09a.structures in sand and sandstone 1

8/10/2019 09a.structures in Sand and Sandstone 1

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Sand and sandstone are rock formed by clastic elements between 2 mm and 0.06 mm.

Sand can be transported by fluids in movement, as water or wind, or by gravitational

forces.

SAND AND SANSTONE



The sand deposition occur at the decreasing moment of the fluid in movement for

excessive grain in the flow or for progressive decreasing of the flow energy, which

does not sustain the grain.The grain can be transported in suspension or on the bed for saltation, rolling and

sliding (caused by bump of the other grain).

STRUCTURES FORMED IN THE SAND

BY FLUIDS IN MOVEMENT



The sand only deposited by suspension, while the flow energy decreases, forms a

very typical structure, characterized by coarse grains in the lower part progressively

decreasing upward of grain-size.

This structure is called GRADATION or NORMAL GRADATION.

GRADATION



The turbulence is the cause of the gradation. It is linked to the “casual” (turbulent)

movement of the fluid particles in natural flows.

In a flow the turbulence result is a downstream movement of the mass, but the fluid

(or the solid particles transported) have an irregular path. This generates a push

upward, that sustain the clasts.

All the natural “Newtonian fluid flows” (flows with low density and low viscosity) are

turbulent.

Beyond the gradation other sedimentological consequence is the selection.



The sand, transported and

deposited on the depositionalsurface, develops bed forms in

equilibrium with the fluid energy.

The first bed form produced on asandy bed, where the velocity of

the overlying flow is increasing, is

called current ripples mark.

CURRENT RIPPLES



The ripple marks are bed forms, asymmetrical and triangular shape in vertical section,

from less than 1 mm to 7 cm high, made up by coarse silt to medium sand.

The current ripples are formed by

eroding and moving grain on the

stoss side and avalanching on the

lee side. Each avalanching produces

a foreset, determined by little grain

variations. In this way the current

ripples migrate and the cross

laminations (foresets) form.



The particular form of the stream lines determine a vortex cell with trapping of

material (fine and/or coarse) and a site with the stream lines reattach they-self with

the bed.

The current ripples are formed by incipient ripples. At first phase of fluid movement,

just above the depositional surface, in correspondence of a thin layer (called viscous

sub-layer), the first movement of sand grain forms little grain aggregations, regularly

spaced, called incipient ripples.

Once they formed, the stream line variation on the bed provokes the growing and the

rhythmic repetition of the ripple form.



In plant, the current ripples have rectilinear, sinuous, linguoid or lunate.

The form also depends on the current velocity.



Dimension and wave length of current ripples.

Ripples quantitative parameters and

Ripple Index.



The geological record of

the current ripples is the

cross lamination, i.e. the

foresets.

Form set (external bedform) is rarely conserved.

CURRENT RIPPLES AND CROSS LAMINATIONS

Cross lamination is a inclined lamina not more

thick of 7 cm; more of 7 cm it is called cross

stratification.



Set and coset of cross

lamination.

Cross laminations:

real and apparent dipping.

CURRENT RIPPLES FORMATION



When sets of cross laminations superimposed each other, they called climbing

ripples. They are produce by an active rate of sedimentation in energetic condition

allowing the superimposition of current ripples.

The angle of

climbing isproportional to the

sedimentation rate.

When the angle of

climbing exceeds

the stoss side angle,

also the stoss sideof the ripple is

conserved.



CLIMBING RIPPLES FORMATION



These structures are linked to various relationships between mud and sand, caused

by energetic variations.

Sedimentary structures

linked to the current

ripples are: flaser , wavy

and lenticular bedding.



Current ripples with herringbone structure are also linked to energetic variation.

But pay attention not all are really herringbone.



The foreset sedimentation is by

traction of grain on the inclined

margin of the ripples and not for

avalanching.

The lamination of waves ripples dipin opposite directions and show

different degree of interdigitations.



On the surface wave ripples show rectilinear

and continuous crests; very typical is the

bifurcation of the crest.



Interference ripples:

wave ripples on current ripples or

wave ripples.



Different type of wind ripples are

known: impact ripples and adhesion

ripples.

WIND RIPPLES AND CROSS LAMINATIONS

Impact ripples. They have low height

and coarse grain concentrated in the

crest, perpendicular to the flow. Their

profile is weakly asymmetric.

Impact ripples form for different impact

angle on the ripples surface, dependingon the if the impact is on lee or stoss

side. On the stoss side fine grains have

a high reflection and are moved away,

whereas in the lee side the low impact

angle allow to stop them. Therefore a

form laminae characterized by ainverse gradation.

Thus wind ripple record is made up by

inverse gradate thin strata weakly

inclined. The wave length of the ripples

depends on the saltation space,

therefore by the wind energy.



Wind impact ripple form planar or

gently inclined laminations, rarely

can be see cross laminations.



Adhesion ripples. When dry sandfall on humid surface, several

grains can be stopped on the

humid surface for capillarity

adhesion. The ripples have very

small dimensions and the inclined

step is wind opposite. The surfacewith adhesion ripples is a

apparently vesicular surface.



