Mid-Neoproterozoic gypsite
Thick sulfate evaporite accumulations marking a mid-Neoproterozoic oxygenation event (Ten Stone Formation, Northwest Territories, Canada) E.C. Turner 1,† and A. Bekker 2,† Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada Department of Earth Sciences, University of California–Riverside, Riverside, California 92521, USA
1 2
ABSTRACT Thick sulfate evaporite accumulations are absent from Proterozoic strata between ca. 2000 and ca. 1000 Ma, and detailed sedimentologic, stratigraphic, and geochemical data for the oldest Neoproterozoic thick marine sulfate evaporite successions are largely lacking. The middle Neoproterozoic Ten Stone Formation (Little Dal Group, Northwest Territories, Canada) consists of ~500 m of pelagic lagoonal gypsite and anhydritite (rocks consisting of the minerals gypsum and anhydrite) deposited shortly before the ca. 811 Ma Bitter Springs carbon isotope anomaly in an intracratonic basin that developed prior to breakup of Rodinia. The thickness of regional stratigraphic subdivisions of this formation, defined by subtle silt- and carbonate-bearing intervals, indicates a minor terrigenous source in the southeast and a silled connection to the open ocean in the northwest. Deposition of the Ten Stone Formation began with abrupt, tectonically triggered subsidence and restriction, and ended equally abruptly, as shown by stratigraphic contacts across which lithofacies corresponding to strikingly different paleoenvironments change sharply, with no evidence for hiatus or erosion. Stratigraphic cyclicity in the evaporite succession is minimal owing to isolation of bottom-hugging, dense lagoonal brine from overlying waters. Deposition of the Ten Stone Formation in a basin that experienced intermittent, basin-scale tectonic adjustments, as recorded by details of its stratigraphy, supports the interpretation that the Mackenzie Mountains Supergroup accumulated in an extensional, tectonically active intracratonic basin whose structure resembled a lowerplate extensional system. The absence of halite from the Ten Stone Formation † E-mails: eturner@laurentian.ca; .bekker@ucr.edu
andrey
contrasts with its abundance in the stratigraphically lower, gypsum-free Dodo Creek Formation, suggesting that deposition of the lower to middle L ittle Dal Group spanned a major oxygenation event, during which the sequence of evaporite mineral precipitation from seawater changed from halite-first to sulfate-first in response to rapid accumulation of atmospheric oxygen and concomitant increase in the global marine sulfate reservoir. The limited range of sulfur isotope values in a new data set spanning hundreds of meters of gypsite indicates a strongly and persistently oxidizing mid-Neoproterozoic atmosphere, an abundance of sulfate in seawater, and marine oxygenation extending below storm wave base. The mineralogy, sedimentology, stratigraphy, and geochemistry of the Ten Stone Formation are virtually indistinguishable from those of thick, Phanerozoic “deep-water” (below wave-base) evaporite successions, and indicate that the tectonic, climatic, and geochemical conditions required for deposition of thick successions of marine sulfate evaporites were well established prior to ca. 811 Ma. Thick sulfate evaporite successions in equivalent stratigraphic positions just below the Bitter Springs carbon isotope excursion elsewhere in Laurentia, as well as on the Congo craton, and in South Australia attest to the global impact of the rapidly increased seawater sulfate reservoir prior to Rodinia’s breakup. High relative burial rates of organic matter prevailed before the breakup of Rodinia and led to oxygenation of the atmosphere-ocean system, growth of the seawater sulfate reservoir, and, in association with a warm and arid climate, deposition for the first time in Earth’s history of thick sulfate evaporites in the middle Neoproterozoic, ~100 m.y. before the first Cryogenian glacial episode. The Neoproterozoic Oxygenation Event may have taken place in several steps, the first of which preceded the Bitter Springs anomaly.
