Sinian through Permian tectonostratigraphic evolution of the ...

Geological Society of America Memoir 194 2001

Sinian through Permian tectonostratigraphic evolution of the northwestern Tarim basin, China Alan R. Carroll Department of Geology and Geophysics, 1215 West Dayton Street, Madison, Wisconsin 53706 USA Stephan A. Graham Edmund Z. Chang Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305 USA Cleavy 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 represented by stratigraphic megasequences divided by major unconformities. Each megasequence is marked by distinctive sedimentary facies, sediment dispersal patterns, sandstone provenance, subsidence history, and in two cases coeval magmatism. The same megasequences are recognized in both the surface Kalpin and largely subsurface Bachu uplifts, indicating that these areas shared an essentially identical history at least through the end of the Paleozoic. The Sinian–Ordovician megasequence overlies an angular basal unconformity with older metamorphic rocks. Siliciclastic facies directly above the unconformity are coarse grained and contain interbedded basalt flows. These facies grade upward into shallow-marine limestone and dolomite and interbedded deeper marine graptolitic shale, apparently as a result of thermal subsidence following a period of extension. Silurian and Devonian facies unconformably overlie Middle Ordovician strata, and are exclusively siliciclastic. They grade upward from 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 adjacent to the eastern Tarim basin, suggesting that they may have been shed from a rising collisional orogen in that area. A pronounced angular unconformity separates Devonian 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 facies that are preserved in the most northwestern outcrop exposures of the Kalpin uplift. These progressively lap out to the southeast, where only thin Lower Permian fluvial and shallow-marine facies are preserved. These facies and thickness relationships suggest deposition in a flexural foreland basin, brought about by an ongoing collision between the Tarim and the central Tian Shan blocks. Lower Permian fluvial facies interbedded 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, and plutons in the Bachu uplift. The significance of this magmatism is unclear, but it may relate 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 of America Memoir 194, p. 47–69.

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INTRODUCTION

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

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

85°

Centra

l Tian

TIAN

Shan South

SHAN

MAP AREA

an Tian Sh

Aksu

CHINA Korla

Kuqa

KALPIN UPLIFT

40°

Manjaer Depression

Bachu Uplift

40°

Ruoqiang GH N TA ALTY

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Minfeng

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Tazhong-1 well

Hotian

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Cenozoic sedimentary cover Pre-Cenozoic rocks

Major subsurface high

FIGURE 2 AKSU UPLIFT SU AK

R RIVE KAN TOX

HALAQI

R VE RI

SHAJINGZI

PIQIANG FAULT

KALPIN

T UL

IAN

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IA AQ

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AWAT AQIA

BACHU SELIBUYA

AULT DI F ANG

XIKEER RESERVOIR

R VE RI

ER

YAS

SUGUN ARTUX

M RI TA

RIV

SANCHAKOU

XIAOHAIZI RESERVOIR

100 km

N

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

Tectonostratigraphic evolution of the northwestern Tarim basin

fects of Cenozoic deformation during the Indian collision are particularly striking, hence a number of studies have considered the 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 sedimentary record of earlier deformations within and adjacent to the Tarim block 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 sides of the Tarim basin (Chen et al., 1985). In contrast to the Paleozoic oceanic or accretionary substrate that underlies the Junggar basin to the north (Hopson et al., 1989; Kwon et al., 1989; Carroll et al., 1990; S¸engör et al., 1993; Allen et al., 1995), the Tarim basin is most likely entirely underlain by Precambrian basement (Zhang et al., 1984; Li et al., 1996), although the nature of this basement beneath the cryptic Manjaer depression remains open to speculation (S¸engör et al., 1996; Fig. 1). Cenozoic thin-skinned folding and thrusting of the Kalpin uplift, which probably occurred above a basal decollement in Upper Cambrian evaporites, is well documented at a reconnaissance scale (Nishidai and Berry, 1990; McKnight, 1993; Yin et al., 1998; Allen et al., 1999). Estimates of shortening derived from these studies range from 20% to 50%. Additional Paleozoic outcrop exposures are available within the Bachu uplift (Fig. 1), a largely subsurface structural high that intersects the Kalpin uplift. Sinian and Paleozoic strata of the northwest Tarim basin have been previously studied by various Chinese researchers, but unfortunately this literature is difficult to access and evaluate by workers outside China (due to difficulties in aquiring and translating Chinese publications, incomplete reporting of spe-

41° 10' N

Wushi

cific field and laboratory data sets, and problems in verifying the accuracy of the data that are reported). The purpose of this paper is to examine the evolution of major sedimentary megasequences exposed adjacent to the northwestern Tarim basin, and to interpret the tectonic record they provide. This study is based on investigations we conducted over a number of field seasons (1987, 1988, 1991, and 1992) in the Kalpin and neighboring Bachu uplifts (Fig. 1). Our studies have focused in two principal geographic areas: the region between Aksu and village of Yingan in the northeastern Kalpin uplift, and near the Xiaohaizi reservoir in the northwestern Bachu uplift. STRATIGRAPHY AND SEDIMENTARY FACIES Sedimentary rocks of the northwestern Tarim basin may be subdivided into four distinct tectono-stratigraphic packages, based on their internal characteristics and on the position of major angular unconformities. Because each of these packages represent major, discrete phases of basin evolution, we refer to them as megasequences (see Hubbard et al., 1985, and Hubbard, 1988, for further discussion of the megasequence concept). Very similar facies are present in both the Kalpin and Bachu uplifts, but surface exposures are stratigraphically less complete and of poorer quality within the Bachu uplift. The following descriptions refer principally to Paleozoic rocks in the northeastern Kalpin uplift near Aksu, Sishichang, Wushi, and Yingan, and in the northwestern Bachu uplift near the Xiaohaizi reservoir (Figs. 1 and 2). Age assignments are supported by a variety of previously reported fossil evidence, summarized in Table 1, and by limited radiometric dating of volcanic units, as described in the following.

