PROPOSED GLOBAL STANDARD
STRATOTYPE-SECTION AND POINT
FOR THE PAIBIAN STAGE AND FURONGIAN
SERIES (UPPER CAMBRIAN)
Prepared on behalf of the International Subcommission on Cambrian
Stratigraphy for the International Commission on Stratigraphy by:
Peng Shanchi, Nanjing
Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East
Beijing Road, Nanjing 210008, China, speng@pub.jlonline.com
Loren E. Babcock, Department of Geological Sciences, The Ohio State University, 125 South
Oval Mall, Columbus, OH 43210, USA, babcock.5@osu.edu
Richard A. Robison, Department of Geology, University of Kansas, Lawrence, KS 66045, USA,
rrobison@ku.edu
Lin Huanling, Nanjing
Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East
Beijing Road, Nanjing 210008, China
Margaret N. Rees, Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV
89145, USA, rees@unlv.nevada.edu
Matthew R. Saltzman, Department of Geological Sciences, The Ohio State University, 125 South
Oval Mall, Columbus, OH 43210, USA, saltzman.11@osu.edu
CONTENTS
Introduction
Proposal: Paibi
(Hunan Province, China) as the GSSP for the base of the Paibian Stage (base of
the Furongian Series)
1. Stratigraphic rank of the boundary
1.A.
Paibian Stage and Furongian Series: new names and historical review
2. Proposed GSSP – geography and physical
geology
2.A.
Geographic location
2.B.
Geological location
2.C.
Location of level and specific point
2.D.
Stratigraphic completeness
2.E.
Thickness and stratigraphic extent
2.F.
Provisions for conservation, protection, and accessibility
3. Motivation for
selection of the boundary level and of the stratotype section
3.A.
Principal correlation event (marker) at GSSP level
3.B.
Stratotype section
3.C.
Demonstration of regional and global correlation
3.C.i.
Agnostoid trilobite biostratigraphy
3.C.iii.
Conodont biostratigraphy
3.C.iv.
Chemostratigraphy
3.C.v.
Sequence stratigraphy
4. Selection process
4.A. Relation of the GSSP to historical
usage
4.B. Other candidates and reasons for
rejection
4.B.i.
Other regional candidates in South China
4.B.ii.
Other extra-regional candidate
5. References
Figures 1-10
Introduction
Regional stage- and
series-level schemes are available and in widespread use for all major Cambrian
continents, but the Cambrian System currently has no formally accepted
subdivisions applicable on a global scale. The base of the Cambrian System, and
the Palaeozoic Eonothem, at the base of the Trichophycus
(or Treptichnus, Phycodes) pedum Zone in Newfoundland has been ratified
(Brasier et al., 1994; Landing, 1994; Gehling et al., 2001), and the base of
the overlying Ordovician System at the base of the Iapetognathus fluctivagus Zone (Cooper & Nowlan, 1999) has been
chosen, but internal divisions within the Cambrian System have not been
determined (Geyer & Shergold, 2000). For much of the Nineteenth and
Twentieth Centuries, the Cambrian was subdivided into three parts, but
recognition of a thick pre-trilobitic lower Cambrian (Landing, 1994, 1998;
Landing et al., 1998), equivalent to roughly half of Cambrian time (Landing et
al., 1998) provides a strong incentive to adopt a four-fold division of the
Cambrian (Landing, 1998; Palmer, 1998; Geyer & Shergold, 2000), with two
series in the lower half, and two series in the upper half, of the system.
Geyer & Shergold (2000), in reviewing the current state of knowledge about
internal Cambrian divisions, emphasized the need to subdivide the system
according to practical, intercontinentally recognizable horizons instead of according
to a tripartition carried over from traditional usage. With the addition of a
large pre-trilobitic interval to the lower part of the Cambrian, subequal
division of the system into three parts could lead to considerable confusion
over newer and older Lower, Middle, and Upper Cambrian series, and would likely
lack adequate horizons for recognition of stage and series boundaries globally.
At
least 11 candidate horizons for global chronostratigraphic correlation are
present in the upper half of the Cambrian System (Geyer & Shergold, 2000),
although not all are equally useful for stage and series boundaries. If a
four-fold division of the Cambrian (Palmer, 1998) is ultimately adopted,
internal boundaries can be expected to roughly correspond to the first appearance
datum (FAD) of trilobites, the FAD of an intercontinentally distributed fossil
(perhaps the trilobite Oryctocephalus
indicus), and the FAD of the agnostoid trilobite Glyptagnostus reticulatus (e.g., Palmer, 1998; Geyer &
Shergold, 2000; Peng & Babcock, 2001; Peng et al., 2001c). Division of the
Cambrian into a greater number of series, however, is possible (Geyer &
Shergold, 2000). Of the suggested and recommended positions of series
boundaries (Shergold & Geyer, 2001), the FAD of G. reticulatus corresponds to palaeo-oceanographic and biotic
events of considerable global importance (e.g., Geyer & Shergold, 2000;
Saltzman et. al., 2000; Peng et al., 2001c). This position is one of the most
clearly recognizable datum points in the Cambrian; a position closely
corresponding to the first appearance of G.
reticulatus is recognizable in strata of eastern and western Gondwana,
Baltica, Kazakhstan, Siberia, Laurentia, and Avalonia (e.g., Palmer, 1962;
Geyer & Shergold, 2000; Peng & Robison, 2000; Peng et al., 2001c; Fig.
