Anomalously diverse Early Triassic ichnofossil assemblages in
northwest Pangea: A case for a shallow-marine habitable zone
Tyler W. Beatty1, J-P Zonneveld2, Charles M. Henderson1
1
Department of Geoscience, University of Calgary, Calgary, Alberta T2N 1N4, Canada
Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
2
ABSTRACT
Early Triassic trace fossil assemblages from the northwest margin of Pangea record a
diverse suite of postextinction infauna. These ichnofossil assemblages occurred within welloxygenated, shallow-marine refuges in a Panthalassa Ocean otherwise characterized by widespread anoxia. We propose an environmentally controlled model for their distribution, in
which wave aeration, enhanced by frequent storms, gave rise to an optimal zone for benthic
colonization. Within this habitable zone extinction pressures were ameliorated and postextinction recovery duration was minimized.
Keywords: Canadian arctic, Permian-Triassic extinction, ichnofossils, anoxia.
INTRODUCTION
The relationship between oxygen stress
and the Permian-Triassic (P-T) boundary
has generated a great deal of interest among
geoscientists. This interval saw unparalleled
extinction in the marine realm, and resulted
in extensive restructuring of communities
during the postextinction recovery (Erwin,
1993). Although debate continues regarding
an ultimate cause (see Knoll et al., 2007, for a
review), the marine Late Permian mass extinction is closely related to the development of
anaerobic and dysaerobic facies, which episodically extended into shallow-marine settings during the Early Triassic (Wignall and
Hallam, 1992; Wignall et al., 1998; Twitchett,
1999; Pruss and Bottjer, 2004; Hays et al.,
2007). Multiple lines of evidence, including
ichnologic data, have been used to implicate
prolonged shallow-marine anoxia in both
the extinction and in delaying postextinction
faunal recovery (e.g., Wignall and Hallam,
1992; Wignall and Twitchett, 1996). These
studies document a reduction in number,
diversity, and size of body and trace fossils
and a concomitant reduction in epifaunal and
infaunal tiering (Ausich and Bottjer, 1982;
Twitchett and Wignall, 1996).
On the northwest margin of Pangea (northern and western Canada; Fig. 1), Early Triassic records of benthic marine fauna are highly
variable, and this is particularly true of the
ichnofossil record. This aspect of the Early
Triassic strata of Pangea forms the principal
focus of our study. Herein we document trace
fossil assemblages within shallow-marine successions and link them to regional physiogeographic properties. In several locations anomalously diverse ichnofossil assemblages from
Griesbachian shoreface environments provide
evidence of oxygenated refuges for benthic
marine organisms.
3
2
1
30°
4
Panthalassa
Paleotethys
Panthalassa
30°
Figure 1. Paleogeographic locations of studied areas on simplified map of Pangea for
Induan stage (modified from Golonka et al.,
1994). Geographic locations: 1—Sverdrup
Basin, Nunavut–Borup Fiord section, lat
81.0°N, long 81.5°W; North Hamilton Peninsula section, lat 80.2°N, long 81.5°W;
Confederation Point section, lat 80.4°N,
long 87.2°W. 2—Liard Basin, Northwest
Territories–Liard River section, lat 60.1°N,
long 124.1°W. 3—Peace River Embayment
of Western Canadian Sedimentary Basin–
Ursula Creek section, British Columbia, lat
56.1°N, long 122.4°W; Kahntah Field, British
Columbia, lat 58.0°N, long 120.2°W; Crooked
River Field, Alberta, lat 55.2°N, long 117.5°W.
4—Kananaskis, Alberta–Hood Creek section, lat 50.4°N, long 115.0°W.
GEOLOGIC SETTING
Outcrop and subsurface exposures of P-T
strata occur in several basins of northern and
western Canada. These successions were deposited along north- and west-facing shorelines on
the northwest margin of Pangea (Fig. 1). During
the P-T transition, these shorelines underwent
a widespread latest Permian transgression,
with maximum coastal onlap during the earliest Triassic (Henderson, 1997; Henderson and
Baud, 1997). Here the P-T interval records
an overall shift from Permian sandy carbonate- and chert-dominated sequences to Triassic
siliciclastic-dominated sequences coincident
with a substantial increase in regional sedimentation rates. Induan (Griesbachian–Dienerian)
sedimentary successions range from ~100 m in
Alberta to >450 m in the Canadian arctic. Conodonts (see GSA Data Repository Table DR11)
provide biostratigraphic control for the studied stratigraphic sections (Henderson, 1997;
Henderson and Baud, 1997; Beatty et al., 2006;
Hays et al., 2007) and palynological data supplement these results (Utting et al., 2005).
