Integrated Chemostratigraphy of the Doushantuo Formation at the Northern

Home , Dengying Formation, Doushantuo Formation, Lantian Formation

Precambrian Research 192–195 (2012) 125–141

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Integrated chemostratigraphy of the Doushantuo Formation at the northern

Xiaofenghe section (Yangtze Gorges, South China) and its implication for

Ediacaran stratigraphic correlation and ocean redox models

a,e,∗ b c d e

Shuhai Xiao , Kathleen A. McFadden , Sara Peek , Alan J. Kaufman , Chuanming Zhou ,

f g

Ganqing Jiang , Jie Hu

Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA

Conocophillips, Houston, TX 77079, USA

Department of Geology, University of Maryland, College Park, MD 20742, USA

Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742, USA

State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China

Department of Geoscience, University of Nevada, Las Vegas, NV 89154, USA

PetroChina Tarim Oilﬁeld Company, Korla, Xinjiang 841000, China

a r t i c l e i n f o a b s t r a c t

Article history: The Ediacaran Doushantuo Formation in the Yangtze Gorges of South China plays an important role in

Received 12 July 2011

our understanding of biological evolution, global correlation, and ocean redox conditions, because of the

Received in revised form 22 October 2011

availability of high-resolution paleontological and geochemical data and numerous radiometric dates.

Accepted 28 October 2011

However, integrated study has been focused on the Jiulongwan section that was largely deposited below

Available online xxx

wave base in a restricted shelf lagoon (Jiang et al., 2011; Zhu et al., 2011). Studies of shallower water

successions are lacking, and this presents a challenge to test Ediacaran stratigraphic correlation and

Keywords:

ocean redox models. To ﬁll this knowledge gap, we conducted a high-resolution integrated study of the

Ediacaran

Doushantuo Formation at the northern Xiaofenghe (NXF) section approximately 35 km to the northeast

South China

and paleogeographically updip of the Jiulongwan section. With the exception of the basal 20 m, NXF

Doushantuo Formation

Carbon, sulfur, strontium isotopes sediments were deposited above normal wave base. Integrated biostratigraphic and chemostratigraphic

Redox data indicate that the 140 m thick NXF section correlates with the lower Doushantuo Formation (Member

Euxinia I and much of Member II; i.e., the lower ca. 70 m of the formation) at Jiulongwan. Geochemical data from

NXF and other Doushantuo sections indicate that euxinic conditions may have been limited to a shelf

lagoon (represented by the Jiulongwan section) that was restricted between the proximal inner shelf and

a distal shelf margin shoal complex, at least during early Doushantuo time following the deposition of the

Doushantuo cap dolostone. Further integrated studies are necessary to test whether euxinic conditions

existed in open marine shelves in South China and elsewhere during the Ediacaran Period.

1. Introduction biostratigraphic, chemostratigraphic, and geochronological data

from the Yangtze Gorges area, particularly the Jiulongwan and

The Doushantuo Formation (ca. 635–551 Ma) on the Yangtze other sections on the southern limb of the Huangling anticline

Craton of South China has played an important role in our (Condon et al., 2005; McFadden et al., 2008; Yin et al., 2011). In

understanding of Ediacaran stratigraphic correlation, biological the area around Jiulongwan, the Doushantuo Formation has been

evolution, and ocean redox history (Xiao et al., 1998; Jiang et al., sampled at high resolution by several research groups, in both out-

2007; Zhou et al., 2007; Zhu et al., 2007, 2011; McFadden et al., crop and drill core, and has been analyzed using a combination

2008, 2009; Li et al., 2010; Liu et al., 2011). Current models of of micropaleontological and geochemical tools (Jiang et al., 2007;

Ediacaran stratigraphic correlation in South China are based upon McFadden et al., 2008, 2009; Sawaki et al., 2010; Liu et al., 2011;

Yin et al., 2011; Zhu et al., 2011). However, the Doushantuo Forma-

tion is characterized by signiﬁcant facies and thickness variations

∗ on the shelf (McFadden et al., 2009; Jiang et al., 2011), and it is com-

Corresponding author at: Department of Geosciences, Virginia Polytechnic Insti-

plicated by the presence of allochthonous olistostromes in upper

tute and State University, Blacksburg, VA 24061, USA. Tel.: +1 540 231 1366;

slope environments (Vernhet et al., 2006; Vernhet and Reijmer,

fax: +1 540 231 3386.

E-mail address: [email protected] (S. Xiao). 2010). Furthermore, Doushantuo fossil occurrences are subject to

126 S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141

Fig. 1. Geological maps, localities, and stratigraphic correlations. (A) Map showing location of the Yangtze, Cathaysia, North China, and Tarim blocks in China. (B) A generalized

paleogeographic map of the Yangtze Craton during the early Ediacaran Period (Jiang et al., 2011). Rectangle at Yichang marks the Huangling anticline. Localities (Baokang, Lan-

tian, Longe, Minle, Weng’an, and Zhongling) mentioned in text are marked. (C) Geological map of the Huangling anticline. Localities (Jiulongwan = JLW, Tianjiayuanzi = TJYZ,

northern Xiaofenghe = NXF, southern Xiaofenghe = SXF, and Zhangcunping) are marked. Note that the location of Xiaofenghe and Zhangcunping were incorrectly marked in pre-

vious publications, including Zhou et al. (2007), Zhu et al. (2007), McFadden et al. (2009), Jiang et al. (2011), and Yin et al. (2011). (D–E) Possible correlation between Doushantuo

Formation in the Yangtze Gorges area (e.g., Jiulongwan section) and in shallower facies (e.g., Weng’an section), highlighting the implications for the biostratigraphic division of

acanthomorph assemblages. (D) Correlation proposed by Zhu et al. (2007, 2011), in which Tianzhushania spinosa is restricted to the lower assemblage. (E) Correlation proposed

by Zhou et al. (2007), in which Tianzhushania spinosa extends to the upper assemblage. (F) Alternative correlation, in which potentially three assemblages can be recognized

␦13 in the Doushantuo Formation. Ccarb curves, radiometric ages, and exposure surfaces are omitted in (E)–(F) for clarity. NT: Nantuo Formation; DY: Dengying Formation.

S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141 127

paleoenvironmental and taphonomic biases (Zhou et al., 2007). As where the well exposed Jiulongwan section on the southern limb

a result, ﬁne-scale litho- and biostratigraphic correlation of the of the Huangling anticline is among the most extensively studied

Doushantuo Formation at the regional scale is not always straight- (Fig. 1). In the Yangtze Gorges area, the Doushantuo Formation can

forward. The difﬁculty is exacerbated by the condensed nature of be more than 200 m thick and generally it has been subdivided into

the Doushantuo Formation: ∼84 million years of geological his- four lithostratigraphic members (e.g., Zhao et al., 1988; McFadden

tory is recorded in ∼200 m of sedimentary rocks, possibly with et al., 2008, 2009). Member I is a 5-m-thick cap dolostone charac-

multiple hiatuses. It is therefore important to test biostratigraphic terized by abundant sheet cracks, tepee structures, macropeloids,

models derived from one area through integrated analysis of high- and barite fans (Jiang et al., 2006). Member II (∼80 m thick) is char-

resolution stratigraphic data from others. Similarly, geochemical acterized by alternating organic-rich calcareous shale and shaley

models derived from the Jiulongwan section also need to be tested dolostone beds with abundant pea-sized fossiliferous chert nodules

against new data from other sections. For example, Li et al. (2010) (Xiao et al., 2010). The boundary between Member II and Member

proposed a stratiﬁed redox model for the Doushantuo basin that III is transitional at the Jiulongwan section, but has been correlated

consists of an oxic surface layer, a thin ferruginous layer at the with a mid-Doushantuo exposure surface in several shallow-water

chemocline, a euxinic wedge, and a ferruginous deep ocean. This sections (e.g., Weng’an and Zhangcunping; see Fig. 1 for location)

model was based on elemental and isotopic data from the Jiulong- (Zhou and Xiao, 2007). This correlation that has been challenged

wan section, the Zhongling section (then interpreted to have been by more recent studies (Yin et al., 2009, 2011; Liu et al., 2011; Zhu

deposited in the outer shelf on a ramp-like platform; see Fig. 1B et al., 2011). Member III (∼60 m thick) consists of massive and lam-

for section location), and two poorly sampled sections at Minle and inated cherty dolostone overlain by interbedded lime mudstone

