do you want to know everything there is to know
about sedimentary structures well in today's video i'm going to be discussing all things sedimentary
structures as part of the sedimentology extra tv playlist here on my channel and this is the sixth
lesson in this playlist and in today's video specifically we'll be going over stratification
and lamination cross bedding planar versus trough cross bedding herringbone cross bedding ripples
versus dunes graded betting normal and reverse and growth betting so let's get started first
things first i want to touch on the differences between beds strata and laminations if you look up
the difference between beds and strata on google it'll tell you that a bed is a piece of furniture
usually flat and soft for resting or sleeping whereas a stratum is a parallel horizontal layer
of material stacked on top of each other so that's not what we're talking about here in
today's geological lesson in terms of geology we're talking about layers of rocks when we say
beds or strata and in terms of the difference between strata and laminations some define strata
as greater than one centimeter in thickness and laminations as less than one centimeter in
thickness however others get a little bit more specific and use the term beds instead of strata
and say that thick beds are over 60 centimeters thin beds are 50 to 60 centimeters and very thin
beds are one to five centimeters and laminations are two millimeters to one centimeter and very
thin laminations are less than two millimeters and so you can see that some people use
the terms of beds and strata as synonyms but it really just depends on what definition you
go by so your professor or whoever you listen to talk about this could use strata and bed as
the same thing but in general i think beds are typically thought of as to be larger scale and
so the larger units of the outcrop we typically go okay those are beds and strata or stratification
and laminations are typically smaller scale just based on what i've experienced and how people
talk about layering of rocks in the field not all rock layers or beds or strata are made equally
though regularity of bedding can greatly vary and also can be described in terms of the uniformity
of thickness of the beds in an outcrop as well as lateral continuity as well as their lateral
uniformity and thickness so basically four classes of beds can be made those equal in thickness
and laterally continuous as well as laterally uniform in their thickness those unequal in
thickness but laterally continuous those unequal in thickness laterally variable in thickness but
still continuous in lateral direction and those beds that are just down equal in everything their
thickness with other beds above and below them as well as their thickness laterally as well as
they are discontinuous if you're wondering what a discontinuous bed looks like there's one that
pinches out here it's this half lens shown in this bottom middle picture and sometimes you can
get lenses in certain depositional settings so you can think of geometry and grain size as useful for
determining proximity to that material source but grain size is something we talked about more in
a couple videos ago i'll link the video up here if you want to check it out and defending geometry
or sedimentary deposit geometry is something we'll talk much more about in a couple videos coming up
in this playlist but in terms of what i'm sure all of you are wondering what is the depositional
environment for planar stratified or laminated sediment planar laminations which obviously
laminations as we just mentioned are very thin are characteristic of fine-grained sediment silt
to shale size and are indicative therefore of calm deep water settings such as liquestrine or
pelagic marine settings now remember that pelagic marine is just deep water marine not coastal
and lycustrine just means a lake type setting a central lake type setting again not a coastal type
of deal because we're getting fine grains we're not getting coarse grains right off the coast um
from delta inputs or fluvial inputs we're just getting the settling of fine grains in the
middle of a calm lake or a marine setting and i say here in regards to marine settings
below the wb the wb is the wave based which is why i show this figure here basically there are waves
that come into shore we all know that if we've been on a beach or seen a beach in movies
or whatever but these waves are obviously seen at the surface but their influence only
goes so deep into the water and the depth at which their influence stops that is the wave base
depth and so below that depth the sediment is not moved or influenced by waves and therefore can
have and preserve planar laminations rather than the sedimentary structures that form due to waves
like symmetrical and further onshore asymmetrical ripples as we'll talk about later and here we
can see the actual depositional environment for this offshore below the wave base this just
stands for fair weather wave base because the wave base increases in depth when there's storms um
so this environment specifically is the offshore sore phase environment and so if you want to
know more about that i talk a lot more about shore face environment both the offshore the shore
face lower middle and upper and then the foreshore in my short phase processes and strategic video
as shown on the screen here and i'll link it up here to the top right if you want to check it out
and sometimes planar laminations can be associated with or actually a lot of times i should say
associated with fine to coarser laminations due to seasonal variation so basically for example
in ice proximal lakes um which varves are common varves just literally means find of course
