Paleostructural geomorphology of the Paleozoic central uplift belt and ...

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Paleostructural geomorphology of the Paleozoic central uplift belt and its constraint on the development of depositional facies in the Tarim Basin LIN ChangSong1†, YANG HaiJun2, LIU JingYan1, PENG Li, CAI ZhenZhong2, YANG XiaoFa1 & YANG YongHeng1 1 2

School of Energy Resources, China University of Geosciences, Beijing 100083, China; Petroleum Exploration and Production Institute, Sino-Petroleum and Chemistry Cooperation, Beijing 100083, China

Inclined eastward and consisting of the Hetianhe, Hetianhedong, Tazhong paleouplifts and Bachu paleoslope, the central paleouplift belt in the Tarim Basin was a large composite paleouplift and paleoslope belt with complicated palaeogeomorphic features during the Middle to early Late Ordovician. A number of paleostructural geomorphic elements have been identified in the paleouplift belt and surrounding areas, such as the high uplift belts, the faulted uplift platforms, the marginal slopes and slope break zones flanking the paleouplift belt, the surrounding shelf slopes or low relief ramps, the shelf slope break zones and deep basin plains. They exerted great influence on the development of paleogeography of the basin. The marginal slopes and slope break zones flanking the uplift belt constrained the formation and deposition of the high-energy facies including reefal and shoal deposits during the Late Ordovician, which comprise the major reservoirs of the Lower Paleozoic in the basin. Toward the end of the Ordovician, the Tazhong paleouplift hinged westward and became a westward-dipped nose as the southeastern margin of the basin was strongly compressed and uplifted. The tectono-paleogeomorphic framework of the central northern basin during the Early Silurian and the Late Devonian to Early Carboniferous changed remarkably in topography from the initial low in east and high in west to high in northeast and low in southwest. The major paleogeomorphic elements developed in these periods included the strong eroded uplift high, the uplift marginal slope, the gentle ramp of the depression margin and the depression belt. The sandstones of the lowstand and the early transgressive systems tracts were deposited along the uplift marginal slopes and the gentle ramps of the depressions comprise the prolific reservoirs in the basin. The study indicates that the distribution patterns of the unconformities within the basin are closely related to the paleogeomorphic features and evolution of the paleouplift belt. From the high uplift belt to the depression, we found the composed unconformity belts at the high uplift, the truncated and onlap triangular unconformity belts along the uplift marginal slopes, the minor angular unconformity or discontinuity belts along the transitional zones from the uplift marginal slopes to depression and the conformity belt in the central depression. The truncated and the onlap triangular unconformity belts are the favorable zones for the formation of stratigraphic trap reservoirs. palaeostructural geomorphology, unconformity distribution patterns, central paleouplift belt, Tarim Basin

Received July 20, 2008; accepted December 5, 2008 doi: 10.1007/s11430-009-0061-8 † Corresponding author (email: [email protected]) Supported by National Basic Research Program of China (Grant No. 2006CB202302), National Natural Science Foundation of China (Grant No. 40372056) and Frontier Research Project of Marine Facies

Citation: Lin C S, Yang H J, Liu J Y, et al. Paleostructural geomorphology of the Paleozoic central uplift belt and its constraint on the development of depositional facies in the Tarim Basin. Sci China Ser D-Earth Sci, 2009, 52(6): 823-834, doi: 10.1007/s11430-009-0061-8

