Introduction to Sedimentology and Stratigraphy
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This document provides an introduction to sedimentology and stratigraphy. It discusses key concepts such as sedimentology focusing on accumulation under uniform conditions while stratigraphy records changes over time. Sedimentary rocks form through weathering, erosion, transport, deposition, lithification and diagenesis. Scientists study facies, depositional systems, and system tracts to interpret ancient environments. Stratigraphy reflects changes in the balance between space creation and filling in sedimentary basins. Correlating rock units across regions is important for stratigraphic research.
Introduction to Sedimentology and Stratigraphy
- 1. Photo by W. W. Little Topic 1: Introduction to Sedimentology and Stratigraphy
- 2. Photo by W. W. Little What do you see?
- 3. Sedimentology vs. Stratigraphy Sedimentology involves all factors that go into the accumulation of sediment at a specific locality under a relatively uniform set of conditions. Stratigraphy records changes in sedimentology through time as a vertical succession. Photo by W. W. Little Photo by W. W. Little Photo by W. W. Little
- 4. Photo by W. W. Little Sediment Production, Preservation, and Alteration A sedimentary rock is the cumulative product of six distinct steps: • Weathering • Erosion • Transport • Deposition • Lithification • Diagenesis
- 5. Environmental Interpretation The primary objectives of sedimentology are to: • Determine the conditions (flow characteristics, water chemistry, climate, topography, sediment source, etc) under which a package of sediment was deposited • Determine how one package of sediment relates to other, time-equivalent packages Modern analog Depositional model Big picture Rock record Photo by W. W. Little Photo by W. W. Little
- 6. Scientists make observations of an event or an object and then try to explain those observations by organizing them into a logical system. As rock bodies can cover extensive areas and represent vast periods of time, much of geological research cannot be done as controlled experiments in a formal laboratory setting. In these situations, we rely on the assumption that chemical and physical laws are constant. That is, the processes operating today are the same as those that operated in the past. For instance, since water flows down hill today, it must have done so in the past. Therefore, if we identify a body of rocks that exhibit characteristics similar to those found in modern geological environments, we assume that they must have formed in a similar manner. Assumes Uniformitarianism Ancient river channelsModern river channels Photo by W. W. LittlePhotographer unknown
- 7. Three Levels of Interpretation Depositional Environment (facies) Depositional System (facies assemblage) Systems Tracts A facies is a body of sediment/rock with characteristics (lithology, texture, structures, fossils) formed under a unique set of physical, chemical, and biological conditions representing a specific depositional environment. A facies assemblage is a body of sediment/rock composed of contemporaneous, genetically-related facies that form part of a depositional system. Often defined as a formation. A systems tract is a series of laterally- equivalent bodies of sediment/rock composed of contemporaneous, facies assemblages. Photo by W. W. Little Photo by W. W. Little Photo by W. W. Little Photo by W. W. Little Photo by NASA
- 8. Three Levels of Models Depositional System Systems Tract Circles denote specific environments A group of environments makes a depositional system. A group of depositional systems forms a systems tract.
- 9. Stratigraphy is the study of temporal relationships in sedimentary rock bodies and reflects changes in the balance between rates at which space is produced and filled. Stratigraphy Photo by W. W. Little
- 10. Photo by W. W. Little Sediment Preservation Potential • To be preserved, sediment must be deposited within a basin. • A basin is space located below base-level. • Space available for sediment accumulation is referred to as accommodation space. • Accommodation space is created/destroyed by relative base- level fluctuations. • Base-level fluctuations are caused by changes eustatic sea-level or uplift/subsidence of the basin floor. • The nature of stratigraphic packages is controlled by a balance between the rate at which is space is created and filled.
- 11. Photo by W. W. Little Stratigraphy Records History The stratigraphic record preserves details pertaining to major geologic events, such as: • Mountain-building events (orogenies) • Eustatically-driven transgressive/regressive cycles (sequences) • Climatic changes
- 12. Floodplain Levee Point Bar Photographer unknown Walther’s Law In a conformable stratigraphic succession, facies that are now in vertical contact represent environments that were once laterally adjacent. Photo by W. W. Little
- 13. Photo by W. W. Little Nested Events A rock body is often composed of laminae (formed in seconds) within lamination sets within beds within bed sets within members within formations within groups within sequences (formed over millions of years).
- 14. Photo by W. W. Little Correlation One of the principle jobs of a stratigrapher is to correlate (or match) equivalent stratigraphic units from one area to another. This has led to a number of different branches of stratigraphy. Some of the formalized types of stratigraphy are: • Lithostratigraphy • Biostratigraphy • Allostratigraphy (sequence stratigraphy) • Chronostratigraphy • Pedostratigraphy • Magnetostratigraphy Rules governing the naming and use of stratigraphic units are found in the North American and international codes of stratigraphic nomenclature.