"Rock of the Month # 263, posted for May 2023" ---

Regional setting:

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.

Detailed studies:

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.