Lecture # 7

Introduction

Turbidites are deposits of a turbidity current i.e

density current moving down slope on ocean

floor driven by gravity that acts on the density

difference between the current and the

surrounding seawater due to suspended

sediment

Turbidite geological formations have their

origins in turbidity current deposits, which are

deposits from a form of underwater avalanche

that are responsible for distributing vast amounts

of clastic sediment into the deep ocean.

Turbidity currents can be initiated by

earthquakes, rivers in flood, and

sediment failure in rapidly deposited

delta front.

The initial sediment in the current will

reflect the source

Major Features

Sandstones and shales are monotonously

interbedded.

Beds tend to have sharp, flat bases, with no

indication of erosion of sea floor.

The sharp bases of beds have abundant

markings (tool marks carved by grid objects)

Within sandstone beds, the grain size

commonly decreases upward (graded bedding

Bouma sequence)

Turbidite FaciesWalker divided deep water rocks into 5 facies associations:

Classical Turbidites: characterized by monotonous interbeds of

sandstone and shale, with no evidence of topography on the

seafloor. All sandstone can be described using the Bouma

sequence (deposits fine upward).

Massive sandstones: much more evidence of erosion of

substrate; beds are commonly associated with channels many

meters deep. The deposits of successive flows also join

together (amalgamation) to make composite beds (the

monotonous aspect of sandstone-shale interbedding is lost).

Most common sedimentary structure of this type is dish and

pillar structure which indicate abundant fluid escape during

deposition.

Pebbly sandstone: beds tend to be well graded with internal stratification fairly abundant and consists of coarse, crude horizontal stratification and commonly channeled and laterally discontinuous.

Conglomerates: imbrications features which typified by clasts whose long axes lie parallel to flow and dip upstream. This signifies that clastshave not rolled on the bed.

Pebbly mudstone, debris flows, slumps and slides: consist of pebbles and distorted clast of sandstone and mudstone, dispersed in a siltymudstone matrix.

Bouma Sequence

Types of submarine gravity

transport mechanisms based on

Cook, et al., 1972 Submarine Rockfall

Submarine slide and slump

Submarine mass flow

Submarine turbidity flow

Submarine Rockfall

Rolling or freefall of

individual clasts.

Sand to boulder sized clasts

Depositional units usually

show distinct boundaries

Poor sorting, no grading

Interclast porosity

Transport distance short

across steep angles

Submarine Slide and

Slumps Displacement of

coherent masses

Movement along

discrete shear planes

Little or no internal

flow

Local folds and faults

Submarine Mass Flow Depositional units show

distinct boundaries

Planar base and top, or

planar base and hummocky

top

Poor sorting, normal grading

rare

Usually has a mud matrix

Clasts jumbled together

during movement and

supported by some type of

non turbulent mechanism.

Transport distance

intermediate across low angle

slopes.

Submarine Turbidity Flow Depositional units usually show

distinct boundaries

Planar base and top, or

channeled base and planar top

Variable sorting, normal grading

and other Bouma sequences

common

May or may not have a mud

matrix

Clasts jumbled together during

movement and supported by

turbulent suspension

Transport distance far across low

angle slopes

Importance of turbidites

Turbidites provide a mechanism for assigning a tectonic and depositional setting to ancient sedimentary sequences as they usually represent deep water rocks formed offshore of a convergent margin, and generally require at least a sloping shelf and some form of tectonism to trigger density-based avalanches.

Turbidites from lakes are also important as they can provide chronologic evidence of the frequency of landslides and the earthquakes that presumably formed them, by dating varves above and below the turbidite.

Economic geology of

turbidites Turbidite sequences are classic hosts for lode gold deposits,

the prime example being Bendigo and Ballarat, Victoria, Australia, where over 2,600 tons of gold have been extracted from saddle reef deposits hosted in shale sequences from a thick succession of Cambrian-Ordovician turbidites. Proterozoic gold deposits are also known from turbidite basin deposits.

Lithified accumulations of turbidite deposits may, in time, become hydrocarbon reservoirs and the petroleum industry makes strenuous efforts to predict the location, overall shape, and internal characteristics of these sediment bodies in order to efficiently develop fields as well as explore for new reserves. Turbidite deposits typically occur in foreland basins.