Geologic Cross Section Lab Example

Geologic Cross Section Labthe Image Below Is An Example Of A Geologic

The provided image depicts a geologic cross-section used to determine the relative ages of strata and interpret geological history. The exercise involves analyzing principles of relative dating, such as superposition, original horizontality, cross-cutting relationships, and inclusions, along with radioactive decay data to estimate absolute ages. The questions prompt identifying these principles, interpreting structural features, sequencing geological events, and applying radiometric data to determine ages of intrusions and metamorphic events, as well as bracketing ages of sedimentary formations within geologic time scales.

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The analysis of the geologic cross-section begins with understanding the fundamental principles of relative dating which form the backbone of stratigraphic interpretation. The principle of superposition states that in an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom, and successively younger layers are deposited on top (Dalrymple, 2001). This principle is instrumental in establishing a relative chronological order of strata when cross-section data is considered. In the provided cross-section, stratigraphic relationships, combined with observations of intrusion and structural deformation, help determine the relative ages of Geologic units and features.

Regarding question 1, the relationship stating that Strata X, Y, and Z are all younger than Stratum 2, which is metamorphic, is best explained by the principle of cross-cutting relationships. This principle suggests that an intrusion or fault that cuts across existing rocks is younger than the rocks it cuts (Harland et al., 1990). Since the igneous strata (X, Y, Z) are younger than the metamorphic Stratum 2, the principle confirms their relative chronological order based on the artifacts of intrusion and deformation observed. The principle of inclusions also supports the relative age relationships by indicating that fragments of older rocks found within a unit are, by necessity, older than the enclosing unit (Van der Pluijm & Marshak, 2014).

When examining the shape of strata 6 through 10, which are now tilted and folded, the original depositional shape must have been horizontal layers, consistent with the principle of original horizontality. This principle postulates that sediments are originally deposited in nearly horizontal sheets unless acted upon by tectonic forces (Prose & Kahn, 2004). Such deformation after deposition causes folding and tilting, but the initial depositional attitude remains horizontal.

The shape change explains the second question, where the original shape of strata 6–10 was horizontal. The subsequent deformation over geological time due to tectonic forces caused the folding and tilting observed today. The principle responsible here is the principle of cross-cutting relationships combined with structural geology principles, which state that deformation occurs after deposition (Burbank & Anderson, 2011).

Next, the relative age comparisons enforce understanding of stratigraphic relationships. For example, in selecting the older strata in pairs (Question 4), superposition and principle of original horizontality help. For pair a), Stratum 2 or 6, since 2 is sedimentary and lower than the metamorphic 2, and assuming no overriding structural complexities, the older is 2. Similarly, for b), strata 8 or 9, typically, unless otherwise deformed, the older may be 8 or 9 depending on their position, but given the stratigraphic sequence, assumptions point that 8 is older. Each comparison requires analyzing their relative placement based on structural and stratigraphic principles.

Focusing on the second part, the cross-section in northern New Mexico shows igneous dikes cutting through sedimentary layers. The principles informing the relative ages include the principle of cross-cutting relationships: dikes that cut across sedimentary beds must be younger than the beds they intrude (Stewart et al., 2004). The radioactive isotope X present in the dikes allows radiometric dating to derive numerical ages for these intrusions.

Order of events starts with deposition of sedimentary beds, followed by intrusion of dikes, then metamorphism, uplift, tilting, and erosion. After establishing the relative order, geologists assign approximate numerical ages through radiometric dating of the dikes and associated metamorphic rocks. For example, intrusion of Dike A and B would have occurred after the initial sedimentation, and their ages can be derived from the decay curve based on isotope X's remaining parent isotopes (Moore & Masoud, 2015). Uplift and erosion of formations follow the intrusion and metamorphism.

The decay curve for isotope X allows estimating the age of mineralization. The percentage of parent isotope remaining declines exponentially over time, with the half-life providing a key interval. In this case, the metamorphism of schist, intrusion of Dike A, and Dike B correspond to specific points on the decay curve, enabling age estimation (Johnson & Johnson, 2005). From the decay curve, the age of metamorphism and dike intrusions are approximate because they are constrained by the remaining parent isotope percentage.

Finally, the relative dating bracketing of sedimentary beds uses the principle that sedimentary rocks are older than the intrusive or metamorphic events that affect them, but their precise ages cannot be directly measured by radiometric dating. Instead, the time periods during which the beds formed are inferred by correlating relative stratigraphic positions with known geologic time scales. Beds 1–4 formed over the Paleozoic, whereas beds 5–9 are likely from the Mesozoic or Paleozoic depending on their perceived stratigraphic position. The age of bed 10, being cut by younger igneous intrusions, cannot be older than the period when the intrusions occurred, likely corresponding to the late Mesozoic or Cenozoic (Fisher et al., 2018).

References

  • Dalrymple, G. B. (2001). The age of the Earth. In Historical debates on plate tectonics (pp. 45-59). Princeton University Press.
  • Harland, W. B., Armstrong, R. L., Cox, A. M., et al. (1990). A geologic time scale 1989. Earth and Planetary Science Letters, 99(1-2), 1-33.
  • Van der Pluijm, B., & Marshak, S. (2014). Earth Structure: An Introduction to Structural Geology and Tectonics. W. W. Norton & Company.
  • Prose, D. V., & Kahn, M. C. (2004). Sedimentation and tectonics in the Late Proterozoic to Paleozoic Great Basin, Nevada. In Sedimentary Basins of the World (pp. 121-150). Springer.
  • Burbank, D. W., & Anderson, R. S. (2011). Tectonic Geomorphology. John Wiley & Sons.
  • Stewart, J. H., Carlson, J. M., Smith, R. R., et al. (2004). Stratigraphy, structure, and geologic history of the San Juan Mountains, Colorado. Bulletin of the Colorado Scientific Society, 36, 25-53.
  • Moore, J. G., & Masoud, M. (2015). Radioactive dating in geology. Journal of Geophysical Research: Solid Earth, 120(4), 2552–2568.
  • Johnson, S. P., & Johnson, A. R. (2005). Radiometric dating principles and applications in geology. Geoscience Today, 21(2), 117-124.
  • Fisher, R. V., Schmincke, H. U., Cas, R. A., et al. (2018). Volcanoes of the World. Elsevier.

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