Unit 4: Sedimentary Petrology Practicals

Part A: Megascopic (Hand Specimen) Identification

This practical involves classifying sedimentary rocks. The primary division is Clastic (made of fragments) vs. Non-Clastic (crystalline/biochemical).

Identification Flowchart

1. Is it Clastic (made of grains/fragments)?
   • YES: Go to Step 2.
   • NO: It's Non-Clastic. Go to Step 3.
2. Clastic Rock ID (based on grain size):
   • Gravel-sized (>2 mm) grains:
     • Grains are rounded → Conglomerate.
     • Grains are angular → Breccia.
   • Sand-sized (gritty) grains: → Sandstone.
     • If pink/red and has feldspar → Arkose.
     • If "dirty" (has a mud matrix) → Greywacke.
     • If mostly quartz → Quartz Arenite.
   • Mud-sized (smooth) grains:
     • Breaks into thin layers (fissile) → Shale.
     • Breaks into blocky pieces (massive) → Mudstone.
3. Non-Clastic Rock ID (based on composition):
   • Fizzes with HCl acid: → Limestone.
     • If made of visible shells → Fossiliferous Limestone or Coquina.
     • If made of tiny spheres (ooids) → Oolitic Limestone.
     • If fine-grained, soft, white → Chalk.
   • Does NOT fizz with acid:
     • Black, lightweight, organic → Coal.
     • Very hard (H=7), conchoidal fracture, microcrystalline → Chert.
     • Crystalline, H=2, can scratch with fingernail → Rock Gypsum.

Part B: Microscopic (Thin Section) Identification

In thin section, you classify clastic rocks by identifying three components:

• Grains: The main fragments (e.g., Quartz, Feldspar, Rock Fragments, Fossils, Ooids).
• Matrix: Fine-grained silt and clay deposited *with* the grains.
• Cement: The crystalline "glue" (e.g., Calcite, Silica) that grew *after* deposition.

Limestone Classification (Dunham): Under the microscope, you check if the grains are "floating" in mud (Mud-supported) or if they are packed together and touching (Grain-supported). This helps determine the depositional energy.

Part C: Study of Sedimentary Structures

These are large-scale features in the rock that tell us about the depositional environment. You will identify these in hand specimens or field photos.

• Bedding / Stratification: Layering in the rock.
• Cross-Bedding: Inclined layers within a main bed. Formed by migrating ripples or dunes. Indicates current direction.
• Graded Bedding: A single bed shows a decrease in grain size from bottom to top. Caused by a current slowing down (e.g., a turbidite).
• Ripple Marks:
  • Asymmetrical: Steep side points downstream. Formed by a one-way current (river, wind).
  • Symmetrical: Sharp crests. Formed by back-and-forth wave action (beach).
• Mud Cracks: Polygonal cracks formed when wet mud dries out. Indicates subaerial exposure (drying).

Part D: Palaeocurrent Analysis

This is a practical exercise to determine the direction of ancient currents (water or wind) by measuring sedimentary structures.

The Exercise (using Cross-Bedding)

1. Data Collection: In the field (or from photos), use a compass to measure the azimuth (direction) of the dipping cross-beds at many locations. (e.g., 045°, 035°, 060°, 040°, etc.).
2. Data Plotting (Rose Diagram):
   • Draw a circle with 360° markings.
   • Divide the directions into "bins" (e.g., 0-30°, 30-60°, etc.).
   • For each bin, draw a "petal" whose length is proportional to the number of readings that fall in that bin.
3. Interpretation: The longest petal (or cluster of petals) points in the mean palaeocurrent direction. This tells you which way the river or wind was flowing.

Part E: Grain-Size Analysis

This is a lab exercise to determine the distribution of grain sizes in an unconsolidated sediment sample (like sand or soil). The most common method is Sieve Analysis.

The Sieve Analysis Procedure

1. Prepare Sample: Dry the sediment sample completely and weigh it (e.g., 100 grams).
2. Sieve Stack: Arrange a stack of standard sieves with the largest mesh size (largest holes) on top and the smallest on the bottom, with a solid "pan" at the very bottom.
3. Sieving: Pour the sample into the top sieve and place the stack in a mechanical shaker for 10-15 minutes.
4. Weighing: Carefully weigh the amount of sediment trapped on *each* sieve and in the bottom pan.

Plotting and Interpretation of Data

You must plot this data to visualize the distribution.

• Frequency Curve (Histogram):
  • Plot: A bar chart showing the weight percentage (Y-axis) for each grain size class (X-axis).
  • Interpretation: Shows the most common grain size (the mode). A sediment with one peak is unimodal; two peaks is bimodal.
• Cumulative Curve:
  • Plot: An S-shaped (sigmoid) curve. Plot Cumulative % Finer (Y-axis) against Grain Size (X-axis, often in phi units).
  • Interpretation: This is the most useful graph. The steepness of the curve indicates sorting. A very steep curve means the sediment is well-sorted (all grains are one size, e.g., a beach sand). A gently sloping curve means it is poorly-sorted (a mix of all sizes, e.g., a glacial till).