Boggs S. Principles of Sedimentology and Stratigraphy
Подождите немного. Документ загружается.
4.3 Stratification and Bedforms 85
If ow takes place over sediment coarser than about 0.9 mm, for example, the rip-
ple phase does not develop. Instead, a lower plane-bed phase forms just prior to
the foation of dunes. Note also from Figure 4.10 that below a grain size of about
0.15 mm, dunes do not form. The ripple phase is succeeded abruptly by the upper
plane-bed phase. For details of these plots as well other data on bed configurations,
e Southard and Boguchwal (1990).
Most studies of bedforms have been carried out laboratory flumes or under
shallow-water conditions in natural environments. Therefore, most available sedi
ment-size/velocity data perta to the formation of bedforms under shallow-water
conditions (commonly less than about 1 m). Much less is known about the devel
opment of bedforms under deeper water conditions. Based on limited available in
formation, Harms, Southard, and Walker (1982) suggest that the nature of small
ripples approximately the same in deep-water flows as in shallow-water ows;
however, the larger bedforms (dunes) can grow much larger deep-water flows.
The hydraulic relationships in deep water are the same as for shallow water; that is,
dunes form at higher velocities than ripples and at lower velocities than plane beds
and antidunes. e exact relationship between grain size, ow velocity, and bed
form phase is not well documented for deeper water, but a generalized relationship
is shown in Figure 4.11. Note from Figure 4.11 that exceedingly high velocities are
required to produce antidunes at a water depth greater than a few meters; there
fore, it appears that antidunes are unlikely to occur under natural conditions in
deep water, except perhaps under some turbidity currents.
Nare of Flow over Bedforms
e mechanisms of sediment transport that are responsible for formation of the
dferent bedforms are very complex. general, the formation of transverse bed
forms is related to a phenomenon called flow separation. Sediment is transported
suspension or by traction up the stoss side of the bedform to the brink or crest.
At the bnk, the flow separates from the bed to form a zone of reverse circulation
0.1
E 0.2
s
�
·
�
0.5
1.0
0.2
0.5
2 5
Flow depth (m)
10
20 50
100
gu 4.1 1
Generalized three-dimensional
depth-velocity-grain-size dia
gram showing the relation
ship among bed phases and
grain size for a wide variety of
flow velocities and flow
depths. Diagram based on
both flume data and observa
tions of natural flows. [After
Rubin, M. D., and D. 5. Mc
Culloch, 1980, Single and su
perimposed bedforms: A
synthesis of San Francisco Bay
and fluvial observations: Sed.
Geology, v. 26, Fig. 11, p.
224, reprinted by permission
of Elsevier Science Publishers,
Amsterdam.]
86
Chapter 4 I Sedimentary Structures
Figure4.12
ripple
_ length
index -
- wavelength
.
1
�lee side _.- stoss side
---
.
1
The terminology used to de
scribe asymmetric ri pples.
[From Tucker, M. E., 1981,
Sedimentary petrology, an in
troduction: john Wi ley & Sons,
Inc., Fig. 2.18, p. 28. Repro
duced by permission of Black
well Science.]
- crest -� trough ----
flow direction
--
cross�
strata
summit
point
bri
nk point
-- �- -
I
point of flow
separation
-
-
.
I
eddy in
trough
\
point of flow
reattachment
avalanching
___ down lee
Figure 4.13
or backflow, producing a separation eddy (Figure 4.12). A zone of diffusion is pre
sent between the zone of backflow and the main flow above, owing to turbulent
mixing with the main flow. Downstream from the point of separation, at a dis
tance several times the height of the bedform, the flow becomes reattached to the
bottom. Flow separation causes separation of the transported sediment into bed
load and suspended load fractions. The bedload fraction accumulates at the ripple
crest until the lee slope exceeds the angle of repose and avalanching takes place.
