Depositional environment of the middle proterozoic velkerri formation in Northern Australia: Geochemical evidence

Precambrian Research, 42 (1988) 165-172 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

165

DEPOSITIONAL ENVIRONMENT OF THE MIDDLE PROTEROZOIC VELKERRI FORMATION IN NORTHERN AUSTRALIA" GEOCHEMICAL EVIDENCE T.H. DONNELLY Baas Becking Geobiological Laboratory, P.O. Box 378, Canberra A. C. T. 2601 (Australia)

I.H. CRICK Division of Continental Geology, Bureau of Mineral Resources, P.O. Box 378, Canberra A. C. T. 2601 (Australia) (Received September 9, 1987; revision accepted March 28, 1988)

Abstract Donnelly, T.H. and Crick, [.H., 1988. Depositional environment of the Middle Proterozoic Velkerri Formation in northern Australia: geochemical evidence. Precambrian Res., 42:165-172. The organic-matter-rich shales of the Middle Proterozoic Velkerri Formation in the McArthur Basin of northern Australia are part of the essentially unmetamorphosed Roper Group, which has many units showing remarkable lithological consistency over vast areas. Jackson et al. reported the discovery of indigenous 'live' oil in the Velkerri Formation, thereby raising considerable interest in its depositional environment. Based on sedimentology alone this unit was suggested to have formed in a deep-water low-energy open marine environment. A geochemical study of the silty mudstones of the Velkerri Formation from Urapunga 3 corehole, which intersects the upper half of this unit, was carried out to examine some aspects of this depositional model. Organic C contents in the core are up to 7.1% and maturity determinations indicate that the sediments have experienced a relatively low organic metamorphic grade. Pyrite S/organic C ratios vary from very low ( < 0.01 ) in the bottom section of the core to high (up to 1.52) in the top part of the core. Disseminated pyrite ~:~4Svalues range from +3.6 to +34.4%c. The geochemical results from the upper half of the Velkerri Formation indicate an initial lowsulfate oxidizing environment. This environment may have been a large lake or a bay which became barred early in its depositional history and later became sulfate-poor as a result of the activities of sulfate-reducing bacteria. Some influx (es) of seawater resulted in increased organic C (and pyrite S ) contents in the anoxic sediments, while the lake or barred bay eventually became euxinic, with the preservation of significant amounts of organic matter. These data indicate the need for an integrated approach to the interpretation of Proterozoic sedimentary environments.

Introduction T h e d i s c o v e r y o f i n d i g e n o u s ' l i v e ' oil i n t h e Middle Proterozoic Velkerri Formation of the southern McArthur Basin, northern Australia ( F i g . 1 ), r e p o r t e d b y J a c k s o n e t at. ( 1 9 8 6 ) , h a s

raised considerable interest in its depositional environment. Based on sedimentology alone, J a c k s o n e t al. ( 1986, 1 9 8 8 ) h a v e s u g g e s t e d t h a t the Roper Group was deposited predominantly in an open marine environment, with the silty mudstones of the Velkerri Formation being

166

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deposited in a deeper-water zone of low energy. This contrasts with the abundance of rift-related continental environments in the underlying McArthur Group (Jackson et al., 1988). Given the difficulties in determining Proterozoic depositional environments (e.g., diagnostic fossils are absent, evaporites are commonly pseudomorphed and of uncertain affinity, and sedimentary sequences are often incomplete), it is important that interpretations be based on as many lines of evidence as possible. In this study we have used some geochemical data from the silty mudstones of the Velkerri Formation, from the Urapunga 3 corehole (Fig. 1 ), with the aim of obtaining more detailed information on the environment of deposition.

Geological setting and sedimentary features The Velkerri Formation (Fig. 1 ) occurs in the upper half of the Roper Group, the Maiwok Subgroup, which is a sequence of essentially unmetamorphosed Middle Proterozoic sedimentary rocks dated at ~ 1.4 Ga (McDougall et al., 1965; Kralik, 1982). Many of the units within the group show a remarkable lithological consistency over vast areas (Plumb et al., 1980) and are considered by Plumb and Derrick (1975) to have formed in a large epicontinental basin. Jackson et al. (1986, 1988) reported that the Roper Group was deposited under open marine conditions, and Jackson et al. (1988) describe the type of sediments in the Roper Group as mainly siliciclastic ranging from conglomerates to mudstones. They define five major depositional cycles which start with mainly fine-grained sediments (including the Velkerri Formation) overlain by coarser sediments, reflecting a gradual increase in energy due to decrease in water depth. During an early period of relatively low sea-level, fluvial sediments of the Limmen Sandstone (Fig. 1) were deposited and during later periods of lower sealevel some sediments in the Mantungula and Mainoru Formations, and the Abner Sand-