79° 30' E

Cenozoic CarboniferousPermian Silurian-Devonian Cambrian-Ordovician Sinian

41° 00' N

Proterozoic Aksu Group

N Thrust fault

Fault - type unknown 91-Su-1,2 31

91-Su-8 35

40° 50' N

km

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15

60 28

91-Si-4

79° 15' E

42

79° 30' E 32

Yingan

49

79° 45' E

40° 50' N

Sishichang 91-Si-6

Figure 2. Geology of Aksu-Yingan area, northeastern Kalpin uplift (modified from unpublished 1:200 000 geologic mapping of Xinjiang Bureau of Geology and Mineral Resources). Triangles indicate position of apatite fission-track samples (see Dumitru et al., this volume).

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Sinan to Ordovician megasequence The oldest rocks of the Kalpin uplift are Upper Proterozoic blueschist-greenschist facies metamorphic rocks of the Aksu Group, exposed locally near Aksu (Figs. 1 and 2). These rocks record high pressure-temperature (P-T) conditions associated with subduction-accretion or collision in a poorly known tectonic setting, and have yielded metamorphic ages ranging from 698 to 754 Ma (K-Ar and Rb-Sr ages of 698–728 Ma from phengite, 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 sedimentary sequences. They are intruded by a series of northwestsoutheast–trending diabase dikes. Liou et al. (1989, 1996) interpreted these dikes to be pre-Sinian, but the dikes have not been directly dated. Proterozoic sedimentary rocks have been subdivided into a lower clastic section assigned to the lower Sinian (the Qiaoenbulak and Yulmenack Formations) and a lithologically diverse upper Sinian section (Sugaitebulake and Qegebulake Formations; Gao et al., 1985; Fig. 3). We have not observed the lower Sinian section, but Gao et al. (1985) reported that it locally

reaches &2000 m in thickness and includes glaciogenic turbidite facies of the Qiaoenbulake Formation that were deformed prior to the deposition of tillite facies of the Yulmenack Formation. The upper Sinian rocks are exposed only at the east end of the Kalpin uplift (Fig. 2) and include a basal conglomeratic phase (Sugaitebulake Formation) that laps unconformably onto the Aksu Group. The character of these deposits is locally variable, ranging from boulder conglomerate to pebbly, coarse sandstone. The conglomerate contains clasts of underlying lithologies, including diabase similar to the Aksu Group dikes. The clasts are moderately to well rounded, with a maximum diameter 1.3 m. The conglomerate grades upward into interbedded red mudstone and fine- to coarse-grained sandstone of the Sugaitebulake Formation (Fig. 4A). The sandstone beds are typically 20 cm to 1 m thick, lenticular, and contain meter-scale planar and trough cross-beds (Fig. 4B), amalgamated beds, and ripples. We interpret these facies to represent braided stream deposits. This interval also contains at least three stratiform basaltic units (Fig. 4A), which we interpret to be flows on the basis of their vesicular character and possible columnar jointing. However, it is also possible that some units could be sills (R. Ressetar, 1999, personal commun.).

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THICKNESS (km)

5.5

Kaipaizileke

CARBONIFEROUSPERMIAN MEGASEQUENCE

Up. Carb.

Devonian

Sishichang-Kangkelin

4.5

Mid. L. Cam. M.-U. Cam. L. Ordovician Ord. U. Sinan U. Prot.

SEMIARID FLUVIAL PLAIN

4.0

3.0

Kalpintage

SILURIANDEVONIAN MEGASEQUENCE

91-Si-4

ANGULAR

91-Sh-6 91-Sh-5 91-Sh-2

Yingan+Qilang Kanling+Saergan

Qiulitage

SHOREFACE INNER SHELF SILICLASTIC MID-SHELF

HIGH-PRODUCTIVITY CARBONATE SHELF

1.5

SINANORDOVICIAN 1.0 MEGAXiaoerbulake+ SEQUENCE Wusongger 0.5

CARBONATE INNER SHELF TO ARID BEACH

91-Su-8 PALEOKARST 91-Su-2 91-Su-1

Sugaitebulake 0 Aksu Group

BRAIDED FLUVIAL

CARBONATE SHELF ANOXIC, STARVED SHELF

Awatage+ Xiaoerbulake

Qegebulake

FLUVIAL AND COAL-SWAMP EPISODIC BASALTIC ERUPTIONS CARBONATE BEACH FLUVIAL

COASTAL BRAID PLAIN 2.5

2.0

NEAR YINGAN

SEMI-ARID FLUVIAL PLAIN

Yimungantawu Tataaiertage

0

ALLUVIAL FAN

EPISODIC BASALTIC ERUPTIONS

3.5

Kupukuziuman

0.5

91-Si-6

Shajingzi

Kontaiaiken 0.5 Formation

Keziertage Formation

6.0 ANGULAR

1.5

1.0

INTERPRETED ENVIRONMENT

LITHOLOGY

5.0

Lower Permian

Bijingtawu Formation

1.0 Devonian

CENOZOIC MEGASEQUENCE

KM

L. Silurian

L. Carboniferous

U. Carboniferous

NEAR WUSHI

Upper Permian Neogene

AGE FORMATION

ANGULAR

CARBONATE INNER SHELF TO BEACH PHOSPHORITE AT BASE CARBONATE INNER SHELF/ BEACH BRAIDED FLUVIAL TO BEACH IN UPPER PART. EPISODIC BASALT ERUPTIONS

BRAIDED FLUVIAL SUBDUCTION COMPLEX (?)

