2_Geologic time .pdf

Politecnico di Torino - Civil and Environmental Engineering GEOLOGY Course code: 01PZGTR Dr. MARTINA GIZZI Prof. ADRIANO FIORUCCI E-mail: [email protected]...

Politecnico di Torino - Civil and Environmental Engineering GEOLOGY Course code: 01PZGTR Dr. MARTINA GIZZI Prof. ADRIANO FIORUCCI E-mail: [email protected] E-mail: [email protected] Tuesday 13:00 - 14:30 (1.5h) 29 Room Timetable Friday 10.00-13.00 (3h) 2M Room  Politecnico di Torino - Civil and Environmental Engineering Syllabus GEOLOGY Section I (Prof. Fiorucci) : The Earth System The geochemical composition of the crust and the subdivision of the Earth on a geophysical basis (Preliminary Reference Earth Model). The general classification of minerals and rocks: igneous (intrusive and effusive), sedimentary, and metamorphic rocks. The rock cycle. Structure, texture, and orientation of rocks. Weathering of rocks. (2 credits). Section II (Dr. Gizzi): Principles of Sedimentary Geology Sedimentary environments, landscape morphologies, and deposits: Alluvial plains and alluvial fans, meanders, deltas, and coastal areas. (1 credit). Section III (Dr. Gizzi): Principles of Structural Geology Folds, faults, geodynamics (volcanism, seismicity, plate tectonics). Elements of Stratigraphy and Geological Mapping: orientation of geological layers and their relationships with topography (topographic map). (1.8 credit). Exercises (1.2 credits): Macroscopic recognition of rock samples (Section I). Interpretation of geological maps at different scales and construction of geological sections (using traditional methods and/or available open-source software) (Sections II and III).  GEOLOGY Introduction to Geologic maps Geologic map: map that shows the distribution of various types of rocks. It consists of a topographic map (a map giving information about the form of the Earth’s surface) which is colored to show where different geological units occur. The colors represent geological units or formations which refer to a rock type of a given age. These units are separated from each other by contacts which can be sedimentary, igneous, tectonic (faults). Lines on the map are drawn to show the contacts between each of the geological units. Each unit has a symbol to help identification. From “Geological map of the upper Pellice Valley (Italian Western Alps)” → 3  GEOLOGY Introduction to Geologic maps In a geological map we can distinguish five components: 1. Topographic and geological maps 2. Caption of lithologies 3. Caption of geological structures 4. Sketch maps 5. Examples of geological cross sections In the caption of lithologies, the younger units will always be at the top left and the oldest ones at the bottom right. Caption of main geological symbols 4  Caption of litholgies Caption of geological symbols “Geological map of the upper Pellice Valley (Italian Western Alps)” 5  Tectonic sketch map Cross sections “Geological map of the upper Pellice Valley (Italian Western Alps)” 6  GEOLOGY Introduction to Geologic maps Two types of surface were represented in the geological map: the geological (or stratigraphic) surface and the topographic (ground) surface. Topographic maps depict the shape of the ground usually by means of topographic contours which are drawn usually for a fixed interval of height. Stratigraphic contours record the height of geological surfaces. → See Exercises 1 and 2 7  GEOLOGY Geologic maps: online resources U.S. Geological Survey World Geologic Maps The U.S. National Geologic Map Database (NGMDB) The U.S. National Geologic Map Database (NGMDB) serves as the authoritative, comprehensive resource for information about paper and digital geoscience maps and reports on the Nation's geology and stratigraphy. The European Geological Map – online service WMS service: https://services.bgr.de/wms/geologie/igme5000/ 8  GEOLOGY Geologic Time How do we plot the events of the universe and the earth on a time scale? The geologic time scale relates stratigraphy (layers of rock) to periods of time. The geological time scale is used by geologists and many other scientists to date certain historical events on Earth. As humans we have a hard time understanding the amount of time required for geologic events. The Earth is approximately 4.5 billion years old: this age is estimated by radiometric dating. Earth's past has been split into different sections based on events that happened during this time. An example is the boundary between the Cretaceous period and the Paleogene period (formerly the Tertiary period) which are separated by an extinction event, where the dinosaurs and many other species went extinct. Historical Geology examines the origin and evolution of the Earth system through time.  GEOLOGY Geologic Time Geologic time is divided into different types of time units. Note that each Era, Period, Epoch represent a different amount of time: The Cambrian period encompasses ~65 million years whereas the Silurian period is only ~30 million years old. The change in different periods is related to the changing characters of life and environments on Earth. The beginning of the Phanerozoic represents the explosion of life. The time before the Phanerozoic is commonly referred to as the PreCambrian (over 4 billion years of time). The Phanerozoic eon (abundant life) represents only the last 13% of Earth time. GEOLOGY Geologic Time Movement of the plates over geologic time  GEOLOGY Measurement of geologic time Geochronology is used to assign dates to geological events. It is based on four fundamentally different methods: 1. Radiometric