stone (Fig. 1), were exposed to weathering which produced red beds or palaeosols. Jackson et al. (1988) suggest that the overall cyclicity of the Roper Group and its great lateral extent is comparable with Phanerozoic examples of sediments deposited in response to sea-level cycles along a passive continental margin. Many of the coarser units have bimodal, bipolar palaeocurrent directions and crossbedding styles characteristically formed by tidal currents. Jackson et al. (1988) point out that there is commonly a rapid transition from these tidally influenced sandy units into lower-energy shelf muds and that beds suggestive of a shoreface facies are commonly absent. Such a rapid transition was observed by them between the Bessie Creek Sandstone and the overlying Velkerri Formation. Overlying the Velkerri Formation is the Moroak Sandstone Member of the McMinn Formation which to the north has pebbly intervals and oolitic ironstone-rich units containing pot-hole erosion surfaces, clearly indicating very shallow water environments and intermittent emergence (Jackson et al., 1988). To the south this member is more uniform and in one exposure displays large-scale trough cross-bedded pebbly sandstones with persistent northerly directed palaeocurrents in about the middle of the member, which could have formed under marine or fluvial conditions. A continental water influence was recognized by Kralik ( 1982 ) in his Sr isotope study on sedimentary carbonate from the Kyalla Member of the McMinn Formation. The Velkerri Formation contains two subcycles, each consisting of a lower subdivision of black, organic-rich laminated siltstone and mudstone and an upper subdivision of organicpoor siltstone. Both subdivisions contain thin, convoluted and contorted beds with steeply inclined laminae interpreted as slumped beds resulting from periodic tectonic activity in an outer shelf unit, and glauconitic silty intervals, containing graded beds with basal scoured surfaces and low-angle cross-lamination, probably deposited from turbidity currents (Jackson et

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al., 1988). This unit contains 'live' oil, which was observed bubbling out of core from Urapunga 4 (Jackson et al., 1986) (Fig. 1). Urapunga 3, which is situated some 60-70 km west ofUrapunga 4 (Fig. 1 ), passes through 210 m of the Velkerri Formation, which in this corehole consists of an upper 137 m of dark grey to black siltstone and mudstone, and a lower 73 m of light grey silty mudstone. Using the Jackson et al. (1988) model, Urapunga 3 appears to have passed through the lower subdivision of the upper subcycle into the upper section of the lower subcycle.

Analytical techniques To obtain samples of disseminated pyrite, selected core samples were crushed and treated with high-density liquids. The pyrite, which was the only S-containing mineral in the heavy mineral residues, was treated by standard methods (Kaplan et al., 1970) to obtain purified SO2 samples. S isotope compositions were determined from S02 using a SIRA 12 ratio mass spectrometer. These compositions are expressed as j34S values relative to the Cation Diablo troilite standard, and the overall precision of the determination is 0.1%o. Total S and organic C were determined by a Leco C-S analyzer.

tary organic matter as a C source (Berner, 1984). In modern marine terrigenous sediments overlain by oxygenated ocean water (normal marine conditions), the amount of bacterially metabolizable organic matter is the most important factor limiting pyrite formation (Berner, 1984). Under such conditions a significant positive correlation has been found between pyrite S and organic C; the line of best fit extends through zero and the mean S/C ratio is 0.36 (Fig. 2). A S/C plot typical of sedimentation under an anoxic water column is shown by studies of the Recent sediments in the Black Sea (Leventhal, 1983 ). In this type of environment pyrite can form in the water column as well as within the sediment, thus producing a regression line with a positive intercept on the S-axis. In freshwater low-sulfate continental environments S contents remain low regardless of organic C contents. Berner and Raiswell (1983) have shown that S/C ratios can also be used as a guide to understanding ancient depositional environments. Holland (1984, pp. Oepth (m) o

13-80

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Results and discussion In this study two relationships have been used to examine the environment of deposition of the silty mudstones of the Velkerri Formation. They are (1) the relationship between pyrite S and organic C contents in anoxic sediments, and (2) the relationship between the j34S value of pyrite in anoxic sediments and the environment of pyrite formation. Before presenting the geochemical results from this study these relationships will be discussed. Pyrite in anoxic marine sediments is formed by the reaction of Fe oxides with H2S produced by sulfate-reducing bacteria, using sedimen-

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Fig. 2. Pyrite S/organic C ratios from the organic-rich silty mudstones of the Velkerri Formation compared with the 0.36 line and an envelope around this line which contains the majority of the S/C ratios found from modern normal marine sediments (Berner and Raiswell, 1983).

