16-Well_Logs_and_Magnetostratigraphy

Module 3: Stratigraphy & Sedimentary Systems

I. Stratigraphy

  • Study of layered sedimentary rocks and the sequence of geological events represented in them.

  • Understanding the vertical and lateral changes in rock layers helps decipher Earth’s history.

II. Lithostratigraphy

  • Focus on the physical and petrographic properties of rock layers.

  • Classification based on lithology and composition helps in identifying different sedimentary formations.

III. Stratigraphic Relationships

  • Investigate the layers' relationships to one another, which aids in reconstructing geological histories.

  • Key to stratigraphic analysis:

    • Basin Analysis Exercise

    • Deerfield Basin Field Trip

IV. Vertical and Lateral Successions of Strata

  • Observations and interpretations of how sedimentary layers accumulate over time.

  • Important for long-term geological studies.

V. Stratigraphic Correlation

  • Techniques to correlate rock layers across different locations:

    1. Seismic Stratigraphy

    2. Biostratigraphy

    3. Magnetostratigraphy

A. Well Logs

  • An instrument (sonde) measures properties of surrounding rock in the subsurface, including:

    • Lithology: Rock composition

    • Fluid Content: Type and presence of fluids within the rock

    • Porosity: Measure of voids within the rock, affecting fluid storage

    • Radioactivity: Specific elements contribute to radioactivity

  • Resistivity: Measurement of how strongly the rock opposes electricity flow.

    • Example: Marine shale with salty pore fluids exhibits lower resistivity compared to porous sandstone/limestone with freshwater, which shows higher resistivity.

B. Gamma Ray Logs

  • Measure radioactivity of the lithology to identify formations (e.g., shale is most radioactive due to potassium, uranium, thorium).

C. Sonic Logs

  • Assess the velocity of sound signals through rocks.

  • Can indicate porosity based on sound speed variations.

VI. Magnetostratigraphy & Paleomagnetism

  • Magnetic iron-rich minerals align to the Earth’s magnetic field as they cool or are deposited:

    • Thermal Remanent Magnetism (TRM): Residual magnetism in igneous/metamorphic rocks.

    • Detrital Remanent Magnetism (DRM): Orientation of magnetic minerals in sedimentary rocks.

  • Earth's magnetic field operates similarly to a dipole bar magnet and is influenced by rotation.

    • Magnetic poles wander and are offset from geographic poles, impacting magnetic declination.

  • Inclination (or magnetic dip): Angle between the orientation of a magnetic grain and Earth's surface. Inclination is 0° at the equator and 90° at the poles.

  • Changes in magnetic declination: Indicate geomagnetic wandering and historical shifts in Earth’s magnetic field.

VII. Geomagnetic Polarity Time Scale (GPTS)

  • A composite geomagnetic polarity sequence, calibrated with radiometric age dates, depicts normal and reverse polarity over the past 17 million years.

  • Recognition of magnetic polarity "stripes" led to the development of the GPTS:

    • Polarity zones (rocks) / chron (time)

    • Polarity subzones (rocks) / subchron (time)

    • Chrons denote divisions of normal/reverse polarity periods.

    • Excursions occur during short-lived events with significant shifts of magnetic poles.

VIII. Observations of Paleomagnetic Records

  • Past magnetic field reversals are frequent; the sedimentary record displays polarity changes rapidly.

  • Two core site studies demonstrate mirrored patterns of polarity reversals across the Northern and Southern Hemispheres.

  • Magnetic inclination is steeper at higher latitudes, indicating geographical influences on sediment analysis.

IX. Assumptions for Correlation

  • For accurate correlation of Site 1208 paleomagnetic data with the GPTS:

    • Assumption is that sediment section is complete (no breaks or unconformities).

    • Physical sedimentology and biostratigraphy provide clues about sedimentary continuity.