Stratigraphic and microfacies analysis of the Kaili Formation, a candidate GSSP for the Cambrian Series 2–Series 3 boundary

Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183

Contents lists available at SciVerse ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo

Stratigraphic and microfacies analysis of the Kaili Formation, a candidate GSSP for the Cambrian Series 2–Series 3 boundary Robert R. Gaines a,⁎, John A. Mering a, Yuanlong Zhao b, Jin Peng b a b

Geology Department, Pomona College, 185 E. Sixth St., Claremont, CA 91711, USA College of Resource and Environment Engineering, Guizhou University, Guiyang, 550003, China

a r t i c l e

i n f o

Article history: Received 27 April 2011 Received in revised form 15 August 2011 Accepted 27 August 2011 Available online 3 September 2011 Keywords: Global Stratotype Section and Point Cambrian Stratigraphy Cambrian Kaili Formation Cambrian Series 2 Cambrian Series 3

a b s t r a c t The Kaili Formation of South China is a ~ 200–300 m thick succession of fine-grained siliciclastic sediments with minor carbonates that spans the proposed Cambrian Series 2–Series 3 boundary interval, formerly the Early-Middle Cambrian boundary. It was deposited at low latitude on a slope lying between the Yangtze carbonate platform to the northwest and deep-water outer shelf facies of the Jiangnan Basin to the southeast. Because the Kaili Formation contains abundant and well-preserved fossils throughout, most importantly oryctocephalid trilobites that have a broad geographic distribution, and because it appears to be less condensed than other contemporaneous sections found on other paleocontinents, a section near Balang Village, Jianhe County, Guizhou Province, has been proposed as a candidate Global Stratotype Section and Point (GSSP) for the Cambrian Series 2–3 boundary. However, because the Kaili Formation is mudstone-dominated, it is not possible to determine from outcrop study whether a change in depositional regime or a condensed interval may be present around the proposed boundary. Because the proposed boundary interval coincides with the maximum flooding stage of a global transgression, it is possible that subtle, yet significant facies changes may occur within the mudstones that comprise the proposed boundary interval, rendering its utility as a potential GSSP questionable. Here, we present an analysis of a composite section of the complete Kaili Formation exposed along two ridges and in a road cut near Balang Village. The section was measured at the cm-scale and sampled every 1 m throughout the complete thickness of the unit, except where covered. Microfacies analysis of 138 samples, using thin sections, polished slabs, acetate peels, X-radiographs and scanning electron microscopy was conducted in the laboratory. These analyses confirm that depositional processes within the Kaili Formation were consistent and unshifting throughout complete thickness of the formation. Event-driven deposition was maintained across the Kaili Formation with no evidence for condensation present, even at the 5–20 μm scale. The signal of the global transgression in the boundary interval is manifest as a slight thinning of individual millimeter-scale event-deposited lamina from 50 to 55 m above the base of the formation, around the proposed GSSP boundary at 52.8 m. The entire Kaili Formation appears to have been deposited below storm wave base, as no silt-sized or coarser clastic particles are present, and no evidence of cross-bedding or graded bedding occurs within any of the facies present within it. The great majority of its thickness, including the entirety of the proposed boundary interval, is comprised of mm-laminated calcareous claystones that exhibit randomly oriented clay microfabrics characteristic of deposition from turbid suspension by sediment-gravity flows. © 2011 Elsevier B.V. All rights reserved.

1. Introduction By recent decision of the International Subcommission on Cambrian Stratigraphy (ISCS), the traditional subdivision of the Cambrian into lower, middle, and upper epochs is currently undergoing revision toward a new chronostratigraphic division of the period into four series of relatively equal durations (Babcock et al., 2005; Peng, 2006; Babcock and Peng, 2007). This change was proposed following the realization

⁎ Corresponding author. E-mail address: [email protected] (R.R. Gaines). 0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2011.08.018

that, under the traditional three-epoch division, the Lower Cambrian represents a disproportionately large segment of time, encompassing over half the period (Babcock et al., 2005). Accordingly, representative stratigraphic sections that will serve to define the upper and lower boundaries of each of the new Cambrian series must be selected as Global Stratotype Sections and Points (GSSPs). Once agreed upon, these reference sections will provide a basis for global correlation. A section of the Kaili Formation, exposed along mountain ridges near Balang Village, Guizhou Province, China has been proposed as a candidate GSSP for the boundary between the unnamed Series 2 and the unnamed Series 3 of the Cambrian (Fig. 1) at ~510 ma (Zhao et al., 2001). The Kaili Formation appears to satisfy many of the requirements

B

Kaili Formation Tsinghsutung Fm.

Cambrian Series 2

Cambrian Series 3

A

Jialo Fm.

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183

Balang Fm.

172

C

52.8 m

Fig. 1. The geological setting of the proposed Cambrian Series 2–Series 3 boundary in Jianhe County, Guizhou Province, China. A. Stratigraphic nomenclature of prominent Cambrian formations in the study area, showing the proposed boundary of Cambrian Series 2 and Series 3 in the lower Kaili Formation. B. Outcrop of the proposed boundary interval at the Wuliu-Zengjiayan section, with the boundary horizon at 52.8 m marked in the upper right of the photograph scale = 50 cm. Numerals on the outcrop indicate collection numbers. C. Close up of the proposed boundary interval shown in B, scale = 20 cm.

for GSSP status, as discussed below. However, the Kaili Formation is dominated by mudstones, including the entirety of the Series 2–3 boundary interval. Mudstones present unique challenges to outcrop study (e.g. Schieber and Zimmerle, 1998), and as a result, it is possible that the proposed boundary interval may represent a cryptic shift in depositional regime or a condensed interval that has not been identified by previous stratigraphic study (e.g. Zhao et al., 2001, 2007; Wang et al., 2006; Lin, 2009), rendering interpretation of faunal shifts, which define the proposed boundary, questionable in terms of their global significance. Here, we present the results of a new stratigraphic and microfacies analysis of the complete Kaili Formation at the proposed GSSP locality that incorporates lab-intensive micro-scale analysis of the depositional environments and physical depositional processes that were operative during the accumulation of the Kaili Formation. We use this data to evaluate the suitability of this section as a potential GSSP for the Cambrian Series 2–3 boundary. 1.1. Global Stratotype Section and Point criteria Since the ratification of the first GSSP at the Silurian–Devonian boundary in Czechoslovakia more than three decades ago, numerous

others have been placed to define key stratigraphic boundaries around the globe (Hedges and Kumar, 2009). To date, GSSPs have been defined for three of the five series boundaries of the Cambrian. The Ediacaran–Cambrian boundary GSSP at Fortune Head, Newfoundland, is marked by the first appearance datum (FAD) of the trace fossil Treptichnus pedum at 542 ma (Brasier et al., 1994; Landing, 1994). The GSSP for the Cambrian Series 4 (Furongian)–Ordovician boundary (488 ma) was established at Green Point, Newfoundland, based on the FAD of the conodont Iapetognathus fluctivagus (Cooper et al., 2001). The GSSP for the Series 3 (unnamed, proposed as Guizhouan)–Series 4 (Furongian) boundary, located at Pabai, China, is defined by the FAD of the trilobite Glyptagnostus reticulatus at 499 ma (Peng et al., 2004). GSSPs have not yet been ratified for the Cambrian Series 1 (Terreneuvian)–2 (unnamed) boundary at 521 ma or the Cambrian Series 2–3 boundary at ~510 ma (Babcock and Peng, 2007). Before a section may be considered as a GSSP, a number of criteria must be demonstrated (Remane et al., 1996; Cooper et al., 2001). In this manuscript, we focus on the primary sedimentary and stratigraphic criteria: 1. Ideally, sedimentation should have been continuous without hiatuses or condensed intervals; 2. vertical facies changes should not be present; 3. sedimentation rates must have

