Key petrographic features
Cryptozoon displays three kinds of microbially-induced layering-lamination (Figs. 1D, 2A; 3C): 1) densely clotted–pelletoidal micrite containing vertical filaments, 2) loosely clotted–pelletoidal micrite with vesicular structure, and 3) structureless microcrystalline carbonate. The transition between filamentous and vesicular fabric is gradual both vertically and horizontally, while laminae of microcrystalline carbonate remain distinct and continuous (Fig. 1D). Layers dominated by filaments tend to exhibit tufted structures, resulting in subtle positive accretionary relief (Fig. 1D). Siliciclastic detritus occurs locally, including silt- to sand-sized quartz, muscovite and chlorite, with minor amounts of feldspar and argillaceous material. The carbonate sedimentary rock surrounding the stromatolites is essentially a packstone with a variety of skeletal bioclasts, stromatolite rip-up clasts, and ooids.
The filamentous and vesicular fabrics contain early centripetal calcite spar cement after primary fabric-selective porosity. Three kinds of dolomite are associated: 1) matrix microdolomite, 2) coarsely crystalline (zoned) dolomite associated with swarms of dikelets (Fig. 3A), and locally, 3) pervasive warped to saddle dolomite (see supplementary information file).
Following the formation of crystalline dolomite, pressure-dissolution associated layer-parallel calcite veins were formed (Fig. 3B, C; Fig. 4B). Significant recrystallization (mineral-preserving) or neomorphism subsequently followed. This transformation is highly selective and heterogenous (Fig. 2), and preferentially affected primary vesicular layers. It resulted in relatively translucent layers and patches with commonly diffuse edges, but locally shows distinctly protruding to interfingering, or even sharp boundaries (Figs. 1D, 2, 3C). Locally, this transformation progressed through the other layers, ultimately generating, de novo at microscopic scale, a localized spongy-looking microfabric (Fig. 2).
The dolomitization is subtle to pervasive and, when combined with recrystallization (neomorphism), did not obliterate early fabric relationships as indicated by manganese-maps (micro-XRF, see supplementary information file). The spar component of the secondary spongy microfabric exhibits a centered granoblastic (= polygonal, subtly interlobate) crystalline fabric (Fig. 3C–E). Sporadically, authigenic white mica is present (Fig. 4C). There are (at least) two subsequent events: shearing and microfaulting (Fig. 3B) and the formation of Fe-calcite cemented microfissures.
Cathodoluminescence reveals microcrystalline, dark-bright red luminescent zoned-euhedral and anhedral dolomite (Fig. 4A, B). Vein-related calcite is characterized by a zoned, bright orange-red luminescence, with local resorption of dolomite evident (Fig. 4B). Recrystallization occurs preferentially along vesicular layers, and is shown by patchy bright-red to bright-orange luminescent granoblastic calcite, locally progressing into the other layers (Fig. 4A).
Fluid inclusions, U–Pb dating
Fluid inclusion data were obtained from relatively early coarse-crystalline (zoned) dolomite and subsequent layer-parallel calcite veins. The measured values (Fig. 4D) provide a Th range of 119–141° C for the dolomite, with a salinity of 7.6–9.5 wt.% NaCleq, and for the vein-calcite 138–155° C with 1.1–6.5 wt.% NaCleq. U–Pb dating yields a dolomite age of about 466 ± 16 Ma and 458 ± 10 Ma (Fig. 4E, F). This roughly brackets the Middle–Late Ordovician boundary interval and ranges through most of the Late Ordovician, which would include local onset of the Taconian orogeny14. The vein-calcite formed at around 432 ± 9 Ma (Fig. 4G) and brackets most of the lower Silurian (i.e., lower Llandovery–most of Wenlock). The earlier part of the Silurian age bracket likely overlaps the end of the Taconian orogeny in the eastern New York area.