Longe representing slope and basinal facies. This model makes cer- and dolomitic mudstone. Member IV (∼10 m thick) is composed of

tain predictions about the ocean redox conditions in the onshore black organic-rich calcareous shale, siliceous shale, or mudstone.

part of the basin, but these predictions cannot be tested without The four-fold division of the Doushantuo Formation, however, is

high-resolution data from the Doushantuo Formation deposited in not easily applicable to sections beyond the Yangtze Gorges area.

shallow-water environments. Three depositional cycles (sequences) are recognized in the

To address the need for high-resolution, integrated data from a Doushantuo Formation in the Yangtze Gorges area (Zhu et al., 2007;

shallow-water inner shelf environment, we investigated the north- McFadden et al., 2008). The boundary between the ﬁrst two cycles

ern Xiaofenghe (NXF) section. This section was chosen because (1) is marked by a mid-Doushantuo exposure surface in shallow-water

detailed micropaleontological studies have already been published sections elsewhere in the Yangtze Craton, such as the Weng’an and

(Yin et al., 2007; McFadden et al., 2009), (2) the micropaleon- Zhangcunping sections (Liu et al., 2009b; McFadden et al., 2009;

tological sample horizons were marked on the outcrop so that Jiang et al., 2011; Zhu et al., 2011). In the Yangtze Gorges area,

geochemical data can be precisely integrated with paleontological however, no easily recognizable exposure surfaces have been iden-

data, and (3) according to current basin architecture reconstruc- tiﬁed, and various authors have correlated the mid-Doushantuo

tion (Jiang et al., 2011), this section is about 35 km up-dip from the exposure surface at Weng’an with a lithofacies change near the

Jiulongwan section, allowing an exploration of paleoenvironmen- Members II–III boundary (Zhou and Xiao, 2007; McFadden et al.,

tal conditions in shallower parts of the Yangtze Craton. Here we 2009; Jiang et al., 2011) or with a Member II horizon where a car-

present carbonate carbon, organic carbon, pyrite sulfur, carbonate bonate unit is overlain by black shale with chert and phosphatic

associated sulfate sulfur, and strontium isotopic data from the NXF nodules (Zhu et al., 2007, 2011). The third cycle starts at a major

section to address basinwide correlations (Jiang et al., 2011) and to lithofacies change at the Members III–IV boundary and is termi-

test predictions of the stratiﬁed redox model (Li et al., 2010). nated by a widespread karst surface in the Hamajing Member of the

Dengying Formation (Zhu et al., 2007), although Zhu et al. (2011)

have recently discovered “a 30 cm thick interval of silty shale” in

2. Geological background and review of previous Member III at the Jiulongwan section and interpreted this silty shale

stratigraphic work “to mark a sequence boundary” and the beginning of the third cycle.

These uncertainties highlight the challenge in identifying cycles or

2.1. Physical stratigraphy and radiometric age constraints sequences in the Yangtze Gorges area where exposure surfaces are

poorly developed.

Neoproterozoic successions in South China developed on a The depositional environment of the Doushantuo Formation

south–southeast facing (present-day position) passive margin in the Yangtze Gorges area is a matter of debate. The abundance

on the Yangtze Craton during the breakup of Rodinia about of saponitic smectite and unusually low concentration of redox-

∼

830–750 Ma (Wang and Li, 2003; Zhao et al., 2011). The Cryo- sensitive elements (Mo, U, V) in the lower Doushantuo Formation in

genian glacial and interglacial successions (750–635 Ma) record a the Yangtze Gorges area has been interpreted as evidence for depo-

rift-drift transition (Jiang et al., 2003a), and subsequent deposi- sition in alkaline lacustrine environments (Bristow et al., 2009).

tion of the early Ediacaran Doushantuo Formation (635–551 Ma) This view, however, is at odds with the spatial continuity of the

occurred in marginal marine environments and focused on topo- Doushantuo Formation across much of the Yangtze Craton, the

graphic lows created by block faulting during the development marine afﬁnity of Doushantuo fossil assemblages, and the similar-

of the Nanhua Rift System (Cao et al., 1989; Wang and Li, 2003; ity of Doushantuo lithostratigraphic and chemostratigraphic trends

Vernhet, 2007). As a result, the Doushantuo Formation shows sig- with Ediacaran successions beyond the Yangtze Gorges area (Jiang

niﬁcant variations in facies and thickness (Zhu et al., 2007, 2011; et al., 2003a, 2007; Kaufman et al., 2006; Zhou and Xiao, 2007; Zhou

Jiang et al., 2011); its thickness ranges from 40 m at Weng’an et al., 2007). Recent analysis of facies variations favors the interpre-

to ∼300 m at Zhongling (see Fig. 1B for locations). The Yangtze tation that the Doushantuo Formation (with the exception of the

Platform eventually evolved into an open shelf extending to a deep- cap dolostone) at Jiulongwan was deposited in a restricted shelf

water basin during the deposition of the late Ediacaran Dengying lagoon (Ader et al., 2009; Vernhet and Reijmer, 2010; Jiang et al.,

Formation (551–542 Ma) and its deepwater equivalent Liuchapo 2011).

Formation, which are overlain by basal Cambrian chert, phospho- Several radiometric dates constrain the depositional age of

rite, and shale (Dong et al., 2008, 2009; Ishikawa et al., 2008). the Doushantuo Formation. A U–Pb zircon age from the Cryo-

Although the Doushantuo Formation outcrops widely in South genian Nantuo Formation (636.4 4.9 Ma) (Zhang et al., 2008)

China, previous work has been centered on the Yangtze Gorges area, places a maximum age constrain on the Doushantuo Formation.

128 S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141

− ␦

On the southern limb of the Huangling anticline, zircon U–Pb values as low as 10‰. The overall Ccarb chemostratigraphic

TIMS ages have been reported from Member I cap dolostone proﬁle has been corroborated in multiple sections in the southern

± ±

(635.2 0.6 Ma), lower Member II (632.5 0.5 Ma), and uppermost Huangling anticline although local variations exist (Condon et al.,

Member IV (551.1 0.7 Ma) (Condon et al., 2005). Two Re–Os ages 2005; Jiang et al., 2007; McFadden et al., 2008; Lu et al., 2009; Yin

± ±

of 598 16 Ma (Kendall et al., 2009) and 593 17 Ma (Zhu et al., et al., 2009, 2011; Sawaki et al., 2010; Zhu et al., 2011). Beyond

␦

2010) have been reported from the Member IV black shale at the southern Huangling anticline, Ccarb proﬁles of the Doushan-

Jiulongwan, although Kendall et al. (2009) cautioned the interpre- tuo Formation at the Zhangcunping and Weng’an sections, as well

tation of their Re–Os age because of possible heterogeneity of the as sections of the equivalent Lantian Formation in southern Anhui

black shale samples. Province, are broadly similar to those near Jiulongwan (Zhou, 1997;

Additionally, Liu et al. (2009b) reported a zircon U–Pb SHRIMP Zhou and Xiao, 2007; Zhu et al., 2007; Zhao and Zheng, 2010; Yuan

age of 614 7.6 Ma from an ash bed in the Doushantuo Forma- et al., 2011), but precise chemostratigraphic correlation is insecure

tion at the Zhangcunping section on the northeastern limb of the because of the lack of independent stratigraphic markers. Major

Huangling anticline, about 60 km to the northeast of Jiulongwan. departure from the Jiulongwan proﬁle has been described in deep-

This ash bed was collected within a dolostone that overlies the water slope and basinal sections, and these have been interpreted

lower Doushantuo black shale but underlies the mid-Doushantuo as evidence for major geochemical gradients in Ediacaran seawater

exposure surface. Finally, a Pb–Pb age of 599 4 Ma (Barfod et al., (Jiang et al., 2007).