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183

been sufficiently high to provide a clear record of paleobiologic and geochemical changes across the interval; 4. abundant, diverse and well-preserved faunas must be present, including cosmopolitan fossil taxa that are favorable for long-range correlation. Other important criteria include lack of significant metamorphism and diagenesis as well as lack of tectonic and synsedimentary deformation, and easy access to sections that is unrestricted by local laws. Supporting biostratigraphic, chemostratigraphic, paleogeographic, facies-relationship, and sequence-stratigraphic data should also be available from the candidate section as well as other contemporaneous sections in the region (Remane et al., 1996; Cooper et al., 2001). The Kaili Formation appears to meet the majority of these requirements (Fletcher, 2007; Zhao et al., 2007). It preserves a thicker section of the boundary interval than is present on other paleocontinents (e.g. Wang et al., 2006; McCollum and Sundberg, 2007), and it contains a well-preserved, abundant and diverse biota that can be correlated to similar outcrops regionally. Among these are key cosmopolitan fossil taxa, such as the trilobites Oryctocephalus indicus and Ovatoryctocara granulata, which allow for global correlation (McCollum and Sundberg, 2002, 2005; Fletcher, 2007; Peng et al., 2009). The FAD of O. indicus, which occurs at 52.8 m above the base of the Kaili Formation, is a leading candidate to define the Cambrian Series 2–3 boundary, although the FAD of O. granulata, which occurs at 10 m above the base of the section is considered a better GSSP candidate by Fletcher (2007). Recent taxonomic study, however, suggests that specimens of O. granulata from the lower Kaili formation are significantly different from the holotype of O. granulata from Siberia, rendering the biostratigraphic utility of specimens assigned to of O. cf. granulata questionable (Yuan et al., 2011; Zhao et al., 2011). For this reason, most attention has focused on the FAD of O. indicus at 52.8 m above the base of the Kaili Formation. Biostratigraphy of the candidate section as well as several other companion sections exposed regionally is very well established (Yuan et al., 1997, 2002; Yang and Yin, 2001; Zhao et al., 2007, 2008; Lin, 2009). The chemostratigraphic framework of the candidate section and several companion sections is also well documented (Zhang et al., 1996; Zhu and Zhao, 1996; Zhu et al., 1999; Guo et al., 2001; 2005, 2010; Yang et al., 2003). By law, access to the outcrops for academic purposes is unrestricted (Zhao et al., 2001), and well-maintained roads provide ready access to the section. Below, we consider the suitability of this proposed GSSP from a stratigraphic and depositional perspective by evaluating the sedimentary characteristics across the formation in millimeter–micron scale detail. 1.2. Geologic and depositional setting During deposition of the Kaili Formation, South China was an isolated continental landmass lying near the equator and inundated by an epicratonic sea (Wang et al., 2006; Lin, 2009). Most of the area of the South China Block was dominated by shallow water carbonate deposition on the Yangtze Platform, to the present day northwest of the study area. The Kaili Formation was deposited along a facies belt known as the Jiangnan Slope that was transitional between the shallow water facies of the Yangtze Platform and deeper water facies of the Jiangnan Basin to the southeast (Wang et al., 2006). The Jiangnan Slope was the site of dominantly fine-grained siliciclastic deposition, mixed with deep-water carbonate sediments transported from the platform (Zhang et al., 1996; Zhu et al., 1999; Lin, 2009). This setting is analogous to the “outer detrital” facies belt of Laurentia, which lay offshore of an “inner detrital belt” of mixed silicilastics, and a carbonate platform belt (Palmer, 1960; Robison, 1960). In the study area, the Kaili Formation is ~ 198 m in thickness. Its base overlies a scoured surface at the top of the Tsinghsutung Formation (Fig. 1), a succession of fine-grained, thin bedded dolostones (Wang et al., 2006). Kaili deposition represents the onset of flooding

173

on the slope. The lower portion of the Kaili Formation has been interpreted to represent a transgressive interval, with maximum flooding in the interval surrounding the proposed boundary at 52.8 m above the formation (the FAD of O. indicus). The remaining thickness of the middle and upper Kaili Formation is interpreted as a highstand systems tract with gradual shallowing accompanying seaward progradation of the Yangtze carbonate platform, manifest as an overall increase in the presence of thin, interbedded carbonates up section, toward the contact with the overlying mixed-siliciclastic carbonate Jialo Formation. The Kaili-Jialo succession has been interpreted to represent a complete 3rd order depositional cycle, representing transgression, maximum flooding, and a protracted period of regression accompanied by basin filling (Wang et al., 2006). Several claystone-carbonate cycles are apparent at the 10 m scale in the top of the formation, which is conformably overlain by the Jialo Formation (Fig. 1). 2. Methods 2.1. Field methods A composite section through the complete Kaili Formation was measured in cm-scale detail and sampled at three exposures at Balang Village. The uppermost Tsinghsutung Formation (4 m) and the lower 58 m of the Kaili Formation were measured at the WuliuZengjiayan section (26°44.843′N, 108°24.830′E), which is exposed along a ridge and has been recently excavated in order to provide nearly complete exposure. Ninety-six meters through the middle portion of the formation were measured at the Miaobanpo section, which lies along a ridge ~500 m to the northeast (26°45.014′N, 108°24.982′E). At the Miaobanpo section, the Kaili Formation lies in fault contact with the Tsinghsutung Formation. Projection along the structural trend using a Brunton compass suggests that the lowermost exposure of the Kaili Formation at the Miaobanpo section corresponds to ~58 m above the base of the Formation at the WuliuZengjiayan section, assuming that both belong to the same structural block, as indicated by all available evidence. The uppermost 48 m of the formation as well as 5 m of the overlying Jialo Formation were measured at the Lu section, here named, for an exposure along a roadcut descending to the south from Balang Village (26°44.532′N, 108°24.783′E). The Miaobanpo and Lu sections are precisely correlated by the base of a succession of cliff-forming thin-bedded carbonates that appear at 96 m above the base of the Miaobanpo section, and could be traced along strike to the Lu section. Since the base of the Kaili Formation is not exposed at the Miaobanpo or Lu sections, the height above the base of the formation at both sections is given as “~”, indicating the height above base of the formation is approximate. Representative hand samples were taken within every 1 m except where covered or highly weathered and returned to the laboratory for analysis. A total of 187 oriented hand samples were collected and wrapped in the field. 2.2. Laboratory methods A total of 138 samples were analyzed for microfabric (Appendix 1). Samples were cut perpendicular to bedding and polished, if necessary. Sedimentary fabric and bedding thickness data were taken by direct examination of slabs. Thirty-four samples were rejected because of advanced weathering or damage during shipping that precluded confidence in analysis of primary features. Thin sections were prepared from 40 representative samples of mudstone, using epoxy impregnation. The same samples were also imaged by X-radiography prior to thin section preparation. Thin sections were used to determine grain size, the presence of microstructures such as scour or cryptic grading, and the abundance, distribution and size of authigenic calcite and pyrite. Cathodoluminescence (CL) microscopy was

