Geological Society of America Special Papers Canyon

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Geological Society of America Special Papers
The Neoproterozoic Earth system revealed from the Chuar Group of Grand
Canyon
Carol M. Dehler, Susannah M. Porter and J. Michael Timmons
Geological Society of America Special Papers 2012;489;49-72
doi: 10.1130/2012.2489(03)
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Notes
© 2012 Geological Society of America
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The Geological Society of America
Special Paper 489
2012
The Neoproterozoic Earth system revealed from
the Chuar Group of Grand Canyon
Carol M. Dehler*
Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84333, USA
Susannah M. Porter*
Department of Earth Science, University of California at Santa Barbara, Santa Barbara, California 93106, USA
J. Michael Timmons
New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, New Mexico 87801, USA
roborate data from rock units around the world, suggesting that at
this time the supercontinent Rodinia was breaking up, Earth’s climate was undergoing glacial and interglacial cycles, there were
massive perturbations to the global carbon cycle, and singlecelled protists were diversifying. Its rich, well-preserved record
is one of the few well-dated successions of this time period, making the Chuar Group a world-class “type section” for this pivotal
interval in Earth system history (Karlstrom et al., 2000). Here we
present an overview of current knowledge on the environmental,
climatic, biological, and tectonic context of the Chuar landscape
as it was ~800–740 m.y. ago and discuss how the Chuar Group
rock record has implications for global change during this time.
INTRODUCTION
The Chuar Group is known today for its beautiful patterns
of Martian-like colors, most commonly seen from the air or the
canyon rims. Now far inboard from the ocean and framed within
the eastern Grand Canyon, it is hard to imagine that these strata
represent part of a calm ocean inlet near the equator during Neoproterozoic time. Field geologic data indicate that the Chuar sea
was affected by tides and waves and was in a seismically active
basin. The repetitions in stratigraphic patterns indicate that sea
level slowly rose and fell in tempo with global changes in climate. Although animals would not appear for another ~200 million years (m.y.) and land plants for another ~300 m.y., fossil
data show that these shallow ocean waters were teeming with
single-celled life, most of it microscopic. This is the scenario,
ca. 750 Ma (mega-annum, or millions of years ago), that is
revealed from recent research on the striking strata of the Chuar
Group in eastern Grand Canyon.
Ongoing and recent research on Chuar Group rocks not only
provides insight about the Chuar basin, but it also contributes
to our understanding of the greater Earth system during midNeoproterozoic time. Tectonic, stratigraphic, sedimentologic,
geochemical, and paleontologic studies of the Chuar Group cor*E-mails: [email protected]; [email protected].
GEOLOGIC BACKGROUND
The Chuar Group is exposed exclusively in several rightbank, east-flowing tributaries to the Colorado River in eastern
Grand Canyon, Arizona, USA (Fig. 1; Sheet 1, map on inserts
accompanying this volume1). This exposure is bounded on the
east by the Butte fault zone (East Kaibab monocline system),
and on all other sides by the “Great Unconformity” marked by
the overlying Cambrian Tapeats Sandstone (Fig. 2). Locally,
the Chuar Group is overlain by the Neoproterozoic Sixtymile
1
Geologic Map of Eastern Grand Canyon, Arizona is also available as GSA
Data Repository Item 2012287, online at www.geosociety.org/pubs/ft2012.
htm, or on request from [email protected] or Documents Secretary, GSA,
P.O. Box 9140, Boulder, CO 80301-9140, USA.
Dehler, C.M., Porter, S.M., and Timmons, J.M., 2012, The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon, in Timmons,
J.M., and Karlstrom, K.E., eds., Grand Canyon Geology: Two Billion Years of Earth’s History: Geological Society of America Special Paper 489, p. 49–72,
doi:10.1130/2012.2489(03). For permission to copy, contact [email protected]. © 2012 The Geological Society of America. All rights reserved.
49
Approximate trace of Laramide monocline
Laramide monocline
Re
d
ine
n
d
35° 58'N
111° 47'W
Pal
is a
N
es
ult
Figure 1. Location map showing outcrop extent of the Grand Canyon Supergroup and major tectonic elements in eastern Grand Canyon (modified from Timmons et al., 2001).
35° 58'N
112° 35'W
n
h
Vis
n
yo
an
C
u
Buried Proterozoic fault
10 km
Proterozoic monocline
Proterozoic normal fault, ball on the
downthrown side
Granite Gorge Metamorphic Suite
m
a
te f
Unkar Group
on
ocl
But
Cardenas Basalt and Diabase
bab
B
r
ig
ht
A
n
ge
l
111° 47'W
36° 25'N
50
Chuar Group
K7-115-3
Bass Canyon
ai
East K
Ca
ny
o
112° 35'W
36° 25'N
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Dehler et al.
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The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon
mi. 54
1b
eap
2
Pz
4b
3
mi. 55
6
Y
X
4a
rs
ua
Ch
an
ko w
1a
ek
Cre
N
mi. 56
5
yn
n
cli
e
6
12
Pz
51
7B
9 8
0
10
1
11
Pz
mi. 57
7
mi. 58
k
ee
Cr
M
bi
atu
Aw
Sixtymile
5
A
Unkar
Meso-
1
line
4
Lava Chuar Creek
9a
9b
9c
Ba
n
yo
an
lt C
1
2 kilometers
Pz
mi. 64
mi. 65
Pa
li
sad
10
mi. 66
mi. 67
11
0
mi. 63
sa
2 3a
Laramide syncline
Measured section (see text), number in square
linked to section line from Dehler et al. (2001)
Pz
Little Colorado
River
A’
ne
cli
Proterozoic normal fault (ball on
downthrown side)
Laramide monocline
2
3
A
Proterozoic syncline
mi. 61
mi. 62
Line of
cross section
Zn Nankoweap Formation
Yc Cardenas Basalt
Yd Dox Formation
mi. 60
o
Kaibab Mon
East
Chuar Group
Galeros Kwagunt
Pz
sync
Walcott Member
Awatubi Member
Carbon Butte Member
Duppa Member
Carbon Canyon Member
Jupiter Member
Tanner Member
ar
Zkw
Zka
Zkc
Zgd
Zgc
Zgj
Zgt
8
Z
Chu
Neoproterozoic
Zs Sixtymile Formation
ver
Paleozoic rocks undifferentiated
Butte fault
Pz
mi. 59
alg
os
a
EXPLANATION
Ri
rado
Colo
nt
gu
a
Kw
es
fau
lt
Pz
Tanner
graben
mi. 69
mi. 68
mi. 70
Figure 2. Geologic map of the Chuar Group with measured section and cross-section locations (modified from Timmons et al., 2001).
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basalt
crystalline
basement
interbedded sandstone and
(or) siltstone beds <1 m thick
dolomite and (or) silty dolomite
beds typically <1 m thick
vase-shaped microfossil
acritarch
xxx
ash bed
Carbon
Butte
Mbr
mafic dikes and sills
prolonged subaerial exposure
unconformity
paleochannel
denotes diagnostic tidal feature
flaky dolomite
Baicalia-Boxonia
white sandstone
basal red sandstone
**
Baicalia
(UPPER)
CARDENAS
BASALT
mr-var
ss-1
dol-mas
dol-lam
Ba-Box
mr-var
ss-1
ss-1&2
mr-var
mr-dark
ss-1
dol-mas
dol-lam
Baic
polygonal marker bed
mr-var
mr-dark
ss-lam
dol-mas
dol-lam
*
*
mr-var
mr-dark
ss-lam
Stratifera/Inzeria
Inzeria
mr-var
mr-dark
ss-lam
NANKOWEAP
FORMATION
UNKAR
GROUP
Mesoproterozoic
flaky
dol.
mr-dark
Tanner dolomite
?
crystalline
basement
dol-lam
pis/ooid
Tanner
Member
100
0
400 m
upper dolomite couplet mr-dark
lower dolomite couplet dol-mas
*
200
meters
GRAND CANYON SUPERGROUP
Nko.
CHUAR GROUP
UNKAR GROUP
Fm
Six.
Fm.
Tonto
Gp.
CHUAR GROUP
*
GALEROS FORMATION
large mudcrack
Middle Neoproterozoic
large cross-beds
karsted dolomite
742±6 Ma
d-crse
1070±70 Ma
intertidal
supratidal
x x x
FACIES
stromatolites
breccia
Sixtymile Fm
Jupiter Member
sandstone
dolomite
?
Kwagunt Formation
Awatubi Walcott Member
Member
black mudrocks
distal subtidal
Neo
variegated and black mudrocks
Dehler et al.
(Basal) Tapeats
Sandstone
proximal subtidal
C
Duppa
Carbon Canyon Member
Member
52
PALEOENVIRONMENT
Figure 3. Generalized stratigraphic column (on left) of the Chuar Group, showing relationships with underlying
and overlying units. To right, composite measured section of the Chuar Group, including facies interpretations
from Dehler et al. (2001).
