sedimentary structures- i

Sedimentary structures- I

Outlines

1. A bit a brainstorming…

2. Then, some classification: how and why?

3. A huge variety of sedimentary structures

Brainstorming

What kind of sedimentary structures do you know?

Definition and Classification

1. Suspension is falling in a calm or slowly moving fluid:

Stokes’ law (spheric particles < 0.2 mm)

2. « Entrainment Flow », when FF > FG

3. Gravity flow within a calm or moving fluid

Three types of sediment deposition


In entrainment flow…

Classification based on:

• The location: internal vs.

bedding

• Physical or biological

process

Definition and Classification: cross bedding

CROSS-BEDDING is a feature that occurs at various scales,Sedimentary.Rocks are

normally deposited as horizonal layers. However, you may see very layers that are at an

angle to the horizontal. These tilted layers layers are termed cross bedding.


In entrainment flow…

A series of strongly

different structures

develop, as a function

of the mean flow

velocity (laminar,

turbulent, subcritical,

supercritical), flow

depth and grain size

of transported particles

Depth: 10-22 cm; Temperature: 10°C

Current RipplesFlow (imagined as streamlines) within the sub-viscous layer

form clusters of grains that modify the flow dynamic.

A region of boundary layer separation get formed

between the flow separation point and the flow attachment

point.

Ripples are bedformscreated by the effect of

boundary layer separation on a bed of sand.

• Adsorbed layer: at margin, fluid particles attached to solid surface (a few molecules thick).

• Boundary layer: zone of bottom-ward decreasing velocity. This contains a viscous sub-layer, a reduced turbulence zone.

Layers within a flow

Current Ripples

H: 0.5 – 3 cm

L: 10 – 20 cm (max. 60 cm)

Ripple Index: 10 (sand) – 20 (silt)

How do ripples form?

Crest

1) Sand grains roll up orsaltate up to the crest on the upstream side of the

ripple.

2) Avalanching of grains occurs on the downstream side as accumulated grains become unstable at the crest, over the maximum critical slope angle (circa 30° for sand-size grains).

Stoss sideLee side

Trough

Current Ripples constrains

• Require moderate velocity within the viscous sub-layer under hydraulically smoothed conditions.

• Independent from the water-depth.

• The dominant grain size is finer than 0.6 mm; no lower grain-size limit but sediment must be non-cohesive.

• Can be up to 40 mm high and the wavelengths range up to 500 mm. Wavelength/height is usually around 10 and 40.

Current RipplesCurrent flow causes downstream migration of the ripple and all its components, creating sand layers at the angle of the slope.

These layers are called cross-laminae and form a specificsedimentary structure named cross-lamination.

3 types of ripple crest

Current Ripples

A huge variety of sedimentary structures

Current ripples

Current Ripples

First ripples (low flow velocity): parallel, long and regular crests

Increase in velocity: sinuous, then linguoid crests

Cross-laminationMainly dependent by the flow velocity/duration and the ripple shape.

Planar cross-laminationfrom straight ripples.

Trough cross-laminationfrom sinous to isolated

Ripples.

Planar cross-lamination

Trough cross-lamination

Trough to planar cross-stratification

Deep-water current ripples

Terminology for cross-stratified beds

Climbing ripplesEach ripple migrate up the stoss side of the ripple form in front,

when the sand-addition rate ≥ migration rate.

Upward decreasing of sedimentation rate

Deposition occurs on both the stoss and the lee side.

Indicators of high sedimentation rate.

Climbing ripples

Kurt Grimm, UBC

Cojan & Renard, 2000


Current ripples Transport > input

Transport = input

Transport < input

current

Subaqueous Dunes• Require higher flow velocities to appear and do not form at all in

sediments < 0.1 mm.

• Range of wavelengths: 600 mm to hundreds of meters.

• Range of height: few centimeters to more than 10 meters.

• Appear in river channels, deltas and shallow-water environments with quite strong and sustained flows.

There is a relationship between flow depth and

the dimension of the dunes

• Originate in one of two ways:

– From ripples at flows ~ 50 cm/sec

– From a plane bed of sediment coarser than

~ 0.8f


Sand Waves: Dunes

Both dunes and ripples can occur when the grain-size ranges between 0.1-0.4

mm.

Current velocity: 40 to 150 cm/s, grain size > 0.15 mm (2.8 Φ)

H = up to several 10s meters

L = up to several 100s meters

Index = ca. 5 (fine sand) - 50

(coarse sand)

Oblique stratification, often

tangential at the base of the

structure (bottomset)

www.nasm.si.edu


Dunes

Ripples vs. Dunes

Clear separation (no overlap) in both wavelength and bedform height between ripples and subaqueous dunes.

The two bedforms are not the same!

Dunes seem to be more dependent by water depht because are generated by large-scale turbolence within the whole flow.

Cross-beddingMainly dependent by the flow velocity/duration and shape.

Planar cross-beddingfrom straight dunes.

Trough cross-beddingfrom sinuous to isolated

dunes.

Plane beds

• A bed with active sediment transport but no obvious topography.