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

The upper Sugaitebulake Formation contains interbedded red mudstone and sandstone with planar-parallel lamination to low-angle cross-stratification, and limestone containing flatpebble intraclast conglomerate and stromatolite. We interpret this succession as marking a transition from nonmarine to shallow-marine depositional conditions. The uppermost Sinian Qegebulake Formation consists of interbedded limestone, dolomite, and mudstone. The carbonate facies are characterized by abundant stromatolite (Fig. 4C) and flat-pebble conglomerate, and are interpreted as intertidal to supratidal. The Qegebulake 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 eastern Kalpin uplift constitute a 1–1.5-km-thick section of predominantly carbonate facies (Figs. 2 and 3). Except for a very thin, oolitic phosphorite-shale interval at its base, Lower Cambrian facies largely are dominated by stromatolite and flat-pebble conglomerate facies, interpreted as representing intertidal to supratidal environments. Dolomitic Middle to Upper Cambrian strata are also stromatolitic (Fig. 5A) and apparently record periods of restriction and emergence, based on reported interbedded evaporite (Fan and Ma, 1991) and observed red mudstone facies. Topographically prominent massive limestone beds of the overlying Lower Ordovician Qiulitage Formation consist of thin-bedded micrite to calcarenite with diverse, normal marine fauna (Fan and Ma, 1991) indicating shallow shelf to supratidal

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Figure 4. Outcrop photographs of Sinian sedimentary facies southwest of Aksu. A. Siliclastic strata of Sugaitebulake Formation, interbedded with basalt flow in lower third of view. Thickness of exposed section is approximately 200 m. B: Cross-beds in basal upper Sinian Yurmeinake Formation sandstone, interpreted to represent braided fluvial environment. Note that sense of flow in this view is predominantly left to right (to southeast) 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.


53

Figure 5. Outcrop photographs of Cambrian and Ordovician facies between Sishichang and Sanchakou (see Figs. 1 and 2 for locations). A: Stromatolite 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 Ordovician Yingan Formation. D: Storm-deposited bed in Middle Ordovician Yingan Formation (carbonate intraclasts concentrated in layer marked by compass; also see Fig. 7).

environments. Similar Qiulitage Formation facies represent the oldest strata exposed within the Bachu uplift at several localities, including Sanchakou (Figs. 1 and 5B). The Middle Ordovician generally reflects deeper water, but probably still shelfal environments, in the Kalpin uplift. The Saergan Formation is a graptolitic, pyritic, carbonaceous, laminated black shale interpreted to represent anoxic to suboxic depositional conditions (Graham et al., 1990). The Saergan Formation has been reported to occur in the Bachu uplift (Xinjiang Stratigraphic Table Compiling Group, 1981), but was not investigated during this study. Overlying Ordovician formations represent a return to a well-oxygenated carbonate shelf environment (Fig. 6). These rocks are extensively bioturbated and occasionally punctuated by conglomeratic and fossiliferous beds 5–10 cm thick, interpreted as storm deposits (Figs. 5, C and

D, and 7). Ordovician units above the Saergan Formation have not been reported from Bachu uplift outcrops. Silurian to Devonian megasequence Lithology changes sharply and markedly across an unconformity that separates the Middle Ordovician carbonate sequence from the overlying Lower Silurian green siliciclastic strata of the Kalpintage Formation (Figs. 6, 7, and 8, A and B). This unconformity locally removes all or part of the Ordovician Yingan and Qilang Formations (Fan and Ma, 1991), indicating erosional relief of tens to hundreds of meters. At Sishichang, a discontinuous, fossiliferous conglomeratic lag 61 m thick is on the unconformity surface (Fig. 7). The lower Kalpintage Formation consists of interbedded fissile green siliciclastic

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B M IOT U U D R DY B SH ATE EL D F

D ST O O M SH IN RM EL ATE F D

Figure 7

FL U VI

A L

PL A IN

100 M

F EL SH E W N O LI LL ND L A A A SH TR VI U S FL

NW

M A B R IN FL RA EU IDE N VI D O A N L M A R IN E Figure TR S A 9 N SA HE SH S IT ND LF O I O O RE FL R N AT FA CE

SE

Figure 10

SH AN E O BA LF XIC SI O N R

FORMATION AGE

S K

QILANG Y MIDDLE ORDOVICIAN

KALPINTAGE LOWER SILURIAN

TATAAIERTAGE

YIMUNGANTAWU DEVONIAN

KANGKELIN

"Upper Carboniferous" ANOXIC MARINE SHALE

SHELFAL CARBONATE

SILICLASTIC SHELF DEPOSITS

FLUVIAL SANDSTONES

REDBEDS

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

KALPINTAGE FORMATION

LOWER SILURIAN

40

INTERBEDDED GREEN RIPPLED TO LAMINATED SANDSTONE AND SILTSTONE

PLATY-WEATHERING, GREEN SILTSTONE AND MUDSTONE 30

RIPPLED SANDSTONE LENTICULAR, GLAUCONITIC,\ CONGLOMERATIC, FOSSILIFEROUS SANDSTONE

THIS STUDY

TRADITIONAL

SILICICLASTIC SHELF

M

LAG

"TEMPESTITE"

"TEMPESTITE"

10

CARBONATE SHELF

YINGAN FORMATION

MIDDLE ORDOVICIAN

20

NODULAR-WEATHERING, EXTENSIVELY BIOTURBATED FOSSILIFEROUS LIMESTONE AND THINLY INTERBEDDED GREEN MUDSTONE

0

Figure 7. Measured outcrop section though Ordovician–Silurian contact near Sishichang.

mudstone, siltstone, and sandstone, interpreted to be deposited on a storm-dominated shelf. The siltstone is calcareous, planarparallel to wavy laminated, and locally nodular and bioturbated. Thin fine- to medium-grained sandstone beds are planarparallel laminated to rippled, and locally contain coarse-grained glauconitic layers, bedding-plane trace fossils, and centimeterscale grooves on bed soles. This facies association is interbedded with medium- to coarse-grained sandstone with pervasive trough cross-beds to 1 m in amplitude, interpreted as shoreface deposits. The upper Kalpintage Formation includes a lenticular, upward-fining succession of white sandstone beds deposited above a basal scour, with meter-scale trough cross-beds and 15–20 cm mud balls at the base, interpreted as a tidal channel deposit. Possible tidal deposits have also been reported near the village of Yingan (Fig. 2; S.J. Vincent, 1999, personal commun.). Redbeds with local mudcracks and paleosols increase in frequency upward near the top of the Kalpintage Formation; the rocks are mapped as the Tataaiertage Formation at the point where redbeds dominate. The Tataaiertage Formation was formerly assigned to the Devonian (Xinjiang Stratigraphic Tables Compiling Group, 1981), but the transitional nature of the contact with the underlying green, marine Silurian rocks suggests that it is fully conformable with the underlying Kalpintage Formation. Fan and Ma (1991) assigned a Silurian age to the Tataaiertage Formation based on the gradational nature of the contact and the local occurrence of Silurian marine fossils (Table 1). They inferred that previously reported plant fossils (Lepidodendropsis) were actually not found in place. The Tataaiertage section contains mostly unfossiliferous, trough cross-bedded sandstone interpreted to represent braided fluvial deposits (Figs. 8C and 9). Finer-grained overlying facies of the Yimungantawu Formation may represent deposits on a broad fluvial plain. The Yimungantawu Formation has been assigned to the Lower Devonian on the basis of nonmarine bivalves (Fan and Ma, 1991; Table 1). The overlying Keziertage Formation,


