Boggs S. Principles of Sedimentology and Stratigraphy
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316
Chapter 9 I Marginal-Marine Environments
A Trans
g
ression by shoreface retreat
Flgure 9.26
Advancing
shoreface/shelf sand
)Rising
sea level
Barrier-island facies generated by transgres
sion and regression. A. Transgression owing
to shoreface retreat during gradual sea-level
rise. B. Effects of rapid sea-level rise, produc
ing in-place drown ing (SL = sea level).
B Tra ns
g
ression by in-place drownin
g
New barrier
)
Rising
C. Facies formed as a result of progradation
under conditions of high sediment supply
relative to sea-level change. [A and B after
Rampino, M. R., and ). E. Sanders, 1980,
Holocene transgression in south-central
Long Island, New York: )our. Sed. Petrology,
v. 50, Fig. 8, p. 1 075, reproduced by per
mission of SEPM, Tu lsa, Okla.; Elliott, T.,
1986, Siliciclastic shorelines, in Reading,
H. G. (ed.), Sedimentary environments and
facies: Blackwell Scientific Publications, Fig.
7.33, p. 180. Part C after Galloway,
W. E., and D. K. Hobday, 1983, Terrigenous
clastic depositional systems: Springer
Ve rlag, New York, Fig. 6.1 0, p. 126.]
sea level
C Re
g
ression
plains, producing dominantly sandy facies in which beach (backshore and fore
shore) deposits overlie foreshore deposits. Galveston Island, Texas (Fig. 9.26C), is
an example of such a progradaonal deposit. A generalized succession of facies
deposited in an ancient back-barrier environment is illustrated in Figure 9.27. The
cyclic natu of this deposit suggests recurring episodes of transgression and re
gression. Such a succession might be confused with some deltaic successions, al
though the absence of delta-front muddy sands and prodelta clays and mud
turbidites should help to distinguish it from deltaic deposits.
Many ancient examples of beach and barrier deposits are known. For exam
ple, the Cretaceous Gallup Sandstone of northweste New Mexico is reported to
be
a progradational (regressive) succession of barrier deposits (McCubbin, 1982),
whereas,
the Cretaceous Cliff House Sandstone in the San Juan Basin of north
western
New Mexico has been interpreted as a transgressive barrier complex
(Donselaar, 1989; McCubbin, 1982). Other siliciclastic sedimentary successions
identified as beach and barrier-island complexes are present in rocks of widely
differing ages in North America. Such successions have been reported from sever
al Penylvanian formations the Appalachian Basin of Kentucky, Virginia, West
Virginia, and Te nnessee; the Lower Cretaceous Muddy Sandstone of Wyoming
and Montana; the Eocene Wilcox Group of east Te xas; and the Quaterna of Cali
fornia (e.g., Davis, 1992). Several ancient carbonate deposits have also been inter
preted as beach complexes. The Lower Cretaceous Edwards Formation of west
Texas, the Lower Cretaceous Cow Creek Formation of central Texas, the Mississip
pian Newman Formation in eastern Kentucky, and the Mississippian Mission
Canyon Formation in the Williston Basin in the Montana area are examples of an
cient stratigraphic units that contain putative carbonate beach deposits.
Tidal flat
Lagoon
Lagoon
Swamp
Tidal channel
Coal, with underclay
Siltstone with quazose
sandstone flasers
Clay shale, with siderite bands;
bioturbated, fossiliferous
Coal with underclay
Sandstone, quazose, planar-bedded
Shale and siltstone, coarsening
upward, bioturbated
Clay shale, siderite bands
Limestone, bioturbated, fossiliferous
Coal with underclay
Sandstone, quartzose, fining upward,
rippled and cross-bedded
Siltstone with sandstone flasers
Flood tidal delt
a
�
Sandstone
,
bioturbate
d,
sideritic
Sandstone, quartzose, cross-bedded
Tidal flat
Lagoon
9.4 ESTUARINE SYSTEMS
Introduction
Shale and siltstone, coarsening
upward, bioturbated
Clay shale with siderite bands,
bioturbated, fossiliferous
9.4 Estuarine Systems
317
Figure 9.27
Generalized succession of facies deposited
in a back-barrier environment, Carbonifer
ous of eastern Kentucky and southern
West rgi"ia. Such successions range from
7.
to 24 m thick. [After Horne, J. C., j. C.
