Glendonites in Neoproterozoic low-latitude, interglacial,
sedimentary rocks, northwest Canada: Insights into the Cryogenian
ocean and Precambrian cold-water carbonates
Noel P. James
Guy M. Narbonne
Robert W. Dalrymple
T. Kurtis Kyser
Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada
ABSTRACT
Stellate crystals of ferroan dolomite in neritic siliciclastic and carbonate sedimentary
rocks between Sturtian and Marinoan glaciations in the Mackenzie Mountains are interpreted as replaced glendonites. These pseudomorphs after ikaite indicate that shallow
seawater at that time was near freezing. Stromatolites verify that paleoenvironments were
in the photic zone and physical sedimentary structures such as hummocky cross-bedding
confirm that the seafloor was repeatedly disturbed by storms. Glendonites within these
low-latitude, continental shelf to coastal sedimentary deposits imply that global ocean
water during much of Cryogenian time was likely very cold. Such an ocean would easily
have cooled to yield widespread sea ice and, through positive feedback, growth of lowlatitude continental glaciers. In this situation gas hydrates could have formed in shallowwater, cold shelf sediment, but would have been particularly sensitive to destabilization
as a result of sea-level change. Co-occurrence of pisolites and glendonites in these rocks
additionally implies that some ooids and pisoids might have been, unlike Phanerozoic
equivalents, characteristic of cold-water sediments.
Keywords: glendonite, Neoproterozoic, limestone, Mackenzie Mountains, snowball Earth.
INTRODUCTION
The nature of the world during middle Neoproterozoic time (Cryogenian) is one of the most contentious issues in modern geology
(Hoffman and Schrag, 2002; Kennedy et al., 2001; Eyles and Januszczak, 2004). Not only were there profound fluctuations in the carbon
cycle (e.g., Kaufman, 1997), but the presence of glacial deposits worldwide has led to the snowball Earth hypothesis (Kirschvink, 1992; Hoffman et al., 1998), wherein the globe is postulated to have been completely covered with glacial ice for periods as long as 10 m.y.
Resolution of this enigma is fundamental to understanding evolution
of the biosphere (Narbonne et al., 1994; Runnegar, 2000; Hoffman and
Schrag, 2002; Narbonne and Gehling, 2003) and the ocean-atmosphere
system (Kaufman and Knoll, 1995; Kennedy et al., 2001; James et al.,
2001). Although many explanations are plausible, constraints for any
theory must come from the sedimentary rock record. To date most
attention has been focused on cap carbonates, whereas interglacial
rocks, with some notable exceptions, have received much less study.
Neoproterozic sedimentary rocks in the Mackenzie Mountains
constitute a well-exposed, easily accessible, relatively unaltered succession that includes both glacial and interglacial deposits (Fig. 1). The
Rapitan strata are correlated with similar Sturtian diamictites elsewhere, whereas the Ice Brook strata are considered coeval with Marinoan glacigene deposits (Young, 1992; Kaufman et al., 1997; Kennedy
et al., 1998; Hoffman and Schrag, 2002). The Windermere Supergroup
is otherwise a series of terrigenous-clastic and carbonate continentalmargin sedimentary deposits (Narbonne et al., 1994; Narbonne and
Aitken, 1995) that accumulated within 108 of the paleoequator (Park,
1997; Evans, 2000). The purpose of this paper is to report on the
discovery of dolomitized glendonite in interglacial strata and discuss
implications for Cryogenian Neoproterozoic paleoceanography and
Proterozoic carbonate deposition.
GEOLOGICAL SETTING
Sedimentary rocks between Rapitan (Sturtian) and Ice Brook
(Marinoan) glacigene strata (Fig. 1) compose an ;1.2-km-thick grand
cycle of shale grading upward to carbonate. The Twitya Formation is
a succession of lower dark shales (;400 m), middle siliciclastic turbidites and shales (;300 m), and upper sandstones, conglomerates,
carbonates, and minor shales (;350 m). The Keele Formation is a
mostly carbonate unit, the ;300 m thick lower part of which is markedly cyclic at the decameter scale; each package grades upward from
dark, deeper-water lime mudstone to intraclast-oolitic shallow-water
grainstone (Day et al., 2004). Ooids have a texture indicating that the
original mineral was aragonite. The ;150-m-thick upper part (locally
called the Keele clastic wedge) is a shallow-water shelf, shoreface, and
fluvial sandstone with conspicuous carbonate flat-pebble tempestites at
the top, just beneath Ice Brook diamictites.
