sinian through permian tectonostratigraphic …carroll/publications/pdf/carroll et...paleozoic and...

47

Geological Society of AmericaMemoir 194

2001

Sinian through Permian tectonostratigraphic evolution of the northwestern Tarim basin, China

Alan R. CarrollDepartment of Geology and Geophysics, 1215 West Dayton Street, Madison, Wisconsin 53706 USA

Stephan A. GrahamEdmund Z. Chang

Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305 USACleavy McKnight

Department of Geology, Baylor University, P.O. Box 97354, Waco, Texas 76798 USA

ABSTRACT

Sinian through Permian sedimentary rocks of the Kalpin and Bachu uplifts,northwest Tarim basin, record three major periods of basin evolution, as representedby stratigraphic megasequences divided by major unconformities. Each mega-sequence is marked by distinctive sedimentary facies, sediment dispersal patterns,sandstone provenance, subsidence history, and in two cases coeval magmatism. Thesame megasequences are recognized in both the surface Kalpin and largely subsur-face Bachu uplifts, indicating that these areas shared an essentially identical historyat least through the end of the Paleozoic. The Sinian–Ordovician megasequence over-lies an angular basal unconformity with older metamorphic rocks. Siliciclastic faciesdirectly above the unconformity are coarse grained and contain interbedded basaltflows. These facies grade upward into shallow-marine limestone and dolomite and interbedded deeper marine graptolitic shale, apparently as a result of thermal subsi-dence following a period of extension. Silurian and Devonian facies unconformablyoverlie Middle Ordovician strata, and are exclusively siliciclastic. They grade upwardfrom green shelfal siltstone and sandstone into red fluvial sandstone and mudstone;paleocurrent indicators within the fluvial facies indicate derivation from the east.These deposits correspond in age with a proposed suture in the Altyn Tagh range ad-jacent to the eastern Tarim basin, suggesting that they may have been shed from a ris-ing collisional orogen in that area. A pronounced angular unconformity separatesDevonian strata from Carboniferous to Lower Permian fluvial and marine facies,which contain quartz-rich sandstone derived from recycling of underlying strata.Carboniferous–Permian rocks include relatively deep marine Carboniferous faciesthat are preserved in the most northwestern outcrop exposures of the Kalpin uplift.These progressively lap out to the southeast, where only thin Lower Permian fluvialand shallow-marine facies are preserved. These facies and thickness relationships sug-gest deposition in a flexural foreland basin, brought about by an ongoing collision be-tween the Tarim and the central Tian Shan blocks. Lower Permian fluvial faciesinterbedded with basalt flows sharply overlie the marine facies in the Kalpin uplift.The basalts are closely tied in age with northwest-southeast–trending dikes, sills, andplutons in the Bachu uplift. The significance of this magmatism is unclear, but it mayrelate to limited extension normal to the collisional front.

Carroll, A.R., et al., 2001, Sinian through Permian tectonostratigraphic evolution of the northwestern Tarim basin, China, in Hendrix, M.S., and Davis, G.A., eds.,Paleozoic and Mesozoic tectonic evolution of central Asia: From continental assembly to intracontinental deformation: Boulder, Colorado, Geological Society ofAmerica Memoir 194, p. 47–69.

48 A.R. Carroll et al.

INTRODUCTION

The Tarim basin of northwest China (Fig. 1) contains asmuch as 13–14 km of Sinian through Quaternary sedimentaryfill deposited over several major episodes of basin subsidence,and represents one of the most important tectonic elements ofcentral Asia. It provides one of the longest and most complete

records of regional tectonics and paleogeography available onthe continent. The Tarim basin also contains significant petro-leum reserves, which recently have been the subject of explo-ration interest (e.g., Gao and Ye, 1997). Several authors havewritten general summaries of the basin as a whole (e.g., Tian et al., 1989; Wang et al., 1992; Li et al., 1996), focusing on itslarge-scale tectonic evolution and petroleum potential. The ef-

TARIM BASIN

Manjaer

Depression

TIBETAN PLATEAU

KuqaAksu

80° 85°

40°

40°

35° 200 km

ThrustStrike-Slip

MajorFault Suture

Cenozoic sedi-mentary coverPre-Cenozoicrocks

Major sub-surface high

Korla

N

KALPIN UPLIFT

TIANSHAN

CHINA

MAPAREA

AWAT

SHAJINGZI

KALPINAQIA

BACHU

ARTUXSUGUN

PIQIANG

SANCHAKOU

SELIBUYA

XIAOHAIZIRESERVOIR

AKSU UPLIFT

HALAQITOXKAN RIVER

TARIM RIVER

AKSU RIVER

HO

TIAN

RIV

ER

XIKEERRESERVOIR

YA

SA

NG

DI F

AU

LT

AQ

IA FA

ULT

KALPINTAGH

FAULT

PIQIANG FAULT

100 km

ALTYN TAGHALTYN TAGH FAULT

Hotian

Minfeng

Tazhong-1well

South Tian ShanCentral Tian Shan

FIGURE 2

RuoqiangBachu

Uplift

N

85°

Figure 1. Location of Tarim basin (modified from Chen et al., 1985; McKnight, 1993). Yasangdi and Aqia faults (dashed) are known from sub-surface seismic studies and have little surface expression.

Tectonostratigraphic evolution of the northwestern Tarim basin 49

fects of Cenozoic deformation during the Indian collision areparticularly striking, hence a number of studies have consideredthe role of these structures in accommodating regional strain(e.g., Tapponnier and Molnar, 1979; McKnight et al., 1989;Nishidai and Berry, 1990; McKnight, 1993; Dong et al., 1998;Yin et al., 1998; Allen et al., 1999). In contrast, the sedimentaryrecord of earlier deformations within and adjacent to the Tarimblock has received far less attention (cf. Hendrix et al., 1992;Carroll et al., 1995; Allen et al., 1999)

Precambrian rocks crop out in ranges surrounding all sidesof the Tarim basin (Chen et al., 1985). In contrast to the Paleozoicoceanic or accretionary substrate that underlies the Junggar basinto the north (Hopson et al., 1989; Kwon et al., 1989; Carroll et al., 1990; Sengör et al., 1993; Allen et al., 1995), theTarim basin is most likely entirely underlain by Precambrianbasement (Zhang et al., 1984; Li et al., 1996), although the nature of this basement beneath the cryptic Manjaer de-pression remains open to speculation (Sengör et al., 1996; Fig. 1).Cenozoic thin-skinned folding and thrusting of the Kalpin uplift,which probably occurred above a basal decollement in UpperCambrian evaporites, is well documented at a reconnaissancescale (Nishidai and Berry, 1990; McKnight, 1993; Yin et al.,1998; Allen et al., 1999). Estimates of shortening derived fromthese studies range from 20% to 50%. Additional Paleozoic out-crop exposures are available within the Bachu uplift (Fig. 1), alargely subsurface structural high that intersects the Kalpin uplift.

Sinian and Paleozoic strata of the northwest Tarim basinhave been previously studied by various Chinese researchers,but unfortunately this literature is difficult to access and evalu-ate by workers outside China (due to difficulties in aquiring andtranslating Chinese publications, incomplete reporting of spe-

cific field and laboratory data sets, and problems in verifying theaccuracy of the data that are reported). The purpose of this paper is to examine the evolution of major sedimentary mega-sequences exposed adjacent to the northwestern Tarim basin,and to interpret the tectonic record they provide. This study isbased on investigations we conducted over a number of fieldseasons (1987, 1988, 1991, and 1992) in the Kalpin and neigh-boring Bachu uplifts (Fig. 1). Our studies have focused in twoprincipal geographic areas: the region between Aksu and villageof Yingan in the northeastern Kalpin uplift, and near the Xiao-haizi reservoir in the northwestern Bachu uplift.