The suspended load fraction is transported downcurrent where the coarser parti
cles in the suspended load settle through the zone of diffusion into the zone of
backflow and are deposited in the lee of the ripple. It is these processes that cause
development and movement of the bedforms.
Classication of Ripples
Ripples are common sedimentary structures in modern environments, where they
occur in both siliciclastic and carbonate sediments. They can form by both water
and wind transport. Ripples can develop in granular material under either w1idi
rectional current flow or oscillatory flow (wave action). Ripples that develop in re
sponse to unidirectional flow are asymmetrical in shape, and the steep or lee side
faces downstream in the direction of current flow (Fig. 4.13). Asymmetrical ripples
formed in this fashion are called current ripples. Under natural conditions they
form
by river and stream flow, by backwash on beaches, and by longshore cur
rents, tidal currents, and deep-ocean bottom currents. In plan view, the crests of
current ripples and dunes have a variety of shapes: straight, sinuous, catenary,
linguoid, and lunate (Fig. 4.14). The plan-view shape of ripples and dunes is ap
parently related to water depth and velocity (Allen, 1968); however, the factors
Asymmetrical current ri pples formed on a riv er bar,
Rogue River, southern Oregon. The current flowed from
left to ri ght.
4.3 Stratification and Bedforms
87
STRAIGHT
TRANSVERSE
STRAIGHT
SWEPT
TRANSVERSE SINUOUS
IN PHASE
RSE SINUOUS
T PHASE
TRANSVERSE CATENARY
TNSVERSE CATENARY
IN PHASE
OUT OF PHASE
UNGUOID
CUSPATE
LUNATE
CATENARY
SWEPT
that control their shapes are not \veil undertood. It ha been observed under nat
ural conditions that the more omplex forms tend to develop in shallower water
and at higher velocities than the less eomplex farms; the order in which the suc
ssion of bedforms develops with decreasing water depth and velity is straight
suous to symmetric linguoid to asymmetric linguoid for ripples and straight
to sinuous to catenary to lunate for dunes. Ripples that form by wave action under
oscillat01y flow are called oscillation ripples. Oscillation ripples tend to be nearly
symmetrical in shape and have fairly straight crests (e.g., Fig. 4.15).
Ripples are most ommon shallow-water environments; however, they
have been photographed on the floor of the modern ocean at depths of a few thou
sand meters. Ripples have relatively low preservation potential because they tend
to be eroded and destroyed by current erosion before burial. Therefore, ancient rip
ples such as the modern ripples shon in Figure 4.13 are not extremely abundant
the sedimen tary record. nes are even less commonly preserved; nonetheless,
ancient dtmes are present in some thick sandstone units (e.g., Fig. 4.16).
Cross-Stratification Structures
Cross-Bedding
Cross-bedding forms primarily by migration of ripples and dunes in water or air.
pple or dune migration leads to formation of dipping foreset laminae owing to
Figure 4.15
Figure 4.14
Idealized classification of cur
rent ripples and dunes the
basis of plan-view shape.
Flow is from the bottom to
the top in each case. [After
Allen, ]. R. L., 1968 Current
ripples: Their relation to pat
terns of water motion: North
Holland Pub., Amsterdam,
Fig. 4.6, p. 65, reprinted by
permission of Elsevier Sciee
Publishers, Amsterdam.]
Oscillation ripples on the surface of fine-grained marine sand
stone, Elkton Siltstone (Eocene), southern Oregon coast. Note
hammer (lower-left corner) for scale.
88
Chapter 4 I Sedimentary Structures
Figure 4.16
Large dunes on the surface
of a sandstone bed, Tyee
Formation (Tertiary), ex
posed along the Umpqua
River, southern Oregon
Coast Range. The dunes
are about 15 em high and
70
em from crest to crest.