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The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon
53
Formation and rests upon the Nankoweap Formation (Figs. 2 and
3). These strata are gently folded by the north-trending Chuar
syncline, which parallels the trace of the Butte fault (Figs. 1
and 2). The Chuar Group is also present in the subsurface along
the Arizona-Utah border where three wells have penetrated as
much as 700 m of Chuaria-bearing mudrock interbedded with
dolomite and sandstone (Rauzi, 1990; Wiley et al., 1998). Chuar
strata equivalents are found regionally in northern Utah (Uinta
Mountain Group and Big Cottonwood Formation), in Death Valley, California (middle Pahrump Group), and in northern Canada
(Little Dal and Coates Lake Groups; Link et al., 1993; Dehler
et al., 2001, 2010; Dehler, 2008). Strata of Chuar age are found
on most continents; the Australian equivalents may be closely
related to the Chuar Group, as these two continents may have
been juxtaposed during deposition (Karlstrom et al., 1999).
The Chuar Group is a 1600-m-thick, apparently conformable, fossiliferous, unmetamorphosed succession composed of
~85% mudrock, with interbedded meter-thick sandstone and
dolomite beds (Fig. 3). It makes up most of the upper Grand Canyon Supergroup and is subdivided into the Galeros Formation
(lower) and the Kwagunt Formation (upper), with the contact at
the base of the prominent, thick sandstone unit of the Carbon
Butte Member (Ford and Breed, 1973a) (Fig. 3). The Galeros
Formation is further divided into the Tanner, Jupiter, Carbon
Canyon, and Duppa Members; the Kwagunt Formation is divided
into the Carbon Butte, Awatubi, and Walcott Members (Ford and
Breed, 1973a).
Multiple data sets collectively indicate that the Chuar Group
is mid-Neoproterozoic in age (ca. 800–742 Ma). The age of the
basal Chuar Group is constrained by preliminary U-Pb analyses
of diagenetic monazite in the Tanner Member that indicate that the
base of the Chuar Group is ca. 800 Ma (Williams et al., 2003). A
direct age for the top of the Chuar Group comes from a U-Pb zircon date from an ash bed of 742 ± 6 Ma (Karlstrom et al., 2000).
Chuar Group paleomagnetic data, which record the original magnetization of the rocks acquired during their deposition and thus
their latitude at that time, correlate with other Neoproterozoic
successions from North America, indicating that Chuar deposition occurred between ca. 850 and ca. 740 Ma at paleolatitudes of
5°–20° north of the equator (Weil et al., 2004). These constraints
are consistent with Chuar stromatolites and microfossils, which
correlate with middle Neoproterozoic fossiliferous successions
elsewhere (Vidal and Knoll, 1983; Vidal and Ford, 1985; Porter
and Knoll, 2000; Nagy et al., 2009). Microfossils and carbonisotope composition suggest robust correlation with the Red Pine
Shale, Uinta Mountain Group, Utah, which is known to be no
older than 770 Ma on the basis of detrital zircons in an informal
formation lower in the group (Dehler et al., 2007, 2010).
and 1890s. He described the Chuar “terrane,” discovered and
named the microfossil Chuaria circularis, and noted the presence of stromatolites (Walcott, 1894, 1899). A more recent comprehensive study of the Chuar Group strata was conducted in the
late 1960s and early 1970s (Ford and Breed, 1973b and references therein), yielding information on the general stratigraphic,
sedimentologic, and paleontologic characteristics. These workers
and most others have interpreted all or part of the Chuar Group
to represent a nearshore protected marine setting (Cook, 1991;
Dehler et al., 2001; Ford and Dehler, 2003; Vogel et al., 2005).
Sedimentary structures are the foremost features for interpreting Chuar paleoenvironments. The best preserved and most
abundant sedimentary structures are in the middle part of the
Chuar Group: the upper Carbon Canyon Member, the Carbon
Butte Member, and the lower Awatubi Member (Fig. 3). One
of the most informative and common sedimentary features are
ripple marks that are symmetrical in cross section (Fig. 4). These
are visible on bedding planes of dipping sandstone beds along the
THE CHUAR SEA
Figure 4. Photographs of symmetrical ripple marks from sandstone
units in the middle Chuar Group. The shape of the ripple marks requires oscillatory flow and suggests deposition along a shoreline.
(A) Mud cracks and ripple marks on the sole of a bed from the Jupiter
Member. These associated features most likely formed in a tidal setting. (B) Ripple marks in the Carbon Butte Sandstone.
Research on the Chuar Group strata started in the late 1800s
and continues today. Charles D. Walcott was the first to study the
stratigraphy and paleontology of the Chuar Group, in the 1880s
A
B
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54
Dehler et al.
Butte fault, especially between Carbon and Lava Chuar Canyons.
The symmetrical shape of these ripple marks requires a backand-forth flow of water. Therefore, we know these sediments
(silt and sand) were subjected to oscillatory currents within the
wave zone along a shoreline. Waves can form in lakes, oceans,
and even rivers, so these features alone do not help to pinpoint a
specific paleoenvironment, yet they do indicate a shoreline setting of some sort. However, Chuar ripple marks are commonly
draped with a thin veneer of mudstone with mud cracks (Fig. 4).
Mud cracks require a wet environment to deposit the mud and a
dry environment to dry out and crack the mud. This combination
of sedimentary features—ripple marks with mud-cracked mud
drapes—requires a continuum of changing physical conditions.
The rippled sands require a fast oscillatory current (e.g., flood
flow) that then must be followed by a slow current (e.g., slack
water) to deposit the mud. The entire mud-armored ripple mark
must then dry out for mud cracks to form (e.g., low tide). By far,
the easiest place for this combination of sedimentary features to
form, and be preserved in the rock record, is along a tidally influenced (marine) shoreline.
The Carbon Butte Member, marked at the base by a big red
cliff of sandstone, preserves other sedimentary features that also
suggest a tide- and wave-influenced environment (Fig. 5; Dehler
et al., 2001). Cross-bedding in these sandstones shows opposing
directions of paleoflow, indicating that underwater dunes were
moving in opposite directions, again a diagnostic tidal feature
(Fig. 5A). Associated with these decimeter-scale cross-beds are
several meter-scale, very low angle cross-bed sets (Fig. 5B); these
are likely the sides of shifting tidal channels. The Carbon Butte
Member also hosts soft-sediment deformation features, indicative of either very fast deposition of sediment or seismic shaking. The best place to see these features is in Kwagunt Canyon,
either right along the Butte fault or west of the Butte fault where
Kwagunt Creek flows over the lower cliff-forming red sandstone
(Fig. 5C).
Stromatolites are a persistent sedimentary feature in the
Chuar dolomites and are found in most members (Figs. 3 and 6).
The laminations reflect the episodic growth of these structures,
and their overall shapes are strongly influenced by the physical
conditions of the environment in which they formed. Stromatolites are much rarer today than they were during Chuar time, perhaps because of destructive grazing by animals (Garrett, 1970;
Awramik, 1971), but modern examples can be found in Shark
Bay, Western Australia, and in the Gulf of California, off the
Baja Peninsula.
There are at least six different types of stromatolites in the
Chuar Group (see Ford and Breed, 1973a, and Cook, 1991;
Dehler et al., 2001), some more recognizable and accessible than
others (Fig. 6). Most Chuar stromatolites appear only once in
the 1600-m-thick succession, making them unique stratigraphic
markers (Fig. 3). Probably the best known of the Chuar Group
stromatolites is Boxonia, a specimen that looks like a giant brain
(Fig. 6A) and can most easily be seen on a stroll up Kwagunt
Canyon in the low hillsides past the Butte fault. (A spectacular
and oft-visited single “brain” at the top of the Carbon Canyon slot
canyon was washed away in a recent flash flood; however, smaller
blocks from the same unit are still scattered in the streambed.)
Another spectacular and prominent stromatolite, Baicalia,
appears in the Carbon Canyon Member (Ford and Breed, 1973a).
Although typically <0.5 m in all dimensions, this stromatolite
is not only important for paleoenvironmental information (discussed below), it is a useful marker bed for mapping and interpreting tectonically controlled thickness changes in Chuar strata
(Figs. 6B and 7). When viewed from the top (on rare beddingplane exposures), Baicalia buildups look like a forest of large
broccoli heads. In many places the Baicalia heads are broken up
and in chaotic orientations, indicating reworking by storm waves.
A more subtle but unnoticed stromatolite interval is present
at the base of the Jupiter Member and is prominently exposed as
a white cliff (or a waterfall on rainy days) along the Lava Chuar–
Carbon Canyon loop hike in Lava-Chuar Canyon. This stromatolite interval has a complex assemblage of stromatolites, ranging
from marble-sized (centimeters in diameter) to river-raft–sized
(meters in diameter). Two species have been identified in this
interval, Inzeria and Stratifera (Ford and Breed, 1973a). Typically, the forms are arranged within one another—domes within
domes within domes. There are also pockets of broken stromatolites within this interval, indicating again the occurrence of infrequent large storms.
These microbial reefs or mounds are useful indicators of
water depth—and thus paleoenvironment—because they only
grow when submerged. Thus we can constrain minimum water
depth to be at least as deep as the height of a mound (if the microbial laminae continue to the base of the mound). For example,
because the “brain” stromatolites can be 2 m (6.56 ft) tall, we can
infer that water depth had to be at least 2 m. Another environmental condition required by stromatolites is that they receive enough
light for microbes to photosynthesize. Thus water depth could
not have been too great—less than ~100 m at an absolute maximum (328 ft), the lower boundary of the photic zone today, or
even shallower in Proterozoic time owing to lower solar luminosity (Sagan and Mullen, 1972). The water also must not have been
too cloudy with sediment, because photosynthetic organisms
need relatively clear water to receive sunlight. This latter factor
has implications for the climatic conditions present when these
microbial communities were living (i.e., low clastic sedimentation rates, and hence more arid during deposition of carbonates).