STRATIGRAPHY AND SEDIMENTARY FACIES

Sedimentary rocks of the northwestern Tarim basin may besubdivided into four distinct tectono-stratigraphic packages,based on their internal characteristics and on the position of major angular unconformities. Because each of these packagesrepresent major, discrete phases of basin evolution, we refer tothem as megasequences (see Hubbard et al., 1985, and Hubbard,1988, for further discussion of the megasequence concept). Verysimilar facies are present in both the Kalpin and Bachu uplifts,but surface exposures are stratigraphically less complete and ofpoorer quality within the Bachu uplift. The following descrip-tions refer principally to Paleozoic rocks in the northeasternKalpin uplift near Aksu, Sishichang, Wushi, and Yingan, and inthe northwestern Bachu uplift near the Xiaohaizi reservoir(Figs. 1 and 2). Age assignments are supported by a variety ofpreviously reported fossil evidence, summarized in Table 1, andby limited radiometric dating of volcanic units, as described inthe following.

0 km 15

N

Wushi

Sishichang

Cenozoic

Carboniferous-Permian

Silurian-Devonian

Cambrian-Ordovician

Proterozoic Aksu Group

Yingan

41° 10' N

79° 45' E

79° 30' E

41° 00' N

40° 50' N

79° 15' E

40° 50' N

79° 30' E

35

31

Sinian

Thrust fault

60

28

Fault - type unknown

32

42

91-Si-6

91-Sh-2,5,6

91-Su-1,2

91-Su-8

91-Si-4

Figure 2. Geology of Aksu-Yinganarea, northeastern Kalpin uplift (mod-ified from unpublished 1:200 000 geologic mapping of Xinjiang Bureauof Geology and Mineral Resources).Triangles indicate position of apatitefission-track samples (see Dumitru etal., this volume).


Sinan to Ordovician megasequence

The oldest rocks of the Kalpin uplift are Upper Proterozoicblueschist-greenschist facies metamorphic rocks of the AksuGroup, exposed locally near Aksu (Figs. 1 and 2). These rocksrecord high pressure-temperature (P-T) conditions associatedwith subduction-accretion or collision in a poorly known tec-tonic setting, and have yielded metamorphic ages ranging from698 to 754 Ma (K-Ar and Rb-Sr ages of 698–728 Ma from phen-gite, and a 40Ar/39Ar age of 754 Ma from crossite; Liou et al.,1989, 1996; Nakajima et al., 1990). These rocks compose crystalline basement for the overlying unmetamorphosed sedi-mentary sequences. They are intruded by a series of northwest-southeast–trending diabase dikes. Liou et al. (1989, 1996)interpreted these dikes to be pre-Sinian, but the dikes have notbeen directly dated.

Proterozoic sedimentary rocks have been subdivided into alower clastic section assigned to the lower Sinian (the Qiaoen-bulak and Yulmenack Formations) and a lithologically diverseupper Sinian section (Sugaitebulake and Qegebulake Forma-tions; Gao et al., 1985; Fig. 3). We have not observed the lowerSinian section, but Gao et al. (1985) reported that it locally

reaches &2000 m in thickness and includes glaciogenic turbiditefacies of the Qiaoenbulake Formation that were deformed priorto the deposition of tillite facies of the Yulmenack Formation.

The upper Sinian rocks are exposed only at the east end ofthe Kalpin uplift (Fig. 2) and include a basal conglomeraticphase (Sugaitebulake Formation) that laps unconformably ontothe Aksu Group. The character of these deposits is locally vari-able, ranging from boulder conglomerate to pebbly, coarsesandstone. The conglomerate contains clasts of underlyinglithologies, including diabase similar to the Aksu Group dikes.The clasts are moderately to well rounded, with a maximum di-ameter 1.3 m. The conglomerate grades upward into interbed-ded red mudstone and fine- to coarse-grained sandstone of theSugaitebulake Formation (Fig. 4A). The sandstone beds are typically 20 cm to 1 m thick, lenticular, and contain meter-scaleplanar and trough cross-beds (Fig. 4B), amalgamated beds, andripples. We interpret these facies to represent braided stream de-posits. This interval also contains at least three stratiformbasaltic units (Fig. 4A), which we interpret to be flows on thebasis of their vesicular character and possible columnar jointing.However, it is also possible that some units could be sills (R.Ressetar, 1999, personal commun.).


0

NEAR WUSHI

U. C

arbo

nife

rous

L. C

arbo

nife

rous

Dev

onia

n

BijingtawuFormation

KontaiaikenFormation

KeziertageFormation

1.5

KM

NEAR YINGAN

0

0.5

1.0

0.5

1.0

Aksu Group

BRAIDED FLUVIAL TO BEACHIN UPPER PART. EPISODIC

BASALT ERUPTIONS

AGE FORMATION

Neo

gene

Upp

er P

erm

ian

Low

er P

erm

ian

Up.

C

arb.

Dev

onia

nL.

Silu

rian

Mid

.O

rd.

L. O

rdov

icia

nM

.-U.

Cam

.L.

Cam

.U.

Pro

t.U.

Sin

an

Shajingzi

Kaipaizileke

Kupukuziuman

Yimungantawu

Tataaiertage

Kalpintage

Yingan+QilangKanling+Saergan

Qiulitage

Awatage+Xiaoerbulake

Qegebulake

Sugaitebulake

Xiaoerbulake+Wusongger

THICK-NESS (km)

LITHOLOGY

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

ANGULAR

INTERPRETED ENVIRONMENT

ANGULAR

ANGULAR

PALEOKARST

ALLUVIAL FAN

SEMI-ARIDFLUVIAL PLAIN

EPISODIC BASALTICERUPTIONS

SEMIARIDFLUVIAL PLAIN

FLUVIAL ANDCOAL-SWAMP

EPISODIC BASALTICERUPTIONS

CARBONATE BEACHFLUVIAL

COASTAL BRAID PLAIN

BRAIDED FLUVIAL

SHOREFACEINNER SHELF

SILICLASTIC MID-SHELFCARBONATE SHELF

ANOXIC, STARVED SHELF

HIGH-PRODUCTIVITYCARBONATE SHELF

CARBONATE INNER SHELFTO ARID BEACH

CARBONATE INNER SHELFTO BEACH

PHOSPHORITE AT BASECARBONATE INNER SHELF/

BEACH

SUBDUCTION COMPLEX (?)

BRAIDED FLUVIAL

Sishichang-Kangkelin

SINAN-ORDOVICIAN

MEGA-SEQUENCE

SILURIAN-DEVONIAN

MEGA-SEQUENCE

CARBON-IFEROUS-PERMIAN

MEGA-SEQUENCE

CENOZOICMEGA-

SEQUENCE 91-Si-6

91-Si-4

91-Sh-6

91-Sh-5

91-Sh-2

91-Su-8

91-Su-291-Su-1

Figure 3. Stratigraphy of eastern Kalpin uplift (based on Xinjiang Stratigraphic Table Compiling Group, 1981, and our field investigations). Tri-angles indicate positions of apatite fission-track samples (see Dumitru et al., this volume).

