Carbonate Fault Rock Composition and Mechanics

Alice Bandeian

University of Rochester, Department of Earth and Environmental Sciences

1. Introduction

Fold-thrust belts (FTB) are a major component of mountain belts. Structures commonly found in these systems are folds and thrusts, as well as basal decollements. Typically forming in weak geologic units and at low depths in Earth’s crust as a major thrust, basal decollements transport material away from a tectonic convergent boundary and towards the stable interior of a continent. How this type of fault forms and mechanically deforms rocks is not well known due to its typical formation at depth in the Earth’s crust.

The Hudson Valley Fold-Thrust Belt (FTB), near Catskill, NY, is a north-south trending, Acadian or Alleghanian age FTB. Located between the Hudson River to the East and the Catskill Mountains to the west, involves deformation of Middle Ordovician to Middle Devonian age sediments. Within this belt is the Rondout decollement, a basal decollement forming in Silurian age dolostone, a mechanically competent unit. Directly above the dolostone are a series of limestones, a mechanically weak lithological unit. For this project, samples from the dolostone and the limestone units were analyzed using scanning electron microscopy and x-ray elemental analysis to gather information on mineral composition and deformation structures in order to determine why the decollement formed where it did.

Figure 1: Schematic Cross-section of a basal decollement of a fold-thrust belt. (Van Der Pluijm, 2004)

Geologic setting, Hudson Valley Fold-Thrust Belt.

Figure 2: Geologic map of Hudson Valley Fold-Thrust Belt. Yellow boxes indicate sampled areas.(Adapted from Marshak, 1986)

2. Techniques

Several techniques were used for this study. These techniques included sample preparation and coating, petrographic microscope analysis, backscatter electron imaging of geologic thin sections, elemental mapping using x-ray microscopy, secondary electron (SE2) imaging of calcite veins from sliding surfaces within the dolostone unit, and colorization of SE2 images with Adobe Photoshop.

2.1 Sample Preparation and Coating

Samples were collected from the dolostone and the limestone units as described above. Seven of these samples were then made into geologic thin sections and three of the samples were made into chips of veins from sliding surfaces. The thin sections were prepared by Gerry Kloc and later were further polished for proper SEM imaging. Vein chips were cut using a hammer and chisel to expose desired planes for imaging. The thin sections and chips were then adhered to metal stubs using carbon tape.

Prior to SEM imaging, the geologic thin sections were analyzed using an Olympus BH-2 petrographic microscope.

Figure 3: Cross-polarized microscope petrographic microscope image of thin section from dolostone unit.

The samples were prepared for SEM imaging using carbon coatings and gold coatings. The thin sections were coated with carbon using graphite and the University of Rochester Optics Department’s carbon coater. The vein chips were coated with 60 Angstroms of gold/min at 15 mA current for 1.6 minutes.

Figure 4: Denton Sputtering tool used for sputtering gold (left). LADD HV Evaporator used for carbon sputtering (right).

3. Backscatter Electron Imaging

The geologic thin sections and vein chips were imaged using the Zeiss Auriga Cross-Beam SEM-FIB of the University of Rochester Optics Department. Images were collected of the polished thin sections using backscatter electron (BSD) imaging.

Backscatter electron imaging was used to depict different phases within the mineral makeup of the samples collected. The variation in brightness between different phases indicates heavier versus lighter atomic masses. Bright regions indicate a heavier atomic number due to high amount of backscattering produced as the electron beam interacts with the high-density nuclei. Darker regions indicate a smaller atomic number as there is less backscattering produced and there are less collisions of the nucleus with the electron beam.

The geologic thin sections were all imaged using backscatter imaging to identify the various phases within the samples. The thin sections were not imaged using SE2 becuase they were polished and would show minimal to no surface topography.


Figure 5-8: Images of samples taken from the limestone and the dolostone units using BSD. Top left image, from the dolostone unit shows a a vein of different composition (bright phase) cutting dolostone. Top right image shows sample from dolostone unit with vein of different phase and crack cutting through.Bottom left image shows sample from dolostone and vein cutting the rock. Bottom right shows image of sample taken from limestone unit.

The BSD images above show the different phases of samples taken from the dolostone as well as the limestone units. The samples from the dolostone unit depict two phases, a dark phase and a light phase. The dark phase shows the matrix of the rock. Relative to the brighter phase, the matix has a small atomic mass for elemental composition. The brighter phase depicts veins that are cutting through the sample. These veins appear to have a uniform composition that is of a heavier atomic number than the matrix. The brightest spots that appear in the images are artifacts left over from polishing the samples with 1 micron aluminum grit. The BSD images of the dolostone samples also show that the minerals in the veins have a relatively coarser grain size than the crystals comprising the matrix. The last image above (Fig. 7) is a BSD image of the limestone unit. The limestone unit has a relatively unitform composition and an overall finer grain size than the dolostone unit's grain size. The bright phase particles that appear in the BSD image of the limestone unit are artifacts of the aluminum grit used for polishing prior to imaging.