2002) has been reported from the upper Doushantuo phosphorite

at Weng’an, above the mid-Doushantuo exposure surface. These

two ages constrain the mid-Doushantuo exposure surface between 2.3. Biostratigraphy

614 ± 7.6 Ma and 599 ± 4 Ma.

The biostratigraphy of the Doushantuo Formation in South

2.2. Chemostratigraphy China, and particularly in the Yangtze Gorges area, has simi-

larly been a focus of previous studies (McFadden et al., 2009;

The chemostratigraphy of the Doushantuo Formation in the Yin et al., 2009, 2011; Liu et al., 2011). Combining biostrati-

Yangtze Gorges area has been the focus of intensive investigation. graphic data from multiple sections in the Yangtze Gorges area,

Several sections on the southern limb of the Huangling anticline, McFadden et al. (2009) recognized two possible acritarch bio-

␦

including the Jiulongwan section, show nearly identical Ccarb zones. The lower is dominated by the species Tianzhushania spinosa,

proﬁles (Jiang et al., 2007; Zhou and Xiao, 2007; McFadden et al., whereas the upper is characterized by greater species evenness

2008; Sawaki et al., 2010). The cap dolostone of Member I is char- and includes abundant occurrence of Ericiasphaera rigida, T. spinosa,

␦ acterized by a negative Ccarb excursion (EN1) down to around Eotylotopalla dactylos, Meghystrichosphaeridium “perfectum”, and

− ␦

4‰, with localized occurrence of extremely negative Ccarb val- Tanarium conoideum. Yin et al. (2009, 2011) also recognized

ues as low as −48‰ (Jiang et al., 2003b; Wang et al., 2008; Zhou two assemblages in the Doushantuo Formation in the Yangtze

␦ et al., 2010). Member II is characterized by a positive Ccarb excur- Gorges area, with the lower assemblage dominated by the genus

␦

sion (EP1) with values up to +5‰. One or two negative Ccarb Tianzhushania whereas the upper devoid of Tianzhushania but dom-

values have been reported punctuating EP1 at Tianjiayuanzi (Chu inated by leiospheres, diverse acanthomorphs, and the tubular

et al., 2003) and Jiulongwan (Sawaki et al., 2010; Zhu et al., 2011). fossil Sinocyclocyclicus guizhouensis. Similarly, Liu et al. (2011) rec-

We are uncertain whether such sporadic occurrences of negative ognized two assemblages in the Doushantuo Formation (the lower

␦13

Ccarb values are of secular or diagenetic origin. Zhu et al. (2007, T. spinosa assemblage and the upper Tanarium anozos–T. conoideum

␦

2011), however, believe that these negative Ccarb values have assemblage) and they also regarded that “the genus Tianzhushania

chemostratigraphic signiﬁcance and they have named this negative does not extend into the upper assemblage”. Thus, these studies

excursion WANCE (=Weng’an negative carbon isotope excursion). differ in whether Tianzhushania and T. spinosa extend to the upper

WANCE at Weng’an occurs at the mid-Doushantuo exposure sur- biozone. This discrepancy is a result of difference in methodol-

face. As discussed above, the correlation between Weng’an and ogy and stratigraphic correlation (see Fig. 1D–F for an example).

the Yangtze Gorges area is ambiguous: some correlate the mid- First, McFadden et al.’s (2009) method was aimed at quantitatively

Doushantuo exposure surface at Weng’an with the Members II–III testing whether the lower and upper biozones – which are rec-

boundary in the Yangtze Gorges area (Zhou and Xiao, 2007) and ognized independent of biostratigraphic data, but on the basis of

thus WANCE at Weng’an would be equivalent to EN2 in the Yangtze sequence stratigraphic and chemostratigraphic correlation – are

Gorges area, whereas others correlate this exposure surface with a taxonomically distinct. As emphasized by McFadden et al. (2009),

horizon within Member II in the Yangtze Gorges area (Zhu et al., their method was not designed to locate the optimal boundary

␦

2007, 2011), implying a negative Ccarb excursion within EP1. between the two biozones. In contrast, Yin et al. (2009, 2011)

Even if one accepts the latter correlation, among the 11 Doushan- started off deﬁning the two assemblages by the presence/absence

tuo sections where WANCE has been identiﬁed by Zhu et al. (2007, of Tianzhushania, which may introduce circularity because the

␦

2011), only three actually have negative Ccarb values at WANCE stratigraphic and environmental ranges of Tianzhushania has not

(three data points at Weng’an, one at Xiangdangping, and two at been independently tested before it is used to deﬁne biostrati-

Jiulongwan by Sawaki et al., 2010; the latter two are essentially graphic units. Thus, because McFadden et al. (2009) divided their

the same section). At all the other sections, WANCE is represented biozones based on depositional cycles and chemostratigraphic fea-

␦

by positive Ccarb values. Thus, it remains to be tested whether tures whereas Yin et al. (2009, 2011) separated their assemblages

WANCE is a truly negative excursion of chemostratigraphic signiﬁ- based on the presence/absence of Tianzhushania, the exact biozone

cance or just a kink within EP1. or assemblage boundaries may be different between the two stud-

␦13

A more consistent negative Ccarb excursion (EN2, down to ies. Second, although McFadden et al. (2009), Yin et al. (2009, 2011),

−

7‰ and perhaps −9‰) occurs at the Members II–III boundary in and Liu et al. (2011) all claimed that the two biozones/assemblages

the Yangtze Gorges area (Jiang et al., 2007; Sawaki et al., 2010; occur in lithostratigraphic units Members II and III and are asso-

Yin et al., 2011; Zhu et al., 2011), followed by another positive ciated with EP1 and EP2, respectively, the correlation of these

␦13

Ccarb excursion (EP2) in dolostone of lower Member III. Car- lithostratigraphic units and chemostratigraphic features beyond

bonates of upper Member III and carbonate nodules in Member IV the Yangtze Gorges area is different between these studies, result-

are characterized by a strongly negative ␦ C excursion (EN3) with ing in different conclusions with regard to the stratigraphic range

S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141 129

of Tianzhushania. This pertains to the Weng’an and Zhangcunping favored chemostratigraphic correlation of the NXF section to the

sections, where acanthomorphs (including Tianzhushania and T. lower Doushantuo at Jiulongwan would be further strengthened

spinosa) occur in the upper Doushantuo Formation above a mid- by the dominance of Tianzhushania at NXF.

Doushantuo exposure surface (Xiao and Knoll, 1999; McFadden

et al., 2009; Chen et al., 2010; Liu et al., 2011). McFadden et al.

(2009) correlated this exposure surface (and the associated neg- 3. Methods

␦ ative Ccarb excursion at Weng’an) with the II/III boundary

◦ ◦

(and associated EN2) at Jiulongwan on the basis of cyclostratigra- The NXF section is located at 30 56.693 N, 111 13.988 E (Fig. 1).

phy and chemostratigraphy, thus placing the Tianzhushania fossils It is the same as the lower ∼135 m of the Xiaofenghe section in

from Weng’an and Zhangcunping acritarchs in their upper bio- Yin et al. (2007) and Zhu et al. (2007), the lower ∼140 m of the

zone (Fig. 1E). In contrast, partly driven by the assumption that Xiaofenghe section in Jiang et al. (2011), and the northern hillside

Tianzhushania is restricted to their lower assemblage, Yin et al. Xiaofenghe section of Zhu et al. (2011). The Xiaofenghe strato-

(2009, 2011) and Liu et al. (2011) followed Zhu et al.’s (2007, 2011) columns in Yin et al. (2007, 2011), Liu et al. (2011), and Jiang et al.

