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Eigenbrode and Freeman 10.1073/pnas.0607540103. |
Fig. 6. General stratigraphy of the Hamersley Province. (Modified from refs. 55, 69, and 70.) The sections covered in the three cores (WRL1, RHDH2a, and SV1) and outcrops are marked. References for SHRIMP zircon U-Pb dates [±2-20 megaannum (Ma) before present] and impact layer (IL) correlation are as follows: A (60), B (67), C (64), D [ref. 62,], E (53) (age of a northwestern deposit), and F (55) (age of a southeastern deposit). These published data were used to constrain core and outcrop correlations for dker data in Fig. 2, assuming a linear depth-time relationship between correlated and dated strata. Fm., formation.
Fig. 7. Geological map of the Hamersley Province on the Pilbara Craton noting core and outcrop locations. Beabea Creek (BC), Spider Creek (SC), and Mulga Hill (MH) are Tumbiana Formation outcrop locations. (Modified from ref. 69.)
Fig. 8. d13C of kerogen and total organic carbon abundance show no correlation. Data for the Tumbiana Formation (Fm.), which has been partially silicified, is plotted separately.
Supporting Literature for Fig. 1
The Archean-Paleoproterozoic global organic-carbon isotopic (d13Corg) record presented in Fig. 1 is a compilation of kerogen and total organic carbon (TOC) d13C values for all sedimentary rock types reported in the published literature (1-50). It is based on an updated geochronology (51-68, ,).
Supporting Materials and Methods
Samples from the Hamersley Province, Western Australia (Fig. 7), were collected from three diamond-drill cores (RHDH2a, WRL1, and SV1) encompassing five different formations (Tumbiana, Jeerinah, Marra Mamba Iron, Carawine Dolomite, and Wittenoom Dolomite; Fig. 6). Core samples were supplemented with 11 outcrop samples of partially silicified, stromatolitc-bound grainstone (n = 10) and shale (n = 1) collected from the western Tumbiana Formation at three localities: Beabea Creek, Spider Creek, and Mulga Hill (Fig. 7). Stratigraphy and lithofacies (Fig. 6) are based on hand-specimen analyses, and they were cross-checked with stratigraphic observations. Stratigraphic notes were supplemented with core log descriptions.
Core samples were cut into 1-cm chips, which were sonicated in distilled deionized water three times for 10 min. After drying, chips were lightly rinsed in dichloromethane and then powdered to <200 mm in a steel disk mill. The mill was cleaned with quartz sand (annealed at 800°C for 24 h) and rinsed with dichloromethane between samples.
Approximately 5 g of shale rock powders (25-40 g of carbonate or silica-rich material) was demineralized using 6 N HCl, 6 N HCl/28 N HF (1:1), 6 N HCl, and 1 N HNO3, with each treatment separated by rinses in distilled deionized water. Demineralization was followed by sonication extraction (three times, 30 min each in dichloromethane) to remove bitumen. Isolated kerogens were analyzed for hydrogen and carbon abundances using an NC2500 elemental analyzer (Carla Erba, Milan, Italy). d13C compositions were determined on a Finnigan MAT Delta+ XP mass spectrometer (ThermoElectron Corporation, Waltham, MA) interfaced with an ECS 4010 elemental analyzer (Costech Analytical Technologies, Inc., Valencia, CA) fitted with a 3-m chromatographic column and macroO2 loop. Kerogen samples were wrapped in silver foil with Pb3O4 for combustion. NIST CO2 gas standards (NIST RM8562, RM8563, and RM8564) and EA-combusted NIST (8541, 8542, NBS 22) and in-house solid standards, having values of -25.84 to -48.2‰, were used to calibrate reference gases and determine instrumental accuracy of 0.04 ±0.03‰ (1s) (n = 18). Blanks and calibrated working standards were analyzed frequently; and analytical precision (1s), accuracy, and sample reproducibility (>25 duplicates) were 0.04‰, 0.10‰, and <0.13‰, respectively.
Total sulfur abundance (wt %) was determined by combustion of whole-rock powder by using an elemental analyzer operated under conditions similar to those described above. TOC (wt %) was determined by combustion of a Leco C analyzer (Humble Geochemical, Inc., Humble, TX) for a large subset of samples.
Inorganic carbon isotope measurements were determined on microdrilled carbonate samples. Analyses were performed on a Finnigan MAT252 mass spectrometer (Finnigan MIT Instruments, Inc., Bremen, Germany) fitted with a Fairbanks device. NBS standards were used for calibration.