The upper Sugaitebulake Formation contains interbeddedred mudstone and sandstone with planar-parallel lamination tolow-angle cross-stratification, and limestone containing flat-pebble intraclast conglomerate and stromatolite. We interpretthis succession as marking a transition from nonmarine to shallow-marine depositional conditions. The uppermost SinianQegebulake Formation consists of interbedded limestone,dolomite, and mudstone. The carbonate facies are characterizedby abundant stromatolite (Fig. 4C) and flat-pebble conglomer-ate, and are interpreted as intertidal to supratidal. The Qegebu-lake Formation is truncated by an unconformity that Gao et al.(1985) inferred to represent only a brief period of time, but that is noteworthy for the paleokarst features associated with it(Fig. 4, D and E).

Cambrian and Lower Ordovician strata exposed in the east-ern Kalpin uplift constitute a 1–1.5-km-thick section of predomi-nantly carbonate facies (Figs. 2 and 3). Except for a very thin,oolitic phosphorite-shale interval at its base, Lower Cambrianfacies largely are dominated by stromatolite and flat-pebble con-glomerate facies, interpreted as representing intertidal to suprati-dal environments. Dolomitic Middle to Upper Cambrian strataare also stromatolitic (Fig. 5A) and apparently record periodsof restriction and emergence, based on reported interbeddedevaporite (Fan and Ma, 1991) and observed red mudstone facies.

Topographically prominent massive limestone beds of theoverlying Lower Ordovician Qiulitage Formation consist ofthin-bedded micrite to calcarenite with diverse, normal marinefauna (Fan and Ma, 1991) indicating shallow shelf to supratidal


Figure 4. Outcrop photographs of Sinian sedimentary facies southwest of Aksu. A. Siliclastic strata of Sugaitebulake Formation, interbedded withbasalt flow in lower third of view. Thickness of exposed section is approximately 200 m. B: Cross-beds in basal upper Sinian Yurmeinake For-mation sandstone, interpreted to represent braided fluvial environment. Note that sense of flow in this view is predominantly left to right (to south-east) C: Stromatolite in lowermost part of uppermost Sinian Qegebulake Formation, documenting shallow-marine depositional conditions. D: Sinian-Cambrian boundary (marked by change to more resistant-weathering massive Cambrian carbonate facies in upper half of exposure).Thickness of exposed section is approximately 50 m. E: Brecciated Sinian carbonates in karst zone immediately below contact with Cambrian.


Figure 5. Outcrop photographs of Cambrian and Ordovician facies between Sishichang and Sanchakou (see Figs. 1 and 2 for locations). A: Stro-matolite hemispheroids exposed on bedding-plane surface in Middle–Upper Cambrian Shayilike Formation dolomite, southwest of Aksu. B: Lower Ordovician Qiulitage Formation limestone facies of Bachu uplift, exposed at Sanchakou. C: Nodular shelf limestone of Middle Or-dovician Yingan Formation. D: Storm-deposited bed in Middle Ordovician Yingan Formation (carbonate intraclasts concentrated in layer markedby compass; also see Fig. 7).

environments. Similar Qiulitage Formation facies represent theoldest strata exposed within the Bachu uplift at several locali-ties, including Sanchakou (Figs. 1 and 5B).

The Middle Ordovician generally reflects deeper water, butprobably still shelfal environments, in the Kalpin uplift. TheSaergan Formation is a graptolitic, pyritic, carbonaceous, lami-nated black shale interpreted to represent anoxic to suboxic de-positional conditions (Graham et al., 1990). The SaerganFormation has been reported to occur in the Bachu uplift (Xin-jiang Stratigraphic Table Compiling Group, 1981), but was notinvestigated during this study. Overlying Ordovician formationsrepresent a return to a well-oxygenated carbonate shelf envi-ronment (Fig. 6). These rocks are extensively bioturbated andoccasionally punctuated by conglomeratic and fossiliferousbeds 5–10 cm thick, interpreted as storm deposits (Figs. 5, C and

D, and 7). Ordovician units above the Saergan Formation havenot been reported from Bachu uplift outcrops.

Silurian to Devonian megasequence

Lithology changes sharply and markedly across an uncon-formity that separates the Middle Ordovician carbonate se-quence from the overlying Lower Silurian green siliciclasticstrata of the Kalpintage Formation (Figs. 6, 7, and 8, A and B).This unconformity locally removes all or part of the OrdovicianYingan and Qilang Formations (Fan and Ma, 1991), indicatingerosional relief of tens to hundreds of meters. At Sishichang, adiscontinuous, fossiliferous conglomeratic lag 61 m thick ison the unconformity surface (Fig. 7). The lower KalpintageFormation consists of interbedded fissile green siliciclastic


ANOXIC

SHELF OR

BASIN

BIOTURBATED

MUDDY SHELF

SHOREFACE

SHELF OR

SANDFLAT

MARINE-NONMARINE TRANSITION

BRAIDED

FLUVIALFLUVIAL PLAIN

FLUVIAL

STRANDLINE

SHALLOW SHELF

FORMATIONAGE

S K QILANG YMIDDLE ORDOVICIAN

KALPINTAGELOWER SILURIAN

TATAAIERTAGE YIMUNGANTAWU

DEVONIANKANGKELIN

"Upper Carboniferous"

ANOXIC MARINESHALE

SHELFALCARBONATE

SILICLASTIC SHELFDEPOSITS

FLUVIALSANDSTONES

REDBEDS

STORM-

DOMINATED

SHELF

SENW

Figure7

Figure9

Figure10

100 M

Figure 6. Outcrop transect through Paleozoic sedimentary rocks near Sishichang.

LAG

YIN

GA

N F

OR

MA

TIO

N

MID

DL

E O

RD

OV

ICIA

NL

OW

ER

SIL

UR

IAN

KA

LP

INTA

GE

FO

RM

AT

ION

0

10

20

30

40

M

INTERBEDDED GREENRIPPLED TO LAMINATEDSANDSTONE AND SILTSTONE

PLATY-WEATHERING, GREENSILTSTONE AND MUDSTONE

RIPPLED SANDSTONE

LENTICULAR,GLAUCONITIC,\CONGLOMERATIC, FOSSILIFEROUS SANDSTONE

"TEMPESTITE"

"TEMPESTITE"

NODULAR-WEATHERING,EXTENSIVELY BIOTURBATEDFOSSILIFEROUS LIMESTONEAND THINLY INTERBEDDEDGREEN MUDSTONE

THIS STUDY

TRADITIONAL

SIL

ICIC

LA

ST

IC S

HE

LF

CA

RB

ON

AT

E S

HE

LF

Figure 7. Measured outcrop section though Ordovician–Silurian con-tact near Sishichang.

mudstone, siltstone, and sandstone, interpreted to be depositedon a storm-dominated shelf. The siltstone is calcareous, planar-parallel to wavy laminated, and locally nodular and bioturbated.Thin fine- to medium-grained sandstone beds are planar-parallel laminated to rippled, and locally contain coarse-grainedglauconitic layers, bedding-plane trace fossils, and centimeter-scale grooves on bed soles. This facies association is interbed-ded with medium- to coarse-grained sandstone with pervasivetrough cross-beds to 1 m in amplitude, interpreted as shorefacedeposits. The upper Kalpintage Formation includes a lenticular,upward-fining succession of white sandstone beds depositedabove a basal scour, with meter-scale trough cross-beds and15–20 cm mud balls at the base, interpreted as a tidal channel de-posit. Possible tidal deposits have also been reported near the vil-lage of Yingan (Fig. 2; S.J. Vincent, 1999, personal commun.).