169

352-355), from an examination of Early Proterozoic S / C ratios in anoxic sedimentary rocks from Finland and Canada (Peltola, 1968; Cameron and Garrels, 1980), concluded that the ratios are similar to those found in normal Recent sediments. The S isotopic compositions of pyrite and sulfate from a wide range of modern sedimentary environments have been shown to have large A:~4S (sulfate-pyrite) values ( ~ 25-40%c; Chambers and Trudinger, 1979), the smallest values coming from intertidal evaporative environments where the supply of sulfate becomes limiting (Chambers, 1982 ). Certainly, by the Middle Proterozoic, ocean sulfate contents were high enough for this to be a major S pool, for example, for the formation of major metal sulfide ore deposits in northern Australia (Muir et al., 1985). Muir et al. (1985) from a review of the literature and their own studies suggested Middle Proterozoic ocean sulfate ~34S values were 20-25%~; that is, similar to present-day ocean sulfate values where pyrite in deeper-water anoxic sediments shows significantly :~2S-enriched 6:~4S values. The 634S values of pyrite formed in anoxic waters are controlled by the availability of sulfate. For example, in the Black Sea, sulfate is not limiting and sulfide 6:~4S values are significantly :~2S-enriched (-30.87i~, -26.9%c, Yu Lein et al., 1983 ). In contrast to the marine environment, pyrite formed in low-sulfate lacustrine environments (e.g., Green River Formation, U.S.A., Cole and Picard, 1981; see also Ivanov, 1983) is characterized by significantly :~4S-enriched 6a4S values as a result of the total bacterial reduction of small amounts of sulfate and the Rayleigh distillation effect in micro-environments causing :~4S-enrichment in the residual sulfate reservoir. The geochemical results from the black shales of the Velkerri Formation are shown in Table I. The lithology of the clastic sediments in Urapunga 3 remains essentially the same (silty mudstone) throughout the whole of the core, and pyrite was the only S-containing mineral present. The b o t t o m 60 m of the 197 m of core

examined, however, has unusually low contents of both organic C and pyrite S (Table I), and is light gray in colour. The organic matter in the top section of the core is mature, having equivalent vitrinite reflectances of between 0.7 and 1.3%, which rapidly increase in the lower section of the core to a maximum of 3% (Crick et al., 1988). The reflectance values in the lower section of the core indicate that the organic matter is slightly overmature, and the rapid increase in reflectance values is probably due to the thermal effects during emplacement of a nearby underlying dolerite sill (Crick et al., 1988). The equivalent vitrinite reflectance values are based on methyl phenathrine indices for organic matter in this and another core from the McArthur Basin (Crick et al., 1988). These thermal conditions may have slightly raised the S / C ratio owing to loss of C (Raiswell and Berner, 1986). Fe contents of the 'Lansen Creek Shale' (Velkerri Formation equivalent; Jackson et al., 1988) from the Amoco Broadmere 1 corehole (Fig. 1) are generally high, ranging from about 3 to 9% total Fe (unpublished results); this, and the high pyrite S contents (up to 7.1% ) in parts of the Urapunga 3 core (Table I), suggest that Fe was available for pyritization and was not limiting for pyrite formation in this environment. If, as suggested by Jackson et al. (1986, 1988), deposition of the black shales of the Velkerri Formation occurred in a low-energy deep-water marine environment, then disseminated pyrite in these black shales should show significantly :r~S-enriched 6:~4S values. However, the 6:~4S values for disseminated pyrite in the Velkerri Formation in Urapunga 3 range from +3.6 to +34.4%( (Table I). Such positive values are common amongst sulfides analysed from Proterozoic black shales (see Lambert and Donnelly, in press). These authors have suggested that this may reflect the abundance of intracratonic basin environments, many of which could have been partly or wholly closed off from the open oceans at various stages of their development. Similar positive values have been found for dissemi-

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TABLE I Geochemical results from the organic-rich silty mudstones of the Velkerri Formation, Urapunga 3 corehole Sample no.

Drill hole depth

Org. C

Pyrite S

(m)

(%)

(%)

S/C ratio

~:~4S

~i~,

84420001 84420003 84420004 84420005 84420006 84420007 84420 008 84420009

13.26-13.30 30.31-30.36 37.56-37.58 37.68 37.74 39.82-39.89 43.10-43.15 49.80-49.86 54.40-54.46

2.02 1.22 5.40 5.10 6.20 4.70 6.25 5.10

1.15 0.70 4.40 4.12 4.33 7.12 4.20 2.88

0.57 0.57 0.82 0.81 0.70 1.52 0.67 0.57

+34.4 + 27.8 + 9.5 + 7.4 -+ 17.4 + 21.1 --

84420010 84420011 84420012 84420013 84420014 84420015 8442(1016 84420017 84420018 84420019 84420021 84420022 84420023 84420024 84420033 84420025 84420026 8442(/034 84420027 84420028 84420029 84420030 84420031 84420032

