Presentation on theme: "“One of the largest wind-deposited formations in the geologic record.""— Presentation transcript:
1 “One of the largest wind-deposited formations in the geologic record." Navajo SandstoneAndrew InfanteDennis TittertonZoe GentesAmy LombariJennifer Sullivan“One of the largest wind-deposited formations in the geologic record."Picture:
2 What you need to know:Navajo Sandstone is a sedimentary rock formation located mainly in Utah, Arizona, and Colorado.It is famous for its beautiful color variations caused by different types and amounts of iron oxides which were precipitated through the rock.Crimson, vermillion, orange, salmon, peach, pink, gold, yellow, and white.
3 Iron OxidesThe precipitated iron oxides also form layers, columns, and pipes of ironstone.The ironstone does not weather as easily as the sandstone and forms recognizable structures that jut out from the local rocks.
4 HistoryThe Navajo Sandstone was originally named as the uppermost formation of the La Plata Group by Gregory and Stone in Later, Baker reassigned it as the upper formation of Glen Canyon Group in 1936.The age has always been a complicated subject and had been speculated many times but then tweaked later.Originally it was called the Glen Canyon Sandstone but was renamed Navajo Sandstone for the Navajo area it is located in.
5 Stages“The ages of strata deposited in succession within a given age of time. They are often defined by a consistency found within the rock, such as index fossils or the polarity. In other words, stages are ‘groups’ of strata containing the same major fossil assemblages.”*Boggs Jr., S., 2006, Principles of Sedimentology and Stratigraphy: Upper Saddle River, NJ, Prentice Hall, p. 481.
6 ProblemBeing sedimentary rock, how can we date the Navajo Sandstone and how can we assume the environment of deposition?The sediments comprising this formation are from multiple sources so how did they reach this area and organize into a wide-spread rock structure?
7 DepositionRipples in the sandstone show that the sediment was laid down by wind.The ripples are directed southwest and show us the prevailing winds were northeast.Both Pictures:
8 DepositionSize of grains are similar to grain sizes of modern day sand dunes. Shows NSS is aeolian deposited sand dunes.Previously believed to be from Ancestral Rockies, but just recently have been proved to be from elsewhere…Photo:
9 Radioisotope-datingJeffrey Rahl and Peter Reiners used two forms of radiometric dating to find when zircon grains were formed and cooled.“Using uranium-lead dating, the researchers found that most of the grains were formed between 1.2 billion and 950 million years ago. And using helium dating, they found that the grains cooled, and therefore eroded, between 500 million and 250 million years ago. Rahl and Reiners say there is only one place in North America with zircons like these — the Appalachians.”*Pratt, Sara. Geotimes. November 2003.
10 Coast to CoastHow is it possible that quartz from the East Coast largely makes up the Navajo Sandstone?According to geologists Greb and Archer, 300 million years ago North America had an expansive river system rivaling the Amazon.This river began in Ontario, Canada, flowed through the Appalachians, and ended in Arkansas.The key to this discovery is quartz pebbles located throughout the path of this ancient river system.In areas that have no other natural process to obtain these quartz pebbles, all of similar size and shape, they must have been brought by a river.It makes perfect sense for the pebbles to stay in these areas, and for the finer sands to be carried to the Southwest.
11 BiostratigraphyThere are many fossilized remains in the Navajo Sandstone, mostly trace fossils, because the erosive nature of the sandstone destroyed most of the body fossils.The particular fossilized remains, tracks, and burrows lead scientists to believe the formation dates to sometime in the Jurassic Period( million years ago).Picture:
12 BiostratigraphyAmmosaurus tracks are abundant in the sandstone. This animal lived during the early Jurassic Period.More specifically, the Pliensbachian stage ( million years ago) through the Toarcian stage ( million years ago)They averaged a length of 13 feet which is small for their suborder but left them to be one of the most versatile dinosaurs of their time.Fossils also abundant in Portland Formation of Connecticut.
13 BiostratigraphyOrnithischia, classified by their pelvic bones, were large, beaked herbivorous dinosaurs. Their tracks can be found in the Navajo Sandstone and this brings us to an interesting cross-section. With the abundance of animal tracks, herbivores in particular, how desert-like could the sand dunes have been?The Pelvic Bone
14 Semi-Aquatic Ecosystem Stromatolites!!-found in marine environments!Protosuchus-first crocodileNavajo Sandstone must have at least had some wet periods with a seasonally saturated ground and possibly ponds.Photo:
15 Not as arid as believedSeasonal monsoons and rainfall lead to slumping in sand dunes.Same seasonal slumps are found in Navajo Sandstone.Photos:
16 SettingWith an approximate age, a good idea of how sediments arrived, and significant fossil evidence we can come to the conclusion that the Navajo Sandstone is…An aeolian sand duneMainly composed of sediments from the distant AppalachiansContains fossils that are indicative of the Jurassic PeriodHaving had periodic seasonal changes, sometimes including a wet seasonFormed between 192 and 178 million years ago, towards the end of the Pliensbachian Stage and the beginning of the Toarcian Stage.
17 Limitations: The exact age of the NSS is controversial. Radiometric dating has restrictions pinpointing age of sediments.Erosion, few body fossilsFor a more specific age of the Navajo Sandstone, we will have to continue to strive for more accurate dating methods in the future.Photo:
18 Works CitedChan, M.A., and A.W. Archer, 2000, Cyclic Eolian Stratification on the Jurassic Navajo Sandstone, Zion National Park: Periodicities ... in D.A. Sprinkel, T.C. Chidsey, Jr., and P.B. Anderson, eds., Geology of Utah's Parks and Monuments. Utah Geological Association Publication 28:1-11.Kocurek G Limits on extreme Eolian systems: Sahara of Mauritania and Jurassic Navajo Sandstone examples. in M. Chan and A. Archer, eds., Extreme Depositional Environments: Mega End Members in Geologic Time.Rainforth, E.C., 1997, Vertebrate ichnological diversity and census studies, Lower Jurassic Navajo Sandstone PDF ver... Unpublished masters thesis, Department of Geological Sciences, University of Colorado, Boulder.