The fact that Chuar stromatolite taxa have been found in marine
deposits of Proterozoic age worldwide suggests a marine origin
for Chuar Group stromatolites (Dehler et al., 2001).
In addition to stromatolites, there is evidence for other
microbially influenced carbonate precipitation throughout the
Chuar Group. These sedimentary structures are common in the
Chuar dolomite beds and show horizontal laminations that are
crinkly, not smooth, in appearance (Fig. 6C). The flat, crinkly
laminations are interpreted to represent broad, flat microbial
mats that formed in the shallow subtidal, intertidal, and supratidal areas (Dehler et al., 2001).
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The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon
A
A
B
B
C
C
Figure 5. Photographs of sedimentary structures in the Carbon Butte
Sandstone, including (A) bipolar cross-bedding in the white sandstone
marker bed; pencil points in main direction of flow, and climbing dune
below pencil shows flow in the opposite direction; (B) low-angle crossbeds (cliff ~5 m tall); and (C) soft sediment deformation. These latter
features suggest deposition in a tidal setting concurrent with sediment
loading (caused by seismic activity or rapid sedimentation).
55
Figure 6. Photographs of various microbial marker beds in the Chuar
Group. (A) The stromatolite Boxonia, aka the “brains,” Kwagunt Canyon. If you had been snorkeling in this Neoproterozoic sea, you might
have scraped your belly on these microbial mounds (now dolomite).
(B) This example of the stromatolite Baicalia shows that the microbial
buildup was onlapped and eventually covered by fine-grained sediment. (C) Crinkly laminated dolomite, which formed in a microbe-rich
peritidal environment. All of these microbial deposits (photos A–C)
likely formed in shallow, clear water, perhaps in a setting similar to the
Persian Gulf coast today.
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Dehler et al.
20 cm
56
Baicalia marker bed
SITE 3
SITE 5
SITE 2
WEST LIMB
CHUAR SYNCLINE
SITE 1A
AXIS CHUAR SYNCLINE
EAST LIMB
BUTTE
CHUAR SYNCLINE FAULT
3 km
Figure 7. Stromatolite cycles from the Carbon Canyon Member, showing sedimentary response to movement associated with the Chuar syncline
(modified from Dehler et al., 2001). Note that the carbonate interval is thickest in the axis of the syncline.
Another special feature in the Carbon Canyon Member, which
occurs tens of meters below the Baicalia bed, is a dolomite bed that
contains deep and contorted mud cracks (Fig. 8). These strange
cracks, arranged in polygonal shapes in bedding-plane view, likely
represent a time when sea level was lowered, and this part of the
land was exposed for a long time. These cracks are similar to carbonate mud cracks forming on supratidal flats in the Persian Gulf
today. This marker bed and the Baicalia marker bed can be seen
along the north face of the north fork of Kwagunt Canyon.
Figure 8. Photograph of a large contorted mud crack in a dolomite
bed (the “polygonal marker bed,” Carbon Canyon Member, Kwagunt
Canyon). This feature is similar to modern mud cracks found in the
supratidal zones of the Persian Gulf.
The extensive and colorful shale in the Chuar Group does
not reveal much information about Proterozoic Earth conditions
when studied in the field. All shale, because of its fine-grained
nature, does indicate a low-energy depositional environment,
but many of the beautiful Martian colors apparent on the Chuar
hillsides were created by post-depositional processes. Although
much of this shale weathers to a red or green color, it is commonly gray to black on a freshly exposed surface, indicating that
most of the Chuar shale was originally organic rich. Geochemical
studies of the Chuar shale reveal high total-organic-carbon percentages (up to 9.39 wt%; Dehler, 2001; Nagy et al., 2009). The
organic content found throughout the Chuar Group requires high
biologic productivity in combination with oxygen-poor waters
near or below the ocean floor. Carbon-isotope composition and
related clay composition from these shales have implications for
climate change and the carbon cycle of the Chuar Group (Fig. 9;
see below).
In summary, the sedimentary rock types, or facies, in the
Chuar Group indicate a variety of depositional environments
(Fig. 10). The facies indicate deposition in an overall low-energy
marine embayment that was influenced by tidal and wave processes, infrequent large storms, microbial activity and carbonate precipitation, and quiet water deposition of mud and organic
matter. All of these facies suggest relatively shallow water (tens
of meters or less) or times of intermittent exposure on a tidal flat.
Not all of these environments were present at the same time:
there were sand-dominated (siliciclastic) intervals and dolomitedominated intervals. Organic-rich mud was deposited throughout
Chuar time, dominantly offshore and also in lagoonal and tidal
0
100
200
Neo
?
Middle Neoproterozoic
Sixtymile
Formation
Nankoweap
Formation
xxxx
1070±70 Ma
ca. 770 Ma
Tanner dolomite
ca. 760 Ma
Stratifera/Inzeria
Baicalia
polygonal bed
red sandstone
Baicalia/Box.
flaky dolomite
dolomite couplet
karsted dolomite
742±6 Ma
dolomite
abundance
1
2
3
4
-6 -4 -2 0
2 4
6
8 10
A
B
C
D
E
60
80
no data
no data
no data
no data
70
CIA
90
0
quartz
quartz
25
75
no data
no data
no data
no data
kaolinite
illite
illit
50
p
m
Mineralogy (cumulative wt %)
100
Climate
interpretation
Figure 9. Lithostratigraphic sequences, carbon-isotope curve, weathering indices (CIA, chemical index of alteration), and shale mineralogy of the Chuar Group. The high-magnitude
variability in carbon-isotope values is characteristic of Neoproterozoic strata. These excursions are commonly found associated with ancient glacial deposits, but this is not the case with
the Chuar Group. This group’s excursions and correlative data sets do, however, suggest changes in local climate and global ice volume. See text for discussion (from Dehler et al., 2005).
Squares indicate isotope values from organic carbon that are best fit to the carbonate carbon curve (shown as circles).
meters
Chuar Group
Kwagunt Formation
Galeros Formation
Walcott
Member
Awatubi
Member
Duppa Carbon
Butte
Member Mbr.
Carbon Canyon
Member
Jupiter Member
Tanner
Member
Mesoprot.
Unkar
Group
(upper)
Cardenas
Basalt
Walcott
Member
Awatubi
Member
Carbon
Butte
Mbr.
Duppa
Member
Carbon Canyon
Member
Jupiter Member
Tanner
Member
δ13C
features
wet
δ13 Ccarb (0 /00 PDB)
(warm)
Composite curve
dry
Lithostratigraphic
sequences
(cool)
C Tapeats Sandstone
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The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon
57
e
pe
sho
re
site of cycle
generation
fault
stromatolite
tidal
channel
re
fore
al
ritid
offs
e
e
hor
hor
s ho
ce
ce
refa
refa
pe
r
sh o
fo re
al
ritid
e
sandstonecapped
cycle
dolomitecapped
cycle
R E G R E S S I O N (subtle sea-level fall)
deposition of sand
faster weathering rates, relatively humid
shoreline
offs
sho
re
sho
fo re
l
itid a
p er
Figure 10. Depositional models for Chuar Group cycles, showing organic-rich mud deposition during sea-level rise (transgression), dolomite-rich deposition during major sea-level
fall (regression), and sand deposition during more subtle sea-level fall (regression). These models can be used to explain small-scale (meter-scale) and large-scale (lithostratigraphic
sequences—100 m scale) cyclicity observed in the Chuar Group. The change in lithology of shallow water deposits (i.e., some sandstone, some dolomite) is controlled by local
changes in climate that are linked to global changes in climate. See text for discussion.
sand
organic-rich
mud
mudcrack
dune
hummock
dolomite
KEY:
h or
sho
fac e
shoreline
58
offs
shoreline
deposition of organic-rich mud,
faster weathering rates, humid
TRANSGRESSION
R E G R E S S I O N (significant sea-level fall)
deposition of dolomite,
slower weathering rates, arid
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Dehler et al.
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The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon
flat areas. Understanding the relationship among these paleoenvironments involves looking closely at how the different sedimentary facies are vertically and laterally related to one another
in the rock record (stratigraphy). This will be addressed in the
next section.
CHUAR SEA LEVEL AND CLIMATE
The stacking of different facies, or stratigraphy, in the Chuar
Group reveals information about how the different depositional environments were related, and how sea level and climate
changed through Chuar time. The Chuar strata are markedly
cyclic, meaning that there are many repetitions of stacked facies,
and therefore repetitions of environmental change (Figs. 3, 9,
11A). Most Chuar cycles consist of shale overlain by dolomite or
sandstone, and are meters to tens of meters thick (Fig. 11B). The
shale was originally deposited offshore as mud, and the sandstone or dolomite was originally deposited near the shoreline as
sand or precipitated as carbonate, respectively. The changes in
facies indicate changes in water depth; because Chuar strata are
laterally continuous across the region (in the subsurface), these
water-depth changes likely reflect sea-level oscillations (Figs. 3
and 9). More specifically, the sandstone cycle tops represent
more subtle sea-level changes, whereas some dolomite cycle
caps indicate times of significant exposure (Figs. 6 and 11B) and
therefore greater drops in sea level.
