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ICHNOFOSSILS*
Ronald McDowell, West Virginia Geological and Economic Survey
*Pronounced, “Ick! No Fossils!”
Description*
Oldest “official” trace fossil genus in the Treatise on Invertebrate Paleontology dates
from 1820, so trace fossils were being observed and described before this date . . . just
not recognized as anything worth special note.
Vertebrate trace fossils were probably recognized as actual vertebrate footprints long
before invertebrate trace fossils were associated in any way with invertebrates.
*(a good drawing in your notes is worth a thousand words)
Dinosaur trackway in the Cretaceous Dakota Sandstone, north of Morrison, CO.
From Hantzschel, 1975
COMMON BIOLOGICAL SEDIMENTARY STRUCTURES
Xiphosuran (horseshoe crab) trackway from a shale interbedded with the Pennsylvanian
Mary Lee Coal, west of Knoxville, TN.
Diplichnites - a trilobite locomotion trace. Coin is 3 cm in diameter.
Paleodictyon (upper right) - a complex feeding-dwelling trace of an unknown organism. Bac-
teria or fungi are farmed by the maker of this trace. One Russian paleontologist suggested
that Paleodictyon was a lithified tadpole egg mass (upper left) or nest. Coin is 1.5 cm in dia-
meter.
Zoophycus – an intensive feeding trace from Upper Paleozoic rocks, southern WV.
Taxonomy*
*(the part I hate)
Everything was cool in the Linnaean taxonomic sense until paleontologists started
recognizing that trace fossils weren’t actually animal (Vermes) or plant (Fucoides)
fossils. Then all hell broke loose . . .
The ICZN (International Commission on Zoological Nomenclature) ruled in the late
1980’s that trace fossils, although not actually the remains of plants or animals, are the
remains of plant or animal behaviour . . . and, thus fall under ICZN rules for naming.
Unfortunately, the ICZN didn’t have any rules in place . . . so they made some up. As
a result, we currently name trace fossils using ichnogenus and ichnospecies. There’s
been some talk of creating ichnofamilies but nothing has been formalized.
#1 Rule: Be certain that what you are naming actually is a trace fossil.
#2 Rule: Don’t give a trace fossil same name as the organism that made it.
#3 Rule: Follow all the other rules of taxonomy as if you were naming a plant or
animal.
Raccoon tracks and rain impressions in Recent flood deposits along the Solomon River, KS.
Coin is 1.4 cm in diameter.
Burrowing wasp nests in Recent desert stream deposits, north of Las Vegas, NV.
Pyritized coprolite (arrow) along with tool mark and assorted horizontal feeding trace
fossils preserved in the Devonian Foreknobs Formation, Elkins, WV. Coin is 3 cm in
diameter.
Recent crab “pellets” in tidal flat sediments, Cape Romaine, SC. Coin is 3 cm in diameter.
Aestivation (earthworm’s version of
summertime hibernation) burrow
filled with fecal pellets. Coin is 2
cm in diameter. Next slide shows a
closeup of the area enclosed with
the rectangle.
Note the annulations (arrow) where burrow wall molded against the worm’s body as
the sediment dried out.
Gordia - a fecal string from a sediment feeder. White rectangle on the bottom left is 1 cm in
length.
Recent borings in a piece
of driftwood (above) and
in a spruce log (left). Coin
is 3 cm in diameter.
This starfish impression may
be the trace fossil Asteriacites
or it may be a sitzmark (an
incidental impression of a
starfish) - you make the call.
From a shale interbedded with
the Pennsylvanian Mary Lee
Coal, west of Knoxville, TN.
Bird tracks and the illusive trace Mermia (fishtail drag marks) in Recent Green River
channel sediments, UT. Coin is 1.4 cm in diameter.
Gas bubbles escaping from Recent sediment after a clumsy geologist stepped nearby (arrow).
Coin is 1.4 cm in diameter.
Informal Classification*
Think of this an easy way of pigeonholing or grouping trace fossils. Unfortunately,
interpretation starts to creep into the process.
Informal classification schemes usually involve the preservational topology (shape) of
the trace fossil, an interpretation of the type of behaviour the trace fossil represents, or
an interpretation of the paleoenvironment in which the trace fossil was formed.
