Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks
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OFFSHORE SANDS
495
(C)
Figure O7 Scenarios lor interriiil structure ot' large ( >
3
ml sand dunes (sand waves), based on seismic anci (.ore data (modified from Berne,
1991). The circles represent Hippinji angles and orientations of mej^aripple (small dunel cross-beds, for a crest of the lar^e dune with an
arbitrary E-W direction. lAl Asymmetrical dune with cosets several meler-thjck. The master beddinj; results from erosion surface formed
during
slorms.
Tbe cross-beds within each coset may have
cin
an^^leof dip as hif"h as 34 . Superimposed bedforms create an erosion surface M the
t()[} of the sand wave, and have dip angles in the same range as that ot the large sand wave. Medium-scale cross-beds at the bottom of the sand
wave have dip angles .ilmost perfiendicular to that of the sand wave, due to the secondary circulation along the lee-side, (B) Symmetrical
dune where llood and ebb net sediment tr<ins[)ort during spring lides are in opposite directions and of simiiar magnitude. Medium-scale cross-
beds with divergent directions constitutes the core of the sand wave. Major bounding surfaces are related to migration of superimposed small
dunes (megaripplesi, as in estuarine dunes iDalrymple and Rbodes,
I
995|. Sucb dunes are enc:ountered along the great axis of several sand
ridges, or in zones of beriload parting. (C) Reversing dunes. This structure corres[)onds lo cases where seasonal changes create inversion of net
bedload transport. Large-scale cross beds correspond to periods when asymmetry of the dune is well pronounced. Major bounding
surtaces form during periods of reversal of net bedload transport, when the bed form becomes symmetrical. Because no avalanche occurs
at that time, megaripple cross-beds (usually dipping parallel or slightly oblique to the great axis of the dune) are preserved. Such structures are
observed in estuaries where seasonal changes of stratilication occur, hut also on open shelves where wincJ-rlriven t urrents reverse seasonally
ae Bot
et.il.,
2001)1.
Sea, il is believed that ridges formed between the last sea-level
lowstatid and the early stage of the deglacial sea-level rise
(i.e.,
between ca. 20ka and 14ka) (Bouysse etui.. 1976; Pantin
atid Evans, 1984; Stride etal.. 1982). This interpretation is
supported by numerical modeling of the M2 tidal constituent
for a sea level lowered by 100 m, which indicates that the shelf
currents would have been about twice as strong as at present,
with an offset of 12" to 23' between the regional peak tidal
flow and the axis of the ridges (Belderson, 1986). A similar
numerical tiiodcl has shown that the orientation of tidal ellipse
for a sea-level lowered by 80 m matciies the orientation of tidal
ridges in the Hast China Sea (Oh and Lee. 1998), SotTie sand
ridges display on seismic records an internal structure
characterized by clinoforms dipping at about 5 in the same
direction as the steeper side (presutnably the direction of siind
i-idgc migratioti) (Houbolt. 1968), Others present a core of
coastal deposits suggesting that the ridges cannibalized pre-
existing deposits (Berne elal.. 1994: Laban and Sehiittenhelni,
1981).
This observation led Snedden and Dalrymple (1999) to
elassify sand ridges as a function of the amount of their
tnigration: juvenile ridges retain their initial nucleus that
represent the initial irregularity necessary for ridge develop-
ment in the Hitthnance model, whereas "fully evolved"" ridges
have tiiigrated sufftciently that they contain no trace of their
origin. Alternatively, il has been proposed that "cored" ridges
represent situations where substrate is erodible and sand
supply litrtited {Bernd etal., 1998). In such situations where the
spatial derivative of the transport rate is negative, ridges (or
any other type of bedforms) are migrating with a negative
angle of elimb and incorporate previous deposits of different
origins (Rubin, 1987). In any event, an important observation
is that such sand ridges may in fact contain a relatively large
fraction of non-sandy tTtaterial.
Most of the eores frotn tnodern satid ridges show massive
sand with an overall coarsening upward trend. In the East
China Sea, the bottomsets of the clinoforms from sand ridges
496 OFFSHORE SANDS
lying
at
about 90 tn display alternating sand
and
silt bedding
with burrowed tidal rhythmites, whereas
the
(opsets consist
of
homogeneous tine
to
medium sand (Bcrnc etal.. 2002). Cores
from
the
Middelkcrke bank
in the
North
Sea
show,
in the
upper part
of the
ridge, ripple
to
medium dune bedding, with
some herringbone cross-beds (Trentesaux
et al..
1999).
The
lower part
of the
bank consists
of
estuarine sediments
incorporated into
the
ridge.
