Sedimentary Geology ELSEVIER
Sedimentary Geology 114 (1997) 189-222
Sequence stratigraphy, sea-level changes and depositional systems in the Cambro-Ordovician of the North China carbonate platform Xianghua Meng a, Ming Ge a, Maurice E. Tucker b,* a China University ofGeosciences, Beijing, China t~Department of Geological Sciences, Durham UniversiO; South Road, Durham, DHI 3LE. UK
Received 13 November 1996; accepted 2 July 1997
Abstract The Cambro-Ordovician strata in North China were deposited over a very extensive craton, extending some 1500 km east-west and 1000 km north-south. The dominantly shallow-water carbonate succession reaches up to 2000 m in thickness and two megasequences (transgressive-regressive cycles) can be distinguished: Lower Cambrian through Lower Ordovician strata, and Middle through Upper Ordovician strata, separated by a major palaeokarst. The first megasequence consists of nine sequences which are generally 50-150 m in thickness. The Lower Cambrian sediments consist of phosphorites and phosphatic sandstones, deposited during the flooding of the craton. Carbonates, mudrocks and evaporites were deposited in the Early Cambrian under an arid climate, laying the foundation for the subsequent long period of shallow-water carbonate deposition which lasted some 70 m.y. The Middle and lower Upper Cambrian sequences consist predominantly of mudrocks and storm deposits ('tempestites') in the lower part and oolitic grainstones and tidal-fiat lime mudstones in the upper part; these represent outer-mid and mid-inner-ramp depositional systems, respectively, of the transgressive and highstand systems tracts. These sequences have strong similarities with the 'Grand Cycles' in the Cambro-Ordovician successions of North America. In the Upper Cambrian, there is a distinctive unit of glauconitic purple-red mudstone several metres thick which is interpreted as the deposits of the maximum flooding of the first megasequence. In the Upper Cambrian, there was a phase of tilting of the North China Carbonate Platform to the north, and storm deposits, especially intraclastic conglomerates and hummocky cross-stratified grainstones-packstones, were very common at this time. Also common in upper Middle and lower Upper Cambrian strata are stromatolitic-thrombolitic bioherms, several metres in diameter. The upper Upper Cambrian through Lower Ordovician strata are dominantly fine-grained limestones and dolomites deposited in shallow-subtidal and inter- to supra-tidal environments on a low-energy epeiric-sea platform. This part of the succession is the regressive part of the first megasequence, so that overall the platform shows an evolution from platform initiation to platform foundation, to a ramp-depositional system and then an extensive epeiric platform. On a small scale, the succession is composed of metre-scale shallowing-upward cycles (parasequences) arranged into cycle sets of 10-30 m thick. The Middle-Upper Ordovician megasequence consists mostly of shallow-water carbonates with several thick evaporite units. Minor palaeokarsts and palaeosoils separate the sequences, which are composed of metre-scale cycles. Regional uplift affected the North China Platform in the Ordovician and sedimentation did not resume until the Carboniferous. There is a good correlation of the two transgressive-regressive megasequences described here with the global 2nd-order relative sea-level curve for the Cambro-Ordovician. Keywords: North China; Cambro-Ordovician; carbonate sedimentology; sequence stratigraphy; Grand Cycles
*Corresponding author. Fax: +44 (91) 374-2510; E-mail:
[email protected] 0037-0738/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0037-073 8 ( 9 7 ) 0 0 0 7 3 - 0
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X. Meng et al. / Sedimentary Geology 114 (1997) 189-222
1. Introduction
The North China Carbonate Platform developed over some 70 m.y. from the Early Cambrian to the Late Ordovician, from around 530 to 460 Ma, and the up to 2000 m of limestones and dolomites deposited there record changes in depositional environments, palaeoclimate, relative sea-level, tectonic uplift and subsidence. The aim of this paper is to document the broad stratigraphic and depositional history of this platform, and to interpret the largerscale sequence stratigraphy (megasequences and sequences) within the tectonic context of the platform. This approach will help understand the longevity of the platform and the dominant controls on platform evolution. The North China Platform provides an excellent case-history of how carbonate deposition responded to long-term changes in relative sea-level and the environment, largely brought about by changes in the regional geotectonic regime and tectonoeustasy. This larger-scale perspective of carbonate platform evolution is one that has not been widely documented, although the various stages recognised in the development of this platform can be seen in platforms of other regions and ages. The Chinese CambroOrdovician carbonates were chiefly deposited in ramp and epeiric platform environments. The terms megasequence and sequence are used here for packages of sediment deposited through 2nd- and 3rd-order cycles of relative sea-level rise and fall (i.e., over timescales of 10-50 m.y. and 0.5-3 m.y., respectively). Much of the Chinese succession consists of metre-scale sedimentary cycles (parasequences, i.e., 5th-order cycles, deposited over a timescale of 0.01-0.1 m.y.), and these are commonly arranged into units of 10-30 m thick, which can be termed cycle sets or parasequence sets, of 4th-order duration (0.1-0.5 m.y.). 2. North China Carbonate Platform: general features and context
The North China Platform is situated in northeastern China, with the city of Beijing in the northeastern part and the city of Xian on the southern margin (Fig. 1). It covers an area of around 1500 km east-west and 1000 km north-south. It includes
the provinces of Liaoning, Jilin, Beijing, Tianjin, Hebei, Shandong and Shanxi, and parts of Anhui, Henan, Ningxia and Inner Mongolia. The regional geology and context of the platform are reviewed in Wang and Liu (1980), Yang et al. (1986), Meyerhoff et al. (1991), Chen and Rong (1992), Wang and Mo (1995) and Meng et al. (1996a); Wang (1985) presents geological maps of the region. The ancient foundation of the North China Platform was consolidated at the end of the Early Proterozoic during the Luliangian orogenic phase (Meyerhoff et al., 1991). The platform comprises several Archaean continental nuclei: Ordos in the northwest, Yan-Liao in the northeast and Hehuai in the southeast. The North China Platform was effectively a microplate in Late Precambrian through early Palaeozoic time. The northern margin is now delineated by a major fault and suture zone (the Northern Tectonic Belt of Junggar-Hinggan), with the Mongolian foldbelt to the north consisting largely of lower Palaeozoic deep-water mudrocks and volcanics (see Fig. 2). A subduction zone, directed southwards, existed along this northern margin in the lower Palaeozoic, and collision with a marginal basin took place in Late Cambrian-Early Ordovician time. Ophiolites occur along this boundary (Wang and Mo, 1995). On the western side, in the region of Helanshan (locality 14, Fig. 2), thick platform-margin and basinal facies were deposited. The southern margin of the platform is bounded by another major fault and suture zone (the Middle Tectonic Belt of Kunlun-Qilian-Qin Ling, Fig. 2) and here the Qin Ling foldbelt is composed of thick lower Palaeozoic basinal sediments and volcanics (Zhang et al., 1995, 1996). Subduction here was directed northwards. To the south occurs the Yangtze Platform (Fig. 1), another lower Palaeozoic stable craton, but smaller than the North China Platform. Deep-water Cambro-Ordovician sediments were also deposited on the eastern side of the platform, where there is a major strike-slip fault (the Tanlu fault). 3. Stratigraphy of the North China Carbonate Platform
There are seven formations in the type Cambrian of the North China Platform (Table 1; Fig. 3): from bottom to top they are the Xinji, which is divided
X. Meng et al/Sedimentary
Geology 114 (1997) 189-222
MCJNWJLIA
W
Cambrian
outcrop:
Fig. 1. Map showing the major geological provinces within China, the location of the North China Platform, and the outcrops of Cambrian strata. Geological regions: 1 = North China Platform. 2 = Tianshan-Hingan, 3 = Qilian. 4 = Kunlun-Qin Ling, S = Yangzi. 6 = Jiangnan.