INTRODUCTION Recent investigations of Precambrian Earthsurface environments have focused predominantly on their geochemical evolution, yielding important data for the timing and means by which a consistently oxygenated atmosphere and ocean evolved, with attendant implications for the eventual emergence of metazoans. The drive to decipher the tectonic events associated with the late Mesoproterozoic assembly and Neoproterozoic breakup of Rodinia has been equally strong. Although these disparate yet temporally related geochemical, biological, and tectonic events may be causally linked, the study of the physical and chemical sedimentology of the sedimentary rocks that encode much of the information used in these lines of research now lags behind the geochemical, paleontological, and geochronological studies that have been the source of many recent advances. Chemical sedimentary rocks uniquely preserve geochemical information that can and should be augmented by a nuanced understanding of associated lithofacies, which have yet to yield their complementary information. Although the redox history of Earth’s atmosphere and oceans involves a broad temporal trend from largely anoxic in the Archean to a well-oxygenated state by the early Paleozoic, Proterozoic oceanic redox evolution is understood to have been spatially and temporally complex, and many proxies are exploited in the attempt to produce a temporally well-constrained and consistent history (e.g., Farquhar et al., 2013). The oxygenation of Earth’s atmosphere-ocean system occurred predominantly in two steps: the Paleoproterozoic “Great Oxygenation Event” (GOE, ca. 2.3 Ga), which refers to the transition from a pervasively reducing Earth-surface system to one with an oxygenated atmosphere and shallow seas, and the “Neo proterozoic Oxygenation Event” (NOE), when the Earth’s atmosphere and ocean are understood to have become persistently oxygenated,
GSA Bulletin; January/February 2016; v. 128; no. 1/2; p. 203–222; doi: 10.1130/B31268.1; 7 figures; 3 tables; Data Repository item 2015240; published online 8 July 2015.
For permission to copy, contact
[email protected] Geological Society of America Bulletin, v. 128, no. 1/2 203 © 2015 Geological Society of America
Turner and Bekker with attendant implications for multicellular animal life (e.g., Johnston et al., 2012; Sperling et al., 2013; Planavsky et al., 2014). Although the protracted GOE is temporally adequately constrained, the exact timing of the geochemical inflection point or points represented by the NOE remains unclear, and the focus is generally on the Ediacaran Period (Fike et al., 2006; Canfield et al., 2007; McFadden et al., 2008; Scott et al., 2008; Sahoo et al., 2012). The redox state of the atmosphere-ocean system is closely linked to the relative burial rate of organic carbon, which is ultimately reflected by temporal variations in the ratio of stable carbon isotopes (13C and 12C) in carbonate rocks. Excessive sequestration of 12C-rich organic matter with buried sediment drives positive carbon isotope excursions in contemporaneous marine carbonate precipitates, and is accompanied by commensurate accumulation of molecular oxygen in the atmosphere and ocean. Conversely, lower rates of relative organic carbon burial with sediment on the global scale, due to either environmentally induced decrease in global biological productivity or oxidative remineralization of isotopically light organic matter (and methane) back into the marine reservoir, result in an isotopically light (negative) signature in marine carbonate precipitates and accompanies diminished oxygen levels in the atmosphere-ocean system. The resulting global carbon isotope variations through time seem to record important events in the biogeochemical evolution of Earth, and are now routinely used to date and correlate Paleoproterozoic to Cenozoic carbonate strata, commonly without adequate understanding of the material analyzed or the isotopic system involved. One of the most conspicuous Proterozoic carbon isotope anomalies is the “Lomagundi event,” a positive carbon isotope excursion between ca. 2.22 and 2.06 Ga that is interpreted to be the result of high organic carbon burial and attendant accumulation of atmospheric oxygen (Karhu and Holland, 1996; Konhauser et al., 2011; Bekker and Holland, 2012; Scott et al., 2014). The Lomagundi event is understood to have been followed by a long Paleoproterozoic and Mesoproterozoic interval with much lower levels of atmospheric oxygen and little variation in carbon isotope values, which ended in the late Neoproterozoic with dramatic fluctuations, of escalating magnitude, in the biogeochemical carbon cycle. Carbon isotope values in carbonates rose in the late Mesoproterozoic, with a further increase after ca. 900 Ma (e.g., Bartley et al., 2001, 2007; Bartley and Kah, 2004). This trend of rising carbon isotope values was interrupted by the mid-Neoproterozoic (ca. 811 Ma) Bitter
