Transcript Lecture 4
1 Stratigraphy • The study of strata (layers) of rocks with an eye toward interpreting the geologic history of the region • Closely tied to dating methods • Uses a variety of methods - fossils, stable isotopes, paleomagnetics, sedimentary cycles - to correlate and distinguish layers • Very important for oil exploration and mining 2 Basin Analysis • Use stratigraphic methods to work out sequence and timing of deposition of rocks • usually sedimentary • Synthesis of data from multiple disciplines • sedimentology to determine environment of deposition • paleontology to get time • Used to be main objective • Petroleum industry 3 Correlation Correlation is determining that rocks are the same formation (may mean rocks are the same age) 4 CHRONOSTRATIGRAPHY Geochronological units 5 6 LITHOFACIES 7 Stratigraphic Contacts • Contacts • Plane or irregular surfaces between different types of rocks • Separate units • Conformable • Unconformable 8 Types of Contacts • Conformable boundaries • Conformable strata form unbroken depositional sequences • Layers are deposited by ~ uninterrupted deposition • Abrupt or gradational • Abrupt • Sudden distinctive changes in lithology • Often, local change • Gradational Gradual change in depositional conditions with time progressive gradual contact One lithology grades into another e.g., ss becomes finer upsection until it becomes a siltstone 9 Lithostratigraphic units • Supergroup • Group • Formation – a mapable unit with distinctive lithic characteristics • Member • Bed 10 Facies • “aspect” or “appearance” • Can be genetic (fluvial facies) or descriptive (sandstone facies) • Lithofacies – a constant lithological character within a formation E.G an evaporite • Walther Law (1894)- facies that occur in conformable vertical succession also occur in laterally adjacent environments 11 Preservation potential of rocks that are deposited • Majority of sediments in fossil record • Marine • Most sub-aerial environments • Erosional • WHY? • Accommodation space! Space available controls accumulation no place to put it, then no deposition base level • balance between erosion and deposition 12 Walther’s Law of Correlation of Facies • Relationship between vertical and lateral variations • The fact that there is lateral variation in facies leads to vertical variation in facies Walther’s Law of Correlation of Facies • Lateral variations are expressed in the vertical due to the succession of facies 13 Walther's Law of Correlation of Facies • “Only those lithofacies which are a product of sedimentary environments found adjacent to one another in the modern can be occur superimposed in continuous, uninterrupted stratigraphic succession.” 14 Walther’s Law: TransgressionRegression • Transgression Landward movement of shoreline (progessive deepening) Stand on beach Over time, you would be under water as shoreline moved landward • Regression Seaward movement of shoreline (progessive shallowing) • Results in lateral and vertical changes 15 Transgression • Geometric relationship of "graded, shore parallel facies belts“ • Fining Upwards Sequence: FUS • More basin-ward facies overlie more landward facies Compared to depositional systems models 16 Transgression and Regression • Shallowing upwards, shoreline moves basinward through time--> Regression • Sea level drop +/- uplift +/- sediment supply Progradation excess sediment supply relative to accommodation space Forced Regression • Relative sea level drop and formation of erosion surfaces: Unconformity (surface of subaerial exposure) • Soils; kaolinitized, clay-rich layers • Angular discordance with underlying units (disconformity) • Plant remains, rooted zones • Non-genetic stratal relationships: basinward shift in sedimentary facies • Strata across lithologic boundaries NOT in accordance with Walther’s law 17 Regression • Geometric relationship of "graded, shore parallel facies belts“ • Coarsening Upwards Sequence: CUS • More landward facies overlie more basin-ward facies Compared to depositional systems models 18 Transgression - Regression • What drives transgression/ regression? • can’t tell from this information! • • • • sea level change has so many components relative = local eustatic = global sediment supply • can drive a regression/ transgression • ONLY KNOW that shoreline has shifted position • multiple factors responsible for sea- level change • Say sea level rise or fall and you are WRONG! • Say transgression or regression! 19 Relative Change Eustatic Change Causes of Sea Level Change 20 Sea Level Cycles • 1st Order Cycles • 100’s my • 100’s of meters • 2nd Order Cycles • 10’s my • 100’s of meters • 3rd Order Cycles • 1-10 my • 10’s of meters Falling Rising 21 Biostratigraphy If two rocks contain the same fossils they must be the same age 22 Evolution • Variations exist within a population • Result from mutations and other genetic accidents • Some variations are advantageous but others are not • Some are neutral • Natural Selection works on these variations • Characteristics of population shift through time = evolution 23 Bio-Events • First appearances of new species • First appearances of new higher taxa • Extinctions of species • Mass extinctions of multiple taxa • Bio-events are unique points in geologic time 24 Index Fossils • Some fossils are more useful than others for relative age determinations • Fossils that are most useful are called INDEX FOSSILS • What factors would maximize a fossil’s usefulness? (i.e., What makes a good index fossil?) 25 Illustration of Principle of Faunal Succession What makes a good index fossil? • Distinctive appearance/easy to recognize • Short duration between first appearance and extinction (a.k.a. RANGE) • Widespread geographic distribution (makes correlation possible across a wide area/multiple continents) 26 27 Characteristics of Index Fossils • Limited Stratigraphic Range • Widespread Geographic Distribution • Commonly Pelagic • Or tolerant of a wide variety of environments (found in many facies) 28 29 30 Unconformities Unconformities are surfaces in rock that represent periods of erosion or non-deposition. In other words, time has been left out of the physical geologic rock record. There are three (3) principal types of unconformities: • Angular Unconformity Rocks above and below unconformity have different orientations. Shows that there was a period of deformation, followed by erosion, and then renewed deposition. Easiest of the three types to recognize because the units are at an angle truncated with the units above them. 31 • Nonconformity Rocks in a horizontal fashion were eroded down to igneous bedrock material at which time subsequent deposition of sedimentary layers commenced. Shows that there was a period of deformation, followed by erosion, and then renewed deposition. Represents the greatest amount of time left out of the geologic rock record. • Disconformity Rocks in a nearly horizontal fashion were eroded and an erosional profile remains covered by subsequent sedimentary deposition. Shows that there was a period of erosion and then renewed deposition in nearly horizontal layers. Most difficult to recognize because the units are nearly horizontal and only a small discontinuous layer can be observed (rubble zone or soil profile). 32 Angular Unconformity 33 Disconformity 34 Nonconformity 35 Unconformity Types Using Grand Canyon as Example 36 Stratigraphic Thinking From D. McConnell, Geologic Time, http://lists.uakron.edu/geology/natscigeo/Lectures/time/gtime1.htm 37 One possible interpretation... 38 39 Look Bob – a shooting star! Lets make a wish…. 40 41 Were The Dinosaurs Failures? • Dinosaurs: 150,000,000 years • Recorded History: 5000 years • For every year of recorded history, the dinosaurs had 30,000 years • For every day of recorded history, the dinosaurs had 82 years • For every minute of recorded history, the dinosaurs had three weeks