Stellate dolomite occurs in the upper Twitya Formation (Fig. 1),
rocks also referred to as the Twitya-Keele transition. These siliciclastic
and carbonate strata are interpreted to have accumulated in a series of
neritic paleoenvironments, many of which were storm dominated, that
ranged from shallow open shelf to shoreface to fluvial. Pebble to granule
conglomerates, sandstones (locally glauconitic), siltstones, carbonates,
and minor shales are interbedded at the meter scale. Storm deposition is
indicated by abundant hummocky cross-stratification, local swaley crossstratification, load casts, and ball and pillow structures. Wave ripples are
common, as are gutter casts, syneresis cracks, and parting lineations.
Fining-upward tempestites with erosive bases are organized into upwardcoarsening progradational successions (i.e., parasequences). Fluvial sed-
q 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; January 2005; v. 33; no. 1; p. 9–12; doi: 10.1130/G20938.1; 3 figures.
9
Figure 2. Dolomite pisoids, thin-section photomicrographs, plane
light. A: Compound pisoid with several smaller ooid fragments enclosed by laminated rind. B: Compound particle composed of numerous ooids cemented together and surrounded by laminated cortex. C: Single superficial pisoid with nucleus replaced by coarse and
saddle dolomite; cortex is relatively well preserved as microcrystalline dolomite. D: Laminated concentric pisoid cortex with vague
brick-like texture indicating that original mineral was aragonite.
particles are classified as cerebral coated pisoids (cf. Richter, 1983) because the cortex is not circumnuclear isopachous, but lumpy and irregular. Cores, where locally near mimetic, are composed of ooids with a
grapestone-like arrangement (Fig. 2B). The cortex is generally less than
one-third the diameter of the grain, and in the best-preserved samples is
laminated concentric (Fig. 2D) with a brick-like microtexture, indicative
of precursor aragonite (cf. Davies and Nassichuk, 1990).
Figure 1. Inset: Location of study area. Left: Stratigraphic column,
lower part of Windermere Supergroup. Right: Stratigraphic column
at 648409300N, 1298549150W, upper part of Twitya Formation (TwityaKeele transition) showing lithologies and location of glendonite
pseudomorphs (stars).
imentary deposits are trough cross-bedded with coarser conglomeratic
intervals having seaward-directed paleocurrents and imbricated clasts.
Alternating distinctively different thin-bedded strata attest to highfrequency sea-level oscillation that caused oscillation between offshoretransitional, shoreface-coastal, and fluvial paleoenvironments.
Meter-scale carbonates are stromatolitic and pisolitic dolostone. Laterally linked hemispherical stromatolite units are as thick as 1 m, have
erosional bases, and are nucleated on flat pebbles. Most 1–2-m-thick
storm deposits comprise a thick lower pisoid (giant ooid) rudstone that
grades upward to an intraclast-peloid grainstone with a centimeter thick
sandstone cap. The capping sandstone has hummocky and swaley crossstratification, climbing ripples, and wave ripples with wavelengths as
great as 50 cm. Pisoids are dolomite with crystals ranging from coarse
rhombs to saddle to mimetic (Fig. 2). Grains are 2.0–4.5 mm in diameter
and vary from single (Fig. 2C) to compound (Fig. 2A) pisoids. Such
10
STELLATE DOLOMITE AND ITS INTERPRETATION
Crystal pseudomorphs occur as stellate clusters of brown, recessiveweathering, iron-rich, medium- to coarse-crystalline dolomite (Fig. 3),
in a stratigraphic section (Fig. 1) along a tributary of the Stoneknife
River. They were observed at three horizons (Fig. 1), in dark siltstone,
medium-grained sandstone with granule layers, and pisoid rudstone.