WANCE concept and correlated the mid-Doushantuo exposure sur- (2011) represent composite sections consisting of the NXF and SXF

◦ ◦

face at Weng’an and Zhangcunping with a poorly deﬁned horizon in (southern Xiaofenghe section, 30 55.945 N, 111 13.266 E, Fig. 1),

middle Member II at Jiulongwan, thus permitting the Tianzhushania although splicing of these two sections is not straightforward.

fossils from Weng’an and Zhangcunping acritarchs to be correlated Because of the splicing problem, there are major inconsistencies in

with the upper EP1 or the upper part of the lower assemblage published stratigraphic thickness of the composite Xiaofenghe sec-

(Fig. 1D). tion, ranging from <100 m (Vernhet and Reijmer, 2010) to >200 m

The biostratigraphic division proposed by Yin et al. (2009, 2011) (Zhu et al., 2011). To avoid this problem and the stratigraphic rep-

and Liu et al. (2011) is appealing because of its simplicity: the etitions resulting from modern landslides at the SXF section (Jiang

presence/absence of a single genus (Tianzhushania) deﬁnes the et al., 2011), we focused here only on the NXF section, which was

two assemblages. However, one needs to consider taphonomic carefully measured and sampled at sub-meter scale (ca. 0.5 m inter-

and environmental biases, as well as uncertainties in stratigraphic vals) for petrographic, paleontological, and geochemical analysis.

correlation, when testing the biostratigraphic model proposed by Petrographic thin sections were made for each sample and

Yin et al. (2009, 2011) and Liu et al. (2011). Certainly, at Jiulong- were examined under a transmitted light microscope. Thin section

wan and perhaps in the Yangtze Gorges area, Tianzhushania and examination allowed us to quantitatively estimate the mineralog-

T. spinosa are absent from Member III (McFadden et al., 2009; ical (e.g., calcite, dolomite, silica, quartz, phosphate, and clay) and

Liu et al., 2011). But a more important question is whether the petrological compositions (e.g., mudstone, wackestone, packstone,

Tianzhushania-bearing strata above the mid-Doushantuo exposure and grainstone). These data, along with sedimentary structures

surface at Weng’an, Zhangcunping, and Baokang (Xiao and Knoll, observed from ﬁeld investigation, are plotted as three separate

2000; Yin et al., 2004; Zhou et al., 2007; McFadden et al., 2009; Chen columns in the stratigraphic log (Fig. 2). All chert-phosphatic nod-

et al., 2010) can all be unambiguously correlated with Member II ules in thin sections were thoroughly examined for microfossils

at Jiulongwan. As discussed above, correlation of these sections (McFadden et al., 2009).

13 18

␦ ␦

based on sequence stratigraphic and chemostratigraphic data is For Ccarb and Ocarb analysis, powders were micro-drilled

indeed ambiguous. Furthermore, different taxa may have conﬂict- from the ﬁnest-grained portions of polished slabs determined by

ing stratigraphic ranges at different sections due to environmental petrographic observation of corresponding thin sections. Approxi-

◦

and taphonomic biases (Zhou et al., 2007). As an example, Yin mately 100 ␮g of carbonate powder was reacted for 10 min at 90 C

et al. (2009) claimed that S. guizhouensis ﬁrst appears in their with anhydrous H3PO4 in a Multiprep inlet system connected to

upper assemblage; however, this taxon has so far been known a GV Isoprime dual inlet mass spectrometer. Isotopic results are

only from the Weng’an biota (indeed ﬁrst appearing right above expressed in the standard ␦ notation as per mil (‰) deviation

the mid-Doushantuo exposure surface) and Member III in the from V-PDB. Uncertainties determined by multiple measurements

Yangtze Gorges area (Xiao et al., 2000; Liu et al., 2008, 2009a). of NBS-19 were better than 0.05‰ (1) for both C and O isotopes.

Yin et al.’s (2009) claim about S. guizhouensis would actually sup- Total organic carbon content (TOC) and ␦ Corg were deter-

port a correlation of the Tianzhushania-bearing Weng’an biota with mined following procedures described by Kaufman et al. (2007).

their upper, not the lower, assemblage in the Yangtze Gorges area. Approximately 1 g of whole rock powder was weighed and then

Indeed, this correlation can better explain the taxonomic differ- quantitatively digested with 6 M HCl. Residues were washed with

ence between the Weng’an biota and the lower assemblage in deionized water, centrifuged, isolated, dried, and weighed to deter-

the Yangtze Gorges area: the latter is dominated by T. spinosa mine abundance of carbonate in the samples. TOC abundance

whereas the former is not. Finally, even if one accepts Zhu et al.’s (wt% of rock powder) was calculated from measured abundance

(2007, 2011) WANCE and sequence boundaries, the Tianzhushania- of carbon in the residues quantiﬁed by an elemental analyzer (EA)

bearing Weng’an and Zhangcunping biotas are restricted to Zhu relative to the urea standard. Isotope abundances were determined

et al.’s sequence 2, whereas the Tianzhushania-dominated lower on a continuous ﬂow GV Isoprime mass spectrometer. ␦ Corg val-

assemblage in the Yangtze Gorges area (e.g., Jiulongwan, Tianji- ues are reported as per mil (‰) deviation from V-PDB. Uncertainties

ayuanzi, and Wangfenggang) is mostly if not entirely restricted based on multiple extraction and analyses of a standard carbonate

to Zhu et al.’s sequence 1 (Liu et al., 2011). Thus, it is possible are better than 0.15% (1) for TOC and 0.3‰ (1) for ␦ Corg.

to envision three Doushantuo assemblages when all evidence is Disseminated pyrite concentrations and ␦ Spy were analyzed

considered (Fig. 1F): a lower assemblage dominated by T. spinosa using both combustion and chromium reduction methods (Canﬁeld

(e.g., Member II in the Yangtze Gorges area), a middle assemblage et al., 1986). For pyrite-rich samples, decalciﬁed residues were sub-

where T. spinosa is still present but become less abundant (e.g., jected to direct EA combustion. For pyrite-poor samples, powdered

upper Doushantuo at Weng’an), and an upper assemblage with- samples were reacted with 50 ml of 1 M CrCl2 and 20 ml of 10 M

out T. spinosa (e.g., the upper assemblage of Liu et al., 2011). For HCl in an N2 atmosphere. H2S produced from pyrite reduction by

now, we readily accept that the lower Doushantuo biozone is dom- CrCl2 was bubbled through a 1 M silver nitrate trap and precipitated

inated by the species Tianzhushania (McFadden et al., 2009). If in as Ag2S. The Ag2S was centrifuged, washed, dried, and weighed.

the future it is conﬁrmed that Tianzhushania exclusively occurs Pyrite sulfur isotopes and concentrations were determined by EA

◦

in the lower Doushantuo Formation (Yin et al., 2009, 2011), our combustion at 1030 C on a continuous ﬂow GV Isoprime mass

130 S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141

Fig. 2. Lithostratigraphic log of the NXF section. Section was logged at 0.5 m intervals and drafted using the Dunham classiﬁcation (Dunham, 1962). M, mudstone; W,

wackestone; P, packstone; G, grainstone. The left edge shows dominant percentage of mineralogy between calcite, dolomite, phosphate, and terrigenous clay. The center

column shows sedimentary structures, and the right edge dominant lithology. Stratigraphic datum at base of the Doushantuo Formation.

spectrometer. 100–500 ␮g of decalciﬁed residue (for direct com- from the supernatant by gravity ﬁltration through 8 ␮m What-

bustion samples) or 100 ␮g Ag2S (for chromium reduction samples) man ﬁlters. Total volume of supernatant was measured, and 10 ml

was dropped into a quartz reaction tube packed with quartz chips of 0.1 M BaCl2 was added to 40 ml of supernatant to precipitate

and elemental copper for quantitative oxidation and O2 resorption. barite. The precipitates were then centrifuged, washed, dried, and

Water was removed from combustion products with a 10-cm mag- weighed.

nesium perchlorate trap, and SO2 was separated from other gases CAS concentrations were calculated from ICP–AES (inductively

with a 0.8 m PTFE GC column packed with Porapak 50–80 mesh coupled plasma atomic emission spectrometer) analysis of small

◦

heated at 90 C. Pyrite concentrations were calculated from SO2 aliquots of the leached solutions at Virginia Tech Soil Testing Labo-

yields and reported as wt% pyrite. ␦ Spy values are reported as ratory. CAS concentrations were calculated from sulfur abundances

2−

per mil (‰) deviation from V-CDT. Uncertainties determined from and reported as ppm SO4 in carbonate, with uncertainties bet-

multiple analyses of NBS 127 interspersed with the samples are ter than 5%. Barite precipitates were combusted with a Eurovector

34 32

better than 0.3% (1) for concentration and 0.3‰ (1) for isotope EA and S/ S ratios were determined using a continuous ﬂow GV

composition. Isoprime mass spectrometer. ␦ SCAS values are reported as per mil

Extraction of CAS was conducted using techniques modiﬁed (‰) deviation from V-CDT. Uncertainties determined from multiple

from Burdett et al. (1989), Gellatly and Lyons (2005), and Shen et al. analyses of NBS 127 are better than 0.3‰ (1).

