
subtidal factory
always underwater, reefs and lagoons
intertidal
between tides, beach and tidal flats
supratidal
above high tide, wet: algal marsh and arid: sabkha
where do evaporites occur in tidal environments
the arid Sabkha area
what conditions needed for coral reef growth
warm, clear, agitated water with plenty of sunlight. neat the equator. carbonates are good climate indicators as it was probably the same in the past
temperature range for reefs
best between 25 and 29, but has to fall between 18 and 36
what is chert
inorganic precipitation, microscopic siliceous fossils. it often occurs as ‘nodules’ within limestone or dolostone
evaporites
evaporation of sea water, resulting in sediments such as halite, gypsum, and anhydrite
where is lime-mud and sand made
lagoons
where is reef sediment made
at the reef front
how wide is the lagoon
0.1 - 2 km
how wide is the reef core (reef front)
50 m
how wide is the forereef slope
0.1-1km
what can be said about the growth patters of reef builders
they are similar for corals, sponges, and stromatolites, which is an example of convergent evolution. zonation is also similar between builders
how are fossils created
- shells burried and preserved unaltered for <100 millions years. cavities are filled with silica, calcium carbonate, or iron, replacing any original substance with mineral matter and any soft tissues by carbon.
- they can be preserved as molds, imprints, and casts in resin.
- tracks, trails, burrows and borings preserved
ichnology
study of trace fossils
bioturbation
process of disturbing sediment
two types of fossils
trace fossils and body fossils
trace fossils
tracks, trails, burrows and borings preserved
body fossils
remains of living things (bones). often these remains have undergone some alteration process