Studied sections represent shorelines from
the margins of large continental embayments
with relatively wide shallow shelves. Shoreline
deposits are predominantly fine grained, comprising laminated mudstone to siltstone in offshore
settings and fine- to very fine-grained sandstone
in shoreface settings. Skeletal debris is relatively
uncommon, comprising fragmentary remains
of inarticulate brachiopods, bivalves, and rare
ammonoids. Lower shoreface and offshore transition settings commonly preserve hummocky
cross-stratified bedforms and, based on climate
modeling of Golonka et al. (1994), this region
was likely seasonally storm dominated.
EARLY TRIASSIC TRACE FOSSILS
AND ICHNOFABRICS FROM
NORTHWEST PANGEA
In contrast to other trace fossil studies across
the P-T interval (Twitchett, 1999; Twitchett
et al., 2001; Pruss and Bottjer, 2004), many of
the Induan ichnofossil occurrences documented
herein display high degrees of both diversity
and bioturbation intensity (Table 1). It is significant that in each of these occurrences peak
ichnofossil diversity and bioturbation intensity
are associated with a narrow range of sedimentary environments (i.e., lower shoreface
to offshore transition, and isolated event beds
in offshore settings; Table 1). Distal to these
environments, bottom-water anoxia is indicated by laminated facies that contain abundant
framboidal pyrite and are characterized by rare
occurrences of diminutive (<5 mm) horizontal burrows such as Planolites and low ichnofabrics [level 1–2 in either the semiquantitative
ichnofabric index (ii) of Droser and Bottjer
(1986) or bedding plane bioturbation index
(bpbi) of Miller and Smail (1997)]. This pattern
1
GSA Data Repository item 2008200, Table DR1,
conodont, facies, and sedimentary environment data,
is available online at www.geosociety.org/pubs/
ft2008.htm, or on request from editing@geosociety.
org or Documents Secretary, GSA, P.O. Box 9140,
Boulder, CO 80301, USA.
© 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
GEOLOGY,
October
2008
Geology,
October
2008;
v. 36; no. 10; p. 771–774; doi: 10.1130/G24952A.1; 3 figures; 1 table; Data Repository item 2008200.
771
TABLE 1. ICHNOFOSSIL OBSERVATIONS
Location
Borup Fiord,
Nunavut
North Hamilton
Peninsula, Nunavut
Confederation Point,
Nunavut
Liard River,
Northwest Territories
Kahntah Field,
British Columbia
Crooked River Field,
Alberta
Hood Creek,
Alberta
Unit
Blind Fiord
Formation
Bjorne
Formation
Blind Fiord
Formation
Bjorne
Formation
Blind Fiord
Formation
Toad
Formation
Montney
Formation
Montney
Formation
Sulphur Mountain
Formation
Age*
C-G
Facies†
Offshore shale
Diversity§
L
Fabric#
ii: 1-2
ii: 5-6
bpbi: 5
ii: 4
bpbi: 5
G
Lower shoreface sandstone
H
G
Lower shoreface sandstone
H
G
Upper shoreface sandstone
M
D
Offshore shale
L
ii: 1
D
Gravity flow sandstone
M
bpbi: 4
G
Lower shoreface sandstone
H
bpbi: 5
G
Offshore transition shale
M
ii: 1
G
Lower shoreface sandstone
H
ii: 3-4
G
Lower shoreface sandstone
M
ii: 1 or 5
ii: 1
G-D
Offshore transition shale
L
D
C-D
Offshore transition sandstone
Distal offshore shale
M
L
ii: 2
ii: 4
Ursula Creek,
Grayling
ii: 1
British Columbia
Formation
*C—Changhsingian; G—Griesbachian; D—Dienerian; G-D—Induan. Supporting biostratigraphic data provided
in Table DR1 (see footnote 1).
†
Supporting lithologic and sedimentologic data provided in Table DR1.
§
Number of ichnotaxa present: L, low = 0–2 taxa; M, moderate = 3–10 taxa; H, high = >10.
