Figure 1

Thin sections of the beige limestone (BL) and the grey limestone (GL) microfacies. Thin sections GP/L 21 (a,d), GP/L 19 (b), GP/L 18 (g), GP/L 172 (c), and GP/L 16 (e,f). (a) BL is composed of thin laminasets of diffuse dark clay laminae (detail in c), interlaminated with pale pure microspar laminae. Elongated to round organic matter-rich dark lenses¹¹ are indistinctly scattered. This microfacies is interpreted as the laminated limestone (LL) Sm5 microfacies¹². (b) GL is composed of dark-grey undulated laminasets formed by thin laminae with fine blackish scattered material, likely clay/organic matter impurities^{10, 12} (detail in d). Laminasets are interlaminated with paler microspar-dominated laminae. Scattered non-oriented detrital quartz is indicated by arrow. GL microfacies is interpreted as Sm1, a clay-carbonate rythmite (CCR)¹². Besides GL having significantly fewer dark lenses than BL, the clay/organic matter-rich laminasets are more frequent, regularly distributed, thicker and have more and closer-packed laminae. (c) Detail of BL dark laminae. (d) Detail of GL dark laminae, showing concentration of pyrite¹⁰. (e) Thin section of GL depicting microspar-dominated level (top) and clay-rich laminaset (bottom). (f) Image in (e) with crossed-nicols showing neomorphic sparry crystals (arrows). (g) GL clay-rich level with peloids. Scale bars: (a)– 1 mm; (b)– 2 mm; (c)– 0.02 mm; (d)– 0.1 mm; (e)– 0.5 mm; (f)– 0.5 mm; (g)– 0. 2 mm.