*(the part I hate a lot less)
Two preservational topology
classification schemes – top is
from Webby, 1969; left is from
Martinsson, 1970. Illustrations
are from Hallam, 1975, Figs.
4.2 and 4.4, p. 58 and 59.
Classification scheme
based on paleoethology
(animal behaviours) and
the type of trace or trace
fossil associated with
each. From Frey, 1975,
Fig. 3.2, p. 49.
Seilacher’s (1967) classification scheme based on interpreted paleoenvironment. From
Basan, 1978, Fig. 7, p. 197.
Interpretation*
Since trace fossils are usually intimately linked to the sediment containing them and
since they were formed nearly contemporaneously with the deposition of the sediment,
it’s only natural to use them to help interpret the environment of deposition.
Because trace fossils are indications of plant or animal behaviour, deciphering this
behaviour and putting it into an environmental context helps us understand the en-
vironment of deposition. And . . . looking at traces being formed today, helps us link
trace, tracemaker, behaviour, and environment. Think Uniformitarianism!!!
Trace fossils were made by organisms that were significantly more sensitive to their
environment than sedimentary grains. Consequently, trace fossils as biogenic sedi-
mentary structures have a lot more environmental information content than do non-
biogenic sedimentary structures.
Interpreting tracemaker behaviour is a chance to exercise Aktuo-Palaontologie, a term
coined by Schäfer in 1962. For ichnofossils and ichnology, this amounts to “thinking
like the tracemaker,” a concept emphasized over and over again by Seilacher.
*(aaaaah, the fun part)
Seilacher’s (1967) original depth vs complexity diagram. NOTE: in discussions, Seilacher
said repeatedly that he did not intend to imply that depth was the primary control on the type
of traces encountered in different paleoenvironments. From Basan, 1978, Fig. 7, p. 197.
from Frey and Pemberton, 1985
Trace fossil assemblages based on
observed environment or interpreted
paleoenvironment but ultimately
based on water depth.
Horizontal
Traces
Vertical
Traces
Transitional
Suspension load predominant
Bottom consolidated
- Shallow O2 penetration
- Low porosity
Traction load predominant
Bottom unconsolidated
- Relatively deep O2 penetration
- High porosity
Cruziana, Planolites,
Scalarituba, etc.
Skolithos, Diplocraterion,
Conostichus, etc.
Arenicolites, Zoophycus,
Chondrites, etc.
Grouping of trace fossils based
on ease of bioturbation and
accessibility of oxygen for
respiration. From McDowell,
1982.
Application*
*(the part that keeps us supplied with pizza and beer)
Sure, ichnology is interesting from an academic standpoint and trace fossils are fun to
look at and collect. But, once you start recognizing them and interpreting what they
mean, people will actually pay you for this information!
The information associated with trace fossils has a bearing on stratigraphy, sedimen-
tology, paleontology, geochemistry . . . and also on the coal, petroleum, and mining
industries.
Examples of Bifungites
preserved on the tops and
bottoms of beds in the
Devonian Foreknobs For-
mation, Elkins, WV.
Casestudy1 – Purely Academic
Plan View (3x)
Center of
U-tube
Variants of Bifungites, each given a different species name and in the case of the trace in the
lower right corner, a different genus name (Arthraria). What the hell is going on here? Part
of the answer lies in the type of sediment in which the trace fossil is preserved. But part of
the answer also lies in the vertical portion of this trace fossil which may not be preserved.
Distinctive arrowhead shape in extremely fine sediments
Dumbbell shape in coarser or water-logged sediment
When the sediment in which Bifungites is made is fairly cohesive and fine-grained, the barbs
of the trace fossil and indications of the vertical tube associated with the trace are well pre-
served. When the sediment in which Bifungites is made is fluid-saturated or coarse-grained,
the trace becomes poorly preserved and indistinctive to the point of being unrecognizable.
Cross-sectional View (3x)
Sediment-Sediment Interface
Sediment-Water Interface
Cross-sectional View (3x)
Sediment-Sediment Interface
Sediment-Water Interface Plan View (3x)
Center of
U-tube
Bifungites with deep u-tube
Bifungites with shallow u-tube
Dorsal view of a “generic” polychaete
worm showing the numerous chaetae
attached to segments of the worm’s
body. From Fretter and Graham, 1976,
Fig. 47, p. 163.
