Walker J. Facies models, Canada, 1992

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EOLIAN SYSTEMS

Figure

Wind ripples (maximum wave length seen is

cm) in fine sand (El Gran

Desierto, Mexico)

Figure

Wind ripples (maximum wave length seen is

cm) in coarse sand (El Gran

Desierto, Mexico).

Figure

Adhesion structures at margin of permanent salt lake (El Gran Desierto, Mexico).

aqueous deposits (Fig.

5).

2) Climbing

ripple lamination closely resembles

subaqueous varieties, but the ripple

foresets are difficult to recognize

because of their low relief except in

oblique sections (Fig.

6).

Hunter (1977)

distinguishes translatent strata (where

only the bounding surfaces between

migrating ripples are visible) from

rip-

pleform strata (where the ripple fore-

sets can be identified). Both are in-

versely graded and relatively closely

packed.

Grainfall lamination is pro-

duced in zones of flow separation

(commonly in the lee of dune crests)

by deposition from suspension (Fig. 7).

Segregation of different grain sizes

is relatively poor and laminae and

grading are difficult to see. The aver-

age porosity is about

per cent, due

to a packing that is intermediate be-

tween the closely packed traction

laminae of

planebed and climbing rip-

ple laminae, and the loosely packed

sandflow strata described below.

Sandflow lamination (avalanche cross

bedding) is caused by slumping and

consequent grainflow down slopes

(Figs. 7 top,

8).

Sandflow cross strata

are loosely packed (average porosity

45 per cent), interfinger with grainfall

laminae near their base, and form

lenses parallel to the slipfaces (Fig.

9).

Internal structures are difficult to

study in modem dunes, hence the em-

phasis on stratification models.

Large-

scale cross bedding was formerly con-

sidered typical of eolian sands, but

large

bedforms also occur in shallow

shelf seas (Chapter 12) and in large

deep rivers (Chapter 7), where the

cross beds may resemble eolian cross

bedding (Allen, 1982). Nevertheless,

many superimposed sets, each thicker

than

10 m, occur far more cqm-

mG eolian than in other depo-

-1

environmegts. Eolian cross

bedding can be deformed into huge

convolutions, caused by slumping of

water-saturated dune sands. Convolute

stratification is found in ergs being inun-

dated by the sea, but may also form by

seasonal melting of ice in cold-climate

dunes (Koster, 1988).

Each type is found on different parts

of a dune and can be recognised even

where the dune is deflated. For ex-

ample, the slipfaces (sandflow cross

strata), saddles (climbing translatent

strata) and the passage between the

slipfaces and saddles (grainfall

lamina-

BROOKFIELD

tion) can be recognized in barchanoid

dunes, even after deflation (Fig. 10).

Further information on stratification

types and their relationship to dune

morphology is given by Hunter

(1981),

Hunter and Rubin (1983), Rubin and

Hunter (1 983).

Our knowledge of the larger internal

structures of modern dunes is due

almost entirely to the work of McKee

(McKee, 1966; McKee and Tibbitts,

1964; McKee and Douglass, 1971;

McKee and Moiola, 1975) and Bigarella

(Bigarella, 1972; Bigarella

a/., 1969).

These studies are summarized in

McKee (1 979).

The most detailed studies were done

in the White Sands dune field of New

Mexico. This area is not analogous to

most ancient eolian deposits because

the dunes are composed of gypsum

and

are

easily stabilized

after

Occa-

Figure

Various lizard and beetle tracks on dune surface (El Gran Desierto, Mexico).

sional wetting by rain. Also, we cannot

Width

biggest

track

set

cm.

be sure that the deeper structures relate

to the modern wind regime. Therefore,

an alternative approach to the under-

standing of internal structures is the

computer modelling of a diversity of mi-

grating

bedforms (Rubin, 1987).

Interd-al part of the

eolian

bedform system (Ahlbrandt and

Fryberger, 1981

;

Kocurek, 1981 b).

In-

des~s~tUlh?&d_sand supply, inter-

dunes consist of

IagAeposits, coaE

garnets

and small isolated dunes

afiidsabknas. Here, the water table may

Emrface and control the

amount and type of sediment deposition

(Stokes, 1968; Fryberger

a/.,

1988).