into the Changping and Mantou formations in many areas, Maozhuang, Xuzhuang, Zhangxia, Gushan, Changshan and Fengshan formations (see Lu, 1962; Lu et al., 196.5; Xiang et al., 1982; Ye, 1983; Zhang, 1983; Yang et al., 1986; Meng et al., 1996a). The abbreviations used in China are: E:~, E?, EF, E:, ‘. E:~, I$, ET’ and Er (where E = Cambrian, 1 = Lower, 2 = Middle, 3 = Upper). Several names are used on the North China Platform for the various Lower Cambrian formations, since deposition was not continuous over the whole platform and it began at different times in different places (Lu et al., 1965). The younger formations were deposited more uniformly over most of the platform, except in areas close to topographic highs, such as in the region of Ordos to the northwest (location shown in Figs. 1 and 4), where they are either very thin or absent through non-deposition, as this area was a contracting island through Cambrian
times. The Upper Cambrian Fengshan Formation is succeeded by the Lower Ordovician Yeli Formation in most areas. There are eight formations in the Ordovician on the platform (Table 2; Fig. 3); from bottom to top they are the Yeli, Liangjiashan (Lower Ordovician), and Majiagou 1 through Majiagou 6 formations (LowerMiddle Ordovician). Several Ordovician formations occur within rift basins, e.g., Yingtaogou (Middle Ordovician) and Tangwangling (Upper Ordovician) in the west, especially in the area of Helanshan (locality 14, Fig. 2) (see Wang, 1980; Lai et al., 1982; Yang et al., 1986; Ge et al., 1996). Fifteen geological sections have been studied in detail and many others visited to document the sedimentology, sequence stratigraphy and cyclostratigraphy of the Cambro-Ordovician strata of the North China Carbonate Platform (see Fig. 2 for location of sections).
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X. Meng et aL / Sedimentary Geology 114 (1997) 189-222
basinal areas
epeiric seas
post-Ordovician ~______~ post-Ordovician uplift uplift post-Cambrian uplift
I I iii
I post-Silurian uplift
major faults
marginal ++ x x x
basin
granite ophiolite
sutures
Fig. 2. Schematic geological and structural m a p of the North China region (adapted from Yang et al., 1986), which also shows the principal localities of C a m b r o - O r d o v i c i a n sections, m a n y of which are referred to in the text: 1 = Changbai, 2 = Hunjiang, 3 = Ginxian, 4 = Pingquan, 5 = Qinhuangdao, 6 = Tangshan, 7 = Dingjiatan, Western Hills, Beijing, 8 = Hunyuan, 9 = Kouquan, Datong, 10 = Jingling, 11 = Taiyuan, 12 = Lishi, 13 = Zhuozishan, 14 = Suyukou, Helanshan, 15 = Mizhi, 16 = Pingliang, 17 = Tangwangling, 18 = Ruicheng, 19 = Lushan, 20 = Fengtai, 21 = Xinji, 22 = Suxian, 23 = JiaGou, 24 = Gushan.
The broad picture of lithologies and the pattern of onlap-offlap for the Lower Cambrian-Lower Ordoviclan megasequence are shown schematically in Fig. 5. Fig. 6 also shows the development of the North China Carbonate Platform during this time, with the appropriate, generalised facies models for each stage.