#
bpbi is bedding plane bioturbation index (Miller and Smail, 1997); ii is ichnofabric index (Droser and Bottjer, 1986).
of poorly to unbioturbated offshore sediments
and moderately to highly bioturbated shoreface
sediments is consistent through many of the
studied sections, regardless of chronological
position within the Induan (Table 1).
This pattern of distribution is well illustrated
in the Borup Fiord section, Canadian arctic
(Fig. 1), where there is only meager evidence
of benthic colonization within the basal 60 m
of the late Changhsingian–Griesbachian Blind
Fiord Formation (Table 1; Fig. 2A). Overlying Griesbachian sandstones deposited in a
lower shoreface setting are highly bioturbated
(Table 1), with a diverse and robust trace fossil
assemblage dominated by dwelling structures
including Spongelliomorpha, Arenicolites, and
B
A
C
D
Figure 2. Core and outcrop photos of Griesbachian strata from northwest Pangea. A: Laminated silty shale and siltstone with rare Planolites burrows, distal offshore environment,
Borup Fiord section. B: Sole of intensely burrowed fine-grained sandstone bed, lower
shoreface-offshore transition environment, Borup Fiord section; large burrow is Spongelliomorpha. C: Bioturbated muddy sandstone, lower shoreface environment, well a-081-A/94-I-3,
Kahntah Field; core face is 12 cm wide. D: Bioturbated and laminated muddy sandstone,
lower shoreface environment, well 15–34–72–25w5, Crooked River Field; core face is 12 cm
wide; black arrow indicates diffuse lower boundary of intensely bioturbated horizon (ichnofabric index 5), white arrow indicates abrupt cessation of intense horizontally burrowed horizon. Overlying muddy laminations (ichnofabric index 1) contain biomarkers indicative of
euxinic conditions (Hays et al., 2007).
772
Rhizocorallium (Fig. 2B). A similar pattern is
observed in Griesbachian strata of the Montney
Formation in the Kahntah River area (Fig. 1),
where offshore successions are depauperate
(with the exception of event beds) and offshore
transitions to lower shoreface successions contain a diverse suite of trace fossils (~24 ichnogenera; Table 1; Fig. 2C). A notable exception
occurs in the Crooked River area, where shallowmarine deposits are characterized by wellpreserved physical sedimentary structures and
bioturbation is limited to discrete but intensely
burrowed horizons (Table 1; Fig. 2D). Biomarker analyses of the Crooked River sediments
reveal lipids diagnostic of anoxygenic photosynthetic bacteria (chlorobiaceae), indicating that
shallow-water photic zone euxinic conditions
were common here during the Induan stage
(Hays et al., 2007). Thus, repeated poisoning
by upward expansion of the euxinic chemocline
limited benthic colonization in this area.
Storm deposits (tempestites) or other sediment
gravity flow deposits emplaced within offshore
settings also contain anomalously high-diversity
or high-intensity (ii ≥4) trace fossil assemblages.
Griesbachian strata deposited in the offshore transition zone in the Kahntah Field, northeast British
Columbia, comprise numerous storm-deposited,
hummocky cross-stratified sandstones, the tops
of which are often intensely burrowed (ii 4–5)
by a diverse suite of trace fossils dominated by
arthropod-constructed forms (e.g., Thalassinoides and Rhizocorallium). Interbedded silty
mudstones are poorly bioturbated and trace fossils are mostly small (burrow diameter ≤5 mm),
horizontally oriented deposit-feeding structures
such as Planolites. Dienerian strata of the Blind
Fiord Formation exposed at Confederation Point,
Canadian arctic (Fig. 1), exhibit a similar trend.
These strata are dominated by laminated, largely
unbioturbated, silty shale deposited in a distal
offshore shelf to slope setting (Embry, 1986).
Rare, massive, very fine-grained sandstone beds,
interpreted as storm-generated sediment gravity
flows, occur in this section. Event beds ≥25 cm
thick are commonly intensely burrowed by lowdiversity or monospecific ichnofossil assemblages (e.g., Haentzschelinia horizon, bpbi 4).
These assemblages are considered to be a result
of burrowing by entrained and transported endobenthos (sensu Föllmi and Grimm, 1990), and
serve as a proxy for the presence of shallower,
more oxygen-rich settings.