[Photograph courtesy of
Ewart Baldwin.]
avalanching or suspension settling in the zone of separation on the lee sides of
these bedforms (Fig. 4.12). lf most of the sediment is too coarse to be transported
in suspension, avalanching of the bedload sediment down the lee side of the rip
ple will cause formation of laminae that are steep and straight. These inclined
foreset laminae make contact with the nearly horizontal, thin bottomset laminae
(deposited from suspension) at a distinct (nontangenal) angle, which is approxi·
mately the same as the angle of repose. Roughly the same effect is achieved iF the
height of the lee slope is large compared to total flow depth, so hat the suspended
load
falls mainly on the lee slope. If the suspended load is 1arge, or if the height of
the lee slope is small compared to flow depth, suspended sediment wm pile up at
the base of the lee slope rapidly enough to keep pace with growth of the avalanche
deposits. This process causes the lower part of the foreset laminae to cue out
ward and approach the bottomset laminae asymptotically (BlaH, Middleton, and
Murray, 1980). Thus, the cross-laminae are said to be tangenial.
The preservation potential of cross-laminae is much higher than that of e
bedforms themselves (because the tops of bedforms tend to be planed off by sub
sequent current or wind erosion); therefore, crss-bedding is a very common type
of sedimentary structure in ancient sedilelh\ny rks. Cross-stratification can be
formed also by filling of scour pits and channels, by deposition on the int bars
of meandering streams, and by deposition on the inclined smfac of beaches and
marine bars. Cross-bedding formed under different environmental conditions can
be very similar in appearane, and it is often difficult field studies of ancient
sedimentary rocks to differentiate cross-bedding formed in fluvial, eolian, and
marine environments.
Cross-beds commonly cur in sets (Fig. 4.4). Cross-bedding sets less than
about 5 em thick is called smal-scale cross-bedding; that in sets thicker tli.an 5 em
is large-scale cross-bedding. Because of their diverse origin, many types of cross
beds occur. Allen (1963) proposed a very elaborate classification of cross-bedding
based upon such propeties as grouping of cross-bed ses, scale, nate of the
c
, Tran�
I
dirtion
flgure4.17
4.3 Stratification and Bedforms
89
The terminoly and defining characteristics o• cross-bedding. Symbols: a, direction paral
lel to average sediment transport direction; c, direction perpendicular to (a) and the trans
port plane (bed) in which (a) lies; S
P
' the principal bedding surface or beddin9 plane; S1,
the forese� surface of cross-bedding. [After Potter, P. E., and F. ). Pe ttijohn, 1977, Paleocur
rent and basin analysis, 2nd ed., �ig. 4.1, p. 91, reprinted by permission f Springer
Verlag, Heidelberg.]
boundin
g
surface of the beds, an
g
ular relation of cross-strata in a set or oset to
the bounding surfces, and degree of grain-size uniformity in different laminae.
The much simpler scheme of McKee and Weir (1953), as modified by Potter and
Pettijohn (1977), is adopted herein. Cross-beds are divided into two principal
, tabular and trough, on the basis of overall geometry and the nature of the
bounding surfaces of the cross-bedded units (Fig. 4.17).
Tabular cross-bedding consists of cross-bedded wuts that are broad in lateral
dimensions with respect to set thickness and that have essentially planar bound
ing surfaces (e.g., Fig. 4.18). The foreset laminae of tabular cross-beds are also
commonly panar, but curved laminae that have a tangential relationship to the
sal
surface also oc. Trough cross-bedding consists of cross-bedded units in
which one or both bounding surfaces are curved (e.g., Fig. 4.19). The units are
t
rough-shaped sets consisting of an elongated scour filled with curved foreset
laminae that commonly have a tangential relationship to the base of the set.