174

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183

also used to assess the distribution of authigenic carbonates in thin section. Acetate peels were prepared from 27 carbonate-rich samples, and examined for the same characteristics using a microfilm viewer and transmitted light microscopy. Thirty-two samples were prepared for analysis by scanning electron microscopy (SEM). Small (~1 cm) chips of rock were broken perpendicular to bedding and imaged on freshly broken surfaces (O'Brien and Slatt, 1990) to determine grain size and clay microfabric. Each of the SEM samples included multiple laminae, which were systematically examined bottom to top for changes in grain size and microfabric. Mineralogy of 16 samples was determined by X-ray diffraction (XRD) of whole-rock powders using a Rigaku Ultima IV spectrometer. Oriented clay separates were prepared for XRD analysis using the method of Moore and Reynolds (1997). Weight percent carbonate was determined using a UIC 5014 CO2 coulometer and digestion in 70% perchloric acid. 3. Results Analysis reveals that the entire thickness of the Kaili Formation is exclusively fine-grained, with no silt or coarser clastic grains present, and only rare allochem content in carbonate facies. Millimetermicron scale investigation revealed the presence of only two facies in all 138 samples analyzed across the entire ~198 m of the Kaili Formation. These facies are defined at the millimeter scale. Although the two are often finely (mm–cm) interbedded, each dominates large portions of the formation. 3.1. Calcareous claystone lithofacies The calcareous claystone lithofacies (Fig. 2A, B) is comprised of thinly-thickly laminated (0.5–6.0 mm) claystone with no silt or coarser grains present (Fig. 2C, D), contrary to previous reports. This silt problem is discussed below. Stratigraphic trends in lamination thickness are shown in Appendix 2. No evidence of grading, scour or cross-bedding was identified in our sample set or by detailed outcrop study. Individual claystone laminae are defined by authigenic carbonate cements that are concentrated at bed tops (Fig. 2C, D). Although the abundance of carbonate has been reduced in our samples by near-surface weathering, transmitted light and CL microscopy

A

B

clearly reveal the primary distribution and authigenic texture of the cements, which are comprised of rhombohedra, 1–50 μm in diameter (avg. ~ 5 μm). XRD analysis of 16 samples indicates that calcite is the primary carbonate phase present in all samples. XRD analysis of clay separates indicates that illite is the dominant clay mineral in Kaili mudstones, with only minor amounts of chlorite and no muscovite present, suggesting that the Kaili Formation has experienced little burial metamorphism. Although black organic grains were reported from the lower part of the formation, by Zhu et al. (1999), no such grains were identified by microscopic study, and no phosphate minerals were identified by XRD analysis. However, abundant oxide pseudomorphs, derived from the weathering of pyrite, are present in this interval and appear opaque in thin section. Oxide pseudomorphs are preferentially associated with carbonates, although they occur commonly throughout the claystone fabric. Samples selected for SEM microfabric analysis span the thickness of the formation, but are clustered most densely around the proposed boundary interval at 52.8 m that is marked by the FAD of O. indicus (Appendix 1). SEM analysis of freshly-broken surfaces of 34 samples confirms that claystones are exclusively fine grained (N20 μm) with no grading across laminae or coarser detrital particles present. Furthermore, SEM analysis reveals that claystones are comprised of randomly-oriented clay microfabrics (Fig. 3). No parallel/subparallel laminated microfabrics were observed in any portions of any of the samples analyzed. The calcareous claystone facies is the dominant facies of the Kaili Formation, occurring across its complete thickness. In the upper portion of the formation, this facies is interbedded with the carbonate mudstone lithofacies at a 0.5–20 cm scale. 3.2. Carbonate mudstone lithofacies The carbonate mudstone lithofacies (Fig. 4), which occurs most prominently in the upper part of the formation, is comprised of thinly-thickly laminated (b0.5–7.0 mm) micrite (Fig. 4C, D), with only rare allochems. Lamination is apparent in weathered hand sample, however systematic analysis of samples by microscopy and acetate peels revealed no evidence for small-scale grading, scour or

C

C P O

D

Fig. 2. The calcareous claystone lithofacies in outcrop (A,B) and thin section (C,D). A. Typical exposure of the calcareous claystone lithofacies, Miaobanpo section, 115 m, scale = 1.20 m. B. Outcrop exposure of a heavily cemented interval of calcareous claystone, Miaobanpo section, 133 m, scale = 45 cm. Heavily cemented intervals, N1 cm–3 cm in thickness, appear as light colored bands in the lower one-third of the photograph. C. Transmitted light micrograph of thin section of typical calcareous claystone lithofacies, Wuliu-Zengjiayan section, 54 m, scale = 2 mm. Carbonate cements appear bright, and are concentrated at the tops of individual event-deposited laminae (arrow C). The abundance of carbonate cements has been reduced by near-surface weathering. Black spots with irregular margins are pyrolusite (arrow P), which precipitated along fractures during weathering. Pyrolusite is also visible at the top center of the image. Very small black spots with regular margins are Fe-oxide pseudomorphs after pyrite (arrow O). Box indicates area shown in D, below. D. Cathodoluminescence (CL) micrograph of thin section of calcareous claystone lithofacies, Wuliu-Zengjiayan section, 54 m, scale = 500 μm. Authigenic carbonate cements, which luminesce under CL, appear bright in the micrograph. CL illuminates carbonate cements at the tops of claystone laminae, whereas clay-rich bed bases appear dark.

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183

A

B

C

D

E

F

G

H

I

175

Fig. 3. Scanning electron micrographs of the calcareous claystone lithofacies, showing microfabrics dominated by randomly-oriented clay mineral grains, characteristic of event deposition from turbid suspension, and showing the absence of silt-sized or coarser grains. A. Wuliu-Zengjiayan section, 26 m, scale = 20 μm. B. Wuliu-Zengjiayan section, 51 m, scale = 50 μm. C. Wuliu-Zengjiayan section, 52.8 m, scale = 20 μm. D. Wuliu-Zengjiayan section, 54 m, scale = 20 μm. E. Wuliu-Zengjiayan, 55 m, scale = 50 μm. F. Miaobanpo section, 58 m, scale = 20 μm. G. Miaobanpo section, 91 m, scale = 20 μm. H. Miaobanpo section, 101 m, scale = 20 μm. I. Miaobanpo section, 147 m, scale = 20 μm.

cross-lamination. The carbonate sediment is comprised of calcite, as indicated by XRD, and is uniformly fine-grained with a narrow range of grain sizes present (Fig. 4D; 10–100 μm, avg. ~ 20 μm). A subordinate amount of clay is present within the carbonate-dominated matrix (≤3–43 wt.%), as determined by coulometry. The carbonate mudstone facies reaches a maximum of 10 cm of uninterrupted, continuous thickness in the uppermost part of the Kaili Formation (above 154 m). This facies is interbedded at the decimeter scale (Fig. 4A, B) with the calcareous claystone facies, which is present across the entire thickness of the formation. 4. Depositional model for the Kaili Formation The Kaili Formation is unusually fine-grained throughout its complete thickness, and was clearly deposited under low-energy conditions. Detailed field description (cm-scale) and systematic analysis (mm–micron scale) of 138 samples did not reveal any evidence of occasional wave influence over the substrate, or any other cryptic evidence of energetic deposition (cross-lamination or grading) in either of the two facies. This suggests that the entirety of the Kaili Formation was deposited below storm wave base. The broad Yangtze Platform (N300 km in width; Lin, 2009) effectively sequestered silt and coarser clastic sediments to its landward side. Only clastics of the clay size fraction were able to bypass the platform, presumably via shallowly dipping bypass zones, to reach the Jiangnan Slope and Basin. Although deposition was not energetic, microfabric evidence reveals that depositional processes were consistently event-driven. SEM analysis of 32 samples from the calcareous claystone facies demonstrates the consistent presence of randomly-ordered clay microfabrics in all samples analyzed (Fig. 3). Randomly-ordered clay microfabrics are characteristic of deposition of flocculated aggregates of clay minerals from turbid suspension by bottom-flowing currents