These cycles are similar to those in younger strata (Phanerozoic, less than 543 m.y. ago) that are caused by Milankovich cycles (e.g., Beach and Ginsburg, 1978; Goldhammer et al.,
1987; Sageman et al., 1997). Milankovich orbital cycles include
the variations in the shape of the Earth’s orbit (eccentricity) and
the tilt and wobble of the Earth’s axis (obliquity and precession, respectively), each complete orbital cycle taking between
10 k.y. and 100 k.y. (Milanković, 1941). The interactions among
these orbital parameters cause changes in the amount of solar
radiation received by the Earth, hence influencing Earth’s climate and, more specifically, causing changes in the ice volume
in polar regions. There are >300 m-scale cycles (hypothesized
to reflect orbital changes) in the Chuar Group, each cycle representing durations on the order of 40,000–100,000 yr (Dehler et
al., 2001). This would suggest that the Chuar Group represents
~30 m.y. of geologic time, consistent with other age estimates for
the Chuar Group (see Geologic Background, above). There are
fewer thicker cycles in the upper Chuar Group, and these cycles
are all capped with dolomite; these features indicate relatively
higher magnitude sea-level change (Fig. 11C). Greater sea-level
change corresponds to melting and freezing of more glacial
ice. Therefore, the Chuar Group cycles indicate the presence of
global ice throughout Chuar time, and an increase in global ice
volume and a lowering of global temperatures in late Chuar time
(Kwagunt Formation).
The Chuar Group cycles can be grouped into four stratigraphic sequences (Fig. 9). Each sequence shows a bundling of
sandstone-rich cycles, followed by a bundling of dolomite-rich
59
cycles. We view these sequences as larger-scale versions of the
Chuar meter-scale cycles and hypothesize that they represent
similar (but longer duration) sea-level fluctuations whereby
the sandstone-rich intervals indicate higher sea level and the
dolomite-rich intervals indicate lower sea level (Fig. 9). These
sequences track the carbon-isotope signatures and shale composition remarkably well and will be discussed below.
Chuar strata also provide information about the carbon
cycle, which in turn can be used to infer both climatic and biotic
changes (e.g., glaciations and extinction events). The carbon
cycle is approximated by measuring the ratio between the two
stable isotopes of carbon (12C, the lighter and most abundant
one, and 13C, the heavier one) preserved in carbonate rocks and
organic matter in shale. A carbon-cycle curve is constructed by
plotting the measured ratios through time. The curve expresses
changes in the rate of organic-carbon burial, which reflects relative rates of primary productivity, sedimentation, and organicmatter decomposition (e.g., DesMarais, 1997). Where the curve
shows more positive values (expressed in parts per thousand, ‰),
it indicates an increase in primary productivity and/or organiccarbon burial; more negative values can indicate lower rates of
primary productivity and/or organic carbon burial. The Chuar
carbon-isotope curve exhibits four major excursions (i.e., rises
and falls), one of which is among the largest ever recorded in
Earth history (cf. Melezhik et al., 1999). This positive excursion
(15‰; Fig. 9), recorded in the lower Awatubi Member, has been
interpreted to indicate high rates of organic-carbon burial, reflecting high primary productivity and high rates of sedimentation
(Dehler et al., 2005). This interpretation is supported by the shale
mineralogy (see below).
The mineral composition of well-preserved shale provides
a view of relative weathering rates in the source area through
Chuar time. Shale rich in the mineral kaolinite (the weathering
product of other unstable minerals, such as feldspar) indicates
relatively high weathering rates, whereas shale rich in the mineral feldspar indicate relatively low weathering rates, because
feldspar is unstable at Earth’s surface conditions. A weathering
“index” or chemical index of alteration (CIA) can be calculated
for shale on the basis of the elements it contains (see discussion
in Chapter 2 [Timmons et al., this volume], and in Nesbitt and
Young, 1982). Shale with a high CIA (>80%) indicates increased
weathering rates, and shale with a lower CIA indicates decreased
weathering rates (Fig. 9). Because rates of weathering are
strongly determined by rainfall, these indices can be used to infer
times of relative aridity and humidity in the area where the Chuar
sediment was weathered from its parent rock (the “source area”).
Shale mineral compositions indicate that weathering rates, and
thus relative humidity, varied significantly during Chuar time,
with less weathering during dolomite-rich times (lower sea level)
and more intense weathering during sandstone-rich times (higher
sea level; Fig. 9; Dehler et al., 2005).
Combining the stratigraphic data with the carbon-isotope
curve and the shale mineralogy has resulted in a compelling climate story (Fig. 9). Sandstone-rich intervals correlate with higher
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60
Dehler et al.
Figure 11 (Continued on facing page).
(A) Photograph of meter-scale cycles
in the Carbon Canyon Member of
the Chuar Group. Geologist for scale
(circled in white). (B) Photograph of
meter-scale cycles in the Carbon Canyon Member, along the north fork of
Kwagunt Creek. White arrows denote
single shallowing-upward cycles. E—
exposure interval at top of one of the
dolomite-capped cycles.
A
E
B
E
m
ps
exposure zone
LOWER CHUAR GROUP
(Jupiter Member)
ps
shallow subtidal
(offshore)
MIDDLE CHUAR GROUP
(Carbon Canyon Member)
ps
ps
m
m
peritidal
shallow subtidal
(offshore)
E
shallow
subtidal
(offshore)
peritidal
exposure
DOLOMITE-CAPPED CYCLES
UPPER CHUAR GROUP
(upper Awatubi/Walcott Members)
shallow subtidal
(offshore)
peritidal/shoreface
SANDSTONE-CAPPED AND
DOLOMITE-CAPPED CYCLES
peritidal/shoreface
SANDSTONE-CAPPED CYCLES
transgressive-prone cycle
regressive cycle
tepee structure
contorted bedding
interference ripplemark
symmetric ripplemark
crinkly lamina
smooth planar lamina
stromatolitic build-up
mudcrack, small and large
planar tabular foresets
trough crossbeds
fluid-escape structure
massive
poorly sorted
intraclast
dark mudrock facies
variegated mudrock facies
laminated sandstone facies
ooid-pisoid grainstone facies
laminated dolomite facies
Explanation
10 m
Figure 11 (Continued). (C) Measured sections of meter-scale cycles in the lower, middle, and upper Chuar Group. Note how the cycles are thicker and become more carbonate dominated in the upper Chuar Group. The change in cycle character indicates an increase in magnitude of sea-level change through Chuar time, likely controlled by an overall increase in ice
volume on the planet.
C
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The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon
61
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62
Dehler et al.
TABLE 1. SUMMARY OF FOSSILS FOUND IN CHUAR GROUP STRATA
Formation
Member
Fossil(s)
Location/Lithology
References
Chuaria
Shales just overlying basal dolomite
unit
Walcott (1899), Ford and Breed
(1973a, 1973b), Nagy et al. (2009)
Acritarchs*
Lower shales?
Ford and Breed (1969) , Vidal and
#
Ford (1985) , Horodyski (1993)**
Bacterial filaments
and unicells
Pisolites, shales, chert nodules in
carbonate
Schopf et al. (1973), Horodyski
(1993), Link et al. (1993)
Dolomite nodules in shales ~15 m
below the top of member; chert
nodules in thin dolomite beds above
basal dolomite; shales above basal
dolomite; shales associated with
pisolite
Bloeser et al. (1977), Bloeser,
(1985), Vidal and Ford (1985),
Horodyski (1993), Porter and Knoll
(2000), Porter et al. (2003)
Hydrocarbon
biomarkers
Argillaceous dolostones 5–6 m above
base of member on Nankoweap Butte
Summons et al. (1988), Vogel et al.
(2005)
Sphaerocongregus
variabilis
Shales ~3 m above flaky dolomite
Nagy et al. (2009)
Chuaria
Shales throughout member
Ford and Breed (1973a, 1973b),
Vidal and Ford (1985), Nagy et al.
(2009)
Acritarchs
Shales throughout member
Downie (1969) , Vidal and Ford
(1985), Horodyski (1993), Nagy et
al. (2009)
Shales throughout member
Vidal and Ford (1985), Horodyski
(1993)
Filamentous bacteria
Shales throughout member
Horodyski (1993)
Possible eukaryotic
algal filaments
Mudstone 50 m below contact with
Walcott
Horodyski and Bloeser (1983)
Sphaerocongregus
variabilis
Shales
Na g y et a l. ( 20 0 9 )
Hydrocarbon
††
biomarkers
Ba se o f me mber
Vog el e t al . (20 0 5)
Acritarchs
Shales throughout member
Nagy et al. (2009)
Unicells and
filaments
Silicified microbial laminated
carbonate
Schopf et al. (1974), abstract cited
in Schopf (1975)
Acritarchs
Shales throughout member
Nagy et al. (2009)
Chuaria
Shales
Vidal an d Ford (1 985)
Acritarchs
Shales throughout member
Vidal and Ford (1985), Nagy et al.
(2009)
Chuaria
Shales throughout member
Ford and Breed (1973b), Vidal and
Ford (1985), Nagy et al. (2009)
Acritarchs
Shales throughout member
Vidal and Ford (1985), Nagy et al.