204
Springs anomaly (BSA; Fig. 1; Hill et al., 2000; Halverson et al., 2005, 2007, 2010; Macdonald et al., 2010), a negative carbon isotope excursion recording a deoxygenation event (Thomson et al., 2015), broadly contemporaneous with the appearance of the oldest eukaryote crown-group biomarkers (Brocks et al., 2014). The subsequent glacial episodes of the late Neoproterozoic (Fig. 1) arguably follow an extensive drawdown of atmospheric CO2 and attendant diminution of greenhouse effects, and are temporally closely associated with the appearance of the earliest macroscopic body fossils of marine metazoans between the two oldest and most severe Neoproterozoic glacial episodes (Fig. 1; Hofmann et al., 1990); the existence of macroscopic metazoans required the presence of a minimum, but disputed, level of dissolved O2. Under Earth-surface conditions, sulfur responds to atmospheric-oceanic redox state via changes in the terrestrial sulfur flux and pyrite burial sink. At a very low atmospheric oxygen level, pyrite would not be oxidized during continental weathering, and detrital pyrite would accumulate in fluvial and shallow-marine deposits. Detrital pyrite disappeared entirely from the rock record (with the exception of highly localized environments with extremely high sedimentation rates) after the GOE at ca. 2.3 Ga (Holland, 2002; Bekker et al., 2004), indicating that the atmospheric oxygen level necessary for oxidative continental weathering had been attained by that time, and that the riverine flux of dissolved sulfur had reached a peak and stabilized. As the riverine flux of dissolved sulfur stabilized, the pyrite burial sink, modulated by the extent of anoxic settings in the oceans, became a major control over the seawater sulfate reservoir and its residence time. The presence of oxidized sulfur compounds (primarily anhydrite and gypsum) in evaporite settings, their sulfur isotope ratios, the range of their sulfur isotope variability, and the order in which evaporite minerals precipitate from seawater correspond to the seawater sulfate level. The presence of sulfate evaporites implies (1) that the atmosphere and deep oceans at the time of their deposition contained enough molecular oxygen to maintain marine sulfate concentrations at a level of saturation with respect to common sulfate minerals such as gypsum and anhydrite, and (2) that their depositional environments were also adequately oxygenated. The Proterozoic history of sulfate evaporite minerals is complex. Sulfate pseudomorphs appeared for the first time in the early stage of the GOE, and sulfate pseudomorphs and layers are particularly conspicuous in rocks contemporaneous with the ca. 2.22–2.06 Ga Lomagundi carbon isotope excursion (Melezhik et al.,
2005; Bekker et al., 2006; Schröder et al., 2008; Morozovet al., 2010; Gorbachev et al., 2011; Scott et al., 2014). The Lomagundi event was followed by deoxygenation and attendant contraction of the seawater sulfate reservoir (Bekker and Holland, 2012; Planavsky et al., 2012), and only minor evidence of sulfate evaporite precipitation (pseudomorphs) has been reported from Paleoproterozoic and Mesoproterozoic rocks younger than 2.0 Ga (Walker et al., 1977; McClay and Carlile, 1978). Thin beds of sulfate evaporites reappeared ca. 1.1 Ga (Jackson and Iannelli, 1981; Whelan et al., 1990; Kah et al., 2004; Turner, 2009a; for depositional age, see Turner and Kamber [2012]), and truly thick accumulations thereof appeared for the first time only in the middle Neoproterozoic (Scott et al., 2014). This overall trajectory is thought to record the first growth of a seawater sulfate reservoir during the GOE, a peak in sulfate evaporite deposition associated with the high atmospheric and oceanic oxygen concentrations of the Lomagundi event, and minimal sulfate precipitation during the long interval of comparatively low atmospheric and oceanic oxygen concentration extending from the Lomagundi excursion to the NOE. This protracted episode of low oxygen concentration and minimal to no sulfate deposition ended in the Neoproterozoic, when Earth-surface oxygenation, the