Organic and inorganic geochemistry of the Mesoproterozoic Roper Seaway Geoscience Australia; 2 Curtin University, 3 McGill University, 4 University of Adelaide, 5 GeoTop 1
Amber Jarrett , Grant M. Cox , Dianne Edwards , Chris Boreham , Peter W. Crockford , Galen P. Halverson , Alan S. Collins , André Poirier , Zheng-Xiang Li 1,2
1
1
3
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Introduction The ca. 1.4 Ga Roper Group of the McArthur Basin, northern Australia, is one of the most extensive Precambrian hydrocarbon-bearing successions preserved in the geological record [1]. It is interpreted to have been deposited in a large epeiric sea known as the Roper Seaway. This study presents biomarkers and high resolution trace element redox geochemistry of immature to mature black shales from the Velkerri Formation to understand the microbial diversity and environmental conditions that persisted during the middle Mesoproterozoic.
a. AGE (Ma)
Eon
Era
Package and dominant lithology
URAPUNGA FAULT ZONE
The combination of an organically hosted negative cerium anomaly (Figure 4A), the presence of cyanobacteria and the absence of aryl isoprenoids (Figure 4B) suggest that euxinia did not extend to the photic zone. 0
1.4
a. Barney Creek Formation
m/z 133
b. Velkerri Formation
m/z 133
c. Velkerri Formation
m/z 133
1.2 38
1.0
471
0.8
SOUTHEASTERN M c ARTHUR BASIN
BATTEN FAULT ZONE North South
2
Oxygenated photic zone
b.
SOUTHERN M cARTHUR BASIN
Beetaloo Sub-basin
5
Ce SN
Geological setting
4
Depth (metre)
1
1300
Roper Group
Hayfield Mudstone Jamison Sandstone Bukalorkmi Sandstone Kyalla Formation Sherwin Formation Moroak Sandstone Velkerri Formation Bessie Creek Sandstone Corcoran Formation Hodgson Sandstone Jalboi Formation Arnold Sandstone Crawford Formation Mainoru Formation Limmen Sandstone Mantungula Formation
0
1685
Paleoproterzoic
1670
Batten Subgroup
lower Balbirini Dolostone 1609±3 Ma 1613±4 Ma
Amos Fm 1614±4 Ma
1625±2 Ma Stretton Sandstone Yalco Formation Lynott Formation
Donnegan Member Hot Spring Member
Caranbirini Member Reward Dolostone
Barney Creek Fm 1640±3 Ma 1639±3 Ma
HYC Pyritic Shale Mbr
W-Fold Shale Mbr
Coxco Dolostone Member
Teena Dolostone Emmerugga Dolostone
Umbolooga Subgroup
McArthur Group
Vizard Group
Saint Vidgeon Formation
Proterozoic
Glyde package
Karns Dolostone
1589±3 Ma
Smythe Sandstone Looking Glass Fm
Nagi Formation
1636±4 Ma
Cooley Dolostone Member
Tawallah Group
Redbank package 1735 1750
Tanumbirini Rhyolite 1713±7 Ma
10 16-9639-4
Nyanantu Formation
?
Echo Sandstone
?
Pungalina Member
Warramana Sandstone
Hobblechain Rhyolite 1725± Ma
Gold Creek Volcanics Wollogorang Formation 1730±3 Ma, 1729±4 Ma, 1723±4 Ma Settlement Creek Dolorite Wuraliwuntya Member
Wununmantyala Sandstone
30
Lower Velkerri
34
38
42
46
50
54
58
TOC (% weight)
Redox sensitive trace metals increase in the middle Velkerri Formation (Figure 5A-B) and appear coeval with increases in the DBT/P ratio (Figure 5C). This suggests a transition from deep water suboxic‑anoxic to euxinic conditions.
Aquarium Formation
McDermott Formation Rosie Creek Sandstone Member
Seigal Volcanics Yiyintyi Sandstone
Middle Velkerri
Figure 4. Left: Shale normalised Ce anomaly vs. TOC [5]. Ce*SN = CeSN/0.5[LaSN + PrSN] after [3]. Normalised values used the Post Archean Australian Shale (PAAS) [4]. Data first published in [5] Right: Partial mass chromatogram of the m/z 133 trace comparing (a) The Barney Creek Formation from drill core GR 7, 38.7 m to the Velkerri Formation in Altree-2 at (b) 471.24 m and (c) 812.11 m demonstrating trimethyl arylisoprenoids (triangles) are below detection limits in the Velkerri Fm. Circles represent a series of trimethyl alkylbenzenes.
25
5
a.