• Occurs in two different physical settings

– All sediments coarser than circa 0.4 mm near the initiation of motion (lower regime) (Still not so not clear process)

– Sediments of any grain size at very high velocity (upper regime) (planar configuration is associated with the planing or flattening of the bed)

Dune

Upper plane bed

Trias, Nevada


Entrainment current, upper flow regime: Upper Plane Bed

Flow velocity between 60 and 180 cm/s

Upper flow regime: supercritical fluid and Fr>1

Formation of Upper Plane Beds, with motion of sediments by traction or

saltation, inducing the development of sorting lineations

Internal Structures in Plane Beds• The internal structures produced are a series of

thin, parallel and sub-horizontal laminae (parallel or planar lamination).

• Internally, the laminae can comprise alternations of slightly finer and coarser sediments.

Current velocity > 120 cm/s:

formation of antidunes,

bedform in phase with surface

waves

Occurrence: tidal current and

flooding events


Entrainment current, upper flow regime: Antidunes

current

Tidal channel, California

Antidunes are commonly observed in small streams that flow across beaches

into the ocean. Flume studies have shown that they can also occur in

submarine environments beneath density flows like turbidity currents.



Entrainment current, upper flow regime: Antidunes

Antidunes: slow

height, length (L) up

to 5 m

Index: between 7

and > 100, usually

high

A: laminae on the

lee side (low

angle slope)

B: laminae wrapping the

entire antidune

C: slightly inclined laminae

on the stoss side: upstream

motion of the antidune

water surface

water surface

water surface

The water surface is

strongly in phase with the

bed. A train of

symmetrical surface

waves is usually

indicative of the presence

of antidunes.


Entrainment current, upper flow regime

Very high current velocity: Chutes and pools, mainly an erosive

feature

Chutes: high flow regime inducing upper plane beds

Pool: downstream, a thicker package of sediment corresponds to

an hydraulic threshold

Bedforms stability diagram

Very slow flow: parallel, planar beds, no mouvement,

deposition by settling of suspension: lower plane bed

Flow velocity between 5-60 cm/sec: small ripples for

sediments < 0.8 mm (0.3 ϕ)

WavesAn ocean wave is an oscillatory motion of the sea surface

caused by wind and involving transfer of energy between particles but no mass transport.

Wave Parameters– Wave crest

– Wave trough

– Wave height

– Wave length

– Wave Amplitude

– Wave period (time interval between arrival of consecutive crests at a

stationary point).

Amplitude

Motion of Water Particles beneath Waves

The oscillatory motion generates a circular pathway.

With increasing depth internal friction reduces the motion and the effect of the

surface waves dies out.

The depth to which surface waves affect a water body is

referred to as the

wave base

Interaction of waves and shoreline

Motion of Water Particles beneath Waves


Wave ripples and orbital velocity

Vertical decrease in velocity

from water surface to the wave

base

Wave base = L/2, where L =

wave length

Wave ripple cross-laminationIn cross-section wave ripples are generally symmetrical in profile,

laminae within each ripple dip in both directions and are overlapping.

Wave ripples can form in any non-cohesive sediment and are principally seen in coarse silts and sand. If the wave energy is high enough wave

ripples can form in granules and pebbles

Wave ripples

Several features are common in wave-ripple lamination. Foresets on

the parallel-to-flow face may dip in opposite directions, sometimes in

adjacent ripples, sometimes within a single ripple. Laminae may be

interleaved at an individual ripple crest.

Wave ripples

Modern wave ripples

Current ripples vs. Wave ripples


Mixed ripples


« Hummock »« Swale »


Hummocky cross stratifications (HCS)

HCS: undulating set of cross laminae both concave up

(swales) and convex up (hummocks)

storm- generated wave causing strong oscillatory flows and/or

combined flow. Erosion of the seabed into Hummocks and

swales.Swales and hummocks are superimposed on top of

each others

Erosive base, low angle (10-15°), rippled top

Summary

1. Storms with oscillatory flows and unidirectional currents

2. Erosive base with angle usually <10°3. Laminae parallels to the base

4. Changes from concave up to convex up

5. Crests not straigth but more curved

6. Coarse silt to fine-medium sands

Ferron Sandstone, Gentile Wash, Utah



Ashgill, Ordovician, Anti-Atlas, between Merzouga and Erfoud




HCS

Upper plane bed



Cross-stratifications

Mud

Bioturbation

Conglomerate

Groove, flutes

« Tempestite »

Linked to a rapid

increase of the orbital

velocity of wavesFAIR WEATHER

STORM

WEATHER

IDEAL SEQUENCE

Tempestite, Carboniferous, east of Tineghir, Anti-Atlas, Morocco



Tides

Tides are formed by the gravitational attraction of teh moon and sun on teh Earth

combined with the centrifugal force caused by the movement of the Earth around

the center of mass of teh Earth-moon system

Semi diurnal or diurnal tidal cycles

Neap-spring tidal cycles

Annual tidal cycles

Tidal sedimentation

Herringbone cross bedding

Tidal sedimentation


Current ripples in tidal environments

FLASER BEDDING

LENTICULAR BEDDING

WAVY BEDDING

Lenticular and wavy lamination in dark mudstone and pale sandstone

Entrainment flow - Synthesis

sedimentary structures- i

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