Lowstand
and
transgressive shorefaces
and delta fronts
In areas where erosion
due to
wave
and
tidal ravinement
is
limited, shoreface deposits
and
sandy delta fronts that formed
at period
of
lower sea-level tnay
be
preserved
and
represent
another type
of
offshore sand bodies, where
the
initial
geometry
is
only partly preserved.
Forced-regressive
and
lowstand deposits
are
preserved
at
depths close
to the
position
of
shorelines during
the
Last
Glacial Maximum (i.e.. about 90tn
to I2()m
water depth).
In
the moderate energy, stortn-dotninated Western Mediterranean
Sea, they form large sand bodies,
up to
35
m
thick
and
15
km
wide, that extend parallel
to
bathymetrie contour lines along
distances more than 100km (Rabineau ctai.. 1998).
On
seistnic
profiles, they display internal clinoforms dipping seaward
at an
angle
of 2 to 5'
(I'igurc
08).
They fortned
as
prograditig
sandy deposits tnigrating seaward
in
response
to
sea-level fall,
following
a
mechanism initially described from outcrop studies
in
the
stratigraphie record (Plint, 1988).
The
clinoforms consist
of glauconitic sand with shells
and
shell debris, passing
progressively
to
silty bottomsets corresponding
to a
lower
shoreface/prodeltaic etivirontiient (Berne
et af.
1998).
The
"sand ridges'" that
lie
parallel
to the
shelf edge
at
water depths
belween
75m and 100m in the
Indian Ocean
off
Bombay
(Wagie
and
Veerayya. 1996) could have
the
same origin,
but in
this ease, higher wave energy
has
subsequently reworked
the
deposits. Deltas that formed during lowstand
of the sea
have simiiar shapes
in two
dimensions,
but
more lobate
geometries. They
are
described from examples
in the
Eastern
Mediterranean
Sea
(Aksu etal..
1987;
Skene
etitf,
1998),
the
Gulf
of
Mexieo (Suter atid Berryhill, 1985),
the
South China
Sea (Hiscoti, 2001).
Such stranded sand bodies
do not
only form during
the
falling
and low
stand stages
of
glacio-eustatic cycles. During
the last deglacial period, slowing
of
sea-level rise and/or
increase
in
seditnent supply have also favored lortnation
of
tratisgressive shoreface deposits that
are now
isolated
on the
tniddle
or
inner
shelf. For
instance, sand banks
of the
inner
shelf Trinity/Sabine incised valley system
in the
Gulf
of
Mexieo
are interpreted
as
stranded shoreface deposits that formed
during
a
phase
of
relatively slow sea-level rise,
and
where
stranded
on the
shelf during
a
rapid sea-level rise {Anderson
and Thotnas,
1991;
Rodriguez
el af.
1999).
An
altertialive
interpretation
for
shore-parallel sand shoals lying
at
about
50m water depth
in the NC
Gulf
of
Mexico
is
that they
developed
at the
position
of an
escarpment formed during
the
transgression,
but
that they consist solely
of
post-transgressivc
deposits (McBride
ciaf,
1999).
Sand sheets
and
sand patches
Sheet-like bodies
of
sand largely covered
by
sand waves (large
dunes)
are
described
in "iow
energy" tidal environments
(where current velocity near
the bed is
around
55
cm/second)
sttch
as the
Southern Bighi
of the
North
Sea
(Belderson.
1986:
Stride
ctaf.
1982). They have
no
preferential orientation
and
their internal structure
is
that
of
(tidal) dunes (large
and
mediutn seale eross beds).
In
areas dominated
by
unidirec-
tional currents, such
as the
southeasl African
shelf,
sand sheets
with very asymmetric
bed
fortns
are
observed (Flemming,
1980).
Where
the
sand cover
is
discontinuous, sand patches
resting
on a
coarse
lag of
fluvial
or
transgressive deposits
are
observed. The.se patches
may or may not
have
a
preferential
orientation, depending
on the
loeal hydrodynamic regitne.
On
the Caiifornian shelf dominated
by
swells
of
the Pacific Ocean.
they form transverse sand patches
or
very flat dunes migrating
seaward
in
response
to
large bottom stresses generated
by
eotnbined waves
and
currents (Cacchione
etaf.
1987). Similar
sand patches
are
also described
in the Bay of
Biscaye
at
depths
ranging from 30 m
to
90 m. Cores display stacked fining
upward elementary sequences I
cm to 10em
thiek, with
erosional base, typical
of
storm deposits (Cirac
elaf.
2000).