Research on the paleostructural geomorphology and its constraint on the paleogeography of sedimentary basins has been an international active subject of sedimentary geology and petroleum-bearing basin analysis. The technique, called seismic paleogeomorphology, has recently been widely used to reconstruct the paleogeomorphic features of carbonate platforms or clastic depositional systems based on high-resolution seismic ― data[1 4]. The Tarim Basin is a large, petroleum-rich superim― posed basin in Northwest China[5 8]. The basin experienced multi-phased important tectonic reformations since Phanerozoic and as a result, is very complicated in basin architecture. One of the regional distinctive features of the basin is the development of a series of regional tectonic unconformities and basin-scale paleouplift belts. Many years of petroleum exploration and investigation in the basin have found that the formation and evolution of these paleouplifts and paleoslopes, genetically closely related to the basin tectonic backgrounds, have played an important role in the petroleum accumulation. The study of these paleouplifts in the basin has been intensified over the last ten years. The origin and evolution of paleouplifts, in fact, has long been an internationally investigated subject and studied in depth in most oil- and gas-bearing basins in the world. There are also some previous works concerning the characteristics of the paleouplifts and unconformities in ― the Tarim Basin[6 9]. However, there are few reports to date on the paleogeomorphic features of these paleouplifts and particularly, their control on the development of sedimentary facies in the basin. Based on borehole and networked seismic profile data, we attempt to reconstruct the paleogeomorphic features of the Paleozoic central uplift belt of the Tarim Basin, mainly using the method flattening the overlying sedimentary horizon (paleo-water level) of the paleouplifts together with integrated analysis of the contact and organization of unconformities. We try to reveal the control of paleostructural geomorphology on the development and distribution of sedimentary facies, providing evidence and analytical method for the further study of the formation and evolution of the paleouplifts, tectonic paleogeography and the prediction of the distribution of stratigraphic traps in the basin. 824

1 Geological setting and tectono-stratigraphic sequences The superimposed Tarim Basin resting on the pre-Sinian continental crust underwent a long-term geological history including a series of development stages of prototype basins from the Sinian, Early and Late Paleozoic, Triassic, Jurassic to Cenozoic times, with significant changes in the basin nature and tectonic settings. During the Sinian-Paleozoic, the tectonism was active and generated a number of large paleouplifts within the basin, such as the Tabei, Taxinan, Tazhong and Bachu uplifts due to the strongly parted basement and the complicated plate tectonic backgrounds. During the development stage of inland depressions from Mesozoic to Cenozoic times, the basin experienced in turn several phases of strong deformation, uplifting and erosion, which resulted in the superimposition and reformation of the different prototype basins and generated a distinctive basin architecture complicated by important tectonic unconformities and differently-orientated paleouplifts and paleodepressions[8]. The formation and evolution of these uplifts and depressions determined the fundamental characteristics of the paleotectonic framework and paleogeography of the basin. The development of a few of large scale NWW-, NEE- or NE-trending paleouplifts and paleodepressions characterized the structural framework of the Tarim Basin in the Paleozoic[6]. The composite west-to-east orientated central uplift belt is about 1000 km long and 80－150 km wide, and consists of the Tazhong and the Bachu uplifts, comprising faulted and folded complicated structural zones with obviously different evolutionary history (Figure 1). The NWW-trending Tazhong uplift is about 300 km long, and 80－100 km wide. It is relatively wide and gentle to the west, and steep and narrow to the east, and bounded by No.1 fault belt to the north, the southern-marginal fault belt to the south. The uplift generally was formed during Paleozoic and encompasses mainly NWW-trending faulted belts, which spread westward and extend converge to the east. The major faults of the uplift generally developed within the middle and lower part of the Paleozoic system, mainly including base-involved thrust faults and high-dip thrust ones. These fault belts amalgamated with the NEtrending fault belts along the southeastern margin. The Bachu uplift belt including a series of NW or

Lin C S et al. Sci China Ser D-Earth Sci | Jun. 2009 | vol. 52 | no. 6 | 823-834

Figure 1

Schematic map showing the distribution of the sub-tectonic units and the central uplift belt of the Tarim Basin.

NWW-trending faults is a large structural nose orientated northwest to westward and plunged northwestward to the central basin. It is about 500 km long, 80－150 km wide, and confined by NEE-trending Keping fault belt to the west, the Selibuya-Majiatage fault belt to the south and the Tumuxiuke fault belt to the north, displaying as a whole a back-thrust folded belt in N-S cross section. The uplift belt occurred initially in the Paleozoic and took its final shape in the Neogene. Based on the angular or minor angular unconformities identified in the central uplift belt of the Tarim Basin, the Paleozoic system can be divided into eight supersequences (second-order sequences) or tectono-sequences, consisting respectively of the Lower Cambrian, Middle to Upper Cambrian, Middle to Lower Ordovician, Upper Ordovician, Silurian, Middle to Lower Devonian, Upper