Redbeds with local mudcracks and paleosols increase infrequency upward near the top of the Kalpintage Formation; therocks are mapped as the Tataaiertage Formation at the pointwhere redbeds dominate. The Tataaiertage Formation was for-merly assigned to the Devonian (Xinjiang Stratigraphic TablesCompiling Group, 1981), but the transitional nature of the con-tact with the underlying green, marine Silurian rocks suggeststhat it is fully conformable with the underlying Kalpintage For-mation. Fan and Ma (1991) assigned a Silurian age to theTataaiertage Formation based on the gradational nature of thecontact and the local occurrence of Silurian marine fossils(Table 1). They inferred that previously reported plant fossils(Lepidodendropsis) were actually not found in place. TheTataaiertage section contains mostly unfossiliferous, troughcross-bedded sandstone interpreted to represent braided fluvialdeposits (Figs. 8C and 9). Finer-grained overlying facies of theYimungantawu Formation may represent deposits on a broadfluvial plain. The Yimungantawu Formation has been assignedto the Lower Devonian on the basis of nonmarine bivalves (Fanand Ma, 1991; Table 1). The overlying Keziertage Formation,


Figure 8. Outcrop photographs of Silurian and Devonian facies near Sishichang. A: Unconformable contact between Middle Ordovician (lightcolored rocks on right) and Lower Silurian (dark colored rocks on left). Apparent angularity is artifact of angle of view at this location. B: Overview of siliciclastic facies of Kalpintage and Tataaiertage Formations, looking south (standing near base of section; also refer to Fig. 6).Total thickness of strata in this view is &500 m. Foreground consists of Kalpintage Formation green mudstone and sandstone, interpreted as storm-dominated shelf deposits. Similar facies continue into partially covered strike valley, where they are interbedded with trough cross-beddedshoreface sandstone. Ridge in background contains lenticular, cross-bedded sandstone facies interpreted as shoreface to tidal deposits, overlainby tidal to coastal plain red mudstone mapped as base of Tataaiertage Formation. Resistant beds at ridge on skyline (left) consist of TataaiertageFormation red sandstone facies interpreted as braided fluvial deposits. C: Cross-bedded sandstone of Tataaiertage Formation, interpreted asbraided fluvial deposits. These cross-beds indicate predominant flow direction to west.

which locally was completely eroded away, contains troughcross-bedded sandstone facies interpreted as a return to braidedfluvial deposition.

We observed a very similar Silurian–Devonian section inthe Bachu uplift near the Xiaohaizi reservoir (Fig. 1). The Xi-aohaizi section, however, is cut by numerous diabase dikes andsills, which obscure stratigraphic relationships. The Xiaohaizisection also includes several intervals of fluvial quartz-pebble

conglomerate beds with scoured bases. These are presumed tobe Devonian, but are unfossiliferous.

Carboniferous to Permian megasequence

Kalpin uplift. Carboniferous and Permian facies of thenortheast Kalpin uplift were described in detail by Carroll et al.(1995); these descriptions are therefore not repeated here.


SH

AL

LO

W, B

RA

IDE

D-F

LU

VIA

L S

YS

TE

M

17

15

M

10

5

0

DE

VO

NIA

N

TATA

AIE

RTA

GE

FO

RM

ATIO

NPALEOCURRENTDIRECTION

MEANPALEOCURRENTDIRECTION (N=8)

MEDIUM-SCALETROUGH CROSS-BEDDED SANDSTONE

MUDSTONE CHIPCONGLOMERATE

RED MUDSTONE

LOW ANGLE, PLANARCROSS-BEDDED SANDSTONE

Figure 9. Measured section in Tataaiertage Formation 5 km northeastof Sishichang (see Fig. 6 for stratigraphic position).

Devonian (and possibly Silurian) strata are extensively trun-cated beneath an angular unconformity cut across the entireKalpin uplift. The oldest strata above the unconformity are pro-gressively younger to the southeast. Near Wushi (Fig. 2), at least2000 m of carbonate and siliciclastic facies as old as Early Car-boniferous unconformably overlie Cambrian through Silurianrocks. This succession deepens upward from fluvial gravels tosiliciclastic turbidites, which then grade into shelf sandstoneoverlain by carbonate olistostrome. On the basis of regional re-lationships, the latter facies are interpreted to have been shedfrom northwest-facing carbonate platforms. At Sishichang (Fig. 2) the Lower Permian Sishichang Formation uncon-formably overlies Devonian redbeds (Carroll et al., 1995). Flu-vial conglomerate and sandstone grade upward into tidalsandstone facies and shallow-marine algal and skeletal lime-stone of the Kangkelin Formation (Fig. 10; Carroll et al., 1995).Clasts in the conglomerates are mostly sedimentary, and havelithologies similar to underlying Devonian strata.

The Kangkelin Formation is sharply overlain by variegatedred-green, nonmarine Permian strata of the Kupukuziman,Kaipaizileke, and Shajingzi Formations (Fig. 3). Carbonaceousstrata, rooted fossil trees, fossil leaf horizons, and thin coal bedsoccur sporadically throughout the lower half of this section,supporting the inference of at least a seasonally humid environ-ment. Two series of basaltic lavas, each of which consists ofmultiple thin flow horizons, occur within the Kupukuziman and

Kaipaizileke Formations and underlie a large area of the north-west Tarim basin (Chang, 1988; Liu and Li, 1991; Wang andLiu, 1991; Fig. 11). Each series totals &150–200 m in thickness,but is interbedded within thicker intervals of nonmarine sedi-mentary rocks; marine interbeds occur to the west (M.B. Allen,1999, personal commun.). Liu and Li (1991) and Wang and Liu(1991) presented geochemical evidence indicating that theflows range in composition from tholeiites to alkali basalts, con-sistent with within-plate magmatism. A variety of K-Ar whole-rock ages have been reported for these flows, ranging from293.25 ; 7.70 to 285.24 ; 6.67 Ma for the lower series and295.13 ; 7.1 to 228.83 ; 5.15 Ma for the upper series (Liu andLi, 1991; Wang and Liu, 1991). Carroll et al. (1995) reported an40Ar/39Ar age of 277.53 ; 0.46 Ma for plagioclase separatedfrom a flow in the lower series 20 km southwest of Sishichang,indicating an Early Permian age for these flows. Basalts of the northeast Kalpin uplift thicken from Sishichang to Yingan(Fig. 2), where dikes may indicate Permian vent areas. The Yin-gan area is the site of a major tear fault in the Kalpin thrustsheets (Figs. 1 and 2), possibly suggesting a long-lived zone ofstructural reactivation.

Age relationships within the nonmarine Permian exposuresabove the basalt flows are subject to controversy. Upper Per-mian rocks are most often depicted overlying an angular un-conformity above the Lower Permian (Xinjiang StratigraphicTable Compiling Group, 1981; unpublished 1:200 000 geologicmapping of the Xinjiang Bureau of Geology and Mineral Re-sources), but some workers maintain that this entire section maybe Lower Permian (Li Wunfeng, 1992, personal commun.). Thetotal original thickness of this interval is unknown; it is ero-sionally truncated and overlain by Cenozoic deposits.