Stage
6
shoreface sands
Last lowstand shoreiace sands
Figure O8 Recent and buried shoreface sands in (ho Western Mediterranean Sea. Offshore sands in ihe Ckilf of Lions cover the outer shelf at
water depths from *)()m lo about 12t)m. Note ihe abrupt seaward transition hetween sands and mud, marked hy a step in the sea-floor
morphology. Similar sand bodies formed during other gla< iai periods Icorresijonding lo isotopic stage 8] are now buried and encased into
prodeltaic muds. Vertical scale is in milliseconds two way travel time (mstwtt).
OHFSHORE SANDS
497
(A)
100
150
200
Ui
E
250
1
km
(B)
m.f.S-
5-15m
"foresets"
fine to
medium
well-sorted
X-bedded
sand
2-5m
"bottomsets"
I
coarse lag
bioturbated
tidal
rhythmites
silt and
sand
Figure O9 Seismic (Al <incl synthetic log lUl for a sand rid^e un the outer shelf of the EasI China Sea (water depth about JiOmj. The seismic profile,
ptTfjendii ular lo thej^reat axis of the sand ridj^e, display ititernal clinoforms dipping al alwut 2-3 ' in the same direction
as
the "steep" side of the
ridge.
A buried Pleistocene sand ridge Ibsri is also observed, overlain by lower shorefate or prodeltaic silts (ts/pdi and deltaic/estuarine
heterogeneous sedimenls with numerous channel structures (d/e). The exiernal morphology of this buried has been preserved, "m" is the seismic
multiple.
Vertical senile is in milliseconds two way travel time (mstwHi. The synthesis of several tores across the ridge shows (Figure O9(B)I thai
Ihe upper parl of the clinoforms i"foresets") consists of fine to medium sands, whereas Ihe "bottomsets" display tidai rhythmites. The top of
the ridge is blanketed by a carpet t)f sediment in equilibrium with modern hydrodynamic regime, but the ridge is believed lo have form during the
last deglacial sea-level rise.
Preservation
of
Pleistocene offshore sands
There
are
contiacjiclory explanation aboin
the
origin
of
many
SLttid bodies encased
in
mudslones.
for
instance
the
Shatinon
Sandstone
in
Wyoming, that
are
interpreted cither
as
fossil
sand ridges (Tillman.
1999) or as
regressive shoreface deposits
(Bergman
and
Walker. 1999). Examples from tnodern
(Pleistocene) cotitinental shelves demonstrate that both types
of sand body
may be
preserved. This discovery
is
rather recent,
beeause tnost
of the
early stitdies
oti
continental shelves
focused
on NW
Europe
and
North American tnargins where
low subsidence, limited seditnent supply
and
stt"ong bottom
currents favor reworking
of
deposits during each glacial cycle.
Iti contrast, seismic surveys
and
geotechnical boreholes
on
(deltaic) margins with high sedimentation rate
and
relatively
high subsidence rates (around 25ntn/miliion years
at the
shelf
edge) demotistrate
the
high potential
of
preservation
of
shelf
sand bodies
in
such areas, where they
are
generally encased
in
prodeltaic/lowcr shoreface regressive silty deposits. Kxamples
of Fleistoeene buried sand bodies include regressive, lowstand
and transgressive shoref^aces (Rabineau
etaf.
1998). lowstand
(shelf edge) deltas (Hiscott. 2001: Sketie
vtaf.
1998; Suter
and
Berryhill.
19S5;
Sydow atid Roberts,
1994:
Winn
etaf. 1998)
and transgressive sand ridges (Berne
eutf.
2002: Yang, 1989).
These sand bodies
are
typically
lOm (o .'^Om
thick, with
a
lateral extent
of
several
km to
tnore than 100 ktn.
and
their
initial shape
is
sotiietimes preserved (Figure
09).
Most authors
agree
in
attributing
a
major role
to 4th
order glacio-eustatic
cycles
(i.e..
about lOOkyr)
in the
construction
of
these
sand bodies, even though shorter titne-periods
are the
498
OrrSHORE SANDS
domiiiiinl lactors controlling
Iht:
iip-hiiilding
of
tniiisgressive
parnsequcnces.
Serae Berne
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lo a
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evolutionary model.
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68
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Cross-references
Coasial Sedimentary Eacies
Cross-Slratilication
(jiaucony
and
Verdine
Milankovitch C~ycles
Ripple. Ripple Mark, Ripple Structure
Sediment Transport
hy
Tides
Sedimeni Transport
by
Waves
Suiface Forms
Tides
and
Tidal Rhvthmites
OIL SANDS
Oil sands constitute an immense petroleum resource on a
worldwide scale (Figure OlO) and Canada, at 1.700 billion
barrels (AFUB, 1946), dominates the current estimates
(Me\er. 1995, 1996), The petroleum resource in ihe Orinoco
011 Belt (OOB) of Venezuela is thought lo bo similar in size at
1,200 billion barrels (Fiorillo, 1987) and dominates the world
heavy oil resources (Meyer, 1998),
Oil sands (also known as tar sands) are delined as petroleum
deposits where the viscosity of the oil (bilumcn) is greater than
10,000 centipoises (cP) at reservoir conditions (Meyer, 1995).