4. Energy-dispersive X-ray Spectroscopy

Energy-dispersive x-ray spectroscopy was used to characterize the elemental composition of the geologic thin sections as well as the vein chips in the form of elemental maps and spectra. X-rays are generated when the high-energy electron beam hits the sample; the interaction of the beam and sample molecules causes an inner shell electron to get ejected and then an outer shell electron falls in towards an inner shell, resulting in the generation of x-rays. These x-rays are then collected by the x-ray detector and used in EDAX to identify elemental composition of samples.This technique was used to determine exact compositional makeup of the dolostone unit as well as to identify the composition of the vein chips.

Figure 9: Elemental maps of dolostone sample.

Figure 10: Elemental spectra of fine grained material on vein. Left image shows SE2 image of vein chip with growth of fine grained crystals on coarser calcite grains. Right image is the spot elemental spectra of fine grained crystals.

Results from the elemental mapping in Fig. 8 shows the elemental distribution in the dolostone sample. The matrix of the rock, as seen as the darker phase in the BSD image, is mainly composed of magnesium, calcite, silica, oxygen,and aluminum. The presence of magnisum and calcite, along with analyses with the petrographic microscope, indicates that the matrix of the sample is composed of dolomite, a mechanically strong carbonate mineral. The presence of silica and aluminum, along with observations made with the petrographic microscope, indicate that there are also a lot of clays within the matrix. The composition of the mostly calcium, indicating the vein is calcite. The calcite vein in the dolomite matrix could have been precipitated from fluids outside of the dolostone or only calcium and not magnesium was dissolved from the matrix and then deposited as calcite veins. Although dolostone is stronger than limestone, the presence of clays in the dolostone can make this unit behave mechanically weaker than the limestone, which does not have clays present.

Results from Fig. 9 show the composition of a fine mineral grow on the face of calcite crytals of a calcite vein chip. From the spot x-ray elemental spectrum, the composition of the fine crystal is silica and oxygen. These fine crystals are then identified as quartz (SiO2).

5. Secondary Electron Imaging

Secondary electron (SE2) imaging was used to collect topographic information of the vein chip samples. Secondary electrons are generated in an inelastic collision as the electrons from the high-beam electron beam hits the surface of a sample. These secondary electrons are low energy and escape from the surface of the sample. These electrons are then collected and generate information about the sample's surface topography.

Figure 11: SE2 images of vein chips from sliding surfaces, within the dolostone unit.

SE2 images were collected in order to determine the structure of the vein growth along sliding surfaces. Prior to SE2 imaging, it was thought that veins that form along sliding surfaces, in this case fault planes, grew in a firbous structure. This fibrous structure would be seen as elongate, equal width crytals that grow in the sliding along the plane. From SEM imaging with SE2 it was found that the veins along the sliding planes did not grow in the fibrous structure as orignally thought. From the rough and blocky texture of the sample's surfaces it is seen that these veins grew in a blocky fashion. This tells us about the structure of the veins and how they formed, however, we cannot use these for further deformation analyses due to their blocky structure. The top middle image in Fig. 10 shoes a calcite crytal face with one set of parallel twin running up and down the crystal. The rhombohedral structure of the calcite grains can be seen in the top right image as well as the middle and right lower images in Fig. 10. This rhombohedral structure of the calcite crytals further demonstrates the blocky texture of the veins.

5. Colorization

Adobe Photoshop was used to colorize the fine quartz crytal growth on the surface of calcite crystals on a vein chip imaged with SE2.

Figure 12: The colorization of these SE2 image of a vein chip.

7. Conclusions

The SEM along with the various tecniques described were used to determine the composition of the dolostone unit as well as understand the microstructure of the veins that formed along sliding surfaces within the dolostone unit. From these analyses with x-ray spectroscopy, it was determined that the dolostone unit is composed of dolomite, a mechanically strong mineral, as well as clays, mechanically weak minerals. Decollements typically form in weak horizons; the presence of clays within the dolomite matrix weakens the unit as a whole and thus is a probably aid for the decollement forming within the dolostone unit rahter than the limestone unit. SE2 imaging of the vein chips revealed that the veins grew in blocky structure rather than the previously interpretation of fibrous growth.

Acknowledgments

I would like to thank my instructor,Brian McIntyre and my TA, Nursha for answering all of my questions, guiding us through the course, labs, and projects, as well as being patient with all of us students.

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References

Marshak, 1986, Structure and tectonics of the Hudson Valley fold-thrust belt, eastern New York: NYSGA.

Van Der Pluijm, Ben A. (2004). Earth Structure. New York, NY: W.W. Norton. p. 457