Microfacies association A
Description. — This microfacies association, widely distributed in the study area, consists of black, 5 to 40 cm-thick radiolarian wackestone and packstone (H1) beds characterized by multiple chert interlayers and nodules (Fig. 4a) particularly common in the lower part of the section. Radiolarians, consisting of spumellarians varying in size from 50 to 200 μm, represent the main bioclasts, along with a few monaxon sponge spicules, ostracods, rare small foraminifers, and crinoid ossicles (Fig. 4b).
Interpretation. — The mud-supported microfacies (H1) displays a well-preserved thin bedding and contains abundant radiolarians and a few monaxon sponge spicules. The abundant micrite, well preserved ostracods with their both valves, and the absence of cross-bedding points to very low hydrodynamic conditions, likely below storm wave base. These characteristics, along with the abundance of organic matter and scarce burrows, are indicative of a relatively deep, quiet, and poorly oxygenated environment (Devleeschouwer et al. 2002; Berra 2007).
Microfacies association B
Description. — This microfacies association consists of massive dark-gray lithoclastic-bioclastic packstone (H2), thick-bedded bioclastic grainstone (H3), and breccias (H4), which conformably overlie microfacies association A.
Figure 4: Photographs of microfacies association A and B in Helv section. (a) Field photograph of radiolarian wackestone and packstone with chert interlayers (red arrow). (b) Photomicrograph of radiolarian wackestone and packstone: radiolari-an (Ra), sponge spicules (SS), ostracod (O). (c) Field photograph of thick dark-gray lithoclastic-bioclastic packstone with chert nodules (red arrow). (d) Photomicrograph of lithoclastic-bioclastic packstone (H2): brachiopod (B), foraminifer Tetrataxis (F), lithoclasts (L). (e) Photomicrograph of bioclastic grainstone (H3).
The dark-gray, 0.5 to 2 m thick massive beds are composed of lithoclastic-bioclastic packstone (Fig. 4c). Reworked, poorly sorted, and angular lithoclasts and bioclasts represent the main compo-nents (Fig. 4d). The high diversity organisms include common brachiopods, crinoids, foraminifers (e.g. Tetrataxis sp.), and ostracods, along with rare bryozoans, gastropods, sponges, calcareous algae, trilobite, coral debris, and radiolarians. Multiple skeletons exhibit borings. Some peloids occur in the matrix. The bioclastic grainstone (H3) is similar to the packstone (H2) above in terms of components but differs from it by the well-rounded, well-sorted and smaller grains (Fig. 4e).
Breccias form a layer about 3 m in thickness (Fig. 5a). Two types of clasts are recognized, mud-stone and domal stromatolite clasts (Figs. 5b, c). The domal stromatolite clasts are the dominating form, which are usually upside down and angular. The darker mudstone clasts, from 2 to 4 cm in size, also show angular outlines. A few bioclasts occur. The matrix of the breccias consists of peloidal micrite (Fig. 5d).
Interpretation. — The diverse reworked lithoclasts and skeletal grains with various primary source areas (e.g. platform margin, lagoon, and open marine) indicate a transport from the shallow marine environment. According to Wilson (1975), H2 is classified as bioclastic to lithoclastic microbreccia (SMF4) that can be found on the foreslope talus. The spar cement and well-rounded grains in H3 are interpreted to reflect an environment with high water-energy, common on slopes (Wilson 1975;
Blomeier and Reijmer 2002). Angular mudstone and domal stromatolite clasts in microfacies H4 suggest deposition near the source area. Collectively, the characteristics of microfacies association B allow to infer a platform marginal slope with high hydrodynamic conditions.
Microfacies association C
Description. — Within association C, two microfacies types were distinguished: stromatolite bindstone (H5) and oncoid-intraclast dolowackestone (H6).
Stromatolite bindstone consists of laminar, domal or hemispheroidal, and regular flabellate co-lumnar forms as reported in Shen and Qing (2010). Laminar stromatolites appear as 5 to 50 cm-thick beds overlying the breccias (Fig. 5a), with thin laminae (2 - 5 mm) recognizable on polished slab surfaces. Stromatolites show the typical alternation of thin dark (0.05 - 2 mm) and thick light (2 - 5 mm) laminae (Fig. 5e). No fossils are recognized except for rare problematic small debris in the dark laminae.