The general definition applied to oil sands is a petroleum
deposit where oil viscosity is so high ihat there is no
commercial inflow to a well at reservoir conditions (AEUB,
1996;
Butler, 1997).
Bitumen is deficient in the lighter hydrocarbon molecules
(Meyer. 1998) and is generally in the 6 API (1,029kg/m') to
12 API (986kg/m ) range. However, oil viscosity is greatly
affected by temperature, and heavier bitumens e.\perience
large reductions in viscosity with an increase in temperature.
Bitumen in the CIcarwatcr Deposit. Alberta. Canada Is
lighter (10 12 API) than the oil in many o\' the heavy oil
deposits in the OOB (8-14 API: Starr etal., 1981). However.
higher reservoir temperatures (40 70"^C) in the OOB give the oil
mobility.
Al!
of the significant oil sands deposits in Canada oeeur in
the northeastern part of Alberta (F-igure Oil). The bitumen is
thought to be a eonvcntional oil that was degraded, mainly
through biodegradaiion, as it migrated iipdip into the
shallower portions of the basin (Creaney and Allan, 1992),
The Wabiskaw-McMurray and Grand Rapids deposits in the
Figure
O10
World
oil
sands resources, Origin.il
Oil In
Pl.ite (OOIP).
OIL SANDS
USA Border
Figure Oil Oil sands areds and OOIP estimates for deposits in
Aiberla, Canada,
Athabasca Oil Sands Aren have no discernible structural or
stratigraphic closure; apparently the oil was biodcjzradcd into
immobility and trapped itself (Wightman
ctttf.
1995).
Dala are sparse for many other oil sands deposits but most
occur in the shallow (less than I.OOOm), updip or marginal
portions of basins (Phizackerley and Scott, 1978: Roadifcr.
1987). These deposits also represent conventional oils that
have been biodegraded into bitumen through contact with
meteoric water (Meyer. 1996).
Most of Alberta's oil sands are comprised of consolidated
(but mostly uncementcd) sand, bitumen and water. The
exceptions occur in Mississippian and Devonian carbonates
but these are classified as oil sands because of the bitumen
(AEUB. 1996). With the sand and carbonate, water eoats the
mineral matter and bitumen tills the remainder of the pore
space (Figure OI2).
Bitumen distribution and oil sands quality (porosity,
permeability and oil saturation) are directly controlled by
depositional facies in most of the clastic oil sands and heavy oil
deposits around the world (Phizackerley and Scott. 1978). This
is the result of shallow depths of burial (limited diagenesis) in
basin margin settings. Clean, well sorted sands form the best
oil sands whereas sands with abundant matrix material,
mudstones and shales eontain little or no oil, Depositional
environments are generally continental to marine shoreline
(Pliizackerley and Scott. 1978),
Figure O12 Schematic diagram of
thin
sections of water wet oil sands,
(A) Clean quartz sand with low water saturation. (B) Arkose or
litharenite with authigenic clay minerals and higher water saturation:
porosities (aii be higher than in (A).
In Alberta, the Lower Cretaceous Mannvillc Group
contains most of the oil sands resource and within it. the
Wabiskaw-McMurray Deposit is the largest with 902 billion
barrels (AEUB. 1996), The rich oil sands generally occur in
channel deposits, so lateral continuity is problematic. Trough
cross bedded sand, with little or no interbedded mudstone. in
the lower parts of channel deposits represents the best oil sands
(33 38 pereent porosity. SO 95 percent pore oil saturation and
5 lOD of permeability, up to 20D in gravel; Wightnian
L't al., 1995). Ihe overlying lateral accretion beds are com-
prised of interbedded sands and mudstones. The mudstones
reduce ore grade for surface mining operations and the lower
permeabilities (generally 10-300 niD) have a significant,
adverse effect on i/i.^itti operations (Strobl eial.. 1997).
Tlie sedimentological characteristics of a deposit influence
the type of itt sitti recovery process that is most applicable.
Fining upward channel deposits are particularly suited to
Steam Assisted Gravity Drainage (SAGD) as the best oil sands
are located in the lower part of the reservoir (F'igure 013),
Cyclic Steam Stimulation (CSS) is being utilized in the
coarsening upward Clearwater Deposit of Alberta (Butler.
1997); pressure parting induces vertical permeability, especially
in the lower portions of the reservoir (Kigure 014).