Because of the way in which sand is

transported through the system, longi-

tudinal

bedforms tend to have coarse

lag and coarse sand sheets and dunes

in the interdune areas. Transverse bed-

forms tend to have sabkhas and fine

sand dunes between them (Glennie*

Figure

Concentric grooves (0.5 to 1.0 m in diameter) created by plants moving in the

.1970; Sharp, 1979). The size of inter-

wind. photo courtesy Rodney Stevens.

dunes is also dependent on sand

supply and on the stage of develop-

m&twof the surrounding erg. ~ost

modern deserts have extensive inter-

dunes because of their relatively

recent development (Mainguet and

Chemin, 1983).

Surface structures are common on

eolian

bedforms and include. ripple

marks (Figs.

11, 12), adhesion struc-

tures (Fig.

13), animal tracks (Fig. 14)

and trails, and marks made by vegeta-

tion (Fig.

15), rain, hail and lightning

(Fig. 16). All may be preserved by

rip

Figure

Fulgurite-sand fused by light-

ening strike (Permian, Arran, Scotland).

Pen is

long.

SIMPLE DUNE TYPES

BARCHAN Longitudinal

wind Transverse

Plan

G;a

1-10m

Section

10-1 00m

OOm

BARCHANOID

loom 10-1 00m

TRANSVERSE

srrrmrr

-----

10-1 00m loom

LINEAR oblique

10-1

00m 1 OOm

No oblique migration

COMPLEX AND COMPOUND

BARCHANOID DRAA

wind

lnterdune

"Saddle"

100-1000m 100-1 000m

LINEAR DRAA

Longitudinal Transverse

lateral

ation

100-1000m

Transvew

Longitudinal

(

to migr. direction)

STAR

DRAA

100m

1000-5000m 1000-5000m

Grainfall and

Bounding

suhca

mclimbing

translatent m~dhesion structures

asand flow B~udcracks Coarse lag sands

ple climbing and grainfall deposition

(Ahlbrandt and Fryberger, 1981;

Kocurek and Fielder, 1982;

McKee,

1982; Steidtmann, 1973). Between

sand layers there may be soil horizons

and plant and animal traces

(Loope,

1 988).

The features described above

suggest that the input for a facies mo-

del should consist of

1) the different

types of

bedforms and interdunes in

modern deserts, and

the types of

stratification formed by the migration of

different parts of the various bedforms.

However, a good model must also con-

sider how the characteristic strata of

various combinations of migrating

bed-

forms can form thick, extensive ancient

eolian sandstones.

STRATIFICATION AND BOUNDING

SURFACE MODELS

Stratification results from the migration

bedforms as they climb at various

angles and directions over one another

(Fig. 17). Simple stratification models

involve simple migrating

bedform sys-

tems and form simple architectural as-

semblages. For example, simple trans-

verse dunes migrating across a desert

form tabular cross beds separated

by planar bounding surfaces. Simple

transverse dunes migrating over larger

transverse draa form slightly more com-

plicated patterns, including

downwind-

dipping second-order surfaces (dis-

cussed below). However,

bedforms

generated by moving sand are rarely

simple and are also affected by existing

structures. Changing winds cause mod-

ification of existing

bedforms and these

changing

bedforms also interact with

the wind to produce a variety of dis-

equilibrium forms (Mader and Yardley,

1985).

The models of Figure 17 assume uni-

directional climbing of ideal

bedforms in

a fully developed dune, draa and inter-

dune system, but only indicate some of

the possible stratification styles.

Rubin

(1987) has used computer models to

show the enormous variety and com-

plexity possible if the

bedforms are

neither climbing unidirectionally nor fully

developed.

Figure

Stratification models for simple

and

complex/compound dune types. Longi-

tudinal and transverse sections are parallel

and perpendicular to resultant wind direc-

tions respectively.

Bounding surfaces

Where

bedforms climb at angles lower

than their windward slopes, the

underlying and preceding

bedforms are

eroded. This leads to the formation of

bounding surfaces

that define sets of

cross strata

(Brookfield, 1977; Figs. 18,

19). These bounding surfaces occur at

all scales, and even translatent strata

are really only ripple bounding sur-

faces. Bounding surfaces can only farm

from a migrating train of bedforms.

First-order surfaces

(Fig. 18) are flat-

lying bedding planes cutting across all

underlying eolian structures and are at-

tributed to the migration of draa.

Second-order surfaces

(Fig. 18) com-

monly lie between first-order surfaces

and usually dip downwind with variable

inclinations. These surfaces are attri-

buted to transverse dunes climbing

down the lee slopes of draa, or to lateral

migration of linear dunes across the

draa lee slope.