methods that gives an absolute age (primarily for crystalline and metamorphic rocks); 2. Stratigraphic methods that give a relative age (sedimentary and metamorphic rocks); 3. Paleontological methods that use fossils to give a biologic age that is calibrated to absolute ages (sedimentary rocks); 4. Paleomagnetic dating methods, particularly applicable to oceanic basalts  GEOLOGY Measurement of geologic time It is based on four fundamentally different methods: 2. Stratigraphic methods that give a relative age (sedimentary and metamorphic rocks); Age relations can be inferred from geologic relationships. To understand this concept, we must examine 3 fundamental concepts set forth by Steno in 1669. These principles are applied to particles deposited in sedimentary basins, as in seas or lakes that are far from coastlines. Principle of Original Horizontality: Sediments are deposited in nearly horizontal layers. Oblique strata or those in vertical positions are evidence of deformation after deposition. Principle of Lateral Continuity: The same type od sediments is deposited throughout a basin during a given time period. Principle of Superposition: In a sedimentary basins, recent contributions are deposited on top of older sediments. Thus, the deeper the strata, the older it is.  GEOLOGY Measurement of geologic time It is based on four fundamentally different methods: 2. Stratigraphic methods that give a relative age (sedimentary and metamorphic rocks); Principle of Original Horizontality Principle of Lateral Continuity Principle of Superposition  GEOLOGY Measurement of geologic time It is based on four fundamentally different methods: 2. Stratigraphic methods that give a relative age (sedimentary and metamorphic rocks); Principle of Cross-cutting relationships: When a dike (geological element) cuts across a rock, the rock was present before. An erosional period preceded its deposition. Principle of Inclusion: If a detrital rock contains particles from a recognizable rock, that rock predates the detrital rock. All the principles are fundamental to the understanding of sedimentary and tectonic phenomena.  GEOLOGY Sedimentology vs Stratigraphy Sedimentology: the study of the processes of formation, transport and deposition of material that accumulates as sediment in continental and marine environments and eventually forms sedimentary rocks Stratigraphy: the study of rocks to determine the order and timing of events in Earth history. It provides the time frame that allows us to interpret sedimentary rocks in terms of dynamic evolving environments Sedimentary Geology  GEOLOGY Sedimentology vs Stratigraphy Sedimentology focuses primarily on facies and depositional environments (how were sediments/sedimentary rocks formed?) Smaller temporal and spatial scales Stratigraphy focuses on the larger scale strata and Earth history (when and where were sediments/sedimentary rocks formed?) Larger temporal and spatial scales The stratigraphic record is nearly always very incomplete due to a limited preservation potential, that decreases with increasing time scales. The combination of sedimentology and stratigraphy allows us to build up pictures of the Earth’s surface at different times in different places and relate them to each other. We can thus build up pictures of the palaeogeography, the appearance of an area during some time in the past, and establish changes in palaeogeography through Earth history.  GEOLOGY Larger temporal and spatial scales Stratigraphy Stratigraphy provides the temporal framework for geological sciences. The relative ages of rocks, and hence the events that are recorded in those rocks, can be determined by: Simple stratigraphic relationships ▪ younger rocks generally lie on top of older ▪ If an igneous intrusion or a fault cuts through the existing rocks is younger than rocks it cuts Fossils that are preserved in strata Measurements of processes such as the radioactive decay of elements that allow us to date some rock units.  GEOLOGY Sedimentary facies The formation of a body of sediment involves: Transport of particles to the site of deposition by gravity, water, air, ice or mass flows or the chemical or biological growth of the material in place. Accumulation of sediments in place which is largely influenced by the chemistry, temperature and biological character of the setting. The processes of transport and deposition can be determined by looking at individual layers of sediment. The size, shape and distribution of particles all provide information to the way in which the material was carried and deposited. If the laws that govern physical and chemical processes have not changed through time (Uniformitarianism), detailed measurements of sedimentary rocks can be used to make estimates (to varying degrees of accuracy) of the physical, chemical and biological conditions (salinity, depth and flow velocity in lake or seawater, the strength and direction of the wind) that existed at the time of sedimentation.  