alternating laminations in these settings you have summer bring in a lot of coarser sediment when
ice that's proximal to the lake is melting and all the sediment entrained in that ice is
being released and these flows are picking up more coarse sediment and just bringing it
into the lake and causing these currents to you know bring sediment to the lake whereas during
the winter you're getting all of the fire green things that are suspended in the water column
settling down very slowly making very thin dark layers during that season and then you'll get the
thicker layers of coarser grains in the summertime you can also recognize when it is varves that
form in a glacial liquestrian or glacio marine setting that sometimes there's drop stones which
is really indicative of that kind of setting but the vargs themselves are already pretty indicative
of that kind of setting however you can also get alternating seasonal-ish laminations when you
have tidal rhythmites preserved these are rarely preserved but when they are they represent
the spring and neap tides basically spring tides are stronger tides and so they'll bring
coarser sediment and make those thicker coarser uh lamination deposits and then the neat tides
are much weaker and will bring finer grained and thinner laminations however again i talk much more
about these processes and their stratigraphy in my title my glacio marine slash glacio la custeren
and my lacustrine lacustrine um videos and i will just link the depositional environments playlist
up here to the top right because all of those videos are in that playlist for you last thing
i want to say about planar laminations is just that their preservation often represents an anoxic
environment which allowed for the laminations to be produced without being interfered mixed up and
ruined by bioturbation which is just the burrowing the digging the grazing and all that of organisms
that live in these settings these aquatic settings the reason that anoxic environments help
with this which just means a lack of oxygen is because animals that do this bioterration
need oxygen and so when there's not oxygen those animals are not around to do the bioturbation and
therefore the laminations get preserved much more prettily and are not messed up or distorted in any
way and so i talk more about anoxic environments and how they help with preservation in this way
in my hydrocarbon preservation video which talks more about hydrocarbon preservation but also
other minerals associated with those hydrocarbons and why anoxia is important for this type of
preservation moving on now to cross betting cross betting or cross stratification again
sometimes these are used synonymously they form when ripples or dunes migrate due to either wind
or water currents it's important to remember it doesn't have to be water when we're talking about
currents wind in for example eolian environments can cause dunes obviously because we see you
know sand dunes in aeolian or desert environments and so when these ripples or dunes migrate due to
wind or water that pushes grains from their lee side to their side it causes the formation
of cross bedding or cross stratification however cross bedding comes in many flavors two
of the main flavors that cross bedding comes in is planar cross bedding and trough cross bedding
these are common in many depositional environments heolian or desert environments shore face
environments at or above the wave base remember that below the wave base planar laminations or
planar stratification occurs and at or above the wave base where the wave base or the waves are
interfering with the sediment is where we can get some ripple formation and the ripple migration
that causes the cross bedding whether it be trough or planar and then also in tidal environments
and i say if strong enough sometimes as i'll show later the flood and ebb currents aren't
strong enough in tides to form cross bedding but sometimes if they can pick up coarse enough
sediment for example sand to form the cross betting they do form planar cross betting and
then also fluvial environments can form both planar and trough cross betting and so you might
be wondering if planar and trough cross betting is common in all of these environments then what
is the difference between planar and trough cross betting in terms of their formation processes
well it's pretty simple basically straight or linear ripples and dunes produce planar
cross stratification or planar cross betting and linguloid or tongue shaped ripples and dunes
produce trough cross stratification and they don't even have to be fully lingualoid or tongue shaped
they can just be a little bit curvy and you can get some weak to strong trough cross betting which
basically just means that the trough crossed beds cut across the troughs of the other beds as we can
see in this figure here and this is in contrast to don't get this confused with hamake or swalely
cross bedding so humikin swalicross bedding or cross stratification is not the same thing as
trough cross bedding and we can see that again the trough cross beds cut across the troughs of other
beds whereas in hamake and swelley crosstrack the beds don't cut across each other sharply and the
curves are at a much shallower angle i remember when i was taking substrate and we were in the
field my professor would always say look at the angle look at the angle so look at the angle if
it's a much shallower angle and it's not cutting across the other beds sharply it's probably
humming key or swelling crosstrack and if it is cutting across sharply and the angle is steeper
then it's probably trough crosstrack and humic and swelly cross bedding is common in some short face