Oil sands have several types of associated water sands
(Figure O15) which occur within and adjacent to the
bitumen (Wightman etal.. 1995; Barson etal.. 2001), Water
sands reduce overall reservoir quality; in particular, top water
can be very detrimental to iti sit ti operations (Barson etal.,
2001).
Oil sands currently (September. 2001) contribute about
650.000 barrels of oil/day in Canada, with 340.000 barrels of
synthetic crude from mining and 310.000 barrels of itt situ
bitumen.
Production is increasing, with predietions of up to
1.650.000 barrels/day by 2015 (NEB. 2000). Worldwide
production from oil sands is also expected to increase as
conventional oil supplies decline in the future.
Darvl M. Wichtman
OIL
(A)
I
MUDSTONT
I SCHEMATIC
-^1 CROSS SECTION
OLSANDS .•••:-:•:
<<^.^.
PRODUCTION WELL INJECTION WELL
(B)
HtAI tU OIL AND
CONDENSATE
Figure O13 Scht'matit diagram of Steam Assisted Graviiy IJraJnagc (SACiDi in 2 slacked i hannel deposits. lA} Longiludin.il profile of
horizontal well pair. |B| Cross secrlion ol (A) showing devolopmeni of steam chamber retarded by laterai accretion niudstones, Sandy to
muddy ch<innel "iile cycle" from Wightman and Pemberton (19')7I.
INJECTION / PRODUCTION WELL
I
Figure O14 Schematic di<)gr<ini ol Cydic Steam SliniuLilion (CSSi, during t^lcom injection, in a coarsening upv\ar(l cleposil.
502 OOl ITF AND COATED (iRAINS
(A)
Meteoric Water
Hydraulic
Head
Figure O15 Schematic diagram
of
types
of
water sands.
(Al In
paleotopographic lows
(I i.
behind
low
permeability strala
(2)
and
in
"leaked
off"
gas zones
(.51.
(B)
Regional water sands
(4)
and flushed
zones (5) with
low or
remnant oil saturations; flushing greatest
in
high
permeability sands/gravels.
of Heavy Crude Oils
and
Ttir
.Sands. First LINITAR Inlernalional
ronference, McGniu-Hill.
pp. 168 173.
Strohl,
R,S,.
Miivviiis.
W.K..
Wightman.
D.M..
Cotterill. D.K...
and
Yuan.
L., 1997.
Application
of
outcrop analogues
and
detailed
reservoir characterization
to the
AOSTRA I'lidergroitnd Test
Fticilily. McMurray Formation. Northeastern Alberta.
In
Pemberton.
S.G., and
James.
D,P,
(eds,). Petroleum
Geology
of the
CretacciHis
Mainnille
Group,
ll'e.stern
Canada. Canadian Society
of
Petroletim Geologists Memoir. 18.
pp. 375 }9\.
Wighlman.
D.M.,
^Attalla.
M,N,.
Wynne,
D.A..
Strobl.
R.S..
Berhane.
H.,
Cotterill.
D.K.. and
Berezniuk.
T.. 1995.
Re.wuree
Characterization
of the
McMio-ray- liuhi.sknw Deposit
in the
Athahiisea
Oil
Sands Area:
A
Synthesis. ,Mberta
Oil
Sands
Technoloiiy
and
Research Authority Teehnieal Publication
Series.
10?
Wiglitman. D.M,.
and
Pemberton.
S.G..
1997.
Tbe
Lower Cretaceous
(Aptian) McMiirniy Formation:
an
overview
of ihe
Kort
MeMurray area, nortlieastern Alberta-
In
Pemberton,
S.G..
iind
James.
D.P.
(eds.). Petroleum Geology
of the
Cretaceous
Mannville
Group.
IVestern
Canada. Canadian Society
of
Petroleum
Geologists Memoir. 18.
pp. 312 344.
Cross-references
Coiistal Sedimentary Facies
Deltas
imd
Estuaries
Rivers
and
Alluvial Fans
Sands
and
Sandstones
Bibliography
AHUB (Alberta Energy
and
Utilities Board).
1996,
Crude Bitumen
Re.serves
Atlas. St;itistieal Series. 96-38.
Barson.
D..
Bachu.
S,. and
Esslinger,
P.,
2001. Flow syslcms
in ihe
Mannville Group
in the
east-central Aihabast-a area
and
implica-
tions
for
steam-assisted graviiy drainage (SAGD) operations
for //;
situ bitumen production. Buttetin
of
Canadian Petroleum Geotogy,
49:
376-392.
Butler.
R.M.. 1997.
Ttiermid Reeovery
of
Oit
and
Bitiaiien. Calgary:
CjravDrain
Inc.
Creaney.