1), and can be identified with precision using multiple lines of evidence.
Despite ongoing discussions concerning the total number and placement of series
boundaries in the Cambrian System, a decision on the interval comprising the
uppermost series is unaffected, and it is appropriate to move forward with
ratification of a boundary position for the base of the uppermost Cambrian
series and its corresponding lower stage.
The purpose of this proposal is to seek
formal recognition for the base of a global stage and series boundary that
represents the base of the uppermost Cambrian series. The proposed global
standard stratotype-section and point (GSSP) for the base of the Paibian Stage
(new name) and the Furongian Series (new name) is at the FAD of G. reticulatus in the section near
Paibi, Hunan Province, China (Peng & Robison, 2000; Peng et al., 2001c,
2001e; Figs. 2-7). The proposed GSSP position in the Paibi section fulfills all
of the geological and biostratigraphic requirements for a GSSP (Remane et al.,
1996). Among other methods that should be given due weight in the selection of
a GSSP (Remane et al., 1996), chemostratigraphic, palaeogeographic,
facies-relationship, and sequence-stratigraphic information is available. The
section is accessible, and of unrestricted access for research. Upon
ratification of this proposal, it is expected that a permanent monument marking
the GSSP position will be erected, and that the site will be permanently
protected in a national geological reserve. This will ensure continued free
access to the site for research purposes.
The
Furongian Series differs in content from the upper Cambrian of the most recent
versions of the preliminary Cambrian time scale (Geyer et al., 2000) and the
global standard scale (Cowie and Bassett, 1989; Remane et al., 2000). Following
internationally accepted practice for defining global chronostratigraphic units
(Hedberg, 1976; Salvador, 1994), the choice of a boundary-stratotype at the
base of the Glyptagnostus reticulatus Zone
is the best available for defining the lower boundary of an upper Cambrian
series (see review in Geyer & Shergold, 2000). Although this position
differs from most traditional, and regionally applicable, positions of the base
of the upper Cambrian (Fig. 1), this position has the important advantage of
being identifiable on a global scale, thus overcoming the problem of
conflicting regional stratigraphic standards.
The
relative suitability of available horizons for the base of the uppermost
Cambrian series has been discussed by members of the Cambrian Stage Subdivision
Working Group of the International Subcommission on Cambrian Stratigraphy
(ISCS), and the base of the G.
reticulatus Zone has been selected as the base of the upper Cambrian series
by an 85% majority of the Voting Members of the ISCS (Shergold & Geyer,
2001). Choices for sections containing the base of the G. reticulatus Zone are available in South China, Kazakhstan,
Siberia, Australia, and Laurentia (Shergold & Geyer, 2001). In 2001, the
Cambrian Stage Subdivision Working Group visited sites in South China
containing the proposed boundary position. After examining the Paibi section,
the Working Group met in Zhijin, Guizhou Province, China, and discussed the
merits of the available candidate sections. The consensus of opinion was that
only two viable candidates exist for the GSSP for the base of the upper
Cambrian series: the Paibi section, China (Dong, 1990; Peng & Robison,
2000; Peng et al., 2001c, 2001e), and the Kyrshabakty River section, Malyi
Karatau, Kazakhstan (Ergaliev, 1980, 1990).
In
early 2002, Voting Members of the ISCS were asked to vote on whether to forward
to the International Commission on Stratigraphy (ICS) the proposal to establish
the FAD of Glyptagnostus reticulatus
in the Paibi section, Hunan, China, as the base of the Paibian Stage and
Furongian Series. By 1 April 2002, 17 votes had been received; 14 votes (82%)
were in favour of the proposal. Two Voting members voted against the proposal,
one Voting Member abstained, and two Voting Members did not reply. In light of
this favourable vote on the proposal, we now respectfully request the ICS for a
vote on ratification of this proposal.
PROPOSAL: PAIBI (HUNAN PROVINCE, CHINA) AS THE GSSP
FOR THE BASE OF THE PAIBIAN STAGE (BASE OF THE
FURONGIAN SERIES)
1.
Stratigraphic rank of boundary.
Base of the Paibian
Stage, and base of the Furongian Stage (Fig. 1). The Paibian Stage is the
lowermost stage of the Furongian Series. The Furongian Series is the uppermost
series of the Cambrian System. The boundary is a standard stage/age, and
series/epoch GSSP.
1.A. Paibian Stage and Furongian Series: new names and historical review.—The Paibian Stage (and Age) is a new name for the lower stage (and
age) of the Furongian Series (and Epoch; also a new name). The name Furongian
replaces in concept and content the traditional upper Cambrian (e.g., Cowie
& Bassett, 1989; Geyer & Shergold, 2000; Remane et al., 2000), and the
various concepts of the upper Cambrian used regionally around the world (see
Geyer & Shergold, 2000; Fig. 1).
