Boggs S. Principles of Sedimentology and Stratigraphy
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4.4 Bedding-Plane Markings
105
Tr ypa nites
Gfossifungites
Skolithos
Cmziana
Zo ophycos
Figure 4.38
Schematic
representation of the relationship of characteristic trace fossils to sedimentary
facies and depth zones in the ocean. Borings of 1, Po/ydora; 2, Entobio; 3, echinoid bor
ings; 4, Tryponites; 5,6, pholadid burrows; 7, Diplocraterion; 8, unlined crab burrow; 9,
Skolithos; 10, Diplocraterion; 11, Thoiossinoides; 12, Arenicolites; 1 3, Ophiomorpho; 14,
Phycodes; 15, Rhizocorallium; 16, Teichichnus; 1 7, Crossopodia; 18; Asteriocites; 19;
Zoophycos; 20, Lorenzinia; 21, Zoophycos; 22, Paleodictyon; 23, Tap hrhefminthapsis; 24,
He/minthoida; 25, 5pirohaphe; 26, Cosmorhaphe. [From Ekdale, A. A., R. G. Bromley, and
S. B. Pembeon, 1984, Ichnology: Trace fossils in sedimentology and stratigraphy: Soc.
Econ. Paleontologists and Mineralogists Short Course No. 15, Fig. 15.2, p. 187, reprinted
by permission of SEPM, Tulsa, Okla. Modified from Crimes, T. P., 1975, The stratigraphical
significance of trace fossils, in Crimes, T. P. , and ). C. Harper, eds., The study of trace fos
sils, Fig. 7.2, p. 118: Springer-Verlag, New York.]
Significance of Tr ace Fossils. Trace fossils are important paleoecological indica
tors; however, they are not infallible paleodepth indicators. In general, organisms
in the littoral or intertidal zones adapt to harsh conditions resulting from high
wave or current energy, desiccation, and large temperature and salinity fluctua
tions by burrowing into sand to escape. Thus, vertal and U-shaped dwelling
burrows, some wi protective linings, characterize the Skolithos ichnofacies of this
zone. The neritic zone or subtidal zone extending from the low-tide zone to the
e
d
ge of the continental shelf (at about 200 m water depth) is a less demanding en
vironment, although erosive currents may be present. Ve rtical dwelling burrows
an
d
protected, U-shaped burrows are less common in this zone. Burrows tend to
be shorter, and surface markings made by organisms such as crustaceans (or trilo
bites during early Paleozoic time) are more common. In the deeper part of the ner
itic zone, organic matter becomes abundant enough for sediment feeders to
become established and produce feeding burrows. In these deeper waters, vertical
Nereites
106
Chapter 4 i Sedimenta Structures
Box 4.1 Important Ichnofacies
SKOLITHOS ICHNOFACIES
Trace fossils of this association are characterized especially by vertical, cylin
drical, or U-shaped burrows (e.g., Ophiomorpha, Diplocraterion, and Skolithos,
Fig. 4.4.1}. Overall diversity of ichnogenera is low and few horizontal struc
tures are present. This ichnofacies is developed primarily in sandy sediment
where relatively high levels of wave or current energy are typical. Organisms
in this environment construct deep burrows to protect
against desiccation or
unfavorable temperature or salinity changes during low tide, and as a means
of escaping the shifting substrate of the surface (Pemberton, MacEachern, and
Frey, 1992). The Skolithos ichnofacies is typical of sandy shoreline environment
(Fig. 4.38), but it may grade seaward into shallow shelf environments. It has
also been reported from some deeper-water environments, such as deep-sea
fans and bathyal slopes.