Ferm, F. T. Caruccio, and B. P. Baganz,
1978, Depositional models in coal explo
ration and mine planning in Appalachian
region: Am. Assoc. Petroleum Geologists
Bull., v. 62, Fig. 4, p. 2385, reprinted by
permission.]
Relatively small, partly enclosed coastal embayments are loosely called coastal
ba
y
s. Tw o broad types of coastal bays are reco
g
nid: estuaries and lagoons.
Estuaries (term derived fm the Latin word aestus, meaning tide, and from the ad
�tive aestuarium, meaning tidal) are considered in a general sense to be the lower
courses of ri\lers open to the sea (e.g., Fiure 9.28}. Dalrymple, Zaitlin, and Boyd
(1992) suggest that the concept of net landward movement of sediment derived
fm outside the estuary mouth is necessary to distinguish estuaries from deltas.
Therefore, they define an estuary as "the seaward portion of a drowned valley
system which receives sediment from both uvial and marine sources and which
Figure 9.28
Wave-dominated estuary of the Klamath
River, northern California coast. Note the
large, northward-projecting (towa rd bottom
of photograph) spit that partially blocks the
mouth of the estuary.
318
Chapter 9 I Marginal-Marine Environments
Figure 9.29
contains facies influenced by tide, wave and fluvial presses. The stuary is con
sidered to extend from the landward it of lida1 facies at its head to the seaward
limit of coastal facies at its mouth." According to Dalrymple, Zaitl, and Boyd, es
tuaries can form only in the presence of a re1ative sea-level rise {i.e., transgression).
Progradation tends to fill and desoy estua ries, causing them to change into deltas.
Physiographic, Hydrologic, and Sediment
Characteristics of Estuaries
Based on the physiographic characteriscs of te1ative relief and degree of channel·
mouth blocking, seven basic types of modem estuaries are recognized (Fig. 9.29;
Fairbridge, 1980). Fjords are high-relief estuaries with a U-shaped valley profile
formed by drowning of glaciaUy eroded valleys during Holene sea-level rise.
Fjards or firths are related to ords but have lower relief. Estuaries developed
winding valleys vvith moderate relief are rias. Coastal-plain estuaries are low-relief
estuaries, funnel-shaped in plan view, that are open to the sea. Low-relief estuar
ies that are L-shaped plan view and that have lower courses parallel to the coast
are bar-built estuaries. Similar estuaries that are seasonally blocked by longsho
drift or dune migration are called blind estuaries. Bar-built estuaries and blind es
tuaries are very similar to lagoons and might be considered lagoons by some
workers. Deltaic estuaries occur on delta fronts as ephemeral distributaries.
Flask-shaped, high-relief rias backed by a low-relief plain created by tectonic ac
tivty are called compound estuaries or tectonic estuaries.
(1b) Fjo
(2) Ria
(3)
(4) Bar-built Estuary
Principal ty pes of estuaries based on physio
graphic characteristics. [From Fairbridge, R.
W., The estuary: Its definition and geody
namic cycle, in Olausson, E., and I. Cato
(eds.), 1980, Chemistry and biochemistry of
estuaries, Fig. 2, p. 9, John Wiley and Sons,
New York, reprinted by permission.]