Hemispheroidal forms were subdivided into two types according to the scheme of Logan et al.
(1964): laterally linked hemispheroids and discrete hemispheroids. The laterally linked hemispher-oids are tight and characterized by low relief and small size with 0.1 - 2 cm diameter and 0.2 - 3 cm vertical height (Fig. 5f). Discrete symmetric hemispheroids are the dominant forms (Fig. 6a). In most case, they are 0.5 - 3 cm in diameter and 2 - 5 cm high, growing regularly. Under the microscope, light laminae show about 90 - 150 μm thickness and the dark partings are less than 50 μm. The space between the stromatolites is filled with peloids.
Flabellate columnar stromatolites, about 2 m in thickness, occur in the upper part of Helv section as reported by Shen and Qing (2010). Individual columns can be 0.5 - 3 cm in diameter and more
Figure 5: Photographs of microfacies association B and C in Helv section. (a) Field photograph of breccias (red arrow) and horizontal stromatolites (green arrow) from Helv section, show the microfacies association C overlying B. (b) Polished slab of breccias with mudstone clasts (white arrow) and domal stromatolite clasts (black arrow). (c) Interpretative sketch of photo b. (d) Photomicrograph of the matrix consisting of peloids (P) in breccias. (e) Photomicrograph of horizontal stromatolites: thinner-dark (red arrow) laminae, thicker-light (green arrow) laminae. (f) Photomicrograph of laterally linked hemispheroids (red arrow) with microbial peloids (P).
than 10 cm high (Fig. 6c). Columns display regular, flabellate-expanded overlapping growth patterns (Shen and Qing 2010), with synchronous laminae characterized by interconnection and low relief, virtually lacking space between individuals (Figs. 6b, d). Only a few peloids and algal debris occur in the scarce space (Fig. 6d).
Figure 6: Photographs of Helv stromatolites. (a) Photomicrograph of hemispheroidal stromatolites: exhibiting a discrete hemispheroid with abundant microbial peloids (white arrows) in the base. (b) Photomicrograph of flabellate columnar stromatolites. (c) Polished slab showing flabellate columnar stromatolites. (d) Photomicrograph of flabellate columnar stromatolites: peloids (white arrow) existing in the scarce interspace. (e) Polished slab of oncoid-intraclast stone: intraclasts with angular outlines (red arrow). (f) Photomicrograph of oncoids in the oncoid-intraclast dolowacke-stone: algal debris (red arrow).
Oncoid-intraclast dolowackestone forms a gray, 0.5 m-thick bed in the upper part of Helv section.
Intraclasts are dark gray, angular and poorly sorted, imbedded in a lighter matrix (Fig. 6e). Tubes (possibly algae) are common in the intraclasts. Oncoids vary in size, ranging from 1 to 3 mm, and exhibit two kinds of nuclei, namely algal debris and intraclasts (Fig. 6f).
Interpretation. — The various morphologies of stromatolites in MFA-C were interpreted to be deep-water forms situated within a platform margin to marginal slope setting, intercalated with deep-water talus breccia (Shen and Qing 2010). The laminae of stromatolites are regular and symmet-ric, similar to the deep-water stromatolites reported in Canada (Hoffman 1974), Siberia (Petrov and Semikhatov 2001), northwestern Australia (George 1999), and Argentina (Gòmez-Pérez 2003). The talus breccias (H4) and thin-bedded radiolarian wackestone and packstone (H1) with sponge spicules are common in the study area (Shen and Qing 2010). The type of deposits indicate a deep-water, low energy marine setting (Shen and Qing 2010). The morphology of the stromatolites are thought to be controlled by water energy and sedimentation rate (Hofmann 1994; Lee et al. 2000; Altermann 2008; Planavsky and Grey 2008). The change in vertical sequences from the laminar stromatolites to the hemispheroidal stromatolites, and to the flabellate columnar stromatolites in the Helv section is interpreted to reflect a change in depositional environment, from deep subtidal to proximal upper slope, and to a deep-water, lower slope setting, respectively, in agreement with Shen and Qing (2010).
The repeated successions comprising different morphological stromatolites indicate the fluctuations of depositional environments most likely related to eustatic sea-level changes.