CRUZIANA ICHNOFACIES
The Cruziana ichnofacies commonly occurs in somewhat deeper water than
the Skolithos ichnofacies within subtidal zones below £air-weather wave base
but above storm wave base (Frey and Seilacher, 1980), typical of the middle
and outer shelf. It may also be present in sediments from some nearshore en
vironments. It is characterized by a mixed association of traces that may in
clude nearly vertical burrows, inclined U-burrows (Rhizocorallium), horizontal
structures (Cruziana}, the traces of organisms that move about on or near the
Figure 4.4.1
Trace fossil association characteristic of the Skothos ichnofacies. 1) Ophiomorpha, 2)
Diplocraterion, 3) Skolithos, 4) Moncraterion. [From Pemberton, S. G., J. A. MacEachern,
and R. W. Frey, 1992, Trace fossil facies models: Environmental and allostratigraphic
significance,in Facies models, Walker, R. G., and N. P. james (eds.), Geological Associa
tion of Canada, Fig. 9, p. 53. Reproduced by permission.]
4.4 Bedding-Plane Markings
107
sediment surface (Thalassinoides}, and other odd traces having star shapes
(Asteriacites) or C-shapes (Arenicolites} (Fig. 4.4.2}. Note: Some authors (e.g.,
Bromley, 1996, p. 249) recognize a separate Arenicolites ichnofacies. e Cruziana
inofacies commonly has high diversity and abundance of traces (Fig. 4.4.2);
fact, a profusion of burrows may be present. It is typically developed in well
sorted silts and sands but may be present muddy sands or silts.
ZOOPHYCOS ICHNOFACIES
is ichnofacies appears most typical of quiet-water environments with mod
erately low oxygen levels and muddy bottoms but can occur in other sub
strates.
It is characterized by traces that range from simple to moderately
complex, such as Spirophyton (Fig. 4.4.3). Individual aces may be abundant,
but overall diversity is low. Sediments of the Zoophycos ichnofacies may be
totally bioturbated (Bromley, 1996, p. 250). Although commonly considered to
indicate deeper water (fig. 4.38), it is known to occur also in shallow water.
Thus, its value as a paleodepth indicator is problematical. Its distribution ap
pears to be tied more closely to oxygen levels and bottom sediment type than
water depth.
Rre 4.4.2
Trace
fossil association characteristics of the Cruziana ichnofacies. 1) Asteriacites, 2)
Cruziana, 3) Rhizocorallium, 4) Aulichnites, 5) Th alassinoides, 6) Chondrites, 7)
Teichichnus, 8) Arenicolites, 9) Rosse/la, 1 0) Planolites. [From Pemberton, S. G., j. A.
MacEachern, and R. Frey, 1992, Trace fossil facies models: Environmental and al
lostratigraphic significance, in Facies models, Walker, R. G., and N. P. james (eds.), Geo
logical Association of Canada, Fig. 10, p. 54. Reproduced by permission.]
1 08
Chapter 4 I Sedimentary Structures
Fi 4.4.3
Trace fossil association characteristic of the Zoophycos ichnofacies. 1) Phosiphon, 2)
Zoophycos, 3) Spirophyton. [From Pemberton, $. G., ]. A. MacEachern, and R. Frey,
1992, Trace fossil facies models: Environmental and allostratigraphic significance, in Fa
cies models, Walker, R. G., and N. P. james (eds.), Geological Association of Canada,
Fig. 11, p. 55. Reproduced by permission.]
NEREITES ICHNOFACIES
The Nereites ichnofacies is characteristic of deep water and is
apparently re
stricted to turbidite deposits. It is distinguished by complex horizontal crawl
ing and grazing traces and patteed eding or dwelling structures. The
ichnogenera are ote and complicated, such as Paleodictyon, Spirorhaphe, and
Nereites (Fig. 4.4.4). To tal diversity of traces is high, but the abundance of indi
vidual traces is low. The Nereites ichnofacies develops initially in sandy (tur
bidite) substrates but may later colonize parts of some muddy (pelagic)
deposits that form on top of sandy turbidites.
OTHER ICHNOFACIES
The
Psilonichnus ichnofacies (Fig. 4.4.5) is a softground ichnofacies developed
under nonmarine to very shallow marine or quasi-mare conditions. It is
characterized by h Y-, or U-shaped burrows of marine organisms, vertical
shafts, and horizontal tunnels of insects and tetrapods; tracks and trails of in
sects, reptiles, birds, and mammals; and root traces. The other ichnofacies list
ed in Table 4.3 are distinguished by development in firm but uncemented
substrates, rocky substrates, or woody material. Scoyenia ichnofacies, which
occur in both terrestrial and aquatic environments, are characterized by di
verse traces that include small, horizontal, curved, or tortuous feeding bur
rows, sinuous crawling traces, tracks, trails, and vertical cylindcal to
irregular shafts (see Hasiotis, 2002). The Trypanites ichnofacies develops in
fully lithified marine substrates (beachrock, rocky coasts, hardgrounds, reefs).