(O'Brien et al., 1980; O'Brien and Slatt, 1990). Although randomlyoriented fabrics may be easily lost and transformed to laminated microfabrics during compaction, randomly-oriented microfabrics can be generated secondarily only through extensive bioturbation (O'Brien and Slatt, 1990). Extensive bioturbation is rarely present within limited horizons of the calcareous claystone facies of the Kaili Formation (Lin, 2009; Lin et al., 2010), and is absent from all samples analyzed by SEM. The great majority of the thickness of the formation is characterized by the absence of bioturbation and the preservation of thin, continuous laminae (Lin, 2009; Lin et al., 2010). Therefore, randomly-oriented clay microfabrics must represent a primary depositional texture, the preservation of which may have been aided by carbonate cementation. While deposition of the calcareous claystone facies was not energetic enough to transport coarser grains to the slope or to produce grading or cross-lamination, deposition was clearly event-driven, as also evidenced by the rare presence of tool marks within this facies (Zhu et al., 1999). Individual gravity-driven depositional events must have been set up by storm wave disturbance of shallower water environments upslope. Microfabric and thin section evidence also indicates that no significant pelagic sedimentation occurred between depositional events, resulting in the accumulation of amalgamated event-deposited laminae. Each lamina would have accumulated on the timescale of hours to days, providing favorable conditions for the burial and preservation of fossils, which were not subsequently disturbed by physical processes within this facies, as scour and other evidence for sediment re-working are absent. The carbonate mudstone facies is also comprised of thin, continuous laminae interpreted to represent event-based deposition. The presence of carbonate sediments in this deep-water setting requires transport from carbonate-producing environments in shallow waters upslope to the locus of deposition on the slope, which lay below storm wave base, and by extension, below the photic zone. Although

176

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183

Fig. 4. Examples of the carbonate mudstone lithofacies. A. Road cut exposure of the upper Kaili Formation at the Lu section, 34–37 m above base of section (~189–191 m above base of formation), dominated by resistant ledges comprised of the carbonate mudstone lithofacies, scale = 1 m. B. Base of the uppermost depositional cycle of the Kaili Formation. Carbonate mudstone ledges 2–4 cm thick are interbedded at the 5–10 cm scale with the calcareous claystone lithofacies, scale = 10 cm. C. Outcrop exposure of carbonate mudstones, Lu section, 8 m, showing millimeter-scale lamination within this facies, as expressed most clearly on weathered surfaces, scale is in cm. D. Transmitted light micrograph of acetate peel taken from carbonate mudstone facies, Lu section, 27 m, showing uniformly fine-grained carbonate (avg. 20 μm) and homogenous texture, scale = 500 μm. Location of light source is responsible for bright spot at center.

in situ production of deep-water carbonates as mud mounds has been documented in offshore Cambrian strata (Elrick and Snider, 2002), the thin, parallel laminated character of Kaili carbonate mudstones excludes the possibility of microbially-mediated in situ precipitation. Primary depositional textures (laminae) and common burrows are easily observed in hand sample, polished slab and in acetate peel. Analysis by light microscopy indicates that the thin laminae are ungraded and composed of very fine-grained micrite (10–100 μm), which may be primary, although it is possible that modest recrystallization has occurred. Because micrite grains are relatively equant in shape compared to sheet-like clay minerals that possess one axis that is profoundly shorter than the other two, the arrangement of micrite grains provides no information of depositional process, even if recrystallization is absent. Light microscopy and coulometric analysis indicates that there is a significant clay component (≤3–43 wt.%) present within individual carbonate-dominated laminae. The presence of clays mixed throughout the micrite fabric of individual

laminae provides additional evidence of rapid deposition from turbid suspension (O'Brien et al., 1980; O'Brien and Slatt, 1990). The carbonate mudstone facies must have been derived from storm wave disturbance of carbonate-producing environments in shallower waters upslope, as well as of muddy sediments lying on the upper slope, which were subsequently transported downslope under the influence of gravity and deposited on the Jiangnan Slope by bottom-flowing currents. Whereas previous interpretations have suggested that alternating carbonate and claystone beds in the upper part of the Kaili Formation reflect discrete storm generated couplets (Zhang et al., 1996; Zhu et al., 1999), our analyses indicate that each carbonate horizon (up to 10 cm thick) is comprised of tens to hundreds of millimeterscale event beds, as also described from the similar “rhythmite” facies of the Marjum Formation (Elrick and Snider, 2002). The slightly greater modal thickness of individual carbonate mudstone laminae probably reflects less compaction in this facies compared to the calcareous claystone facies, due to early lithification of

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183

5. Stratigraphic interpretation The calcareous claystone facies dominates the lower and middle parts of the Kaili Formation (Figs. 5–8). The first appearance of the carbonate mudstone facies occurs at ~154 m above the base of the

meters 60

50

40

KAILI FORMATION

carbonate sediments. Therefore, the original thickness of event-deposited laminae was probably comparable across facies, suggesting similar depositional energy was maintained throughout the formation. Although the Kaili Formation is frequently described as silt-bearing (e.g. Zhang et al., 1996; Zhu et al., 1999), our analyses and a recent study by Lin et al. (2010) confirm that no silt sized or larger clastic grains occur within the formation. Confusion arises as a result of extensive authigenic carbonate fabrics that are present within the calcareous claystone facies, as revealed by thin section and CL microscopy (Fig. 2C, D). Authigenic carbonate cements, originally concentrated at individual bed-tops, are preferentially lost to weathering in many samples, along with authigenic pyrite that commonly precipitated within the cemented portions of laminae, resulting in the presence of thin (b0.5–5 mm) weathered bands that appear orange to yellow-brown in outcrop. In fresh exposure, these weathered and iron-stained bands superficially resemble thin silt “stringers” that are common in other mudstones deposited in more proximal depositional environments (e.g. Gaines and Droser, 2002), but instead represent the preferential loss and remobilization of authigenic components during recent near-surface weathering. The origin of this “yellow-brown sediment” was recently considered in depth by Lin et al. (2010), who noted that this material was also preferentially concentrated in burrows. Authigenic cements commonly replace burrows and preserve them in three dimensions in other similar calcareous claystones of Cambrian age (Gaines et al., 2005; Gaines and Droser, 2010; Garson et al., in press). Lin et al. (2010) suggested on the basis of insoluble residues from burrow fillings that locallyderived fecal pellets may constitute a significant proportion of the volume of yellow-brown sediment contained therein. While no evidence of fecal pellets was revealed in this study, it is certainly plausible that fecal pellets may have been deposited in burrows and subsequently surrounded by cements. However, at the bed-to-bed scale, fecal pellets do not comprise any recognizable portion of the yellow-brown stained horizons at the tops of discrete claystone laminae. The difference in color and texture did not arise from differences in sediment grain size, but instead resulted from the preferential weathering of bed-capping cements, which produced a network of small, often inter-connected voids left behind by mineral dissolution that were stained by iron oxides derived from the oxidation of pyrite. While conspicuous and stratigraphically significant alternations between the calcareous claystone facies and the carbonate mudstone facies occur across the section, the accumulation of both facies resulted from the same, event-driven depositional process. Turbid suspensions of fine-grained sediment set up by wave disturbance in shallower settings flowed downslope under the influence of gravity, resulting in the deposition of fine-grained laminae below storm wave base on the Jiangnan Slope. This depositional setting is notably similar to that of other “outer detrital belt” deposits, including the Spence Shale (Liddell et al., 1997; Garson et al., in press) and the Wheeler and Marjum Formations of Utah (Rees, 1986; Gaines and Droser, 2010), all belonging to Series 3 of the Cambrian. Each of these Laurentian units, however, contains carbonate wackestones and packstones in some intervals, indicating a higher maximum energy of deposition. The depositional settings of the Kaili and these similar deposits were favored by high global eustatic sea level and the establishment of broad carbonate platforms that effectively sequestered coarse clastics on the landward side, promoting mixed claystone-carbonate deposition on the seaward facing slopes descending from the platform margins.