Third-order surfaces

form the bound-

aries of bundles of laminae within co-

sets of cross laminae, and are attributed

to erosion followed by renewed deposi-

tion due to local fluctuations in wind di-

rection and velocity. They are reac-

tivation surfaces.

Simple dune and draa systems

should lack the second-order surfaces.

However, they occur only because few

systems are in the dynamic equilibrium

assumed in climbing

bedform theories.

Second-order surfaces can be formed

by dunes periodically overtaking one

another and coalescing (Mader and

Yardley,

1985), and occur especially in

seasonally reversing dune systems (Fig.

20). All dune-draa systems must show a

resultant sand drift except at the sand-

flow nodes (Wilson, 1971). Net unidirec-

tional climbing is thus a near certainty

(Hesp

eta/.,

1989), although some star

draa systems may only climb very

slowly. Theoretical rnodels suggest very

low rates of dune and draa climbing,

since most eolian sandstones seem to

preserve only a small basal part of the

original bedforms. Nevertheless, the

rnodels can be modified for steep rates

of climb where necessary

(Rubin and

Hunter, 1982;

Rubin, 1987). These

eolian sandstones, truncating all under-

lying surfaces. These

supersurfaces

form in response to external controls on

the desert and are discussed in a later

section.

In deserts, many different systems

are present, and it is necessary to con-

sider the possible stacking of different

systems during the evolution of a

desert. This requires knowledge of the

typical areal distribution of

bedform

types in a desert, and the vertical and

lateral varisations in stratification formed

as they migrate and the desert evolves.

FACIES MODELS

The facies models developed here are

based on the two

fundamental concepts

discussed above. First, strata are gen-

erated by hierarchies of migrating bed-

Figure

First-order bounding surfaces are

order surfaces are generated by the movemer

lee slope. Third-order surfaces represent eoliar

forms of differing sizes and shapes.

These climb over one another at dif-

ferent angles and in differing directions.

The resulting bounding surfaces can be

used to recognize the nature of the

bedform systems in ancient eolian

sandstones (Brookfield, 1977;

Rubin

and Hunter, 1982; Rubin, 1987). Se-

cond, the stratification types vary in pro-

portion and location in different types of

bedforms in modern deserts. The strati-

fication types, together with bounding

surfaces, can be used to identify bed-

forms in ancient eolian sandstones

(Hunter, 1977).

Application of studies of

modern sediments

There are many problems in using

studies of modern eolian sediments to

PATH

DRAA

generated by the migration of draa. Second-

1t of individual dunes climbing down the draa

lamination. From Brookfield (1977).

stratification models are for uniform as-

Figure

Bounding surfaces, mostly first

Figure

Reversing barchanoid dunes

semblages

Of One

type

dune-inter-

order, in the Jurassic Navajo Sandstone at

between rows of large star draa; one is

dune or draa-dune-interdune system.

Page, Utah. Note decrease in thickness

being traversed by the truck, others can be

the

surfaces

dis-

upwards toward contact with overlying fluvial seen in the distance. Note deflation sur-

~~ssed above, there may be other ex-

sediment, suggesting decrease in bedform

faces on lee slopes of both draas and

tensive surfaces that cut across ancient

size with time.

dunes (El Gran Desierto, Mexico).

formulate general facies models. Gen-

are usually Quaternary in age (Wilson,

era1 models must incorporate modern

1973). No thick eolian deposits compa-

and ancient examples, as well as theo-

rable to ancient examples seem to be

retical results. Modern large eolian bed-

forming today. Deposits observed today

forms commonly rest on alluvium and

are unlikely to be even approximately in

ALLUVIAL FANS

ALLUVIAL

FANS

WIND

DIRECTION

TRANSVERSE

BARCHANOID ORAA

LONGITUDINAL

SECTION

(

PARALLEL

TROUGH

TRANSVERSE

SECTION

Figure

Synthetic model for desert with transverse bedforms. Vertical scale is exagger-

ated, such that

bedforms appear to be climbing too steeply, and the troughs shown in the

transverse section are too concave. Only first-order surfaces are shown on the transverse

section; some second-order surfaces are shown on the longitudinal section. For scale, the

basin is at least

km across.

.,..

I...,

,Dc

.......

-;.y?:&i$;y:;!F,i:<q>q::::

<,.,..

;.',.;

a,.....,.

)":-.

...

MI,.?.

<--..

.x.>:*;

,,,,;:

::.,.>.

