UNDERSTANDING THE ORIGIN OF INCLINED STROMATOLITE COLUMNS

Stromatolites are layered sedimentary structures that represent the lithification of microbial communities, and the diverse morphology of stromatolites provides a window into the interaction of microbial systems with their environment. Here we investigate unusual, inclined to sinusoidal columnar stromatolites from the Mesoproterozoic (~1.0 Ga) Chandi Formation (Chhattisgarh Basin, India) and the Mesoproterozoic (~1.4 Ga) Altyn Formation (Belt Supergroup, USA). Previous investigation of inclined stromatolite morphologies has suggested mechanisms for inclination that range from heliotropism, to post-depositional deformation, to the influence of current activity.

Detailed photogrammatic analyses of inclined stromatolites of the Chandi and Altyn formations reveal that changes in morphology are recorded on the timescale of formation of individual stromatolitic laminae. We find that, in both of these formations, stromatolite growth most commonly occurs by the lateral translation of successive microbial laminae, which results in a primary inclined morphology. Within the Chandi Formation discrete episodes of stromatolite growth are also identified that reflect upward growth, followed by toppling of columns to an inclined position. Similarly, within the Altyn Formation, re-orientation in the direction of growth is commonly associated with disruption of the sedimentary substrate. In both the Chandi and Altyn formations, termination of discrete stromatolite beds is growth correlated with periods of enhanced detrital input. Together these observations suggest that inclined stromatolite morphologies reflect a complex interaction among microbial growth, hydrologic activity, and sediment deposition, with hydrologic activity a key element to the growth of inclined morphologies. Continued investigation is needed to identify the extent to which disruption and re-orientation of columns reflects local or regional environmental effects.