Sly Creek Sandstone 1800
8
Suboxic to sulfidic at depth
Goyder package
1708±5 Ma
6
1639±6 Ma
Mitchell Yard Member Mara Dolostone Mbr Myrtle Shale Leila Sandstone
Tooganinie Formation Tatoola Sandstone 1648±3 Ma Amelia Dolostone Mallapunyah Formation Masterton Sandstone
Burash Sandstone
4
Upper Velkerri
Abner Sandstone
Upper Balbirini Dolostone
Yalwarra Volcanics Knuckey Formation Mount Birch Sandstone
2
TOC (% weight)
Dungaminnie Formation
Walmudga Formation
812
0.6
Nathan Group
Favenc package
Collara Subgroup Maiwok Subgroup
1500 1575
Wilton package
Nathan Group
Mesoproterozoic
Chambers River Formation
Carolina Sandstone Member
60
b.
Westmoreland Conglomerate
20
c.
50
4
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Biomarkers
10
E
-T TOC rate ation e d o ri M cova
5
E OC-T T k a n We variatio co
2
-TE TOCon g n i Stro ovariat c
1
0
All core extracts contain a large UCM in addition to high ratios of monomethyl alkanes to regular n-alkanes (Figure 2). Regular C27 – C35 hopanes and 2α-methylhopanes were detected (Figure 3) demonstrating that bacteria including cyanobacteria and methanotrophic bacteria were the dominant primary producers.
3
Sulphur (ppm)
15
40
Euxinic threshold
Figure 1. (a) Stratigraphy of the McArthur Basin with major lithographic packages, stratigraphic units and oil and gas occurrences modified from [2]. (b) Location map of the Altree 2 drill hole in the Beetaloo sub basin, the main depocentre of the Roper Group, McArthur Basin.
U(ppm) / Al (% weight)
Source rock
Euxinic threshold
Reservoir Unconventional reservoir
Mo(ppm) / Al (% weight)
Gas discovery Oil show
20
-TE TOCion g n t Stro ovaria c
10
0 0
2
4
6
8
10
30
16-9639-5
0 0
2
TOC (% weight)
4
6
8
10
0
0.25
TOC (% weight) Upper Velkerri
Middle Velkerri
0.50
0.75
1.00
DBT/P Lower Velkerri
Figure 5. Evidence for the onset of euxinia based on trace elemental enrichments with respect to TOC and the Dibenzothiophene/ Phenanthrene ratio with respect to Sulfur. (a) Mo and (b) U show similar trends, initial linear covariation with respect to TOC up to ~4% TOC followed by a breakdown of this linear covariation consistent with the onset of euxinia [6,7] (Algeo and Maynard, 2004). Data first published in [5]. (c) The increase in DBT broadly correlates with increasing sulfide concentrations in the deep water column which was subsequently incorporated into the organic matter [7], broadly consistent with a transition to euxinia.
Summary • Bacteria were the dominant primary producers, including cyanobacteria and methanotrophic bacteria. • The water column was stratified with an oxic photic zone and suboxic-anoxic and deep waters in the upper Velkerri Formation. • A transition to deep water euxinia occurred in the middle Velkerri Formation. • Water column geochemistry in the Mesoproterozoic was dynamic and heterogeneous.
Figure 2. GC-MS full scan chromatogram of the saturated hydrocarbon fraction of a representative middle Velkerri Fm extract from 750.28 m in the Altree 2 drillcore.
Figure 6. A conceptual model of the ocean chemistry based on trace elemental ratios and characteristic hydrocarbons from the Roper Seaway through the (a) Upper, (b) Middle, and (c) Lower Velkerri Formation.
Acknowledgments We thank Junhong Chen, Ziqing Hong, Tara Webster (GA), Jochen Brocks and Janet Hope (ANU) for technical assistance and Darryl Stacey (NTGS) for facilitating access to core.
References [1] Munson 2014. NTGS Report 22. [2] Carr et al 2016. GA Record 2016/004. [3] Bau & Dulski 1996. Precambrian Research, 79, 37-55. [4] Nance & Taylor 1976. Geochimica et Cosmochimica Acta, 40, 1539-1551. [5] Cox et al. 2016. Chemical Geology, Accepted manuscript. [5] Alego & Maynard 2004. Chemical Geology, 2016, 289-318. [6] Lyons et al. 2009. Annual Review of Earth Sciences, 37, 507-534. [7] Hughes et al. 1995. Geochimica et Cosmochimica Acta, 59, 3581-3598. Figure 3. Distribution of the C27 to C35 hopanes in the combined m/z 370, 384, 398, 412, 426, 440, 454, 468, 482 Ò 191 traces in drillcore Altree 2, 410.55 m. Peak assignments define the C-22 stereochemistry (R and S), ɑβ indicates 17ɑ(H),21β(H), Ts = C27 18ɑ(H),22,29,30trisnorneohopane, Tm = C27 17ɑ(H),22,29,30-trisnorhopane, and * = diahopanes. GA 16-9640 | GeoCat 90047
© Commonwealth of Australia (Geoscience Australia) 2016. This material is released under the Creative Commons Attribution 4.0 International Licence. http://creativecommons.org/licenses/by/3.0/au/deed.en
For Further Information: Amber Jarrett & Grant Cox Email:
[email protected] Ph: +61 2 6249 9529 Web: www.ga.gov.au