Significant along-strike variations in Carboniferous–Permian stratigraphy occur within the Kalpin uplift. It generallyappears that thicker Carboniferous strata are present to thesouthwest (Xinjiang Stratigraphic Table Compiling Group,1981; unpublished 1:200 000 geologic mapping of the XinjiangBureau of Geology and Mineral Resources), although detailedfield data have not been reported.

Bachu uplift. Carboniferous and Permian rocks of theBachu uplift at Xiaohaizi are extensively intruded (Fig. 12, A and B), and generally are less well exposed than their Kalpinuplift equivalents. A mafic composite sill &50–100 m thick (Fig. 12A) either coincides with the position of the Devonian-Carboniferous contact, or else is slightly above it. The angularunconformity seen in the Kalpin uplift is not clearly visible atXiaohaizi. However, we measured &10° of dip discordance between Devonian strata beneath the sills and Carboniferousstrata above. The sills are cut by a composite syenitic to graniticpluton that radiates felsic dikes into the surrounding rocks (Fig. 12C). Lower Carboniferous to Lower Permian shallow-marine deposits overlie the sill complex and comprise inter-bedded mudstone, limestone, and poorly exposed gypsum (possibly nodular). We interpret these facies to represent back-barrier and lagoonal environments in which carbonate grains de-


ALLU

VIA

L F

AN

LO

WE

R

PE

RM

IAN

KA

NG

KE

LIN

SIS

HIC

HA

NG

F

OR

MAT

ION

M35

30

25

20

15

10

5

0

DE

VO

NIA

N

YIM

UN

TAN

TAW

UF

OR

MAT

ION

RED MUDSTONE

ALGAL LIMESTONE

MICRITE

WHITE, CROSS-BEDDEDAND RIPPLED SANDSTONEWITH THIN MUDSTONE BEDSORGANIZED, PEBBLECONGLOMERATE

POORLY ORGANIZED CGL.CLASTS COARSEN UPWARDTO 20 cm

RED MUDSTONE WITHTHIN RIPPLEDSANDSTONE BEDS

RED PLATY SANDSTONEAND MUDSTONE

UPWARD - FININGSANDSTONE BEDS

GRAY MUDSTONE GRADINGUPWARD TO SANDSTONE

ORGANIZED PEBBLE CONGLOMERATE

YELLOW SILTSTONE

ORGANIZED, IMBRICATEDCONGLOMERATE WITHCLASTS TO 40 cm

RED, POORLY ORGANIZEDCONGLOMERATE GRADINGTO UPPER COARSE SAND

ANGULAR UNCONFORMITY

Figure 10. Measured section through an-gular contact between Yingan andSishichang Formations 5 km northeast ofSishichang (see Fig. 6 for stratigraphicposition). CGL.—conglomerate.

rived from storm washover alternated with mudstone and gyp-sum deposition under restricted conditions. The upper part of thesuccession also contains brecciated limestone, algal laminites,and flat-pebble conglomerate indicating supratidal environ-ments. All of these facies are extensively cut by northwest-southeast–trending mafic dikes, which locally compose as muchas 20% of the total outcrop width. These dikes, and gabbroicdikes associated with an alkali igneous complex to the south ofXiaohaizi, have 40Ar/39Ar ages that are essentially identical tothe lower series of basalt flows discussed here (Carroll et al.,1995). However, the Xiaohaizi exposures do not include flows.

Groves and Brenckle (1997) used graphical correlationtechniques to infer that the Carboniferous–Permian successionat Xiaohaizi is actually far less stratigraphically complete thanit appears in outcrop. They argued that the Xiaohaizi section andseveral sections within the Kalpin uplift contain hiatuses thatrepresent at least as much geologic time as the preserved sedi-mentary rocks. Furthermore, these hiatuses are generally longerthan can be explained by third-order sea-level changes, and sug-gest instead control by local geologic processes. We did not ob-serve any obvious unconformities within this succession.

PALEOCURRENTS AND SANDSTONE PROVENANCE

Paleocurrent measurements were collected at several loca-tions from Sinian through Permian facies in the Aksu-Sishichang area (Fig. 13). Note that although the numbers ofmeasurements reported here are not statistically significant,they are representative of larger, visually identified populationsof similar features. Sandstone point-counts were conducted using a modified Gazzi-Dickinson method (Table 2; see Graham et al., 1993, and Ingersoll et al., 1984, for complete dis-cussions of techniques). The raw data were recalculated into detrital modes and plotted on standard ternary diagrams (Fig. 14) to permit comparison with previously recognizedprovenance types (cf. Dickinson, 1985).

Sinian paleocurrent measurements generally indicate trans-port directions to the south or southeast (Fig. 13A). These mea-surements represent mostly decimeter-scale planar cross-bedswithin coarse-grained, fluvial sandstone facies (e.g., Fig. 4B).Sinian sandstone samples vary widely in composition and re-flect mixed provenance (Fig. 14A), related both to metamorphicbasement lithologies and to interstratified Sinian volcanic rocks.


Kuqa0 300

km

N

Basalt Gabbro Fault

Conglomerate

Sandstone

Shale

Limestone

Basalt

Tertiary

Kaipai-zileike

Kupuku-ziman

0

1000

m

Sishi-chang

Wushi

Yingan

Kashgar

Shache

Yecheng

Hotian

Bachu

Heshen-2 well

Heshen-1 well

Shacan-1 well

Kaipai- zileike

Heshen-2well

Heshen-1well

YinganKaipaizileike

SishichangShacan-1

well

Sis

hich

ang-

Kan

gkel

inK

upuk

uzim

anFo

rmat

ion

Kai

paiz

ileik

eFo

rmat

ion

Low

er P

erm

ian

Figure 11. Occurrence of Lower Per-mian basalt in northwest Tarim basin(modified from Xinjiang StratigraphicTable Compiling Group, 1981; Chang,1988; Liu and Li, 1991; Wang and Liu,1991; J. Liu, 1995, personal commun.).Basalt intervals represented in upperpart of figure are actually composite in-tervals of interbedded basalt and sili-clastic sedimentary rocks; totalthickness of basalt flows is therefore lessthan represented.

The relatively high content of polycrystalline quartz (Qp) mostlikely reflects input from metamorphic rocks of the Aksu Group.

Cross-beds, ripples, and groove casts in Silurian sandstonefacies display mixed transport directions (Fig. 13B). In contrast,fluvial Devonian sandstone facies show strong west to south-west transport directions based on trough cross-beds, partinglineation, and grooves (Figs. 8C and 13C). Silurian and De-vonian sandstone contains mostly quartz and lithic rock frag-ments (Fig. 14B), suggesting a recycled orogenic provenance(cf. Dickinson, 1985). Together these observations circumstan-tially support the presence of a Devonian orogenic sediment

source area east of the study area. Uniformly high plagioclase/total feldspar (P/F) and locally high lithic volcanic/total lithic(Lv/L) ratios in Silurian–Devonian sandstones likely record apartly volcanic provenance, probably south of the Altyn Taghfault (Fig. 1).