The names Paibian and Furongian are
derived from geographic localities in South China, where the base of the G. reticulatus Zone is well exposed, and
well constrained in stratigraphic position. The name Paibian Stage (and Age) is
derived from Paibi, a village near the proposed GSSP site, in Hunan Province,
China. The Paibian Stage, proposed for global use, has the same lower boundary
as the Waergangian Stage as used in South China (Peng et al., 1999, 2000,
2001c; Geyer et al., 2000; Peng & Babcock, 2001; Fig. 1). The upper
boundary of the stage is currently undefined (Fig. 1), and will be defined by
the base of the succeeding stage, which has yet to be determined. The name
Furongian is derived from Furong, which means lotus, referring to Hunan, the Lotus State. Furong has been used as
a nickname for Hunan since about A.D. 800, during the late part of the Tang
Dynasty (A.D. 618-907). The Furongian Series is the global equivalent of the
Hunanian Series as used in South China (Peng et al., 1999, 2000, 2001c; Geyer
et al., 2000; Peng & Babcock, 2001; Fig. 1). The upper boundary of the
Furongian Series is the base of the Tremadocian Series (and the Ordovician System).
Numerous
stadial and series schemes for the upper part of the Cambrian have been used
regionally (e.g., Westergård, 1946; Henningsmoen, 1957; Öpik, 1966, 1967;
Rosova, 1968; Robison, 1976; Rushton, 1978; Ergaliev, 1980; Shergold, 1982;
Ludvigsen & Westrop, 1985; Zhang & Jell, 1987; Chang, 1988; Ahlberg
& Ahlgren, 1996; Palmer, 1998; Peng et al., 1999, 2001c; Geyer &
Shergold, 2000; Peng & Robison, 2000; Peng & Babcock, 2001; Fig. 1),
but the decision to place the base of a new globally applicable upper Cambrian
stage and series boundary at the base of the G. reticulatus Zone (Shergold & Geyer, 2001) follows the
determination that this is one of the most widely recognizable and distinct
horizons in the Cambrian System (e.g., Shergold, 1982; Geyer & Shergold,
2000; Peng et al., 2001c). Correlation of this position is discussed in section
3.C.
2. Proposed GSSP – geography and physical geology
2.A. Geographic location.—The Paibi section
(Figs. 2-7), situated in the Wuling Mountains (Wulingshan), Huayuan County,
northwestern Hunan Province, China. Its geographic coordinates are latitude
28°23.37´ N, longitude 109º31.54´ E of Greenwich, England. The Paibi section
consists of a nearly continuous series of roadcuts, small quarries, and
hillside outcrops (Fig. 6A, B) located approximately 35 km west of the city of
Jishou along the north side of the Jishou-Huayuan highway (Chinese National
Highway 319), and approximately 28 km south of Huayuan. Beginning just west of
the village of Sixin (Sixicun), the section extends approximately 1.7 km to
just west of the village of Paibi (Figs. 2-5). The Paibi section extends from
the middle Cambrian through the Lower Ordovician. The stratotype section is
represented on the Paibi topographic map (Hunan Branch of State Topographical
Surveying Bureau, Map number H49 G 087025, 1:10,000 scale; Fig. 3). Strata of
the Huaqiao Formation extend from road level through the top of a hill that
rises approximately 100 m above road level (Fig. 6A). The proposed GSSP is at
an elevation of approximately 774 metres (Fig. 6B).
2.B. Geological location.—The Wuling
Mountains consist of an extensive series of folded and thrusted slices
resulting from post-Devonian compressional tectonics that extend through parts
of northwestern Hunan, eastern Guizhou, and southeastern Sichuan provinces,
China. The Paibi section is located along the northwest limb of the
Liexi-Zhuitun Syncline.
Cambrian
strata of South China are assigned to three major depositional environments
along a platform-to-basin transition (e.g., Pu & Ye, 1991; Peng &
Robison, 2000; Peng & Babcock, 2001). Relatively shallow environments of
the Yangtze Platform were flanked by deeper environments of the Jiangnan Slope
Belt, and still deeper environments of the Jiangnan Basin. The proposed GSSP
occurs within the Huaqiao Formation, which consists of a thick succession of
carbonate beds deposited in the outer part of the Jiangnan Slope Belt.
2.C. Location of level and specific point.—The
base of the first calcilutite layer containing the cosmopolitan agnostoid
trilobite Glyptagnostus reticulatus
in the Huaqiao Formation in the Paibi section (Peng et al., 2001e) is proposed
as the GSSP of the Paibian Stage, and of the Furongian Series (Figs. 6-9).
Except for containing the lowest occurrence of G. reticulatus in the
Paibi section, this calcilutite bed is essentially indistinguishable from other
beds of similar lithology in this largely monofacial succession. The FAD of G.
reticulatus in the Paibi section corresponds to a position 369.06 m above the
base of the Huaqiao Formation according to the measured section of Peng et al.
(2001c). The base of the Paibian Stage and Furongian Series, as proposed for
global chronostratigraphic purposes, corresponds to the base of the Waergangian
Stage and Hunanian Series as used in South China (Peng, 1999; Peng et al.,
1999a, 2000; Peng & Babcock, 2001).