Polychaete worm in its burrow (Arenicolites). The worm maintains a current through the
burrow to oxygenate its gills and to feed. From Fretter and Graham, 1976, Fig. 70, p. 203.
Variations in size and shape of apical
chaetae from different polychaete
worms. Different shapes serve dif-
ferent functions. From Marshall and
Williams, 1972, Fig. 271, p. 310.
Cross-sectional View (3x)
Sediment-Sediment Interface
Sediment-Water Interface
The barbs of Bifungites may represent open cavities into which the tracemaker (a polychaete)
inserted anterior or posterior (or both) appendages. Thus “braced” against the overlying sedi-
mentary layer, the tracemaker could push its ventral surface into the underlying sediment to
maintain an open tube. Movement by the tracemaker’s chaetae would create circulation in the
“improvised” u-tube.
Scenario 1 – Bifungites as a dwelling-maintenance trace
Cross-sectional View (3x)
Sediment-Sediment Interface
Sediment-Water Interface
Anterior or posterior appendages were inserted into the barbs of Bifungites. However, in this
scenario, the tracemaker would resort to this behaviour only when a predator penetrated one
of the limbs of the u-tube, attempting to grasp the tracemaker to extract it from the burrow.
The tracemaker could resist by bracing its terminal appendages against the enclosing sediment.
Scenario 2 – Bifungites as a defensive trace
On the left, an outcrop of Devonian Brallier Fm. from a quarry just east of Brandywine, WV.
Closeup photograph shows weathered (tan) and unweathered shale from the Brallier. On the
right, an outcrop of Devonian Foreknobs Fm. from a quarry ~3 mi. east of Brandywine, WV.
Closeup photograph shows unweathered silty shale (mudstone) from the Foreknobs. If
you’re mapping these strata, how you tell them apart?
Casestudy 2 - Helping to Create a Geologic Map
Pteridichnites biseriatus – (left) illustration from a 1913 Maryland Geological Survey
publication describing the Devonian fossils of that state; (middle and right) examples
of Pteridichnites from the Devonian Brallier Formation at Elkins, WV.
Upper Devonian stratigraphic terminology for eastern WV. Lithologic descriptions from field
observations and Rossbach and Hall, 1998. Greenland Gap Group from Dennison, 1970.
Occurrence of Pteridichnites biseriatus from field observations and Rossbach and Hall, 1998.
Hampshire Formation
Greenland Gap Group
Foreknobs Formation
Red Lick
Member
Pound
Member
Blizzard
Member
Briery Gap
Member
Mallow
Member
Scherr
Formation
Nonmarine, dark red sandstones, shales, and mud-
stones w/ occasional plant fossils.
Fossiliferous marine siltstones and sand-
stones grading into nonmarine siltstones
and sandstones.
Conglomeratic medium to coarse-grained
sandstone.
Very fossiliferous, marine siltstones, sandstones,
and minor shale. Several thick sand beds in
the middle of the unit.
Conglomeratic fine to coarse-grained
sandstone.
Fossiliferous marine sandstones and
siltstones. Base is marked by first occur-
ence of medium to coarse sandstone.
Brallier
Formation Marine, turbiditic, dark grey to light olive grey
shales with interbedded siltstones. Rare inverte-
brate fossils.
Scherr is recognized as siltstone with
shale and fine sandstones, all of which
weather a light olive grey.
Devonian
FrasnianFamennian
Series Stage Stratigraphic Units General Lithology
Millboro
Shale
Marine, anoxic waters, black, carbonaceous, silty
shale with calcareous lenses and nodules. Contains
a depauperate invertebrate fauna.
Occurrence of
P. biseriatus
Acme
Zone
?
?
Hampshire Formation
Greenland Gap Group
Foreknobs Formation
Red Lick
Member
Pound
Member
Blizzard
Member
Briery Gap
Member
Mallow
Member
Scherr
Formation
Nonmarine, dark red sandstones, shales, and mud-
stones w/ occasional plant fossils.
