How to identify Fossiliferous Sandstone

To correctly identify Fossiliferous Sandstone (Sandstone with marine fossils (likely gastropods or bivalves)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Variable, often light tan, brown, gray, or reddish, depending on the dominant mineral grains and cementing agent. Fossils may appear lighter or darker than the matrix.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, reflecting the clastic nature of the grains and the typically non-metallic luster of the cementing material.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, granular, and often gritty to the touch. Individual sand grains are visible, and the embedded fossils provide a distinct bioclastic texture. Grain size ranges from 0.0625 mm to 2 mm.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the rock as a whole; individual mineral grains (e.g., quartz) may show subhedral to anhedral forms. Fossils retain their original shell morphology.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Absent in the rock as a whole. Individual mineral grains may exhibit cleavage (e.g., feldspar), but the rock breaks irregularly along grain boundaries and fossil planes.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Shallow marine environments, including beaches, tidal flats, continental shelves, deltas, and nearshore areas where clastic sedimentation and biogenic accumulation occur simultaneously.

Key Facts

Hardness

Variable, typically 6-7 on Mohs scale for quartz grains, but the overall rock hardness is lower (3-5) due to the cement and fossil material. Calcite-cemented varieties are softer (3).

Specific Gravity

2.2 - 2.8 g/cm³, depending on mineral composition and porosity.

Crystal System

Not applicable for the rock as a whole; individual mineral grains (e.g., quartz) are trigonal.

Color

Variable, commonly light tan, brown, gray, or reddish.

Luster

Dull to earthy.

Transparency

Opaque.

Fracture

Irregular to conchoidal, breaking around grains and fossils.

Cleavage

Absent in the rock; present in some constituent minerals (e.g., feldspar).

Composition

Predominantly quartz (SiO₂), feldspar, and lithic fragments, with a significant proportion of biogenic calcium carbonate (CaCO₃) from fossils. Cementing agents include silica (SiO₂), calcite (CaCO₃), or iron oxides.

Physical characteristics

• Crystal Habit: Not applicable for the rock; individual grains are typically anhedral to subhedral. Fossils retain their original biogenic forms.
• Cleavage Type: None (rock); perfect in some constituent minerals (e.g., feldspar).
• Fracture Type: Irregular to conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

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