Geology 101 Quiz 7 Flashcards
most of our oil, natural gas, and coal comes from
sedimentary deposits
almost everything we know about past life on this planet comes from
sediments & sedimentary rocks
fossils do not survive melting (and are often lost during metamorphism as well), so we depend on ? to preserve a record of past life
sedimentary systems
sedimentary rocks can form in one of two ways
1) by accumulation and lithification of particles
2) by precipitation from solution
clastic sediments (also called detrital)
sediment formed by the accumulation of particles
chemical sediments (or biochemical)
sediments formed by precipitation of dissolved ions
“biochemical” simply means
organisms aided in precipitating minerals from solution
? sediments are about ten times more abundant on Earth than ? sediments
clastic 10x more abundant than chemical
the composition and appearance of clastic sediments is determined by
composition of the source rock, weathering conditions, distance of transport, mode of transport
with more aggressive weathering, there will be fewer ? minerals and more ? minerals
fewer primary, more secondary
with more aggressive weathering, the grains are more
rounded
the longer the travel distance, the more ? will take place
weathering
long travel distance tends to result in
a decrease in easily weathered primary minerals, more secondary minerals, breakdown of larger particles, and rounding of grains
modes of transport
by wind, water, or ice
the mode of transport will affect
weathering conditions and the degree of sorting sediments are likely to experience
sorting
the degree to which grains are of a similar size
poorly sorted sediments
large range of grain sizes
well sorted sediments
smaller range of grain sizes
wind as a mode of transport
wind can normally only pick up clay, silt, and sand size particles; as the wind slows, the larger particles will fall out first resulting in well sorted deposits
sand dunes
example of wind as a mode of transport; typically occur in deserts and near beaches
loess
wind blown deposits of glacial silt; here in Mississippi; pulverized rock dried out after the ice age, allowing winds to pick up the loose material
desert pavement
not technically deposited by wind, but created by wind; as wind blows over desert sediments, the finer grains are picked up and blown away; gravel and larger rocks are left behind in what looks like a pavement of gravel on the surface
wind is most effective in ? climates
dry
why is wind most effective in dry climates
water causes particles to stick together and supports vegetation that protects the underlying sediment from blowing away
water as a mode of transport
water is effective in carrying even large boulders during floods; as water slows, the larger particles drop out first resulting in well sorted deposits
water moving on the (inside/outside) edge of a bend moves faster
outside
water on the inside edge slows and drops some of its sediment in well sorted
sand bars
wind will tend to etch the surfaces of the grains, giving them a
frosted appearance (how best to tell whether ancient dunes were deposited by wind or by water)
ice as a mode of transport
transport by ice means transport by glaciers; glaciers are the bulldozers of erosion - everything in its path is pushed forward and left in a mixed heap when the glacier retreats
moraines
bulldozed glacial deposits with very angular particles and that are very poorly sorted
glacial outwash deposits
as a glacier melts, water will carry sediments away and form more sorted deposits
the composition and appearance of chemical sediments is determined by
ion concentrations, temperature, water pressure, presence of oxygen, presence of organisms
examples of ions that will only precipitate if the concentration of ions is very high
Na+ and Cl-
examples of ions that will precipitate at low concentrations of ions
Ba2+ and SO4 2-
if seawater is cold,
calcite may precipitate
if seawater is warm,
a polymorph (aragonite) may precipitate instead
calcite becomes more soluble at higher
water pressure (lower depth)
sediments found on the deep ocean floor are not likely to be made of
calcite (sediments could be silica though)
the presence of oxygen can be important for some minerals such as those containing iron:
if oxygen is present, ? may form, but if oxygen is absent, ? may form instead
Fe2O3 hematite
FeS2 pyrite
organisms can be divided into
microorganisms and macro-organisms
micro-organisms
require a microscope to see
macro-organisms
visible without magnification
reefs grow because
the organisms build a new calcite shell above the old and move into the new space - only the surface layer is “living”
sedimentary environment (AKA a depositional environment)
a geographical location characterized by a particular combination of geologic processes and environmental conditions
elements that characterize a sedimentary environment
geographic location and plate tectonic setting, transport agent and medium, organic processes and organisms that modify sediments, climate
examples of clastic environments
beach, river (fluvial), glacial, desert, lakes (lacustrine), deltas, and the deep sea
where would we expect to find shell fragments and sand dunes