Fossiliferous marine siltstones and sand-
stones grading into nonmarine siltstones
and sandstones.
Conglomeratic medium to coarse-grained
sandstone.
Very fossiliferous, marine siltstones, sandstones,
and minor shale. Several thick sand beds in
the middle of the unit.
Conglomeratic fine to coarse-grained
sandstone.
Fossiliferous marine sandstones and
siltstones. Base is marked by first occur-
ence of medium to coarse sandstone.
Brallier
Formation Marine, turbiditic, dark grey to light olive grey
shales with interbedded siltstones. Rare inverte-
brate fossils.
Scherr is recognized as siltstone with
shale and fine sandstones, all of which
weather a light olive grey.
Devonian
FrasnianFamennian
Series Stage Stratigraphic Units General Lithology
Millboro
Shale
Marine, anoxic waters, black, carbonaceous, silty
shale with calcareous lenses and nodules. Contains
a depauperate invertebrate fauna.
Occurrence of
P. biseriatus
Acme
Zone
?
?
Acme
Zone
?
??
?
Phyllogeny of the echinoderms
showing the connection between
asteroids, ophiuroids, and crinoids.
From Marshall and Williams, 1972,
Fig. 799, p. 834.
Transverse cross-section
through one arm of a typical
asterozoan (starfish). From
Fretter and Graham, 1976,
Fig. 146, p. 386.
Tube feet
1 cm
Illustration of single-arm locomotion
by an ophiuroid. Tube feet near the tip
of the arm are anchored in the bot-tom
sediment and then the arm is arched,
pulling the body of the animal forward.
Modified from Schäfer, Fig. 117, p.
210.
1
2
1. Ophiuroid anchors tube feet at the
distal end of one arm in the bottom
sediment.
2. Ophiuroid arches the arm and at the
same time pulls its body forward.
Enlargement of the anchored tube feet
shows that the bottom of each foot
distorts as the feet attempt to hold their
position within the sediment.
Casestudy 3 – Solving Engineering Problems in an Oilfield
Chondrites
Teichichnus
Unidentified
Bivalve
Crinoid
Pieces
Spiriferid?
Reverse grading
Reverse grading
Scour
High-angle (~30º)
crossbeds
Pay Sandstone
Siderite
Non-pay
Sandstone
Microfault
ANALYSIS OF
CORE PLUG:
He= 7.2%
kh= 0.24 mD
kv= 0.03 mD
ANALYSIS OF
CORE PLUG:
He= 13.1%
kh= 3.10 mD
kv= 0.08 mD
ANALYSIS OF
CORE PLUG:
He= 22.5%
kh= 0.05 mD
kv= 37.0 mD
Chondrites
Teichichnus
Unidentified
Bivalve
Crinoid
Pieces
Spiriferid?
Reverse grading
Reverse grading
Scour
High-angle (~30º)
crossbeds
Pay Sandstone
Siderite
Non-pay
Sandstone
Microfault
ANALYSIS OF
CORE PLUG:
He= 7.2%
kh= 0.24 mD
kv= 0.03 mD
ANALYSIS OF
CORE PLUG:
He= 13.1%
kh= 3.10 mD
kv= 0.08 mD
ANALYSIS OF
CORE PLUG:
He= 22.5%
kh= 0.05 mD
kv= 37.0 mD
Drill core from an active oilfield in WV. Production is from the Devonian Gordon sandstone
reservoir. There are problems predicting whether or not new wells will be productive and why
injection of water to increase production is so unpredictable. Can trace fossils help?