Traces include cylindrical, tear-, or U-shaped borings, commonly vertical to
branching, most of which are dwelling structures for suspension-feeding or
ganisms (Fig. 4.38, 1). Other structures in this ichnofacies include rasping
and saping traces made by feeding organisms, holes drilled by predatory
gastropods, and micro borings made by algae and fungi. The Glassngites ich
nofacies develops in a vaety of marine environments in firm, but unlithified,
re 4.4.4
4.4 Bedding-Plane Markin
109
Trace fossil association characteristics of the Nereites ichnofacies. l) Spirorhaphe, 2)
Uroheiminthoidea, 3) Lorenzinia, 4) Megagrapton, 5) Pa/eodictyon, 6) Nereites, 7)
Co smorhaphe. [From Pemberton, S. G., ]. A. MacEachern, and R. W. Frey, 1992, Trace
fossil facies models: Environmental and allostratigraphic significance, in Facies models,
Walker, R. G., and N. P. James (eds.), Geological Association of Canada, Fig. 12, p. 56.
Reproduced by permission.]
re 4.4.5
Tra fossil association characteristic of the Psilonichnus ichnofacies. 1) Psilonichnus, 2)
Macanopsis. [After Pemberton, S. G., j. A. MacEachern, and R. W. Frey, 1992, Trace fos
sil facies models: Environmental and allostratigraphic significance, in Facies models,
Walker, R. G., and N. P. james (eds.), Geological Association of Canada, Fig. 8, p. 53.
Re pruced by permission.]
substrates that typically consist of dewatered, cohesive muds. It is character
by vertical, cylindrical, U- or tear-shaped borings and/ or densely branch
g buows of suspension feeders or caivores such as shrimp, crabs, worms,
d pholadid bivalves (Fig. 4.38, ). Individual structures may be abundant
but diversity is low. Te redolites ichnofacies are restricted to woody substrates
(alled woodground) commonly in estuarine or very nearshore environ
mts where substantial amounts of woody material can accumulate on the
m. The traces consist of profuse club-shaped borings that may be stumpy
elongate and subcylindrical to subparallel.
11 0
Chapter 4 I Sedimentary Structures
Figure 4.39
escape or dwelling burrows thus tend to give way to horizontal feeding burrows.
This zone of the ocean is distinguished by the Cruziana ichnofacies, characterized
by
such traces as those shown in Figure 4.38, 14-18 and Fig . 4.4.2. The deep ba
thyal and abyssal zones of the ocean exist below wave base where low-energy
conditions generally prevail, although erosion and deposition can occur in ese
zones owing to turbidity currents or deep-bottom currents. Complex feeding bur
rows, such as those of the Nereites ichnofacies (Fig. 4.38, 22-26; Fig. 4.4.4), are par
ticularly common in these zones.
Although each of these marine ichnofacies tends to be characteristic of a par
ticular bathymetric zone of the ocean, as shown in figure 4.38, e now know that
individual trace fossils can overlap depth zones. No smg�e biogenic structure is an
infallible indicator of depth and environment. The basic controls on the formation
of trace fossils are not simply depth but include nature of the substrate, water en
ergy, rates of deposition, water turbidity, oxygen and slity levels toxic sub
stances, and quantity of available food (Pemberton, MacEache, and Prey, 1992).
addition to their role as envonmental indicators, trace fossils are also useful
in several other ways. They may, for example, serve as indicator of relative sedi
mentation rates based on the assumption that rapidly deposited sediments contain
latively fewer trace fossils than slowly deposited sedents. They can also help to
show whether sedentation was contuous or marked by erosional breaks, and
they provide a record of the behavior pattes of extct organisms. They may even
be useful in paleocurrent analysis; study of the orientation of restg marks of organ
isms that may have preferred to face into the current while restg establishes the pa
leocurrent-flow direction. Some trace fossils such as U-shaped burrows, which
opened upward when formed, can be used to tell the top and bottom orientation of
beds. Tr ace fossils also have biostratigrapc and conostratigraphic signcance for
zoning and correlation, and they may be useful for recotion of botmding disconti
nuities between stratigrapc successions (Pemberton, MacEachern, and Frey, 1992;
also see Fy and Pemberton, 1985, and Frey and Wheatcroft, 1989).