55

Figure 8. Outcrop photographs of Silurian and Devonian facies near Sishichang. A: Unconformable contact between Middle Ordovician (light colored 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 stormdominated shelf deposits. Similar facies continue into partially covered strike valley, where they are interbedded with trough cross-bedded shoreface sandstone. Ridge in background contains lenticular, cross-bedded sandstone facies interpreted as shoreface to tidal deposits, overlain by tidal to coastal plain red mudstone mapped as base of Tataaiertage Formation. Resistant beds at ridge on skyline (left) consist of Tataaiertage Formation red sandstone facies interpreted as braided fluvial deposits. C: Cross-bedded sandstone of Tataaiertage Formation, interpreted as braided fluvial deposits. These cross-beds indicate predominant flow direction to west.

which locally was completely eroded away, contains trough cross-bedded sandstone facies interpreted as a return to braided fluvial deposition. We observed a very similar Silurian–Devonian section in the Bachu uplift near the Xiaohaizi reservoir (Fig. 1). The Xiaohaizi section, however, is cut by numerous diabase dikes and sills, which obscure stratigraphic relationships. The Xiaohaizi section also includes several intervals of fluvial quartz-pebble

conglomerate beds with scoured bases. These are presumed to be Devonian, but are unfossiliferous. Carboniferous to Permian megasequence Kalpin uplift. Carboniferous and Permian facies of the northeast Kalpin uplift were described in detail by Carroll et al. (1995); these descriptions are therefore not repeated here.

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A.R. Carroll et al.

M 17

PALEOCURRENT DIRECTION

DEVONIAN

TATAAIERTAGE FORMATION

MEAN PALEOCURRENT DIRECTION (N=8)

10

MEDIUM-SCALE TROUGH CROSSBEDDED SANDSTONE

MUDSTONE CHIP CONGLOMERATE 5 RED MUDSTONE

SHALLOW, BRAIDED-FLUVIAL SYSTEM

15

LOW ANGLE, PLANAR CROSS-BEDDED SANDSTONE 0

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

Devonian (and possibly Silurian) strata are extensively truncated beneath an angular unconformity cut across the entire Kalpin uplift. The oldest strata above the unconformity are progressively younger to the southeast. Near Wushi (Fig. 2), at least 2000 m of carbonate and siliciclastic facies as old as Early Carboniferous unconformably overlie Cambrian through Silurian rocks. This succession deepens upward from fluvial gravels to siliciclastic turbidites, which then grade into shelf sandstone overlain by carbonate olistostrome. On the basis of regional relationships, the latter facies are interpreted to have been shed from northwest-facing carbonate platforms. At Sishichang (Fig. 2) the Lower Permian Sishichang Formation unconformably overlies Devonian redbeds (Carroll et al., 1995). Fluvial conglomerate and sandstone grade upward into tidal sandstone facies and shallow-marine algal and skeletal limestone of the Kangkelin Formation (Fig. 10; Carroll et al., 1995). Clasts in the conglomerates are mostly sedimentary, and have lithologies similar to underlying Devonian strata. The Kangkelin Formation is sharply overlain by variegated red-green, nonmarine Permian strata of the Kupukuziman, Kaipaizileke, and Shajingzi Formations (Fig. 3). Carbonaceous strata, rooted fossil trees, fossil leaf horizons, and thin coal beds occur sporadically throughout the lower half of this section, supporting the inference of at least a seasonally humid environment. Two series of basaltic lavas, each of which consists of multiple thin flow horizons, occur within the Kupukuziman and

Kaipaizileke Formations and underlie a large area of the northwest Tarim basin (Chang, 1988; Liu and Li, 1991; Wang and Liu, 1991; Fig. 11). Each series totals &150–200 m in thickness, but is interbedded within thicker intervals of nonmarine sedimentary 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 the flows range in composition from tholeiites to alkali basalts, consistent with within-plate magmatism. A variety of K-Ar wholerock ages have been reported for these flows, ranging from 293.25 ; 7.70 to 285.24 ; 6.67 Ma for the lower series and 295.13 ; 7.1 to 228.83 ; 5.15 Ma for the upper series (Liu and Li, 1991; Wang and Liu, 1991). Carroll et al. (1995) reported an 40 Ar/39Ar age of 277.53 ; 0.46 Ma for plagioclase separated from 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 Yingan area is the site of a major tear fault in the Kalpin thrust sheets (Figs. 1 and 2), possibly suggesting a long-lived zone of structural reactivation. Age relationships within the nonmarine Permian exposures above the basalt flows are subject to controversy. Upper Permian rocks are most often depicted overlying an angular unconformity above the Lower Permian (Xinjiang Stratigraphic Table Compiling Group, 1981; unpublished 1:200 000 geologic mapping of the Xinjiang Bureau of Geology and Mineral Resources), but some workers maintain that this entire section may be Lower Permian (Li Wunfeng, 1992, personal commun.). The total original thickness of this interval is unknown; it is erosionally truncated and overlain by Cenozoic deposits. Significant along-strike variations in Carboniferous– Permian stratigraphy occur within the Kalpin uplift. It generally appears that thicker Carboniferous strata are present to the southwest (Xinjiang Stratigraphic Table Compiling Group, 1981; unpublished 1:200 000 geologic mapping of the Xinjiang Bureau of Geology and Mineral Resources), although detailed field data have not been reported. Bachu uplift. Carboniferous and Permian rocks of the Bachu uplift at Xiaohaizi are extensively intruded (Fig. 12, A and B), and generally are less well exposed than their Kalpin uplift equivalents. A mafic composite sill &50–100 m thick (Fig. 12A) either coincides with the position of the DevonianCarboniferous contact, or else is slightly above it. The angular unconformity seen in the Kalpin uplift is not clearly visible at Xiaohaizi. However, we measured &10° of dip discordance between Devonian strata beneath the sills and Carboniferous strata above. The sills are cut by a composite syenitic to granitic pluton that radiates felsic dikes into the surrounding rocks (Fig. 12C). Lower Carboniferous to Lower Permian shallowmarine deposits overlie the sill complex and comprise interbedded mudstone, limestone, and poorly exposed gypsum (possibly nodular). We interpret these facies to represent backbarrier and lagoonal environments in which carbonate grains de-