settings um for example when there's storms and the wave base deepens but i talk much more about
that again in my short face environments video if you want to check it out i also talk about the
different morphologies or shapes of dunes in my aeolian video which can also to some extent be
extrapolated to ripples as well so if you want to know about all the different shapes that ripples
and dunes can form you can check that video out moving on now to herringbone cross stratification
now herringbone cross stratification as we can see in for example the upper left image is basically
just planar cross stratification that goes in an opposite direction to the bed above or below it so
this is common for example in tidal environments where you have a flood and an ebbtide those are
opposite direction water flows and the flood tide can deposit a cross stratification in this example
going in flow direction to the left of the screen and the egg tide can then come backwards
and deposit cross-stratification in the opposite direction in this case showing the flow
direction to the right of the screen in the upper left image however most tidal situations don't
have an appetite as strong as their flood tide typically as we see in the far right picture
the flood tide is much stronger than the eptide and all the eptide can do is deposit mud because
it can't carry coarse enough sediment like sand to make cross stratification here are a few
pictures to see each situation that i showed in the previous slide basically in the top right we
have a herringbone cross stratification relatively equal in direction as flood direction and then we
have to the middle a sort of strong flood and a lightly ebb crosstrack ability um with the
eptide being less strong but not totally too weak to carry sand and so that shows mostly
crosstrack in one direction the flood direction and then a little bit in the other or ebb
direction and then lastly the most common scenario where the eptide is only strong enough
to pick up mud and not make cross stratification we have just single direction flood direction um
cross stratification and in this top right picture again it would pretty much just be called
planar cross betting it wouldn't even be called herringbone at that point because you don't
have the opposing direction but where you can tell that it is tidal at least is in between the cross
beds there are layers of mud that represent the ebb tide and so if you have those layers of mud
in between you know you're at least in a partially aqueous environment uh it wouldn't be eolian cross
stratification for example which is typically fully sand and does not contain mud again guys if
you want to know more about this you can check out my title uh depositional environments video so
i've talked now a lot about cross stratification or cross betting which is due to the migration
of dudes or ripples but what exactly is the difference between dunes and ripples rachel well
guys it is very simple it's literally just scale um basically ripples are much smaller than dunes
and in terms of definitions dunes are typically over half a meter in their height and ripples are
confined to a height of one to two centimeters now you might be thinking there's a lot of room
between two centimeters and half a meter what is that defined as well this is an interesting
concept because this is called the forbidden wavelength gap typically stable bed forms cannot
form in that size and that's why ripples are so tiny and dunes are so large as we just haven't
really observed processes here on earth that form stable ripples or dunes that are two centimeters
to half a meter um i'm sure there are bizarre exceptions to this that i just haven't seen in
the literature if you guys know of some please comment them down below but based on what i read
this is the case and here we can see a picture of a raw camera showing ripple marks in the bedding
here and then over here to the bottom left we see a picture of preserved dunes you can just see
the difference in these two pictures of the scale i mean here's a little person down there and this
is just absolutely humongous and these ripples are just very tiny unless that hammer is humongous and
just before we move on from ripples i want to talk a little bit about how ripples can be symmetrical
or asymmetrical and i want to talk about this because if you do see one or the other types of
these ripples in the rock record it can be very indicative of the environment because symmetrical
ripples are caused by bidirectional flow again just like the title environment we talked about
when there is a flood and an ebb current that's bi-directional there's also mid to upper shore
face environments that can cause symmetrical ripples it's not necessarily bi-directional flow
that causes the symmetrical ripples in this case it's just that the waves before they break
and hit the breaker zone they're oscillating in a very symmetrical circular pattern in this
forms symmetrical ripples whereas once they break they become unidirectional because the flow
becomes unidirectional the current at the breaker zone after the waves break is fully going
toward the shore whereas the oscillatory motion of the waves themselves in the upper to mid
shore face before they break is not technically unidirectional it's kind of hard to explain it's
more of an oscillatory thing than a directional thing that's why there's symmetrical ripples in
that zone and i talk much more about that in my short face environment video however symmetrical
ripples are rarely preserved in the rocker which kind of sucks because they are so indicative
of this type of wave base environment and again asymmetrical ripples are caused by unidirectional