4. Biostratigraphy and international correlation The Cambrian stages in China are defined on the basis of trilobites (Table 1), as elsewhere in the world. The lowest part of the Cambrian (the Meishucunian stage) is not present on the North China
X. Meng et al. / Sedimentary. Geology 114 (1997) 189-222
193
Table 1 Stage names, formations and fossil zones for the Cambrian of North China, along with the probable equivalent fossil zones for Britain and North America (in part from Yang et al., 1986) CHINA Formations
Stage
Fossil Zones
Britain
North America Tremp
Saukla
ealeauan Fengshanian
Fengshan
Calvinella-Mictosaukia Quadratlcephalus-DictyellaPtychaspis-Tsinania
Pettura
Saratogia Franconlan
Taenicephalus
Upper Cambdan Changshanian
Kaolishania Changshania
Leptoplastus Parabolinaspinuiosa
ChuangJa
01enus
OundeFoer~la
Changshan
Gushanian
Gushan
Zhangxlan
Crep~cephalus
Zhangxia
Crepicephalus
Middle
Baitiella
Cambrian Xuzhuangmn Meazhuangian
Xuzhuang
Pofiagraulos
Maozhuang
Sunaspis Kochaspis Shantungaspis
Mantou
HoffetellaRedlichia
Changping
Palaeolenus Drepanuroides
Longwangmiaoian
Lower Cambrian
Qiongzhusian Meishucunian
B
Dresbachian
Drepanura
Blackwelderia Damesella Amphoton-Taitzuia
Canglangpuan
Elwnia
Ceaann
Le]opycje
Solenopleura Goniagnostus Ptychagnostus
Bolastpidella
Hypagnostus Tomagnostus Ptychagnostus
BathyunscusElrathlha Glossopleura Albertella
Eccaparadoxides
PtagmraPofella
Protolenid-Strenuellid
Bonnia-Olenellus Nevadetla
YdiangeltaYunnanasis Eoredlichia Parabadiella Allatheca-Yunnano Anabarites- Circotheca
Platform; there is a large stratigraphic gap with the Sinian below. There has been much discussion over the placing of the Lower-Middle Cambrian boundary and the view now is to include the Maozhuang Formation in the Middle Cambrian on the basis of the distinct biotic change from redlichid trilobites in the Mantou Formation to ptychoparid trilobites in the Maozhuang (Yang et al., 1986). The Cambrian-Ordovician boundary occurs
A~nostus
Olenellid
Fal/otaspis
Non trilobite
Pre-tritobite zone
within a continuous succession in North China and is placed between the Calvinella-Mictasaukia zone and the Onychopyge-Leiostegium zone. The Ordovician zone fossils are mainly graptolites (Table 2); this can make correlations difficult since they are rare in the platform carbonates. Conodonts are useful, however. The Yeli Formation broadly correlates with the Tremadoc stage of Britain and the Gasconada stage of North America; the Liangjiashan Formation
194
X. Meng et al. / Sedimentary Geology 114 (1997) 189-222
Sequence Stratigraphy Stratigraphic Units Coastal onlap curve and systems tracts seqs Formations
M sH
sa
Sb~~rS T
T
S
\
T
I
T
SB ~
Majiagou Fm. 5
HST OS4
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2
OSSO7 Majiagou Fro.6 .-~,-v--Karst.~vv-.
,_XLEL_
T
£
T
Sedimentary System
O83
37 Majiagou Fm. 4
::)7 uajiagouFrn.3
Evaporitecarbonate -epeiric
Majiagou Fm. 2
platform
37'
R
Majiagou Fm. 1
-
•~v~-Kars t Lian~shan
Sb(
TST HST TST HST TST HST
H S Sb [~_ T M ;b i, E mfs ~, GAleS- ~ . . . . . . S Sb~'~'~-TST (_____HS_T_. . . . . . E :_:_:_:_
eE
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N
T[
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. . . . . .
OSl
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£S7 F
Fengshan Fm.
~3
cs Changshan Fm EseF-~e ~ 3 Gushan Fm. ES5 z~
carbonate platform
PF LC AR TIC
LR Storm-dominated A A ramp TM system EP
Zhangxia Fm.
S b[____HST ' ~ T StSZ T ...... \--
Epeiric
~Oy
L A T F O
ii £s4
HST . . . . ES3 e ,
Xuzhuang Fm.
Maozhuang Fm.
ER Oolite-dominated A A ramp R IV system Lp Y
Sb
1
L S "1" "
Platform foundation: / ~N, HST ES2 a Up.Mantou Xinji Fm carbonates-evaporites TST Sb ~ ~' Changping/ / ~ HST cP Platform initiation: ~ TST E;Sl ~1 carbonates-phosphorites SB ~ ~,~.,,
Fig. 3. Stratigraphy, sequence stratigraphy, pattern of onlap-offlap and major phases of carbonate sedimentation on the North China Carbonate Platform.
correlates with the lower part of the Arenig and Beekmantown stages (Table 2).
5. Sedimentology and sequence stratigraphy in the Cambro-Ordovician of the North China Carbonate Platform
5.1. Sedimentology and sequence stratigraphy of the Lower Cambrian There are two formations in the Lower Cambrian and both are interpreted as corresponding to sequences. The Lower Xinji Formation in the southeast of the platform, which is the Changping Forma-
tion in the Beijing area (and their equivalents, the Shuidong and Jianchang formations in the northeast, and Suyukou Formation in the west) is interpreted to be the first sequence, eS1 (i.e., e for Cambrian, S for sequence and 1 the sequence number). The Mantou Formation (the Upper Xinji Formation in the southeast, and Wudaotang Formation in the west) is interpreted to be the second sequence (eS2).
5.1.1. The Lower Xinji/Changping Formation, ecP, Sequence eS1 The basal sediments of the Lower Xinji/Changping Formation were deposited during the initial Cambrian transgression which was confined to the
195
X, Meng et al./Sedimentary Geology 114 (1997) 189-222
c? ocP j /
o I
(" L s ~ S - ~ "
.
/ /
50 I
I
I
I
I
Fig. 4. Palaeogeography and location of shorelines for the Lower Cambrian (~P, Changping Formation), the Middle Cambrian (eMZ, Maozhuang Formation; eTM, Zhangxia Formation) and the Upper Cambrian (E3 F, Fengshan Formation) on the North China Platform. The Early Cambrian transgression on to the platform came from the southeast and northeast (open arrows). Successive shorelines through the Cambrian were farther west, as shown, with the Ordos region a contracting land area. Tilting of the platform to the north at the end of the Middle Cambrian resulted in the shoreline then located along the southern edge of the platform in Fengshan times (~), although two small islands were left in the Ordos area (diagonal shading).