KANGKELIN


57

M 35 RED MUDSTONE ALGAL LIMESTONE MICRITE 30

WHITE, CROSS-BEDDED AND RIPPLED SANDSTONE WITH THIN MUDSTONE BEDS ORGANIZED, PEBBLE CONGLOMERATE

25

20

15

RED MUDSTONE WITH THIN RIPPLED SANDSTONE BEDS

ALLUVIAL FAN

SISHICHANG FORMATION

LOWER PERMIAN

POORLY ORGANIZED CGL. CLASTS COARSEN UPWARD TO 20 cm

UPWARD - FINING SANDSTONE BEDS

Figure 10. Measured section through angular contact between Yingan and Sishichang Formations 5 km northeast of Sishichang (see Fig. 6 for stratigraphic position). CGL.—conglomerate.

GRAY MUDSTONE GRADING UPWARD TO SANDSTONE 10

ORGANIZED PEBBLE CONGLOMERATE YELLOW SILTSTONE

DEVONIAN

YIMUNTANTAWU FORMATION

5

0

ORGANIZED, IMBRICATED CONGLOMERATE WITH CLASTS TO 40 cm RED, POORLY ORGANIZED CONGLOMERATE GRADING TO UPPER COARSE SAND ANGULAR UNCONFORMITY RED PLATY SANDSTONE AND MUDSTONE

rived from storm washover alternated with mudstone and gypsum deposition under restricted conditions. The upper part of the succession also contains brecciated limestone, algal laminites, and flat-pebble conglomerate indicating supratidal environments. All of these facies are extensively cut by northwestsoutheast–trending mafic dikes, which locally compose as much as 20% of the total outcrop width. These dikes, and gabbroic dikes associated with an alkali igneous complex to the south of Xiaohaizi, have 40Ar/39Ar ages that are essentially identical to the 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 correlation techniques to infer that the Carboniferous–Permian succession at Xiaohaizi is actually far less stratigraphically complete than it appears in outcrop. They argued that the Xiaohaizi section and several sections within the Kalpin uplift contain hiatuses that represent at least as much geologic time as the preserved sedimentary rocks. Furthermore, these hiatuses are generally longer than can be explained by third-order sea-level changes, and suggest instead control by local geologic processes. We did not observe any obvious unconformities within this succession.

PALEOCURRENTS AND SANDSTONE PROVENANCE Paleocurrent measurements were collected at several locations from Sinian through Permian facies in the AksuSishichang area (Fig. 13). Note that although the numbers of measurements reported here are not statistically significant, they are representative of larger, visually identified populations of 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 discussions of techniques). The raw data were recalculated into detrital modes and plotted on standard ternary diagrams (Fig. 14) to permit comparison with previously recognized provenance types (cf. Dickinson, 1985). Sinian paleocurrent measurements generally indicate transport directions to the south or southeast (Fig. 13A). These measurements represent mostly decimeter-scale planar cross-beds within coarse-grained, fluvial sandstone facies (e.g., Fig. 4B). Sinian sandstone samples vary widely in composition and reflect mixed provenance (Fig. 14A), related both to metamorphic basement lithologies and to interstratified Sinian volcanic rocks.

58

A.R. Carroll et al.

Kaipaizileike Yingan Conglomerate

Limestone

Sandstone

Basalt

Shale

Heshen-1 well Sishichang

Shacan-1 well Tertiary

1000

Kupukuziman Formation

Kaipaizileike m Kupukuziman

0

SishichangKangkelin

Lower Permian

Kaipaizileike Formation

Heshen-2 well

Basalt

Gabbro

Fault Sishichang

300

0 km

Figure 11. Occurrence of Lower Permian basalt in northwest Tarim basin (modified from Xinjiang Stratigraphic Table Compiling Group, 1981; Chang, 1988; Liu and Li, 1991; Wang and Liu, 1991; J. Liu, 1995, personal commun.). Basalt intervals represented in upper part of figure are actually composite intervals of interbedded basalt and siliclastic sedimentary rocks; total thickness of basalt flows is therefore less than represented.

Kuqa

Wushi Yingan

Shacan-1 well

Kaipaizileike Kashgar

Bachu

Heshen-1 well

Heshen-2 well Shache Yecheng N Hotian

The relatively high content of polycrystalline quartz (Qp) most likely reflects input from metamorphic rocks of the Aksu Group. Cross-beds, ripples, and groove casts in Silurian sandstone facies display mixed transport directions (Fig. 13B). In contrast, fluvial Devonian sandstone facies show strong west to southwest transport directions based on trough cross-beds, parting lineation, and grooves (Figs. 8C and 13C). Silurian and Devonian sandstone contains mostly quartz and lithic rock fragments (Fig. 14B), suggesting a recycled orogenic provenance (cf. Dickinson, 1985). Together these observations circumstantially 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 a partly volcanic provenance, probably south of the Altyn Tagh fault (Fig. 1). Net northwest sediment transport in the Kalpin uplift area during the Carboniferous is expected from regional considerations, but our paleocurrent data (Fig. 13D) are too sparse to lend further support to this interpretation. Allen et al. (1999) reported northwest-directed paleocurrent directions in turbidites of the Upper Carboniferous Sasikebulake Formation (located west