marine sulfate reservoir, and sulfate evaporite precipitation at last reached levels comparable to those of the Phanerozoic. Sulfate evaporite minerals belong to an assemblage of minerals that precipitate sequentially during evaporative concentration of marine water: each mineral reaches saturation at a different stage in the evaporation of an aliquot of seawater (Hardie and Eugster, 1970; Harvie et al., 1980). In the modern, well-oxygenated world, as in evaporite successions deposited throughout the Phanerozoic, the sulfate minerals gypsum and anhydrite precipitate from evaporating seawater after aragonite or calcite, but before halite. At a lower seawater sulfate level, however, gypsum and anhydrite precipitate after halite, under Na+ and Cl– concentrations similar to those of the modern ocean. Some postLomagundi, Paleoproterozoic marine evaporite successions show clear evidence of halite precipitation before gypsum or anhydrite or even in the absence of associated gypsum or anhydrite (e.g., ca. 1.88 Ga Stark Formation; Pope and Grotzinger, 2003, and references therein), which is construed as evidence for a limited marine sulfate reservoir and low atmospheric oxygen (Scott et al., 2014). In contrast, Lomagundi-age sedimentary successions contain evidence for sulfate precipitation before halite (Melezhik et al., 2005; Bekker et al., 2006;
Geological Society of America Bulletin, v. 128, no. 1/2
Ediacaran
Cambrian
Mid-Neoproterozoic gypsite
500
A
[O2] %PAL
B
C
δ C ‰ VPDB marine carbonate 13
δ34S ‰ VCDT marine CAS & evaporite
Ara anomaly
5 Gaskiers glaciation
Shuram anomaly
600
4 Maieberg anomaly
Cryogenian
700
Marinoan glaciation
Trezona anomaly oldest uncontested Tayshir metazoan body fossils anomaly (Hofmann et al., 1990) Rasthof anomaly
3
Sturtian glaciation
vase-shaped microfossils (Strauss et al., 2014)
Islay anomaly
oldest crown-group metazoan biomarkers (Brocks et al., 2014)
800
900
Bitter Springs anomaly
Ram Head Fm.
Snail Spring Fm. Ten Stone Fm.
100 -10
(Canfield, 2005; Shields-Zhou & Och, 2011)
oldest thick marine sulfate evaporite successions; oldest gypsum-first Neoproterozoic evaporite successions (Ten Stone Fm., MMSG); Minto Inlet Fm., SSG)
1
This study
oldest petrographically discernable metazoan-grade tissue (Neuweiler et al., 2009)
Ma 10
Halverson, 2006
-5
gypsum-first marine evaporite (Kilian Fm., SSG)
2
Gayna Fm
Tonian
Yudomski event
0
(pink = Halverson, 2006) (red = Halverson et al., 2007, 2010; Macdonald et al., 2010; Saltzman & Thomas, 2012)
5
5
15
25
youngest halite-first marine evaporite (Dodo Creek Fm., MMSG)
(Gorjan et al., 2000; Halverson et al., 2010)
35
45
Figure 1. Neoproterozoic geochemical events and position of stratigraphic units referred to in this paper (MMSG—Mackenzie Mountains Supergroup; SSG—Shaler Supergroup). Fm.—Formation. (A) General understanding of late Neoproterozoic atmospheric oxygen concentration (source references at bottom). Detailed trajectory of Neoproterozoic atmospheric oxygen concentration has yet to be determined, including identification of the inflection point at which an accelerated rise in global Neoproterozoic oxygen concentrations started. %PAL—percentage of present atmospheric level. (B) Positions of Gayna, Ten Stone, and Snail Spring Formations relative to established δ13C curves (pink curve, Halverson [2006], data from MMSG; red curve, references in figure, global composite curve) are based on formation contacts and δ13C values in Halverson (2006). A composite curve of δ13C values obtained in this study (blue line) fills part of a gap in the pre-existing MMSG data (Halverson, 2006), and delineates new, small-scale pre–Bitter Springs carbon isotope anomaly (BSA) excursions that might be of global significance. VPDB—Vienna Peedee belemnite. (C) Green curve indicates composite δ34S curve for the Ten Stone Formation (this study). Pale green dots for existing δ34S sulfate (evaporite and carbonate-associated sulfate) data are from compilation of Halverson et al. (2010), but data for sulfate deposited after the BSA (lower Loves Creek Member of the Bitter Springs Formation) have been adjusted to a younger depositional age (younger than ca. 811.5 Ma; Macdonald et al., 2010) than was available when the data were originally published (Gorjan et al., 2000). Numbered intervals in the sulfur isotope field refer to stages in deposition of mid- to late Neoproterozoic bedded sulfate evaporites referred to in the text; horizontal arrows indicate the range of isotopic values for each stage. VCDT—Vienna Canyon Diablo Troilite.