Gordon Lithofacies Mean Primary Secondary Mean Sorting Cements (in order
Permeability Porosity Porosity Grainsize of abundance)
Conglomeratic ss 4.042 mD 4.68% 0.63% 1.170 mm Poorly sorted Silica & clay
Laminated ss 3.889 mD 4.98% 1.53% 0.132 mm Well-sorted Clay & siderite
"Featureless" ss (PAY) 43.326 mD 15.98% 2.25% 0.128 mm Well-sorted Clay
Visibly Bioturbated ss 0.000 mD 0.40% 0.32% 0.340 mm Mod. well-sorted Calcite, clay, & siderite
Gordon as a whole 7.334 mD 4.95% 0.99% 0.541 mm Mod. poorly sorted Clay & siderite
Gordon Lithofacies Mean Primary Secondary Mean Sorting Cements (in order
Permeability Porosity Porosity Grainsize of abundance)
Conglomeratic ss 4.042 mD 4.68% 0.63% 1.170 mm Poorly sorted Silica & clay
Laminated ss 3.889 mD 4.98% 1.53% 0.132 mm Well-sorted Clay & siderite
"Featureless" ss (PAY) 43.326 mD 15.98% 2.25% 0.128 mm Well-sorted Clay
Visibly Bioturbated ss 0.000 mD 0.40% 0.32% 0.340 mm Mod. well-sorted Calcite, clay, & siderite
Gordon as a whole 7.334 mD 4.95% 0.99% 0.541 mm Mod. poorly sorted Clay & siderite
Here are the petrophysical characteristics of all of the different lithologies recognized in the
Gordon sandstone. The key features are porosity and permeability – that’s where the oil is
(was) and where water can be injected. But, what’s with this Featureless sandstone?
Sometimes bioturbation is obvious in thin section (upper left). Sometimes it’s not. Photo-
micrograph (upper right) shows featureless sandstone (the pay zone) from our oilfield. The
~uniform grain size makes it practically impossible to spot bioturbation. However, the elon-
gate grains (arrows) rotated 90° to bedding say “Feeding time!”
1 mm
1 c m
TOP
1 c m1 c m
TOPTOP
A piece of drill core
showing a variety of
feeding traces. The
permeability of indi-
vidual traces has been
measure by injecting
nitrogen gas. Core is
10 cm wide.
CALCITE
CLAY
SILICA
SIDERITE
SHALLOW
BURIAL
DEEPER BURIAL - COMPACTION
AND DISSOLUTION OF FELDSPARS
CALCITE
CLAY
SILICA
SIDERITE
SHALLOW
BURIAL
DEEPER BURIAL - COMPACTION
AND DISSOLUTION OF FELDSPARS
Here is the cementation history for the Gordon sandstone in our oilfield. We think that
bioturbation may have played a role in how and when the rock got cemented, but what?
EARLY LATE
Trace fossils, especially those with backfillings, may have a different cement than the
surrounding sediments. Skolithos from the Cambrian Lodore Sandstone, Jones’ Hole,
UT, are readily visible in this outcrop because of their light color. The burrow fillings
consist of quartz sandstone but lack the iron oxide cement present in the enclosing rock.
1.2 m
In this outcrop of Cambrian Flathead Sandstone, Windy Point, WY, Skolithos and Areni-
colites are readily visible because their fillings have been lost during weathering. The burrow
fillings probably consisted of quartz sandstone; the cement was probably predominantly clay.
0.15 m
“Active” Sedimentary Layer
Recently Buried
Sediment
Burrows are filled with sediment from the active sedimentary
layer. Fill may include organic matter and clay.
Sandstone
“Active” Sedimentary Layer
Recently Buried
Sediment
Burrows are filled with sediment from the active sedimentary
layer. Fill may include organic matter and clay.
SandstoneSandstone
“Active” Sedimentary Layer
Not-so-recently
Buried Sediment
Early calcite cementation seals the boundary between sedimen-
tary layers from vertical migration of pore fluids. Where vertical
trace fossils are present, this cementation may originate in prox-
imity to burrow fillings thus allowing calcite to penetrate deeper
into sedimentary layers. NOTE: once this seal is present, perme-
ability may be preserved in the buried layers.
Recently Buried
Sediment
Early Calcite
Cement
Sandstone
“Active” Sedimentary Layer
Not-so-recently
Buried Sediment
Early calcite cementation seals the boundary between sedimen-
tary layers from vertical migration of pore fluids. Where vertical
trace fossils are present, this cementation may originate in prox-
imity to burrow fillings thus allowing calcite to penetrate deeper
into sedimentary layers. NOTE: once this seal is present, perme-
ability may be preserved in the buried layers.