9.4 Estuarine Systems
319
Perillo (1995a) proposes a slightly different classification scheme in which he
divides estuaries into the following main types: (1) Estuaries of former river val
leys, a) Coastal plain, b) Rias; (2) Estuaries of former glacial valleys, a) Fjord, b)
Fjard; (3) River-dominated estuaries, a) Tidal river, b) Delta front; and (4) Structural
estu
aries. The characteristics of many of these estuaries are described in detail in
Perillo (1995b ).
Dalrymple, Zaitlin, and Boyd (1992) present a model for estuaries, based
on
dominant hydrologic characteristics and the kinds of sediment and sediment bod
ies formed in the estuary. They suggest that estuaries can be classed as wave dom
inated, tide dominated, or mixed wave and tide dominated.
Wa ve-Dominated Estuary
a wave-dominated estuary, tidal influence is small, and the mouth of the estu
ary experiences high wave energy. Hydrologic conditions may range from partial
ly mixed to well stratified. Sediments tend to move alongshore and onshore into
e mouth of the estuary, where a subaerial barrier /spit or submerged bar devel
ops. This barrier prevents most of the wave energy from entering the estuary;
thus, only inteally generated waves are present behind the barrier. Depending
upon
tidal range and current velocity, a small number of inlets may be kept open
this barrier. Alternatively, the barrier may close the estuary entirely at times to
produce a blind estuary or coastal lake. The relative influence of marine and river
presses and the kinds of sediment bodies formed in different parts of a wave
dominated estuary are illustrated in Figure 9.30.
A
MARINE-DOMINATED
100
>
ES
T
UAR
Y
MIXED-ENER
G
Y
RIVER-DOMINATED
--·-
100
z
�0
�
c
:
�
�
Figure 9.30
�/
so
Distribution of (A) energy
50
>
�
B
c
/
types, (B) morphological
J-��==�
� �2=
T
o= '=
A
L=
E
=
N
=
E
=
R
G
=
Y
�
��
:
J.
_l
components in plan view,
o
and (C) sedimentary facies in
longitudinal section within
an idealized wave-dominated
estuary. The shape of the es
tuary is schematic. The barri
er/sand plug is shown as
headland attached; however,
on low-gradient coasts, it
may be separated from the
mainland by a lagoon. The
section in Part C represents
the onset of estuary filling
following a period of trans
gression. [From Dalrymple,
R. W., B. A. Zaitlin, and R.
Boyd, 1992, Estuarine facies
models: Conceptual basis
and stratigraphic implica
tions: Jour. Sed. Petrology, v.
62, Fig. 4, p. 1134, repro
duced by permission of
SEPM (Society for Sedimen
tary Geology), Tulsa, Okla.]
320
Chapter 9 I Marginal-Marine Environments
Figure 9.31
Distribution of (A) energy
types, (B) morphological
components in plan view,
and (C) sedimentary facies in
longitudinal section within
an idealized tide-dominated
estuary. UFR = upper-flow
regime; M.H.T. = mean
high tide. The section in
Part C is taken along the
axis of the channel and does
not show the marginal
mudflat and salt-marsh fa
cies; it illustrates the onset
of progradation following
transgression. [From Dal
rymple, R. W., B. A. Zaitlin,
and R. Boyd, 1992, Estuar
ine facies models: Concep
tual basis and stratigraphic
implications: jour. Sed.
Petrolog v. 62, Fig. 7, p.
1136, reproduced by per
mission of SEPM (Society for
Sedimentary Geology),
Tulsa, Okla.]
Note that marine-derived sands are carried landward only a short distance
beyond the barrier as washover deposits or flood-tidal delta sands. Muddy sedi
ments, supplied mainly by the river, accumulate in the central part of the estuary
where total energy is lowest. Deposition of mud is enhanced by mixing of river
and marine water, which causes flocculation of clay particles owing to positively
charged ions in seawater that neutralize the negative charges on clay particles.
