sedimentary rocks form from layers of sediment sediment is any loose particles or fragments of rocks or minerals at earth's surface produced by weathering processes that we discussed in the first part of this module so over here we see loose sediment sand and gravel right those are going to undergo a process of lithification or becoming a rock that is going to end up cementing that loose sediment together to form solid rocks like sandstone or this rock with the rounded rounded gravels in it is called conglomerate the first type of sedimentary rocks that we're going to discuss are the clastic sedimentary rocks so clasts are rock fragments so the clastic sedimentary rocks are composed of fragments of rocks and minerals that were formed by physical weathering processes that we discussed in the first part of this module the clastic sedimentary rocks are basically classified on their grain size or the size of the clasts okay so here we have the terms for the various sizes of clasts the largest size class are boulders okay so very very large particles and then gravel sand and the finest particles are silt and clay these usually aren't separated from each other so together they are referred to as mud all right so we're decreasing in size from boulders to gravel to sand to silt and clay all right so these do these terms they do have specific size ranges for the particles you don't need to worry about that what you need to know is the terms and which order they go in that boulders are larger than gravel gravel is larger than sand etc so the particle size is going to tell us something about the environmental energy that these sediments have gone through all right so what we're seeing in this picture is the um the transfer of material from its original source area where it's being weathered and being transported in this case in a stream all the way to the ocean all right as that transport occurs or that erosion we're going to go from large grain sizes larger particles to more medium-sized particles than to very very fine sized particles okay so the grain size is going to decrease the farther that these particles have traveled that's because the particles as they're moving in this in this case in a stream the particles are going to be banging against each other they're going to be bumping against the bed of the stream and that is going to cause the grain size to get smaller and smaller okay so the smaller the grain sizes the farther that sediment has traveled from its original source area in this case the mountains okay so the reason that those particles are getting smaller is due to this process of abrasion so abrasion is just meaning that the rocks are either hitting each other or scraping against each other or grinding against each other and that is going to remove material from the outside of that particle okay so that is going to cause weathering as the material is being transported and it's going to cause the particles to get smaller as we talked about in the previous slide abrasion is also going to cause the particles to become more rounded all right so here we kind of see the two extremes of rounding where we have angular particles over here all right so they have sharp edges and corners to a very well-rounded sedimentary deposit of these pebbles here these would have been tumbled in a river losing all of their sharp edges and corners okay so abrasion is causing the particles to get smaller but it's also causing the particles to become more well-rounded so the farther that the material has traveled from its source area the more well well-rounded it would be if we see a very angular deposit like these rocks here that indicates that these have not been transported very far away from their source area they have not gone undergone a lot of abrasion and so they're still very large and they're still very angular the next thing that's going to happen is the particles are going to become what we call more well sorted so sorting refers to the distribution of the sediment size meaning are all of the particles in this deposit about the same size or are they all very different in size so so the sorting is going to become better the farther the material is transported so here we can see the two extremes of this sorting over here this is a very poorly sorted deposit right we've got really massive boulders some smaller boulders we've got gravels we've got sand and we've probably got silt and clay in there as well so this is very poorly sorted we have all of the different sizes of grains the sand dune however is very well sorted right these are all about the same size sand grains um so that would be very well sorted there there's not a distribution in size they're all about the same size as each other so the sediments are going to become more well sorted the farther they are transported so in this case we're up in the mountains we're very close to the source region for these for these sediments for these um different grain sizes and so they haven't traveled very far and so we get a very poorly sorted deposit the sand in the sand dune has traveled a great distance so in this case the method of transport is the wind not not running water and wind is going to be very good at sorting sand grains because wind is only going to be able to pick up certain sizes of grains the wind's not going to be able to pick up big old boulders like we see over here so the wind picks up the sand and then eventually the sand gets deposited the wind deposits the sand in the sand dunes forming these very well sorted sand deposits okay so the sorting is going to get better as well okay so if we look at these put this all together we see that the rounding is going to increase as we go farther with transport so here these would not have been transported very far from their original source area they're very angular after they've traveled a bit they start to become rounded so now you can see that they've started to lose their sharp edges and corners but they're not really round yet okay they're not as angular but they're not quite round and then eventually we get very well-rounded grains where all of those edges and corners have been smoothed out and the greens are actually approaching a spherical shape the sorting is also going to increase as we saw all right so here we have a poorly sorted deposit we've got a wide range of sediment sizes a moderately sorted deposit right we're getting closer to the the sizes being about the same and then a well sorted deposit where all of the particles are about the same size okay so the size is going to