177

30

20

10

0

Tsinghsutung Fm. Sh Si SS M W

P

Fig. 5. Log of the Wuliu-Zengjiayan section. The lowermost 63 m of the Kaili Formation measured here overlie an erosive surface at the top of nodular dolomites comprising the uppermost Tsinghsutung Fm. The stripes from 0 to 18 m above base represent intervals of calcareous claystone that are extensively cemented by authigenic calcite. Arrow indicates the location of the proposed GSSP for the Cambrian Series 2–3 boundary at 52.8 m above the base of the Kaili Fm., marked by the first appearance of Oryctocephalus indicus. Meters given are height above the base of the Kaili Formation. Sh = shale, Si = siltstone, SS = sandstone, M = carbonate mudstone, W = wackestone, P = packstone.

formation (74 m above the base of the Miaobanpo section; Fig. 6), and this facies becomes increasingly prominent upsection. Intervals comprised of amalgamated, thinly-thickly laminated micrites are interbedded at the 1–10 cm scale with the calcareous claystone facies throughout the upper part of the formation (Figs. 7–8). The sudden appearance and subsequent vertical persistence of carbonate mudstone indicates that a significant source of micrite became available to the depositonal system of the Jiangnan Slope at this point in the section. This influx of carbonate sediment is interpreted to reflect seaward progradation of the Yangtze Platform (toward the southeast) across the upper Jiangnan Slope, commensurate with basin filling and an overall shallowing trend of the section, as previously interpreted by Wang et al. (2006) to represent a part of the transgressive portion of a 3rd order depositional cycle comprising the Kaili and Jialo Formations.

meters 150

JIALO FM.

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183

meters Covered

50

140

40

130

30

120

20

KAILI FORMATION

178

10

110

Covered

0

100

Sh Si SS M W P Fig. 7. Log of the Lu section showing the uppermost 48 m of the Kaili Formation, overlain by the mixed siliciclastic–carbonate Jialo Formation. This section is dominated by the carbonate mudstone lithofacies, with four calcareous claystone to carbonate mudstone-dominated cycles expressed as increasing carbonate:shale upsection. Meters given are height above the base of the Lu section, which lies ~ 154 m above the base of the Kaili Formation. Sh = shale, Si = siltstone, SS = sandstone, M = carbonate mudstone, W = wackestone, P = packstone.

90

80

70

60

Sh Si SS M W P Fig. 6. Log of the middle portion of the Kaili Formation at the Miaobanpo section. The stripes from ~ 132 to 136 m above the base of the formation represent intervals of calcareous claystone that are extensively cemented by authigenic calcite. Because it is uncertain how much stratigraphic overlap exists between the measured intervals of the Wuliu-Zengjiayan section and the Miaobanpo section, meters shown are approximate height above the base of the Kaili Formation (± 5 m). Sh = shale, Si = siltstone, SS = sandstone, M = carbonate mudstone, W = wackestone, P = packstone.

Carbonate-rich horizons are present in the lower 18 m of the formation (Fig. 5), forming prominent cm-scale ledges in outcrop. Acetate peel and thin section microscopy, however, reveal that these intervals are comprised of heavily-cemented calcareous claystones, that contain 5.9 to 42.5 wt.% CaCO3 (Fig. 9). A similar interval of heavily cemented calcareous claystone (9.3–40.9 wt.% CaCO3) occurs from ~132 to 136 m above the base of the formation at the Miaobanpo section (Figs. 2B, 6). We consider it likely that these heavilycemented claystone laminae may have contained a significant primary micrite component, that was subsequently recrystallized and at least partially redistributed within the claystones during early diagenesis (e.g., Hallam, 1986). If this interpretation is correct, then the vertical distribution of these heavily-cemented horizons in the lower, transgressive portion of the formation, and the middle portion, belonging to the early part of the regressive phase, have stratigraphic significance at the scale of the prominent 3rd order depositional cycle. Their distribution indicates that Yangtze Platform lay close enough to the Jiangnan Slope during the deposition of these two intervals to facilitate the periodic input of micrite to the site of deposition, although this carbonate sediment was mixed with terrigenous clays in individual event-deposited claystone laminae. These observations are consistent with previous sequence stratigraphic interpretations (Guo et al., 2005, 2010; Wang et al., 2006), and imply that the

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183

179

meters Jialo Fm.

200

Lu Section

190

180

170

160

Fig. 9. Transmitted light micrograph of thin section of a heavily cemented interval of the calcareous claystone lithofacies, Miaobanpo section, ~ 133 m above base of the Kaili Formation, scale = 5 mm. Carbonate cements appear bright, clay-dominated portions of laminae appear dark.

150

140

130

Miaobanpo Section

120

100

90

KAILI FORMATION

110

80

70

60

?

50

W uliu-Zengjiayan Section

40

30

20

10

0

Tsinghsutung Fm. Sh Si SS M W P

Fig. 8. Generalized composite section of the Kaili Formation in the Balang area. Ranges of the three individual measured sections are shown, with ± 5 m uncertainty in correlation of the base of the Miaobanpo section to the Wuliu-Zengjiayan section, as indicated by a question mark. Arrow indicates the FAD of O. indicus at 52.8 m. Sh = shale, Si = siltstone, SS = sandstone, M = carbonate mudstone, W = wackestone, P = packstone.

seaward edge of the Yangtze Platform was progressively drowned and ultimately buried by fine-grained siliciclastics during late transgression and highstand. Transgression resulted in progressively restricted micrite input to the slope during the interval spanning 0– 18 m above the base of the formation, and in restriction of micrite from the locus of deposition from 18 to ~132 m, during the latest transgressive phase, maximum flooding, highstand, and the early regressive phase. Following maximum flooding and highstand, the seaward progradation of the Yangtze Platform resulted in increasing micrite input to the slope during regression. Within the broader context of the 3rd order depositional cycle, 4th order depositional cycles (parasequences) are clearly manifest across parts of the section. In the upper part of the formation, cycles are most conspicuously expressed as changes in the proportion of carbonate mudstone to calcareous claystone, accompanied by changes in thickness of carbonate mudstone intervals comprised of 10's to 100's of individual micrite-dominated laminae. In this part of the formation, four prominent calcareous claystone to carbonate mudstone cycles ranging in thickness from 3.5 to 20.5 m are apparent. These cycles are interpreted to represent progradation and drowning of the seaward edge of the Yangtze Platform, resulting in systematic variation in supply of carbonate sediments to the slope. The heavily cemented interval of from ~132 to 136 m may represent a small depositional flux of micrite to the slope that caps another, weakly expressed 4th order cycle at a similar scale. It is likely that 4th order depositional cycles or parasequences are present throughout the formation, but are only cryptically expressed because water depth and the horizontal distance from the carbonate platform were sufficient to preclude transport of micrite to the locus of deposition. Although cyclic depositional patterns are present, the entire Kaili Formation represents deposition by a single process. Even during the deepest parts of cycles, event-driven deposition was maintained with no evidence of pelagic deposition, condensation or hiatus observed in the field or lab. During the shallowest parts of cycles, this location on the slope remained below storm wave base, and was not subject to erosion or large-scale facies changes resulting from a change in depositional regime. 6. Implications for the proposed GSSP for the Cambrian Series 2–3 boundary The proposed GSSP boundary at the maximum flooding surface, which lies at 52.8 m above base of the formation at the WuliuZengjiayan section (FAD of O. indicus), is part of a thick interval dominated by the calcareous claystone facies that spans 0–~154 m. As described above, two intervals occurring on either side the proposed

