Facies and Stratigraphy sedimentary rocks.pptx

About facies and startigraphy

1. Chapter-6 Facies and Stratigraphy

2. Facies and Walther’s Law Facies All the properties of a body of rock that allow us to differentiate it from those above, below or laterally adjacent to it Properties include Lithology – rock type, including color, etc. Composition – mineral content Texture – grain size, sorting, roundness Sedimentary structures Fossils Facies means aspect – same Latin root as “face” Overall appearance of a rock body Facies are the products of depositional environments Examples: Planar laminated fine quartz arenite facies Bioturbated , poorly sorted muddy skeletal limestone facies Cross-stratified arkosic conglomerate facies Stromatoporoid -tabulate coral reef facies

3. Facies : Can be genetic (fluvial facies ) or descriptive (sandstone facies ) Lithofacies – a constant lithological character within a formation E.G an evaporate

4. Facies Model An idealized description of a facies Constructed from modern environments and ancient rocks Serves as a Norm for comparison Framework for observation Predictor of patterns

5. Facies Patterns Groups of facies commonly show patterns Proximal Facies (near the source) tend to be coarse grained Distal Facies (far from source) tend to be finer grained This pattern is displayed upstream and down in rivers and onshore to offshore in coastal areas Facies are arranged according to distribution of depositional environments

6. Facies Migration Facies migrate through space and time Migration is in response to environmental factors Sediment supply Sea level change Subsidence Facies become stacked during migration A single facies is likely to be different ages in different locations

7. Walther’s Law of the Correlation of Facies Named after Johannes Walther (1860-1937), a German geologist, who in 1894, noted a fundamental relationship between the vertical and lateral distribution of facies .

8. Sedimentary environments that started out side-by-side will end up overlapping one another over time due to transgressions and regressions. The result is a vertical sequence of beds. The vertical sequence of facies mirrors the original lateral distribution of sedimentary environments.

9. But… Walther's Law can only apply to sections without unconformities.

10. Walther’s Law of Correlation of Facies Only works where there are no unconformities Only facies that were laterally adjacent during deposition (result of laterally adjacent environments) can be stacked vertically Vertical arrangement of facies gives us information on Distribution of environments How environments migrated through space and time Used as a basis to build facies maps or paleogeographic maps Accurate time correlation of facies is essential Time lines provide framework for correlation Bio-events Volcanic ashes Other thin, unique lithologies or marker beds

11. Only those lithofacies which are a product of sedimentary environments found adjacent to one another in the modern can occur superimposed in continuous, uninterrupted stratigraphic succession.”

12. Walther’s Law: Transgression-Regression Transgression Landward movement of shoreline (progressive deepening) Stand on beach Over time, you would be under water as shoreline moved landward Regression Seaward movement of shoreline (progressive shallowing ) Results in lateral and vertical changes

13. 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

14. 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

15. Regression Shallowing upwards, shoreline moves basin ward 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

16. Causes of Sea Level Change/transgression and regression Relative Change Eustatic Change

17. Stratigraphy The study of strata (layers) of rocks with an eye toward interpreting the geologic history of the region Types of stratigraphy Lithostratigraphy Biostratigraphy CHRONOSTRATIGRAPHY

18. Lithostratigraphic units Supergroup Group Formation – a mappable unit with distinctive lithic characteristics Member Bed

19. Biostratigraphy 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

20. 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

21. 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?)

22. Characteristics of a good index fossil: Distinctive appearance/easy to recognize Short duration between first appearance and extinction. Widespread geographic distribution (makes correlation possible across a wide area/multiple continents

23. Limited Stratigraphic Range Commonly Pelagic Or tolerant of a wide variety of environments (found in many facies )

24. CHRONOSTRATIGRAPHY Geochronological units 27

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26. Stratigraphic Contacts Contacts Plane or irregular surfaces between different types of rocks Separate units

27. Types of Contacts Conformable Unconformable

28. 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

29. 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.

30. 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).

31. Angular Unconformity

32. Disconformity

33. Nonconformity

34. Unconformity Types Using Grand Canyon as Example

35. Stratigraphic Thinking

36. One possible interpretation …