Supporting Discussion
Detailed Lithofacies Descriptions and Environmental Interpretations. WRL1 core. The Warrie Member of the Jeerinah Formation overlies basalt, and it is composed of calcareous and tuffaceous peloidal grainstones (coarse sand to silt) interbedded with silicified, calcareous, pyritic black shale (up to ~5 wt % TOC). The uppermost grainstone contains massive sulfide in the form of crystalline pyrite (up to 50%), nodules, and bands (up to 4 cm). Thorne and Tyler (71) noted tabular beds of graded, dolomite/tuff peloid sandstone beds with erosional bases, and undulatory, parallel, and ripple laminations in the Warrie Member, suggestive of wave and storm action. Similar peloids are reported in the Woodiana Sandstone, a transgressive shoreline deposited to the north (72). The Warrie Member has been interpreted as a shallow-water deposit (73), likely of marine affinity in the south, based on ripple structures and orientation (74). These interpretations suggest that it was an incipient shelf built on submerged basalt flows.
The massive sulfide at the top of the Warrie Member is indirect evidence of early diagenetic pyritization in deepwater (i.e., hardground formation), and it represent a hiatus correlative to the erosional unconformity between the Warrie and the Roy Hill Members noted by Blake and colleagues (55, 70, 74).
The Roy Hill Member of the Jeerinah Formation consists of silicified, pyrite- and organic-rich (3-12 wt % TOC), finely laminated black shale and thicker laminae of gray silt fining upward to black shale (i.e., d-e unit Bouma sequences), indicative of deepwater pelagic and hemipelagic deposition associated with a slope environment. The Roy Hill shale gradually changes upward into iron chert dolomite shale units that make up the Marra Mamba Iron Formation (MMIF).
The lower two members of the MMIF are mostly black shale, whereas the top member is dolomite chert banded iron formation (BIF). From its base to top, the entire MMIF is characterized by an increase in hematite, an increase in detrital dolomite, and a decrease in shale. The abundance of shale and the absence of graded sediments at the base of the MMIF indicate pelagic deposition in deep water. The gradual shift to dolomite-rich BIF at the top of the formation indicates increased chemical deposition from hydrothermal fluids and greater proximity to a shallow-water dolomite source. Thus, the MMIF probably records a deep basin to slope environment.
The Wittenoom Dolomite conformably overlies the MMIF, and it is composed of grainstones having tuff and dolomite peloids and intraclasts and rare carbonaceous shale beds (<10 cm), more common at the base. Gradual loss of chemical sediments across the MMIF-Wittenoom Dolomite contact suggests that shallowing continued upsection. The predominance of dolomitic grainstone indicates even greater proximity to the shallow-water dolomite source. We concur with other researchers who have interpreted the Wittenoom Dolomite grainstones as shallow-water carbonate detritus deposited in deep water as massive turbidites (75, 76). Reported inorganic-carbon isotope values from the Wittenoom Dolomite (d13Ccarb) are -0.5‰ (±2.0‰, 1s; n = 66), and they are consistent with d13Ccarb for well preserved marine carbonates from five other Archean provinces (-0.3 ± 0.7‰; n = 324) (77).
RHDH2a core. The Jeerinah Formation is dominated by organic-rich black shale, indicating mostly quiescent deposition below storm-wave base. Within this unit, two lithofacies were observed. The first consists of pyritic, massive black shale (2-4 wt % TOC), interpreted as a deposit from a relatively deep-basin environment. The second lithofacies, interpreted as a slope environment, is composed of alternating fine laminae (<50 mm) of pyritic carbonaceous mud (£9 wt % TOC) and dark gray tuff and dolomitic silt. Subordinate interbeds of subaqueous basalt, graded pelloidal grainstones, and brecciated diamictites indicate volcanic activity and episodic high-energy currents (e.g., turbiditic or storm surge) that transported shallow (shelf) sediments to the slope depositional environment. In the Carawine Dolomite, massive bedded dolomite (0.1 wt % TOC) with rare stromatolites (<3 cm) is interpreted as a peritidal shelf facies. A mixed organic-rich black shale and dolomite (marl) lithofacies is stratigraphically associated with the bedded shelf dolomites. Sublithofacies include massive dolomitic black shale (2-5 wt % TOC), interpreted as a lagoon facies, and dolomitic black shale interbedded (0.5-2 cm) with laminated dolomite, interpreted as deposition adjacent to a lagoon or between large stromatolitic reefs, where circulation may be restricted. Our environmental interpretations are generally consistent with those made by Simonson et al. (76). Reported inorganic-carbon isotope values from the Carawine Dolomite (d13Ccarb) are 0.0‰ (±0.8‰, 1s; n = 32), matching our mean value of 0.1‰ (±0.7‰, 1s; n = 10, nonslope samples), and they are consistent with d13Ccarb for well preserved marine carbonates from five other Archean provinces (-0.3 ± 0.7‰, n = 324) (77).