S.. and
Allan.
J., 1992.
Petroleum systems
in ihc
Foreland
Basin
of
Western Canada,
In
Macqueen.
R,W., and
Lcckie,
D.A.
(eds,).
Foretand Basins
and
Fotd Betts. American Association
of
Petroleum Geologists Memoir.
55. pp, 279 308.
Fiorillo,
G..
1987. Explor;ition
and
evaluation
of
the Orinoco Oil Belt,
In Meyer.
R.F.
(ed,). E.\ploration for Heavy Crude Oil
and
Natural
Sitianen. American Association
of
Peiroleum Geoloiiists Studies
in Geology.
25. pp. 103 114,
Meyer.
R,.
1995. Bitumen.
In
Eneyelopedia of Energy Tirhnology and
ihe
Environment.
4
Volume
Set, New
York: John Wiley
&
Sons,
pp.
433-454.
Meyer. R.F.. 1996. Tars.
lar
sands
and
extra heavy oils.
In
Diiseh.
E.J,
(ed.).
Encvctopediaof Earth Sciences. Simon
&
Schuster Maemillan.
pp.
1072-1075.
Meyer.
R,F,,
1998, World heavy crude
oil
resources.
In
World
Petro-
leum
Congress
Proceedings.
15th,
Beijing,
pp.
459-471,
NEB (National Energy Boiu'd). 2000. Canada's Oil Sands: A Supply
and
Market Ontlook to 2015.
Phizackerley. P.H.,
and
Scott,
L.O..
1978. Major tar-sand deposits
of
the world.
In
Chilingariaii.
G.V.. and Yen. T.F,
(eds.). Bitumens.
Asphalts
and Tar
.Sands. Developments
in
Petroleum Science.
Volume
7.
Elsevier.
pp. 57 92.
Roadifer.
R.E.,
1987. Size distributions
of the
world's largest known
oil
and tar
accumulations.
In
Meyer.
R.F.
(ed.). Exploration tor
Heavy Crude Oil
and
Natural Bitumen. American Association
of
Petroleum Geologists Studies
in
(ieoiogy,
25, pp. 3 24.
Starr.
J,,
Prats. J.M..
and
Messulam.
S.A..
1981. Chemical properties
and reservoir characteristics
of
bitumen
and
heavy
oil
from Canada
and Venezuela.
In
Mever, R.F.,
and
Steele.
CT.
(eds.). The Future
OOLITE AND COATED GRAINS
Oolite is a sedimentary rock, most often limestone, composed
of smoothly laminated, spherical to ellipsoidal coated grains
thai arc cemented together (Figure O16). The grain compo-
nents of oolite are called ooids (or ooids or ooliths), terms
derived from the Greek word for fish roe ("oon"), whieh ooids
resemble. The terms "oolite" and "ooid" are considered
distinct with oolite pertaining to rock and ooid referring to an
individual grain (Teichert. 1970). Not all workers adhere lo
this technical distinction. In geologic descriptions, the term
"oolitic"" refers either to ooid-rich sediment or to ooid-bearing
rocks, Tlie term "pisoid"". by convention, refers to ihose ooids
larger than 2 mm. Historically, the term oolite was first nsed in
1727 by F.E. Brticktnan (Cayeux. 1935. p.2ll). -Oolilic
grains"" were described by Lyell (1854, p.l2) and Sorby
(1879,
p.74), The term ooid was first introduced by Kalkowsky
(1908,
p.72).
Ooids have rounded grain shapes and generally spherical to
ellipsoidal grain forms (Figure O17), Internally, ooids consist
of a tutdeit.s surrounded by an evenly laminated cortex which
increases in sphericity with distance from the nucleus. The
nucleus commonly eonsists of a shell or fossil fragment, a
peloid or a detrital sand grain whereas the cortex consists of
concentric layers of caleium earbonate or other minerals,
Ooids are distinguished from other coated grains by
descriptive and/or genetic criteria depending upon classifica-
tion scheme. Most workers readily distinguish ooids from
"oneoids"" by the even lamination of the ooid cortex and the
general absence of biogenic features such as algal filaments. Of
these, evenness ol" cortex lamination is probably the most
OOLITE AND COATED GRAINS
Figure O16 Mis^^issippian oolite, Slo. Genevieve Limestone, Kenlucky.
Nolc the liiminaled cortices surroundinj; the nuclei ol some of the
ooids (scv tirrowheaHs). Sc:ale =
1
mm.
Figure O17 Modern ooids, loultcrs Cay, Bahamas. Note the spherical
lo ellipsoid j^rain forms and the smooth, polished surfaces of the ouids.
