Coward, Andrew J1; Slim, Anja C1; Brugger, Joël1; Wilson, Siobhan A2; Pillans, Bradley J3; Williams, Tim4;
1School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC 3800, Australia, 2Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2R3, Canada, 3Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia, 4Monash Centre for Electron Microscopy, Monash University, Clayton, VIC 3800, Australia
Zebra rock is an Ediacaran paleosol unique to the East Kimberley region of Western Australia, renowned globally for its highly regular and unusual red and white banding. The underlying mechanisms behind the self-organisation of zebra rock patterns has remained an enduring question ever since the first scientific studies of the ornamental stone in 1926 and continues to fascinate geologists to this day. Many theories have been proposed to explain the formation of these unique patterns, including the infilling of ripple beds, Liesegang banding, and ferronematic liquid crystals, but no definitive consensus has yet been reached, in part due to the lack of supporting chemical and mineralogical analysis.
Twenty-five zebra rock samples were analysed from five different outcrops across the Lake Argyle region using a variety of analytical techniques, including XRD, SEM, TEM, LA-ICP-MS and X-Ray CT imaging. The principle pigment of zebra rock was found to be exclusively hematite in all cases, a result consistent with previous studies. Within the dark banding, hematite manifested as large (1-10μm) aggregates of nanoscale (200-500nm) hexagonal platelets distributed within the interstitial spacing between larger quartz and kaolinite grains. Large (100 μm), polycrystalline hematite was also observed within the white bands, although at significantly reduced abundance (<1 wt%). Hexagonal platelets and needle-like crystals of hematite were observed in the overlying, iron-rich host shales, while the comparatively iron-poor, underlying shales exhibited a random arrangement of large iron-oxide nodules and dissolution voids.
Zebra rock mineralogy was found to be highly variable on a regional scale. Across the five deposits examined, four distinct mineralogical assemblages were identified, each defined by the absence or presence of one or more of alunite, muscovite and dickite as major phases. Each of these four assemblages contained key indicator minerals suggestive of argillic and advanced argillic hydrothermal alteration, most prominently dickite, kaolinite, alunite and, in low amounts, svanbergite and pyrophyllite. All four mineral assemblages can each be correlated with a different degree of argillic and advanced argillic alteration, suggesting a gradual neutralisation of acidic hydrothermal fluids as they progressed laterally along permeable bedding. On a local, outcrop scale, zebra rock mineralogy was found to be largely homogeneous, with no mineral phase other than hematite exhibiting consistent concentration gradients between the dark and light bands. Trace elements followed a similar pattern, with no consistent concentration gradients outside of significant enrichment in the dark banding of V51, Cr52, Mn55, Co59, Ni60 and Mo95, elements all known to strongly adsorb to hematite. Curiously, no HREE enrichment was detected, as might otherwise be expected in hydrothermal systems.
On the basis on the above findings, we propose two alternative hypotheses to explain the development of zebra rock patterns: 1) high temperature, argillic hydrothermal alteration in conjunction with the formation of the 510 Ma Kalkarindji large igneous province or 2) low temperature acid-sulphate weathering in anoxic, waterlogged soils during the Ediacaran. Dating techniques, such as paleomagnetic analysis and stable O isotopes, are required to determine which of these two mechanisms is responsible for the hematite patterns of zebra rock.
Andrew Jeremy Coward is a doctoral candidate at Monash University. Mr. Coward specializes in studying self-organization mechanisms in geochemical and biogeochemical systems with a specific interest in various banded siltstones and clays in north-western Australia.