2.D. Stratigraphic completeness.—Detailed
bed-by-bed correlation of the middle-upper Cambrian through northwestern Hunan,
coupled with detailed biostratigraphy, sedimentology and carbon-isotope
chemostratigraphy (e.g., Dong, 1990; Rees et al., 1992; Fu et al., 1999; Peng
et al., 2000, 2001a, 2001b, 2001c, 2001d, 2001e; Peng & Robison, 2000;
Saltzman et al., 2000; Dong & Bergström, 2001a, 2001b; Peng & Babcock,
2001), clearly demonstrate the stratigraphic continuity of the basal Paibian in
the Paibi section. Biostratigraphic studies within Hunan and globally
demonstrate that the succession of Glyptagnostus
species (e.g., Geyer & Shergold, 2000; Peng & Robison, 2000; Peng &
Babcock, 2001; Peng et al., 2001b, 2001c, 2001d, 2001e), other trilobite
species (Peng & Babcock, 2001; Peng et al., 2001a, 2001b, 2001c, 2001d,
2001e), and conodont species (Dong, 1990; Dong & Bergström, 2001a) in the
Paibi section is undisturbed. The Paibi section lacks evidence of
synsedimentary and tectonic disturbances near the proposed GSSP. A few thin and
laterally discontinuous matrix-supported calcirudite beds (representing debris
flows of shelf-derived intraclasts) are intercalated between calcilutite beds
below the boundary, but their bases appear to be non-erosional (compare with
the experimental work of Marr et at., 2001), and none is present at the
boundary point. The biostratigraphic succession in the section is unaffected by
the interbeds. Likewise, evidence of metamorphism and strong diagenetic
alteration are absent.
2.E. Thickness and stratigraphic extent.— The
basal Paibian contact, proposed as the GSSP, occurs in a mostly monofacial
succession of dark grey to black, thin-bedded, calcilutite beds. The contact
where Glyptagnostus reticulatus first
appears is subtle, occurring at the base of a layer of dark grey calcilutite
overlying a layer of black calcilutite (Fig. 6C, D). The basal Paibian in the
Paibi section is observable in a rather prominent cliff face in a hillside
outcrop along a bedding plane length of more than 200 metres.
2.F. Provisions for conservation, protection, and accessibility.—The exposure containing the proposed GSSP is not subject to building,
landscaping, or other destruction. Rapid vegetative growth is typical in South
China, and minor effort is needed to keep vegetative cover off some areas of
the Paibi section. However, the near-vertical cliff face containing the
proposed GSSP shows significantly less vegetation than most areas of the
hillside. In keeping with the usual practice in China, it is expected that the
Paibi section will be designated as a national geological reserve, and receive
appropriate conservation, if the proposed GSSP at Paibi is ratified. Local
authorities have already expressed their willingness to proceed with protection
of the section.
Access
to the hillside outcrop at the Paibi section is unrestricted in all seasons.
Travel to Huayuan County, Hunan Province, China, is open to persons of all
nationalities, and travel for scientific purposes is welcomed and encouraged by
local authorities. Major highways in Hunan Province are currently under
construction, and their completion will significantly enhance access to the
Paibi locality in the near future.
3. Motivation for selection of the boundary level and of the stratotype
section
3.A. Principal correlation event (marker) at GSSP level.—The agnostoid trilobite Glyptagnostus
reticulatus has one of the broadest distributions of any Cambrian
trilobite, and its first appearance has been acknowledged as the most
favourable level for a GSSP defining the base of a global Cambrian series
(Robison et al., 1977; Peng & Robison, 2000; Geyer & Shergold, 2000;
Peng et al., 2001c; Shergold & Geyer, 2001). Agnostoid trilobites provide
the best and most precise tools for intercontinental correlation in the upper
half of the Cambrian System (e.g., Robison, 1984; Peng & Robison, 2000).
Recent recalibration of radiometric ages for the Cambrian (Grotzinger et al.,
1995; Davidek et al., 1998; Landing et al., 1998, 2000), scaled against the
number of agnostoid zones now recognized in the upper half of the Cambrian
indicates that the average duration of an agnostoid-defined biochron is about
one million years (Peng & Robison, 2000). Glyptagnostus reticulatus has been identified (Geyer &
Shergold, 2000; Peng et al., 2001c) from China, Australia, Antarctica,
Kazakhstan, Russia, South Korea, Sweden, Denmark, Norway, the United Kingdom,
the United States, Canada, and Argentina, and has been used as a zonal guide
fossil in South China, Australia, Kazakhstan, Siberia, and Laurentia (Geyer
& Shergold, 2000; Peng & Robison, 2000). Co-occurrences with other
trilobites allow correlation into such regions as Baltica (Homagnostus obesus Zone; Ahlberg and Ahlgren, 1996; Ahlberg, 1998)
and Argentina (lower Aphelaspis
Zone/lower G. reticulatus
Zone-equivalent; Shergold et al., 1995).