(2008). Roughly 100 g of sample was leached with 10% NaCl solu- For strontium isotope analyses, micro-drilled carbonate pow-

tion, washed, dried, powdered, and weighed. Dried powders were ders (ca. 5 mg) were repeatedly leached (3×) with ultrapure 0.2 M

∼

treated with 30% hydrogen peroxide for 48 h to remove dissem- ammonium acetate (pH 8.2) to remove exchangeable Sr from non

inated pyrite and organically bound sulfur that could potentially carbonate minerals (Montanez˜ et al., 1996) and then treated with

contaminate CAS extractions. Leached powders were quantita- ultrapure 0.5 M acetic acid for 12 h. The supernatant was separated

tively dissolved in 3 M HCl and insoluble residues were separated from insoluble residues by centrifugation, decanted, dried, and

S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141 131

subsequently dissolved with 200 ␮l of 3 M HNO3. Small polyethy- complicated by many faults/landslides and is inappropriate for

lene columns were prepared with quartz wool and ∼1 cm of Sr spec high-resolution chemostratigraphic sampling. Yin et al. (2007)

resin and washed with 400 ␮l of 3 M HNO3. After loading, the sam- and Zhu et al. (2007) placed a black shale unit in the uppermost

ple was sequentially eluted with 3 M HNO3 (200 ␮l), 7 M HNO3 Doushantuo Formation in their SXF stratocolumns, which were

(600 ␮l) and 3 M HNO3 (100 ␮l) to primarily remove Ca and Rb, adopted by McFadden et al. (2009). This black shale unit would be

and then 400 ␮l of 0.05 M HNO3 was passed through the column regarded as equivalent to Doushantuo Member IV recognized at the

to collect the Sr fraction. The samples were then dried and loaded Jiulongwan section. However, we were unable to verify this black

with 0.7 ␮l of TaO onto a Re ﬁlament for analysis in a VG Sector 54 shale unit at SXF (see also Zhu et al., 2011). Instead, the Doushantuo-

thermal ionization mass spectrometer at the University of Mary- Dengying boundary at SXF is represented by the transition from

land. Sr beams of 0.5 V on mass 88 were obtained at temperatures Doushantuo oolitic grainstone/packstone to Dengying cherty dolo-

◦ ◦

between 1450 C and 1650 C allowing for at least 75 stable ratios. stone with dissolution structures. It is possible that this black shale

Repeated analysis of NBS 987 during the analytical session yielded unit is missing because of bedding-parallel faults, but it is more

an average value of 0.710238 ± 0.000006 (1). likely that a facies change led to the disappearance of the black

shale in shallow water environments such as at the Weng’an section

(Zhou et al., 2005; Jiang et al., 2011).

4. Lithostratigraphy Comparison between the NXF and Jiulongwan section suggests

that the former is dominated by dolomitic phosphatic wackestone

Overall, the NXF section (Fig. 2) contains greater abundances and packstone whereas the latter by argillaceous mudstone, which

of carbonate, phosphatic clasts, silt and sand, but less shale than is consistent with the paleobathymetric inference that the NXF

the Jiulongwan section. An important marker bed of the Doushan- section was more proximal than the Jiulongwan section (Jiang

tuo Formation, the basal Doushantuo cap dolostone (Member I), is et al., 2011). Although the Nantuo-Doushantuo boundary is unam-

present at the NXF section and overlies diamictite of the Cryoge- biguously identiﬁable at NXF, the three sedimentary cycles are

nian Nantuo Formation. The cap dolostone at NXF is very similar not easily recognizable because the exposure surfaces are poorly

to Ediacaran cap dolostones described elsewhere in South China: developed. Sequence boundaries or erosional surfaces have been

it is a 3.9 m thick massive-bedded microlaminated dolomicrite, marked on Xiaofenghe stratocolumns at ∼57 m (Fig. 2 in Yin et al.,

with peloids, abundant tepee-like structures (Fig. 3A), and quartz- 2011; Liu et al., 2011), ∼62 m (Fig. 3 in Yin et al., 2011), ∼74 m

ﬁlled sheet cracks (Fig. 3B–D). The crest of the tepee-like structures (Jiang et al., 2011), and ∼88 m (Yin et al., 2007; Zhu et al., 2007,

◦

strikes at 90–110 , consistent with measurements taken at Jiulong- 2011). These surfaces are all based on a single possible erosional

wan. The upper part of the cap dolostone becomes thinner-bedded, surface identiﬁed at SXF and correlated to NXF strata, and the vari-

more limey, and partially siliciﬁed. The cap dolostone passes into ation in stratigraphic height is an indication of the difﬁculty in

a 20-m thick organic-rich shale interbedded with ﬁne-grained splicing the NSF and SXF sections. Our stratigraphic analysis of

phosphatic siltstone (Fig. 3E). Phosphatic grains are silt to fine sand- NXF identifies a possible flooding surface at ∼96 m. However, no

size, subangular to rounded, well sorted, and occur in thin lenses erosional unconformity has been identiﬁed at NXF (see also Zhu

and layers 1–3 cm thick. Small wave ripples were observed at the et al., 2011). Similarly, because of the absence of the black shale

base of this unit around 4.1 m above the formation. Thin dolo- unit in the uppermost Doushantuo Formation, the surface separat-

stone interbeds also occur locally and contain abundant pea-size ing the second and third cycles is also ambiguous at Xiaofenghe.

chert nodules. The phosphatic component increases upsection from From the lithostratigraphic and cyclostratigraphic point of view,

around 10% at the base of the unit to greater than 50% near the top. the cap carbonate and overlying organic-rich mudstone/siltstone

In outcrop, the phosphatic lenses increase in density and thickness and phosphatic wackestone/packstone (0–96 m) could be equiva-

up to 5–7 cm thick. lent to the ﬁrst cycle (Members I and II) at Jiulongwan, whereas the

Overlying the siltstone is a thick unit composed of cherty dolomitic mudstone and overlying limestone (96–140 m) equiva-

dolomitic mudstone and wackestone (25–106 m). Dolostones are lent to dolostone and overlying ribbon rocks of the second cycle or

locally argillaceous, thin to medium bedded, and contain minor Member III at Jiulongwan (Fig. 5; see also Jiang et al., 2011). Alterna-

to moderate amounts of quartz and phosphate grains (Fig. 3F). tively, Zhu et al. (2007, 2011) propose that the sequence boundary

Chert nodules (Fig. 4A and B) are 1–5 cm in diameter, spheroidal at ∼88 m in SXF could be correlated with a horizon within Member

to oblong in shape and oriented parallel to bedding. Chert nod- II at Jiulongwan, implying that much if not all of the NXF section

ules contain abundant acritarchs (Fig. 4C–E) and phosphatic grains. is equivalent to Member II at Jiulongwan. The biostratigraphic and

The abundance of phosphatic grains increases upsection, although chemostratigraphic data, reported below, indicate that the entire

it is lower than the underlying phosphatic siltstone. A sharp NXF section is more likely correlative to the lower Doushantuo,

lithofacies change occurs at ∼96 m, where phosphatic-dolomitic consistent with Zhu et al.’s (2011) correlation. This correlation indi-

wackestone–packstone gives way to dolomitic mudstone and then cates that lithostratigraphic differences between the upper NXF and

lime mudstone that dominate the rest of the section (96–140 m); its potential correlative at Jiulongwan could be related to facies

this change may represent a ﬂooding surface. The uppermost part changes between the inner shelf lagoon and nearshore subtidal

of the NXF section (106–140 m) was measured along a satellite sec- carbonates (Jiang et al., 2011).