(2009)
§
Walcott
VSMs
†
Kwagunt
§
Awatubi
Carbon
Butte
Duppa
Carbon
Canyon
Galeros
Jupiter
VSMs
†
No fossils reported
Tanner
*There are no confirmed reports of ornamented acritarchs from the Walcott Member. See footnotes below.
†
VSMs—vase-shaped microfossils.
§
In an appendix to Ford and Breed (1969), Downie (1969) describes small round bodies with surface textures that are “sometimes
smooth or shagrinate [sic] or wrinkled and ‘spongy’” from a single sample collected from the “Chuaria horizon.” According to Ford et al.
(1969), this interval is “20 feet below the ‘Corn Flake Bed’ to just above silicified oolite,” i.e., the upper Awatubi through lower Walcott
Members. The exact position of this sample within the interval was not recorded.
#
Vidal and Ford (1985) report Chuaria, VSMs, and ‘cf. Stictosphaeridium sp.’, a “‘wastebasket’ grouping” comprising “single or
clustered thin-walled spheromorphs without any distinctive primary ornamentation” (p. 377) from a single sample at an unspecified zone
in the Walcott Member.
**Horodyski (1993) reported scarce to common acritarchs from several samples collected from the lower ~80 m of the Walcott
Member, but did not provide taxonomic lists.
††
Vogel et al. (2005) report gammacerane and extreme enrichment of ααα C27 steranes.
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The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon
(more positive) carbon-isotope values and kaolinite-rich shale,
and dolomite-rich intervals correlate with lower (more negative)
carbon-isotope values and feldspar-rich shale. We hypothesize,
therefore, that the sandstone-rich intervals indicate deposition
during locally wetter times, when more clastic sediment was
delivered to the Chuar basin at faster rates, and during globally
warmer times, when sea level was higher and there was less glacial ice. The dolomite-rich intervals indicate increased carbonate
precipitation during locally drier times, when less sediment was
delivered to the basin (as silt and sand), and during globally cooler
times, when sea level was lower and there was more glacial ice.
In the past decade, much attention has been paid to Neoproterozoic glacial deposits, some of which are about the same age
as the Chuar Group (see Hoffman and Li, 2009, for a summary of
age constraints). These deposits are anomalous for many reasons,
most notably the fact that many were deposited at sea level in
equatorial regions and are associated with extreme variability in
the carbon-isotope curve (Hoffman et al., 1998a; Evans, 2000).
There is an array of hypotheses to explain these relationships (see
Hoffman and Schrag, 2002, for a review of these hypotheses);
the best known is the “snowball Earth” hypothesis (originally
suggested by Harland, 1964, extended by Kirschvink, 1992, and
revived by Hoffman et al., 1998b), which suggests that Earth’s
oceans were completely frozen over for at least 10 m.y. at a time
and that these special conditions explain the large variability
in the carbon cycle. It has been hypothesized that all extreme
carbon-isotope excursions in the Neoproterozoic—even those
not stratigraphically tied to glacial deposits—may indicate glaciation (Kaufman et al., 1997). The Chuar Group has such excursions, and although it lacks glacial deposits, its stratigraphic
data sets (Figs. 9 and 11C) independently suggest that glaciers
were on the planet (Dehler et al., 2001). Importantly, the Chuar
Group indicates that there was ice on the planet between 800 and
742 Ma (at least at the poles), yet not in lower latitudes as suggested by the “snowball Earth” hypothesis. These findings not
only help to constrain the timing of a “snowball Earth” (if this
hypothesis holds), they also support the idea that there is more
than one way to get large-scale changes in the Proterozoic carbon cycle. The variability in the Chuar carbon cycle appears to
be ultimately controlled not by “snowball Earth” conditions, but,
rather, by concomitant changes in local humidity and global ice
volume (Dehler et al., 2005).
63
Chemical byproducts of photosynthesis encouraged precipitation of carbonate coatings on the grains; as a result, the grains
are made up of concentric rings of carbonate (known as ooids or
pisoids, depending on their size). They are preserved today, along
with the carbonate grains in cherty oolite and pisolite units, distributed at a number of intervals in the Walcott Member (Schopf
et al., 1973; Cook, 1991).
In shallow, low- to high-energy waters in the Chuar basin
grew stromatolites; these organo-sedimentary structures are
described in the previous section. Living within—and probably dining upon—the bacterial mats growing on the surface
of these stromatolites were amoebae, single-celled organisms
that move about on pseudopods, i.e., finger-like extensions of
the cell. Many of these lived in tests, protective “houses” they
built themselves, with only their pseudopods protruding outside
(Fig. 12). Although the cells decayed long ago, the tests are abundantly preserved in association with bacterial mats in dolostones
(the “flaky dolomite”; Cook, 1991) near the base of the Walcott
Member (Porter and Knoll, 2000) and in upper Awatubi mudstones (Bloeser et al., 1977; Bloeser, 1985). However, they are
most abundant (preserved by the billions!) in carbonate concretions ~15 m below the top of the Walcott Member on Nankoweap
Butte (Fig. 13A). These concretions formed in situ after the surrounding shale had been deposited; it is likely that the decay
of the amoebae themselves promoted carbonate precipitation,
essentially entombing their tests in carbonate rock (Porter and
Knoll, 2000). Their remains are distinctive; the tests look like
tiny vases or bags with an opening at one end. They are known in
the literature as vase-shaped microfossils or VSMs, and are often
referred to informally as Bonnie Bags, after Bonnie Bloeser, the
woman who first discovered them in the Chuar Group (Bloeser
et al., 1977; Bloeser, 1985). At least 11 species are known from
these concretions, and an additional species has been found in
Awatubi mudstones (Bloeser, 1985; Porter et al., 2003). Species differ in the shape of the test itself (Fig. 13B versus 13G
versus 13E), the shape of the test opening (Figs. 13B and 13C),
A
B
LIFE IN CHUAR WATERS
The Chuar fossil record (Table 1) indicates not only that
there was a diversity of life in the Chuar sea but also that organisms lived in a variety of different habitats, from high-energy
shallow waters to low-energy offshore waters, attached to sand
grains, grazing on microbial mats, or floating in the water column. Although primarily single celled, life by this time was
highly diverse, both taxonomically and ecologically.
In the shallow, high-energy, warm-water environments of
the Chuar sea lived cyanobacteria attached to carbonate grains.
Figure 12. Representative sketches of (A) arcellid testate amoebae,
and (B) euglyphid testate amoebae. Note the difference in pseudopod
shape. Both arcellid and euglyphid testate amoebae may have mineralized scales embedded in their tests; these are shown here only in image
B, however. Modified from figure 2 in Ogden and Hedley (1980).
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64
Dehler et al.
A
B
C
D
15 µm
15 µm
E
F
G
15 µm
15 µm
H
L
K
I
15 µm
10 µm
10 µm
10 µm
20 µm
Q R
P
30 µm
15 µm
N
20 µm
O
J
25 µm
M
15 µm
15 µm
30 µm
S
300 µm
5 µm
Figure 13. Representative body fossils from the Chuar Group. (A) Dolomite concretion bearing billions of vase-shaped microfossils (VSMs).
Upper Walcott Member, Nankoweap Butte. Geologic hammer for scale. B–E, G, I, K, VSMs, all from upper Walcott Member dolomite concretions. F, H, J, modern testate amoebae. L–Q, acritarchs from the Jupiter Member (L, Q) and the Tanner Member (M–P). Note circular excystment structure in P and medial split in Q. R, Chuaria circularis. Specimen from the Awatubi Member. S, Sphaerocongregus variabilis, from the
Awatubi Member. Image in A is courtesy of A. Knoll.
and the presence of indentations (Fig. 13D) or scales (Fig. 13I).
Some of the tests look identical in form to those of lobose amoebae today (Fig. 13H), a group closely related to slime molds
(Baldauf, 2003). Others are similar to those of euglyphid amoebae (Fig. 13J), close relatives of the Foraminifera and Radiolaria
(Nikolaev et al., 2004). Interestingly, the modern groups most
similar to these ancient forms live not in the ocean but on moss,
in leaf litter, or in lakes, suggesting that several ancient lineages
of testate amoebae may have moved onto land, perhaps following
the food when land plants evolved, some 300 m.y. later (Porter et
al., 2003). By analogy with their modern counterparts, Neoproterozoic testate amoebae probably ate any or all of the organisms
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The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon
in Chuar waters. They may have eaten each other, as well, as
some amoebae do today. Evidence that some testate amoebae
may have been preyed upon comes from semicircular holes in
the test walls of many specimens (Fig. 13K), although it is not
clear who may have made these or how (Porter et al., 2003). In
any case, VSMs provide some of the earliest evidence for predators in the fossil record, indicating that relatively complex food
webs were in place ~740 m.y. ago.
Farther offshore lived a diversity of planktonic microorganisms. These likely included ciliates, voracious single-celled
micropredators covered in hair-like projections called cilia.
These particular species likely lived in low-oxygen waters or at
the boundary between oxygen-rich and oxygen-depleted waters,
dining on bacteria that thrived in the low-oxygen environment.