get smaller the rounding is going to increase the sorting is going to get better and also the proportion of certain very stable minerals like quartz is going to increase so if you remember from our discussion in the weathering part of this module quartz is very stable at earth's surface because it forms close to the surface close it forms at low temperatures close to the surface and so it's very stable at the surface and doesn't really undergo any chemical weathering process so what's going to happen as the the sediment deposit travels farther the more susceptible grains um are going to undergo things like chemical weathering they're going to break down and so ultimately by the time we have traveled very very far distance the proportion of the stable mineral quartz is going to increase okay so this is seen um you could see this at the beach when you go to the beach if it's a nice white sandy beach basically what you're seeing there is just a lot of sand-sized grains of quartz because quartz is very very stable at the surface conditions of the earth okay so to sum all of that up the grains are going to get smaller they're going to become more round they're going to become more well sorted and they're going to be composed of more durable minerals like quartz with increasing transport distance so at our source area in the mountains we are going to tend to have larger bolder or gravel sized particles that are angular in shape as they are carried in a river they start to become more rounded and better sorted we get to the beach and we're basically just left with sand side grains and then offshore into the ocean we get very fine grain material mud deposited like we see here okay so once that material is deposited it has to undergo a process to become a sedimentary rock okay so the sediments are going to undergo this process of compaction and cementation which is going to lithify the sediments into a sedimentary rock so if we look over here in this scenario we have a lake okay and we've got sediments being deposited into the lake probably by a river so the sediments are going to settle down onto the bottom of the lake and at the top up here they're going to be pretty uncompacted meaning that they're forming layers but there's still going to be a lot of water in between the grains of sediment here okay and so we can kind of look at a column here and see what that would look like so up here uncompacted sediments as more and more sediment continues to be deposited on the bottom of this lake eventually the pressure is going to increase such that the sediments start to become compacted right and so that's shown here so more and more particles are are piling up that is increasing what's referred to here as the overburdened pressure on the materials at the bottom and they start to get compacted right so they start to fill in all the little pore spaces in between and the grains get very very close to each other as that's happening the porsche base is decreasing and the water that was in between these grains is escaping upwards to a place of lower pressure okay so that's the compaction part here right so we can see here if we have clay sized particles and they're just kind of randomly oriented and there's lots of water filling the pore spaces so that would be what we would have up here and the uncompacted sediments as more and more sediments get layered on top that over overburden pressure increases and that is going to compact the sediment and get all of that water is going to escape out okay so that's the first part of this lithification process is compacting the sediments the second part is the cementation process okay so here what we if we look over here we have groundwater influx so groundwater is water that moves through pore spaces or fractures in bedrock underground and in this case we've got igneous crust okay so there's groundwater that's moving through this igneous crust and it's able to seep out of that crust into these sediments that groundwater has chemicals in it it has elements in it things like silica or calcite in that groundwater okay so here if we look at our grains our compacted grains in this case we've got this mineral bearing groundwater that's able to circulate through the small pore spaces that are left after the grains have been compacted okay eventually what's going to happen is this material is going to crystallize it's going to precipitate out minerals that forms what is referred to as the cement so the cement is what is gluing the grains together and is what is filling in any pore spaces that are left behind after the compaction process okay so it's this two parts the grains get compacted together and then mineral bearing groundwater can move through precipitate out mineral cement that glues the rock together to form something like a sandstone okay finally the clastic sedimentary rock classification is based on grain size so there's just four clastic sedimentary rocks that i want you to know about the first two both have gravel-sized particles okay so you can see here that's the common name it could have a wide range of particle but we're just going to compile those all together into gravel the difference between these two rocks is the shape of the gravel conglomerate has rounded gravels okay so the gravel size particles within that conglomerate are round this rock breccia has angular-sized particles okay so both are gravel size their their dominant grain size is gravel but conglomerate has rounded grains and breccia has angular grains so that means that the particles that the gravel that forms the conglomerate had to have traveled farther than the particles that are forming the brecha right so they because they become more rounded due to abrasion sandstone is pretty easy because the name of the grain size is right in the name of the rock so sandstone is formed of sand grains all cemented together there are there are different types of sandstones uh if you're in the lab you'll probably be seeing different types of sandstones but for our purposes we're just gonna we're just gonna worry about just a generic sandstone and then finally the finest grain clastic sedimentary rock that i want you to know about is the rock that's called shale it's also sometimes referred to as mudstone because it is made of the mud particles right remember silt and clay together they often don't separate from each other and so together those are referred to as mud so shale is formed from mud-sized particles it's very very fine grained right so that's the four clastic sedimentary rocks that you should know