southern, eastern and western margins of the North China Platform (see Fig. 4). From a reconstruction of the palaeoshoreline based on facies distribution (Fig. 4), it would appear that the transgression mainly came from the Qin Ling sea to the south and southeast. The Xinji Formation unconformably overlies Precambrian basement, either Archaean gneiss or Sinian metasediments, and a weathered zone or palaeosoil is commonly present. Lower Xinji sediments are characterised by the presence of phosphorite, up to 25% phosphate, and important localities for phosphorites are shown in Fig. 4. The phosphoritic sandstones contain sandsized grains of collophane and there are many biogenic grains within the phosphatic sediments includ-
ing hyolithids, inarticulate brachiopods (such as Lingulella) and trilobites. Glauconite is usually present. C o m m o n sedimentary structures include cross-bedding, cross-lamination and planar bedding. Phosphoritic conglomerates have pebbles of phosphoritic sandstone, amorphous phosphorite and chert, and form lenses and beds up to 1 m thick. The thickness of these phosphorite beds varies greatly along strike and individual beds reach a thickness of 10 m. Associated facies, especially in the middle and upper parts of this unit, are siliceous shales, cherts and chert breccias, black shales and sandy shales (Ye et al., 1989). Deposition of these basal phosphoritic sands and gravels probably took place in a shallow-to-moderate
196
X. Meng et al./Sedimentary Geology 114 (1997) 189-222
Table 2 Stage names, formationsand fossil zones for the Lowerand Middle Ordovicianin China, and the probable correlationswith stages in Britain and North America(in part from Yanget al., 1986) CHINA Stages
Middle Ordovician
Guniutanian
Zones
Pterograptus elegans / Didymogrraptus rnurchisoni Ampexograptusconfetus Cardiograptus Didymograptus
Formations
Britain
North America
Llanvirn
Whiterock
Majiagou
nexus
Dawanian
Lower
Ordovician
Hunghuayanian
Yichangian
Oncograptus rn~nus Didymograptus abnormis / Azygograptus suecicus Didymograptus protobifidus Didymograptus deflexus Tetragraptus fruticosus / DIdymograptus hliformis Z (Etragraptus)approximatus Adelograptus Clono~raptus Aletograptus Tri~raptus Staurograptus Anisocjraptus
depth, marine sand-wave environment, with periods of intense sediment reworking, at a time of sediment starvation. Minor changes in sea-level may also have been instrumental in the precipitation of the phosphorite and in the concentration of phosphorite grains (e.g., Meng, 1985; Ye et al., 1989). The generally high phosphate content may have been the result of deep weathering and erosion of the Precambrian basement to the platform, coupled with high nutrient supply in the transgressing sea. In several areas, such as Suyukou in the Helanshan region (Ningxia Province, locality 14, Fig. 2) in the western part of the platform, Changbai (Liaoning Province, locality 1) in the northeastern part of the platform, Xinji (Henan Province, locality 21) in
:!:i~:i::.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:~i!iiiii~!i~iiiiii~i!i~i!i~ ~iiii Arenig ~:.~:~:~:~:~:.:~:~:~:~:~:~:`:.:~:~:.:`:~:~::~:~:~:~:`:~:~:~:~:~:+:~:~:~:~:~:~:
Beekmantown
i!i!i!!!i!i!i!i!i!i!i!!!i!ili!!!i!i!i!i ¸ :::::::::::::::::::::::::::::::::::::::: .,.........................
::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::
Liangjiashan
Yell
Tremadoc
Gasconada
the southeastern part, and Fengtai (Anhui Province, locality 20) in the south, fining-upward, shallowing-upward cycles, 10-20 m thick, are present in the succession. From bottom to top, they generally consist of a phosphoritic breccia passing up into phosphoritic silty shale and then oolitic and pisolitic quartzitic phosphorites. Such cycles (parasequences) are interpreted as the result of shoreline retrogradation and progradation, with the breccia representing a basal transgressive deposit produced by reworking and lag concentration and the oolitic and pisolitic phosphorite deposited in a regressive high-energy sand-wave environment. The Changping Formation in the Western Hills, Beijing, the Jianchang Formation of Liaoning and
X. Meng et al./Sedimentar)' Geology 114 (1997) 189-222 Ordos
Lishi
Datong
t97
Fengtai onlap ~
~
;; eF
offlap
°s'/ L
ES7
~30
E~
ES6
~
~ss ES4
[-~
limestone
[7~7]
dolomite
sandstone
~
mudrock
.-~bioherm AAAA^ evaporites
- marl
o
o °
oolite
" ~. -. .~. -. . .-. . ~ , ~ ~, . ,,, . ., . , • ~ : : ; :~---'-'-'-'-'-'-'-'-~I { E~MZ ~.'.'r~.~L:,=~....I. .... I........ '. . . . . I . . . I 2 ~"~:~.." 7.--7-. ; , - - '~-- ' r - - ;-r- ; ~ - . , ~ " ..- " L/_ A ~ A ~ A-z- A - L ^--z. ^ ~ M - .~'__.'-'-'-'-'-"~,,.~-~A:--TA'':--:A'-7'ATIE1
"e" ~*
"
"
Ef
formations
"%-
ES3 F~S2 1
sequences
Fig. 5. Schematic stratigraphy, lithofacies and pattern of onlap-offlap for the Cambrian-Lower Ordovician strata of the North China Platform.
Jilin provinces in the northeast, the mid-Xinji Formation in the south/southeast, and the upper part of the Suyukou Formation in the west, generally consist of fine-grained carbonates, with limestones decreasing and dolomites increasing upwards (Fig. 7). In addition, there is a change from more subtidal limestones in the lower part, with intraclasts and burrows, and locally oncoids, in some cases bituminous, to more intertidal-supratidal dolomites in the higher part with fenestrae and desiccated microbial laminites. In the Western Hills, Beijing, the Changping Formation (60 m thick) is capped by a palaeokarst horizon and overlain by a breccia at the base of the Mantou Formation (Fig. 7). An equivalent and similar irregular, hummocky surface with overlying breccia occurs in the west of the platform at Helanshan, close to the boundary of the Suyukou and Wudaotang formations. Elsewhere, the boundary is a conformity, although overlain by glauconitic sediments of the next formation (Mantou, see below). Nodular anhydrite/gypsum occurs within the Changping equivalent, the Zhushadong Formation, in the southwest part of the platform near Lishi and Ruicheng (localities 12 and 18, Fig. 2).