beach clastic environments
where would we expect to find flood plain deposits and sand bars
river (fluvial) clastic environments
where would we expect to find moraines and outwash deposits
glacial clastic environments
where would we expect to find desert pavement and sand dunes (with etched grains)
desert clastic environments
examples of chemical sedimentary environments
carbonate, siliceous, swamp, soil B horizon, evaporite
where would we expect to find calcite shells and reefs (mostly shallow ocean)
carbonate chemical environments
where would we expect to find silica tests (shallow and deep ocean)
siliceous chemical environments
where would we expect to find fine grained sediment and vegetation
swamp chemical environments
where would we expect to find precipitated minerals (like caliche) leached from A horizon
soil B horizon chemical environments
where would we expect to find precipitated minerals (like caliche) leached from A horizon
evaporite chemical environments
lagoon
a shallow body of water partially protected from a larger lake or sea
in arid climates where seawater washes into a lagoon and evaporates, ? can form
evaporite deposits
common evaporite deposits
halite, gypsum, selinite, calcite
playa lake
a lake with an inlet stream but no outlet that periodically dries up (usually seasonally)
playa
a playa lake when it is dry
in a playa lake, water is lost only to
evaporation and seepage into the ground
mixed environments
where clastic and chemical sediments are being deposited together
examples of mixed environments
deep sea, playa lake
deep sea as a mixed environment
chemical: silica tests from swimming microorganisms
clastic: volcanic ash drifting over the ocean
playa lake as a mixed environment
chemical: evaporite minerals
clastic: sand and silt washed in from the inlet stream
bedding structures
features preserved in lithified sediments that provide information on the type of environment, and even the original orientation of the strata if it has been shifted
examples of bedding structures
ripple marks, mud cracks, graded bedding, cross bedding, bioturbation structures, bedding sequences
two types of ripple marks
symmetrical and asymmetrical
symmetrical ripple marks
formed by wave action (indicated near shore environment)
asymmetrical ripple marks
formed by wind or water current (indicates river environment or current in larger water body)
mud cracks
indicates a shallow water environment that occasionally dries up
in mud cracks, if a flood suddenly brings in muddy water, the mud may
fill the cracks before they swell and seal - if this layer is buried, the mud cracks may be preserved
graded bedding
a single layer with coarse grains near the bottom grading up to finer near the top
what type of bedding structure is common anytime sediment is mixed in water suddenly and allowed to settle?
graded bedding
turbidity flow
sediment that begins to slide underwater, entrains water in the sediment and begins to flow rapidly down slope
turbidite
deposit laid down by a turbidity flow
graded bedding if often seen in ? flows
turbidity
cross bedding
sloping layers in dune deposits are truncated by sloping layers in another direction
bioturbation structures
“bio” means life, “turbation” means stirred up - so bioturbation refers to preserved structure that show that organisms have moved sediment around
examples of bioturbation structures
preserved tracks, tunnels, or burrows
bedding sequences
a pattern of layers in a specific sequence is often an indication of a particular process
lithification (turning sediment into rock) involves
compaction and cementation
compaction
squeezing particles closer together (especially significant for clays)
cementation
precipitation of minerals between grains
common cements
calcite and silica
diagenesis
processes that alter sediments after deposition (including lithification)
in addition to compaction and cementation, diagenesis also includes alteration of minerals, such as
changing crystal structure to turn aragonite into calcite; swapping half the Ca atoms for Mg atoms to calcite to turn it into dolomite
sedimentary rocks are divided into two categories
clastic and chemical
clastic rocks
conglomerates, sandstone, siltstone, mudstone and shale
conglomerates
made from poorly sorted sediments, including gravel
sandstone
made from sand
siltstone
made from silt
mudstone and shale
both made from clay (mud), but shale breaks into thin layers
clastic rocks can also have
descriptive identifies or unique names
example of descriptive identifier for mud with calcite cement
calcareous shale
example of descriptive identifier for mud with silica cement
siliceous shale
example of unique names for sandstone made of mostly quartz
quartz arenite
example of unique names for sandstone made of quartz and feldspars
arkose
chemical rocks
limestone, dolostone, chert, organic, iron formation, evaporite
limestone
calcareous muds, shells and coral (reefs)
dolostone
limestone (calcite) turned into dolomite
chert
siliceous sediments (opal, chalcedony, quartz)
organic
organic debris (peat, coal)
iron formation
iron oxides (hematite)
evaporite
evaporite minerals (halite, gypsum, selenite, calcite)
the Mediterranean Sea and Black Sea were once
dried up