Evidence for this comes from the presence of gammacerane, a
hydrocarbon molecule preserved in lower Walcott dolostones
(Summons et al., 1988; Vogel et al., 2005). Gammacerane is the
geologically stable form of tetrahymenol, a lipid produced primarily by ciliates that consume significant quantities of bacteria
and live in or close to anoxic environments (Sinninghe Damsté
et al., 1995). Dinoflagellates, a group comprising both photosynthetic and heterotrophic protists, may also have been present in
the Chuar sea, as suggested by the presence of dinosterane biomarkers in Walcott samples (Vogel et al., 2005). (Dinosteranes
are the geologically stable form of dinosterols, which today are
almost exclusively formed by dinoflagellates [Volkman et al.,
1993].) Additional hydrocarbon molecules preserved in both
basal Walcott and basal Awatubi strata are consistent with the
presence of chemoautotrophic bacteria (i.e., bacteria that fix their
own carbon and get energy from chemical reactions rather than
from the sun) and of abundant red and/or golden-brown algae
(Summons et al., 1988; Vogel et al., 2005).
Additional microorganisms are represented in Chuar strata
by beautifully ornamented organic-walled microfossils known as
acritarchs. Acritarchs form the bulk of the Proterozoic fossil record
and are found throughout the Chuar Group in shale and mudstone.
Although it is likely that these ornamented forms are eukaryotic
(Javaux et al., 2003), paleontologists aren’t exactly sure what kinds
of eukaryotes they represent. It has long been assumed that they
are the remains of phytoplankton—tiny algae that float in the water
column—but recent discoveries suggest that at least some species
may have been fungi (Butterfield, 2005), and some may represent
animal eggs and embryos (Xiao and Knoll, 2000; Yin et al., 2004).
It is also clear that not all floated in the water column; some fossils
are preserved attached to sand grains, suggesting that they lived
on the seafloor (e.g., Butterfield, 1997). Many acritarchs appear to
have been cysts, dormant structures formed when a cell is exposed
to stressful conditions, such as nutrient depletion or changes in
water salinity or temperature. Evidence that some acritarchs are
cysts comes from excystment structures, circular “escape hatches”
(Fig. 13P) or medial splits (Fig. 13Q) through which the cell exited
the cyst once its dormant stage was over. The diversity of ornamented acritarchs—including species covered with tiny cone-like
projections, concentric circular ridges, and intricate wrinkles—and
65
their abundance in certain beds suggests that protists were important components of the Chuar ecosystem.
One organism living in the Chuar sea appears to have been
a giant relative to the rest: the ~1–3-mm-diameter Chuaria circularis (Fig. 13R). Although its remains are now found as flat
discs on shale bedding planes, Chuaria was originally a smooth,
featureless, probably planktonic sphere. The discovery in early
Neoproterozoic strata from India of probable Chuaria specimens attached to the end of another cm-sized ribbon-like fossil, Tawuia, suggests that Chuaria may represent a reproductive
stage in the life cycle of a seaweed (Kumar, 2001). Holdfast-like
structures on some Tawuia specimens suggest that the seaweed
lived attached to the sediment; once mature, Chuaria would have
become detached from its parent seaweed and dispersed planktonically (Kumar, 2001). This may explain why Chuaria is much
more widespread in rocks of this age than is Tawuia (Kumar,
2001). Although Chuaria occurs in both shallow and deeper
water environments preserved in the Chuar Group, Tawuia has
never been found in the Chuar Group.
Upper Chuar shales preserve ~5–20 µm aggregates of even
tinier (<1 µm) organic-walled spheres, sometimes surrounded
by a membrane (Fig. 13S; Moorman, 1974; Cloud et al., 1975;
Nagy et al., 2009). These structures, similar to those described
under the names Sphaerocongregus variabilis and Bavlinella
faveolata, are thought to be the remains of bacteria, although an
origin from framboidal pyrite cannot be ruled out (Nagy et al.,
2009). Sphaerocongregus variabilis (= Bavlinella faveolata)
has an unusual distribution: It is extremely rare in diverse acritarch assemblages, but it occurs sporadically in high concentrations commonly by itself in rocks characteristic of low-oxygen
environments and/or interbedded with glacial diamictites
(Knoll et al., 1981; Vidal and Nystuen, 1990). Its occurrence
in the Chuar Group is consistent with this pattern; S. variabilis
is absent from the high-diversity acritarch assemblages found
in lower Chuar strata, but it is recovered in high numbers from
upper Awatubi and lower Walcott mudstones that are otherwise
devoid of diverse acritarch assemblages (Table 1; Nagy et al.,
2009). Two taxonomic interpretations have been suggested for
S. variabilis. The first view is that it was a cyanobacterium that
bloomed under high-nutrient (eutrophic) conditions, living in a
thin layer of oxygenated surface waters above an anoxic water
column (e.g., Moorman, 1974; Cloud et al., 1975; Knoll et al.,
1981; Mansuy and Vidal, 1983). The second view is that it was
an anoxygenic (non-oxygen producing) photosynthetic bacterium that thrived in anoxic waters much like sulfur bacteria in
stratified lakes today (Repeta et al., 1989; Vidal and Nystuen,
1990). In either case, its presence in the upper Chuar Group
suggests a transition to eutrophic and anoxic conditions during late Chuar time, an inference corroborated by the presence
of gammacerane, a biomarker indicative of anoxic waters, in
upper Chuar rocks, and by recent iron speciation and sulfur isotope analyses (see above and Table 1; Summons et al., 1988;
Sinninghe Damsté et al., 1995; Vogel et al., 2005; Johnston et
al., 2010).
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Dehler et al.
SHAKING SANDS AND MOVING CONTINENTS
In Chapter 2 (Timmons et al., this volume) the history of
Unkar Group sedimentation and deformation was described, providing a framework for understanding the relative importance
and timing of the subsequent Chuar-age basin formation. We now
know that late Unkar Group deformation predated Nankoweap
and Chuar Group deposition and was different in deformational
style from Chuar-related faulting and syncline development.
Unkar Group rocks were faulted and tilted along NW-trending
fault systems like the Palisades fault (Fig. 2). In contrast, Chuar
rocks were folded by the Chuar syncline, paralleling the northtrending Butte fault. The different styles of deformation and field
observations suggest that the Unkar and Chuar Groups record
two separate deformational events in the middle to late Proterozoic. The remainder of this section will further describe the style
and relative timing of Chuar Group deformation and its importance for understanding the tectonic history of the western United
States in late Proterozoic time.
The relative importance of Chuar Group deformation,
including movement of the Butte fault, development of the Chuar
syncline, and formation of intraformational faults was documented by Timmons et al. (2001). In the absence of absolute age
determinations from the Chuar Group, early workers lumped the
faulting and tilting of the Unkar Group (described in Chapter 2
[Timmons et al., this volume]) and faulting and folding in the
Chuar Group into a single tectonic episode that affected much
of western North America (Noble, 1914). This tectonic episode
propagated through the geologic literature as the Grand Canyon
“Revolution” (Maxson, 1961), “Disturbance” (Wilson, 1962;
Elston and McKee, 1982), and “orogeny” (Elston, 1979). The
common model envisioned for this tectonic event was broadly
analogous with Basin and Range deformation seen in the modern landscape in the western United States. All these early workers placed the timing of this deformational event in latest Chuar
Group and Sixtymile Formation time. More recently, workers
have come to realize through a combination of better geochronology, more detailed geologic mapping, and sedimentary-tectonic
interpretation that the Unkar and Chuar Groups record distinct
tectonic and sedimentary events separated by more than 200 m.y.
of time (Timmons et al., 2001, 2005).
The Butte fault is a major north-trending normal fault that
records west-side-down Neoproterozoic displacement (see Chapter
2 [Timmons et al., this volume] for description of common fault
types). The amount of displacement across the Butte fault is large
by Colorado Plateau standards and varies along the trace of the
fault. The maximum amount of Proterozoic displacement is estimated at 1800 m; however, the actual amount of displacement may
have been greater. Erosion of the late Proterozoic landscape prior to
Cambrian (Tapeats) time precludes an accurate estimation of fault
displacement. Subordinate faults within the Chuar basin parallel the
trace of the Butte fault and also record normal-sense movement.
The Chuar syncline is a broad asymmetric fold comprising Chuar Group strata (Fig. 14). Some of the best places to see
the Chuar syncline and Butte fault are in Nankoweap, Kwagunt,
Carbon, and Lava Chuar drainages. The Carbon–Lava Chuar
loop hike offers a fabulous view, but the ultimate view may be
from the North Rim (e.g., Point Imperial), where views into Nankoweap Canyon display the syncline in stunning detail. Desert
View on the South Rim also presents an impressive view; from
there the Chuar syncline can be seen within the Carbon and Lava
Chuar Canyons. The hinge line of the fold is doubly plunging,
meaning that within the axis of the syncline, beds in some areas
(Nankoweap Canyon) dip toward the south, and in other areas
(Lava Chuar Canyon) beds dip toward the north (Fig. 2). The
Chuar syncline parallels the trace of the Butte fault, suggesting
a genetic relationship between the syncline and the fault. The
relative timing of syncline development can be determined in the
field by examining the contact of the Chuar Group with the overlying Tapeats Sandstone where the Tapeats truncates the Chuar
syncline, indicating that the Chuar syncline is Proterozoic in age
(Fig. 14). Another key characteristic of the Chuar syncline is that
lower formations of the Chuar Group are more tightly folded
than upper beds (Fig. 15). This is an unusual relationship in most
folded terranes and suggests that the syncline was forming during deposition of the Chuar Group. To test the hypothesis that
the Chuar syncline, Butte fault, and Chuar Group deposition are
linked, we must examine the preserved rock record.