60.31-60.46 69.81 69.91 70.06- 70.46 72.84-72.94 72.96-73.06 75.10-75.22 79.37-79.43 84.92-84.98 89.32-89.37 99.74-99.81 109.74-109.80 119.98-120.03 125.56-125.65 129.88-129.94 132.58-132.68 137.40-137.44 140.04-140.08 140.62 150.10-150.16 172.56-172.60 179.72-179.80 199.42-199.52 203.92-204.02 210.60 210.70

3.42 6.45 5.25 2.02 4.75 5.25 6.45 3.54 2.82 2.86 2.02 3.98 2.88 2.44 2.74 5.35 0.37 -0.32 0.28 0.07 0.98 3.10 0.34

0.35 1.39 5.69 0.59 0.89 1.20 1.01 1.55 0.46 1.49 0.50 1.23 0.97 1.43 2.97 2.02 0.02 -< 0.01 < 0.01 < 0.01 < 0.01 < 0.01 <0.01

0.10 0.22 1.08 0.29 0.19 0.23 0.16 0.44 0.16 0.52 0.25 0.31 0.34 0.59 1.08 0.38 0.05 -< 0.03 < 0.03 < 0.10 < 0.01 < 0.01 <0.03

+ 19.6 -+ 11.5 ----+ 22.0 ---+ 15.8 --+ 3.6 + 12.5 + 11.4 + 8.5 ----+ 7.9 --

nated pyrite in black shales from those parts of the McArthur Basin suggested to be nonmarine environments in such an intracratonic basin (Logan and Williams, 1984; Muir et al., 1985; Donnelly and Jackson, in press). The bottom 60 m of silty mudstones in Urapunga 3 have low C and S values (Table I). There is no evidence to suggest that secondary alteration was responsible for these low values. Organic matter in the upper section of the core is contained within dark laminae whereas in the lower 60 m it commonly occurs as dark disseminated flecks up to several millimeters in length.

Disseminated pyrite in this section of the core has 6:34S values of + 7.9, +8.5 and + 11.4%c and together with the low organic C content suggests their formation in an oxidizing lake or barred basin with low organic C preservation and a low sulfate content. Pyrite S/organic C ratios are very low in this bottom 60 m section of the Velkerri Formation (Fig. 2). Above this section the most :~2S-enriched 634S value (+3.6!i~:) was found for disseminated pyrite and may reflect some marine water influx at this time. Pyrite S/organic C values above 133-85 m depth are generally within the range of an-

171

oxic marine sediments formed below oxic bottom waters (Fig. 2). The upper 80 m of the core is characterized by having the highest organic C and pyrite S contents (6.5 and 7.1% respectively), but at times organic C contents remained high while the pyrite S contents became relatively low (Table I, Fig. 2). Such high organic C contents and, for a number of the samples, high S contents suggests that the water column may have become anoxic. As available Fe for pyrite formation does not appear to be limiting in this environment, i't is suggested that sulfate in the water column, at times, decreased to relatively low values. The disseminated pyrite d:~S values in this upper section show the greatest :~4S-enrichment (up to + 34.4(i,, Table I ). While it is possible there was some addition of' geothermally formed pyrite (see Muir et al., 1985; Donnelly and Jackson, in press ) the high organic C and pyrite S contents are typical of euxinic environments (Leventhal, 1983). Disseminated pyrite (~348 values as high as +34.4~/~, and at times low sulfate contents, suggest a closed to partially closed environment where sulfate in the basin was pushed to greater and greater :~4S-enrichment by the effects of the sulfate-reducing bacteria.

amounts of organic matter (upper section Urapunga 3 corehole). This interpretation does not conflict with the sedimentological evidence given by Jackson et al. (1986, 1988), except in their interpretation of the depositional environment of the Velkerri Formation as being wholly open marine. This study shows the value of combining geochemical and sedimentological evidence to obtain a better understanding of Proterozoic depositional environments.

Acknowledgments The authors wish to thank Drs. M.R. Walter and T.G. Powell for reviewing an early draft of this paper and Drs. I.B. Lambert and P.N. Southgate for helpful discussions. Total C, S and Fe analyses were carried out by AMDEL, Adelaide, Australia. The Baas Becking Laboratory is supported by the Bureau of Mineral Resources (BMR), the Commonwealth Scientific and Industrial Research Organisation and the Australian Minerals Industries Research Organisation Limited. I.H.C. publishes with the permission of the Director of the BMR.

References Conclusions The data from this study indicate that the silty mudstones of the Velkerri Formation, with low organic C content (bottom section Urapunga 3 corehole), were deposited initially in a low-sulfate oxidizing environment. This environment may have originally been a large lake, or a bay which was barred at the start of deposition of the Velkerri Formation, resulting in gradual depletion of sulfate as a result of the activities of' the sulfate-reducing bacteria. Some influx(es) of seawater into the lake, or bay, caused it to become more saline, and the organic C (and pyrite S) content in the anoxic sediments increased. Later, owing possibly to increased organic matter production, it became euxinic with the preservation of significant

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