Recently Buried
Sediment
Early Calcite
Cement
SandstoneSandstone
Scenario 1 – Bioturbation of Sandstone w/o Interbedded Shale
Permeability within the Gordon is isolated from pore fluids
migrating across stratal boundaries. This isolation may prevent
additional cements from forming (e. g., Gordon pay sandstones) and
also acts as a barrier to vertical migration of petroleum. Horizontal
or intrastratal migration of fluids is unaffected.
Scenario 2 – Bioturbation of Sandstone w/ Interbedded Shale
“Active” Sedimentary Layer
Recently Buried
Sediment
Sandstone
Shale
Burrows are filled with sediment from the active sedimentary
layer. Fill may include organic matter and clay.
Not-so-recently
Buried Sediment
“Active” Sedimentary Layer
Recently Buried
Sediment
Sandstone
Shale
Burrows are filled with sediment from the active sedimentary
layer. Fill may include organic matter and clay.
Not-so-recently
Buried Sediment
“Active” Sedimentary Layer
Recently Buried
Sediment
Sandstone
Shale
Not-so-recently
Buried Sediment
Older
Buried Sediment
Early calcite cementation begins to seal the boundaries between
sedimentary layers from vertical migration of pore fluids. Where
vertical trace fossils are present, this cementation may originate in
proximity to burrow fillings thus allowing calcite to penetrate
deeper into sedimentary layers. In addition, where the density of
trace fossils is greater (i.e., bioturbation is more intense), the seal
is more extensive.
Early Calcite
Cement
“Active” Sedimentary Layer
Recently Buried
Sediment
Sandstone
Shale
Not-so-recently
Buried Sediment
Older
Buried Sediment
Early calcite cementation begins to seal the boundaries between
sedimentary layers from vertical migration of pore fluids. Where
vertical trace fossils are present, this cementation may originate in
proximity to burrow fillings thus allowing calcite to penetrate
deeper into sedimentary layers. In addition, where the density of
trace fossils is greater (i.e., bioturbation is more intense), the seal
is more extensive.
Early Calcite
Cement
Sandstone
Shale
Older
Buried Sediment
During diagenesis, compacted and altered shales shed dissolved
silica to pore fluids. Sandstones adjacent to interbedded shales
and not previously sealed with early cement, may be subject to
late-stage silica cementation. Additionally, clay cement may also
form.
Late Silica
Cement
SiO2
SiO2
SiO2
SiO2
Sandstone
Shale
Older
Buried Sediment
During diagenesis, compacted and altered shales shed dissolved
silica to pore fluids. Sandstones adjacent to interbedded shales
and not previously sealed with early cement, may be subject to
late-stage silica cementation. Additionally, clay cement may also
form.
Late Silica
Cement
Sandstone
Shale
Older
Buried Sediment
During diagenesis, compacted and altered shales shed dissolved
silica to pore fluids. Sandstones adjacent to interbedded shales
and not previously sealed with early cement, may be subject to
late-stage silica cementation. Additionally, clay cement may also
form.
Late Silica
Cement
SiO2
SiO2
SiO2
SiO2
SiO2
SiO2
SiO2
SiO2
Permeability within the Gordon may be isolated from pore fluids
migrating across stratal boundaries. The initial density of ichno-
fossils (i.e., intensity of bioturbation) in combination with early
calcite cementation help determine the completeness of the seal.
Relatively complete seals prevent silica-saturated fluids from mi-
grating out of diagenetically altered shale layers to form late-stage
silica and clay cements - permeability is preserved (e.g., Gordon
pay sandstones). Incomplete initial seals allow these fluids to
penetrate adjacent sedimentary layers - late-stage cementation may
remove all remaining permeability (e.g., other Gordon
sandstones). Horizontal or intrastratal migration of fluids may or
may not be effected depending on the pervasiveness of late-stage
cementation.
BIOTURBATION IS RESPONSIBLE FOR SOME OF THE RE-
SERVOIR’S UNPREDICTABLE PRODUCTION AND INJEC-
TION BEHAVIOUR – IT HAS ADDED HETEROGENEITY!
Scenario 1 – Bioturbation of Sandstone w/o Interbedded Shale
Scenario 2 – Bioturbation of Sandstone w/ Interbedded Shale