Net northwest sediment transport in the Kalpin uplift areaduring the Carboniferous is expected from regional considera-tions, but our paleocurrent data (Fig. 13D) are too sparse to lendfurther support to this interpretation. Allen et al. (1999) reportednorthwest-directed paleocurrent directions in turbidites of theUpper Carboniferous Sasikebulake Formation (located west


Figure 12. Outcrop photographs of intrusions at Xiaohaizi reservoir (see Fig. 1 for location). A: South-dipping diabasic sills intruding Devonianand Carboniferous strata (utility poles in foreground are &10 m high). B: Small dike cutting cross-bedded Devonian fluvial sandstone, and feeding sill that has intruded cross-laminae. C: Sills shown in A (right), cut by pluton (left). Height of ridge at right is approximately 50 m aboveforeground.

of Halaqi; Fig. 1). Carboniferous to Lower Permian sandstoneolder than the Kupukuziman Formation is relatively quartzose(Fig. 14C), reflecting derivation from low-lying areas within the Tarim craton. Carboniferous subcrop maps indicate that Precambrian basement was nowhere exposed within the interiorof Tarim (Lu and Qi, 1994, personal commun.); pre-Kupukuzi-man Formation sandstone therefore was almost certainly derived from weathering and reworking of underlying Silurian–Devonian sandstone and conglomerate.

In contrast, Lower Permian sandstone of the KupukuzimanFormation marks a dramatic shift in both paleocurrents andprovenance (Figs. 13E and 14D). They are dominated by felsicvolcanic material derived from the northwest. Felsic-volcanicrock fragments with pyroclastic textures, granitic rock frag-ments, angular monocrystalline quartz, and potassium-feldspargrains are all common constituents. Carroll et al. (1995) inter-

preted these sandstones to be derived from rhyolitic volcanicsand related granites in the southern Tian Shan. However, northwest-directed paleocurrents have also been noted in time-equivalent sandstone facies with similar modal compositions located to the southwest of our study area (S.J. Vincent, 1999,personal commun.). This suggests the presence of another rhy-olitic or granitic detrital source to the southeast, most likelywithin the buried Bachu uplift.

SUBSIDENCE HISTORY

The subsidence history of the Aksu-Sishichang and Xiao-haizi areas was reconstructed using the backstripping methodsof Bond and Kominz (1984; also see Steckler and Watts, 1978;Van Hinte, 1978; Schlater and Christie, 1980, for further detailsof the method). Absolute ages are based on the time scale of


0 km 30

Sishichang

Cenozoic

Carboniferous-Permian

Silurian-Devonian

Cambrian-Ordovician

Proterozoic Aksu GroupYingan79° 45' E

40° 50' N

35

31

Sinian

60

28

0 km 30

Yingan79° 45' E

40° 50' N

35

31

28

Yingan79° 45' E

40° 50' N

35

31

28

Yingan

35

31

28

Yingan

40° 50' N

35

31

60

28

sishichang xbeds

N

N=12

NE crossbeds

N

Cross-beds

N=26N=4

Parting lineation

Cross-beds

N=7

N

Grooves

N=4Cross-beds

N=23Ripples

N=6

N

Crossbeds

Cross-beds

N=52

N=13

N

N

Imbrication

N=3

N

sish crossbedsCross-beds

N=7

N

N

LEGEND

A. SINIAN

B. SILURIAN C. DEVONIAN

D. PERMIAN-SISHICHANG FORMATION E. PERMIAN-KUPUKUZIMAN FM.

N=44

N

Cross-beds

Cross-beds

Figure 13. Paleocurrent summary for Aksu-Yingan area (see Figs. 1 and 2 for location).


Lv Lsm P K

Qp QmLv Lsm P K

Qp Qm

Lv Lsm P K

Qp QmLv Lsm P K

Qp Qm

Qm

F Lt

Qm

F Lt

Qm

F Lt

Qm

F Lt

D.Permian

(Nonmarine)(n = 11)

C.Carboniferous-

Permian (Marine)(n = 14)

B.Silurian-Devonian(n = 11)

A.Sinian(n = 10)

Figure 14. Modal compositions of sand-stone samples (see Table 2 for raw dataand text for further explanation). Qm—monocrystalline quartz, F—totalfeldspar, Lt—total lithics + polycrys-talline quartz, Qp—polycrystallinequartz, Lv—volcanic lithics, Lsm—sedimentary + metamorphic lithics, P—plagioclase, K—potassium feldspar.

Harland et al. (1990), except for the Permian. Permian ages arebased on Ross et al. (1994) because this time scale more closelyfits known Lower Permian paleontological and radiometric con-straints at Sishichang. The lower age limit of Sinan sedimentaryrocks near Aksu is very well defined by radiometric dating ofmicas contained in pelitic schist underlying the basal uncon-formity (Nakajima et al., 1990). A late Sinian (Vendian) age isfurther supported by reported acritarch and stromatolite occur-rences (Gao et al., 1985). The amount of missing time repre-sented by the basal unconformity, however, is unknown.Stratigraphic thicknesses for all units are based on appropriatereference sections (Xinjiang Stratigraphic Table CompilingGroup, 1981), modified on the basis of our field investigations.

Because of the incompleteness of individual outcrop sections,the reference sections each represent local composites of morethan one actual measured section. Some errors (on the order of10%–20%) may therefore have been introduced due to strati-graphic thickness variations, but generally the backstripped sec-tions are representative of the true vertical thickness of stratapresent at each location.

Paleobathymetries are based on our interpretation of ma-rine depositional environments, and approximate error rangesare estimated. Paleoelevations during deposition of nonmarineunits represent best guesses, based on the occurrence of similardepositional environments elsewhere and on the present eleva-tion of northwest Tarim; estimated error ranges are correspond-


ingly large. Unconformities are based on missing strata as indi-cated by the Xinjiang Stratigraphic Tables, paleontologic andradiometric data of Carroll et al. (1995), graphic correlation ofCarboniferous and Permian marine fauna by Groves andBrenckle (1997), and our field investigations.

The total and tectonic subsidence histories at both locationswere very similar at both localities during lower to middle Pa-leozoic time (Fig. 15), both in terms of timing and magnitude.Sinian through Cambrian rocks do not crop out within the Bachuuplift; direct comparison of subsidence during this intervaltherefore is not possible. We know of no reported well penetra-tions of these units in the Bachu uplift, but their presence hasbeen inferred in the subsurface (e.g., Fan and Ma, 1991). Tec-tonic subsidence rates in the Aksu-Sishichang area gradually de-creased from Sinian through Cambrian time, and appear to haveremained relatively constant and slow during the Ordovician.Greater Early Ordovician subsidence occurred at the Bachu up-lift, resulting in preservation of strata that are &50% thicker thanin the Aksu-Kalpin area. Upper Ordovician strata are not re-ported from the Xiaohaizi area, due either to erosional removalby the base-Silurian unconformity or simply to lack of expo-sure. We were not able to locate an exposed Ordovician-Siluriancontact in this area.

Subsidence rates increased in both areas during the Silurianfollowing development of the widespread basal-Silurian uncon-formity, and increased subsidence continued into the Devonian.The thickness of preserved Devonian strata varies greatly withinthe Kalpin uplift, depending on the magnitude of erosion at thebasal Carboniferous unconformity. For example, the Upper De-vonian Keziertage Formation is absent at Sishichang, but locallyreaches nearly 1200 m in thickness above the YimungantawuFormation (Xinjiang Stratigraphic Table Compiling Group,1981). At its maximum the Silurian–Devonian megasequence inthe Kalpin uplift reaches more than 2200 m in thickness in theKalpin uplift, and therefore records a significant period of tec-tonic subsidence.