Interpretation. The Lower Xinji-Changping Formation is interpreted as the first Cambrian sequence; it consists of two parts, a lower transgressive succession characterised by the phosphoritic sediments of the Lower Xinji Formation (and equivalents elsewhere) and the regressive carbonates and local evaporites of the Changping Formation (and equivalents elsewhere), which would belong to the highstand systems tract. Some of the phosphorite deposits could represent lowstand or early transgressive sediments, produced during the initial flooding of the craton and deposited in topographic lows, before the major transgression which spread phosphoritic muds and sands more widely across the platform. Formation of phosphorites during transgressive periods generally is common (e.g., MacQuaker et al., 1996) and they are widely developed generally in Lower Cambrian strata (e.g., papers in Cook and Shergold, 1986; Brasier, 1990). The exposure horizon and palaeokarstic surface at the top of the Changping would represent a sequence boundary. In this Early Cambrian time, there was probably significant relief upon the platform.
198
X. Meng et al./Sedimentary Geology 114 (1997) 189-222
NW
SE megasequence boundary
karst + soils
.....~, , , ,
sea-level
:,
fa.
Lowstand ~ and exposure
, ,,,,,,,,,,,,
.....
End Lower Ordovician
Platform exposure
HIGHSTAND muddy subtidal carbonates ]
S H[ghsta.n.d.. L eolmentatlon ! rate > relative s e a - l e v e r se
/
microbial - fenestral inter-supra:,-tidal carbonates
STAGE
.............
,
Lower
O1L Epeiric
Ordovician
carbonate platform O1v
maximum flooding
tidal flats
'tempestites', hcs-grain/packstones, intraclast breccias
Continued
tra.nsgre.s,sion., t-FWB . . . . . . . . . . . . . . . . . . ~ Sedimentation rare
EaF
r ¢cs 8aPmPe~rian
maximum I'lOOQinq.
< relative sea-level rise
[ SWS
~
Storm-dominated
"-~ I /
camonate ramp E OlJ
\ TRANSGRESSIVE
\
I Tilting event I
STAGE peritidal carbonates oolite grain/packstones storm Continue.d transclresslon, ~ ' ~ ° . . ~ , • "i . . . . . . _ .a.ggr~.dation.. ~ ~ealmentatlon ra~e ~ = r,elat[v~ i carbonate ramp ~ ~ " " ~ -
facies ~z.
sea-ievel nse
evaporites
Sea-level Iowstand and transgression
E,x 1 ~lidmd/erian e:,~zlj ....
clastics/ carbonates / phosphorites
rm initiation
._
~
l,ower
LOWSTAND
E1cPSCambrian megasequence boundary
NW
,
~200km
!
SE
Fig. 6. Schematic diagram showing the evolution of the first Cambro-Ordovician megasequence on the North China Carbonate Platform with sketch facies models for each stage and the broad pattern of onlap-offlap.
5.1.2. The Upper Xinji/Mantou Formation, e~, Sequence eS2 The second Cambrian formation, the Mantou Formation (the Upper Xinji Formation in southeast Hebei Province and Wudaotang Formation in Helanshan, Ningxia Province) is interpreted as the second sequence (eS2). The Mantou Formation lies within the Redlichia chinensis trilobite biozone, that is uppermost Lower Cambrian. The Mantou Formation occurs more widely across the North China Platform as a result of another major transgression following the regression and local exposure at the end of the Changping Formation. Mantou facies vary considerably from southeast to northwest, with normal-marine carbonate ramp and shelf facies in the east and south (lime mudstones, nodu-
lax limestones, ribbon rocks and bioclastic-oolitic packstones-grainstones), and low-energy, evaporitic lagoonal shoreline, coastal plain to muddy shelf facies in the more northern part of the platform, such as in the Beijing region, Tangshan and Pingquan (localities 7, 4 and 6, Fig. 2). The thickness in the Western Hills, Beijing, is 50-70 m. Continental areas still existed in the western parts of Shanxi and Hebei provinces (Fig. 4). During Mantou times, evaporitive dolomites, locally laminated-nodular gypsum beds and redpurple dolomitic silty mudstones were deposited under arid conditions in coastal plain, tidal-flat and shallow-subtidal lagoonal environments around the embayed shoreline of the North China Platform. Collapse breccias are present in the Mantou Forma-
X. Meng et al./Sedimentary Geology 114 (1997) 189-222
199
Western Hills, Beijing Lower-Middle Cambrian Maozhuang Changping
Mantou
60
- sb
220.