Scale =
I
mm.
reliable criterion lo distinguish ooids from oneoids. as biogenie
features such as mucus films are evident in many ooids (e.g..
Newell I't al., 1960). Although the arbitrary size distinction
separating ooids from pisoids is clear, the term pisoid is
additionally used by many workers to refer to nonmarine
oolitic grains (e.g.. F-1iigel. 19S2). Because of this, some recent
workers (Sumner and Grotzinger. 1993) have begun to use the
term "giant ooid" when deseribing large (>2mm) oolitic
grains that appear lo have formed in marine environments.
Comprehensive reviews and information about ooids and
other coated grains can be found in the works of Bathurst
(1975).
Simone (1981). Kliigcl (1982). Peryt (1983) and Tucker
and Wrighl (1990). Reviews of ooids composed of iron-bearing
minerals ean be found in the work of Young (1989). Reviewed
here are ealeareous ooids and oolitic roeks as these form the
majority of modern and aneient ooid-bearing deposits.
Modern environments of formation
Ooids fortn in a wide variety of modern depositional settings.
Most form in shallow, tropical marine settings of moderate to
high wave activity, at depths <
10
m {often <2m). where the
water is supersaturated with respect to calcium carbonate.
Well-studied areas of modern marine ooid formation ineSitde
the Great Bahama Bank and the Persian
Gulf.
Oolitic sands in
these settings typically occur along the edges of plalfortns. in
lagoons, in tidal deltas and mid-ehannel bars, and in beaches
and tidal Hats. Most marine ooids oeciir in or near Ihe
environment in which they are forming, although pelagic and
eolian occurrences clearly indieate significant transportation.
Ooids and pisoids also form in hypersaline lakes and lagoons,
such as the Great Salt Lake. Utah and Baffin Bay. Texas as
well as in brackish waier lakes such as Pyramid Lake, Nevada
(Sandberg. 1975; Land eial.. 1979: Popp and Wilkinson.
1983),
Other environments where oolitic grains form include
freshwater lakes, calcareous soils (calcrete). rivers, caves, hot
springs and human-constructed features such as drainage pipes
and water treatment plants.
Mineralogy and microfabric of modern ooids
Calcareous ooids are composed of aragonite, high Mg-ealcile.
low Mg-ealeile or. in the case of bimineralic ooids. sotne
cotnbination of aragonite and calcite. Modern marine ooids
have pritnarily an aragonite cortex: those with a high
magnesian calcite cortex occur but are tnuch less common.
The eortex of ooids from nonmarine settings has a variable
eotnposition depending on the Mg/Ca ratio of the precipitating
water and the salinity. Ooids from hypersaline settings with a
high Mg/Ca ratio are dotninantly aragonite. and to a lesser
degree high Mg-ealeite. or they are bitninL'ralie with a cortex
composed of a cotnbination of aragonite and high Mg-calcite
(e.g.. Sandberg, 1975; Land etal.. 1979). Ooids from Pyramid
Lake. Nevada, a brackish water ensialic lake with a fluctuating
Mg/Ca ratio, are bimineralie with cortices composed of
aragonite and low Mg-calcite (Popp and Wilkinson. 1983).
Ooids from more dilute lakes as well as streams, caves and
calcareous soils are comtnonly cotnposed of low Mg-calcite.
The ooid cortex exhibits a range of crystal fabrics depending
upon original mineralogy, the energy level of the depositional
setting, organie iniluences and subsequent diagenesis. Most
modern aragonitic ooids from the Bahamas have a tatigetitial
tuiirafabrie consisting of small (1 3i.mi) aragonite rods and
submieron nannograins oriented parallel to the eortex laminae.
In thin section under cross-polarized light, modern tangential
ooids commonly display a distinctive pseudouniaxial cross
(Figure O18). The tangential microfabric appears limited lo
those ooids composed of aragonite and is considered by
most workers to rellect relatively high energy eonditions
504
OOLITE AND COATED GRAINS
Figure O18 Thin section view of modi'in laiij^enlial ooids under cross-
pol.irized light, Joulters Cay, Bahtimas. Note ihe [jseudouniaxial
crosses exhibited by several ot the ooids. Stale =
0,1
mm.
eharaeterized by numerous grain collisions. Sotne workers
assert that tangential microfabrics are influenced by biological
processes
(e.g.,
Pabricius, 1977).
Modern ooids may also have a
radiul tnicrofabric
consisting
of fibrous or bladed erystals of aragonite oriented normal to
the ooid nucleus. Aragonitic ooids with radial microfabrics are
well described from the Persian Gulf, the CJreat Barrier Reef
and frotn the Great Salt Lake (Loreau and Purser, 1973;
Davies and Martin. 1976; Satidberg, 1975). High Mg-calcite
ooids with radial microfabrics are nncotnmon but have been
described frotn the Great Barrier Reef (Marshall and Davies.
