Depositional processes and sea level changes

Dans le document Carboniferous carbonate platforms and coral reefs: New insights from southern China (Page 145-151)

The comparison of sedimentary sequences in Helv, Dujie, and Mengcun (Shen and Qing 2010), allows stratigraphic correlation, displayed in figure 12. The Helv section, a deep-water succession, starts with microfacies association A in a deep-water setting. This facies is overlain by massive pack-stone and fore-reef talus breccias beds (MFA-B) forming the substrate of stromatolites. The vertical transition from MFA-A to MFA-B reflects an increase in wave and current energy interpreted as shallowing upwards, which may relate to a sea-level drop rather than just progradation. The repeat-ed successions comprising laminar, domal or hemispheroidal, and flabellate columns stromatolites

Figure 11: Stratigraphic column of Dujie section, Long’an, with δ13C and δ18O data. MFA-D: microfacies association D.

MFA-E: microfacies association E. MFA-F: microfacies association F.

in Helv and Mengcun suggest high-resolution fluctuations of depositional environments (Shen and Qing 2010).

In Dujie section, representing the shallow-water succession coeval to the Helv section, microfacies association D is interpreted as subtidal and dominates the lowest part (Figs. 12, 13a). As the water shallows, microfacies association E with shallow-water components is encountered (Fig. 13b). The vertical transition from MFA-D to MFA-E, from a platform margin to a shallow subtidal environ-ment, reflects a decrease in water depth. Microfacies association F, mainly consisting of intertidal and supratidal deposits, forms a distinct layer in Dujie succession, most likely recording a sea-level lowstand (Fig. 13c). Four cycles consisting of MFA-E and MFA-F have been recognized in Dujie

Figure 12: Correlation of the sedimentary sequences in Dujie section, Helv section, and Mengcun section (Shen and Qing 2010).

Figure 13: Depositional model for the late Visean - early Serpukhovian interval in Dujie. (a) Sea-level highstand in late Visean. (b) Sea-level fall, leading to the shoal deposits in Dujie. (c) Sea-level lowstand during end Visean-Serpukhovian, tidal flat environment.

section, which may reflect high frequency sea-level oscillations during this period as recorded else-where (Veevers and Powell 1987; Dvorjanin et al. 1996; Bishop 2009, 2010). This pattern, similar to that of the Helv section, is thought to record a sea-level fall rather than progradation.

A distinct sea-level fall is identified in both the lower part of Helv and Dujie sections. This strati-graphic level may correspond to the distinct regressive event in the latest Visean recorded from several sites (Bisohop et al. 2009, 2010; Wang et al. 2013; Chen et al. 2016). Chen et al. (2016) analyzed the sedimentary facies from five late Visean-early Serpukhovian outcrop sections including both shallow and deep successions in Guizhou, South China. The major depositional hiatus characterized by paleokarst and associated siliciclastic mudstone was interpreted as the result of the significant sea-level drawdown across the Visean-Serpukhovian boundary (Chen et al. 2016), corresponding to the paleosol in the Arrow Canyon succession (Bishop et al. 2009).

Dvorjanin et al. (1996) published a series of paleogeographic maps of the Dnieper Donets Basin in Ukraine during the late Visean-Serpukhovian. These authors identified four to six short shallow-ing-upward T-R cycles. They correlated the cyclicity to Western Europe and North America and concluded that tectonics and (glacio-)eustatic sea-level fluctuations contributed to the genesis of the cyclic series. The distribution of subaerially exposed subtidal sediments in Arrow Canyon Range, southeastern Nevada, also records the apparent sea-level fluctuations in the latest Visean-Serpuk-hovian (Bishop et al. 2009, 2010). Across the belt of low latitudes north of Gondwanaland in upper Mississippian, widespread shallow-marine and paralic deposits is interpreted as synchronous deposi-tion of transgressive-regressive (T-R) sequences in different parts of Euramerica (Veevers and Powell 1987). This widespread cyclicity occurring in several settings in South China, Western Europe, and North America, is most likely the results of (glacio-)eustatic sea-level fluctuations, possibly with local influences of tectonic movements, corresponding to glacial deposits in western Argentina and South Tibet region (Veevers and Powell 1987; Garzanti and Sciunnach 1997; Fielding et al. 2008b, c; Loinaze et al. 2010; Fielding and Frank 2015). Based on complete sections, the next step for future research will be to identify.