120 50 210110 40 200100 30 190-
90 20 180-
80 10 170-
70
E
16060
~
~
limestone
~
dolomite
sandstone
mr]
mudrock/marl
ribbon rock
~
shale
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tion of the Western Hills, Beijing (Fig. 7), the result of evaporite dissolution. They are characterised by angular carbonate clasts, an absence of sorting, a red mudrock matrix, vugs, and patches of calcite and quartz containing evaporite relics. Seven, decimetrescale cycles of red, muddy collapse breccia passing up into thin-bedded dolomite occur in the succession (Fig. 7) and were the result of subaqueous saltern evaporite precipitation and intertidal-supratidal carbonate deposition (similar to the modern Marion Lake of South Australia, Warren, 1982, and the Cretaceous Ferry Lake Anhydrite of the Texas subsurface, Loucks and Longrnan, 1982). Around the shoreline in some areas where there was some topographic relief, debris-flow breccias of alluvial talus and karstic material are interbedded with the evaporitic facies. Gypsum-beating strata of the Mantou Formation also occur in the Taizhe, Tonhua-Huinan and Liaoyang-Benxi depressions in Jilin and Liaoning Provinces in the northeastern part of the platform, where there was some extensional faulting in Early Cambrian time, leading to the formation of small rift basins. Up to 50 m of gypsum and dolomite are present, and four, decimetre-scale cycles (parasequences) can be recognised; each shows a vertical facies change from limestone-dolomite to dolomitic mudstone-gypsum, representing flooding and shallow-marine deposition followed by tidal-fiat progradation and evaporite precipitation. The four cycles can be correlated for a considerable distance along the northern side of the platform. Trilobites tolerant of higher salinity occur in the upper part of this sequence, and they indicate fluctuations between the somewhat restricted waters of this area and the open sea. The marine incursion probably came from Pyongyang (Korea), the direction of the open ocean in northeast China. Interpretation. The Mantou Formation represents the second rise and fall of sea-level on a 3rd-order scale in which extensive terrigenous, low-angle ramp and saline lagoonal facies were deposited (schematically shown in Figs. 5 and 6), In the south of the region (e.g., Suxian, Anhui Province, and Lushan, Henan Province, localities 22 and 19, respectively, in Fig. 2), the lower boundary of Sequence eS2 is a conformity overlain by a lenticular glauconitic limestone. However, in the north, at Beijing, Tang-
shan and Pingquan, as noted above, the lower sequence boundary is a karst unconformity with a breccia locally reaching 10 m or more in thickness, resulting from dissolution of the underlying Changping Formation. The upper boundary of Sequence eS2 is a sharp undulating surface in northern localities, representing a short hiatus, but it is again a conformable boundary without any indication of lengthy exposure in the southern outcrops. In most areas of the platform, the transgressive deposits are thin, marine mudrocks and carbonates, locally glauconitic. By way of contrast, the much thicker evaporitic sediments and restricted inter- to supra-tidal dolomites represent regressive (highstand) deposits of a long-term relative sea-level fall, upon which were superimposed shorter-timescale relative sealevel changes which produced the metre-scale cycles (parasequences). The climate in the Early Cambrian was clearly arid. 5.2. Sedimentology and sequence stratigraphy of the Middle Cambrian Three formations are present within the Middle Cambrian succession and each is interpreted as representing one sequence: the Maozhuang Formation is Sequence eS3, the Xuzhuang Formation is Sequence eS4 and the Zhangxia Formation is Sequence eS5. 5.2.1. The Maozhuang Formation, e2Mz, Sequence ES3 The area occupied by the Maozhuang Formation is much greater than the Changping/Mantou formations because the transgression now extended farther to the north and west (Figs. 4 and 5). The Maozhuang Formation, 50-150 m thick, occupies the Shantungaspis and Kochaspis trilobite biozones (Table 1). The sediments of the Maozhuang Formation are a mixture of clastics and carbonates. Quartzitic sandstones up to 20 m thick (the Huoshan Sandstone) occur at the base of the formation in areas close to the palaeocontinent ('Oldland') which was inundated for the first time. They are planar-bedded and cross-bedded beach-shoreface facies consisting of well-rounded and sorted quartz grains. In Shanxi Province (e.g., Lishi, locality 12, Fig. 2), they occur above a basal conglomerate, probably also of early Middle Cambrian age.
X. Meng et al./Sedimentary Geology 114 (1997) 189-222
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Fig. 8. MaozhuangFormation,Western Hills, Beijing. Pale units (right and left of centre) are muddy, oolitic carbonates which pass up into red, playa/lacustrinemarls (dark beds, left and centre). Stratigraphictop to the right. In the Beijing area, the Maozhuang Formation is 80-100 m thick and begins with a 30 m purplish-red silty mudstone with salt pseudomorphs (see Fig. 7). This was deposited in a marginal-marine sabkhaplaya. It is overlain by two dolomitic units, around 10 m thick, with intraclasts and microbial laminites passing up into red marl. The central part consists of several cycles of muddy, locally oolitic, limestone passing up into red marl of playa/lacustrine origin (Fig. 8), and then grey-green silty mudstone, with glauconite, passes up into an oolitic and bioclastic limestone. The formation is capped by fine-grained, tidal-flat dolomite, consisting of high-frequency, metre-scale cycles of laminated to massive lime mudstone. In the Datong area (Kouquan section, locality 9, Fig. 2), a marginal part of the platform at this time, the Maozhuang Formation is composed of four parasequences. The lowest parasequence (18 m) consists of mainly purple shales and siltstones, with thin sandstones, passing up into marl and dolomite with salt pseudomorphs, evaporite dissolution breccias, mudcracks and lamination. Deposition took place
in a supratidal mudflat and restricted lagoon. The thicker (30 m), 2nd parasequence shows a similar mudrock to dolomite upward change, but three, metre-scale cycles of mudrock to marl are present within it. The thinner, 3rd and 4th parasequences are mostly marl to fine dolomite, with lenses of stromatolites and intraclasts, and wave-rippled crosslaminated sandstones. At the JiaGou section in Xu county, Shandong Province (locality 23, Fig. 2), in the southeast part of the platform, there are also four parasequences in the formation. The lower part is formed of two, metre-scale cycles with cross-laminated quartzitic siltstones in the lower part and thin silty bioclastic oolitic limestones in the upper part. There is a little glauconite (less than 3%) in the second oolitic limestone. The upper part of the formation consists of two cycles largely of thick-bedded dolomites and micritic limestones deposited in shallow subtidal to intertidal environments. Interpretation. The Maozhuang Formation represents a 3rd-order sequence and, on a broad scale, the more muddy, locally oolitic, lower part represents
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the transgressive package, and the more dolomitic, tidal-flat upper part with locally coarser clastics the regressive highstand. The sabkha-playa unit at the base in the Beijing Western Hills would represent a lowstand deposit. The muddy, glauconitic layers in the central part of the formation are the deepest-water facies and would represent the condensed section of this 3rd-order sequence.