Stratigraphically,
G. reticulatus always succeeds G. stolidotus (Peng et al., 2001c), and
it is desirable to select the position of a GSSP in a section showing a
complete succession from the G.
stolidotus Zone through the G.
reticulatus Zone. Consistent upsection changes in morphology, notably
increased reticulation (Fig. 9), and consistent stratigraphic occurrence below G. reticulatus (except where inferred
hiatuses exist in Baltica and Avalonia), together strongly suggest that G. stolidotus was ancestral to G. reticulatus. Selection of the FAD of G. reticulatus as the base of the
uppermost Cambrian series ensures that the boundary will fall within the
stratigraphic interval bearing Glyptagnostus,
and at an arbitrary, but readily identifiable, point in an evolutionary series.
Globally, the stratigraphic interval bearing Glyptagnostus species is relatively narrow but widely exposed. This
allows the boundary to be tightly constrained as long as Glyptagnostus-bearing strata are present in a region.
Selection
of a GSSP in a slope environment, and particularly a low-latitude Gondwanan
slope environment such as the Jiangnan Slope Belt, is desirable because it
provides faunal ties (and correlation tools) with low-latitude shelf areas,
high-latitude shelf areas, and low- or high-latitude, slope-to-basinal areas.
In the latter half of the Cambrian, stratification of the world ocean according
to temperature or other factors that covary with depth (e.g., Cook &
Taylor, 1975, 1976; Babcock, 1994) led to the development of rather distinct
trilobite biofacies in shelf and basinal areas. Low-latitude shelf areas were
inhabited mostly by endemic polymeroid trilobites and some pan-tropical taxa.
High-latitude shelf areas, and basinal areas of low and high latitudes, were
inhabited mostly by widespread polymeroid trilobites and cosmopolitan agnostoid
trilobites. Slope areas are characterized by a combination of some
shelf-dwelling taxa and basin-dwelling taxa. The combination of cosmopolitan
agnostoids, which have intercontinental correlation utility, Gondwanan
shelf-dwelling polymeroids, pan-tropical polymeroids, and widespread
polymeroids in the Jiangnan Slope Belt (Egorova et al., 1963; Peng &
Robison, 2000; Peng et al., 2001a) allows for the precise correlation of the
base of the G. reticulatus Zone (and
other marker horizons) into such Gondwanan shelf areas as the North China
Platform (Zhang & Jell, 1987), and Australia (Öpik, 1963, 1966, 1967;
Shergold, 1982; Jago & Brown, 1992); and into such slope areas as
Kazakhstan (Ergaliev, 1980, 1990), France (Shergold et al., 2000), Iran (Peng
et al., 1999), Oman (Fortey, 1994), and Victoria Land, Antarctica (Cooper et
al., 1996). Correlation into high-latitude shelf areas of Baltica (Westergård,
1946; Ahlberg & Ahlgren, 1996), and shelf-edge regions of Laurentia
(Palmer, 1999) and Siberia (Ivshin & Pokrovskaya, 1968; Rosova, 1968, 1984)
is also precise.
3.B. Stratotype section.—The FAD of G. reticulatus in the Paibi section,
Hunan Province, China, occurs in the Huaqiao Formation (at a level 369.06 m
above the base of the formation according to the measured section of Peng et
al., 2001c; Figs. 7, 8). At this section, the Huaqiao Formation rests in
conformable succession above the Aoxi Formation. Agnostoid trilobite zonation
of the Huaqiao Formation in the Paibi section reveals a complete, tectonically
undisturbed, marine succession from the Ptychagnostus
(or Acidusus) atavus Zone through the upper part of the G. reticulatus Zone (Peng & Robison, 2001). Formerly, the FAD
of G. reticulatus was included in the
lower part of the Bitiao Formation in northwestern Hunan, but following
revision of stratigraphic nomenclature (Peng & Robison, 2000), the Huaqiao,
Chefu, and Bitiao formations of Hunan Province and adjacent areas of Guizhou
Province (Peng & Babcock, 2001) have been grouped into a single unit, the
Huaqiao Formation. The Huaqiao Formation at the Paibi section (Rees et al.,
1992; Peng & Robison, 2000; Peng et al., 2001c, 2001e) is a mostly
monofacial succession of alternating thin-bedded, dark grey to black argillaceous-
and lime-rich calcisilitites and calcilutites. Thin- to medium-bedded
calcarentites containing Bouma divisions, and matrix-rich, clast-supported
boulder- to pebble-calcirudites, are sporadically present below the FAD of G. reticulatus and are more common above
that position. The calcirudites commonly have non-erosive flat bases and
uniform thicknesses over distances of tens of metres. Lenticular and
channelized calcirudite beds, some of which display downslope textural
transformations, are less common. Many calcarenites and calcirudites contain
identifiable shelf-derived allochems, as well as resedimented slope deposits.
