37) Depositional Environments


(Opening Music) Welcome back geology fans! Throughout this series on sedimentary rocks I’ve
mentioned various environments of deposition, but it is of use for us to summarize these
environments before continuing on to to our final rock type. We will use this
model of terrestrial to marine environments to describe the various
features we will see in each, and we will progress from higher to lower
elevation settings. Our high elevations are dominated by the erosional agents of
ice and gravity. Rock fall from the face of a cliff produces a steep slope of
poorly sorted and angular rock fragments that form a deposit we call “talus” or “scree”. Landslides and gravity driven deposits accumulating at the base
of hillsides have a more fine grained material in it, ranging often from silt
to those larger rock fragments which still show angularity and poor sorting.
Such gravity eroded deposits are called “colluvium”. Glaciers are also encountered
at high elevations in high latitudes. Glacial material starts as sub-angular,
poorly sorted sediment resulting in glacial till. Talus, or scree, colluvium,
and till are all poorly sorted, but as long as these deposits are exposed at
the surface they can be acted on by the other two erosional agents: wind and the
dominating water. Wind can act all the way from the top of the highest
mountains all the way down to sea level. And where it causes depositional
accumulations, such sediments are called aeolian deposits, which are
characteristically well-sorted sand sized or smaller mineral grains. Not so
for water. As glaciers and snow melt they feed streams. Together, glaciers and
streams do the most geomorphic work on Earth; that is shaping of the Earth’s
surface. As streams move eroded pieces down out of the highlands, they round
them a deposit moderately to poorly sorted conglomerates to start with in
the headwaters of the stream. At the base of mountains, hill slopes, and
in down dropped basins, the deposits that appeared to be water eroded but not
dominated by main river systems are called alluvium or alluvial deposits.
A special environment involving alluvium is the base of steep mountains where
energy levels suddenly drop, and these are called alluvial fans. Such fans are
dominated by braided streams due to having more sediment supply than the streams
can usually transport, a common feature of alluvium. The largest, most angular
clasts (rock fragments) are near the mountain source and the smaller and
rounder clasts deposit further and further from the mountain’s face. But
don’t think the stream spreads out in all directions simultaneously to make
this fan shape. This is sediment dominated, not water dominated. one shallow
wide stream track is taken through time depositing eroded material from the
nearby highlands until the stream steepness lowers and another steeper
route is taken. If you could see an alluvial fan through time, you would see
the stream jumping from location to location slowly building upward and
outward. Ephemeral flows join together downstream to form environments that
become more and more water dominated with two main flavors on land: rivers and
lakes. The further downstream clasts are eroded, the smaller and rounder and
better sorted the clasts get. So we start to get coarser material in the
fast-moving stream channel often sand sized or smaller, but occasionally better
sorted conglomerates. The edges of the stream are called floodplains and are
normally exposed to air, but going underwater and becoming more
depositional during floods. Floodplains usually get the finer silts to muds
settling out of this more stagnant water that has left the faster moving stream
channel. Together the channel and floodplain
deposits of river systems are called fluvium or fluvial deposits. Coarse
sediments which are not laterally continuous, often containing cross-beds,
surrounded by finer grained muds, both of which may contain terrestrial fossils
(fossils of organisms that lived on land) all point to a river or fluvial
environment. Whether in the head waters or downstream, we meet lakes and swamps
with still stagnant or standing water. With a greatly reduced energy level we
see coarser fluvial sediments at the shore, but the interior is dominated by
finely layered clays and silts and even evaporates or other chemical
precipitates can form. Lacustrine is the term for lake deposits in general
but when deposited in a repeated rhythmic pattern we call those deposits
varves. The Castile formation’s environment 250 Million
years ago was actually a lake formed near the shore of an ancient
ocean, but damned by an extinct coral reef. When the conditions were arid,
this closed basin evaporated and deposited evaporites of mostly halite
and anhydrite, but during more humid conditions sea water could leak through
the reef replenishing the lake with nutrients to the plankton grew and died
and formed carbonates. And if enough runoff could make it into the basin, silt and
clay deposited. The general rule for carbonates is they can only dominate an
area if the clastic supply is low to none. Just a little mud will overwhelm carbonate production, and
make a mudstone though perhaps a calcitic mudstone. Pollen can be thought
of as a clastic particle which gets incorporated into sediments in lakes,
and looking at pollen, which can often be identified to the species level within
these annual varves in lakes gives us wonderfully detailed information about
the changing eco-systems surrounding such lakes through time. The study of pollen in such sedimentary
deposits is called palynology, and the study of inland waters in general and
their interactions with the rest of the environment is called limnology. Lakes
are often damned, and dams can break and cause flooding. No matter their cause, flood deposits are less well sorted and
often have larger clast sizes. I say “often” because it depends on what material is
available for the flooding waters. Glacial outwash could be lumped as a
special case of these overly energetic water deposits and flood deposits in
general can be lumped as a special category of fluvial deposits. But more
often the streams leaving a lake are at typical flow, where we get these mixed
sediment sizes in fluvial deposits we previously discussed, which persist down
to the beaches of the world. All along the way there are soil deposits which
develop mudstones, and we mean mudstone and not shale as the plate silicate minerals