Muddy sediment may include biogenic debris such as molluscan shells, wood
fragments, and organically produced pellets. Horizontal and subhorizontal strati
fication and bioturbation traces are common structures. In the head of the estuary,
coarse, river-derived sediments are deposited in channels; marsh deposits may
form adjacent to the channels. Examples of modern wave-dominated estuaries in
clude San Antonio Bay, United States; the Miramichi River, Canada; and Hawks
bury Esh1ary, Australia.
Ti de-Dominated Eshwries
Tide-dominated estuaries occur mainly on macrotidal coasts where tidal-current
energy exceeds wave energy at the mouth of the estuary, creating energy condi
tions in the estuary higher than those typical of wave-dominated estuaries. Wa ter
in the estuary commonly well mixed. Elongate sand bars develop parallel to the
length of the eshwry from sand carried into the estuary from marine sources.
These bars tend to dissipate tidal energy. On the other hand, constricon of in
coming tidal currents between the tidal bars causes an increase in their velity for
some distance up the estuary. Figure 9.31 shows the hydrologic conditions and
sediment bodies developed in tide-dominated estuaries.
A
100
.
w
z
w
50
w
>
3
w
B
c
c
E S T U A R Y
MARINE-DOMINATED
100
50
'
,
I
I , -
I
o
0
UFR
SAND FLAT
I
I
"STRAIGHT" MEANDERING"STRAIGHT"
I
I
I
I
I I
I
· I
I I
I
TIDAL
LIMIT
ALLUVIAL
VALLEY
ALLUVIAL
SEDIMENTS
9.4 Estuarine Systems
321
In addition to forming tidal bars in the mouth of the estuary, sand may be
transported landward through the estuary in e tidal-fluvial channeL Bedforms
ranging in size from ripples to large dunes develop on sandy sediment in the bars
and tidal channels, and cross-bedding generated by migration of these bedforms
can dip in either a landward or a seaward direction. Haser bedding may form dur
ing slack water owing to deposition of suspended mud over sand ripples. Muddy
sediment is deposed also in lower energy parts of the estuary floor and in salt
marshes adjacent to the chael along the edges of the estuary. Muddy sediments
are characterized by nearly planar alternations of silt, clay, very fine sand, and car
bonaceous (plant) debris. Bioturbation by burrowing and feeding organisms may
locally mix and homogenize these layers. Estuarine sediments typically contain a
brackish-water fauna that may include oysters, mussels, other pelecypods, and
gastropods. Examples of tide-dominated estuaries include Cook Inlet, Alaska;
Ord River, Australia; Gironde Estuary, France; and the Seve River, United
Kingdom.
Mixed Wa ve- and Tide-Dominated
Many estuaries have characteristics that are intermediate between wave-domi
nated and tide-dominated types. For example, as tidal energy increases relative
to
wave energy, the barrier system of wave-dominated estuaries becomes pro
gressively more dissected by tidal inlets and elongate sand bars develop in loca
tions previously occupied by barrier segments and the channel-margin linear
bars of ebb-tidal deltas. Marine-derived sand is transported greater distances up
the estuary, and the generally muddy central basin is replaced by sandy tidal
channels
fl anked by marshes. Examples of mixed-energy estuaries include the St.
Lawrence River, Canada; Willipa Bay, United States; and Oosterschelde Estuary,
The Netherlands.
Ancient Euarine Facies
Estuaries and lagoons are both ephemeral features. Because they tend to fill with
sediments in geologically short periods of time, the preservation potential of estu
arine and lagoonal sediments is generay high. Nevertheless, relatively few estu
arine deposits have been reported from the geologic record, possibly because they
have not been widely recognized and distinguished from associated fluvial, deltaic,
lagoonal, or shallow marine deposits.