flow typically in the upper or breaker zone short face to foreshore environment as well as
fluvial type environments obviously fluvial or river environments are going in one direction
and then an environment i forgot to mention here is submarine fans or turbidity currents
which can also cause unidirectional ripples another type of ripple i want to talk about
really quick before i move on to other structures is climbing ripples climbing ripples form by the
net deposition basically when rate of movement of sediment is greater than the rate of entrainment
or basically when everything is becoming deposited when a flow is decelerating or waning and that
is typically associated with fluvial floods or turbidity currents so when fluvial environments
or basically rivers flood they undergo a decrease in flow velocity when they become higher than
their confining walls or banks and so when they go over the banks they slow down significantly and
deposit all their sediment while also still moving forward and this depositing and moving forward and
decelerating um type of math equation when it all comes together causes climbing ripples climbing
ripples are just freaking beautiful in the rock record i mean you can see a picture over here to
the bottom right um showing the climbing ripples and the flow direction to the right basically it's
exactly what it sounds like they're like climbing each other's lease signs and you can see that over
here as well in the middle picture going to the left and you can see their difference between
like the normal planar cross stratification forming ripples when they migrate they form these
like planar cross strap beds above and below each other and then the climbing ripples form this like
ripply bedding that's just beautiful and again this can happen in both you know floods of rivers
as well as turbidity currents which are basically submarine fan flows that have all sorts of
sediment contained with them and then they deposit all their coarse sediment first and further and
further out in the flow they're depositing finer and finer sediment and they have this really
predictable sequence called a turbidite sequence or even the boma sequence which we'll talk
about in a second when we get to the graded bedding because that's like really significant
for graded bedding but you can see in the sequence that there is a rippled section that can
contain unidirectional ripples that are migrating in a planar bedded fashion but also sometimes can
contain climbing ripples because it does indicate waning flow finally moving on from ripples
and dunes and cross strad and all that we have heterolithic vetting heterolytic netting is pretty
well defined i mean it's heterodifferent lithic rock benny and these are interbedded deposits of
sand and mud and they can have different amounts of sand and mud which define them differently in
each case basically it's called flaser bedding if the mud is present in a sandy matrix or there's
mostly sand but a little mud and it's wavy bedding when the mud to sand ratio is around 50 50 and of
course there's a little bit of variation in this and then it's lenticular bedding when it's mostly
mud but there's also sand present in the money matrix these heterolytic bedding deposits are
typically deposited in environments such as tidal environments like tidal flats or thai dominated
deltas or estuaries sometimes glacial environments fluvial environments rare but occurs in that
case and occasionally in deeper water settings with weak storm action like i mentioned in my
title video if you want to confirm that this heterolytic betting was caused by tides or in
a title depositional environment you can look for neap and spring cycles or the presence of
bi-directional current ripple orientations for example if you look at these beddings um you
can see most of the cross stratification is um directed toward the right of the screen but
then there's also some that's directed toward the left of the screen and when you see that you know
there's bi-directional flow that means potentially a tidal environment where you have a flood
and an bi-directional system moving to graded betting again i mentioned that i would talk
about the bulma sequence or the predictable sequence deposited by turbidity currents which
are basically like slope failures or just debris flow type things that are sub aqueous in a
continental shelf in deep sea environments or the intersection between those two environments
and they're called or at least what they form is called submarine fans and the current is called
the turbidity current and then the deposit itself that has that predictable sequence is either
called like a boma sequence deposit or a turbidite or a turbidite sequence whatever you want to call
it but this is a predictable sequence that often contains rated betting or actually i should
say always contains graded betting again it's predictable however sometimes they don't get fully
preserved and they get truncated either off the top their sequence or bottom of their sequence and
so the graded betting isn't always preserved but when preserved you can typically see you know one
turbidite sequence above another and here we can see the sequence which i briefly showed earlier um
to the right here this sequence contains basically they call the sections t a t b t c t d n t e and
the sequence of sediments goes from graded betting of relatively coarse um compared to the rest of
the beds sands and gravels and then tb contains high energy planar laminated sands now we did talk
earlier about how plano laminations are typically indicative of low energy environments so what
the heck are high energy planar laminated sands well again i was talking earlier about finer grain