Interpretation of carbon and oxygen stable isotopes

Carbon and oxygen isotope values obtained from carbonate rocks are linked to paleoclimate, paleoenvironment, element cycling and events in the geological past (Popp et al. 1986; Bruckschen and Veizer 1997; Bruckschen et al. 1999; Mii et al. 1999, 2001; Saltzman et al. 2000, 2004; Saltz-man 2002, 2003a, b; Campion et al. 2018). Carbon isotope excursions usually occur in the critical periods (e.g. during glacial stages), which are used to reflect the organic burial and primary produc-tivity (Saltzman et al. 2000; Saltzman 2002; Buggisch et al. 2008; Qie et al. 2011, 2015, 2016; Yao et al. 2015). In the present study, most δ13C values overlap with those values of the North American brachiopod (Mii et al. 1999), with seven exceptions showing lighter values (Fig. 14). In contrast, the δ18O values from Dujie section tend to be more negative than those of well-preserved brachiopods in Mii et al. (1999). Both δ13C and δ18O values in this study overlap with those of micro-drilled micrite

from Yashui platform succession in Guizhou, China (Chen et al. 2016). The lighter isotopic values may point to extensive meteoric diagenesis of these shallow subtidal and peritidal carbonate, while the other excursions could record primarily shifts in seawater δ13C values (Mii et al. 1999; Chen et al. 2016).

Figure 14: Cross-plots of carbonate δ13C and δ18O values obtained from the study area.

High-resolution δ13C time series of late Visean-early Serpukhovian from carbonate platform-to-slope successions in Guizhou province, South China, were documented in Chen et al. (2016). They recognized several negative shifts and a long-term decrease of δ13C, in slope sections and shallow platform Yashui succession, respectively, across the Visean-Serpukhovian boundary interval (Chen et al. 2016). The notable negative δ13C excursions at the base of first MFA-F most likely correspond to the most negative δ13C value below the Visean-Serpukhovian boundary in Yashui section (Fig.

15) (Chen et al. 2016). Biostratigraphic data, based on foraminifera, is limited in peritidal facies, which make it difficult to determine the precise Visean-Serpukhovian boundary. However, the most prominent shift in microfacies and δ13C of the measured sections of the present study could be used as a marker.

Qie et al. (2011) proposed a Lower Carboniferous carbon isotope stratigraphy for South China based on sections studied from Long’an and Baping. The maximum δ13C positive excursion with a 4.3‰ value in the present study is correlated to the third major positive δ13C excursion in Qie et al.

(2011) (Fig. 15). This significant positive shift was also recorded in Yashui section, Guizhou (Chen et al. 2016). The early Serpukhovian δ13C positive shift in Dujie section coincides with the positive δ13C excursions obtained from the brachiopod shell calcite in Europe, U.S. Mid-continent, and Russian Platform (Popp et al. 1986; Bruckschen and Veizer 1997; Bruckschen et al. 1999; Mii et al. 1999, 2001; Grossman et al. 2008), from the conodont apatite in Europe and Laurentia sections (Buggisch et al. 2008), and from bulk rock samples in western U.S. (Dyer et al. 2015) and Northern Spain (Campion et al. 2018). These positive excursions are accompanied by a global depositional hiatus with large-scale karstic exposure in the western U.S., unconformities on carbonate shelves in the U.K., North Africa, and western U.S. (Saunders and Ramsbottom 1986; Bishop et al. 2009; Dyer et al. 2015; Campion et al. 2018). This major positive excursion also corresponds to the time-equivalent lowstand system tract deposits in South China (Wang et al. 2013) and extensively distributed glacial deposits (Bonorino 1992; Garzanti and Sciunnach 1997; Isbell et al. 2003; Campion et al. 2018), which have been interpreted to mark the onset of Glacial Maximum by Qie et al. (2011).

The positive shifts and excursions of δ13C may result from massive organic burial or enhanced primary productivity (Saltzman 2003a; Stanley 2010). However, the isotopically light carbonate platforms from the late Mississippian are inconsistent with this simple model (Dyer et al. 2015).

Dyer et al. (2015) proposed an improved carbon box model on the basis of the canonical carbon box model described by Kump and Arthur (1999) and argued that the subaerial exposure of massive carbonate platforms affected the 13C of oceanic dissolved inorganic carbon (DIC) by trapping enough terrestrial carbon through meteoric diagenesis. Campion et al. (2018) tested the hypotheses of Dyer et al. (2015) with a δ13C record from carbonates in Northern Spain that are assumed to be not affected

Figure 15: Comparison of carbon isotopic data originating from micritic limestone from Dujie section (this study), bulk carbonate from Long’an section, South China (Qie et al. 2011), micro-drilled micrite from Guizhou, South China (Chen et al. 2016) and brachiopod shells calcite (average values) from U.S. Midcontinent and Russian Platform (Grossman et al. 2008) and Glacial events are based on Fielding et al. (2008c) and Isbell et al. (2003).

by meteoric diagenesis. The δ13C values from these sections show a 2‰ increase responding to the widespread meteoric diagenesis of marine carbonates during the early Serpukhovian (Campion et al.

2018). The early Serpukhovian δ13C positive excursion might be connected with enhanced organic carbon burial in other regions (Buggisch et al. 2008; Qie et al. 2011) and a significant light carbon sink through widespread meteoric diagenesis of subaerially exposed paleotropical marine carbonates triggered by glacioeustatic fall when ice growth expanded (Dyer et al. 2015; Campion et al. 2018).

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