SV1 core and Tumbiana Formation outcrops. The Tumbiana Formation is composed of two lithofacies: a cross-bedded tuffaceous sandstone lithofacies, interpreted as a high-energy fluvial deposit (30, 69, 78, 79), and a stromatolitic boundstone, interpreted as a very shallow-water environment. Boundstone beds show high-resolution variability in structure, both vertically (10-cm scale) and laterally (in outcrop). The stromatolitic boundstone facies is composed of shallowing upward cycles of silicified, black shale or chert grading into silicified, sandy to silty, planar and wavy mat-bound lime grainstones, which grade into silicified, stromatolite-bound lime grainstones (i.e., nondolomitized) with small (microdigitate) stromatolites, fenestrae, mud cracks, and coarse sand. Very fine disseminated pyrite (0.1-0.8% total sulfur) is present, particularly in tuffaceous sands and in microbial mat laminae. Desiccation features noted in the SV1 core, indicative of shallow-water deposition, are consistent with other reports, which also note halite casts (79) and, possibly, other microscopic evaporites (30) at some localities, suggesting episodes of at least local hypersalinity.
Researchers have presented contrasting environmental interpretations for this stromatolite-rich carbonate facies, including intermontanne or ephemeral lacustrine (79-81,) and shallow marine (supra- to intertidal) (30, 82, †), both evolving from a fluvial/deltaic system. The overall geological framework (55, 69, 70) is consistent with a large lake or a marine basin such that the coastline initiated as an eroded basaltic plain dotted with granitic paleotopographic highs; the valleys filled in as the basin subsided to the south. The north to south distribution of subaerial and subaqueous basalts, respectively, before, during, and after the Tumbiana deposition reflects the tectonic regime and architecture of the margin. The geological framework sets the Tumbiana deposits apart from the minor, laterally discontinuous, stromatolitic carbonates and sandstones in the underlying Kylena Basalt and overlying Maddina Basalt (cf. 55, 69, 70), although all of these units exhibit very shallow-water facies. Whereas some were perhaps ephemeral pond deposits on a basaltic plain, others may have been fluvial lacustrine sediments deposited in fault-block basins formed by crustal extension during an initial phase of continental breakup. These depositional settings may have persisted during the time of Tumbiana Formation deposition, particularly in paleovalleys on the northern portion of the craton that experienced less subsidence. Diachronous deposition of the Tumbiana Formation suggested by U-Pb geochronological data (53, 55) is consistent with the regional tectonic regime.
Disparity in the observance of particular sedimentological features from one locality to the next adds to the environmental debate. Most important is the absence at most localities of bidirectional current structures indicative of tidal activity. This absence is consistent with a lacustrine interpretation (79); however, river/wave-dominated shallow marine deposits would also likely lack these structures (83). Notably, the upper Carawine Dolomite, having the architecture and stratigraphy clearly diagnostic of a marine platform that dips to the southwest, lacks these structures (76). Bidirectional current structures have been identified by Sakurai et al. (84) near our Beabea Creek and Spider Creek localities, and they suggested that at least the southern outcrops of the Tumbiana carbonates were marine (83). Blake et al. also suggests mixed marine and lacustrine associations for the Tumbiana carbonates (55).
The d13Ccarb values reported for the southern portion of the Tumbiana Formation average 0.0‰ (±1.2‰, 1s; n = 36) (30), match previously reports values (82), and they are similar to the global mean (77) for marine carbonates, although more 13C-depleted values have been also been noted (85). Rare-element distributions, which can be used to discern a marine association, are compromised by the terrestrial flux of tuff. Geochemical indicators of lacustrine and/or marine environments applied to post-Precambrian sediments (e.g., curtailed evaporite paragenesis; ref. 79) are inapplicable to the Archean because of likely low sulfate concentrations in the ocean at this time (8, 86).
An emerging view is that the Tumbiana Formation paleoenvironment was spatially and temporally heterogeneous, consistent with its complex tectonics and inferred paleotopography. Regardless of whether it was lacustrine or marine (or both), it is clear that the Tumbiana carbonate units sampled in our study record very shallow deposition, which is of foremost importance to our study of microbial signatures in the late Archean.
d13Cker, d13Ccarb, and TOC Abundance. Fig. 8 shows that there is no correlation on the dker-% TOC plot for our Hamersley data (even with the Tumbiana removed). A similar observation was made by Beukes and coworkers (7, 41) for 2.55-2.45 gigaannum for Transvaal shales and carbonates. The lack of a dker-% TOC correlation in part reflects differences in inorganic sedimentation rates and/or diagenetic mineral precipitation. A majority of the samples contain transported tuffaceous and carbonate detritus or have been silicified. The difference here is that Strauss and Beukes (41) interpreted the upper envelope of data (-30 to -20‰ values) as a thermal alteration trend. Hamersley sediments with near -30‰ values have a 0-2% TOC range that is inconsistent with thermal alteration. Moreover, d13Ccarb values for some Carawine marls with low dker values are similar to adjacent Carawine shallow-water carbonate (e.g., RHDH2a -136.0 and -141.7, respectively), despite major variations in % TOC (2.5% and 0.1%), suggesting that carbonate-carbon exchange has not enriched dker values.
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