Reservoir Geoscience

PDB 2012

Dr. Solomon

Office Block L-01-22

Ext. 7121

RECAP

Sediment texture

and

Sand and sandstones

Rock composed mainly of silicate

particles from weathering of rocks

Clastic (siliciclastic)

- Sandstones

- Mudstones (shales)

- Conglomerates

Chemical/Biochemical

- Carbonates

- Siliceous Oozes/Cherts

- EvaporitesClassification of

sedimentary rocks- Grain size

The Pettijohn classification of

sandstones (Pettijohn 1975).

Transport, Deposition of Sediments and Sedimentary Structures

Learning Outcomes

At the end of this lecture, students should be able to:

Analyze the mechanism of sediment transport.

Relate sediment texture and structure with hydrodynamic condition.

Differentiate the different types internal sedimentary structures.

Outline

•Physical process of transportation

• Bed forms and sedimentary structures

- Internal sedimentary structures

Physical process of transportation

Most sedimentary deposits are the result of transport of material as particles.

Sediments are eroded, transported and deposited by a variety of agents that flow across

Earth’s surface.

- Movement of detritus may be purely due to gravity but more commonly it is the

result of flow in water, air, ice or dense mixtures of sediment and water.

Cont….

Gravity: The simplest mechanism of sediment transport is the movement of particles

under gravity down a slope.

Water: Transport of material in water is by far the most significant of all transport

mechanisms.

Air: Wind blowing over the land can pick up dust and sand and carry it large distances.

Ice: is high viscosity fluid that is capable of transporting large amounts of clastic

debris.

Dense sediment and water mixtures: When there is a very high concentration of

sediment in water the mixture forms a debris flow.

Sediment Flow/Transportation

• There are two types of fluid flow.

• Laminar

• orderly, parallel flow lines

• all molecules within the fluid move parallel to each other in the direction of transport.

• in a heterogeneous fluid almost no mixing occurs during laminar flow.

• Turbulent

• particles everywhere; flow lines change constantly.

• molecules in the fluid move in all directions but with a net movement in the transport direction.

• heterogeneous fluids are thoroughly mixed in turbulent flows.

• Why are they different?

• Flow velocity, bed roughness, type of fluid

Nichols (2009)

No mixing between layers

• Laminar and Turbulent flows can be characterized and quantified using Reynolds Number.

• The equation to define the Reynolds number is:

- laminar when Re < 2000

- transient when 2000 < Re < 4000

- turbulent when Re > 4000

Reynolds Number

Transport of particles in a fluid

Particles of any size may be moved in a fluid by one of three mechanisms:

• Rolling/Traction: the clasts or grains move by rolling along a bed surface.

• Saltation: the grains move in a series of jumps along a bed surface.

• Suspension: grains remain in the moving flow above the bed surface.

Particles being carried by rolling and saltation are referred to as bedload, and

the material in suspension is called the suspended load.

Fig. Particles move in a flow by rolling, saltating and in suspension.

- Rolling/Traction: a grain rolls along a

bed surface due to dominating drag

forces caused by flow.

- Saltation: a grain moves in a series of

jumps along the bed surface due to a

lift force caused by differences in

pressure along the grain.

Cont…

The process by which epiclastic sediment transportation is initiated by erosion is called entrainment.

The fluid velocity at which a particle becomes entrained in the flow can be referred to as the critical

velocity.

Fig.1 Hjulstrom’s diagram showing the relationship between the velocity of a water flow and the transport of loose grains (applies specifically to a

20°C, 1m deep aqueous flow ). (modified after Hefferan and O’Brien, 2010)

Entrainment, transportation and deposition

Cohesive silt and clay particles finer than 0.0625mm,

the critical entrainment velocity increases with

decreasing particle size! WHY?

- The higher the amount of surface area/volume, the stronger

the attractive forces, and the more cohesive the sediment will be.

And difficult to entrain.

Transport in

suspension

Deposition

• The threshold velocity of deposition is lower than that required for entrainment for

all particle sizes.

- the velocity at which a particle stops moving, its threshold velocity of

deposition, is proportional to particle size for all particle sizes.

The grain size of the particles in a flow can be used as an indicator of the velocity

at the time of deposition of the sediment.

Geologically Significant Fluids and Flow Processes

• These distinct flow mechanisms generate sedimentary deposits with distinct textures and structures.

• The textures and structures can be interpreted in terms of hydrodynamic conditions during deposition.

Debris flow

Traction deposits

Summary

Water, air and glaciers are main agents of sediment transport.

Particles of any size can be moved in a fluid either by traction or saltation

or suspension.

The grain size of the particles in a flow can be used as an indicator of the

velocity at the time of deposition of the sediment.

Various flow mechanisms impose different texture and structure on

sedimentary deposit.

Outline

• Physical process of transportation

•Bed forms and sedimentary structures

- Internal sedimentary structures

Sedimentary structures

Occur on the upper and lower surfaces of beds as well as within beds.

Can be used to deduce the processes and conditions of deposition, the directions of the

currents which deposited the sediments, and in areas of folded rocks, the way-up of the

strata.

Develop through physical and/or chemical processes before, during and after deposition,

and through biogenic processes.

It is convenient to recognize five categories of sedimentary structure: Erosional,

Depositional, Post-depositional/Diagenetic and Biogenic.

Main Types

• Bedding surface structures Bedding undersurface (sole) structures

- Ripples - Flute casts

- Shrinkage cracks - Groove casts (continuous/discontinuous ridges)

- Parting lineation (primary current - Tool marks

lineation)

- Rainspot impressions - Load casts: bulbous structures

- Tracks and trails: crawling, walking, - Scours and channels: small- and large- scale

grazing, resting structures

• Internal sedimentary structures

- Bedding and lamination - Graded bedding - Cross-stratification - Massive bedding

Analysis of borehole

images across a wide

range of sedimentary

environments reveals a

consistent set of

commonly recognized

sedimentary features.