Estuarine deposits tend to have restricted faunal assemblages that include
brackish-water species and that may be characterized by trace fossil assemblages
flecting brackish to stressed conditions (Reinson, 1992); however, no unique
physical criterion exists for these deposits. Depending upon location within an es
tuary, estuarine deposits may consist almost entirely of cross-bedded sands, lami
nated or bioturbated muds, or combinations of sand and mud. Gradation from
uvial channel sands at the base of a vertical section through mixed fluvial-marine
muds in the middle of the section to marine (tidal) sands at the top suggests a
transgressive estuarine deposit. The exact vertical succession of facies that devel
ops in estuaries depends, however, upon the kind of estuary (wave or tide domi
nated) and the location within the estuary. Facies dominated by cross-bedded,
bioturbated sand are present near the mouths of estuaries and in fluvial-tidal
chnels, whereas laminated to well-bioturbated muds occupy the nonchannel
middle and upper parts of the estuary. Many estuaries are subjected in time to
ansession. Tr ansgression brings about a landward shifting of environments,
sulting in vertical stacking of estuary-mouth sands on top of middle-estuary
muds and/ or uvial-tidal chael sands. By contrast, regression causes filling
and destruction of the estuary and seaward progradation, changing it into a delta.
322
apter 9 I Marginal-Marine Environments
Figure 9.32
Model for a tide-dominated,
transgressive estuarine
embayment depositional
system based on the
Woburn Sands (Lower Cre
taceous), southern England.
(a) Idealized vertical section
showing facies in the more
seaward part of the estua.
(b) Block diagram showing
sand body characteristics of
the inner and outer estuarine
embayments. {After johnson,
H. D., and B. K. Levell, 1995,
Sedimentology of a trans
gressive, estuarine sand
complex: The Lower Creta
ceous Woburn Sands (Lower
Greensand), southern Eng
land, in Plint, A. G. (ed.),
Sedimentary facies analysis:
A tribute to the research and
teaching of Harold G. Read
ing, International Association
of Sedimentologists Spec.
Publ. 22, Blackwell Science,
Fig. 18, p. 41, reproduced
by permission.]
b)
a)
Basic characteristics
large�scale, complex
cross�bedding;
ebb-dominated
paleurrt directiors:
mainly minor bioturbation
Outer estuarine/ Sliv Sands and
embayment Udal
Red Sands
shoals
Moderately sorted sands
Inner estuarin
with clay lays; ebb- and
Heterolithic Sands
flood�directed paleocurrerts;
embankment tidal
Orange Sands
channels d shoals
moderate-strong bioturbio'l
Pre-transgressive deposits
• • Lag gravels
_ SCoured surface
Large-scale cross�beddig
�Trough cross-bedding
Flaser bedding
akly bioturbed
+ Strongly biurbated
Figure 9.32 shows facies developed in a tide-dominated, transgressive estu
arine sand complex in Lower Cretaceous Woburn Sands of southrrn England
Oohnson
and Levell, 1995). Johnson and Levell suggest that e Orange and Het
erolithic Sands were deposited in ebb and flood tidal channels and intervening
tidal shoals in an inner estuarine environment. The Silver and Red sands were
de
posited in the higher energy outer reaches of an estuary where greater water
depth allowed large-scale bedforms to build. Slowly deposited fossiliferous ma·
rine beds of the Transition Series and Basal Gault were then laid down on p of
the succession as transgression proceeded.
9.5 LAGOONAL SYSTEMS
A coastal lagoon is defined as a shallow stretch of seawatersuch as a sound,
channel, bay, or saltwater lake--near or commicatg with the sea and partly or
completely separated from it by a low, narrow elongate strip of land, such as a
reef, barrier island, sandbank, or spit (Bates and Jackson, 1980), e.g., Figure 9.33.
Most mode lagoons are formed behind spits or offshore barriers of some type
and thus are elongated bodies lying parallel to the coast with a narrow coection
to the open ocean. Lagoons also form behind barrier reefs and atolls. Lagoo
commonly extend parallel to the coast, in contrast to estuaries, which are oriented
approximately perpendicular to the coast. Many lagoons have no significant
freshwater runoff; however, some coastal embayments that otherwise satisfy the
general definition of lagoons do receive river discharge. Lagoons may occur in
ose association with river deltas, barrier islands, and tidal ats.