Ripples give a qualitative idea of the

velocity of the producing flow, but is not

still possible associate ripples to thequantitative parameters because ripple

forms depend on vary factors beyond the

velocity of the flow, as:

granulometry, density and viscosity of the

water, flow duration, sedimentary input.

GEOLOGICAL MEANING OF THE RIPPLES AND CROSS LAMINATIONS

Climbing ripples give information on the

quantity of deposited sediment.

Ripples give well information about the

way-up.



Ripples give well information about the way-up.

Nevertheless, pay attention with the structures called ripples load-cast.



One of the most utility of the ripples is the

palaeocurrent information.

In current ripples the dipping of the foreset is

quasi-parallel with the flow which originated theripple, therefore we can take this data as

palaeoflow sense direction.

However, pay attention with the palaeocurrent data from ripples. The ripples are formed by weak current,

which could be influenced by small morphological irregularity and not linked with the main depositional

flows.

In wave ripples the crest of the ripples give information on the axis of the wave

motion, but not the sense.



The dunes are a sedimentary bed form geometrically similar to the ripples, but of

major dimensions and of different hydraulic conditions.

Dune are associated to cross stratifications.

DUNES AND CROSS STRATIFICATIONS IN SUBAQUEOUS ENVIRONMENT

D di i i t f i l b h i ht d l th



Dunes discriminate from ripples by height and wave length.

Cross stratification distinguish from cross lamination for a boundary of 7 cm of

height and a wavelength of 60-70 cm.

Dunes form in granulometric and

energetic conditions differentrespect the ripples.



Dunes can cohabit with ripples, as in eolic dunes.

Dunes as the ripples always are in group, because represents a equilibrium

condition between the overlying flow and the sandy bottom.

So, isolated cross stratification could not be dunes.



Dunes form as ripples, with transport on stoss side and avalanching on lee side.

The transport stoss side can be produced by ripples also.

Normally the height of the dunes and relative cross-stratifications are between 0.1-

1 m.

Once, dunes were called megaripples or sand wave or bar.The external shape shows rectilinear, lobate, sinuous, lunate or linguoid crests.

In sedimentology we are able to distinguish only bi-dimensional (2D) and three-

dimensional (3D) dunes.



They are dunes with rectilinear

crest.

The sedimentological record is thetabular cross stratification.

The set contact are rectilinear

(plane or weakly inclined), the

foreset bottom is angular.

Bi-dimensional (2D) dunes.



Dunes with irregular crest.

The sedimentological record is

the trough cross stratification.The set contact are concave,

(especially in cut perpendicular to

the flow), the foreset bottom is

tangential.

Three-dimensional (3D) dunes.

T h t tifi ti f i hi h ti diti th t b l



Trough cross stratification form in higher energetic conditions than tabular cross

stratification. The particular form is linked to the concentration of the reattachment

point of the stream line.

The upper surface is called

rib and furrow structure.



DUNES FORMATION



The cross stratification are not organized in simple forms. Frequently we meet

composite forms, as great eolic dune or fluvial bars or tidal bars.



The vortex cell in the lee side could

produce inverse flow generating

structures migrating in opposite

sense, called countercurrent cross

laminations.

H ft th t t d d b ll it t I h t



However often these structures are produced by a really opposite current. In what

kind of environment?

REACTIVATION SURFACES



REACTIVATION SURFACES

The reactivation surfaces are cut

surface on the foreset surface.

These surfaces have less

inclinations of the cross

stratifications.

They are formed as erosion surfaces

on foreset plane by ebb or flood

currents in tidal environment or

during the waning flow in fluvial

environments.

Where is the reactivation surface?

Could you determinate the genetic

condition of formation?



SIGMOIDAL CROSS STRATIFICATION OR TIDAL BUNDLES

They are sinusoidal sandy foresets

separated by pelitic laminae. The pelitic

laminae are cyclically organized in the

foresets.They are typical structures that form in

tidal environments. The pelitic laminae

form during the stacking water phase.

The different concentrations of pelitic

laminae is linked to the cyclical spring

(sizigia) tide and neap (quadratura) tide.



tidal bundles

GEOLOGICAL MEANING OF THE DUNES AND CROSS STRATIFICATIONS



Cross stratification formed by

dunes give a qualitative idea of the

movement energy, as showed in

the classical experiment of Simons

et al. (1965).

The flow height is proportional tothe dunes high, being the flow

height two or more times greater of

the dune height.

Dunes as ripples give information

on the way up. How?

GEOLOGICAL MEANING OF THE DUNES AND CROSS STRATIFICATIONS

If the flow height is proportional to the

dunes high, how do you interpret this

cross stratification variation?



The great utility of the cross

bedding is as palaeocurrent

indicator.

The use of cross bedding is better

than cross lamination as it respects

greater and more general flow

conditions.

To take palaeocurrent

data is easy form rib andfurrows structures.



Is not always easy take palaeocurrents data from cross stratification, especially

when appear only apparent cross stratifications.

In this case the use of stereogram of Wulff is necessary.

09a.structures in sand and sandstone 1

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