Jurassic Entrada Sandstone - The Rocks of Utah

welcome to another episode of the rocks of Utah my name is Benjamin Berger and in this video we're going to be exploring one of the most iconic rock formations in Utah the Jurassic and trata sandstone I'm here just outside of Moab Utah in arches national park home to the largest and highest concentration of natural bridges and arches in the world this arch behind me is undoubtedly one of the most famous it's called delicate arch and it stands 18 meters high with 14 meters span underneath the arch so this arch has come to symbolize the landscape of Utah with these orange sand stones this orange landscape contrasted with the blue sky it's unlike any other place in the world arches national park is over three times the size of Manhattan Island in New York City and it features spectacular exposures of the Jurassic entrada sandstone which is the primary arch forming sandstone around Moab and within the boundaries of arches national park now not all of the arches in Utah actually formed by the entrada sandstone many are formed by the Permian Cutler group such as the Oregon rock formation the cedar Mesa sandstone forms a lot of arches as well as the Navajo the Windgate and the nugget formations all form arches like these so the the the jurassic entrada formation is the one that really forms a lot of the arches around arches nat'l monument now the jurassic entrada sandstone here in arches national park has a very serendipitous geological history that really promoted the formation of natural which is and bridges so the jurassic entrada sandstone was deposited a hundred and eighty million years ago and it represents layers of sand that were deposited in yo lien sand dunes and there was a great sand sea that existed here during the Jurassic during this period of time here in Utah so what are the cool things about being try sandstone is you can see how the sand laid down was laid down in these sand dunes so you can see these sedimentary structures these really widespread cross trough beds what we call them and these are very indicative of eolian deposits deposits of sand that were laid down in sand dunes so you see you can see how the sand was dipping off of these sand dunes and piling up and it forms these little layers and the difference between these little layers is slight differences in the type of class the type of sand grains so sometimes it might be a little bit more finer grained and other times a little bit more coarser grained and that's what gives us wonderful beautiful banding pattern you see in the entrada sandstone now occasionally there would be marine intrusions of the ocean and they would make their way southward into this area as transgressions of the of the ocean and these sediments were mud flats they were or carbonate rich rocks in the Jurassic and these layers formed mud stones and limestone's and softer sedimentary layers so the jurassic entrada sandstone is deposited on top of a softer unit a mud stone called the the Carmel formation and then it's capped by another soft unit the the Curtis or the Somerville and that's capped by the the jurassic morrison formation which caps us those are composed of softer mudstones so here about the at the base of the entrada sandstone which is up here and this is the Carmel formation it's kind of sandy in this area and it's known as the Dewey bridge member of the Carmel sandstone has more calcium carbonate in it so it it erodes and we can see there's like a little mud stone right in here and that's what you're roading out so I'm in one of these alcoves that eventually might become an arch and if you look up here in in this area we have like a layer of some mud that's interbedded in to the entrada sandstone and what's neat about this is that that mud is gonna erode out faster so it's cutting in the overhanging sandstone and as that cuts in it's gonna form this big arch here so in here we have these little holes that are all along the sandstone here and these are caused by chemical weathering so what it is is these holes that are in here these little cavities these little mini tiny little arches and natural bridges are formed because what's holding the sandstone together is a glue called calcite calcite is composed of calcium carbon and oxygen and it's the glue that's holding everything together but calcium carbonate also known as calcite and mineral form when rain comes down when we have these infrequent rains here in Utah that rain is slightly acidic and it reacts to the carbonate glue and it basically starts to erode it it starts to dissolve some of that calcite forming carbonic acid that slightly acid rain water that's coming down and some snow as well you basically starts to work on those little cavities and once you get a little cavity like this opening up that rain will continue to kind of open expand that out and that's all caused by chemical weathering of dissolving out the calcium carbonate it's the same process that forms caverns and caves but here since we have a sandstone that's composed of calcium carbonate it's making these weird sort of great little little arches and bridges and holes and nooks all throughout the entrada sandstone so it's really kind of a special type of weathering so one of the most important principles in geology is understanding how the two different types of ways that that rocks are deformed or the way that rocks break and when when stress is applied to them now some rocks behave like a lump of clay and this is called ductile deformation so if you hit the clay with a rock hammer just deforms to the end of the rock hammer so this is ductile deformation uncooked lasagna noodles when I hit them with a rock hammer they break into little pieces and this is called brittle deformation so this region around Moab is called the paradox Basin and deep below the subsurface is our thick layers of deeply buried salt now this this salt basically kind of bobbed up moved up and that caused the overlying rock layers to kind of start to swell and buckle before they were actually buried really deep enough to become ductile so here I have kind of a sandwich of the mud stone of the Carmel down here which is a ductile zone mud and then we have the entrada formation formation which is these uncooked lasagna noodles and on top we have another clay layer another mud stone which is the Somerville or Curtis formations so one of the things that happened was that typically rocks will get buried deep underground and they'll all behave ducked early they'll all be you know start to bend and deform the same rate but what's interesting is because of assault tectonics that was pushing up when before these were buried very deeply the sandstone behaved Britta Lee and so as that was coming up it began to break through and you can start hearing the cracking of that sandstone in there mud stone of the overlying mud starts to erode away you can see these fins forming where it had cracked because of the deformation so here we have some of the fins that are in the entrada sandstone and what you can see is how the sandstone has broken apart in these little pieces and between those pieces there's erosion that's going on that's cutting down between those plates this is like the uncooked lasagna that has broken up and it's sort of each of those little pieces of the end cook lasagna is poking up through the mud stones so here we have some of these geological fins coming out these sandstone fins that have been broken apart pulled apart and erosion has worked in between them so making these big thin platy sand stones they're just poking out of the landscape so underneath us we're standing on the the contact with the Carmel formation which erodes pretty easily but these sand stones they acted more brutally they broke and they separated out and they formed these a wonderful erosional cracks that water was able to get down into you and start to erode them and start to dissolve out that glue that holds them together so though these like wonderful monoliths out here in the desert they're just this spectacular sandstone formations so they're they're just beautiful beautiful rocks and what's amazing is that this has created you know in a serendipitous geological Providence where you had this salt deformation that caused the rocks when they were still brittle to start to break apart and make these fins and that set up the weathering that we see in arches national park of all these wonderful jurassic and trata sand stones that break apart and begin to form these arches so there's some fantastic petroglyphs there everywhere but these are kind of neat because they're very late period in fact there they are after the introduction of horses and so you can see there's some horses in these petroglyphs which is really cool there's all sorts of little dogs they're following along and some big horn sheep that you can see delicate arch is eroding by the same processes and eventually those thin sand stones that are holding up the arch will eventually thin to the point where the arch collapses so it'll be a really sad day when the arch eventually collapses so while it's standing i recommend coming and visiting arches national park and coming up and seeing delicate arch while it still stands so I want to thank my supporters on patreon who made this video possible including max McGregor Shawn Corbit Brian clever Pablo Lucifer gays are kotas 18:11 Justin Bovie Emmett Larsson Marlo and Draco and Fred only thank you very much if you like to learn how you can support some of these educational videos on the geology of Utah check out the link below in the description

Channel: Benjamin Burger

Views: 1,577

Rating: 5 out of 5

Keywords: Moab, Geology, Arches, Arches National Park, National Parks, Utah, Jurassic, Dinosaurs, Visit Utah, Moab Utah, Sand Dunes, Natural Bridges, Geological Stories, Science, Lectures, Education, Eastern Utah, Grand County, Canyonlands, Vacation

Id: czb-h4Tf_hs

Length: 13min 1sec (781 seconds)

Published: Thu Jun 13 2019