Stromatolites
Stromatolites are organically formed, laminated structures composed of fine silt
or clay-size sediment or, more rarely, sand-size sediment. Most ancient stromato
lites occur in limestones; however, stromatolites have also been reported in siliciclastic
sediments. Stromatolitic bedding ranges from nearly flat laminations that may be dif
ficult to dferenate from sedimentary linations of other origins to hemispherical
forms in which the laminae are crinkled or deformed to various degrees (Fig. 4.39).
The hemispherical forms range shape from biscuit- and cabbage-like forms to
Stromatolites in limestones of the Helena Forma
tion (Precambrian), Glacier National Park,
Montana.
4.4 Bedding-Plane Markings
111
col. Logan, Rezak, and Ginsburg (1964) classified these hemispherical stro-
matolites into three basic types: (1) laterally linked hemispheroids, (2) discrete,
vertically stacked hemispheroids, and (3) discrete spheroids, or spheroidal struc-
(Fig. 4.40). Laterally linked hemispheroids and discrete, vertically stacked
hemispheroids can combine in various ways to create several different kinds of
compound stromatolites. The term thrombolite was proposed by Aitken (1967)
r structures that resemble stromatolites in external form and size but lack dis-
ct laminations. The laminations of stromatolites are generally less than 1 mm
ick and are caused by concentrations of fine calcium carbonate merals, fe or-
gc matter, and detrital clay and silt. Stromatolites composed of quartz grains
have also been reported (Davis, 1968).
Stromatolites were considered true body fossils by early workers, but they
now known to be organosedentary structures formed largely by the trappg
d bdg activities of blue-green algae (cyanobacteria). They are forming today
ny localities where they occur mainly the shallow subtidal, intertidal, and
Ty pes
terally linked
hemisphoids
Discrete, vertically stacked
ispheroids
Discre sphoids
mbination forms
Description
Space-linked hemispheroids with close-linked
hemispheroids as a microstructure in the
constituent laminae
Discrete, vertically stacked hemispheroids
composed of close-linked hemispheroidal
laminae on a microscale
Spheroidal structures consisting of inverted,
stacked hemispheroids
Spheroidal structures consisting of
concentrically stacked hemispheroids
Spheroidal structures consisting of randomly
stacked hemispheroids
Initial space-linked hemispheroids passing into
discrete, vertically stacked hemispheroids with
upward growth of structures
Initial discrete, vertically stacked
hemispheroids passing into close-linked
hemispheroids by upward growth
Alternation of discrete, veically stacked
hemispheroids and space-linked
hemispheroids due to periodic sediment
infilling of interstructure spaces
Initial space-linked hemispheroids passing into
discrete, vertically stacked hemispheroids;
both with laminae of close-linked
hemispheroids
Initial discrete, veically stacked
hemispheroids passing into close-link
hemispheroids; both with laminae of close
linked hemispheroids
Vertical section of
stromatolite structure
Figure 4.40
Structures of hemispheri
cal stromatolites showing
examples of laterally
linked hemispheroids, ver
tically stacked hemispher
oids, and discrete
spheroids. [After Logan,
B. W., R. Rezak, and R. N.
Ginsburg, 1964, Classifica
tion and environmental
significance of algal stro
matolites: )our. Geology,
v. 72, Fig. 4, p. 76, and
Fig. 5, p. 78, reprinted by
permission of University of
Chicago Press.]