59

Figure 12. Outcrop photographs of intrusions at Xiaohaizi reservoir (see Fig. 1 for location). A: South-dipping diabasic sills intruding Devonian and 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 above foreground.

of Halaqi; Fig. 1). Carboniferous to Lower Permian sandstone older 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 interior of Tarim (Lu and Qi, 1994, personal commun.); pre-Kupukuziman 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 Kupukuziman Formation marks a dramatic shift in both paleocurrents and provenance (Figs. 13E and 14D). They are dominated by felsic volcanic material derived from the northwest. Felsic-volcanic rock fragments with pyroclastic textures, granitic rock fragments, angular monocrystalline quartz, and potassium-feldspar grains are all common constituents. Carroll et al. (1995) inter-

preted these sandstones to be derived from rhyolitic volcanics and related granites in the southern Tian Shan. However, northwest-directed paleocurrents have also been noted in timeequivalent 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 rhyolitic or granitic detrital source to the southeast, most likely within the buried Bachu uplift. SUBSIDENCE HISTORY The subsidence history of the Aksu-Sishichang and Xiaohaizi areas was reconstructed using the backstripping methods of Bond and Kominz (1984; also see Steckler and Watts, 1978; Van Hinte, 1978; Schlater and Christie, 1980, for further details of the method). Absolute ages are based on the time scale of

60

A.R. Carroll et al.

N

N Cross-beds 31

31

35

sish crossbeds Cross-beds

35

N=44

N=7 60 28

40° 50' N

28

N Imbrication

N=3

Yingan

Yingan E. PERMIAN-KUPUKUZIMAN FM.

D. PERMIAN-SISHICHANG FORMATION

N N Ripples N=6

Cross-beds

Grooves

N=23

31

N=4

sishichang xbeds Cross-beds

31

35

35

N=52 km

0

30

40° 50' N 28

N

28

N

40° 50' N

Crossbeds

Cross-beds

N=13

Yingan

Yingan 79° 45' E

79° 45' E

N=7

B. SILURIAN

C. DEVONIAN

N N

Cross-beds

NE crossbeds

Cross-beds N=26

31

N=4

N=12 35

0

km

Parting lineation 30

60 28

40° 50' N

Yingan 79° 45' E A. SINIAN

LEGEND

Sishichang Cenozoic

Cambrian-Ordovician

CarboniferousPermian

Sinian

Silurian-Devonian

Proterozoic Aksu Group

N

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


61

62

A.R. Carroll et al.

D. Permian (Nonmarine) (n = 11)

Qm

Qp

Lt

F

Lsm

Lv

Qm

Qm

K

P

Qp

Qm

C. CarboniferousPermian (Marine) (n = 14)

Lt

F

Lsm

Lv

Qm

K

P

Qp

Qm

B. SilurianDevonian (n = 11)

Lt

F

Lsm

Lv

Qm

K

P

Qp

Figure 14. Modal compositions of sandstone samples (see Table 2 for raw data and text for further explanation). Qm— monocrystalline quartz, F—total feldspar, Lt—total lithics + polycrystalline quartz, Qp—polycrystalline quartz, Lv—volcanic lithics, Lsm— sedimentary + metamorphic lithics, P— plagioclase, K—potassium feldspar.

Qm

A. Sinian (n = 10)

F

Lt

Lv

Lsm

Harland et al. (1990), except for the Permian. Permian ages are based on Ross et al. (1994) because this time scale more closely fits known Lower Permian paleontological and radiometric constraints at Sishichang. The lower age limit of Sinan sedimentary rocks near Aksu is very well defined by radiometric dating of micas contained in pelitic schist underlying the basal unconformity (Nakajima et al., 1990). A late Sinian (Vendian) age is further supported by reported acritarch and stromatolite occurrences (Gao et al., 1985). The amount of missing time represented by the basal unconformity, however, is unknown. Stratigraphic thicknesses for all units are based on appropriate reference sections (Xinjiang Stratigraphic Table Compiling Group, 1981), modified on the basis of our field investigations.

P

K

Because of the incompleteness of individual outcrop sections, the reference sections each represent local composites of more than one actual measured section. Some errors (on the order of 10%–20%) may therefore have been introduced due to stratigraphic thickness variations, but generally the backstripped sections are representative of the true vertical thickness of strata present at each location. Paleobathymetries are based on our interpretation of marine depositional environments, and approximate error ranges are estimated. Paleoelevations during deposition of nonmarine units represent best guesses, based on the occurrence of similar depositional environments elsewhere and on the present elevation of northwest Tarim; estimated error ranges are correspond-


ingly large. Unconformities are based on missing strata as indicated by the Xinjiang Stratigraphic Tables, paleontologic and radiometric data of Carroll et al. (1995), graphic correlation of Carboniferous and Permian marine fauna by Groves and Brenckle (1997), and our field investigations. The total and tectonic subsidence histories at both locations were very similar at both localities during lower to middle Paleozoic time (Fig. 15), both in terms of timing and magnitude. Sinian through Cambrian rocks do not crop out within the Bachu uplift; direct comparison of subsidence during this interval therefore is not possible. We know of no reported well penetrations of these units in the Bachu uplift, but their presence has been inferred in the subsurface (e.g., Fan and Ma, 1991). Tectonic subsidence rates in the Aksu-Sishichang area gradually decreased from Sinian through Cambrian time, and appear to have remained relatively constant and slow during the Ordovician. Greater Early Ordovician subsidence occurred at the Bachu uplift, resulting in preservation of strata that are &50% thicker than in the Aksu-Kalpin area. Upper Ordovician strata are not reported from the Xiaohaizi area, due either to erosional removal by the base-Silurian unconformity or simply to lack of exposure. We were not able to locate an exposed Ordovician-Silurian contact in this area. Subsidence rates increased in both areas during the Silurian following development of the widespread basal-Silurian unconformity, and increased subsidence continued into the Devonian. The thickness of preserved Devonian strata varies greatly within the Kalpin uplift, depending on the magnitude of erosion at the basal Carboniferous unconformity. For example, the Upper Devonian Keziertage Formation is absent at Sishichang, but locally reaches nearly 1200 m in thickness above the Yimungantawu Formation (Xinjiang Stratigraphic Table Compiling Group, 1981). At its maximum the Silurian–Devonian megasequence in the Kalpin uplift reaches more than 2200 m in thickness in the Kalpin uplift, and therefore records a significant period of tectonic subsidence. The record of Carboniferous to earliest Permian subsidence varies greatly depending on location. Most or all of this interval is represented by an unconformity in the Aksu-Sishichang area, whereas sedimentation at Xiaohaizi initially appears to have been more continuous. Deposition of the Kupukuziman Formation initiated a major new episode of basin formation, characterized by rapid subsidence. Fission-track analysis of Sinian through Permian samples from the Aksu-Sishichang area indicates that this entire section cooled below annealing temperature simultaneously during the latest Permian to Early Triassic (details of these analyses and estimates of the timing and magnitude of unroofing are presented in Dumitru et al., this volume; these results are therefore not be repeated here). We infer that substantial erosion of Permian rocks occurred at this time (Fig. 15). There is no stratigraphic or fission-track evidence that large thicknesses of Mesozoic strata ever covered the Kalpin uplift. Fission-track shortening of &15%