Devonian to Carboniferous and Permian System (Figure 2). The Sinian System unconformably rests on the pre-Sinian crystallized and metamorphosed basement consists mainly of shore and shallow marine clastic, carbonate and volcanic clastic deposits. The contact between the Sinian and the overlying Lower Cambrian appears as a minor angular or parallel unconformity. In the central uplift belt, it can be observed on seismic profiles that the Lower Cambrian onlaped unconformably on the Sinian or pre-Sinian basement. The Cambrian is about 1600―2000 m in thickness and dominated by platform or confined platform carbonate deposits encompassing essentially dolomite, micrite and lime mudstone. The basal Lower Cambrian contains phosphorite nodules and chert stripes or lumps, whereas the Middle Cambrian comprises plenty of gypsum beds and carbon-


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Figure 2

Classification of the Paleozoic tectono-stratigraphic sequences and the depositional evolution of the central uplift belt in the Tarim Basin.

ate deposits of evaporite platform. The contact of the Middle Cambrian with the Lower Cambrian displays as a minor angular unconformity, observed commonly on seismic profiles along the central uplift. The Lower Ordovician, onlaped unconformably on the underlying strata, is composed mainly of thick-bedded dolomite or dolomite limestone formed in restricted platform environment. The Middle to Upper Ordovician are widely developed open platform carbonate deposits with reefal and shoal facies, except from the northern depression where mainly received abyssal argillaceous and turbidite deposits. The contact between the Upper Ordovician and the Middle Ordovician shows as a minor angular un826

conformity, which is marked by a purplish or red horizon (Tumuxiuke or Qiaerbake Formations) consisting of micrite or limestone, calcareous mudstone or brownish bioclastic limestone in the outcrop along the northwestern margin of the basin. In addition, the overlying Upper Ordovician Qilang Formation in the section is composed mainly of shelf-slope mudstone or calcareous mudstone. The Silurian is separated from the underlying Ordovician by a widely spread angular unconformity, to which most of the faults in the Tazhong uplift terminated. This tectono-sequence, although faulted locally, was usually deformed in the form of folding. On the uplift high, the Middle Ordovician commonly was entirely


eroded. The Silurian and the Middle to Lower Devonian mainly contain shoreline or tide flat, deltaic and shallow marine clastic deposits. The contact of Upper Devonian with the underlying Middle Devonian appears as a regional angular unconformity. Along the eastern margin of the basin, the Silurian and the Middle to Lower Devonian were seriously deformed and eroded. During the Carboniferous to Permian, the basin received dominantly shore or beach, tidal flat and fluvial deltaic deposits. The middle part of the Carboniferous usually contains some bioclastic limestone, thin-bedded limestone and gypsum or salt beds. The Permian system is mainly composed of fluvial, shallow lacustrine clastic deposits, intercalated with some medium-acidic and basaltic volcanic rocks. Toward the end of the Permian, the basin became an inland basin filled with nonmarine clastics. In the Bachu uplift belt, the basal contacts of the Eogene and the Neogene are all displayed as obvious angular unconformities, with the underlain strata strongly deformed and mostly lack of the upper part of the Mesozoic along the uplift high due to erosion.

2 Methodology for paleogeomorphic reconstruction Here we develop a method by flattening and decompacting the overlying sedimentary layers of the paleouplifts based on well-constrained seismic data to reconstruct the paleouplift geomorphology. The study has preferably reconstructed the paleogeomorphic features and revealed evolution of the central paleouplift belts, and at the same time, documented the relationship between the distribution styles of unconformities and the evolution of the paleostructural geomorphology of the paleouplift belt. In the Tarim Basin, relatively large-scale uplifts usually experienced a polycyclic history from subsidence-infill to uplifting-erosion. A typical evolutionary cycle of an uplift can be divided into the following three stages: (1) the initial uplifting or underwater uplifting stage before the uplift exposes above the water surface, the overlying deposits thin toward the top of the uplift and form a shoaling upward depositional sequence, i.e., “depositional uplift”; (2) the major uplifting and eroding stage after the uplift exposes above the water surface, causing stratigraphic hiatus and generating an unconformity, i.e., “eroded uplift”; (3) the following subsidence stage when the uplift subsides with stratigraphic

onlap on the unconformity, forming a deepening upward depositional sequence, also a depositional uplift. Obviously, it is necessary to distinguish different development stages in order to reconstruct the paleogeomorphology of the uplift. 2.1 Flattening the overlying sedimentary layers of uplift To reconstruct the paleogeomorphology of a depositional uplift, we first need to select a sedimentary layer, just overlying the top of the paleouplift and flatten this horizon. When the paleo-water depth or the sedimentary facies of the selected layer changed laterally, it is necessary to correct the paleo-water depth. Then, we need to decompact the flattened strata in order to obtain the true paleogeomorphic feature[10,11]. When a sediment layer is normally compacted with depth, there is an exponential porosity-depth relationship:

