Fig 1.
Study area (modified after [90], with permission from the copyright holder, the Geological Survey of Western Australia).
The analyzed black chert facies crops out at the Trendall locality in the northern part of Western Australia.
Fig 2.
Stratigraphical context of the Strelley Pool Formation (representative sections; modified after [8], [35], [36], [43], [90]).
The black chert facies corresponds to Member III.
Fig 3.
Field observations of the black chert facies.
(A) Laminated and stromatolitic carbonates (Member II of the Strelley Pool Formation) below the black chert facies. (B-E) Characteristics of the black chert facies include fenestral fabrics (B), intercalated cm-high stromatolitic layers (C) that locally show ductile deformation (dashed line in D), as well as small-scaled cm-sized cross lamination (E). (F) Conglomerate (Member IV of the Strelley Pool Formation) above the black chert facies.
Fig 4.
Petrographic observations on the black chert facies.
(A-D) Thin sections (A, B, D: transmitted light; C: reflected light). (A) Layers are laterally not continuous in thickness but wavy. (B, C) Dark chert layers consist of a fine grained matrix that is enriched in organic matter and pyrite. Note that the organic material is laterally interwoven (B) and closely associated with pyrite (C). (B, D) Fenestrae are filled by blocky cements. (E, F) SEM photographs of the pyrite crystals. Note that the pyrite crystals are enriched in layers (E) and framboidal in shape (F). (G, H) Clusters of silicified acicular crystals, probably representing chert pseudomorphs after aragonite.
Fig 5.
ToF-SIMS ion images of a 300μm × 300μm area of the back chert facies with an organic matter layer in the center.
The pixel brightness reflects the signal intensity of (from the left): Si = [Si]+, representing the chert matrix; Ca = sum of [Ca]+, [CaO]+, and [CaOH]+, representing CaCO3; Corg = sum of major hydrocarbon ions [C2H3]+, [C2H5]+, [C3H]+, [C3H2]+, and [C3H3]+, representing organic matter. Pixel brightness in Ca and color coded overlay of Si (red) and Corg (green) is enhanced to increase image contrast.
Fig 6.
Thin sections (A, C) and corresponding Raman spectra (B, D; point measurements) documenting local occurrences of organic material (A, B) and dolomite (C, D) in some fenestrae.
Fig 7.
CL overlay photographs of the black chert facies.
Note that the calcite aggregates (red luminescence) appear to be linked to organic material (dark colors). (A) Finely laminated layers of organic matter containing CaCO3 spheroids (red luminescence). (B) Close up view of (A). (C) Aggregated CaCO3 spheroids (red luminescence) associated with organic matter. (D) Close up view of (C).
Fig 8.
NanoSIMS isotope enrichment maps of organic layers shown in Fig 7A, revealing the presence of spheroids (TSI: Total secondary ions).
Note that these bodies are generally enriched in C (12C) compared to the surrounding areas. Organic matter (12C14N) and sulfur (32S) are preferentially enriched at the edges, whereas the centers represent carbonate phases. This spatial arrangement of the isotopes is further illustrated in the color-coded overlay maps (12C & 14N; 12C & 32S).
Fig 9.
NanoSIMS isotope enrichment profile across a spheroid (see Fig 8 for orientation of the section).
Organic matter (12C14N) and sulfur (32S) are closely associated and preferentially enriched at the edges of the body, whereas carbon (12C) is also enriched in intermediate spaces due to the presence of carbonate phases.
Fig 10.
Fenestral carbonate facies from the Triassic Dachstein Limestone of the Northern Calcareous Alps.
This facies was formed by microbial mats in peritidal- to shallow lagoonal environments.