112
Chapter 4 I Sedimentary Structures
Figure 4.41
supratidal zones of the ocean. They have also been found in lacustrine environ
ments. Because they are related to the activities of blue-green algae, which carry
out photosynthesis, they are restricted to water depths and environments where
enough light is available for photosynthesis. The laminated structure forms be
cause fine sediment is trapped in the very fine filaments of algal mats. On ce a in
layer of sediment covers the mat, the algal filaments grow up and around sedi
ment grains to form a new mat that traps another thin layer of sediment. This suc
cessive growth of mats produces the laminated structure. The shapes of the
hemispheres are related to wa ter energy and scouring effects in the depositional
environment. Laterally linked hemispheroids tend to form in low-energy environ
ments where scouring effects are minimal. In higher-energy enviroents, scour
ing by currents prevents linking of the stromatolite heads; thus, vertically stacked
or discrete hemispheroids form. Stromatolites are formg in the world's oceans at
the present time, and they have been reported in ancient rocks as old as 3.45 bil
lion years (e.g., Hofmann et al., 1999).
Bedding-Plane Markings of Miscellaneous Origin
Mudcracks in modern sediment are downward-taperg, V-shaped fractures that
display a crudely polygonal pattern in plan view. The area between the cracks is
commonly curved upward into a concave shape. Mudcracks form in both silici
clastic and carbonate mud owing to desiccation. Subsequent sedimentation over a
cracked surface fills the cracks. In ancient sedimentary rocks, mudcracks are com
monly preserved on the tops of bedding surfaces as positive-relief fillings of the
original cracks (Fig. 4.41). Mudcracks occur in estuarine, lagoonal, tidal-flat, river
fl oodplain, playa lake, and other environments where muddy sediment is inter
mittently exposed and allowed to dry. They may be associated with raindrop or
hailstone imprints, bubble imprints and foam impressions, flat-topped ripple
marks, and vertebrate tracks (Plummer and Gostin, 1981).
Syneresis cracks are subaqueously formed markings on beddg surfaces
that superficially resemble mudcracks; however, they are discontinuous and vary
in shape from polygonal to spindle-shaped or sinuous (Plummer and Gostin,
1981). They commonly occur in thin mudstones interbedded with sandstones as
either positive-relief features on the base of the sandstones or negative-relief fe a
tures on the top of the mudstones. Syneresis cracks are subaqueous shrinkage
cracks that form in clayey sediment by loss of pore water from clays that have floc
culated rapidly or that have undergone shrinkage of swellg-clay mineral lattices
because of changes in salinity of surrounding water (Burst, 1965).
Ancient mudcracks on the surface of a rock slab (age?),
Death Va lley, California. [Photograph by james Stovall.]
4.4 Bedding-Plane Markings
113
Small craterlike pits with slightly raised rims commonly cur together with
mudcracks and are thought to be impressions made by the impact of rain
(raindro
p
imprints) or hail (hailstone imprints). They are commonly only a few
miUeters deep and less than 1 em in diameter, and they may occur as either
wid ely scattered pits or very closely spaced impressions. en they can be tmam
biguously recognized, their presence indicates subaerial exposure; however, small
ci rcular depressions created by bubbles breaking on the surface of sediment
(bubble impnts), escaping gas and so types of organic markings can be con
wih raindrop or hailstone imprints.
Kill marks are small dendritic channels or grooves that form on beaches by
discharge of po waters at low tide, or by small streams debouching onto a
sand or mud flat. They have very low preservation potential and are seldom
fo in ancient sedimentary rocks. Swash marks are very thin, arcuate lines or
small ridges on a beach formed by concentrations of fine sediment and organic
debris. They are caused by wave swash and mark the farthest advance of wave
uprush. They likewise have low preservation potential, but when found and rec
ognized in ancient sedimentary rocks, they indicate either a beach or a lakeshore
environment.
