No heliotropism in Neoproterozoic columnar stromatolite growth, Amadeus Basin, central Australia: geophysical implications

An apparent sine wave pattern of columns in a single specimen of the stromatolite Anabaria juvensis (subsequently identified as Kotuikania) from a Neoproterozoic dolomite unit, originally assigned to the ∼ 850 Ma Bitter Springs Formation, in the Amadeus Basin, central Australia, was interpreted previously as recording heliotropic growth, that is, the non-vertical growth of columns throughout the year controlled by averaged incident solar radiation [Vanyo, J.P., Awramik, S.M., 1985. Stromatolites and Earth-Sun-Moon dynamics. Precambrian Research 29, 121-142]. The model of heliotropic growth was used to estimate obliquity of the ecliptic (Earth's axial tilt) and days/year at 850 Ma. Subsequent work, however, casts strong doubt on the heliotropic interpretation. Further field observations and the study of 11 additional specimens of Anabaria = Kotuikania juvensis from the original locality confirm that the columns typically display strong branching, which produces a common divergence and convergence of columns that is incompatible with heliotropic growth. The rare, apparent sinuosity of columns is seen as the fortuitous product of column irregularity and column branching. Moreover, stratigraphic studies indicate that the host dolomite unit does not belong to the Bitter Springs Formation but caps the younger Cryogenian glaciogenic succession in the Amadeus Basin and hence is ∼ 600 Ma. The previous estimate of ∼ 435 (range 409-485) days/year based on extrapolated counts of laminae in the original specimen of A. = K. juvensis conflicts with the figure of 400 ± 7 days/year indicated by high-quality palaeotidal data obtained from the late Cryogenian (∼ 640-600 Ma) Elatina-Reynella tidal rhythmites in South Australia. We conclude that inferences concerning Neoproterozoic obliquity and palaeorotation cannot be drawn from the non-vertical growth patterns of the columnar stromatolite A. = K. juvensis.