Fig. 7

a Photomicrograph showing stromatolites and serpulid (SP) tubes. Walls of serpulid tubes remain micrite but the tube interiors and some adjacent laminae have been infilled by, or recrystallized to, sparry calcite. The outermost peripheral laminae are still micrite which look much darker. 1.25×, plane-polarized light; b Microbes grew surrounding the tube walls and accreted to form columnar stromatolites. 1.25×, plane-polarized light; c, e Fan-shaped stromatolites accreted through the growth of colonies of filamentous microbes (probably cyanobacteria) and with the early and pervasive precipitation of carbonate in the EPS sheaths of their filaments. Grains supplied on the stromatolite surface were not trapped and bound in the microbial EPS, but only deposited between the filament fans (Suarez-Gonzalez et al. 2019). Orange lines represent thin micritic crust. 1.25×, plane-polarized light; d, f Flat-shaped stromatolites accreted through the growth of other microbes (blue lines) and grains were trapped and bound in the uncalcified EPS. Thin micritic crusts (orange lines) separating successive laminae formed during interruptions in accretion. 2.5×, plane-polarized light