180

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183

boundary (0–18 m and ~132–136 m) contain heavily cemented calcareous claystone that may reflect some limited input of micrite, mixed with clay, to the slope. The entire interval from 18 to ~132 m, however, is strictly monofacial, and is comprised of amalgamated mm-scale laminae of calcareous claystone. Examinations of polished slabs and thin sections in combination with SEM microfabric analysis verify that event-driven deposition was maintained throughout this complete interval. The signal of maximum flooding is manifest only as a slight thinning of event beds in the 50–55 m interval immediately surrounding the proposed boundary (Appendix 2). Thinning of event beds implies a more distal depositional setting in this interval, however, no evidence of condensation or even a partial shift to pelagic deposition is present at the micron to outcrop scale. While it is certainly possible that a subtle hiatus surface may have been missed in the field, there is no evidence to support a shift in depositional regime or the presence of a brief condensed interval. The depositional location of the Kaili Formation was ideal to capture a record of continuous event-driven sedimentation that preserved a continuous record of biotic change through the proposed boundary interval as well as across its complete ~198 m thickness, including the FAD of O. cf. granulata at 10 m. 7. Conclusions The Kaili Formation near Balang Village, Guizhou Province, China, is an unusually fine-grained mixed siliciclastic–carbonate succession that straddles the proposed boundary between Series 2 (unnamed) and 3 (unnamed) of the Cambrian period, an interval that coincides with a major global eustatic transgression. Systematic micro-scale analyses were applied to 138 oriented samples collected at 1 m intervals across the complete thickness of the formation in the context of composite section that was measured and described in the field at the cmscale. Analysis reveals that only two facies are present, a calcareous claystone facies and a carbonate mudstone facies. The former contains no silt-sized or coarser clastic particles, and the latter contains only rare allochems. No evidence for grading, scour or crosslamination is present, indicating that the entire Kaili Formation was deposited under low-energy conditions below storm wave base, from gravity driven suspensions of fine-grained sediment that were set up by storm wave disturbance of substrates lying in shallower waters upslope. This conclusion is supported by the consistently thinly-thickly laminated (b0.5–7.0 mm) character of both facies. Sea level changes corresponding to 3rd and 4th order depositional cycles caused migration of the adjacent Yangtze carbonate platform, the proximity of which controlled the amount of carbonate sediment delivered to the Jiangnan Slope upon which the Kaili Formation was deposited. During a global transgression that culminated in maximum flooding at the proposed boundary, the Kaili Formation in the study area remained dominated by event-driven deposition of mm-laminated calcareous claystone, with no evidence for condensation, hiatus, reworking, or cryptic facies change present at the outcrop, millimeter, or micron scales. Due to its favorable depositional setting, the Kaili Formation preserves a continuous record of biotic and geochemical change through this important interval in the early history of metazoan life, and is a well-qualified candidate section for the Cambrian Series 2–3 boundary GSSP. Acknowledgments We thank B. Li, F. Liu, Z. Liu, Q. Qin, Y. Wang, and H. Yang and for assistance in the field, and T. Fletcher, F. Sundberg, M. Webster, M. Zhu and two anonymous reviewers for beneficial comments on an earlier draft of this manuscript. We thank D. Haley and D. Tanenbaum for assistance with SEM analyses, and L. Curtin for assistance with XRF and coulometric analyses. Fieldwork and analyses were supported by the NSF DMR-0618417 and a D.L. and S.H. Hirsch Research Initiation Grant to RRG, and NSFC-406720018 (China), Gui.Co.G. [2008] 700110 and [2010]7001 (Guizhou, China) to Zhao.

Appendix 1. Table showing samples taken from the Kaili Formation, and analytical methods used on each

Sample #

Rejected

Wuliu-Zengjiayan section W0 W1 W2 W3 W4 W5 W6 W7 W8 W9 W10 W11 W14 W15 W16 W17 X W18 W19 W20 W21 W22 X W24 W25 W26 W27 W28 W29 W30 W31 W32 W33 W34 W35 W36 X W37 W38 W39 W40 W41 W42 W43 W44 W45 W46 W47 W49 X W50 W51 W52 W53 W54 W55 W56 W57 W58 W59 W60 W61 W62 W63 X W64 X W65 X W66 X W67 X W68 X Miaobanpo section M58 M60 M61 M62

Polished slab PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS PS

PS PS PS PS

Thin section

Acetate peel

SEM

XRD

%CaCO3

SEM

XRD XRD AP TS TS

18.4% XRD SEM

AP AP

XRD XRD 42.5% SEM XRD

TS

5.9% XRD 0.8%

TS

SEM

TS

SEM XRD

TS

SEM

SEM

TS TS TS

TS TS TS TS

TS TS

SEM SEM SEM SEM SEM SEM SEM

XRD

5.4%

XRD

SEM SEM

SEM

TS

SEM

0.1%

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183 Appendix 1 (continued) (continued) Sample #

Rejected

Miaobanpo section M63 M64 X M65 M66 X M67 M68 M69 M70 X M71 M72 M73 M74 M75 X M76 M77 M78 M79 M80 M81 X M82 X M83 M84 X M85 X M86 X M87 M88 M89 M90 M91 M92 X M93 M94 M95 M96 M97 X M98 X M99 X M100 X M101 M102 M103 X M104 M115 X M116 M117 M118 M119 M120 M121 M126 M127 X M128 M129 M130 M131 M132 X M133 M134 M135 M136 M137 M138 M139 M140 M141 M142 M143 X M144 M145 M146 X M147 M148 M149

181

Appendix 1 (continued) (continued) Polished slab

Thin section

Acetate peel

SEM

XRD

%CaCO3

Lu section L0 L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 L14 L19 L20 L21 L22 L23 L24 L25 L26 L27 L28 L29 L30 L31 L32 L33 L34 L35 L36 L37 L38 L39 L40 L41 L42 L43 L44 L45 L46 L47 L48 L49 L50

PS PS PS

TS SEM

PS

TS

PS PS PS PS

TS TS TS TS

PS PS PS PS PS

TS

SEM

PS

PS PS PS PS PS

TS TS TS

SEM

PS PS PS PS

PS PS

XRD

SEM TS

PS PS PS PS PS PS PS PS

SEM

Rejected

Polished slab

Thin section

Acetate peel

SEM

XRD

%CaCO3

X X PS PS

AP AP

PS PS

AP AP

PS PS PS

AP AP AP

PS

AP

22.4%

PS

AP

46.7%

PS PS

AP

PS PS PS

AP AP AP

PS

AP

PS

AP

PS

AP

PS PS PS PS PS PS PS PS PS

AP AP AP AP AP AP AP

X XRD 57.7%

X SEM

X X X X X X X XRD

79%

X X 67.4%

X X X X X 81.6%

X

96.7% XRD

AP

TS SEM TS TS

SEM

PS PS PS PS

TS

SEM

PS PS PS PS PS PS PS PS PS PS

TS TS

0%

Appendix 2. Range and modal thickness of laminae in polished slabs of Kaili Formation. CC = calcareous claystone lithofacies, CM = carbonate mudstone lithofacies