Tabular cross-bedding is formed mainly by the migration of large-scale,
straight-crested ripples and dunes (Fig. 4.20A); thus, it forms during lower flow
gime conditions. Individual beds range in thickness from a few tens of centime
rs to a meter or more, but bed thicknesses up to 10m have been obseed (e.g.,
Harms et al., 1975). Trough cross-bedding can originate both by migration of small
Flgure4.18
Large-scale tabular uoss-bedding in Permian sandstones,
Canyon de (helly National Monument, Arizona. Note that
bounding surfaces of the cross-bedded units are planar and
that the foreset laminae form mainly tangential contacts with
these planar surfaces. [Photograph by james Stovall.]
90
apter 4 I Sedimentary Structures
Figure4.19
Small-scale trough cross
bedding, Coaledo Forma
tion (Eocene), southern
Oregon coast. Notice that
several episodes of scour
ing produced small, ero
sional troughs (arrows),
which were subsequently
filled with low-angle cross
laminae.
Figure 4.20
Diagram illustrating (A)
large-scale tabular cross
bedding formed by migrat
ing straight-crested dunes
(with rippled surfaces) and
(B) large-scale trough
cross-bedding formed by
migrating, trough-shaped
dunes. Flow is from left to
right in both A and B.
[From Harms, j. C., j. B.
Southard, and R. G. Walker,
1982, Structures and se
quences in clastic rocks:
Soc. Econ. Paleontologists
and Mineralogists Short
Course No. 9, Fig. 3-1 1,
p. 3-23 and Fig. 3-10,
p. 3-1 9, reprinted by per
mission of SEPM, Tulsa,
Okla.]
current ripples, which produces small-scale cross-bed sets, or by migration of
large-ale, trough-shaped dunes (Fig. 4.20B). Trough cross-bedding formed by
migration of large-scale ripples commonly ranges in thickness to as much as a few
tens of centimeters and in width from less than 1 m to more than 4 m.
Ripple Cross-Lamination
Ripple cross-lamination (climbing ripples) forms when deposition takes place
very rapidly during migration of current or wave ripples (McKee, 1965; Jopling
A
B
4.3 Stratification and Bedforms 91
and Walker, 1968). A series of css-laminae are produced by superimposing mi-
grating ripples. The ripples climb on one another such that the crests of vertically
succeeding laminae are out of phase and appear to be advancing upslope. This
prss results in cross-bedded units that hav the general appearance of waves
(Fig. 4.21) in outcrop sections cut normal to the wave crests. In sections with other
orientations, the laminae may appear horizontal or trough-shaped, depending
up on the orientation and the shape f the ripples.
The frmation of ripple cross-lamination appears to require an abundance of
sediment, especially diment in suspension, which quickly buries and preserves
original rippled layers. Abundant suspended sedmment supply must be combined
with just enough traction transport to produce rippling of the bed, but not enough
to cause complete erosion of aminae front the stoss side of ripples. Some ripple
laminae may be in phase (one ripple crests lies directly above the other), indicat
ing that lhe ripples did not migrate. In-phase ripple laminae form under condi
tions where a balance is achieved between raction transport and sediment supply,
and are balanced so that fhe ripples do not migrate despite a growing sediment
surface. Ripple cross-Famination curs sediments deposited in environments
charactered by rapid sedimentation from suspension-fluvial flood plains,
int bars, river deltas subject to periodic flooding, and environments of turbidite
sedimentation. Figure 4.21 shows the sequence of bedforms developed in a river
during waning flood stage. Laminae at the bottom of Figure 4.21 developed dur
ing a plane-bed phase of upper-flow regime transport at high flood velocity. As
velity waned into the lower-flow regime, ripple cross-lamination formed on top
of the plane-bed laminae.
Flaser and Lenticular Bedding
Flaser bedding is a type of ripple bedding in which thin streaks of mud occur
between sets of cross-laminated r lipple-lamlmated sandy or silty sediment
(Fig. 4.22). Mud is concentrated main1y the r•pple troughs but may also partly
ver the crests. Flaser bedding suggts deposition under fluctuating hydraulic
conditions. Periods of curnt activity, when tracon transport and deposion of rip
pl sand take place, alteate with periods of quiescence, when mud is deposited.