:.,;,:,

,,.>

:2.2

...

%.>'"

1::"

ALLUVIAL

FANS

WIND

DIRECTION

TRANSVERSE

SECTION

LONGITUDIML

SECTION

Figure

Synthetic model for desert with longitudinal bedforms in an enclosed basin. Note

that some lateral migration of longitudinal star draas is permitted

without migration, ver-

tical stacks of longitudinal-star draa deposits would be separated by thick interdraa lags,

barchan dunes, and

fluvial deposits (lags, dunes and fluvial deposits shown in black).

For

scale, the basin is at least

km across.

equilibrium with present wind condi-

tions. This is because large dunes and

draa take thousands of years to re-

equilibrate with changing wind condi-

tions. Those observed today could not

possibly have adjusted to the rapid al-

ternations of climate and wind regimes

during and since the Pleistocene (Wil-

son, 1973). Ancient ergs may have

formed under more uniform conditions,

which might explain why ancient eolian

sandstone systems appear simpler

than modern ones.

Stratification in Recent large dunes

and draa of uncemented quartz sand is

almost impossible to examine by trench-

ing, because the trench walls rapidly

collapse. Because of the Recent cli-

mate and wind changes, any stratifica-

tion observed in trenches would be

difficult to relate to processes of forma-

tion. Direct comparison between cross

sections of modern dunes and ancient

eolian sandstones is also difficult be-

cause normally, only the lowest parts of

eolian

bedforms are presetved. Rates of

bedform migration tend to be at least

one order of magnitude greater than

vertical buildup. It follows that only the

lowest parts

the dunes are pre-

served, and reconstruction of large

bedforms depends almost entirely upon

structures formed at their basal lee

slopes.

The two different models presented in

Figures 21 and

are based on the mi-

gration of predominantly transverse and

predominantly longitudinal bedforms, re-

spectively. These models are based

largely on simplifications of the

westem

part of the Gran Desierto (Fig. 2B) and

of the

Fachi-Bilma erg (Fig. 2A).

Transverse bedform

model

The transverse bedform model (Fig. 21)

is based on unidirectional winds, with

upward and lateral buildup of sand

within an enclosed basin with marginal

fans. The constriction of the fans keeps

the first-order bounding surfaces hori-

zontal; by contrast, on an unconfined

extensive plain, these bounding sur-

faces would tend to

be convex-upward

on a large

scale.,The model involves

the initial development of sand patches

and barchan dunes with the onset of

aridity, followed by the successive de-

velopment of transverse dunes and

compound transverse draa at the

climax of aridity. Finally, with decreas-

ing aridity

and/or reduction in sand

supply, there would be a gradual con-

traction of the erg and eventual covering

Boundln~

over by fluvial or lacustrine sediments.

surfaces

Fluctuations during erg development

lead to complex interfingering of

fluvial

__--

Upwind

1st

- ----

and eolian deposits (Langford, 1989;

/',

','

Langford and Chan, 1989).

2nd

/'0

Longitudinal bedform model 3rd

/‘,

In this model (Fig. 22), the bedforms

---Ic--

Downwlnd

rest on alluvium and lag deposits, as

in the

Fachi Bilma erg. The early sand

Barchanold Barchanold

(alkt) lf~aleowinds

Draas dunes QTowards nearest erg margin

patches and barchans are followed by

TRANSVERSE

SECTIONS

linear dunes and linear complex draa,

and eventually (in complex wind

regimes) by star draa. These longitu-

dinal patterns have rarely been recon-

structed in ancient eolian sandstones.

This is possibly because such bed-

forms are characteristic of deserts with

a net through-flow of sand and little de-

position, or because such

bedforms

migrate laterally, laying down deposits

that mimic those of transverse bed-

forms (Hesp

a/.,

1989; Rubin and

Hunter, 1983). Changes in climate over

a long period of time would probably

lead either to partial preservation of the

bedforms below

soil and alluvial

cover, or almost complete deflation.

Isolated sand lenses may be the only

remnants of the large star draa.

In both the transverse and longitu-

dinal models, the thickest units are in

the centre of the erg, where the largest

bedforms occur. Erg migration may lead

to systematic changes in the nature and

thickness of eolian strata in specific

areas (Porter, 1986). In an ideal desert,

the thick compound superimposed

cross strata would characterize the inte-

rior. Toward the margins, there would

be an increasing amount of thicker inter-

dune deposits with smaller, simpler

cross strata and possibly interbeds of

fluvial deposits. The areal relationships

and paleo-winds of such sequences are

shown on Figure 23.