Parting lineation, sometimes called current lineation, forms on the bedding
surfaces of parallel laminated sandstones. It consists of subparallel ridges and
oves a few millimeters wide and many centimeters long (Fig. 4.42). Relief on
e ridges and hollows is commonly on the order of the diameter of the sandstone
grains. The grains in the sandstone generally have a mean orientation of their long
axes parallel to the lineation. The lineation is oriented parallel to current flow, and
its presence in ancient sandstones is useful in paleocurrent studies, although
it shows only that the current flowed parallel to the parting lineations and does
not show which of he two diametrically opposed directions was the flow direc
n. Parg lineation curs in newly deposited sands n beaches and in fluvial
environmen
ts. It is most common in ancient deposits in thin, evenly bedded sand
n. Its origin is obviously related to current flow and grain orientation, proba
bly owing to flow over upper-flow regime plane beds, but the exact mechanism by
ich parting lineation forms is poorly understood.
Figure 4.42
Parting lineation in sandstone, Haymond Formation, Te xas. Paleocurrent
flow was parallel to the lineation. [Photograph courtesy of E. F. McBride.]
114
Chapter 4 I Sedimentary Structures
4.5 OTH ER STRUCTURES
Sandstone dikes and sills are tabular bodies of massive sandstone that fill frac
tures in any type of host rock. They range in thickness from a few centimeters to
more an 10 m. They lack internal structures except for oriented mica akes and
other elongated particles that are commonly aligned parallel to the dike walls.
Sandstone dikes are formed by forceful injection of liquefied sand into fractures,
commonly in overlying rock; however, injection appears to have been downward
in some rocks. Sandstone sills are similar features that formed by injection parallel
to bedding. ese sills may be difficult or impossible to distinguish from normal
ly deposited sandstone beds unless they can be traced into sandstone dikes or be
traced far enough to show a cross-cutting relationship with other beds. Suggesd
causes of liquefaction of sand include shocks owing to earquakes or iggering
eects related to slumps, slides, or rapid emplacement of sediment by mass flow.
Secondary sedimentary structures are structures at form sometime after
deposition during sediment buriaL These structures are largely of chemical origin,
formed by precipitation of mineral substances in the pores of semiconsolidated or
consolidated sedimentary rock or by chemical replacement processes. Concretions
are probably the most common kind of secondary structure. Most concreons are
composed of calcite, but concretions composed of dolomite, hematite, siderite,
chert, pyrite, and gypsum are also known. They form by precipitation of mineral
matter around some kind of nucleus, such as a shell fragment, and gradually
build up a globular mass (e.g., the dark, rounded features in Fig. 4.6), which may
or may not display concentric layering. Shapes of these masses range from spher
ical to disc-shaped, cone-shaped, and pipe-shaped, and they may range in size
from less than 1 em to as much as 3 m. Concretions are especially common in
sandstones and shales but can occur in other sedimentary rocks.
Stylolites are suturelike seams of clay or other insoluble material that com
monly occur limestones owing to pressure solution (discussed in Chapter 6).
Less common secondary structures include sand crystals (large crystals of calcite,
barite, gypsum filled with sand inclusions) and cone-in-cone structures (nested
sets of small concentric cones composed of carbonate minerals). See Boggs (1992,
p. 119-124) for additional descriptions of secondary structures.
4.6 PA LEOCURRENT ANALYSIS FROM
SEDIMENTA RY STRUCTURES
As mentioned, many sedimentary structures yield directional data that show e
diction ancient current flowed at the time of deposition. The dip direction of
cross-bed foresets; the asymmetry and orientation of the crests of current ripples;
and the orientation of flute casts, groove casts, and current lineation are all exam
ples of directional data that can be obtained from sedimentary structures. Cross
bedding is one of the most useful sedimentary structures for determining
paleocurrent direction. Because the foreset laminae in cross-beds are generated by
avalanching on the downcurrent (lee) side of ripples, the foresets dip in the down
current direction. To measure paleocurrent direction from cross-beds requires that
they be exposed in a three-dimensional outcrop. The strike of the foreset laminae
is determined first; the dip direction is 90° to the strike. If cross-beds have been
tilted
by tectonic uplift after deposition, a correction must be made for is lt
(e.g., Collinson and Thompson, 1989, p. 200).
The orientation of directional sedimentary structures is determined in the
field with a Brunton compass by taking measurements from as many dierent out
crops and individual beds as possible and practical. The orientaon of directional
structures determined from a particular bed or stratigraphic unit commonly shows