are not lined up. With water saturating the lower part of a soil stack oxygen
tends to deplete there, but above the saturated portion of the soil air keeps
the oxygen levels high enough to change the basic chemistry of one of the most
common elements on earth; iron. With no oxygen iron becomes
chemically reduced and we call it ferrous iron, which tends to display green
to gray colors in rocks. With oxygen present, iron is oxidized to ferric
iron, which tends to display yellow to red colors. Mudstones with brightly
colored greens and reds are most likely to be ancient soil deposits which we
call paleosols. But land gives way to water at the beaches of the world, and
there we get a range of particles depending on the average energy at the
beach and the source material, but overall tend to be well sorted with the
rolling motion at the beach rounding larger clasts. Of course most beaches accumulate
sand, but very well-sorted is the general rule, and beach deposits extend laterally
much farther than the stream deposits. Terrestrial fossils transition to marine
species at the beach, or even more specific beach species such as certain
crabs, starfish, and coastal plants. Like the alluvial fans, as a river goes from
a constrained land channel into a still body of water like a lake or the sea, it
will deposit a feature called a delta named after the sediment deposit at
the mouth of the Nile River, which looked to ancients like the capital Greek letter
Delta, but there are various types of Delta’s which can be classified as: wave
dominated, tide dominated, or even fluvial – river dominated. The Ganges and
Brahmaputra Rivers join to form a delta that is tide dominated with less
wave action. The Nile Delta is wave dominated with less tidal action. And the
Mississippi birdfoot delta is a fluvial dominated delta, as the Mississippi River
discharges about a ton of sediment per second on to its delta, and the Gulf of
Mexico is relatively low in both waves and tides. Deltaic deposits are usually
fine grained, mostly mud, with various bed features such as the bottomset beds, foreset beds
depositing off of horizontal, and topset beds on top. Evaporites show up again at beaches,
appearing first above the high tide line in sabkhas, which will have carbonates,
evaporites, and some silt to fine sand. Otherwise, evaporites are more common
along hotter/drier shorelines, and where bays can get cut off and evaporate
faster than the water input. If the bays are sheltered and have enough water and
nutrients limestone can also accumulate, but as we noted before, limestone
formation requires little to no clastic input; thus our specifying sheltered
bays – no rivers run in from the shore side and
reefs off to protect from high-energy ocean waves. Limestone in the sheltered
bay makes uniform usually micritic, sometimes oolitic, limestone layers leading
out to the protective reef where the rock becomes coquina; the limestone made
of skeletal fragments of reef builders. Brecciated, that is angular broken up skeletal fragments from the reef, find
their way offshore into deeper waters. But if the shore is unprotected from
clastic input, the classic form is to have sand at the beach, which is much more
uniform and laterally continuous than fluvial sand deposits, and contains
fossils of beach loving species. As we move offshore we get the smaller silts
settling out, and then the clays even farther out. Silts and clays show up in
aeolian, fluvial, and lacustrine systems as well, but none of those extend
quite as far as the marine silts and shales. And the iron in these deep water
deposits is more likely to be reduced than oxidized. The dinosaur Cretaceous
aged Pierre Shale on the Colorado School of Mines campus contains marine fossils,
and can be mapped across the North American continent; larger than any river
floodplain or lake could make. Just the size tells us there was an
intercontinental sea through the middle of our continent in the Cretaceous. With
repeated reminder that limestone forms were there is little to no clastic input,
it should be no surprise that if we can get out past the beach where the sands get
removed, past the nearshore silts, and the offshore clays we can finally get to
where the calcite planktonic skeletons can be the dominant clastic particle
settling to the sea floor, and we see the deep marine limestone forming. In Dallas
where I grew up dreaming of being a geologist, the rock in the nearby creek
bed was Cretaceous aged Austin Chalk limestone with marine fossils. I was hunting for dinosaurs in the right
age rocks, just not the right environment. This limestone tells me in the deeper
parts of the Intercontinental sea, limestone formed. There is a catch here
though. Calcite dissolves better in colder,
higher pressure water, which is exactly what we find in deeper oceans. Thus when
we get down to about 4,200 to 4,500 m depth, calcite begins to dissolve
faster than it precipitates, and no calcite, and thus no limestone gets
preserved. Below this depth, called the Carbonate Compensation Depth (or CCD), calcite
does not accumulate. And so even with very low clastic input, windblown silt and
clays and insoluble organics are the only things to settle into the deeper
seas at very VERY slow rates. But there is one more dramatic sedimentary
deposit that can occur in these deep marine settings; the submarine fans of
coarser material, and often including graded bed turbidites from higher
energy density currents moving sediment down into these deeper waters. Though
turbidites can also occur in lacustrine, lake environments, they’re more likely
marine and often deep marine where the continental slope breaks into the
abyssal plane of the seafloor. The presence or absence of calcite can be a
clue to just how deep these deposits were. And speaking of the solubility of
calcite, it should be no surprise to see buried and pressured limestone take on
stylolites, the result of dissolution we met in the secondary
sedimentary structures, but with that we are seeing the first changes of rock in
the solid state. When we come back next time we will
start our investigation of the last rock type; the metamorphic or changed
form rocks which have been altered physically and/or chemically in the
solid state. We’re going deep next time to meet these mysterious information bombs, here on Earth Explorations.

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