Field observations collectively suggest that the Chuar Group
was deposited in a shallow-marine extensional basin related to
the earliest development of the Cordilleran margin, which was
then the western edge of Laurentia (Sears 1990; Dehler et al.,
2001; Timmons et al., 2001). Chuar Group sediments were
deposited synchronously with movement on a series of N-S–
striking normal faults. The principal fault exposed is the Butte
fault; however, owing to the dominantly fine-grained nature of
the Chuar Group, it is reasonable to conclude that Chuar deposits originally overlapped this fault zone (no fault scarp is indicated by Chuar Group deposits, e.g., thicker and coarser grained
deposits). Faults, though, work as networks, and so what we learn
about the Butte fault and subordinate faults tell us much about
regional stresses and what was happening within the greater
Chuar basin. Some of these subordinate faults are intraformational, meaning that the faults die out up section and lower beds
have more displacement than upper beds, suggesting that extensional faulting was concurrent with deposition. The amount of
displacement across these minor faults is typically very small,
usually <2 m of displacement, and likely did not form scarps at
the surface. Rather, they were likely buried faults during deposition (i.e., they did not rupture the surface), much like the larger
Butte fault. The subordinate faults do, however, provide clues as
to when faulting occurred in the basin. Intraformational faults
are found throughout the Chuar Group strata, including the basal
Tanner Member and middle Carbon Canyon and Carbon Butte
Members, suggesting that faulting was occurring throughout
Chuar Group deposition.
One of the most striking observations is that Chuar Group
deposits are not known to be preserved east of the Butte fault,
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The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon
5
Lava Chuar Canyon
4
3
2
67
1
0
Shale and mudstone of the Carbon
Butte Member
Fine- to medium-grained sandstone
of the Carbon Butte Member
10
Meters
Basal stromatolite marker (Boxonia)
of the Awatubi Member
20
30
40
50
W
E
2000
Walcott Member
Tonto Group
Carbon Butte Member
Elevation (m)
4
*
Duppa
3
r
Carbon Canyon Member
A
A = partial measured
section in Carbon Canyon Member
2
*
Membe
Jupiter Member
1000
1
Awatubi Member
5
Tonto
Group
Dox
?
* = measured sections in
member not presented
A
A′
Figure 14. East-west geologic cross section and measured sections of the Carbon Butte Member in Lava Chuar Canyon. The locations of the
measured sections are numbered on the cross section. The overall form of the Chuar syncline is asymmetric, with a steeper limb on the east
and a shallower limb on the west. The syncline is shown tightening with depth, as suggested by field observations. The Cambrian Tonto Group
is not folded by the Chuar syncline, indicating that the syncline is Proterozoic in age.
Sixtymile
Walcott
Awatubi
70
Figure 15. Field photo of the Chuar
syncline, looking toward the north
along its axis. The Carbon Butte Sandstone (at right) is steeply dipping toward the west, and, at the skyline, the
Sixtymile Formation caps Nankoweap
Butte. Note that the syncline axial plane
dips to the east and that upper beds are
folded less tightly. This is a hallmark of
growth synclines.
Downloaded from specialpapers.gsapubs.org on January 24, 2013
68
Dehler et al.
indicating that movement of the Butte fault preserved deposits
to the west and exposed the deposits to the east to erosion, all
before the Tapeats Sandstone was deposited. When this relationship fully developed is a mystery, but one possibility is that
syncline development, fault movement, and deposition were concurrent. To address this possibility, numerous measured sections
were completed at multiple locations with particular interest in
understanding how deposits vary in thickness and type across
the syncline. An interval bound by marker beds within the Car-
bon Canyon Member was measured on the east limb and within
the axis of the syncline (Fig. 16). The first observation is that
within the axis of the syncline the marker bed interval is thicker
than the same interval on the east limb; however, the thickness
changes are largely confined to mudstones. Carbonate beds and
sandstones do not express this thickness change and likely reflect
either short depositional episodes and/or periods of relative fault
inactivity. Sections measured above in the Carbon Butte Member also reflect a similar relationship, with the thinnest intervals
East limb of Chuar
syncline
B
Axis of Chuar syncline
A
Baicalia
40
?
?
?
Depth (m)
30
?
?
Figure 16. Measured partial sections of
the Carbon Canyon Member in Carbon
Canyon (A) and Kwagunt Canyon (B).
Note lateral continuity of thicker carbonate marker beds and discontinuity of other rock types. Within the synclinal axis
(section A) the mudstone beds are thicker, suggesting that the syncline axis was
creating more accommodation for deposition of mud. Does this reflect episodic
development of the Chuar syncline and
Butte fault, or do mudstone facies record
more geologic time and thus more incremental synclinal development?
20
10
"Polygonal" marker bed
0
GALEROS FORMATION
Partial section of
Carbon Canyon Member
in Carbon Canyon
<0.5 km as projected into cross sections
Duppa
Member
Carbon
Canyon
Member
Jupiter
Member
Partial section of
Carbon Canyon
Member in
Kwagunt Canyon
KEY
variegated mudrocks
Marker bed
interval
black mudrocks
sandstone
dolomite
stromatolites
crinkly laminations
large mudcrack
planar laminations
trough cross-beds
calcareous
intraclasts
symmetric ripple
mudcrack
Downloaded from specialpapers.gsapubs.org on January 24, 2013
The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon
occurring on the eastern limb of the Chuar syncline (Fig. 14).
Here again, the mudstone units reflect the syncline form, with
thicker accumulations within the axis; sandstone beds are relatively uniformly thick across the syncline. Finally, within the
upper units of the Chuar Group the syncline form becomes even
more pronounced, and it is manifested in different units, including carbonate beds (Fig. 17). At this interval, carbonate beds
pinch out to zero thickness toward the Butte fault. If the deposits
were accumulating in an incrementally developing syncline, this
is the relationship one would predict.
The above observations collectively point to the conclusion
that the Chuar syncline was developing during deposition and
faulting of the Chuar Group. The Butte fault and subordinate
structures were instrumental to the development of the Chuar
syncline, but remained buried during deposition of the Chuar
Group (Fig. 18A). Early in Chuar time, intraformational faults
offset Tanner Member deposits and the synclinal form began to
take shape with incremental faulting (Figs. 18A and 18B). By
Nankoweap Butte
(X and Y)
60
(W)
(E)
the end of Chuar time, incremental fault movement forms the
Chuar syncline while the fault itself remains buried. During this
interval, it is possible that some of the fine-grained sediments
deposited east of the fault would have been eroded and recycled
into the deposits on the west side of the fault. Sometime after
Chuar Group time, the fault must have penetrated to the surface
and created a scarp that would encourage the complete stripping
of the Chuar Group from the east side of the fault (Fig. 18C).
The record of the emergent Butte fault may be recorded
in the deposits of the Sixtymile Formation. This formation is
truly a unique section in the Grand Canyon Supergroup. Workers have long recognized that the Sixtymile Formation marks
dramatic faulting along the Butte fault system and represents
the principal extensional event in the Neoproterozoic (Elston,
1979). Although the Sixtymile Formation does reflect a dramatic change in depositional style, we prefer the interpretation
that the Sixtymile Formation records a continuation of Butte
fault movement.
Sixtymile Canyon
(Z)
Cambrian
69
Tapeats
upper member
sandstone with conglomerate
interbeds; breccia at base
red to white siltstone with silicified
beds, breccias and convolute beds
(channel fill)
40
Sixtymile
dolomite
Depth (m)
20
Walcott
middle (siltstone) member
(locally brecciated and slumped)
brecciated siltstone
soft-sediment
deformation
siliceous layers
0
742 Ma ash
shale
slumped horizon
red
black
Walcott
30m
omitted
Walcott
Butte fault scarp
concretion in black shale with
evidence for rotation and
minor translation toward the
axis
~700 m
to the east
30m
omitted
dolomite doublet pinches
out toward Butte fault in
Sixtymile Canyon
syncline axis
Figure 17. Measured sections and schematic diagram of deformational features in the upper Chuar Group and Sixtymile Formation. The view is
toward the north, parallel to the trace of the Butte fault and Chuar syncline. Carbonate beds of the upper Walcott Member pinch out toward the
Butte fault, suggesting that syncline development continued through Walcott time. The end of Walcott Member deposition is marked by very
large, disarticulated blocks of carbonate on a mudstone slip horizon. Sixtymile time is marked by deposition of clastic red beds and the development of incised channels within the formation.
Downloaded from specialpapers.gsapubs.org on January 24, 2013
70
Dehler et al.