HABITABLE ZONE
Deep oceanic waters were stratified and oxygen depleted during the P-T interval (Wignall
and Twitchett, 1996; Isozaki, 1997), thus
Panthalassa Ocean waters were dominantly
aerated by diffusion from the atmosphere.
This produced a vertically restricted oxygenated surface layer within which planktic and
GEOLOGY, October 2008
GEOLOGY, October 2008
UPPER
SHOREFACE
LOWER
OFFSHORE
DISTAL
SHOREFACE TRANSITION OFFSHORE/SLOPE
Ben
thic
BASIN
stress
oxygen
Infa
unal
di
INCREASING
SWASH
versity
Wave stress
Wave aerated
Diffusion aerated
DYSOXIC
Lower Triassic
nektic organisms survived. In modern settings
the rate of diffusion depends on ocean surface
processes. Breaking waves have the greatest effect on air-sea gas exchange, as injected
bubbles significantly increase the interface area
(Wallace and Wirick, 1992; Chanson and Cummings, 1994). The most common environment
for generating breaking waves is the shoreface
under fair-weather conditions, expanding into
the offshore transition during storm-weather
conditions. Wave base thus is an important factor in delineating the habitable zone (Fig. 3).
Wave stress, however, has a significant influence on benthic organism behavior; increased
bottom shear energy and water turbidity, and
shifting sediment substrates, inhibit suspension
feeding and suppress deposit-feeding infauna
(e.g., Howard, 1972). Trace fossil preservation
potential also decreases with increasing wave
energy (Fig. 3). This upper limit of the optimal
zone for colonization and preservation is comparable to that in a normally oxygenated ocean
throughout much of the Phanerozoic.
The habitable zone was well developed along
the northwest margin of Pangea, in part due
to the influence of frequent storms on a broad
continental shelf. Wallace and Wirick (1992)
observed that breaking waves in 38 m of water
were capable of creating supersaturated conditions with respect to oxygen to depths of at least
19 m, with the effects of large breaking-wave
events lasting weeks. These observations serve
only as a qualitative guide for the P-T period,
when atmospheric oxygen levels may have been
significantly lower and sea-surface temperatures
significantly higher than today. Even at reduced
atmospheric concentrations it follows that
the degree and depth of oxygen saturation in
oceanic surface waters would have been greatest after a storm episode. The width of the shelf
was also likely an important contributing factor to the development of a habitable zone. The
most diverse and robust of the trace fossil refugia identified in this study occur within basins
that are situated in large embayments on the
northwestern margin of Pangea (i.e., Sverdrup
Basin, Liard Basin, Peace River Basin). It was
likely a feature of these broad shallow shelves
that storm-aerated waters maintained sufficient
oxygen levels for persistent benthic colonization
during inter-storm periods.
The proposed habitable zone model accounts
for the occurrence and preservation of robust
and diverse ichnofossil assemblages in the Early
Triassic of northwest Pangea (Fig. 3). Previous studies demonstrated that ocean stratification and the development of anoxic and euxinic
conditions greatly limited benthic colonization
during the Induan. Along the northwest Pangea margin, however, we observe that specific
marine environments were capable of supporting
robust communities of endo-benthic organisms.
FWWB
BF
LR
K
NH
Wave stress/poor
preservation
SWWB
HC
EUXINIC?
CP
UC
Habitable zone
Transported/
rare fauna
Extremely rare/
absent
Figure 3. Schematic cross section of typical shoreline from northwest Pangea and relative
environmental position of studied sections (no horizontal or vertical scale implied). NH—
North Hamilton Peninsula, K—Kahntah Field, LR—Liard River, BF—Borup Fiord, HC—Hood
Creek, CP—Confederation Point, UC—Ursula Creek. Habitable zone is depicted with respect
to shoreface position, infaunal diversity peak, and oxygen and wave stress. FWWB—fairweather wave base, SWWB—storm-weather wave base.
The habitable zone provides an explanation for
the occurrence of such benthic communities by
accounting for the physical conditions under
which the development and persistence of an
oxygenated water column can occur.