Clusters are subspherical to elongate and range in size from ;1 to 8
cm. Although typically localized to a layer one rosette thick, they also
occur dispersed over thicknesses of as much as 20 cm. Dolomite crystals
have straight to sweeping extinction and under cathodoluminescence are
black rhombs with phases of dull and bright dolomite cement. Pseudomorphs radiate out from a central nucleus or elongate core in the case
of lozenge-shaped forms (Fig. 3). Margins between crystal pseudomorphs are difficult to discern, but terminations are sharp, either low
pyramidal or flat. There is minor distortion of surrounding sedimentary
laminations, and sand grains are locally enclosed within clusters.
These pseudomorphs are interpreted, on the basis of morphology
and crystal habit, to be dolomitized glendonite. Glendonite is interpreted as a calcite pseudomorph after metastable ikaite (CaCO3·6H2O).
Growth habits herein are similar to those illustrated from rocks
throughout the geological record (for illustrations and reviews see Kaplan, 1979; Shearman and Smith, 1985; De Lurio and Frakes, 1999).
Crystal morphology is compatible with glendonite pseudomorphs of
various names (Dana, 1884; Shearman and Smith, 1985; Swainson and
Hammond, 2001) but is unlike that of aragonite (Riccioni et al., 1996).
Whereas most glendonites are calcite pseudomorphs (Swainson and
Hammond, 2001), interpreted ferroan dolomite crystal pseudomorphs
are documented from Cryogenian Neoproterozoic limestones (Spencer,
1971; Johnston, 1995).
GEOLOGY, January 2005
Figure 3. Dolomitized glendonites (finger is 1.5 cm wide). A: Large stellate crystal clusters with well-preserved pseudomorph terminations
in sandstone (centimeter scale). B: Large pseudomorph in sandstone. C: Large elongate pseudomorph in coarse-grained sandstone with
crystals radiating out from linear core and entombing some siliciclastic granules. D: Several tiny pseudomorphs in dark siltstone. E:
Numerous small clusters with well-preserved pseudomorphic terminations. F: Thin-section photomicrograph, cross-polarized light, showing
dolomite-replaced pseudomorphs radiating out from poorly defined core.
Ikaite occurs naturally only at water temperatures between 21.9
and 17 8C (Pauly, 1963; Suess et al., 1982; Buchardt et al., 1997). In
marine sediments it grows displacively at or just below the sedimentwater interface (Suess et al., 1982; Stein and Smith, 1985). Ikaite stability increases with decreasing temperature (Marland, 1975), unlike
anhydrous minerals such as calcite, aragonite, and vaterite. Ikaite, however, is severely understaturated in modern natural marine waters even
at temperatures approaching freezing, and precipitation requires extraneous Ca11 or HCO23 (specifically a 10-fold increase in alkalinity relative to normal seawater), and elevated concentrations of orthophosphate (Bischoff et al., 1992). In modern marine environments chemical
preconditions are satisfied through the suboxic microbial degradation
of organic matter (Suess et al., 1982). The d13C values for modern
ikaites believed to precipitate from seawater range from ;219‰ to
;232‰ (relative to the Vienna Peedee belemnite [VPDB] isotope
standard), while d18O values are generally 0‰ to 14‰ (relative to the
VPDB isotope standard) (Shearman and Smith, 1985). The d13C and
d18O values of the dolomite replacing the Neoproterozoic glendonites,
however, are ;21‰ and 210‰, respectively, identical to the values
of carbonates throughout the section (James et al., 2001). Either the
d13C values of the glendonites were totally reset during dolomitization
or the glendonites had an unusual contribution from 13C-depleted organic matter when they formed. Preservation of the pseudomorphic
crystal habit in Twitya glendonites further suggests that transformation
to calcite was slow (Larsen, 1994), implying persistent low seawater
temperatures and slow warming.
