Morphometry of microstromatolites in calcrete laminar crusts and a fractal model of their growth

The laminar crust, constituting the upper part of calcretes (terrestrial CaCO₃ accumulations inside surficial sediments), is a succession of thin layers of various colors and shapes resembling micro-stromatolites. The crust structure and its diagenetic evolution are similar to stromatolites. A quantitative study of its structure was made using image analysis. Euclidian parameters were calculated to describe lamina shape. Eight hundred and eighty-six laminae were divided into six classes from the flatest forms to columnar shapes. The geometrical relationships between the shapes are interpreted as steps in the growth process of the microstromatolite. A fractal model of laminar crust growth was developed, using the diffusion-limited aggregation model (DLA) and dilation (an operation of mathematical morphology). This model simulates all growth steps observed in thin section and emphasizes the necessity of an interface with the atmosphere to explain the variety of shapes. This growth model supports the theory of a surficial and biogenic origin for certain calcrete laminar crusts.