Figure

(top) Synthetic transverse

sections

and B, related to the nearest in-

ferred erg margins and resultant wind

direction.

Figure

(right) Three dimensional

sketches of facies based on

foreset orien-

tations and shapes,

Bridgnorth Sandstone,

Permian, Britain. Modified from Karpeta

990).

8. EOLIAN SYSTEMS

Application of models

to ancient rocks

Similar interpretations have been devel-

oped for ancient ergs (Karpeta, 1990;

Kocurek, 1981 a; Ross, 1983). Karpeta

(1 990) distinguished six facies in

the Permian Bridgnorth Sandstone in

Britain, based on the nature of the strat-

ification and orientation of the

foresets

(Fig.

24),

as in the stratification and

bounding surface models discussed

above. He then grouped different facies

into three facies associations, which

were interpreted as various dune-inter-

dune systems (Fig.

25).

Kocurek

(1981a) made a particularly detailed re-

construction of the large Jurassic

Entrada erg in the western U.S.A.

Using stratification types, bounding sur-

faces,

foreset dip dispersion, and inter-

Transverse

Long term

wind

Seasonal

wind

FAClES ASSOCIATION

BARCHANOID DRAA

FAClES ASSOCIATION

DOME DUNE

Figure

Facies associations in the Bridgnorth Sandstone, Permian, Britain. Modified

from Karpeta

(1990).

Crabbing scour pit is a scour moving sideways across the slipface.

Rippled plinth is a flat upwind surface with ripples.

dune characteristics, he estimated the

actual wavelength and sizes of the

bed-

forms and their distribution in the desert

(Figs. 26, 27). This study remains the

best published analysis of an ancient

erg. Kocurek

(1981a) also estimated

the wavelengths of the individual draa

from the intersection of first-order sur-

faces with supposed synchronous

horizons (supersurfaces, see below).

These horizons represent major changes

in the environment within an erg.

SUPERSURFACES

Kocurek's (1 981 a) supposed syn-

chronous horizons that cut all earlier

surfaces are now known as

supersur-

faces.

They may consist only of a thin

coarse lag between identical eolian

sands, and can therefore be very diffi-

cult to detect. These horizons separate

stratigraphic packages and record major

interruptions in basin history (Galloway,

1989). They are

bounding discontinu-

ities

(Chapter I), and can be used to

place eolian deposits within the context

of allostratigraphy and sequence stratig-

raphy. Although often considered iso-

chronous on a geological time scale, the

supersurfaces need not be.

Origin of supersurfaces

The origin of eolian supersurfaces has

been considered most thoughtfully by

Kocurek

(1988), who placed the sur-

faces into three broad categories. The

categories are not mutually exclusive;

for example, sea level changes may

cause changes in climate.

Climatic change

Supersurfaces can form during the re-

gional termination of an erg because

of climatic change. Quaternary exam-

ples are common. In the Sahel, south-

ern Sahara, vegetation-covered dunes

are now being reactivated due to over-

grazing (Talbot, 1985).

similar stra-

tigraphic break occurs in the rolling

vegetation-covered Nebraska sand

hills, which were once a large early

Quaternary erg (Warren, 1976). In

these examples, the form of the inac-

tive erg is preserved beneath covering

vegetation. The supersurface is essen-

tially the complex rolling land surface

that is forming as erosion smoothes the

hills and causes deposition in the de-

pressions. An unusual surface of this

type surmounts the Permian Yellow

Sands of northeastern England. Here,

linear draa were rapidly buried by the

flooding of the Zechstein Sea (Glennie

and Buller, 1983; Steele, 1983). This

flooding was so rapid that rounded pro-

files of individual draa up to

m high

are preserved below the overlying

transgressive mudstones (the Marl

Slate). However, most ancient ergs

have undergone much greater erosion

before being preserved (Langford and

Chan, 1989).