IMPLICATIONS
An important implication of the habitable
zone concept is that, given the right conditions,
available ecologic niches were occupied in the
immediate aftermath of the terminal Permian
mass extinction. Theoretical models suggest
that niches emptied by mass extinction should
refill rapidly after extinction stresses ameliorate
(Erwin, 2001). This reoccupation is initially
exponential, but slows as environmental carrying capacity is approached (Erwin, 2001). The
habitable zone model fulfills these theoretical
expectations wherein the offshore exemplifies the delay (stress has yet to ameliorate) and
the shoreface is quickly inhabited. This depthstratified recovery indicates that oxygen stress
rather that low food resources (Twitchett, 1999)
or reduced biotic diversity (Erwin, 1993) was
the dominant control on colonization.
Habitable zones are not distributed equally
around the world. In many areas, perhaps the
entire tropical Paleotethyan region, successions where habitable zones might occur are
instead characterized by well-developed microbialite facies (e.g., Lehrmann, 1999). Although
a definitive explanation for this trend is uncertain, the reduced oxygen-carrying capacity of
warm ocean waters may have been a control-
ling factor. Thus, successions characterized by
habitable zones may be limited to middle- and
high-latitude coastal areas.
Another implication of the habitable zone
can be drawn from the apparent selection for
arthropod-produced trace fossils. Arthropods
are the most sensitive to oxygen stress among
the common marine invertebrates (Diaz and
Rosenberg, 1995). Thus, trace fossils produced
by marine arthropods are expected to be rare
and sparsely distributed in earliest Triassic
shallow-marine successions. Our findings indicate that earliest Triassic shallow-marine successions on the northwestern margin of Pangea
were characterized by ichnofaunas dominated
by diverse and abundant arthropod-constructed
forms (Beatty et al., 2005). Their apparent dominance in the habitable zone is likely due to two
factors: (1) they are tolerant of higher energy
environments, and more important, (2) oxygen
stress was minimal in the habitable zone. It follows that a steep oxygen gradient existed immediately distal of the habitable zone because
typical low-oxygen-tolerant ichnofossils such
as Chondrites and Zoophycos (Bromley and
Ekdale, 1984) are absent, and temporary replacement of these forms by diminutive Planolites
suggests a diminished dysoxic biocoenose (e.g.,
Wignall et al., 1998).
CONCLUSIONS
Northwest Pangea localities record intensely
bioturbated intervals in the aftermath of the
Late Permian extinction despite overwhelming
773
evidence for widespread shallow-marine anoxia
at that time. Our data show that endo-benthic
colonization patterns are strongly controlled by
environmental conditions, and lower shoreface–
offshore transition environments record the
greatest amount of benthic colonization. This
colonization window is designated as the
habitable zone. Lithologies deposited in depositional environments immediately distal to the
offshore transition are characterized by few trace
fossils, indicating that resident infauna were
rare to absent in these areas. Wave aeration, and
consequent suppression of anoxia, in shallowmarine settings is the root cause for the existence
of the habitable zone. Shorelines on the margins
of basins associated with large marine embayments along northwest Pangea frequently hosted
these ichnofaunas. The broad, storm-dominated
shelf of these shorelines also promoted development of habitable zones. Shorelines on the open
Panthalassa coast were frequently poisoned by
anoxic waters during relative changes in sea
level and had reduced ability to develop a persistent resident benthic community. Thus, areas
buffered by broad shelves were more likely to
support long-term populations of infaunal and
epifaunal organisms. The habitable zone concept,
when adjusted for differences in basin configuration and paleoceanographic conditions, provides a useful model for explaining apparently
anomalous occurrences of ichnofossils in Early
Triassic successions. Latest Permian extinction
levels are reduced and the recovery time is minimized within the oxygenated habitable zone,
supporting the interpretation that oxygen stress
was a major cause of the delay in recovery from
Earth’s greatest extinction.
ACKNOWLEDGMENTS
Our research was funded by a Discovery Grant to
Henderson from the Natural Science and Engineering
Research of Canada (NSERC), the Polar Continental Shelf Project, and a student research grant from
the American Association of Petroleum Geologists.
We thank Benoit Beauchamp for logistical support
in arctic Canada. Beatty acknowledges scholarships
from the Province of Alberta and NSERC. Sara
Pruss, Paul Wignall, and Yukio Isozaki critically read
the manuscript, and their comments are gratefully
acknowledged.
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Manuscript received 24 March 2008
Revised manuscript received 10 June 2008
Manuscript accepted 16 June 2008
Printed in USA
GEOLOGY, October 2008