SEM Sample SEM

TS

SEM

TS TS

SEM

PS PS PS PS PS

Sample #

SEM TS

XRD

XRD

9.3%, 40.9%

0%

Lu section L50 L48 L47 L45 L44 L43 L42 L34 L27 L26 L14 L10 L9 L8 L6

Lamina thickness range (mm)

Modal lamina thickness (mm)

Lithofacies

b0.5–3 0.5–4 0.5–3 b0.5–3 0.5–3 1.0–5.0 1.0–5.0 1.0–4.0 1.0–4.0 1.0–6.0 1.0–4.0 1.0–7.0 0.5–5 b0.5–6 1.0–6.0

0.5 1.0 1.0 1.0 1.0 2.0 1.5 1.0 1.0 1.5 1.5 2.0 1.0 1.0 3.0

CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM (continued on next page)

182

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183

Table 2A (continued) (continued) Sample

Modal lamina thickness (mm)

Lithofacies

1.0–6.0 0.5–6

2.5 1.0

CM CM

Miaobanpo section CKM135 b 0.5–5.5 CKM134 0.5–3.0 CKM133 0.5–5.0 CKM131 0.5–3.0 CKM128 b 0.5–2.0 CKM126 1.0–4.0 CKM118 0.5–3.0 CKM95 1.0–4.0 CKM83 b 0.5–3.0 CKM80 0.75–2.0

1.0 1.5 1.5 1.0 1.0 2.0 1.5 2.0 1.5 1.0

CC CC CC CC CC CC CC CC CC CC

Wuliu-Zenjiayan section CKW62 1.0–3.0 CKW61 0.5–2.0 CKW59 1.0–3.5 CKW57 0.5–3.0 CKW54 0.5–2.0 CKW52 b 0.5–1.5 CKW50 0.5–2.0 CKW47 0.5–2.5 CKW45 1.0–2.0 CKW44 0.5–3.0 CKW41 0.5–1.5 CKW40 b 0.5–1.0 CKW39 1.0–2.0 CKW38 0.5–6.0 CKW34 b 0.5–1.5 CKW33 0.5–1.0 CKW32 1.0–3.0 CKW31 0.5–2.5 CKW30 b 0.5–1.5 CKW29 b 0.5–1.0 CKW28 b 0.5–1.5 CKW27 0.75–2.0 CKW26 1.0–4.0 CKW25 1.0–3.0 CKW24 0.5–2.5 CKW19 0.5–2.0 CKW18 0.5–2.5 CKW16 0.75–3.0 CKW15 1.0–3.0 CKW8 1.0–3.0 CKW7 0.75–2.5 CKW3a 1.0–3.5 CKW3 0.5–2.0

1.5 1.0 1.5 1.5 1.0 1.0 0.75 1.5 1.0 1.0 1.0 1.0 1.5 1.5 0.5 0.75 1.5 1.0 0.5 0.5 0.75 1.0 2.0 1.0 1.0 1.0 1.5 1.0 1.0 2.0 1.5 2.0 1.0

CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC CC

Lu section L5 L2

Lamina thickness range (mm)

References Babcock, L.E., Peng, S., 2007. Cambrian chronostratigraphy: current and future plans. Palaeogeogr. Palaeoclimatol. Palaeoecol. 254, 62–66. Babcock, L.E., Peng, S., Geyer, G., Shergold, J., 2005. Changing perspectives on Cambrian chronostratigraphy and progress toward subdivision of the Cambrian System. Geosci. J. 9, 101–106. Brasier, M.D., Cowie, J., Taylor, M., 1994. Decision on the Precambrian–Cambrian boundary. Episodes 17, 3–8. Cooper, R.A., Nowlan, G., Williams, S.H., 2001. Global stratotype section and point for base of the Ordovician System. Episodes 24, 19–28. Elrick, M., Snider, A.C., 2002. Deep-water stratigraphic cyclicity and carbonate mud mound development in the Middle Cambrian Marjum Formation, House Range, USA. Sedimentology 49, 1021–1047. Fletcher, T.P., 2007. The base of Cambrian Series 3: the global significance of key oryctocephalid trilobite ranges in the Kaili Formation of South China. AAUP Memoirs 33, 29–33. Gaines, R.R., Droser, M.L., 2002. Depositional environments, ichnology, and rare softbodied preservation in the Lower Cambrian Latham Shale, East Mojave. In: Corsetti, F.M. (Ed.), Proterozoic-Cambrian of the Great Basin and beyond: SEPM Pac. Sec. Book, 93, pp. 153–164. Gaines, R.R., Droser, M.L., 2010. The paleoredox setting of Burgess Shale-type deposits. Palaeogeogr. Palaeoclimatol. Palaeoecol. 297, 649–661. Gaines, R.R., Kennedy, M.J., Droser, M.L., 2005. A new hypothesis for organic preservation of Burgess Shale taxa in the Middle Cambrian Wheeler Shale, House Range, Utah. Palaeogeogr. Palaeoclimatol. Palaeoecol. 220, 193–205.

