Turbidite Identification Guide

What Turbidite Looks Like

A turbidite is not a single mineral but a sedimentary rock deposit laid down by a turbidity current — a dense, sediment-laden underwater flow that surges down continental slopes. The hallmark of a turbidite is graded bedding: each bed grades from coarse sand at the base to fine mud at the top, recording a single waning flow event. Stacked repeatedly, turbidites form rhythmic alternations of sandstone and shale/mudstone.

• Color: Tan, gray, brown sandstone layers alternating with darker gray to black mudstone/shale.
• Luster: Dull, earthy.
• Transparency: Opaque.
• Texture: Graded beds (coarse to fine upward), often arranged in the classic Bouma sequence of internal divisions.

Field-ID Checklist

1. Look for graded bedding — each layer is coarse at the bottom, fine at the top.
2. Check for rhythmic alternation of sandstone and mudstone beds.
3. Identify Bouma divisions — graded base, parallel laminae, ripple cross-lamination, upper laminae, then mud cap.
4. Inspect bed bases for sole marks** — flute and groove casts on the underside of sandstone beds.
5. Note sharp bases, gradational tops of sandstone beds.
6. Test the components — sand grains (often quartz/lithic) scratch glass; mudstone is soft.

Key Diagnostic Tests

• Hardness: Depends on grains and cement; quartz-rich sand layers scratch glass (~7), mud layers are soft.
• Acid test: Calcareous turbidites fizz in acid; siliciclastic ones do not. Useful for composition but not for identifying the deposit type.
• Structures (the real diagnostic): Graded bedding, the Bouma sequence (Ta–Te), sole marks (flute/groove casts), and rhythmic sand-mud couplets are what define a turbidite.
• Fracture: Splits along bedding planes between layers.
• Field context: Deep-marine setting, submarine fan environments, often deformed in fold-and-thrust belts (flysch).

Common Look-Alikes

• Ordinary sandstone: Lacks the systematic upward grading and Bouma divisions; turbidites show repeated graded couplets.
• Fluvial/deltaic sandstone: Shows channel forms and cross-bedding but not consistent deep-water graded sequences or sole marks.
• Storm deposits (tempestites): Can grade too, but typically include hummocky cross-stratification and shallow-marine fauna.
• Varves: Fine lake laminations that are far thinner and seasonal, lacking sandy graded bases.
• Contourites: Bottom-current deposits with different, more bioturbated, less sharply graded structures.

Where It's Found

Turbidites form on continental slopes and in deep-sea fans worldwide and are preserved in ancient mountain belts as flysch. Classic studied successions include the Apennines (Italy), the Welsh and Scottish sequences, the Annot Sandstone (France), and California's Great Valley and coastal ranges. They are economically important hosts for oil and gas.