Geol342 sedimentation and stratigraphy

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  • 1. 1/6/13 GEOL342 - Sedimentation and Stratigraphy42: Sedimentation and Stratigraphy2011 Geophysical stratigraphy: Due to the lack of surface outcrop in many areas geophysical methods of correlation have been developed that exploit the methods of physics to mapg the physical properties of rocks, including: Density Permeability Porosity Character of pore fluids Variations in those properties with depth reveal the presence of different rock types and are used to create vertical and lateral sections of rocks that cant directly be examined. There are two general approaches: Well logs: that record information provided by probes that are placed down boreholes Seismic studies: in which physical features of subsurface rocks are approximated based on seismic wave propagation Well logging Direct sampling: Not all well information is remote. Bentonite muds are continually circulated through the drill pipe as a coolant and lubricant for the drill bit. Rock chips are brought to surface with the mud, captured, identified and logged, creating a direct lithologic record. Aditional information comes from devices lowered into the borehole: Caliper: measures the width of the drill hole. This indicates the presence of mudrocks, which are prone to caving and sagging, hence constricting the borehole slightly. Sonde: A probe lowered into hole to measure various electrical and physical properties of the rocks. Gas detectors and gas chromatographs: measure gases in the well. Gamma-ray log: measures natural radioactivity of the strata www.geol.umd.edu/~jmerck/geol342/lectures/19.html 1/11
  • 2. 1/6/13 GEOL342 - Sedimentation and Stratigraphywww.geol.umd.edu/~jmerck/geol342/lectures/19.html 2/11
  • 3. What the sonde records:Spontaneous potential (SP) log: measuresdifference in electrical potential between anelectrode on the sonde and one at the surface.An electrical potential exists between thenatural pore fluids of the rock and the drillingmud that invades those pores. Therefore, SPlogs are a measure of permeability: Shales and limestone nearly impermeable and have a 0 reading Negative deflection for sandstones (high permeability) or fluids with higher conductivity than the drilling mud (like saltwater) Positive deflection for fluids with lower conductivity than mud (like freshwater)Resistivity (R) log: measures resistivity offluids in the surrounding rock to an appliedelectrical current. Resistivity indicates amountof fluid in the pore spaces, therefore R logs aremeasures of porosity. Resistivity increaseswith decreasing pore space. High resistivity: dense rocks with no pores (quartzite, limestone.), rocks with non-conducting fluid in their pores (like petroleum) Low resistivity: rocks with significant porosity (sandstone), rocks with conducting fluid in their pores (like salt water), rocks containing significant amounts of water in their crystal structure (clay-rich rocks).Examples of Spontaneous Potential logs: Fluvial deposit with point bar sequence and overbank mud shows fining-upward sequence.
  • 4. 1/6/13 GEOL342 - Sedimentation and Stratigraphy Deltaic deposit grading into shoreline Coarsening upward sequence with thick sands. Deltaic deposit grading into distributary channel and interdistributary bay Coarsening upward sequence.www.geol.umd.edu/~jmerck/geol342/lectures/19.html 4/11
  • 5. 1/6/13 GEOL342 - Sedimentation and Stratigraphy Deltaic deposit grading into delta plain Coarsening upward sequence with thick sands. Regressive shoreline Coarsening upward sequence. Dipmeter: measures resistivity in four directions. By this means, it locates contacts and identifies their dip direction, allowing identification of folds, faults, and other structure. Gamma-ray log: measures natural radioactivity of the rock. Most gamma radiation comes from decay of 40K. Therefore, the gamma-ray log is sensitive to rocks high in K-bearing minerals (feldspars, micas, clays) including: shales feldspathic and lithic sandstones In contrast, limestones and quartz rich sandstones produce low gamma ray values.www.geol.umd.edu/~jmerck/geol342/lectures/19.html 5/11
  • 6. 1/6/13 GEOL342 - Sedimentation and Stratigraphy Well-logging techniques all have one significant drawback: They require you to drill a borehole. There is a less expensive alternative: Seismic stratigraphywww.geol.umd.edu/~jmerck/geol342/lectures/19.html