like silt to mud planar laminations however there can be typica i think the only case where you get
these high energy planar laminated sands which are sands which are coarser than typical laminations
is in these turbidite sequences so your tb section of turbidite sequences is i think
the only place where you can get this but this is what tb is and then tc contains sands that
exhibit um at least on their top of their section current ripples unidirectional flow ripples
sometimes at the top top you can see climbing ripples indicating weaning flow and at the bottom
part of tc it's typically convolute type bedding sometimes trough cross bedding then td is planar
laminated fine sand and mud which is deposited much more slowly and more calmly and more typical
of the types of planar laminations we talked about earlier and then lastly we get the deep basin
muds which are the really calm and really fine laminated muds that are common in like pelagic
marine settings or calm middle lake settings and if you want to know a lot more about turbidity
currents and where they occur how they occur what causes them and the structures that they produce
in the rock record and how to recognize those structures even if they're truncated and not fully
preserved i suggest you check out my submarine fan depositional environment video before we move on
to the last centimeter structure which is going to be growth betting i do want to mention that there
can be reverse grading reverse grading as opposed to normal grading which we saw in the turbidite
sequences is instead of going from course to fine it goes from fine to coarse grading and i
talk about how this might occur in my alluvial fan video where we talk about this process called
kinetic sieving and it's rare um even in alluvial settings but if it does occur it occurs in a
specific type of alluvial fan setting so you can watch that video to see how reverse grading
might occur but most of the time grading is normal and most of the time normal grading is caused by
turbidity currents moving now on to growth betting here we must make the distinction between chemical
and detrital once again i know i've talked about this distinction many times in my set strat
playlist so far but basically uh rocks are basically all of those we talked about so far
in this video where detritus or loose sediment is transported by some current and deposited and
accumulated and then lithified into rock however chemical sedimentary rocks or sedimentary deposits
warmed by the chemical precipitation of minerals from solution and this for example includes
evaporates evaporative or plyo lakes like like maggady shown down here at the bottom have a
rainy season where they are filled with water as shown in the bottom left and they also have a dry
season where they basically completely evaporate and when they evaporate they leave behind
all their salts from solution because the solution that's evaporating is pure water they
don't want to take their ions with them when they evaporate they leave behind the ions which when
the water is evaporating they start to precipitate out as minerals or evaporate minerals as they're
called because that solution becomes so super saturated that it just can't handle having those
ions dissolved anymore so they precipitate out as solids and these solids that result form these
bedding layers of what's called growth bedding because it's growing in place rather than being
transported and deposited and i talked more about this milo custom video if you want to check that
out the last seminar structure we will talk about today is mud cracks mud cracks form pretty
simply in mud by the shrinkage of that mud upon drying these are also for that reason called
desiccation cracks and basically the presence of mud cracks uh whether it be preserved in the rock
record or just in general when you are see them in the environment like i do all the times i live in
the desert in el paso um that implies an exposed environment aka it's not underwater and it also
implies that there was a mud substrate for example sand does not shrink upon drying so they cannot
form in coarser grains than mud and because mud cracks often preserve as casts of sand filled
mud cracks because mud cracks themselves can't preserve as cracks because a crack is a void it's
nothing how does it preserve as nothing it can't so it has to be filled with something overlaying
on top of it for example sand and the filled mud cracks are what preserve and typically the mud
is weathered away and then you just have the sand filled cracks or the casts of the mud cracks
preserve and you know that mud had to have been underlying that sand because mud cracks can only
form in mud and we'll talk much more about mud crack casts in the next video when we talk about
netting plane markings so stick around for that video coming up soon and i hope you guys enjoyed
this video if you want to know the references i am using i am using the principles of sedimentology
and stratigraphy by sam boggs and sedimentary rocks by fj petty john if you want to check these
out they are linked in my description below and if you want to check out any of the videos listed
here above the white arrows those are all videos that i've already done and are definitely out
on my channel by the time you're watching this and even below the white arrows may be out on
my channel by the time you're watching this depending on when you're watching this so you can
check those out at the cinematology entertainment playlist on my channel that is linked below in
the square that just showed up on the screen at some point that is down there you can click click
on that thanks okay i'll see you guys later bye