Sedimentary structures: Turbidites

A look at turbidites in outcrop

This is part of the How To…series on describing sedimentary rocks

Here are some annotated, close-up outcrop images showing details of sedimentary structures commonly found in turbidites. In each image, I have used the iconic Bouma Sequence notation (A through E intervals); reference is made to the Lowe Model (used for coarse-grained, commonly gravelly turbidites), and the Stow Model for fine, mud-silt dominated turbidites. The three models-schemas are summarized in the diagram below (modified from Stow and Smillie, 2020).

The images are organised approximately as follows, depending on the primary attributes I wish to emphasize:

General attributes of turbidite flow units
Dewatering and soft-sediment deformation

Mass Transport Deposits (MTDs) will be treated in a separate post.

I have no doubt there will be disagreements with some interpretations. Hopefully this will generate some discussion among students.

Most of the structures illustrated are at the bed scale – this is where students get their noses right up to the outcrop. I have not included more panoramic views of turbidite successions – for these I invite you to look at the Atlases.

Most of these images have been cropped from originals featured in the Atlas of submarine fans and channels, and Atlas of synsedimentary deformation.

Related links in this series on outcrops

Sedimentary structures: Alluvial fans

Sedimentary structures: coarse-grained fluvial

Sedimentary structures: Fine-grained fluvial

Sedimentary structures: Mass Transport Deposits

Sedimentary structures: Shallow marine

Volcanics in outcrop: Lava flows

Volcanics in outcrop: Secondary volcaniclastics

Volcanics in outcrop: Pyroclastic fall deposits

Other useul links

Sediment transport: Bedload and suspension load

The hydraulics of sedimentation: Flow regime

Fluid flow: Froude and Reynolds numbers

Liquefaction: More than a sloppy puddle at the beach

The hydraulics of sedimentation: Flow Regime

Describing sedimentary rocks; some basics

Analysis of sediment grain size distributions

Measuring a stratigraphic section

Sediment transport: Bedload and suspension load

References: Includes a couple of recent reviews

A.M. Hessler and A. Fildani, 2019. Deep-sea fans: Tapping into Earth’s changing landscapes. Journal of Sedimentary Research, v.89, p.1171-1179.

E.Meiberg and B.Kneller, 2010. Turbidity currents and their deposits. Annual Review of Fluid Mechanics, v. 42, p. 135-156. Excellent review of flow mechanics, observed and theoretical. Available for downloading.

G. Shanmugam, 2016. Submarine fans: A critical retrospective. Journal of Palaeogeography, v. 5, p. 110-184. Available for downloading

D. Stow and Z. Smillie, 2020 Distinguishing between deep-water sediment facies: Turbidites, Contourites, and Hemipelites. Geosciences, v. 10,. Open Access.

The first four diagrams show some basic sediment descriptors and terminology, and a typical stratigraphic column drawn from outcrop data. These are your starting points for describing and interpreting sedimentary rocks and sedimentary structures in outcrop, hand specimen, and core.

Schematic view of graded bedding, with coarsest clasts at the base, becoming finer toward the top of the bed. The matrix is commonly muddy (clay plus silt particles).

Graded bedding is one of the diagnostic structures of turbidites. This schematic view of graded bedding, shows the coarsest grains at the base, with finer grains toward the top of the bed. The matrix is commonly muddy (clay plus silt particles).

The images below use the Bouma nomenclature for turbidite depositional intervals. These can be cross-referenced with the nomenclatures formalized by D. Stow and D.R. Lowe.

Common turbidite structures