tion offset from the main section. Splicing of this satellite section

with the main section resulted in an approximate 5 m discrepancy

between our stratigraphic thickness measurements and those pub- 5. Biostratigraphy

lished in Yin et al. (2007) and Zhu et al. (2007, 2011). This unit is

characterized by massive limestone with thin (possibly microbial) Acritarch biostratigraphy of the Doushantuo Formation at

laminations interbedded with argillaceous limestone. Cherts in this Xiaofenghe has been previously published (Yin et al., 2007;

unit are oblong to banded, and contain well preserved acritarchs McFadden et al., 2009). Yin et al. (2007) presented acritarch occur-

(Fig. 4F–H). rence data from the lower Doushantuo Formation at NXF and upper

The upper NXF section is not exposed above 140 m in the Doushantuo Formation at SXF. As discussed above, McFadden

study area. Others (e.g., Yin et al., 2007; Zhu et al., 2007, et al. (2009) and Yin et al. (2009, 2011) recognized two potential

2011) continued their stratigraphic measurements up to the acritarch biozones in the Doushantuo Formation. Although there

Doushantuo–Dengying boundary along the SXF section, which is are some differences between the two studies, both recognized

132 S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141

∼

Fig. 3. Sedimentary structures. (A) Field photograph of tepee-like structure in cap carbonate at 0.5 m. (B) Field photograph of quartz-ﬁlled sheet crack structures in cap

∼

carbonate at 1.5 m. (C and D) Thin section photomicrographs of chalcedony quartz and calcite ﬁlling sheet crack structures at 2.5 m, under non-polarized and cross-

polarized transmitted light, respectively. Thin section XF-2.5. (E) Thin section photomicrograph of phosphatic siltstone at 21.9 m. Phosphatic grains in total extinction under

cross-polarized transmitted light. Thin section XF-21.9. (F) Thin section photomicrograph of phosphatic wackestone at 50.8 m. Phosphatic grains in total extinction under

cross-polarized transmitted light. Thin section XF-50.8.

that the lower biozone is dominated by Tianzhushania, particu- −6.4‰ at 29.2 m, and +3.9‰ at 26.0 m); and (3) the rest of the

larly T. spinosa. The dominance of T. spinosa in the NXF section section (35–140 m) with positive values mostly around +5‰ to

∼ ∼ ␦

(McFadden et al., 2009; Yin et al., 2009), from 35 m to 110 m +6‰. This proﬁle is similar to previously reported Ccarb data

(Figs. 4C–H and 5), indicates a biostratigraphic assignment to the from NXF (Liu et al., 2011; Yin et al., 2011; Zhu et al., 2011). Third,

␦34 lower biozone and correlation to the lower Doushantuo Formation Spy values are all positive and vary between +14.2‰ and +32.6‰.

␦34 at Jiulongwan. SCAS values are also positive with irregular variation between

+6.6‰ and +39.1‰. SCAS-py varies from −6.1‰ to 16‰. The

variability in some intervals likely reﬂects diagenetic processes,

6. Chemostratigraphy and correlation with the Jiulongwan

␦

section although if only Spy values of pyrite-rich samples (those with

>2 mg Ag2S yield from chromium reduction extraction) are con-

sidered, sample-to-sample stratigraphic scattering in ␦ S and

Chemostratigraphic data are presented in Table S1 (see Sup- py

34 87 86

S is reduced (Fig. 6). Finally, Sr/ Sr values of limestone

plemental Online Material) and plotted in Fig. 6. The data show CAS-py

samples range between 0.7079 and 0.7081.

several important features that are useful in correlation with the

␦13 There has been extensive discussion on the diagenetic alteration

Jiulongwan section. First, the Corg proﬁle is remarkably constant

− 13 of Proterozoic carbon, sulfur, and strontium isotope signatures

␦

around 28.8‰, despite Ccarb variation up to 10‰, resulting in

␦13 13 (Kaufman and Knoll, 1995; Lyons et al., 2004; Gellatly and Lyons,

a strong correlation between Ccarb and Ccarb-org (Fig. 7A).

␦13 2005; Halverson et al., 2007; Marenco et al., 2008; Shen et al.,

Second, the Ccarb proﬁle can be divided into three segments: (1)

2008, 2011; Knauth and Kennedy, 2009; Derry, 2010; Halverson

the cap dolostone (0–4 m), characterized by negative values around

␦13 34

− 13 et al., 2010). We interpret the C and ␦ S of the NXF section

␦

3‰; (2) the overlying 31 m strata (4–35 m), where Ccarb val-

as representing mainly primary signatures. This interpretation is

ues hover around 0‰ (with three exceptions, −9.6‰ at 25.5 m,

S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141 133

Fig. 4. Chert nodules and microfossils. (A) Field photograph of chert nodules (black color) at 98.5 m. (B–H) Thin section photomicrographs under non-polarized light. (B) Chert

nodule (brown color) at 96.2 m. Thin section XF-96.2. (C–E) Tianzhushania spinosa at 65.8 m. (D and E) Successively enlarged views (arrows in C and D) to show multilaminate

membrane and cylindrical processes characteristic of T. spinosa. Thin section XF-65.8. (F–H) Tianzhushania spinosa at 111.8 m. (G and H) Enlarged views (black and white

arrows in G) to show multilaminate membrane and cylindrical processes characteristic of T. spinosa. Thin section XF-141.5. (For interpretation of the references to color in

this ﬁgure legend, the reader is referred to the web version of the article.)

supported by the well deﬁned stratigraphic trends (particularly sample-to-sample variation (i.e., stratigraphic inconsistency). NXF

13 13

␦ ␦

in Ccarb and Corg) and regional consistency in the Yangtze pyrite is mostly disseminated and likely of authigenic origin. Even

Gorges area (when compared with the Jiulongwan section). Fur- so, the ultimate source of sulfur for pyrite precipitation came from

13 34

␦ ␦ thermore, there is very little correlation between Ccarb and marine sulfate. The highly positive Spy values from the NXF sec-

␦18

Ocarb data from NXF (Fig. 7B), and these data plot outside the tion are similar to those reported from other Ediacaran successions

meteoric diagenetic trend (Knauth and Kennedy, 2009); at least (Shen et al., 2008, 2011; Ries et al., 2009) and may reﬂect unique

␦13

Ccarb data from 35 to 104 m are not strongly altered by mete- marine geochemistry of Ediacaran oceans. The strong sample-

␦34 18

␦ oric diagenesis, although the primary origin of a few negative to-sample variation in SCAS and Ocarb time series likely

␦13

Ccarb values at 25–30 m may be questioned because of the large reﬂects variable degrees of alteration, although the range of sulfate

134 S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141

Fig. 5. Biostratigraphic distribution of Ediacaran acanthomorphic acritarchs at the Xiaofenghe (NXF and SXF) and Jiulongwan sections (Yin et al., 2007; Zhou et al., 2007;

McFadden et al., 2009). NT, Nantuo Formation; DST, Doushantuo Formation; DY, Dengying Formation.

S isotope compositions may also reﬂect the low sulfate availabil- Huangling anticline despite local variations (Jiang et al., 2007; Zhou

87 86

ity in Ediacaran oceans (McFadden et al., 2008). Sr/ Sr ratios and Xiao, 2007; Zhu et al., 2007, 2011; McFadden et al., 2008; Yin

␦ are susceptible to diagenetic processes such as dolomitization and et al., 2009, 2011; Sawaki et al., 2010). The negative Ccarb excur-

87 86

inﬂuence of detrital sediments. Thus, we deem that the Sr/ Sr sion at 25–30 m in the NXF section is only deﬁned by two data points

ratios from limestone samples more faithfully reﬂect the deposi- and could represent diagenetic alteration, although they may be

␦

tional values. correlated with a brief negative Ccarb excursion, also deﬁned by

Consistent with the biostratigraphic match between sections, one or two data points, recovered from the lower Doushantuo For-

chemostratigraphic proﬁles from NXF show similarities to those mation (around 58 m from the base) in a drill core near Jiulongwan

from the lower Doushantuo Formation at Jiulongwan. Carbon iso- (Sawaki et al., 2010) and an equivalent horizon at Tianjiayuanzi in

tope trends of the NXF section can be readily correlated with the Yangtze Gorges area (Chu et al., 2003; Zhou and Xiao, 2007). At

those from the lower Doushantuo Formation (Members I and II) the present, we are inclined to interpret these negative values as

at Jiulongwan (McFadden et al., 2008). ␦ Corg trends are nearly diagenetic artifacts with little chemostratigraphic signiﬁcance.