®
50 km
Figure 9.33
9.5 Lagoonal Systems
323
Cape Hatteras, South Carolina; a lagoonal system enclosed by a barrier-island chain. A, Di�
agram matic sketch of the barrier chain and lagoon. B. Cape Hatteras as seen from Apolo
9; Pamlico Sound is partly obscured by clouds. [A. From Barnes, R. S. K., 1980, Coastal la
goons, Fig. 1.3, p. 5, Cambridge University Press, reprinted by permission; B. NASA pho
tograph, downloaded from the Internet 4/1 9/2004.]
Many factors affect water flow, water mixing, and sediment transport in la
goons, such as tides, wind waves, freshwater runoff, episodic storms, density gra
dients, sea-level changes, and changes in climate and temperature. Even so, water
circulation patterns in lagoons are much less affected by freshwater inflow than
they are estuaries, and many lagoons receive no freshwater discharge. Also, cir
cu
lation with the open ocean is restricted by the barrier. Consequently, the princi
pal movement of water within lagoons is in the form of tidal currents (which move
and out through the narrow inlets between barriers) and wind-forced waves.
On the basis of geomorphology and the nature of water exchange with the
coastal ocean, Kj erfve and Magill (1989) identify three types of lagoons: choked, re
stricted, and leaky (Fig. 9.34). Choked lagoons occur along coasts with high wave en
e
and sikant alongshore drt (e.g., Coorong Lake, southern Australia). They
are characterized by one or more long, narrow entrce channels; long residence
of water within the lagoon; and dominant water movement by wind forcing.
Intense slar radiation coupled with inflow events can cause intermittent vertical
stratification. Restcted lagoons commonly exhibit two or more entrance channels
or inle, have a well-defined tidal cculation, are strongly fluenced by winds, and
are generally vertically mixed (e.g., Lake Pontchartrain, Louisiana). Leaky lagoons
typically occur along coasts where tidal currents are a more important factor in sedi
ment transport than are wind waves (e.g., Belize Lagoon, Belize). They may stretch
along coasts r more than 100 but commonly are no more than a few kilometers
wide. They are characterized by wide tidal passes, eicient water exchange with the
, strong tidal currents, d sharp salty and turbidity fronts.
Except within tidal channels that extend into the lagoon, lagoons are pre
dominantly areas of low water energy. Tidal deltas commonly develop at the ends
324
Chapter 9 I Marginal-Marine Environments
A Choked
Figure 9.34
8 Restricted
C Leaky
Principal kinds of coastal lagoons (choked, restricted, leaky) based on the deg ree of water
exchange with the adjacent coastal ocean. [From Kjerfve, B., and K. E. Magil, 1989, Geo
graphic and hydrodynamic characteristics of shallow coastal lagoons: Marine Geology, v.
88, Fig. 2, p. 190, reproduced by permission.]
of these tidal inlets, both within the lagoon and on the ocean sides, and sandy sed
iment may also deposited within the higher energy tidal channels inside the la
goon. Otherwise, sedimentation within lagoons is dominated by deposition of silt
and mud, although occasional high wave activity during storms can cause
washover of sedent from the barrier.
Salinity within lagoons can range from hypersaline to essentially that of
fresh water, depending upon the hydrologic conditions and the climate. Lagoons
formed arid or semiarid coastal areas, where little freshwater influx curs, a
commonly hypersaline, with salinities well above that of normal seawater.
goons more humid regions may be characterized by brackish water. Salinity
with lagoons may vary in response to seasonal rainfall and evaporation rat.
Also, salinity at a particular time may not be iform throughout a lagoon. La
goons receiving considerable freshwater inflow commonly display distinct, later
al sality zones.
