Fig. 1: From the East Kimberley region of northeastern W.Australia, a striped and spotted rock variously referred to as "zebra rock" or "astronomite", or other unofficial, varietal names. The rock is a siltstone, a fine-grained clastic sediment composed of quartz, clay minerals and iron oxides. In more detail, the mineralogy includes quartz, sericite, kaolinite and dickite, plus perhaps alunite, and some hematite. The sawn slice is 15 x 45 x 3 cm. Oxides, sheet silicates and a sulphate. Royal Ontario Museum sample 16976.
"Rock of the Month # 263, posted for May 2023" ---
The rock is late Proterozoic (early Ediacaran) in age, and was discovered and described by Blatchford in 1924. It is from the Johnny Cake member of the Ranford Formation. The rock is found in a belt of localities that trends SW-NE, south of the town of Kununurra, a region where Lake Argyle drains north via the Ord River into the Cambridge Gulf. These rocks occur as discontinuous lenses in a thin unit extending tens of kilometres, around Lake Argyle and a few km eastwards into the adjacent Northern Territory.
This region of the East Kimberleys is in the far northeast of W.Australia, south of the Timor Sea (Eddison and Loan, 2001). The Kimberley basin is the westernmost of a series of Proterozoic sedimentary basins across northern Australia. Its margins are defined by the King Leopold mobile zone to the south and the Halls Creek mobile zone to the east (Plumb, 1990). The wider region has diverse mineral deposits, such as the Argyle diamond mine, the Sally Malay nickel deposit, the Brockman REE (rare earth element) deposit, lead-zinc-silver and gold-silver occurrences, and the Panton sill, with its resources of PGE (platinum group elements). However, this is all peripheral to,and older than, our immediate focus, the zebra rock outcrops. Important as the metal resources are, a small mine processing a small tonnage of prized collection or lapidary grade mineral specimens likely makes a lot more money per tonne of raw material (ore) moved, which can be attractive for small, even family-scale operations. According to MINDAT, this rock was quarried for lapidary purposes at three sites, as of 2013, and other occurrences have been worked in the past.
Retallack (2020) offers an elegant and detailed account of the zebra rocks around Lake Argyle, with an emphasis on field geology, and studies of material collected from several localities. In a few words, Retallack concludes that these sediments are a form of paleosol, a gleyed soil of Ediacaran age (a gley soil, common in the northern hemisphere nowadays, forms when the soil is waterlogged for much of the year, as beneath a peat bog). Remarkably, there are terrestrial fossils in these strata, near the close of Precambrian time. Ediacaran megafossils associated with zebra rock include Palaeopascichnus and Yangtziramulus. The zebra rock occurs as discontinuous lenses in the shaley Johnny Cake member of the Ranford formation. The underlying sandy Jarrad member overlies the "cap carbonate" of the Moonlight Valley tillite. The Ediacaran age is indicated both by the fossils, and by comparison with the stratotype in S.Australia: model ages for the zebra rocks range circa 635-599 Ma. The zebra rock-bearing unit is seen as a paleosol. Associated features include red sandstone with dolomitic nodules, dessication cracks, and gypsum rosettes. Red-grey banding in such soils forms due to “gleisation” of red soil by pockets of anaerobic bacteria, active during waterlogged conditions. Retallack’s detailed study of the paleoenvironment of the zebra rock finds it to be "a lowland seasonally waterlogged, redoximorphic soil (Wajing pedotype)", developed in an arid and cool to cold temperate climate.
Kawahara et al. (2022a) argue that the striped pattern reflects water-rock interactions, and it is suggested that zebra rock formed in an acid hydrothermal system. In this model, pH buffering of acid fluid with carbonate led to rhythmic Fe oxide precipitation. They distinguish two types of zebra rock according to colour and clay mineral assemblages. For their detailed observations (Kawahara et al., 2022a,b) analyses were made of seven samples bought at a gallery in Kununurra, in 2008. The samples were excavated from a quarry in a small island in Lake Argyle, the locality now flooded. Crystallography (XRD) distinguishes two types of zebra rock, either kaolinite- dominant or alunite- dominant. In a companion study, Kawahara et al. (2022b) examined bleached spots in Fe oxide rich sediments such as red beds, noting that localised reduction by organic matter is often invoked to explain such spots. However, evidence of microbial activity is often lacking. Based on zebra rock samples, they find that such spots (small in this case, circa 1 mm) may instead form by pyrite decomposition and associated pH changes. The original pyrite may be replaced by a pseudomorph of dickite with hematite and goethite. This may have widespread application, though it is a separate issue to the spectacular bands and cm-scale spots seen in Figs. 1-2.
Fig. 2: A close-up shows the strange spots, each ovoid in this plane through the rock, and 1-2 cm in size. What shape are they in 3 dimensions: spheroids, ovoids, rods? And why is each spot delineated by a thin, pale rim? I worry about this phenomenon (below), but honestly do not have the answers at this time.
Structural, chemical, pedological explanations:
While this particular piece is most memorable for the remarkable spotted or star-like pattern (hence "astronomite"), which scarcely looks natural, the stripey component has earned it the market-friendly term, "zebra rock". In the wider World of Rock, this unofficial term risks confusion with other striped lithologies, which in the geological literature are referred to by the well-entrenched terms zebra rock, zebra dolomite or zebra texture. These are most commonly described in the context of dolomite-hosted, Mississippi Valley-type lead-zinc mineralization.
The model of clayey soil horizons, gley soil and Ediacaran paleosol sounds perfectly reasonable. The localised reduction of ferric to ferrous iron (even absent organic matter) is also well-presented. I still have trouble with the occurrence of striped domains (or orientations) versus spotted areas. I can't help but envision the spots as representing the depressions or culminations in an egg box, an allusion which harps back to models of interference folds in structural geology (see, e.g., Hobbs et al., 1976, pp.355-360). What do I know? Soil science and complex structures are above my pay grade. At any rate, a lovely rock, not surprising it sells well in the lapidary market. More to follow when I find an answer I can grasp!
Eddison,F and Loan,G (editors) (2001) Western Australia: Atlas of Mineral Deposits and Petroleum Fields. Geol.Surv.W.Australia, Perth, 36pp.
Hobbs,BE, Means,WD and Williams,PF (1976) An Outline of Structural Geology. John Wiley & Sons, 571pp.
Kawahara,H, Yoshida,H, Yamamoto,K, Katsuta,N, Nishimoto,S, Umemura,A and Kuma,R (2022a) Hydrothermal formation of Fe-oxide bands in zebra rocks from northern Western Australia. Chemical Geology 590, 10pp. Access here.
Kawahara,H, Yoshida,H, Nishimoto,S, Kouketsu,Y, Katsuta,N and Umemura,A (2022b) Bleached-spot formation in Fe- oxide- rich rock by inorganic process. Chemical Geology 609, 7pp. Access here.
Plumb,KA (1990) Halls Creek Province and The Granites Tanami Inlier - regional geology and mineralisation. In `Geology of the Mineral Deposits of Australia and Papua New Guinea' (Hughes,FE editor), Australasian Institute of Mining and Metallurgy Monograph 14, 1828pp., 681-695.
Retallack,GJ (2020) Zebra rock and other Ediacaran paleosols from Western Australia. Australian J.Earth Sci., 25pp. Access here.
Graham Wilson, 11-12,18-19 February 2023
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