Soft-sediment deformation is extremely rare in the succession, and truncation
or slide surfaces are absent, suggesting distal deposition on relatively gentle
slopes. Strata enclosing the proposed boundary position, between 361.5 and
376.5 m above the base of the Huaqiao Formation, include five laterally
discontinuous calcirudite interbeds ranging from 8 to 66 cm in thickness. None
of the calcirudite interbeds occurs at the proposed boundary or disrupts the
stratigraphic appearance of taxa in any way. Trilobite sclerites are common in
the fine-grained limestones but are absent from the calcirudites. The Huaqiao
Formation represents outer-slope deposition in a marine setting, the Jiangnan
Slope Belt, which was adjacent to the Yangtze (South China or Southwest China)
Platform (e.g., Pu & Yi, 1991; Rees et al., 1992; Peng & Robison, 2000;
Peng & Babcock, 2001).
The
proposed GSSP in the Paibi section is placed within a continuous evolutionary
sequence of Glyptagnostus species
(Figs. 8, 10). Successive stratigraphic levels show an evolutionary succession
beginning with G. stolidotus (Fig.
9A, B), and continuing through weakly reticulated (primitive) G. reticulatus (commonly formalized as G. reticulatus angelini; e.g., Palmer,
1962; Ergaliev, 1980; Dong, 1990; Fig. 9C-E), to strongly reticulated (derived)
G. reticulatus (commonly formalized
as G. reticulatus reticulatus; e.g.,
Henningsmoen, 1958; Palmer, 1962; Shergold, 1982; Rushton, 1983; Jago &
Brown, 1992; Peng, 1992; Ahlberg & Ahlgren, 1996; Clarkson et al., 1998;
Fig. 10F-G). Peng & Robison (2000) synonymized the two morphotypes, along
with other named morphological variants of the species. Globally, the weakly
reticulated morphotype of G. reticulatus
always precedes the strongly reticulated morphotype of the species in ascending
stratigraphic order. The FAD of G.
reticulatus in the Paibi section, as well as the base of the G. reticulatus Zone globally, is taken
to be the first appearance of the weakly reticulated morphotype of G. reticulatus (Fig. 10C-E). The base of
the bed containing the FAD of G.
reticulatus at the Paibi section is isochronous along its exposed length,
although lithologically it is essentially indistinguishable from other layers
in a succession of thinly bedded, dark grey to black lime-rich and argillaceous
calcilutites (Fig. 6).
Ranges
of trilobites across the stratigraphic interval containing the proposed GSSP
are summarized in Fig. 8. Besides species of Glyptagnostus, a number of other guide fossils, which have utility
for correlation on a regional to intercontinental scale, help to constrain the
boundary position. They include the agnostoid trilobites Acmarhachis typicalis, Peratagnostus
obsoletus, Pseudoagnostus josepha,
all of which first appear in the G.
stolidotus Zone and range up into (or, in the case of P. obsoletus, through) the G.
reticulatus Zone. Likewise, a polymeroid trilobite, Proceratopyge fengwangensis, first appears near the top of the G. stolidotus Zone, and ranges through
the G. reticulatus Zone. The
widespread agnostoid trilobites G.
stolidotus, Ammagnostus histus, Agnostardis amplinatus, and Agnostus inexpectans occur in the G. stolidotus Zone but none ranges
higher than that zone. Polymeroid trilobites that occur within the
stratigraphic interval containing Glyptagnostus
in northwestern Hunan, China, but that range no higher than the FAD of G. reticulatus, include Chatiania chatianensis, Fenghuangella liostracinala, Paradamesella typica, Protaizehoia yuepingensis, Pseudoyuepingia laochatiensis, and Teinistion posterocosta. Zonation of
conodonts from the Paibi section (Dong & Bergström, 2001a) shows that the
base of the Westergaardodina proligula
Zone occurs just slightly below the base of the G. reticulatus Zone.
3.C. Demonstration of regional and global correlation.—A position at or closely corresponding to the FAD of Glyptagnostus reticulatus in the Paibi
section is one of the most easily recognizable horizons on a global scale in
the Cambrian (e.g., Palmer, 1962; Geyer and Shergold, 2000; Peng and Robison,
2000; Saltzman et al., 2000; Peng et al., 2001c; Figs. 1, 9). Papers discussing
the suitability of the FAD of this species for marking a global stage and
series boundary have been summarized by Geyer and Shergold (2000). Key
correlation tools (Fig. 9) are as follows:
3.C.i. Agnostoid trilobite
biostratigraphy.—Glyptagnostus reticulatus is recognized worldwide (e.g., Kobayashi,
1949; Öpik, 1966; Rosova, 1968; Jago, 1974; Robison et al., 1977; Shergold,
1982 and references therein; Shergold et al., 1995; Geyer & Shergold, 2000;
Figs. 1, 9). It has been identified (Geyer & Shergold, 2000; Peng et al.,
2001c) from China (northwestern Hunan, eastern Guizhou, southern Anhui,
northwestern Gansu, Xinjiang, western Zhejiang), Australia (western Queensland,
Tasmania), Antarctica (Ellsworth Mountains), Kazakhstan (Lesser Karatau),
Russia (northwestern Siberian Platform, northeastern Siberian Platform), South
Korea, Sweden, Denmark, Norway, the United Kingdom, the United States (Alabama,
Alaska, Nevada, Tennessee, Texas), Canada (British Columbia, Northwest
Territories), and Argentina. The species is used as a zonal guide fossil in
South China (Jiangnan Slope area), Australia, Kazakhstan, Siberia, and
Laurentia (Geyer & Shergold, 2000; Peng & Robison, 2000).
