Unit 3: Collection and Separation Techniques

Collection and Separation of Fossil

This unit covers the field and lab techniques used to extract fossils, especially microfossils, from a rock.

Collection (Field)

• Bulk Sampling: For microfossils (like foraminifera or pollen), the best method is to collect a bulk sample (a few kilograms) of promising rock (e.g., shale, mudstone, limestone).
• Macro-fossil Collection: Larger fossils (e.g., shells, bones) are collected individually, noting their exact layer and orientation. If fragile, they may be excavated with a plaster "jacket" for protection.

Separation (Lab)

The goal is to free the fossils from the rock matrix without destroying them.

• Disaggregation of Shales/Mudstones:
  1. The rock is gently crushed into small pieces (1-2 cm).
  2. The pieces are soaked in water, hydrogen peroxide (H₂O₂), or a weak acetic acid to break down the clay matrix.
  3. The resulting "mud" is washed through a set of nested sieves (e.g., 63-micron, 125-micron).
  4. The fossils are collected from the residue caught on the sieves, dried, and then "picked" under a binocular microscope with a fine, wet brush.
• Acid Maceration (for Limestones):
  • Used to extract microfossils with silica or phosphatic skeletons (like conodonts or radiolaria) from a carbonate (limestone) rock.
  • The limestone is dissolved in a weak acid, typically acetic acid (vinegar) or formic acid.
  • This dissolves the calcite matrix (CaCO₃) but leaves the insoluble microfossils behind, which can then be sieved and picked.

Separation of Heavy Minerals

A standard technique in sedimentology. Most common rock-forming minerals (Quartz, Feldspar) have a specific gravity (density) of ~2.65 g/cm³. Heavy Minerals are those with a specific gravity greater than 2.85 g/cm³ (e.g., Zircon, Garnet, Magnetite, Rutile).

They are rare but act as "fingerprints" to tell you the source of the sediment (the provenance).

Lab Separation Technique (Using Heavy Liquids):

1. Prepare Sample: A sediment sample (e.g., sand) is sieved to a specific size fraction (e.g., fine sand) and washed to remove clay.
2. Heavy Liquid: The clean, dry sand is poured into a separatory funnel containing a "heavy liquid" with a specific gravity of ~2.85. The most common (and less toxic) is Sodium Polytungstate (SPT). (Historically, toxic Bromoform was used).
3. Separation:
   • Light Minerals (Quartz, Feldspar) will FLOAT on the liquid.
   • Heavy Minerals (Zircon, Garnet, etc.) will SINK to the bottom.
4. Collection: The heavy minerals are drained from the bottom of the funnel, and the light minerals are skimmed from the top. Both fractions are washed, dried, and weighed. The heavy minerals are then mounted on a slide for microscopic identification.

Use of Chemicals in Geology

Geologists use several simple chemical tests in the field and lab for quick identification.

• Dilute Hydrochloric Acid (HCl):
  • Use: Identifying carbonate minerals.
  • Test: A drop of 10% HCl will fizz vigorously on Calcite (the main mineral in Limestone).
  • Test: It will only fizz on Dolomite if the mineral is first powdered (by scratching it).
• Hydrogen Peroxide (H₂O₂):
  • Use: Disaggregating shales (as described in fossil separation) and checking for organic matter.
  • Test: Fizzes when it comes in contact with organic-rich mud or soil.
• Acetic Acid (CH₃COOH):
  • Use: Gentle dissolution of limestones for fossil extraction.

Construction of Lithologs

A Litholog (or Lithological Log / Stratigraphic Column) is a simple, visual, vertical log that represents a sequence of rock layers (strata). It is the most fundamental way to record geological data from an outcrop.

How to Construct a Litholog:

1. Measure the Section: In the field, use a Jacob's Staff or tape measure to measure the true thickness of each rock layer, from bottom to top.
2. Establish a Vertical Scale: On graph paper, set a scale (e.g., 1 cm = 1 meter).
3. Draw the Log:
   • Draw the layers in their measured thickness, starting with the oldest at the bottom.
   • Use Standard Symbols: Fill in each layer with its correct lithological symbol (e.g., dots for sandstone, dashes for shale, brick pattern for limestone).
   • Grain Size Profile: The horizontal width of the column is used to show grain size. Sandstone would be drawn "wider" (coarser) than shale.
   • Add Notes: On the side of the log, write descriptions: colour, grain size, sedimentary structures (e.g., "cross-bedding"), and any fossils found.