Changes of relative sea level,

or tectonic setting

The Gran Desierto (Fig. 28; 9, in back-

ground) shows deflating

bedforms

around its western and southern

margins due to a combination of cli-

matic, sea level and tectonic causes

(Blount and Lancaster, 1990). This de-

flation results in supersurfaces of local

or desert-wide extent. Many examples

were noted by Blakey and Middleton

(1983) in the Mesozoic of the western

U.S.A. (Fig. 28). Supersurfaces of this

type are especially common in coastal

deserts where sea level changes di-

rectly affect the desert (Chan and

Kocurek, 1988). However, sea level

changes can also affect groundwater

levels farther inland, which in turn

can cause chemical cementation and

"fixing" of interdune and upwind sources

(see Fig. 13). The dunes can then be

entirely deflated, leaving a planar

scoured or Stokes surface (Fryberger

al., 1988; Loope, 1985). Sand sheets

may subsequently form on these sur-

faces (Kocurek and Nielson, 1986).

Migration of sand seas

The lower and upper bounding surfaces

of an eolian deposit may be formed by

the migration of an entire sand sea

through time (Porter, 1986). Within an

extensive inland basin, changes in

climate, sea level, tectonics or source

could trigger the migration of Wilson's

(1971) sand nodes. Migration of an erg

within a basin could therefore lead to

multiple, closely spaced, intersecting

supersurfaces. However, there are no

published examples of this.

DYNAMIC INTERPRETATION

Dynamic interpretations of both modern

and ancient sand seas are controver-

sial, because there is no consensus

about what causes the form, shape

and distribution of

bedforms in modern

deserts. Experimentation is difficult for

bedforms larger than ripples. The larger

the bedforms, the greater the lag

effects, and the harder it is to determine

whether they are adjusted to the cur-

rent wind regime. The development of

theoretical models relating

bedform de-

velopment to atmospheric structure,

sand transport patterns and

bedforml

airflow interactions, may therefore be

an important direction for future re-

search.

ACKNOWLEDGEMENTS

thank the Natural Sciences and

Engineering Research Council of Can-

ada for their generous and continuing

support of my research. Roger Walker

criticized and improved the manuscript.

REFERENCES

Basic sources of information

Allen, J.R.L., 1982, Sedimentary structures;

their character and physical basis:

Amsterdam, Elsevier, v.

1, 594 p. and

v. 2, 644 p.

The most comprehensive text ever

written on sedimentary structures.

Essential reading for all sedimentologists.

Bagnold, R.A., 1941, The physics of blown

sand and desert dunes: London, Meth-

uen, 265 p.

Still an essential, marvellous, thought-

provoking study of recent eolian bed-

forms.

Brookfield, M.E. and Ahlbrandt, T.S., eds.,

1983, Eolian sediments and processes:

Amsterdam, Elsevier, 660 p.

Cooke, R.V. and Warren, A., 1973, Geo-

morphology in deserts: London,

Batsford, 394 p.

Kocurek, G., ed., 1988, Late Paleozoic and

Mesozoic eolian deposits of the western

interior of the United States: Sedimen-

tary Geology, v. 56, p. 1-413.

Many good papers on the best studied,

large extensive ancient sand seas.

Kocurek, G., 1991, Interpretation of ancient

eolian sand dunes: Annual Review of

Earth and Planetary Sciences, v. 19,

p. 43-75.

Koster, E.A., 1988, Ancient and modern

cold-climate aeolian sand deposition; a

review: Journal of Quaternary Science,

v. 3, p. 69-83.

Do not forget that large sand dunes

occur next to glaciers and in Antarctica.

McKee, E.D., ed.,

1979,

A study of global

sand seas: United States Geological

Survey, Professional Paper 1052,429 p.

lot of good stuff.

Rubin, D.M., 1987, Cross-bedding, bed-

forms and paleocurrents: Society of Eco-

nomic Paleontologists and Mineralogists,

Concepts in Sedimentology and Paleon-

tology, v. 1, 187 p.

An essential guide to the relationships of

bedforms, bedding and bounding sur-

faces.

Wilson,

I.G., 1973, Ergs: Sedimentary Geo-

logy,

v. 10, p. 77-106.

The impetus for modern studies on

sand seas.

Figure

lnterpretation of the overall structure, and dimensions of bedforms and interdune

areas, in the central erg facies of the Entrada Sandstone. Draa slipfaces are up to 110 m

high, and individual barchanoid dunes have slipfaces up to

m high. Scales in

Redrawn

from Kocurek

(1981a).

---

EOLIAN

SYSTEMS

155

JURASSIC

20°N

Central erg facies

large, compound crescentic dunes

and broad interdune areas.

Coastal erg facies

small, simple crescentic dunes,

frequently f lmded interdune areas.

Sheet sand facies

rippled sands. small irregular dunes.

....

Inland erg margin facies

smal

simple dunes. sheer sands.

wadis.

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