Stellate pseudomorphs after ikaite in the Twitya are thus thought
to reflect precipitation from very cold bottom waters in shallow-marine
environments. The mechanisms responsible for probable elevated alkalinity and orthophosphate are unclear. Such evidence, however, is
not always preserved in the rock record (cf. Larsen, 1994). In the context of the regional setting (James et al., 2001), the sequence of diagenetic events is interpreted as (1) ikaite precipitation in cold seawater,
(2) slow alteration to calcite in warming waters, and (3) replacement
GEOLOGY, January 2005
by dolomite. Dolomitization destroyed any relict texture that may have
been preserved in an earlier diagenetic calcite.
IMPLICATIONS
The Twitya-Keele transition records mixed siliciclastic-carbonate
deposition on a storm-dominated shelf whose water depths, on the basis
of physical sedimentary structures, were mostly above fair-weather
wave base (i.e., probably ,30 m deep). Stromatolites constrain the
setting to within the photic zone. The fine-scale interbedding of hummocky cross-stratification, swaley cross-stratification, and pisoids indicates a well-mixed water column. Precipitation of ikaite from near
freezing marine waters in such a situation implies either (1) later downward percolation of frigid marine waters (related to the subsequent Ice
Brook glaciation) through these sediments, or (2) very cold marine
waters during deposition. Later subsurface cold water percolation is
unlikely because there is more than 450 m of strata between the glendonites and Ice Brook glacigene deposits, and geothermal heating
would have warmed any subsurface fluids well above freezing. Thus
the shallow-marine shelf waters, and by implication open ocean surface
waters at this tropical latitude, were overall very cold.
Such a situation, given that the ocean and world climate are intrinsically linked, has further implications. A cold tropical ocean would
have been more conducive to widespread sea-ice formation and,
through positive feedback, a concomitant increase in continental glaciation than in a warmer world. Furthermore, cold shelf waters at shallow depths could have allowed the formation and sequestration of gas
hydrates (Kvenvolden and Grantz, 1990) on the shelf. At such locations, clathrates would have been particularly susceptible to temperature perturbations and sea-level fluctuations, especially in the context
of high-frequency sea-level oscillations suggested by Twitya-Keele
transition strata; warming would have led to quick release of methane
and a consequent temperature rise that would have been rapid and
dramatic (cf. Kennett et al., 2003).
The early and middle Neoproterozoic was a time when giant ooids
11
(pisoids) were common in shallow-marine rocks (Sumner and Grotzinger, 1993); these are particularly widespread in Cryogenian carbonates,
where they are typically found directly below glacigene strata and in
interglacial deposits (Grotzinger and James, 2000). Pisoids documented
here (Fig. 2), although locally encompassing smaller ooids, are ‘‘cerebral’’ or lumpy in appearance, an attribute that is speculated to involve
microbial processes (Richter, 1983). The presence of glendonites in these
sediments implies that the pisoids precipitated from near-freezing waters.
Although pisoid formation could be restricted to short warmer-climate
highstand periods, the requisite ;25 8C temperature changes in shallow
seawater necessary to make their environment of formation comparable
to modern ooid settings is highly unlikely. Precipitation from cold seawater could in part be due to the postulated high carbonate saturation of
Neoproterozoic seawater (Grotzinger and James, 2000). Regardless of
specifics, these observations imply that ooids and pisoids in Neoproterozoic, and possibly earlier Precambrian rocks, do not necessarily indicate
warm, tropical paleoenvironments. Since reliable environmental indicators in Precambrian rocks are far fewer than in younger Phanerozoic
carbonates, this conclusion has profound implications for the interpretation of many older successions. It would seem that with elevated
CaCO3 levels, as many have suggested, ooid deposits on the young Earth
could have formed in both cold- and warm-water settings.
ACKNOWLEDGMENTS
This research was supported by Natural Sciences and Engineering Research Council of Canada grants to the authors. We thank Tim Simmons and
Canadian Helicopters for logistical assistance and splendid flying. Research was
carried out under licenses granted by Aurora College, Inuvik, N.W.T. The paper
was greatly improved thorough reviews by P. Pufahl.
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Manuscript received 25 June 2004
Revised manuscript received 4 October 2004
Manuscript accepted 6 October 2004
Printed in USA
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