63

in these samples could have occurred either due to relatively minor Mesozoic burial, or due to burial beneath now-eroded Cenozoic sediments. About 1700 m of Neogene-Quaternary nonmarine sedimentary rocks cover the Bachu uplift near Selibuya (Fig. 1); involvement of these strata in Cenozoic thrusting (Allen et al., 1999) indicates that they may have originally also covered Paleozoic rocks in the Kalpin uplift. DISCUSSION AND CONCLUSIONS Sinian–Ordovician megasequence The basal unconformity that marks the onset of Sinian deposition most likely corresponds with the initiation of rifting of a preexisting Proterozoic continent. Shi et al. (1995) suggested that Sinian extension was followed by a period of thermal subsidence. The Sinian paleogeography of this region is very poorly known, but local variations in the thickness of lower Sinian strata of &2000 m and the presence of basal upper Sinian boulder conglomerate indicates substantial relief on the basal unconformity. South-directed paleocurrents indicate either that Tarim Proterozoic rocks continue to the north of the study area, or that the original source area for these sediments subsequently rifted away. There is no evidence for renewed tectonic activity during the latest Sinian through Ordovician; the observed changes in sedimentary environments were apparently driven mostly by changing relative sea level. The Tarim block occupied low paleolatitudes by the early Paleozoic (Fig. 16A), latitudes that were conducive to extensive shallow-marine carbonate sedimentation. Graptolitic shale deposition probably records maximum sea-level highstands. Silurian–Devonian megasequence The base-Silurian unconformity corresponds to a basinwide event also observed in the subsurface north of the Tazhong structure in the southeastern Tarim basin (site of the Tazhong-1 well, Fig. 1; Li et al., 1996; R. Ressetar, 1999, personal commun.), with relief of at least hundreds of meters. Increased basin subsidence and the reintroduction of siliclastic detritus indicate that this unconformity records a new tectonic episode of basin evolution. The source area for Silurian to Devonian clastic sediments is unknown, but may be a middle Paleozoic orogenic belt developed to the east, within the Altyn Tagh range (Figs. 1 and 16B). Sobel and Arnaud (1999) proposed the existence of a middle Paleozoic suture (termed the Lapeiquan suture) between Archean rocks of the Tarim block and a Proterozoic block to the south. The oldest arc intrusions within the Proterozoic block were dated as 435 ; 20 Ma, and crosscutting postorogenic granite plutons were dated as 383 ; 7 Ma. Sobel and Arnaud (1999) therefore concluded that an intervening ocean basin closed after the Early Silurian and before the Middle Devonian. They also pointed out that sedimentary rocks of Silurian to Middle

64

A.R. Carroll et al.

Ma 700

600

500 ORDOVICIAN

CAMBRIAN

SINIAN

400 SILU- DEVORIAN NIAN

300

200

CARBONIF- PERM- TRIASEROUS IAN SIC

100

JURASSIC

0 1000

CRETACEOUS TERTIARY

PALEO-BATHYMETRY/ELEVATION 0

APPROXIMATE APATITE FISSION-TRACK COOLING AGE

- 1000

TECTONIC SUBSIDENCE

-2000

-4000

KALPIN UPLIFT (SISHICHANGYINGAN)

-5000

ELELVATION (m)

-3000

-6000

-7000

TOTAL SUBSIDENCE

UNCONFORMITIES: Ma 700

600 SINIAN

500 CAMBRIAN

ORDOVICIAN

400 SILU- DEVORIAN NIAN

300

200

CARBONIF- PERM- TRIASEROUS IAN SIC

JURASSIC

100 CRETACEOUS TERTIARY

0 1000

PALEO-BATHYMETRY/ELEVATION 0

(NOT EXPOSED)

"TECTONIC SUBSIDENCE" -2000

-3000

BACHU UPLIFT (XIAOHAIZI) TOTAL SUBSIDENCE

UNCONFORMITIES:

ELELVATION (m)

-1000

-4000

-5000

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

65


B. Devonian L A P E I Q U A N

?

NORT H TIAN

SO

UT

IA HT

NS

HA

D. Late Early Permian SHAN SUTUR E U NS

TU

RE

S U T U R E

?

30° N

15° N

C. Early Carboniferous ? ?

CENTRAL TIAN SHAN

?

# 35° N

# #

?

A. Ordovician

FOREBULGE?

# ?

?