Figure 4.21
Ripple cross-lamination (below ballpoint
pen) in flood deposits of the Illinois River,
southwestern Oregon. Parallel laminae at the
bottom of the photograph developed during
a plane-bed phase of upper-flow-regime
conditions; as current velocity diminished
into the lower-flow regime, ripple cross
lamination formed on top of the laminae. A
later flood pulse deposited upper-flow
regime plane beds on top of the ripple
cross-lamination.
92
Chapter 4 I Sedimentary Structures
Figure4.22
Flaser bedding in tidal-flat sediments of the North Sea. [After Rei
neck, H. E., 1967, Layered sedimen of tidal flats, beaches, and shelf
bottoms of the North Sea, in Lau, G. H. (ed.), Estuaries: Amer.
Assoc. for the Advancement of Science, Washington, D.C., Fig. 8,
p. 195.]
Ffgure4.23
Repeated �pisodes ofurrent activity ede previously deposited ripple crests, al
lowing nw ripp1ed sand to bury and preserve rippled beds with mud flasrs
the troghs (Reineck and Singh, 1980). Lenticular bedding is a structure formed
by interdde mud and ripple cross-laminated sand in which the ripples or sand
lenses a di�ontinuous and isolated m both a verical and a horizontal direction
(fig. 4.23). Reineck and Singh (1980) suggt that flaser bedding is produced in en
vironments in which conditions for deposition and preservation of sand are more
favorable than for mud, but that lenticular bedding is produced envionments
in which conditions favor deposition and preservation of mud er sand. flaser
and lenticular bedding appear to form particularly on tidal flats and in subtidal
environments where conditions of current flow or wave action that cause sand de
position alternate with slack-water conditions when mud is deposited. They also
form in marine delta-front environments, where fluctuations sediment supply
and current velocity are common; in lake environments in front of small deltas;
and possibly on the shallow marine shelf owing to storm-related transport of sand
into deeper water.
Hummocky Cross-Stratification
The name hummocky cross-stratification was introduced by Harms et al. in 1975,
although the struces had been recognized and described under different names
by earlier workers. Hummocky cross;strhficaion is characterized by undulating
sets of cross-laminae that are both concave-up (swales) and �0nvex-up (hum
mocks) (Fig. 4.24). The cross-bed sets cut gently into each other 'ith curved ero
sion surfaces (Fig. 4.25). Hummocky cross-bedding commonly occurs in sets 15 to
50 em thick with wavy erosional bases and rippled, bioturbated tops (Harms et a!.,
1975). Spacing of hummocks and swales is from em to several meters. The
lower bounding surfa�e of a hummocky unit 1s sharp and is commonly an erosion
al surface. Current-focmed so1e marks may be present on the base. Hummocky
Lenticular bedding in tidal flats of the North Sea. Sand
lenses are wave ripples. [After Reineck, H. E., and I. B.
Singh, 1975, Depositional sedimenta environments,
Springer-Verlag, New York, Fig. 176, p. 1 03.]
HUMMOCKY CROSS STRAFICAON- HCS
LONG WAVELENGTH, 1 - 5 M
LOW HEIGHT. FEW 10'S OF CM
4.3 Stratification and Bedforms
93
HUMMKS D SWALES CIRCULAA TO ELLIPTICAL IN PLAN VIEW
SSTS. AVEGE
SSTS. COMMONLY INTERBEDDED
WITH BIOTURBATED MSTS.
CHARACTERIZED BY -
1, UPWARD CURVATURE OF LAMINATIONS
2. ANGLE, CURVED LAMINA INTERSECTIONS
S. VERY LONG WAVELENGTHS, LOW HEIGHTS;
INA OIPS NOMALLY lESS THAN 1
LOW ANGLE CURVED
LAMINA INTERSECTIONS.