Co-occurrences with other trilobites allow correlation into Avalonia (Homagnostus obesus Zone; Rushton, 1983),
and Argentina (lower Aphelaspis
Zone/lower G. reticulatus
Zone-equivalent; Shergold et al., 1995).
3.C.ii. Polymeroid trilobite
biostratigraphy.—The base of the G. reticulatus Zone coincides with
turnovers in polymeroid trilobite faunas recognized at the base of the
Waergangian Stage and the Hunanian Series in South China (Peng et al., 1999,
2001c; Peng & Babcock, 2001; Fig. 8), the base of the Changshanian
(Paishanian) in North China (Walcott, 1913; Öpik, 1967; Qian, 1994), the base
of the Idamean Stage in Australia and Tasmania (Öpik, 1960, 1963, 1967; Jago,
1974; Shergold, 1982; Jago & Brown, 1992), the base of the Sackian Stage
and base of the Upper Cambrian Series in Kazakhstan (Ergaliev, 1990), and the
base of the Kugorian (Kutugunian) Stage in Siberia (Rosova, 1984). The base of
the G. reticulatus Zone corresponds
to the base of the Steptoean Stage and Millardan Series (Palmer, 1965; 1998,
1999; Ludvigsen & Westrop, 1985) in Laurentia. However, shelf successions
lack the appropriate lithofacies for G.
reticulatus. On the Laurentian shelf, the FAD of the trilobite Coosella perplexa, at the base of the Aphelasis Zone, corresponds closely to
the base of the G. reticulatus Zone.
The Aphelaspis Zone can be recognized
across much of the Laurentian shelf (see Palmer, 1999) and in Argentina
(Shergold et al., 1985). The G.
reticulatus Zone corresponds to the lower part of the Homagnostus obesus Zone (Olenus
gibbosus Zone) in Scandinavia (Westergård, 1946, 1947; Henningsmoen, 1957;
Ahlberg & Ahlgren, 1996; Ahlberg, 1998), eastern Avalonia (central England;
Rushton, 1983), and western Avalonia (southeastern Newfoundland; Hutchinson,
1962).
3.C.iii. Conodont biostratigraphy.—The base of a conodont biozone, the Westergaardodina proligula Zone (Dong & Bergström, 2001a),
occurs just slightly below the base of the G.
reticulatus Zone. The intercontinental correlation potential of other
biostratigraphic tools, such as brachiopods, near the base of the G. reticulatus Zone has not been
extensively tested.
3.C.iv. Chemostratigraphy..—The base of the G. reticulatus
Zone closely corresponds with the onset of a large positive shift in δ13C values referred to as the Steptoean positive
carbon isotope excursion (SPICE excursion; Brasier, 1993; Runnegar &
Saltzman, 1998; Saltzman et al., 1998, 2000, 2001; Perfetta et al., 1999). The
precise base of the SPICE excursion is subjective, as the excursion follows a
monotonic positive shift in δ13C
values from values that are indistinguishable from background values. The SPICE
excursion reaches peak values of about +4 ‰ δ13C between the FAD of G.
reticulatus and the FAD of Irvingella
(Saltzman et al., 2000), at a position roughly corresponding to the interval of
peak biotic diversity in the Pterocephaliid Biomere of Laurentia (Rowell &
Brady, 1976), and to an important sea level fall represented in Laurentia by
the Sauk II-Sauk III hiatus (see Palmer, 1981; Saltzman et al., 2000). The
excursion has been documented from sections in South China (Paibi and
Wa’ergang), Kazakhstan (Kyrshabakty River section, Malayi Karatau), Australia
(Queensland), and the United States (Great Basin). Carbonate environments
yielding the SPICE excursion range from slopes where dark, thin-bedded
limestones predominate, through shallow platforms where a variety of carbonate
lithofacies (boundstones, oolitic grainstones, and fenestral limestones) are
present (Saltzman et al., 2000). A rise in seawater 87Sr/86Sr
values, coinciding with the SPICE excursion has been documented from Laurentia
(Montañez et al., 1996, 2000; Denison et al., 1998), and presumably has global
expression.
3.C. v. Sequence stratigraphy.—Work in the Jiangnan Slope Belt of Hunan Province, China, shows that
the base of the G. reticulatus Zone
coincides with the initial stages of a transgressive event (Yang & Xu,
1997a, 1997b, 1997c). Transgression coinciding with the lower part of the G. reticulatus Zone is followed by a
highstand phase and then a shallowing that is expressed in South China, North
China, and Laurentia (Palmer, 1981; Yang and Xu, 1997a); the eustatic sea level
fall is represented in Laurentia as the Sauk II-Sauk III hiatus (Palmer, 1981;
Osleger & Read, 1993; Fig. 9).