5.2.2. The Xuzhuang Formation, ex, Sequence eS4 The Xuzhuang Formation reaches 120 m in thickness and contains the biozones Kochaspis, Sunaspis, Poriagraulos and Bailiella. The stratigraphy of the Xuzhuang can be divided into two parts: the lower part is chiefly mudrocks and thin bioclastic and oolitic limestones, and the upper part is dominated by oolitic limestones with thin shaley interbeds (schematically shown in Fig. 5). By the end of the Xuzhuang stage, most of the North China craton had become an extensive carbonate platform (Fig. 4). The Xuzhuang has a conformable boundary with the underlying formation, with no evidence of exposure. The upper boundary is marked by a basal lag deposit of the Zhangxia Formation. The lower part of the Xuzhuang Formation consists mainly of terrigenous deposits with thin calcareous sandstones, muddy limestones and oolitic limestones. These are well exposed in the Western Hills of Beijing, and in the Hunyuan and Kouquan sections near Datong in the northwestern part of the platform (Figs. 9-11). The terrigenous deposits consist of purple-red silty mudstone and siltstone with many trace fossils, including Cruziana and burrows such as Skolithos. Hummocky cross-stratification is also present, with ripple marks and wave-formed cross-bedding in the thin grainy beds. Broadly, these sediments were deposited on an open, shallow-marine ramp, with deeper-water, more muddy mid-outer-ramp facies occurring in the southern part of the platform and more shallower-water, peritidal facies occurring in the western part. The upper part of the Xuzhuang is mainly composed of oolitic limestone, with subordinate micritic,
bioclastic and stromatolitic limestone. The oolite beds increase in thickness upwards and decimetrescale cross-bedding, herringbone cross-bedding, flat bedding and ripples are common. Deposition took place in shallow-water, tide- and wave-dominated sand shoals. One major east-west shoal was located along the northern part of the platform, from Hunyuan to Beijing (Fig. 2), and tidal channel, tidal delta and beach facies can be recognised (Meng et al., 1996c). Cross-beds are mostly directed to the south (180-200°). Crawling traces and horizontal burrows (including Zoophycos) occur in the interbedded micritic limestone and marl. Internally, the Xuzhuang Formation is composed of numerous metre-scale cycles; 80 cycles occur within the Dingjiatan section in the Western Hills, Beijing, for example. In the lower part of the formation, these '5th-order' parasequences are generally shales passing up into ribbon rocks consisting of storm layers and dolomitic lime mudstones. Thin oolites cap some of these cycles. In the upper part of the formation, the cycles are thin shales or lime mudstones passing up into oolites. The latter generally become thicker, and the ooids larger, up the succession. The metre-scale cycles (parasequences) are arranged into larger-scale units, cycle sets (4th-order cycles), each several tens of metres thick (Fig. 9). Five of these can be recognised along the northern and western margins of the platform from Beijing to Datong to Lishi. These units are basically composed of a lower more muddy part and an upper more calcareous part; in fact, within a parasequence set, the metre-scale cycles show a systematic upward increase in the proportion of limestone to mudstone. The basal layers of the parasequence are commonly glauconitic shales, representing a flooding event. Interpretation. Overall, the Xuzhuang consists of a transgressive lower part, more muddy and dominantly mid- to outer-ramp facies, passing up into a more regressive upper part, characterised by oolites of mid to inner ramp. These represent the TST and HST, respectively, of a 3rd-order sequence, the
Fig. 9. Sketch graphic logs of the Xuzhuang and Zhangxia formations of the Hunyuan section, near Datong, Shanxi Province, with sequence stratigraphic interpretations.The formationsare composedof metre-scalecycles (parasequences)which are organisedinto five units ranging from 20 to 40 m in thickness (parasequencesets).
X. Meng et al./Sedimentary Geology 114 (1997) 189-222
Hunyuan, Datong Middle Cambrian Xuzhuang Fm.
Zhangxla Fm.
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Fig. 10. The Xuzhuang Formation of the KouqUan section, near Datong (location shown in Fig. 2), Shanxi Province. The lower part (centre and left) consists of a more shaley succession, with storm beds and fine-grained limestones, the upper part is dominated by oolites, the thick-bedded limestones to the right.'
eS4. In many sections, including the Western Hills of Beijing, Datong and Lishi (localities 7, 9 and 12), glauconite is present within thin-bedded micrites and oolites, or within sandy mudstones, giving a distinctive green-grey colour, towards the top of the more shaley lower part of the formation. This part of the formation is interpreted as the equivalent of a condensed section, being deposited during the time of maximum flooding. Since the succession is composed of metre-scale cycles there is not one thin, discrete maximum flooding surface, but a zone of strata which can be assigned to this part of the sequence (cf. Montafiez and Osleger, 1993). Long-term relative sea-level was rising during deposition of the Xuzhuang Formation, as shown by the greater area of the platform covered by this formation. Mid- to inner-ramp shoreline and shallow-subtidal carbonates were mostly deposited. Topography on the platform was gradually being reduced. The general facies model for the Xuzhuang Formation is shown in Fig. 6.
5.2.3. The Zhangxia Formation, EZH,Sequence ~$5 The Zhangxia Formation, which contains the zone fossils Crepicephalus, Amphoton-Taitzuia and Damesella, forms the fifth sequence, eS5. The formation reaches 160 m in the Beijing area and 200 m at Lishi. It is characterised by abundant oolite and storm deposits (ribbon rocks), along with calcirudite, skeletal peloidal limestone, micritic limestone and dolomite. As with the Xuzhuang Formation, the Zhangxia generally consists of two parts, a lower more muddy part and an upper part dominated by oolites. Storm beds are common in the lower part, and these are thin, graded, cross-laminated limestones interbedded with dolomitic shales (see later section). The oolites are cross-bedded and flat-bedded, deposited in shoal areas. In the relatively deep parts of the ramp (e.g., the Hunyuan section in Shanxi Province, Fig. 9), black shale, mudstone and micritic limestone were deposited extensively over large areas in the lower part of the formation, whereas in shallower-water areas, as in the Western Hills of Beijing, oolites are common throughout the formation.
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Fig. 11. The lower and middle part of the Xuzhuang Formation of the Hunyuan section, Shanxi Province, with a more shaley lower part with thin limestones, passing up into bedded oolites forming the near vertical cliff.