Garson, D.E., Gaines R.R., Droser, M.L., Liddell, W.D., and Sappenfield, A., in press. Dynamic paleoredox and exceptional preservation in the Cambrian Spence Shale of Utah, Lethaia, p. 37. Available online: http://onlinelibrary.wiley.com/doi/10.1111/ j.1502-3931.2011.00266.x. Guo, Q.J., Yang, W.D., Zhao, Y.L., Zhu, L.J., Yang, R.D., 2001. Geochemical characteristics of the stratotype candidate boundary section of the Middle-Lower Cambrian, Guizhou. Geochimica 30 (4), 383–389. Guo, Q., Strauss, H., Liu, C., Zhao, Y., Pi, D., Fu, P., Zhu, L., Yuan, J., 2005. Carbon and oxygen isotopic composition of Lower to Middle Cambrian sediments at Taijiang, Guizhou Province, China. Geol. Mag. 142, 723–733. Guo, Q., Strauss, H., Liu, C., Zhao, Y., Yang, X., Peng, J., Yang, H., 2010. A negative carbon excursion defines the boundary from Cambrian series 2 to Cambrian series 3 on the Yangtze Platform, South China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 285, 143–151. Hallam, A., 1986. Origin of minor limestone-shale cycles: climatically induced or diagenetic? Geology 14, 609–612. Hedges, S., Kumar, S., 2009. The Timetree of Life. Oxford Univ., New York. Landing, E., 1994. Precambrian–Cambrian boundary global stratotype ratified and a new perspective of Cambrian time. Geology 22, 179–182. Liddell, W.D., Wright, S.H., Brett, C.E., 1997. Sequence stratigraphy and paleoecology of the Middle Cambrian Spence Shale in Northern Utah and Southern Idaho. Brigham Young University Geol. Stud. 42, 59–78. Lin, J.P., 2009. Review of the depositional environment of the Kaili Formation (Cambrian series 2–3 boundary interval: China). AAUP Memoirs 37, 131–149. Lin, J.P., Zhao, Y.L., Rahman, I.A., Xiao, S., Wang, Y., 2010. Bioturbation in Burgess Shaletype lagerstätten—a case study of trace fossil-body fossil associations from the Kaili Biota (Cambrian Series 3), Guizhou, China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 292, 245–256. McCollum, L.B., Sundberg, F.A., 2002. Correlation of the Lower-Middle Cambrian boundary interval in the Circum-Pacific region based on oryctocephalid trilobites. First International Palaeontological Congress, Geol. Soc. Aust., 68, pp. 242–243. McCollum, L.B., Sundberg, F.A., 2005. The use of Oryctocephalus indicus as a “LowerMiddle” Cambrian boundary GSSP: a status report. Acta Micropalaeontol. Sin. 22, 113–114. McCollum, L.B., Sundberg, F.A., 2007. Cambrian trilobite biozonation of the Laurentia Delamaran stage in the southern Great Basin, U.S.A.: implications for global correlations and defining a series 3 global boundary stratotype. AAUP Memoirs 34, 147–156. Moore, D.M., Reynolds, R.C., 1997. X-ray Diffraction and the Identification and Analysis of Clay Minerals, 2nd ed. Oxford Univ., New York. O'Brien, N.R., Slatt, R., 1990. Argillaceous Rock Atlas. Springer-Verlag, New York. O'Brien, N.R., Nakazawa, K., Tokuhashi, S., 1980. Use of clay fabric to distinguish turbiditic and hemipelagic siltstones and silts. Sedimentology 27, 47–61. Palmer, A.R., 1960. Some aspects of the early Upper Cambrian stratigraphy of White Pine County, Nevada and vicinity. In: Boettcher, J.W., Sloan, W.W. (Eds.), Guidebook to the Geology of East Central Nevada. IAPG, Salt Lake City, pp. 53–58. Peng, S.C., 2006. A new global framework with four series for the Cambrian System. J. Strat. 30, 147–148. Peng, S.C., Babcock, L.E., Robison, R.A., Lin, H.L., Rees, M.N., Saltzman, M.R., 2004. Global Standard Stratotype-section and Point (GSSP) of the Furongian Series and Paibian Stage (Cambrian). Lethaia 37, 365–379. Peng, J., Zhao, Y.L., Yuan, J.L., Yao, L., Yang, H., 2009. Bathynotus: a key trilobite taxon for global stratigraphic boundary correlation between Cambrian Series 2 and Cambrian Series 3. Prog. Nat. Sci. 19, 99–105. Rees, M.N., 1986. A fault-controlled trough through a carbonate platform: the Middle Cambrian House Range embayment. GSA Bull. 97, 1054–1069. Remane, J., Basset, M.G., Cowie, J.W., Gohrbandt, K.H., Lane, H.R., Michelsen, O., Wang, N.W., 1996. Revised guidelines for the establishment of global chronostratigraphic standards by the International Commission on Stratigraphy (ICS). Episodes 19, 77–81. Robison, R.A., 1960. Lower and Middle Cambrian stratigraphy of the eastern Great Basin. In: Boettcher, J.W., Sloan, W.W. (Eds.), Guidebook to the Geology of East Central Nevada. IAPG, Salt Lake City, pp. 43–52. Schieber, J., Zimmerle, W., 1998. Introduction and overview: the history and promise of shale research. In: Schieber, J., Zimmerle, W., Sethi, P. (Eds.), Shales and Mudstones I. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, pp. 1–10. Wang, Y., Yu, Y.Y., Peng, J., Wang, P.L., 2006. Discussion on the sequence stratigraphy and sea-level changes of the Kaili Formation at Balang, Taijiang, Guizhou. J. Strat. 30, 34–41. Yang, R.D., Yin, L., 2001. Acritarch assemblages from the Early-Middle Cambrian Kaili Formation of East Guizhou Province and biostratigraphic implication. Acta Micropalaeontol. Sin. 18, 55–69. Yang, R.D., Zhu, L.-J., Wang, S.J., 2003. Negative carbon isotopic excursion on the Lower/ Middle Cambrian boundary of Kaili Formation, Taijiang County, Guizhou Province, China: implications for mass extinction and stratigraphic division and correlation. Sci. China 46, 872–881. Yuan, J.L., Zhao, Y.L., Wang, Z.Z., Zhou, Z., Chen, X.Y., 1997. A preliminary study on the Lower-Middle Cambrian Boundary and trilobite fauna at Balang, Taijiang, Guizhou, South China. Acta Palaeontol. Sin. 36, 494–524. Yuan, J.L., Zhao, Y.L., Li, Y., Huang, Y.Z., 2002. Trilobite Fauna of the Kaili Formation (Uppermost Lower Cambrian-Lower Middle Cambrian) from Southeastern Guizhou, South China. Shanghai Sci. Tech., Shanghai. Yuan, J.L., Lin, J.P., Zhao, Y.L., Peng, J., 2011. Zonation of the Balang and Wuxun (“Tsinghsutung”) formations and the lowermost part of the Kaili Formation

R.R. Gaines et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 311 (2011) 171–183 (Cambrian stage 4) based on oryctocephalid trilobites in Guizhou, South China. In: Hollingsworth, J.S., Sundberg, F.A., Foster, J.R. (Eds.), Cambrian Stratigraphy and Paleontology of Northern Arizona and Southern Nevada: Museum of Northern Arizona Bulletin, 67, pp. 314–315. Flagstaff, Arizona. Zhang, Z.H., Shen, J.W., Gong, X.Y., Zhao, Y.L., Mao, J.R., Yanan, C.-H., 1996. A preliminary discussion on preservation condition of Kaili Fauna, Middle Cambrian, Taijiang, Guizhou. Acta Palaeontol. Sin. 35, 607–622. Zhao, Y.L., Yang, R.D., Yuan, J.L., Zhu, M.Y., Guo, Q.J., Yang, X.L., 2001. Cambrian stratigraphy at Balang, Guizhou Province, China: candidate section for a global unnamed series and Stratotype section for the Taijiangian stage. Palaeoworld 13, 189–208. Zhao, Y.L., Yuan, J.L., Peng, S.C., Babcock, L.E., Peng, J., Lin, J.P., Guo, Q.J., Wang, Y.X., 2007. New data on the Wuliu-Zengjiayan section (Balang, South China), GSSP candidate for the base of Cambrian Series 3. AAUP Memoirs 33, 57–65. Zhao, Y.L., Yuan, J.L., Peng, S.C., Babcock, L.E., Peng, J., Guo, Q.J., Lin, J.P., Tai, T.S., Yang, R.D., Wang, Y.X., 2008. A new section of Kaili Formation (Cambrian) and a biostratigraphic study of the boundary interval across the undefined Cambrian Series 2 and Series 3 at

183

Jianshan, Chuandong village, Jianhe County, China with a discussion of global correlation based on the first appearance datum of Oryctocephalus indicus (Reed, 1910). Prog. Nat. Sci. 18, 1549–1556. Zhao, Y.L., Yuan, J.L., Peng, J., Yang, X.L., Yang, Y.N., 2011. The FADs of trilobite species for the international Cambrian stage 3, stage 4 and stage 5 in Guizhou Province China. In: Hollingsworth, J.S., Sundberg, F.A., Foster, J.R. (Eds.), Cambrian Stratigraphy and Paleontology of Northern Arizona and Southern Nevada: Museum of Northern Arizona Bulletin, 67, p. 316. Flagstaff, Arizona. Zhu, L.J., Zhao, Y.L., 1996. Studies of trace element and REE Geochemistry of rocks in Middle-Lower Cambrian boundary section of Taijiang, Guizhou. Acta Palaeontol. Sin. 35, 623–629. Zhu, M.Y., Erdtmannannann, B.D., Zhao, Y.L., 1999. Taphonomy and paleoecology of the early Middle Cambrian Kaili Lagerstätte in Guizhou, China. Acta Palaeontol. Sin. 38 (supplement), 28–57.