4. Selection process
4.A. Relation of the GSSP to historical usage.—The Paibian Stage (and Age) is a new name for the lower stage (and age) of
the Furongian Series (and Epoch; also a new name). The name Furongian replaces
in concept, and content, the traditional upper Cambrian (e.g., Cowie &
Brasier, 19879; Geyer & Shergold, 2000; Remane et al., 2000), and the
various concepts of the upper Cambrian used regionally around the world (see
Geyer & Shergold, 2000; Fig. 1). The Paibian Stage has the same lower
boundary as the Waergangian Stage as used in South China (Peng et al., 1999,
2000, 2001c; Geyer et al., 2000; Peng & Babcock, 2001; Fig. 1). The
relationship of the Paibian Stage and Furongian Series to other regional stage
and series concepts is discussed in section 3.C.ii, and summarized in Fig. 1.
4.B. Other candidates and reasons for rejection.—Following extensive discussion by members of the Cambrian Stage
Subdivision Working Group of the ISCS, meeting in Zhijin, China, in September
2001, only two sections emerged as viable candidates for designation of a stage
and series GSSP: 1, the Paibi section, China (Peng & Robison, 2000; Peng et
al., 2001c, 2001e; Figs. 4-7, 10A-C); and 2, the Kyrshabakty River section,
Kazakhstan (Ergaliev, 1980, 1990). Review of available published information
and our collective experience leads us to conclude that the Paibi section is
superior to the Kyrshabakty River section, and to all other sections in
northwestern Hunan, China (Paibi-2 section, Peng et al., 2001c; Wangcun
section, Peng et al., 2001d; and Wa’ergang section, Peng et al., 2001b).
4.B.i. Other regional candidates.—Besides the Paibi section, the only South China section nominated and
fully documented as a possible GSSP is that known as Paibi-2 (Peng et al.,
2001c, 2001e), which occurs on a hillside adjacent to the Paibi section (Figs.
3- 5). The base of the Paibi-2 section is in the G. stolidotus Zone; it continues through the G. reticulatus Zone, into the overlying Irvingella angustilimbata Zone, and through to the base of the
Ordovician. Similar to the Paibi section, the first appearance of G. reticulatus is in the Huaqiao
Formation. The lithology of the Huaqiao Formation in the Paibi-2 section is the
same as in the Paibi section. The Paibi-2 section is considered to be less
suitable for a GSSP than the Paibi section because the full evolutionary
succession from the FAD of G. stolidatus
through the FAD of G. reticulatus is
not preserved, and because the FAD of G.
reticulatus occurs about 16 cm above the top of a thick (60 cm) rudstone
interbed. The interbed does not appear to have disrupted the biostratigraphic
succession. Paibi-2 is best regarded as a reference section for the base of the
Paibian Stage and Furongian Series (Peng et al., 2001e).
Other
sections in Hunan Province, China, that show the lower part of the G. reticulatus Zone are considered
unsuitable as GSSP candidates because of structural complications or poor
exposure. In the section near Wangcun (Peng et al., 2001d), a fault is present
in the interval of the Huaqiao Formation between the last observed occurrence
of G. stolidotus and the first
observed occurrence of G. reticulatus.
In the section near Wa’ergang (Peng et al., 2001b), poor exposure hinders easy
access to the interval between the upper G.
stolidotus Zone and the lower G.
reticulatus Zone.
4.B.ii. Other extra-regional
candidate.—The Kyrshabakty River section, Kazakhstan, is a
potential candidate for a GSSP. Published information on the section (Ergaliev,
1980, 1990) indicates that G. reticulatus
occurs in a thick succession of dark grey to black limestones and argillaceous
limestones with numerous rudstone interbeds beginning well below the first
appearance of G. reticulatus and
persisting to the base of the Ordovician. Similar to the Huaqiao Formation at
Paibi, China, the Kyrshabakty River section records deposition in an outer
slope fan environment. The best available zonation of the Kyrshabakty section
(Ergaliev, 1980, 1990) is by trilobite assemblages, and the first observed
appearance of G. reticulatus
(reported as G. reticulatus angelini)
is within the G. stolidotus Zone
(Ergaliev, 1990, p. 18, 19). There is an indication that the normal
biostratigraphic succession has been disrupted in the interval between the G. stolidotus Zone and the superjacent G. reticulatus-Eugonocare Zone according
to Ergaliev (1980). Hadragnostus modestus
(reported as Formosagnostus formosus;
Ergaliev, 1980), Kormagnostella longa,
and Blackwelderia sp., are all
present above the first observed appearance of G. reticulatus (reported as G.
reticulatus angelini; Ergaliev, 1980) in the Kyrshabakty section. In all
other occurrences globally, none of these taxa ranges above the Linguagnostus reconditus Zone. These
occurrences could represent either significant upward range extensions from the
L. reconditus Zone to the G. reticulatus Zone (as used herein;
corresponding to the G. stolidotus
Zone of Ergaliev, 1980, 1990), or a mixing of faunas representing different
agnostoid zones. In addition, Ammagnostus
sp. has been reported from the G.
reticulatus Zone in the Kyrshabakty section (Ergaliev, 1980). Elsewhere in
the world, the genus has not been shown to range above the G. stolidotus Zone.
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Proposal submitted 26 May 2002