GEOLOGY Sedimentary facies Sedimentary Facies is a body of rock with specified characteristics that reflect the conditions under which it was formed (Reading & Levell 1996). Describing the facies involves documenting all the characteristics of its lithology, texture, sedimentary structures and fossil content that can aid in determining the processes of formation. By recognizing associations of facies it is possible to establish the combinations of processes that were dominant. Tidal flats →  Reef  GEOLOGY Sedimentary Geology: the data sources The first step to learn the principles of sedimentology and stratigraphy is the study of outcrop relationships in the field, but modern exploration, particularly for hydrocarbons or mining, involves a range of techniques for finding out what is below the surface. In some cases, this will be direct sampling of what is down below by drilling a hole and bringing pieces of rock back to the surface, but most exploration uses less direct means of investigating the strata hundreds or thousands of metres below ground. These approaches involve making measurements of the physical properties of the rocks and are hence referred to as geophysical techniques. Sources of stratigraphic information: 1. Outcrops (consolidated vs. unconsolidated sediments) 2. Cores (hand-operated vs. power-driven) 3. Geophysical data (seismic surveys)  GEOLOGY Sedimentary Geology: the data sources 1. Outcrops (consolidated vs. unconsolidated sediments) 2. Cores (hand-operated vs. power-driven) 3. Geophysical data (seismic surveys) Sandstone Conglomerate  GEOLOGY Sedimentary Geology: the data sources Sand and gravel deposits A deltaic foresets dipping eastwards, capped by poorly stratified topset gravel  GEOLOGY Sedimentary Geology: the data sources 1. Outcrops (consolidated vs. unconsolidated sediments) 2. Cores (hand-operated vs. power-driven) 3. Geophysical data (seismic surveys)  GEOLOGY Sedimentary Geology: the data sources A drill bit can be designed such that it cuts an annulus of rock away leaving a cylinder in the centre, a core, that can be brought up to the surface. Where coring is being carried out the drilling is halted, and the section of core is brought up to the surface in a sleeve inside the hollow drill string. As each section of core is brought to the surface it is placed in a box, which is labelled to show the depth interval it was recovered from. Recovery is often incomplete, with only part of the succession drilled preserved, and the core may be broken up during drilling.  GEOLOGY Sedimentary Geology: the data sources 1. Outcrops (consolidated vs. unconsolidated sediments) 2. Cores (hand-operated vs. power-driven) 3. Geophysical data (seismic surveys)  GEOLOGY Sedimentary Geology: the data sources 1. Outcrops (consolidated vs. unconsolidated sediments) 2. Cores (hand-operated vs. power-driven) 3. Geophysical data (seismic surveys) The interpretation of seismic reflection profiles provides a model for the stratigraphic and structural relationships that may exist in the subsurface. Data from these sources can provide some indicators of the lithologies in the subsurface, but a full geological picture can be obtained only by the addition of information on lithology and facies. This can be provided by drilling boreholes through the succession and either taking samples of the rocks and/or using geophysical tools to take detailed measurements of the rock properties.  GEOLOGY Sedimentary Geology: the data sources There is a wide range of instruments, geophysical logging tools, that are lowered down a borehole to record the physical and chemical properties of the rocks. These instruments are mounted on a device called a sonde that is lowered down the drill hole (on a wireline) once the drill string has been removed. Data from these instruments are recorded at the surface as the sonde passes up through the formations.  GEOLOGY Sedimentary Geology: interpretation of sediments A distinction can be drawn between sediments and sedimentary rocks. Sediments: generally loose material. Sedimentary rocks: lithified sediment. Lithification or Diagenesis is the process of ‘turning into rock’.  GEOLOGY Sedimentary Geology: interpretation of sediments Lithification (Diagenesis) includes the full range of alterations sediments undergo after deposition, at relatively low temperatures and pressures (gradational to metamorphism) Lithification may occur simultaneously with deposition (in several carbonates, evaporites, and volcanoclastics) Physical and chemical diagenetic processes constitute compaction and cementation, respectively Diagenesis leads to a reduction of porosity and permeability  GEOLOGY Sedimentary Geology: interpretation of sediments The environment governs the sedimentological processes which determine what sort of sediments are formed and deposited. Environment → Process → Sediment In studying older rocks, we base our approach on certain features which we can observe or measure, and attempt to interpret the processes that produced them. The recognition of such sedimentary processes helps us to reconstruct the environments. The sequence of interpretation in studies of older sediments or sedimentary rocks is thus: Study of sediment → Process → Environment e.g., variations in grain size and sedimentary structures in profiles can be interpreted as having been formed through particular processes.  GEOLOGY Depositional environments and sedimentary basins Sediments accumulate in a wide variety of environments, both on the continents and in the oceans. Some of the more important of these environments are illustrated in Figure. The important terrestrial depositional environments and their characteristics Some of the important depositional environments for sediments and sedimentary rocks.  GEOLOGY Depositional environments and sedimentary basins Sediments accumulate in a wide variety of environments, both on the continents and in the oceans. Some of the more important of these environments are illustrated in Figure. Some of the important depositional environments for The important marine depositional environments and sediments and sedimentary rocks. their characteristics