Geological Society of America Bulletin, v. 128, no. 1/2 205
Turner and Bekker Schröder et al., 2008). Sulfur isotope values of sulfates also record expansion and contraction of anoxic settings: a higher burial rate of pyrite in anoxic settings is reflected by positive shift in sulfur isotope values of sulfates, whereas ocean oxygenation results in a negative shift (e.g., Claypool et al., 1980; Strauss, 1997). Furthermore, expansion of anoxic settings decreases the size of the seawater sulfate reservoir, resulting in highly variable sulfur isotope records of sulfate evaporites and carbonate-associated sulfate (Kah et al., 2004). Biogeochemical carbon and sulfur cycles both control and respond to redox changes in surface environments in such way that high relative burial rates of organic matter and pyrite result in positive carbon and sulfur isotope excursions in carbonate and sulfate records, respectively, and oxygenation of the atmosphere-ocean system, whereas episodes of low relative burial rates of organic carbon and pyrite result in negative carbon and sulfur isotope excursions, respectively, with decreased oxygen flux to the atmosphere-ocean. For the Phanerozoic, carbon and sulfur isotope records show an inverse trend, suggesting partial compensation for atmospheric oxygen fluctuations within carbon and sulfur biogeochemical cycles (Veizer et al., 1980; Planavsky et al., 2012). Sulfur isotope records also reflect a secular increase in fractionation between oxidized and reduced sulfur species in association with the GOE and NOE (Canfield and Teske, 1996), whereas carbon isotope records show no irreversible secular trend throughout Earth’s history, despite dramatic, long-term carbon isotope excursions in the early Paleoproterozoic and late Neoproterozoic. This paper integrates sedimentologic and complementary geochemical data to address the broad tectonostratigraphic and geochemical implications of a sedimentary system that appeared for the first time in the mid-Neoproterozoic, lagoonal sulfate evaporites. The >500-m-thick Ten Stone Formation (Northwest Territories, Canada) was deposited ~200 m.y. after assembly of Rodinia, just prior to the BSA, and ~100 m.y. before the Cryogenian glacial events. This formation accumulated in one of a pair of contemporaneous intracratonic basins (Amundsen and Mackenzie Basins) during a time of poorly understood, protracted regional extension and subsidence in northwestern Laurentia that presaged the breakup and dispersal of Rodinia during the Ediacaran. GEOLOGICAL SETTING The ~4-km-thick early to middle Neoproterozoic Mackenzie Mountains Supergroup (Fig. 2; 775 Ma [Jefferson
206
and Parrish, 1989; Heaman et al., 1992; Sandeman et al., 2014; Milton et al., 2014]) was deposited in an irregularly subsiding, moderately extensional, intracratonic basin. The uppermost ~2.5 km of the Mackenzie Mountains Supergroup constitutes the Little Dal Group, a succession dominated by shallow-marine carbonate rocks and minor fine-grained terrigenous strata. The middle of the Little Dal Group contains a ~500-m-thick gypsite and anhydritite (rocks made of the minerals gypsum and anhydrite, respectively) interval, the Ten Stone Formation (Turner and Long, 2012; formerly the informal “Gypsum formation” of Aitken, 1981), the subject of this paper. The Ram Head Formation is stratigraphically above the Ten Stone Formation in the Mackenzie Mountains Supergroup (Fig. 2), and is considered to be chemostratigraphically correlative to part of the upper Fifteenmile Group of Yukon, because both record a carbon isotope anomaly identified as the BSA (Halverson et al., 2005, 2007, 2010; Macdonald et al., 2010). Dating of strata just below the BSA in the upper Fifteenmile Group, together with this chemostratigraphic correlation, suggests that the depositional age of the Ten Stone Formation is slightly older than 811 Ma (Macdonald et al., 2010). The Ten Stone Formation is considered to be correlative to a thick evaporite succession (Minto Inlet