ϕ = ϕ0 e−Cy ,

(1)

where ϕ represents the porosity of the sedimentary layer at the depth of y, ϕ 0 is surface porosity of the sedimentary layer and C is compaction coefficient. Let y1 and y2 be the top and the bottom depths of the sedimentary layer, S is basement depth of the sedimentary layer, when flattened to initial depositional surface, we have

φ0

φ

(e−Cy1 − e−Cy 2 ) + 0 (1 − e−Cs ). (2) C C If the porosity is approximately calculated by using the depth of middle point of the sedimentary layer, S can be expressed as S = y 2 − y1 −

S = (( y 2 − y1 )e −C ( y2 + y1 ) / 2 ) /(1 − ϕ0 ).

(3)

The paleobathymetry correction may need the integrated analysis of benthic microfossils, trace fossils, depositional facies and distinctive geochemical signatures. The paleobathymetry sometimes can be roughly determined from the geometry of sedimentary layers displayed on seismic profiles, such as clinoforms of deltaic front, carbonate platform margin or shelf slopes. By decompacting the clinoforms to their original geometries, we can straightforwardly measure the paleo-water depth in the sections. Figure 3 shows two-selected paleostructural geomorphic profiles reconstructed through the method mentioned above. It should be pointed out that high-resolution seismic data set is the necessary database for reconstruction of paleogeomorphology. We have to calibrate the seismic reflection with well data, to determine the


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Figure 3 Reconstructed late Middle Ordovician paleostructural geomorphology of the profile (a) across the middle part of the Tazhong uplift, and the profile (b) across the northern slope in the eastern part of the paleouplift through flattening the overlying layer just above the top of the paleouplift. Note the reflection architecture of the clinoforms and the onlap sequence architecture.

flattened reflection horizons and trace their 3D distribution for reconstructing paleogeomorphology. 2.2 Distribution pattern of unconformities and relevant uplift paleogeomorphology

The tectonically active, large superimposed basins commonly developed a series of unconformities with complicated arrangement and distributed patterns[12]. The contact relation of a major unconformity may change significantly from high angular, lower angular, parallel to conformable contacts in different structural belts within a basin. This variation generally reflected the paleostructural geomorphic change from the uplift high, marginal slope to depression. In the high uplifted belt, the multiple uplifting may result in composition of numerous unconformities. There are several distinctive unconformity organized belts recognized from the paleouplift to depression based on detailed examination and correlation of the unconformities in the Tarim Basin (Figure 4). The composite unconformity belts are distributed over the paleouplift high; the truncated or the onlap triangular unconformity belts developed along the 828

marginal paleoslope; and the minor angular or parallel unconformity belts situated along the transitional zones from the marginal slope to the conformable area within depression. The composite unconformity belt represents an obviously uplifted and eroded paleogeomorphic high. It encompasses usually a major unconformity and a few of secondary unconformities. The major unconformity, produced as the result of an important tectonic event, is of relatively large amount of erosion and long exposed period. The distribution of different composite unconformity belts reflects the location of the paleouplift highs formed in different evolutionary stages. The wedge-shaped triangle unconformity belts include both the truncated and the onlap ones. The former refers to those formed by the truncation of the secondary compounded unconformities or sequence boundaries by the major unconformity. This unconformity belt separates the downward parallel unconformity or conformable belts with an initial truncated point. The onlap triangular unconformity belt is formed by the onlap of the secon-


Figure 4

Paleogeological map and distribution of the unconformity Tg3 (seismic reflection surface) at the base of the Upper Devonian, the Tarim Basin.

dary unconformities or sequence boundaries on the major unconformity, which sits at the base of the triangle unconformity belt. The angle of inclination of the secondary unconformities or sequence boundaries against the major unconformity has been suggested to reflect the intensity of the uplifting and deformation. The deeper inclination should be undoubtedly the result of relatively strong deformation. The petroleum exploration in the basin has proven that the wedge-shaped triangle unconformity belts are the favorable zones for the formation of large-scale unconformity stratigraphic traps, and it is important to delineate the distribution of these unconformity belts for the reconstruction of paleogeomorphology and the prediction of stratigraphic traps in the basin.

