Fig. 7

Macro- and microscopic characteristics of the columnar stromatolites. a Transverse section of a micro-columnar stromatolite from sample ZK01-b11: the column is nearly circular with a column diameter of approximately 600 μm; and, there are spherical microstructures (possibly algae fossils) between the column; b Transverse section of a columnar stromatolite: the column is nearly circular with a column diameter of approximately 3 mm; and there are micro-columnar stromatolites that grew between the column; c Vertical section of a columnar stromatolite from sample ZK02-b6: the laminar shape is slightly arched, the inheritance degree of laminae is high with gradational light-and-dark laminae transition, and oncolites were present in the laminae; d Vertical section of a columnar stromatolite from sample ZK02-b6 showing the microstructure of the laminae bounded by oolites; e Pillared laminar texture of a columnar stromatolite from sample ZK02-b6, which consists of dendrolites with widths of 60–100 μm and heights of 400–800 μm; f Vertical view of micro-columnar stromatolites from sample ZK05-b20, which grew between the columnar stromatolites, while the black clotted organic matter around the column possibly indicates nutrient absorption by microbial mucus