Formation) in the ageequivalent Shaler Supergroup of the Amundsen Basin (Fig. 2; Rainbird et al., 1996, 1997; Long et al., 2008; Prince, 2014) in northern mainland Canada and adjacent islands of the Canadian Arctic archipelago. Geochronological results from Re-Os dating of black shales in the lower Wynniatt Formation (van Acken et al., 2013), a Shaler Supergroup unit recording the BSA (Thomson et al., 2015) and thought to be ageequivalent to the lower Ram Head Formation, yielded a depositional age of 848 ± 49 Ma, which, within analytical error, is compatible with the previously discussed interpretation that the underlying Ten Stone Formation was deposited just before 811 Ma. The Gayna Formation, underlying the Ten Stone Formation, is thought to be stratigraphically equivalent to the Boot Inlet Formation of the Shaler Supergroup, which yielded a Re-Os date of 892 ± 13 Ma (van Acken et al., 2013). Strata of the recently formalized Mackenzie Mountains Supergroup (Turner and Long, 2012; Long and Turner, 2012) are exposed in the eastern Mackenzie Mountains (Northwest Territories; Aitken, 1981; Long et al., 2008; Turner and Long, 2008; Martel et al., 2011) and in the Wernecke Mountains (Yukon; Thorkelson et al., 2005; Turner, 2011a). The Little Dal Group is, however, exposed almost exclusively in a
narrow band parallel to the northeast-vergent Plateau fault (Figs. 2B–2C); based on isopach data and lithofacies distribution for the Mackenzie Mountains Supergroup (Long et al., 2008; Turner and Long, 2008), this orientation is also assumed to be parallel to a paleo-coastline to the northeast. Evaporite deposits in the Little Dal Group are limited to the thick sulfate succession (gypsite or anhydritite) of the Ten Stone Formation and abundant halite casts in quartz arenite and siltstone of the Dodo Creek Formation (basal Little Dal Group; Figs. 2D, 2F) which entirely lacks sulfate evaporite minerals or their pseudo morphs. None of the carbonate-dominated stratigraphic units of the Little Dal Group contains intercalated sulfate evaporite strata, sulfate pseudomorphs, or even evidence of halite. The obvious difference in the order of evaporite mineral precipitation between the Dodo Creek and Ten Stone Formations suggests that the two formations may bracket an inflection point across which the seawater sulfate reservoir grew dramatically in response to the middle Neoproterozoic oxidation of the atmosphere-ocean system. Study of fluid inclusions in marine halite of the ca. 830 Ma Browne Formation (Officer Basin, Australia) of roughly the same depositional age as the lower Little Dal Group suggests that seawater sulfate concentrations were ≥3 mmol, ~10% of modern seawater sulfate concentration (Spear et al., 2014). The Shaler Supergroup of northern Canada, which correlates with the Mackenzie Mountains Supergroup, contains a second marine evaporite unit, the Kilian Formation (Fig. 2E; Rainbird et al., 1994, 1996; Prince, 2014), which overlies the BSA stratigraphic interval (Jones et al., 2010) and shows a sulfate-first sequence of evaporite minerals. A seemingly correlative post-BSA evaporitic unit in the Mackenzie Mountains, the Redstone River Formation (Fig. 2D), is thought to be at least partly non-marine (Ruelle, 1982; Jefferson and Parrish, 1989) and, because it cannot be assumed to record consistently marine global geochemical conditions, was not the focus of the present study. The regional extent, lateral continuity, and depositional setting of the Ten Stone Formation and its constituent units have been disputed since the Geological Survey of Canada first mapped the area in the 1970s (Aitken, 1981) and contemporaneous exploration work took place at the Gayna River zinc property (Hewton, 1982). No complete exposures of the formation are known, but undeformed lower and upper contacts are exposed in several locations. The present work is based on three stratigraphic sections along a depositional and structural strike length of 185 km (Figs. 2–4; Turner, 2009b).