As with the Xuzhuang Formation the main feature of the Zhangxia is that it consists of numerous metrescale parasequences, 106 in the case of the Dingjiatan section in the Western Hills of Beijing (e.g., Figs. 12 and 13). Many different types of metre-scale cycle can be recognised. The cycles occur within three major facies associations: (1) outer-ramp facies dominated by shale/marl-ribbon limestone cycles; (2) middle-ramp facies characterised by ribbon limestone-oolitic limestone cycles; and (3) innerramp facies dominated by back-shoal muddy oolites to tidal-fiat sediments, which may be fine-grained dolomites or microbial laminites. In the inner-most part of the platform, to the west, more terrigenous clastics occur within the cycles. Some of the oolites of the mid-inner-ramp cycles show evidence of subaerial exposure in the form of local dissolution and occluded oomoldic porosity. In a similar way to the Xuzhuang, the Zhangxia metre-scale parasequences are organised into cycle sets of 20-30 m thick (Fig. 9), and five of these can be distinguished in the northern and western exposures of the North China Platform. Each has a more
clastic lower part and a more calcareous, generally oolitic upper part. Glauconite grains occur within the Zhangxia within some of the parasequences towards the top of the second cycle set. In some localities (e.g., Hunyuan near Datong, and Dingjiatan near Beijing), small thrombolitic mounds a few metres across and up to 1 m high occur in the more muddy parts of the parasequences below the oolites. These are mostly present in cycle sets 3 and 4 (Fig. 9). On the western margin of the platform in the Helanshan area at Suyukou, the small-scale cyclicity is not very apparent. Deeperwater mudrocks with storm beds are more common and the latter consist of ooids, skeletal debris and intraclasts. Interpretation. The Zhangxia Formation is interpreted as a 3rd-order sequence, ES5, consisting of a lower transgressive part and an upper regressive, highstand part. The lower boundary of this Sequence ~$5 as seen in the Western Hills area of Beijing, at Hunyuan near Datong and Lishi, is a sharp surface overlain by a lenticular glauconitic bioclastic limestone. This represents a transgressive surface.
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Fig. 12. Metre-scale cycle of the Zhangxia Formation consisting of ribbon limestone in the lower part, made up of thin storm deposits, and thicker bedded oolite forming the upper part. Cycle 1.8 m thick. Hunyuan section, Shanxi Province.
Fig. 13. Metre-scale cycles of the Zhangxia Formation of the Western Hills, Beijing, each consisting of a shaley lower part (dark) and an oolitic upper part (light grey).
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There is no evidence of exposure, although this may have been removed during the ravinement. The glauconitic horizons in the parasequences of the second cycle set indicate that this part of the succession represents the maximum flooding/condensed section of the sequence. During deposition of the Zhangxia Formation, the Cambrian transgression continued to extend farther across the North China Platform, so that now the whole area was covered by shallow-to-moderate depth subtidal environments, except for some local topographic highs in the western and central part of the platform in the Ordos area, which were still exposed as islands (Fig. 4).
5.3. Sedimentology and sequence stratigraphy of the Upper Cambrian Three formations occur within the Upper Cambrian; the lower Gushan and Changshan formations are interpreted to constitute one 3rd-order sequence, Sequence eS6, and the uppermost formation, the Fengshan, is interpreted to be Sequence eS7. The Upper Cambrian deposits are thicker in the north than in the south and the facies distribution shows that the water depths were now greater in the north than in the south. In general, transgression was more extensive in the Upper Cambrian, compared to the Middle Cambrian (Fig. 4). From south to north, there were now major changes in the palaeogeography with tidal flats in the south passing into a storm-dominated ramp system and a moderately deeper-water marine basin; this is shown schematically in Fig. 6. The ramp-depositional system consisted mainly of thin-bedded limestones with storm deposits and local thrombolitic and reefal mounds.
5.3.1. The Gushan and Changshan formations, ~3G and e~s, Sequence ~$6 The basal surface of the Gushan Formation (Sequence eS6) in the northern part of the North China Platform is a flooding surface with mudrocks above resting on oolites of Sequence eS5 (Zhangxia Formation) below. However, in the southern part of the platform, to the south of a line from Handan to Laiwu (Hebei Province), it is difficult to separate Sequence eS6 from Sequence eS5, since most of the deposits of the late Middle to Late Cambrian
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here are tidal-flat dolomites. This region became the shoreline-inner-ramp part of the platform as a result of a tilting event and change in palaeoslope (see Section 7.4). Indeed, in this area there may well be stratigraphic breaks which are difficult to recognise because of the paucity of fossils. The Gushan Formation has a thickness ranging from 30 to 80 m and the succeeding Changshan Formation is 20-50 m thick. Sketch graphic logs of the Gushan and Changshan formations of Hunyuan, Datong are shown in Fig. 14. In general the Upper Cambrian strata of the northern part of the North China Platform are dominated by storm deposits. Bioherms are also a distinctive feature of the strata, and some show the effects of storm erosion. The mounds are composed of stromatolites, microbial laminites, oncolites, calcareous algae/cyanobacteria (Epiphyton, Girvanella, etc.), crinoids and bryozoans. The size of the bioherms ranges from 0.5 to 6 m across and up to 3 m in height (see Figs. 15 and 16). Three major storm facies occur within the Upper Cambrian sediments (Meng et al., 1986,b; Ma, 1993): Facies A consists of storm density-current deposits ('tempestites'), ranging in thickness from a few cm to 20 cm. They form ribbon limestones, where the storm beds, a few centimetres thick, alternate with thin shales (Fig. 17). Deposition took place below storm wave-base in the deep-water, outerramp environment. Facies B consists of graded beds, up to 30 cm thick, with hummocky cross-stratification (wavelength up to 50 cm). Grazing burrow structures of the Zoophycos type are common. These beds were deposited above storm wave-base by storm currents and waves. Facies C consists of intraclastic conglomerates, typically 5-20 cm in thickness (Figs. 17 and 18), with sorting and imbrication of tabular clasts, and vertical ('edgewise') and radiating arrangements too. These intraclastic, flat-pebble conglomerates occur in other Upper Cambrian successions, e.g., in Virginia (Whisonant, 1987), and an identical 'flakestone' facies is typical of Late PrecambrianCambrian strata generally (see Tucker, 1992). In most cases, the intraclasts are the result of the disruption and reworking by storms of incipient hard-
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