How to identify Fossiliferous Sandstone

To correctly identify Fossiliferous Sandstone (Sandstone with fossil inclusions (likely bivalve or brachiopod impressions)), 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

   Highly variable, depending on the dominant mineral grains and cementing agent. Commonly light tan, buff, gray, white, yellow, brown, or reddish. Iron oxide cements can impart red or orange hues.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, reflecting the granular nature of the sand grains and the typically non-metallic luster of quartz and other common minerals. Fossil material may have a slightly different luster if preserved as original shell material (e.g., pearly for some bivalves).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, granular, and gritty to the touch. Individual sand grains are visible and palpable. The presence of macroscopic fossils (shells, impressions) is the defining textural characteristic. Grains are typically sub-angular to rounded.

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 exhibit anhedral to subhedral forms. Fossils retain their original biological forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Absent in the rock as a whole. Individual mineral grains (e.g., feldspar) may exhibit cleavage, but the rock breaks irregularly across grain boundaries and cement. Fossils do not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Shallow marine environments (beaches, tidal flats, continental shelves, deltas), fluvial systems (river channels, floodplains), and sometimes eolian (desert) environments if terrestrial fossils are present. Requires conditions conducive to both sand deposition and fossil preservation.

Key Facts

Hardness

Variable, typically 6-7 on Mohs scale for quartz grains, but the overall rock hardness is controlled by the cement. Calcite cement (3) makes it softer, silica cement (7) makes it harder.

Specific Gravity

2.65-2.75 (for quartz-rich varieties), but can vary slightly depending on the density of the cement and fossil material.

Crystal System

Not applicable for the rock as a whole. Constituent minerals like quartz are trigonal.

Color

Highly variable: tan, buff, gray, white, yellow, brown, reddish.

Luster

Dull to earthy.

Transparency

Opaque.

Fracture

Irregular to conchoidal (if quartz-rich and well-cemented).

Cleavage

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

Composition

Predominantly quartz (SiO2) grains, with varying amounts of feldspar, lithic fragments, and accessory minerals. Cement can be silica (SiO2), calcite (CaCO3), iron oxides, or clay minerals. Contains fossil remains (e.g., CaCO3 shells, phosphatic material, or organic carbon impressions).

Physical characteristics

• Crystal Habit: Not applicable for the rock. Individual quartz grains are typically anhedral to subhedral, rounded to sub-angular.
• Cleavage Type: None (for the rock).
• Fracture Type: Irregular to sub-conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

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