Geological Society of America Bulletin, v. 128, no. 1/2
Mid-Neoproterozoic gypsite 130°W
74°N
A FIELD AREA
B
CANADA
eR
r
MACKENZIE MOUNTAINS SUPERGROUP EXPOSURE AREA
.
ive
66°N Keele R.
64°N
Fig. 7
TRACE OF PLATEAU FAULT (simplified)
62°N
N
132°W
130°W
128°W
62°N
130°W
126°W
RAPITAN GP.
rift basins
723 Ma (U-Pb)
non-marine evaporite
Natkusiak Fm.
?
?
Kuujjua Fm.
sulfate-first marine evaporite
Kilian Fm.
SHALER SUPERGROUP
Bitter Springs Anomaly
SNAIL SPRING FM.
TEN STONE FM.
CM
sulfate-only marine evaporite
GAYNA FM. STONE KNIFE FM. / SILVERBERRY FM.
Wynniatt Fm.
Minto Inlet Fm.
Boot Inlet Fm. Aok Fm.
halite-first marine evaporite
DODO CREEK FM.
Jago Bay Fm. Fort Collinson Fm.
Nelson Head Fm.
F
Mikkelsen Islands Fm.
377 m thick; incomplete), near the Gayna River zinc camp, the lower contact is in the subsurface, but the upper contact is exposed. At Stone Knife River (SKR; >483 m thick; incomplete), the basal contact is exposed, but the formation is erosionally truncated and overlain by Cambrian redbeds (Nainlin Formation; MacNaughton and Fallas, 2014) and dolostone of the Franklin Mountain Formation (Cambrian–Ordovician; Turner, 2011b). At Nidhe Brook (NB; >301 m thick; incomplete), the upper contact is exposed, and although the basal contact is identifiable, the stratigraphy and true thickness of the lower part of the formation are uncertain due to tectonic deformation. The first two exposures are 55 km apart parallel to both the presumed trend of the basin margin and to the present-day structural grain of the orogen; the second and third are separated by 130 km along strike. The section at SKR is the only location where the uppermost Gayna Formation together with an undeformed contact with the Ten Stone Formation have been identified. Together, these sections span the full depositional thickness of the Ten Stone Formation, exposing the formation’s lower and upper contacts and nine stratigraphic units (Fig. 4). Samples of Ten Stone Formation gypsite were collected at FC and SKR only, because of logistical constraints. Carbon and oxygen isotope analyses on carbonates were performed at Hatch Labs (University of Ottawa) and at Stable Isotopes for Innovative Research Laboratory (SIFIR) Laboratory (University of Manitoba) according to standard protocols. Sulfur isotope analyses of evaporite material and carbon isotope analyses of organic carbon were performed at SIFIR Laboratory following standard protocols. See the GSA Data Repository1 for detailed analytical methods.