The record of Carboniferous to earliest Permian subsidencevaries greatly depending on location. Most or all of this intervalis represented by an unconformity in the Aksu-Sishichang area,whereas sedimentation at Xiaohaizi initially appears to havebeen more continuous. Deposition of the Kupukuziman Forma-tion initiated a major new episode of basin formation, charac-terized by rapid subsidence.

Fission-track analysis of Sinian through Permian samplesfrom the Aksu-Sishichang area indicates that this entire sectioncooled below annealing temperature simultaneously during thelatest Permian to Early Triassic (details of these analyses and es-timates of the timing and magnitude of unroofing are presentedin Dumitru et al., this volume; these results are therefore not berepeated here). We infer that substantial erosion of Permianrocks occurred at this time (Fig. 15). There is no stratigraphic orfission-track evidence that large thicknesses of Mesozoic strataever covered the Kalpin uplift. Fission-track shortening of &15%

in these samples could have occurred either due to relatively minor Mesozoic burial, or due to burial beneath now-erodedCenozoic sediments. About 1700 m of Neogene-Quaternarynonmarine sedimentary rocks cover the Bachu uplift nearSelibuya (Fig. 1); involvement of these strata in Cenozoic thrust-ing (Allen et al., 1999) indicates that they may have originallyalso covered Paleozoic rocks in the Kalpin uplift.

DISCUSSION AND CONCLUSIONS

Sinian–Ordovician megasequence

The basal unconformity that marks the onset of Sinian de-position most likely corresponds with the initiation of rifting ofa preexisting Proterozoic continent. Shi et al. (1995) suggestedthat Sinian extension was followed by a period of thermal sub-sidence. The Sinian paleogeography of this region is very poorlyknown, but local variations in the thickness of lower Sinianstrata of &2000 m and the presence of basal upper Sinian boul-der conglomerate indicates substantial relief on the basal un-conformity. South-directed paleocurrents indicate either thatTarim Proterozoic rocks continue to the north of the study area,or that the original source area for these sediments subsequentlyrifted away. There is no evidence for renewed tectonic activityduring the latest Sinian through Ordovician; the observedchanges in sedimentary environments were apparently drivenmostly by changing relative sea level. The Tarim block occupiedlow paleolatitudes by the early Paleozoic (Fig. 16A), latitudesthat were conducive to extensive shallow-marine carbonate sedimentation. Graptolitic shale deposition probably recordsmaximum sea-level highstands.

Silurian–Devonian megasequence

The base-Silurian unconformity corresponds to a basin-wide event also observed in the subsurface north of the Tazhongstructure in the southeastern Tarim basin (site of the Tazhong-1well, Fig. 1; Li et al., 1996; R. Ressetar, 1999, personal com-mun.), with relief of at least hundreds of meters. Increased basinsubsidence and the reintroduction of siliclastic detritus indicatethat this unconformity records a new tectonic episode of basinevolution. The source area for Silurian to Devonian clastic sedi-ments is unknown, but may be a middle Paleozoic orogenic beltdeveloped to the east, within the Altyn Tagh range (Figs. 1 and16B). Sobel and Arnaud (1999) proposed the existence of a middle Paleozoic suture (termed the Lapeiquan suture) betweenArchean rocks of the Tarim block and a Proterozoic block to thesouth. The oldest arc intrusions within the Proterozoic blockwere dated as 435 ; 20 Ma, and crosscutting postorogenic gran-ite plutons were dated as 383 ; 7 Ma. Sobel and Arnaud (1999)therefore concluded that an intervening ocean basin closed afterthe Early Silurian and before the Middle Devonian. They alsopointed out that sedimentary rocks of Silurian to Middle


1000

0

- 1000

-2000

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-4000

-5000

-6000

-7000

0100200300400500600700

KALPIN UPLIFT(SISHICHANG-

YINGAN)

Ma

APPROXIMATEAPATITE

FISSION-TRACK COOLING AGE

UNCONFORMITIES:

EL

ELV

AT

ION

(m

)

TOTAL SUBSIDENCE

TECTONIC SUBSIDENCE

PALEO-BATHYMETRY/ELEVATION

CAM-BRIAN

ORDO-VICIAN

SILU-RIAN

DEVO-NIAN

CARBONIF-EROUS

PERM-IAN

TRIAS-SIC

JURAS-SIC CRETACEOUS TERTIARYSINIAN

0100200300400500600700

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-2000

-3000

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-5000

BACHU UPLIFT(XIAOHAIZI)

Ma

(NOT EXPOSED)

UNCONFORMITIES:

PALEO-BATHYMETRY/ELEVATION

"TECTONIC SUBSIDENCE"

TOTAL SUBSIDENCE

EL

ELV

AT

ION

(m

)

CAM-BRIAN

ORDO-VICIAN

SILU-RIAN

DEVO-NIAN

CARBONIF-EROUS

PERM-IAN

TRIAS-SIC

JURAS-SIC CRETACEOUS TERTIARYSINIAN

Figure 15. Subsidence histories for Kalpin and Bachu uplifts. Sishichang-Yingan history includes Devonian Keziertage For-mation, which is present at Yingan but eroded at Sishichang. Time scale after Harland et al. (1990); Permian time scale ismodified according to Ross et al. (1994). Apatite fission-track (AFTA) cooling age is from Dumitru et al. (this volume).


A. Ordovician

B. Devonian

15° N

0°

LAPEIQUAN

SUTURE

ALLUVIAL FAN

ALLUVIAL PLAIN

SHALLOW MARINE(MIXED CARBONATE ANDSILICICLASTIC)

SHALLOW MARINE (DOMINANTLY CARBONATE)

SHALLOW MARINE (EVAPORITE)

DEEP MARINELAVA FLOWS

DIKES

THRUST FAULT

SUBDUCTIONZONE

STRIKE-SLIPFAULT

POSSIBLE PERMIANNORMAL FAULT

# #

?

?

?

?

?

?

?

?

SEDIMENT DISPERSALDIRECTION

(NONDEPOSITIONOR LATER EROSION)

D. Late Early Permian

C. Early Carboniferous

30° N

35° N

200 km

#

#

#

#

??

?

?

SOUTH TIAN SHAN SUTURE

NORTH TIAN SHAN SUTURE

CENTRAL TIANSHAN

FOREBULGE?

Figure 16. Schematic paleogeographic reconstructions of Tarim basin and surrounding areas. Orientation and paleolati-tude of Tarim block compiled from paleomagnetic studies and summaries by Bai et al. (1987), Li et al. (1988a, 1988b), Li(1990), Sharps et al. (1989), Fang et al. (1990), and Zhao et al. (1996). Paleogeography from this study and from previ-ous work by Hu et al. (1965), Zhang et al. (1983), Lai and Wang (1988), Allen et al. (1991, 1993), Fan and Ma (1991),Wang et al. (1991), Zhou et al. (1991), Carroll et al. (1995), Chen et al. (1999), and Zhou et al. (this volume).