Jupiter Member
Tanner Member
Tanner dolomite
Unkar Group
A
Carbon Butte Member
Duppa Member
Carbon Canyon Member
Jupiter Member
Tanner Member
Tanner Mem
ber
B
Unkar Group
Sixtymile Formation
Duppa
Member
Carb
o
n Ca
nyon
Jupi
C
ner
Unkar
Group
Awatubi
emb
er
Mem
ber
Duppa
Tanner
D
Unkar
Unkar Group
ber
Paleozoic rocks
Jupiter
Tanner
Mem
ter M
Tan
Jupiter
Walcott
Carbon
Butte
Carbon Butte
Carbon
Canyon
Walcott
Awatubi
Paleozoic
rocks
Unkar
Group
Figure 18. Schematic time slices illustrating the growth history of the
Chuar syncline at key intervals. (A) Time slice shows the truncation
of the Tanner dolomite against a subordinate normal fault, with fault
overlapped by Tanner Member shale, indicating that extensional faulting was synchronous with Tanner Member deposition. (B) Time slice
illustrates the growth nature of the Chuar syncline during deposition of
the Carbon Butte Member; note the postulated depocenter in the syncline and thinning of units over the footwall of a “blind” Butte fault.
(C) Time slice displays our interpretation of syncline/fault relationships during Sixtymile Formation time. Note the pinch-out of dolomite units in the Walcott Member, the apparent truncation of the Chuar
syncline by the Sixtymile Formation, and the tightening of the syncline
at depth. (D) Time slice shows the present fault-syncline geometry in
Lava Chuar Canyon (includes Laramide movement on the Butte fault).
Unshaded layers or portions of layers indicate those rocks that have
been eroded.
The Sixtymile Formation is present in only four places in the
Chuar Valley, with outcrops atop Nankoweap Butte, and within
Awatubi and Sixtymile Canyons (Fig. 2). The thickness of this
formation was measured in multiple localities and is as thick as
60 m, though the actual depositional thickness remains unknown
owing to erosion prior to deposition of the Tapeats Sandstone.
Within this section a number of field observations indicate that the
formation represents a fundamental change in depositional environments and tectonic activity (Fig. 17). Near the base of the section are blocks of carbonate that are surrounded by finer grained
siltstone and mudstone. The blocks lie within a narrow interval,
and individual blocks appear to be rotated, suggesting that they
were transported on bedding-parallel slip planes or slumps. In
fact, early workers postulated that these blocks were derived from
Chuar deposits east of the Butte fault, specifically the “doublet
beds” of the Walcott Member (Elston, 1979). Our more reserved
interpretation is that the carbonate blocks do not appear to be
equivalent to the doublet, and the doublet beds pinch out west of
the Butte fault (Fig. 17). More likely, the carbonate blocks were
derived from another part of the Chuar section, but we have not
been able to correlate these blocks directly to lower beds in the
Chuar Group. Soft sediment deformation of finer grained beds
within the same interval suggests that the blocks and deformation
occurred while the deposits were still soft and wet.
Farther up section the Sixtymile Formation is dominated by
thinly bedded siltstone. The siltstone beds are incised and infilled
by locally derived silicified siltstone clasts. The incised channels
are steep sided and locally as deep as 15 m (Fig. 17). The channel
fill is weakly stratified with interbedded sandstone and locally
derived breccia. The incision into siltstone of the Sixtymile Formation is interesting in that it suggests that there was some sort
of change in relative base level, meaning either the land surface
was uplifted relative to sea level, or sea level dropped. In either
case the change in depositional environments had a profound
effect on the development of the Butte fault and Chuar syncline.
Without fine-grained deposits continually burying the Butte fault
zone, continued incremental movement along the Butte fault
likely reached the surface and created a fault scarp, as suggested
by lower beds in the Sixtymile Formation. The Butte fault likely
continued to move after Sixtymile time, resulting in the complete removal of Chuar deposits from the east side of the fault
(Fig. 18D). The final extensional movement of the Butte fault
system ended late in Neoproterozoic time, prior to Tapeats Sandstone deposition, but much of that record was lost to erosion.
THE NEOPROTEROZOIC EARTH SYSTEM AS
“SEEN” THROUGH THE CHUAR GROUP
The Chuar Group captures a critical “snapshot” in Earth
systems history. The combined data sets suggest that the Chuar
Group was deposited during, or just before, the onset of lowlatitude glaciations, and during the early rifting of the supercontinent Rodinia. Researchers are currently pursuing the Chuar
Group and correlative strata to find clues as to why the ensuing
Downloaded from specialpapers.gsapubs.org on January 24, 2013
The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon
large-scale (possibly snowball-Earth-style) glaciations occurred
and how these changes affected biotic evolution. Understanding
the timing and tectonic style of the Chuar basin provides geologists with more documentation on the complex disassembly
of Rodinia, a problematic question owing to the long duration
(200 m.y.) between initial rifting and the development of the Cordilleran miogeocline (e.g., Bond and Komintz, 1984; Timmons et
al., 2001; Colpron et al., 2002).
The Butte fault and Chuar syncline provide a unique window into the regional tectonics of the western United States during Neoproterozoic time. Along much of the western margin of
North America, remnants of late Proterozoic deposits are preserved in isolated outcrops (Fig. 19). The combined record of
these deposits marks the incipient phases of a continental rift
margin. Starting ~800 m.y. ago, the western margin of the North
American crust began to thin and stretch as North America began
Baltica
.75
Windermere
Group
w
Australia
50° N
71
North
America
0.58
0.78
0.55
0.82
East
Antarctica
800-700 Ma incipient rifting of Rodinia
Ed g e o f N
A
me
r i ca
45° N
40° N
or
th
Uinta Mountain
Group
35° N
Pahrump Group
Chuar Group
30° N
500 km
120° W
Caborca Group
Gr
115° W
110° W
105° W
Figure 19. Index map of Neoproterozoic sedimentary rocks (shown in black)
and inferred Neoproterozoic structures.
The Neoproterozoic N-S tectonic grain
is inferred from the extent of Laramide
(reactivated) structures and other N-S
features. Inset shows a proposed Neoproterozoic plate reconstruction (after
Brookfield, 1993; Karlstrom et al., 1999;
Burrett and Berry, 2000). Neoproterozoic sedimentary basins (900–600 Ma) are
shaded, and ages of mafic dikes are in
billions of years.
Downloaded from specialpapers.gsapubs.org on January 24, 2013
72
Dehler et al.
to rift away from other continental blocks. Within the Windermere Group of British Columbia, the Pahrump Group of Death
Valley, and the Uinta Mountain Group of Utah, workers have
identified syn-extensional deposits, much like the Chuar Group
(Link et al., 1993; Prave, 1999; Rybczynski, 2009; Dehler et al.,
2010). The combined studies point to a major continental-scale
rifting event that affected much of western North America, and is
a proxy for what was going on worldwide at ca. 750 Ma.
The carbon-isotope excursions in the Chuar Group are of
similar magnitude and age to strata in other places in the world,
and indicate that the Chuar curve may represent a global-scale
carbon cycle signature (Dehler et al., 2005). The cyclicity in the
Chuar Group strata also reflects a global phenomenon: fluctuations in global sea level and ice volume throughout the history
of the basins, with increasing ice volume by 750 Ma (Dehler et
al., 2001). On a more local scale, changes in the shale geochemistry suggest changing rainfall patterns throughout Chuar time.
These data sets all point to a climate-regulated carbon cycle.
One possible scenario is that many Chuar-type intracratonic rift
basins formed in low-mid latitudes as the supercontinent was
breaking up. The basins acted as sediment traps and, in concert
with changing sea level and local rainfall patterns, buried enough
carbon to cause the radical shifts in the carbon curve as seen,
for example, in the upper Chuar Group. When enough carbon
was removed from the atmosphere and buried in sediments, this
could have caused a global CO2 drawdown from the atmosphere,
potentially significant enough to bring glaciers to lower latitudes
and elevations.
The paleontological record of the Chuar Group provides
indirect evidence for the onset of low-latitude glaciation as
well. The lower Chuar Group documents a diverse commu-
nity of ornamented acritarchs (Nagy et al., 2009), consistent
with other evidence that suggests that eukaryotes were diversifying during this time (Porter, 2004). In the upper Awatubi
Member, however, ornamented acritarchs disappear, replaced
in both shallow- and deeper water environments by blooms of
the bacterium Sphaerocongregus variabilis. This fossil is typically associated worldwide with “snowball Earth” glaciations;
indeed, blooms of S. variabilis have been recovered from synglacial deposits (sediments deposited during glaciations) (Knoll
et al., 1981). The appearance of bacterial blooms in the Chuar
Group coincides with evidence for increased ice volume on the
globe (inferred from stratigraphic cycles; see above), as well as
biomarker evidence that the Chuar sea may have become euxinic (anoxic and sulfidic) at this time. Interestingly, this shift
also coincides with the first appearance of VSMs—fossils of
heterotrophic protists that may have proliferated because of the
organic-rich environment.
Research continues on the Chuar Group, and perhaps in the
future there will be more concrete answers as to what caused the
unusual changes recorded in these strata. If you are lucky enough
to see the Chuar Group from one of the canyon rims, from the air,
or on a hike, think of these strata not only as the gorgeous painted
rocks of the Grand Canyon desert but as a “snapshot” in time,
~750 m.y. ago. Along this ancient, warm, quiet shoreline, shallow
seas came in and out as glaciers waxed and waned in other nearby
latitudes, diverse protists and bacteria floated in the water or grew
on the surface of carbonate sands, and the ground occasionally
shook from movement on the nearby Butte fault.
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