Figure 4.24
AS HUMMOCKS AND SWALES
MIGRATE SLIGHTLY
Schematic diagram of hummocky cross
stratification, which typically occurs in
terbedded with bioturbated mudstone.
MSTS, mudstones; SSTS, sandstones.
[From Walker, R. G., 1984, Shelf and shal
lw marine sands, i Walker, R. G. (ed.),
Facies models, 2nd ed.: Geoscience Cana
da Reprint Ser. 1, Fig. 11, p. 149, repro
duced by permission.]
Figure4.25
Hummocky cross-stratification in fine-grained
sandstone, Elkton Siltstone (Eocene), southwestern
Oregon. Note the clearly defined erosional surface
with fine, laminated nd draped over the erosion
al hummock. The width of the area photographed
is �60 em.
css-stratification typically occurs in fine sandstone to coarse siltstone that com
monly contains abundant mica and fine carbonaceous plant debris (Dott and
Bourgeois, 1982).
Hummocky cross-stratification has not yet en produced flum or re
rted from modern environments, ut it has been reported in ancient strata from
numerous localities. Harms et a!. (1975, 1982) suggest that this structure is formed
by strong surges of varied direction (oscillatory flow) that are generated by rela
tively large stor m waves in the e. Strong storm-wave action first erodes the
bed into low hummks and swales that laGk any significant orientation. Thls
topography then mantled by laminae of material swept over the huocks and
al. Moreently, Duke, Arnott, and Cheel (1991) and Cheel and Leckie (1993)
suggest that hummky cro-stratification originates by a combination of unidi
onal and oscilfatory flow related to stor activity. Although hunocky
tratification is commonly confined to shallow marine sedimentary rocks,
Duke(1985) reports this structu in some lacustrine sedimentary rks.
94
Chapter 4 I Sedimentary Structures
Figure 4.26
Convolute lamination in
fine-grained sandstone and
shale, Coaledo Formation
(Eocene), southwestern
Oregon. Note absence of
deformation in the underly
ing layers.
Irregular Stratification
Deformation Structures
Convolute Bedding and Lamination. Convolute bedding is a structure formed by
complex folding or intricate crumpling of beds or laminations into irregular, gen
erally small-scale anticles and syncnes. It is commonly, but not necessadly,
confined to a single sedimentation unit or d, and he strata above and bel ow this
bed may show little evidence of deformation {Fig. 4.26). Convolute dding is
most common in fine sands or silty sands, and the laminae can ty pically be traced
through the folds. Faulting generally does not occur, but the onvolutions may be
truncated by erosional surfaces which may also be convoluted. The convolutions
crease in complexity and amplitude upward from undisturbed laminae the
lower part of the unit. They may either die out in the top part of the unit or be
truncated by the upper bedding surface. Beds containing convolute lamination
commonly range in thickness from about 3 o 25 em (Potter and Pettijohn, 1977),
but convoluted units up to several meters thick hav been reported in both eolian
and subaqueous deposits.
Convolute lamination is most common in turbidite successions. It also oc
curs in intertidal-flat sediments, r�ver flo-plain and point-bar diments, and
deltaic deposits. The origin of convolute dding is still not thoroughly under
stood, but it appears to be caused by plastic deformation of partially liquefied
iment soon after deposition. The axes of some convoluted folds have a preferred
orientation which commonly coincides with the paleocurrent direction, suggest
ing that the process that produces convolutions curs during deposition, at least
in
these cases. Liquefaction of sediment can be caused by such processes
as differ
ential overloading, earthquake shocks, and breaking waves.
Aame structures. Ftam�structures ate wavy or flame-shaped tongues of mud that
project upward Into an nverlying layer, which is wmmonty sandstone (Fig. 4.27�.
The
crests of some flames are bent over or overturned; generally, vvertued crests
tend to all point in the same direction, but this is not always the case, as illustrated
in Figure 4.27. Flame structures are commonly associated with other structures