Devonian age are not known from this area. We noted the ab-sence of Silurian or Devonian sedimentary rocks along thesoutheastern margin of the Tarim basin (between Ruoqiang and Minfeng), although Devonian quartz-pebble conglomerates arepresent in the southwestern Tarim near Hotian (Fig. 1). To-gether, these observations suggest that an orogenic belt roughlyparallel to the present southeast margin of the Tarim basin may

have shed Silurian–Devonian clastic sediments to the northwest(Fig. 16B). The compositions of Silurian–Devonian sandstonesamples from the Kalpin uplift (Table 2; Figure 14B) are con-sistent with derivation from the preorogenic lithologies de-scribed by Sobel and Arnaud (1999). However, it is not clearwhat genetic relationship, if any, existed between the Lapeiquansuture and basin subsidence in the Kalpin and Xiaohaizi areas.


Apparently increasing rates of tectonic subsidence during theLate Devonian (Fig. 15) are suggestive of flexural subsidence(cf. Dickinson, 1976; Jordan, 1981), but this interpretation isspeculative due to the poor age data on these nonmarine facies.

Carboniferous–Permian megasequence

The base-Carboniferous angular unconformity is presentvirtually throughout the Tarim basin, including the Kalpin upliftand the southwest Tarim basin near Hotian (our field observa-tions), in the subsurface of the southeastern Tarim basin (Li et al., 1996), and in the Altyn Tagh range (Sobel and Arnaud,1999). Carroll et al. (1995) and Allen et al. (1999) proposed thata flexural foredeep formed along the northern Tarim margin inresponse to a Late Devonian to Early Carboniferous collisionbetween Tarim and the Yili block (Wang et al., 1990; Allen et al., 1991, 1993; Hsü et al., 1994; Gao et al., 1998; Fig. 16C).We further propose that flexural loading of the northern Tarimblock by the collisional orogen may have created a flexural fore-bulge with in the northern Tarim basin, coincident with theSishichang-Yingan area of the Kaplin uplift (Fig. 16C). A fore-bulge would explain the absence of Lower Carboniferous stratain the Sishichang-Yingan area (they are present at both Wushiand Xiaohaizi), and would help restrict marine circulation andpromote the deposition of evaporite facies.

Observations by Zhou et al. (this volume) north of Kuqa(Fig. 1) support at least limited Late Devonian to Early Car-boniferous uplift of the southern margin of the central Tian Shan.They report that the Erbin Shan granite, which intrudes Devo-nian strata and has a reported U-Pb age of 378 Ma (Hu et al.,1986), is unconformably overlain by Carboniferous conglomer-ate and carbonate facies bearing Visean marine fossils. This tim-ing of this nonconformity corresponds with the development ofthe Devonian–Carboniferous angular unconformity in the Tarimbasin, strongly suggesting a causal relationship. The presence ofLower Carboniferous marine facies within the Tian Shan doesnot support continued widespread orogenic uplift, however, asimplied by Carroll et al (1995) and Allen et al. (1999).

Paleomagnetic, stratigraphic, and petrologic evidence sug-gests that this collision was diachronous, occurring later to thewest (Gao et al., 1998; Fang et al., 1990; Li, 1990; Windley et al., 1990; Biske, 1995; Carroll et al., 1995; Chen et al., 1999;Zhou et al., this volume). Zhou et al. (this volume) note the pres-ence of blueschist facies metamorphic rocks as young as EarlyCarboniferous in the western Tian Shan along the suture be-tween the central and southern Tian Shan, and a coeval mag-matic arc along the southern edge of the central Tian Shanterrane. They infer that subduction continued in this areathrough the Early Carboniferous (Fig. 16C), as proposed by Gaoet al. (1998) and Chen et al. (1999). This conclusion requiresthat the foreland basin model proposed for the northern Tarimbasin by Carroll et al. (1995) and subsequently adapted by Allenet al. (1999) be modified in recognition that the actual collisionbetween the northwestern Tarim block and the central Tian Shan

terrane may not have occurred until the Middle Carboniferous.The origin of Early Carboniferous Tarim basin tectonic subsi-dence therefore becomes somewhat enigmatic. One hypothesisis that vertical movements of the northwest Tarim passive mar-gin occurred in response to flexure of the downgoing remnantoceanic lithosphere as the Tarim block approached the trench,but prior to the collision. Further detailed field mapping coupledwith mechanical modeling will be required to evaluate this hy-pothesis. Alternatively, oblique convergence between Tarim andthe central Tian Shan during or after collision may have resultedin strike-slip faulting along their boundary (as yet undocu-mented), thereby altering the original spatial relationships be-tween the Carboniferous arc and the northwestern Tarim basin.It is conceivable that the entire range of depositional environ-ments present at Wushi resulted from changing eustatic sealevel, and that the absence of most or all of the Carboniferous atSishichang reflects coastal onlap. However, global sea levelgenerally appears to have fallen during the Early to Middle Car-boniferous (Ross and Ross, 1988), which seems to conflict withthe overall deepening trend we observed at Wushi.

The Early Permian magmatic event represented by maficintrusions and flows in central and northwest Tarim and by rhy-olite in the southern Tian Shan–northern Tarim coincides withdramatic shifts in sediment dispersal patterns and provenance,and a marked increase in basin subsidence (Fig. 16D). A promi-nent northwest-southeast aeromagnetic anomaly associatedwith the Bachu uplift is most likely related to Lower Permian in-trusive rocks, rather than a middle Paleozoic suture as proposedby Yin and Nie (1994). Lower Permian dikes record continentalextension in a direction approximately normal to the Paleozoicnorthwest passive margin of Tarim; their orientation sug-gests that northwest-southeast compression continued duringemplacement. Continued uplift in the south Tian Shan is also indicated by southeast-directed sediment transport, and by felsic-volcanic lithic grains and granitic rock fragments inLower Permian sandstone. The sandstone grains appear to havebeen derived principally from Lower Permian rhyolite thatcrops out in the south Tian Shan; there were additional contri-butions from uplifted and eroded granitic plutons.

Carroll et al. (1995) suggested that Early Permian extensionmay have resulted from collision along an irregular continentalmargin, similar to the scenario proposed by Sengör et al. (1978)for the upper Rhine graben. Alternatively, extension may haveresulted from mantle flow out of the collisional zone in the leastprincipal stress direction (cf. Flower et al., 1998). An alternatehypothesis may be that the Permian magmatism is unrelated toregional tectonics, but rather reflects the influence of a mantleplume (M.B. Allen, 1999, personal commun.).

ACKNOWLEDGMENTS

We are grateful to many individuals for their assistancewith various aspects of this investigation or for helpful discus-sions, including M.B. Allen, J. Amory, J. Chu, B. Hacker, M.


Hendrix, L. Lamb, J.G. Liou, J. Liu, X. Liu, M. McWilliams,E.R. Sobel, S.J. Vincent, X. Wang, D. Ying, D. Zhou, and X.Xiao. Financial assistance was provided by the Stanford-ChinaIndustrial Affiliates, a group of companies that has includedAgip, Amoco, Anadarko, Anschutz, BHP Petroleum, British Pe-troleum, Canadian Hunter, Chevron, Conoco, Elf-Aquitaine,Enterprise Oil, Exxon, Fletcher Challenge, Japanese NationalOil Corporation, Mobil, Occidental, Pecten, Phillips, Statoil,Sun, Texaco, Transworld Energy International, Triton, UnionTexas, and Unocal. We thank M.B. Allen, S.J. Vincent, R. Res-setar, and M.S. Hendrix for careful and helpful reviews.

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Manuscript Accepted by the Society June 5, 2000

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