200 km

SHALLOW MARINE (MIXED CARBONATE AND SILICICLASTIC)

(NONDEPOSITION OR LATER EROSION)

SHALLOW MARINE (DOMINANTLY CARBONATE)

ALLUVIAL FAN

? ALLUVIAL PLAIN

# #

0°

SHALLOW MARINE (EVAPORITE) DEEP MARINE

LAVA FLOWS

THRUST FAULT

STRIKE-SLIP FAULT

SUBDUCTION ZONE

DIKES

POSSIBLE PERMIAN NORMAL FAULT

SEDIMENT DISPERSAL DIRECTION

Figure 16. Schematic paleogeographic reconstructions of Tarim basin and surrounding areas. Orientation and paleolatitude 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 previous 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 absence of Silurian or Devonian sedimentary rocks along the southeastern margin of the Tarim basin (between Ruoqiang and Minfeng), although Devonian quartz-pebble conglomerates are present in the southwestern Tarim near Hotian (Fig. 1). Together, these observations suggest that an orogenic belt roughly parallel 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 sandstone samples from the Kalpin uplift (Table 2; Figure 14B) are consistent with derivation from the preorogenic lithologies described by Sobel and Arnaud (1999). However, it is not clear what genetic relationship, if any, existed between the Lapeiquan suture and basin subsidence in the Kalpin and Xiaohaizi areas.

66

A.R. Carroll et al.

Apparently increasing rates of tectonic subsidence during the Late Devonian (Fig. 15) are suggestive of flexural subsidence (cf. Dickinson, 1976; Jordan, 1981), but this interpretation is speculative due to the poor age data on these nonmarine facies. Carboniferous–Permian megasequence The base-Carboniferous angular unconformity is present virtually throughout the Tarim basin, including the Kalpin uplift and the southwest Tarim basin near Hotian (our field observations), 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 that a flexural foredeep formed along the northern Tarim margin in response to a Late Devonian to Early Carboniferous collision between 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 Tarim block by the collisional orogen may have created a flexural forebulge with in the northern Tarim basin, coincident with the Sishichang-Yingan area of the Kaplin uplift (Fig. 16C). A forebulge would explain the absence of Lower Carboniferous strata in the Sishichang-Yingan area (they are present at both Wushi and Xiaohaizi), and would help restrict marine circulation and promote 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 Carboniferous uplift of the southern margin of the central Tian Shan. They report that the Erbin Shan granite, which intrudes Devonian strata and has a reported U-Pb age of 378 Ma (Hu et al., 1986), is unconformably overlain by Carboniferous conglomerate and carbonate facies bearing Visean marine fossils. This timing of this nonconformity corresponds with the development of the Devonian–Carboniferous angular unconformity in the Tarim basin, strongly suggesting a causal relationship. The presence of Lower Carboniferous marine facies within the Tian Shan does not support continued widespread orogenic uplift, however, as implied by Carroll et al (1995) and Allen et al. (1999). Paleomagnetic, stratigraphic, and petrologic evidence suggests that this collision was diachronous, occurring later to the west (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 presence of blueschist facies metamorphic rocks as young as Early Carboniferous in the western Tian Shan along the suture between the central and southern Tian Shan, and a coeval magmatic arc along the southern edge of the central Tian Shan terrane. They infer that subduction continued in this area through the Early Carboniferous (Fig. 16C), as proposed by Gao et al. (1998) and Chen et al. (1999). This conclusion requires that the foreland basin model proposed for the northern Tarim basin by Carroll et al. (1995) and subsequently adapted by Allen et al. (1999) be modified in recognition that the actual collision between 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 subsidence therefore becomes somewhat enigmatic. One hypothesis is that vertical movements of the northwest Tarim passive margin occurred in response to flexure of the downgoing remnant oceanic lithosphere as the Tarim block approached the trench, but prior to the collision. Further detailed field mapping coupled with mechanical modeling will be required to evaluate this hypothesis. Alternatively, oblique convergence between Tarim and the central Tian Shan during or after collision may have resulted in strike-slip faulting along their boundary (as yet undocumented), thereby altering the original spatial relationships between the Carboniferous arc and the northwestern Tarim basin. It is conceivable that the entire range of depositional environments present at Wushi resulted from changing eustatic sea level, and that the absence of most or all of the Carboniferous at Sishichang reflects coastal onlap. However, global sea level generally appears to have fallen during the Early to Middle Carboniferous (Ross and Ross, 1988), which seems to conflict with the overall deepening trend we observed at Wushi. The Early Permian magmatic event represented by mafic intrusions and flows in central and northwest Tarim and by rhyolite in the southern Tian Shan–northern Tarim coincides with dramatic shifts in sediment dispersal patterns and provenance, and a marked increase in basin subsidence (Fig. 16D). A prominent northwest-southeast aeromagnetic anomaly associated with the Bachu uplift is most likely related to Lower Permian intrusive rocks, rather than a middle Paleozoic suture as proposed by Yin and Nie (1994). Lower Permian dikes record continental extension in a direction approximately normal to the Paleozoic northwest passive margin of Tarim; their orientation suggests that northwest-southeast compression continued during emplacement. 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 in Lower Permian sandstone. The sandstone grains appear to have been derived principally from Lower Permian rhyolite that crops out in the south Tian Shan; there were additional contributions from uplifted and eroded granitic plutons. Carroll et al. (1995) suggested that Early Permian extension may have resulted from collision along an irregular continental margin, similar to the scenario proposed by S¸engör et al. (1978) for the upper Rhine graben. Alternatively, extension may have resulted from mantle flow out of the collisional zone in the least principal stress direction (cf. Flower et al., 1998). An alternate hypothesis may be that the Permian magmatism is unrelated to regional tectonics, but rather reflects the influence of a mantle plume (M.B. Allen, 1999, personal commun.). ACKNOWLEDGMENTS We are grateful to many individuals for their assistance with various aspects of this investigation or for helpful discussions, 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-China Industrial Affiliates, a group of companies that has included Agip, Amoco, Anadarko, Anschutz, BHP Petroleum, British Petroleum, Canadian Hunter, Chevron, Conoco, Elf-Aquitaine, Enterprise Oil, Exxon, Fletcher Challenge, Japanese National Oil Corporation, Mobil, Occidental, Pecten, Phillips, Statoil, Sun, Texaco, Transworld Energy International, Triton, Union Texas, and Unocal. We thank M.B. Allen, S.J. Vincent, R. Ressetar, 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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