formation’s exposure area, ubiquitous, shallowmarine (above storm wave base to intertidal) sedimentary structures in siltstone and quartz sandstone in the lower half of the Dodo Creek Formation include syneresis cracks, halite casts, hummocky cross-stratification, gutter casts, tool marks (including halite crystal drag marks), and asymmetrical and symmetrical ripple crosslamination. The upper part of the formation contains less sandstone, fewer physical sedimentary structures, rare halite casts, and carbonate mudstone layers. The formation contains no evidence of the former presence of gypsum or anhydrite, but centimetric halite casts are abundant on the soles of sandstone beds (Figs. 2D, 2F). Gayna Formation The Gayna Formation (Figs. 2D, 3, and 4; Turner et al., 2011; Turner and Long, 2012), underlying the Ten Stone Formation, consists predominantly of massive, resistantly weathering, subtly cross-stratified ooid dolograinstone, alternating with recessive, thin-bedded, desiccation-cracked, commonly argillaceous dolomudstone (“platy dolomite lithofacies” of Aitken [1981]; “recessive intervals” of Turner et al. [2011]). At the SKR location (Fig. 2C), massive, cross-stratified, oolitic dolostone of the Gayna Formation with a thin stromatolite layer at the top (Fig. 3E) is overlain by ~54 m of yellowish- to buff-weathering, rippled, desic cation-cracked, quartz-silty dolomudstone (Fig. 3F). Quartz silt content diminishes markedly in the uppermost 14 m of the formation. A simi-
lar lithofacies is present at the top of the Gayna Formation at NB (Fig. 2C), although the overlying evaporite rocks are deformed and the lower part of the Ten Stone Formation cannot be documented. Ten Stone Formation The Ten Stone Formation (Turner, 2009b; Turner and Long, 2012; Figs. 2–5) consists predominantly of weakly banded, white-weathering gypsite (Fig. 5A) together with subordinate gray-weathering anhydritite. The petrographic character of the bedded sulfate is extremely variable, including mosaics of tabular, sandsized crystals (anhydrite) and felted meshes of acicular crystals (gypsum). In outcrop and some gypsite thin sections, cross-lamination is locally outlined by trace amounts of terrigenous silt or carbonate (Figs. 5B and C), in spite of the inferred dehydration and rehydration of the sulfate minerals during burial and exhumation. The contact with dolostone of the underlying Gayna Formation (well exposed only at SKR; Figs. 3B, 3G, 3H) is sharp and lacks erosional or meteoric alteration features. The Ten Stone Formation expresses a subtle stratigraphic pattern defined by color and mineral composition (Figs. 3, 4). The first white interval (29 m thick at SKR; Figs. 3B, 3G, 3H, 4) consists of massive, white- and pale gray–weathering gypsite or anhydritite. A 3-m-thick interval with minor, pale chert nodules, 1–5-cm-thick mediumgray dolostone bands, and sparse argillaceous interlayers is present 10 m from the base.
Figure 3 (on following page). (A–D) Exposures of the Ten Stone Formation at three measured localities and another location showing a more typical exposure. (A) At Fugue Creek, evaporite is sharply but conformably overlain by the Snail Spring Formation dolostone (above yellow line). Dashed red lines show location of the two traverses forming composite section 06-FC (stratigraphic thickness of the exposed evaporite section is ~375 m). View to southwest. (B) At Stone Knife River, basal contact with the Gayna Formation is well exposed, but evaporite is unconformably overlain by the Cambrian Nainlin Formation. Dashed red line indicates location of section SKR (stratigraphic thickness of measured section is 560 m). View to north. (C) Nidhe Brook section, including abrupt but conformable contact with the overlying Snail Spring Formation. Lower part of the Ten Stone Formation is not exposed. LITHOSTRATIGRAPHY Dashed red line shows location of section NB (stratigraphic thickness of measured section is ~250 m). View to northeast. (D) Weathered exposure of steeply dipping evaporite strata on Dodo Creek Formation the ridge south of section NB showing cyclic gray carbonate stripes and red layers containThe Dodo Creek Formation (Turner and ing terrigenous mud. Width of field of view is ~600 m; view to northeast. (E–F) Lithofacies Long, 2012) is a thin (