Depositional Structures

Internal sedimentary structures

Bedforms/Bedding

A bedform/bedding is a depositional feature on the bed of a river or other body of flowing

water.

All sedimentary rocks occur in beds of some kind. Beds are tabular or lenticular layers of

sedimentary rock.

Beds may be characterized internally by the presence of features such as laminae, a lens

of pebbles, or a band of chert.

Laminae and beds are the basic sedimentary units that produce stratification; the transition

between the two is arbitrarily set at 1 cm.

Fig. Terms used for describing the thickness of beds and laminae Boggs (2009)

Bedding and lamination define stratification. Bedding is thicker than 1 cm whereas lamination

is thinner than 1 cm.

Many finely laminated fine-grained sediments are deposited in protected environments such as

lagoons and lakes and in relatively deep water marine basins.

Fig. (a) Bedding planes and bed contacts: the range of possibilities (Tucker, 2003), (b) Lenticular, wavy and flaser bedding in

deposits that are mixtures of sand and mud (Nichols, 2009).

(b) (a)

Flaser lamination is characterised by isolated thin drapes of mud amongst the cross-laminae of a sand.

Lenticular lamination is composed of isolated ripples of sand completely surrounded by mud, and intermediate

forms made up of approximately equal proportions of sand and mud are called wavy lamination.

Flaser, lenticular and wavy bedding

Flaser bedding is where cross-laminated sand contains mud streaks, usually in the ripple

troughs.

Lenticular bedding is where mud dominates and the cross-laminated sand occurs in lenses.

Wavy bedding is where thin-ripple cross-laminated sandstones alternate with mudrock.

These bedding types are common in tidal-flat and delta front sediments, where there are

fluctuations in sediment supply or level of current (or wave) activity.

Fig. Lenticular bedding: thin

lenses of cross-laminated sand.

(Boggs, 2009).

(Nichols, 2009)

Beds that contain internal layers that are essentially parallel to the bounding bedding

surfaces are said to be planar-stratified. Groups of similar planar beds are called

bedsets.

Simple bedsets are characterized by similar compositions, textures, and internal

structures; composite bedsets consist of groups of beds that differ in these

characteristics but that are genetically associated.

Beds displaying internal layers deposited at a distinct angle to the bounding surfaces

are cross-stratified.

Fig. terminology of bedsets (ref. in Boggs, 2009).

Graded Bedding

Graded beds are strata characterized by gradual but distinct vertical changes in grain size.

Normal grading: beds that display gradation from coarser particles at the base to finer

particles at the top.

- can result from the settling of particles out of suspension or as a consequence of a

decrease in flow strength through time.

Reverse grading: those that grade from finer particles at the base to coarser at the top.

- increase in flow velocity through time may result in an increase in grain size up

through a bed.

Fig. Normal and reverse grading within individual

beds and fining-up and coarsening-up patterns in a

series of beds (Nichols, 2009).

Fig. Beds deposited by turbidity currents are called turbidites. Each event produces a single bed

characterized by a decrease in sediment size from bottom to top, a feature known as a graded bed

(Monroe et al., 2007).

Massive Bedding

Massively bedded sediments are rare.

Presumably generated in the absence of fluid-

flow traction transport, either by some type of

sediment gravity flow or by rapid deposition of

material from suspension.

Applied to beds of sedimentary rock that contain few or no visible internal laminae.

Fig. Massive-bedded sandstone (upper part of photograph) lying

above thin, parallel-bedded siltstone and shale (Boggs, 2009).

shale

sandstone

Cross-Bedding

forms during deposition on the inclined surfaces of bedforms such as ripples and dunes, and

indicates that the depositional environment contained a flowing medium (water or wind).

Cross-beds, are strata in which internal layers, or foresets, dip at a distinct angle to the

surfaces that bound the sets of cross-beds.

Tabular cross-bedding: having bounding surfaces that are planar,

Trough cross-bedding, having bounding surfaces that are curved.

Fig. Terminology and defining characteristics of two

fundamental types of cross-bedding. (Sp), the principal

bedding surface or bedding plane; (Sf), the foreset

surface of cross-bedding.

(Boggs, 2009)

Fig. (a) Multiple sets of small-scale planar cross-beds (between arrows) with tangential foresets, (b) Three intersecting

sets of small-scale trough cross-beds in fine, laminated sandstone. The area marked (B) may be a burrow (Boggs, 2009).

(b) (a)

Cross-bedding formed under different environmental conditions (fluvial, eolian, marine) can

be very similar in appearance and thus may be difficult to differentiate in ancient deposits.

Cross stratification

• Cross lamination (small-scale cross stratification) is produced by ripples.

• Cross bedding (large-scale cross stratification) is produced by dunes.

• Cross-stratified deposits can only be preserved when a bedform is not entirely

eroded by the subsequent bedform (i.e., sediment input > sediment output).

• Straight-crested bedforms lead to planar cross stratification; sinuous or linguoid

bedforms produce trough cross stratification.

Fig. (A) Cross beds from a modern beach sand dune, (B) Aeolian cross beds in the Navajo Sandstone,

Zion National Park.

Summary

Bedding and lamination define stratification. Bedding is thicker than 1 cm whereas lamination

is thinner than 1 cm.

Graded beds are strata characterized by gradual but distinct vertical changes in grain size.

Beds displaying internal layers deposited at a distinct angle to the bounding surfaces are cross-

stratified.

Massive bedding applies to sedimentary rock that contain few or no visible internal laminae.

Thank You