␦13 ␦34

identical between the two sections. The negative Ccarb excursion Spy values at NXF are unusually positive, again similar to val-

in the cap dolostone and the positive excursion at 35–140 m can ues from the lower Doushantuo Formation at Jiulongwan, although

␦34 34 be matched with, respectively, EN1 in Doushantuo Member I and Spy and ␦ SCAS proﬁles at NXF show more consistent strati-

␦ EP1 identiﬁed in Doushantuo Member II. Ccarb values at 5–35 m graphic trends and less stratigraphic scattering than the lower

␦13 are also similar to the average Ccarb values in lower Member Doushantuo Formation at the Jiulongwan section. The NXF section

34 34

II at Jiulongwan and other sections on the southern limb of the also has higher ␦ Spy values but lower ␦ SCAS values, resulting in

S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141 135

34 34

Fig. 6. Chemostratigraphic proﬁles of the NXF section. In the ␦ Spy and SCAS-py proﬁles, solid symbols represent samples with >2 mg Ag2S yield in CRS extraction (roughly

87 86

equivalent to >0.02% pyrite). In the Sr/ Sr proﬁle, solid and empty symbols represent limestone and dolostone samples, respectively.

136 S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141

40 10 y = 0.9655x + 28.973 (R² = 0.9423) 35 5

30 0 (‰ VPDB) (‰ VPBD) carb-org

carb -5

C 25 C 13 13 δ Δδ -10 20 Knauth and Kennedy (2009) NXF (35-104 m) A NXF (4-35 m) B

NXF (0-3.9 m cap dolostone)

15 -15

-10 -5 0 5 10 -15 -10 -5 0 5

δ 18 Ccarb (‰ VPDB) δ Ocarb (‰ VPDB) 0.5 Longe (Li et al., 2010) Minle (Li et al., 2010)

Zhongling (Li et al., 2010) 0.4

Jiulongwan lower DST (0- 70 m) (McFadden et al., 2008)

8 Jiulongwan upper DST (70- 155 m) (McFadden et al., 2008) 0.3

Jiulongwan lower DST (0- 70 m) (Li et al., 2010) Jiulongwan upper DST (70- 155 m) (Li et al., 2010) 0.2 NXF (This Study) 7 0.1

6 0

0 0.5 1 1.5 2 5 (Wt %)

py 4 S

0 2 4 6 8 C org (wt %)

13 13 13 18

Fig. 7. Geochemical crossplots. (A) ␦ Ccarb vs Ccarb-org, showing positive correlation with a slope of nearly 1.0. (B) ␦ Ccarb vs ␦ Ocarb, showing poor correlation for much

of the Doushantuo Formation at NXF (35–140 m) and an offset from the meteoric alteration trend identiﬁed in Knauth and Kennedy (2009). (C) Corg (wt%) vs Spyrite (wt%),

showing relatively low TOC, pyrite content, and Spy/Corg ratio at NXF as compared with the Jiulongwan section. Inset shows a close-up view of the lower left corner of the

plot. Pyrite contents reported in McFadden et al. (2008) and Table 1 have been converted to Spyrite contents.

lower SCAS-py values relative to the lower Doushantuo Forma- found in Member III limestone with little clay (McFadden et al.,

tion at Jiulongwan. From a chemostratigraphic point of view, it is 2008; Sawaki et al., 2010). Our analyses (Figs. 6 and 8) of limestone

important to note that the prominent decrease in both ␦ Spy and samples near the top NXF section are consistently low (ranging

␦ SCAS observed in the upper Doushantuo Formation at Jiulong- from 0.7079 to 0.7081). Matched with the trend deﬁned by high Sr-

wan (arrows in Fig. 8) (McFadden et al., 2008; Li et al., 2010) is not content limestone samples from previous studies (including core

seen at NXF, further strengthening the view that only the lower samples analyzed by Sawaki et al., 2010), the upper NXF samples are

Doushantuo Formation is represented at NXF. clearly equivalent to those in the lower Jiulongwan strata, further

As a critical test of these regional correlations, we evaluated supporting our correlation.

variations in strontium isotope signatures in limestone samples The established isotopic contrasts between the lower and upper

throughout the NXF section. Previous studies (Yang et al., 1999; Doushantuo Formation at Jiulongwan are striking. The upper inter-

Jiang et al., 2007; Sawaki et al., 2010) revealed a strong isotopic con- val contains a carbon isotope anomaly (EN3) known globally as

trast between lower and upper Doushantuo Formation limestones the Shuram event, named after a long-lived negative carbon iso-

87 86

−

at multiple sections in the Yangtze Gorges area. Sr/ Sr values tope excursion to values near 10‰ recorded in drill core from

of best-preserved samples rise from ca. 0.7078 to 0.7089 from the Oman (Burns and Matter, 1993; Le Guerroue et al., 2006; Le

bottom to top of the formation (Fig. 8), a transition consistent with Guerroue and Cozzi, 2010; Bergmann et al., 2011; Grotzinger

data derived from other Ediacaran successions (Shields and Veizer, et al., 2011; Verdel et al., 2011). This biogeochemical anomaly is

2002; Halverson et al., 2010). This rise cannot be explained by an recorded in upper Member III and Member IV of the Doushan-

upsection increase in the content of clays that contain more radio- tuo Formation, and stratigraphic data from these intervals also

genic Sr, because overall the lower Doushantuo is more argillaceous record simultaneous depletion of S in both sulﬁdes and sul-

87 86

than the upper Doushantuo and the highest Sr/ Sr values are fates (McFadden et al., 2008). Neither these signiﬁcant anomalies,

S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141 137

Fig. 8.

138 S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141

Table 1

␦13

Statistics of chemostratigraphic data from Doushantuo strata representing the Ccarb feature EP1 at NXF (this study) and Jiulongwan (Jiang et al., 2007; McFadden et al.,

87 86

2008; Li et al., 2010). Statistics of Sr/ Sr data at Jiulongwan also includes data from Tianjiayuanzi (Yang et al., 1999) and a drill core near Jiulongwan (Sawaki et al., 2010).

NXF (34–140 m) Jiulongwan (25.7–66 m) (Jiang et al., Jiulongwan (29–65 m) (Li

(this study) 2007; McFadden et al., 2008) et al., 2010)

TOC (wt%) 0.58 ± 0.39 (n = 163) 1.44 ± 0.81 (n = 52) 0.87 ± 0.21 (n = 9)

Pyrite (wt%) 0.03 ± 0.01 (n = 44) 0.79 ± 1.08 (n = 40) 1.22 ± 0.70 (n = 6)

Spy/Corg 0.04 ± 0.02 (n = 42) 0.3 ± 0.4 (n = 41) 0.6 ± 0.6 (n = 9)

CAS (ppm) 310 ± 557 (n = 159) 591 ± 710 (n = 50) 131 ± 133 (n = 18)

␦13

Corg (‰) −28.8 ± 0.6 (n = 163) −29.8 ± 0.4 (n = 52) −29.9 ± 0.3 (n = 20)

␦13 ±

± ±

Ccarb (‰) 5.5 1.3 (n = 161) 4.8 1.2 (n = 68) 4.0 1.9 (n = 20)

13 ±

Ccarb-org 34.2 1.3 (n = 155) 35.5 ± 0.7 (n = 17) 33.8 ± 2.0 (n = 20)

␦34

Spy (‰) 24.6 ± 3.2 (n = 44) 18.3 ± 9.0 (n = 40) 7.0 ± 9.2 (n = 9)

␦34

SCAS (‰) 28.8 ± 4.5 (n = 110) 32.5 ± 9.9 (n = 36) 38.0 ± 5.8 (n = 18)

SCAS-py 3.3 ± 4.2 (n = 27) 12.7 ± 9.0 (n = 25) 28.9 ± 8.0 (n = 9)

87 86

Sr/ Sr 0.708004 ± 0.000071 (n = 5) 0.708028 ± 0.000156 (n = 24)

Fig. 9. Paleographic reconstruction (A; box diagram modified from Jiang et al., 2011) and stratified redox ocean model (B; cross section modified from Li et al., 2010), with the

approximate location of Doushantuo sections marked on the diagrams. In this model, euxinia is restricted to the shelf lagoon. It remains uncertain whether euxinia extends

the open shelf and to deep oceans.