The sediments deposited in lagoons can be derived from several ur�es,
which can include (depending upon the nature of the lagoon) rivers, the ocean,
shores, and barriers. Sediment can also be derived internally by organic pwduc
tion, chemical precipitation, and erosion of older deposits (Nichols and Bn,
1994). The deposits of lagoons may differ from those of estuaries in several ways.
First, because many lagoons do not receive freshwater discharge from rivers, most
or all the sediment in such lagoons is from marine sources. Lagoons are typically
low-energy environments, although tidal currents move into lagoons th rough in
lets between barriers, winds create some wave action along shorelines, storms
provide occasional episodes of high-energy wa·ves that wash over barriers into the
lagoon, and prevailing winds may more or less continuously blow small amounts
of sediment from barriers into the lagoon. Because of the domance of low-eney
conditions in lagoons, lagoonal deposits consist mainly of fine-graed sediments.
Sandy sediments are confined principally to tidal deltas constructed at the mouths
of the tidal inlets, to some tidal channels that extend into the lagoon, to washover
lobes behind barriers, and to some parts of the lagoonal shoreline (lagoon a) beach
es). Small amots of sandy sediment blown from barriers may also be scattered
throughout the lagoon. Sandy sediment in tidal channels is characterized by cur
rent ripples and internal small-scale cross-bedding that may dip in either a land
ward or a seaward direction. Most of the lagoonal bottom is covered with silty or
muddy sediments, commonly extensively bioturbated, that may contain thin
tercalations of sand brought in by storms or blown in by wind. This sand is gener
ally horizontally laminated, but it may display ripple cross-laminations. The
faunas that inhabit lagoons are highly variable dependg upon the salinity �on
ditions of the lagoon, but they are generally characterized by low diversity. La
goons with normal salinity show faas similar to those of the open ocean,
9.5 Lagoonal Systems
325
whereas brackish-water faunas dominate lagoons in front of river mouths. Hyper-
saline lagns commonly contain few organisms because few species are adapted
to such high salin}ties.
areas where little siliciclastic sediment is available and climac conditions
are favorable, sedimentation in lagns is dominated by chemical and biochemical
deition. Under very arid conditions, lagoonal sedimentation may character
by deposition of evaporites, which a mainly gypsum but may include some
halite and minor ddloruites (e.g., lagoons in the Persian Gulf). Under
less hypersaline
conditions, carnate deposition prevails, particularly in lagns developed behind
barrier reefs (e.g., Australia}, Deposits in such lagoons may consist largely of carbon
ate muds and assiated skeletal debris, although ooids may form in more agitated
parts of the lagn. Algal mats, commonly developed in the supratidal and shallow
intertidal zone, may trap e carbonate or siliciclastic mud to form stromatolites.
Algal mats in supratidaJ zone generally display mudcracks with curled margins.
Ancient Lagoonal Deposits
Lagnal deposits may form in many settings, including parts of barrier-island
c0mplexes. Criteria that I be used to distinguish ancient lagoonal deposits from
estuarine and other deposits include evidence for restricted circulation such as the
presene of evaporites or anoxic facies (e.g., black shales), lack of strong tidal in
uence, slow rates of terrigenous sediment influx and dominantly fine grained
sediments, low fauna� diversity, and extensive bioturbation (Davis, 1983). The St.
Mary ver Formation of southe Alberta, Canada, contains back-barrier de
posits that illustrate some of the characteristics of lagoonal sediments (Fig. 9.35).
Figure 9.35
Composite stratigraphic sec
tion of Cretaceous formations
in southern Alberta, Canada.
Back-barrier lagoonal deposits
of the St. Mary River Forma
tion overlie tidal-inlet and
tidal-delta deposits of the
Blood Reserve Formation.
[From Reinson, G. E., 1984,
Barrier island and associated
strand-plain systems, in Walker,
R. G. (ed.), Facies models, 2nd
ed.: Geoscience Canada
Reprint Ser. 1, Fig. 1 6, p. 1 28,
reproduced by permission of
Geological Association of
Canada.]