  • 7. 1/6/13 GEOL342 - Sedimentation and Stratigraphy Shots, pulses of sound are generated: by explosives or a mechanical thumper on Vibraseis trucks on land by a shipboard air gun at sea. Those waves that are propagated nearly straight downward can be reflected off subsurface interfaces of materials of different densities, such as contacts between rock units. Travel time is recorded by an array of geophones on land or hydrophones at sea. Reflections from each shot are recorded as individual seismic profiles by the geophones. Information from each geophone in the array is correlated, processed to remove noise, and summed up across the array, yielding a vertical line in which reflectors as shown as wave-shaped deviations. This is a one-dimensional plot of reflectors beneath the shot The array and thumper are then moved slightly along a transect and a new seismic shot made, which yields a separate trace. Ultimately individual traces are displayed together as seismic profiles, approximating two dimensional images of reflectors below the transect, each vertical line of which represents one shot. More ambitious seismic techniques involve the deployment of two-dimensional geophone arrays to develop three-dimensional seismic profiles. Definitions: Reflector: boundary that creates a seismic reflection Reflection: acoustic waves created by sounds bouncing off of a reflector Impedance: physical rock property of sound propagating through rock. A function of average sound velocity and rock density Impedance contrast: physical boundary within rocks producing a reflectionwww.geol.umd.edu/~jmerck/geol342/lectures/19.html
  • 8. 1/6/13 Seismic stratigraphy can be used for both deep and shallow structural analysis. Layered reflectors appear as distinct horizons, whereas structureless units or those of uniform density show random reflections. (E.G.: the contrast between marine sediments and a rising salt structure - right.) But what, exactly are these reflectors? Simply, they are density contrasts. These may be caused by: contacts between rock units interface of different pore fluids (E.G.: petroleum and water) unconformities diagenetic features The traces of seismic reflections have numerous aspects that can be measured: amplitude duration (2-way travel time) area, etc.www.geol.umd.edu/~jmerck/geol342/lectures/19.html 8/11
  • 9. 1/6/13 GEOL342 - Sedimentation and Stratigraphy This is pleasingly quantifiable, however a large element of inference goes into the interpretation of seismic profiles, because: Seismic profiles are NOT cross sections because the vertical scale is two-way travel time, not thickness. Reflector horizons neednt be lithologic boundaries. Layers with high concentrations of chert nodules make nice reflectors, for instance. The resolution of seismic stratigraphy is low. A single seismic pulse on a seismic profile may be up to 150 m. thick. (right) As with so much else in stratigraphy, the ability to amass large quantities of information compensates for the uncertainty inherent in the information. In this case, patterns that are likely to be connected to stratigraphy can be observed at great depths in unexposed rock on land or beneath the sea, into which no well has been bored.www.geol.umd.edu/~jmerck/geol342/lectures/19.html 9/11
  • 10. 1/6/13 GEOL342 - Sedimentation and Stratigraphy Of course, if well-log or outcrop information is also available, seismic reflectors can reliably be connected to known lithologies. By this means we have learned that marine sediments tend to contrast strongly with continental ones. (right) The presence of petroleum can be revealed by anomalous horizontal reflectors indicating the interface of petroleum and water, or by a brightening of the profile caused by the presence of gas. Cornell University, through the Consortium on Continental Reflection Profiling (COCORP) has used seismic methods to profile major orogenies. Among the interesting results: Whereas the traditional view was that the Piedmont and Blue Ridge had deep crustal roots, it develops that they are underlain by extensive thrust faults and have actually been thrust onto Paleozoic sediments.www.geol.umd.edu/~jmerck/geol342/lectures/19.html 10/11
  • 11. GEOL342 - Sedimentation and Stratigraphy Seismic sequences: The geometry of unconformities that truncate beds is sufficiently distinctive to be identifiable in seismic profiles, allowing identification of seismic sequences - unconformity bounded "packages" whose presence is revealed by seismic reflections. Indeed, the development of sequence stratigraphy has gone hand-in-glove with that of seismic stratigraphy, because only seismic methods can identify sequence boundaries on a large scale. When connected to lithologic information, these can be correlated with age to identify sea level cycles. (right) For many, the hope has been that these would be caused by global eustatic sea level change, enabling their use in global sequence stratigraphy. In 1977, Vail, Mitchum, and Thompson published a summation of first and second order sea level curves based on major unconformities. Second-order cycles appeared to be markedly asymmetric, because of the depositional asymmetry of transgression and regressions in which transgressions are erosional, but sediment can continue to aggrade up during regressions. Identifying onlap and offlap of sediments onto continents, enabled Haq and colleagues to develop an adjusted curve showing sea level over the last 200 my. As your text makes clear, the reliability and usefulness of the Vail curve is controversial.www.geol.umd.edu/~jmerck/geol342/lectures/19.html 11/11