87 86

nor the high Sr/ Sr values, are recorded in samples from NXF. Jiulongwan, probably because NXF was closer to a carbonate factory

Thus, chemostratigraphic and biostratigraphic data indicate that and detrital sediment sources. This correlation also suggests that

the 140-m-thick NXF section is correlated with the ∼70-m-thick the lithostratigraphic similarity between the dolostone–limestone

lower Doushantuo Formation at Jiulongwan (highlighted in Fig. 8), unit (96–140 m) at NXF and Member III at Jiulongwan does not

implying that the sedimentation rate at NXF was about twice that at imply chronostratigraphic equivalency.

Fig. 8. Comparison of chemostratigraphic proﬁles of the NXF and Jiulongwan sections, with proposed chemostratigraphic correlation (yellow shading). To facilitate compar-

87 86

ison, corresponding chemostratigraphic proﬁles are plotted at the same scale. Highly radiogenic Sr/ Sr values (0.709–0.713) from the cap carbonate at a drill core near

87 86 87 86

Jiulongwan were regarded as altered values (Sawaki et al., 2010) and are omitted from the Jiulongwan Sr/ Sr proﬁle. Sr/ Sr data from the Tianjiayuanzi (see Fig. 1C for

location) (Yang et al., 1999) are scaled to the thickness of the Jiulongwan section based on ␦ C chemostratigraphic correlation (Zhou and Xiao, 2007). (For interpretation of

the references to color in this ﬁgure legend, the reader is referred to the web version of the article.)

S. Xiao et al. / Precambrian Research 192–195 (2012) 125–141 139

7. Test of Ediacaran ocean redox models authigenic pyrite formation at NXF (vs syngenetic pyrite forma-

tion at Jiulongwan) where diffusion of seawater sulfate to pore

Li et al. (2010) proposed a stratiﬁed redox model for the waters limited the isotopic fractionation between sulfate and

Doushantuo basin. In this model, the Doushantuo basin consisted pyrite.

of an oxic surface ocean that was underlain by a thin ferrugi-

nous layer, a wedge of euxinic water resting on an open shelf,

8. Conclusion

and a deep ferruginous ocean. This model was derived from geo-

chemical data from successions deposited in bathymetrically deep

This study highlights the necessity of integrating basin archi-

basinal ferruginous setting (Zhongling, Minle, and Longe sections)

tecture, biostratigraphy, and chemostratigraphy when correlating

and intermediate-depth euxinic setting (Jiulongwan section). How-

Ediacaran strata and reconstructing environmental changes in

ever, geochemical data from the Minle and Longe sections were

Earth’s distant past. Our biostratigraphic and chemostratigraphic

sparse, and no data from proximal shallow facies were available.

data highlight the potential of acanthomorph acritarchs and car-

Further, the Zhongling section, previously interpreted to have accu-

bonate carbon isotopes in Ediacaran correlation (Moczydłowska,

mulated on a distal ramp (Li et al., 2010), is now reinterpreted as

2005; Willman and Moczydłowska, 2008; Vorob’eva et al., 2009;

sedimentation on the lagoonal side of a shelf margin shoal com-

McFadden et al., 2009; Liu et al., 2011; Sergeev et al., 2011; Zhu

plex (Jiang et al., 2011; Zhu et al., 2011). This reinterpretation

et al., 2011), and they show that only lower Doushantuo Forma-

of the basin architecture has implications for the stratiﬁed redox

tion is represented at the NXF section. A positive identiﬁcation of

model.

the upper Doushantuo Formation at the SXF section remains to be

Importantly, the basin architecture reinterpretation and our

veriﬁed by integrated stratigraphic data. Geochemical data indi-

data from NXF indicate that both the NXF and Zhongling sections

cate that the NXF sediments were laid down in non-euxinic waters.

may have been deposited beneath non-euxinic (oxic or ferrug-

Considered with published basin architecture reconstruction (Jiang

inous) waters in shallower environments surrounding a deeper

et al., 2011; Zhu et al., 2011), the new data is consistent with eux-

euxinic lagoon, which developed shortly after the deposition of

inia in a shelf lagoon bracketed by non-euxinic waters in the inner

the Doushantuo cap dolostone (Jiang et al., 2011). The interpre-

shelf (e.g., NXF) and outer shelf margin (e.g., Zhongling), at least

tation that the NXF sediments appear to have been deposited in

during the early Doushantuo time following the deposition of the

non-euxinic waters is supported by their lower TOC and pyrite

cap dolostone. More data are needed to further test whether a eux-

contents (Table 1) and lower S /C ratios as compared with

pyrite org inic wedge, which appears to be an emerging feature in the redox

the lower Doushantuo Formation at Jiulongwan (Fig. 7C), although

structures of late Archean to early Neoproterozoic oceans (Reinhard

this interpretation needs to be further tested by iron speciation

et al., 2009; Johnston et al., 2010; Kendall et al., 2010; Poulton et al.,

data from NXF. A non-euxinic environment for the NXF section

2010; Poulton and Canﬁeld, 2011), also characterizes the Ediacaran

is consistent with predictions of the Li et al. model. However, the

oceans; and if so, whether the Ediacaran euxinic lens or wedge was

euxinic condition at Jiulongwan spatially bracketed by non-euxinic

controlled by oceanic factors such as a sulfate gradient (Li et al.,

conditions at NXF (this study) and Zhongling (Li et al., 2010) can

2010), paleogeographic factors such as basin restriction (Jiang et al.,

alternatively be interpreted in the framework of a recent basin

2011), or a combination of both. Such a test requires geochemical

architecture reconstruction (Jiang et al., 2011): euxinia developed

data from Ediacaran successions beyond the Yangtze Gorges area

in a restricted lagoon and may have come and gone in the Ediacaran

and beyond South China.

Period (Fig. 9). This restricted euxinic lagoon is different in geom-

etry from the euxinic wedge that rested on an open shelf (Li et al.,

Acknowledgements

2010). More high-resolution chemostratigraphic data are needed to

test whether euxinia also occurred in shelf margin and whether the

This research was supported by National Science Founda-

global ocean was stratiﬁed with ferruginous/euxinic deep waters

tion, NASA Exobiology and Evolutionary Biology Program, Chinese

below oxic surface waters.

Academy of Sciences, National Natural Science Foundation of China,

In detail, some of the chemostratigraphic differences between

Chinese Ministry of Science and Technology, and the Guggenheim

NXF and Jiulongwan can also be explained in the framework

Foundation. We would like to thank Jinlong Wang for ﬁeld assis-

of the basin architecture presented by Jiang et al. (2011). First,

tance, James Farquhar for access to CRS extraction facilities, Natalie

although the NXF section and lower Doushantuo Formation at Jiu-

13 13 Sievers, Benn Breeden, Rebecca Ohly, and Susan Drymala for techni-

longwan have remarkably invariant ␦ Corg proﬁles, the ␦ Corg

cal assistance, and two anonymous reviewers for critical but useful

values are slightly greater at NXF than at Jiulongwan (Table 1).

comments.

This subtle difference may indicate that photosynthesis had a

greater contribution to organic production at the non-euxinic

Appendix A. Supplementary data

NXF section, whereas microbial recycling of aged organic car-

bon played a greater role in euxinic conditions at Jiulongwan

Supplementary data associated with this article can be found, in

(Fike et al., 2006; McFadden et al., 2008). This interpretation

13 the online version, at doi:10.1016/j.precamres.2011.10.021.

␦

is also consistent with slightly greater Ccarb values at NXF

than Jiulongwan during the proposed EP1 positive excursion

␦ (Table 1). Second, stratigraphic scattering observed in the Ccarb, References

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