1975). Isolated radial laminae of low Mg-calcite occtir in some
ooids from Pyramid Lake (Popp and Wilkinson. 1983:
Wilkinson
etal..
1980). Although exceptions are known, radial
ooids are more eomtnon to low-energy settings, settings
characterized by fewer grain collisions.
Ooids lacking an orderly tangential or radial microfabric are
described as having a rattihnit
tiucrofahric
and are commonly
referred to as micritic ooids. Modern exatnplcs generally
consist of
a
random arrangetnent of aragonite rods or crystals.
an arrangement that is tnore often a product of micritization
rather than an original depositional fabric. Mieritie ooids have
been deseribed from a variety of low and high-energy settings.
Numerous additional ooid microfabrics have been deseribed
from some tnodern and ancient oolitic deposits
(e.g..
eerebroid
ooids:
half moon ooids). These speeial types are siimtnarized in
Richter (1983).
Origin of ooids
Ooids may fortn by a variety of physical, chemical and
biological processes; however the exael niechanistn(s) by which
ooids form is still unresolved. The strictly physical origin
proposed by Sorby (1879) involving vertical accretion (snow-
balling) of ealeareous grains is often discounted, beeause grain
interactions during ooid growth tend to polish the ooid surfaee
rather than contribute to physical accutnulation of calcium
carbonate. The only site where substantive physical aeeumula-
tion occurs is in borings within the ooid cortex (Gaffcy, 1983).
Formation by chemical (inorganic) processes is favored by
many workers, particularly for radial ooids (Tucker and
Wright, 1990). In the marine environment, inorganic forma-
tion of ooids requires supersaturated water, abundant nuelei.
normal to elevated salinity, and warm temperatures ( >
20^
C).
Signilicant agitation may or may not be required for inorganic
carbonate precipitation. Laboratory experiments mimicking
sueh conditions have produced tangential ooids simitar to
modern Bahamian ooids (Davies
etal..
1978).
Biological processes leading to the formation of ooids have
been suspected since the lale 19th century (Rothpletz, 1892).
However, the degree to which organic compounds and
organisms and are involved in ooid fortnation is still a matter
of debate. Several experimental studies have shown that the
presenee of organic eotnpoimds, partieularly humic acids, can
facilitate the formation of high Mg-calcite radial ooids, at least
under laboratory conditions (Sucss and Fiillerer. 1972: Davies
el ul. 1978: Ferguson ei al., 1978). Interestingly, in those
solutions laeking any humic compounds, only aragonite
needles or aragoniie ooids with tangential fabries formed.
The role of algae and eyanobacteria in ihe formation of ooids
remains largely unresolved, despite stgntfieant study by some
workers
(e.g.,
Fabricius, 1977). While algae and eyanobacteria
certainly have a role in the formation of oneoids. their
importance in the formation of ooids is less clear, especially
those fortning in caves or other settings with no light. Recent
investigations of freshwater ooids frotn Lake Geneva (Davaud
and Girardclos. 2001) demonstrate that nucleation of calcitic
oolitic coatings can occur in close association with mucus filtns
produced by the activities of diatoms and eyanobacteria.
Those films appear to act as a catalyst or at least a substrate
for earbotiate precipitation and cortex growth. Sitnilar
associations between tnucus films and erystal growth were
described by Folk and Lynch (2001). They found tninutc
crystals of aragonite with peculiar "baton" and composite
needle tnorphologies imbedded in the mucus-rich laminae of
sotne Bahatnian aragonitic ooids. The fortnation of these
crystals is attributed to the activities of minute "nannobac-
teria"
in a manner sitnilar to carbonate precipitation indticed
by larger bacteria
(e.g..
Chafetz and Buczynski. 1992). If
nannobacteria truly exist—a matter still under debate—their
presence and their micro-environtnental influence strongly
suggest that biological processes greatly inflttence the fortna-
tion of ooids. Further work is needed to firmly establish the
role of organie cotnpounds and biological processes on the
fortnation of ooids. This work will likely be the most active
area of ooid researeh in the coming years.
Ancient ooids and oolite deposits
Oolite has a long geologic history extending at least to the late
Archean.
The environtnent of deposition of ancient oolitic
deposits has been studied extensively, as has the texture and
mineralogy of
the
ooids they contain. Ancient ooids commonly
have radial textures, with or without elear eoncentric lamina-
tions;
other ancient ooids consist of large neomorphic calcite
crystals or are filled with clear calcite cement. Although radial-
concentric ooids were previously thought to represent a
diagenetic replacement product of aragonitie ooids, researeh