3 Paleozoic paleostructural geomorphology and its constraint on the development of sedimentary facies 3.1 Middle to Late Ordovician

(1) Paleostructural geomorphic units and their distribution. The integrated analysis of seismic profiles, borehole and outcrop data indicates that the central uplift belt in the Tarim Basin began to form in the late Middle Ordovician. The unconformity Tg5-2 (seismic reflection surface) within the basin representing this uplifting event can be compared with the unconformity on the top of the red Tumuxiuke or Qiaerbake Formation observed in outcrop along the northwestern basin margin. The study has recently shown that the Tazhong and Bachu structural belts were gentle slopes or lower relief


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ramps from the Sinian to Early Cambrian, under a stretching background belonging to a passive continental environment with syndepositional faults (inverted later), small-scale grabens or half grabens. Toward the late Middle Ordovician, the tectonic setting of the Tarim Basin evolved from extension to compression. Some early normal faults were inverted into thrust faults in the Tazhong uplift. The three widely distributed unconformities represent three relatively strong tectonic uplifting events occurred at the end of the late Middle Ordovician early Late Ordovician and the end of Late Ordovician, and they appeared to be the later, the stronger. On seismic profiles, these unconformities are clearly displayed and traceable throughout most parts of the central basin. From the reconstructed Middle to Late Ordovician tectono-paleogeomorphic map, we can see clearly that the central uplift belt in fact consists of the northdipping Bachu paleoslope, the NEE-trending Hetianhe paleouplift, the EW-trending Hentianhedong (to the east of the Bachu uplift) and the NW-trending Tazhong paleouplifts. This giant paleouplift-slope belt with complicated paleogeomorphic feature was generally higher to the southwest and lower to the northeast in topography. From the paleouplift belt to the submarine basin, a number of paleogeomorphic units have been identified, including the high uplift belt in the central part of the paleouplift, marginal slope flanking the paleouplift, the shelf gentle ramp or low relief platform, shelf break zone and deep marine plain or basin. They had significant influence on the distribution of the paleogeography of the basin. Formed usually by the thrust horsts or high angular thrust fault blocks, the high uplift belt can be further divided into relatively narrow faulted uplift high and high uplift platform. The high uplift belt of the Hetianhedong paleouplift orientated northeastward, whereas that of the Tazhong paleouplift trended in NWW direction. Both the northern No.1 and the southern marginal fault (Tangbei fault) belts confined the high central platform of the Tazhong paleouplift, which was wider in the middle part, and became narrow and plunged to the west and east ends respectively. The central fault horsts, the Tazhong No.10 and No.5, Tangbei and Tumuxiuke fault belts all formed a faulted uplift high belt about 10－20 km wide. The study has shown that the paleostructural geomorphic features obviously controlled the development and distribution of sedimentary facies. 830

The marginal slope belts of the paleouplift represent the transitional zones from the uplift high to the shelf shallow platform. Along the northern No. 1 fault developed a relatively steep marginal slope or slope break zone during Middle to Late Ordovician. It extended westward along the Tumuxiuke fault belt to the outcrop area in the western margin of the basin. The southeastern marginal slope break zone of the Hetianhe and the Tazhong paleouplifts are distributed along the southern marginal fault, the Badong fault, the Tazhong No.7 and No.8 fault belts, and extended southwestern to join with the southwestern marginal slope belt of the Hetianhedong paleouplift. The platform marginal slope zones had shifted obviously with time as the result of the interplay of sea level change and tectonic subsidence. It is worth to point out that the marginal slope break zones along the margin of the paleouplifts may be genetically the socalled structural slope break zones[11,12], and their formation has been suggested to be related to the activity of the marginal faults. The study indicates that the activity of the northern No.1, Tumuxiuke and the southern marginal faults constrained significantly the development and distribution of the marginal slope break zones formed during Early Paleozoic. Shelf gentle ramps and low rises led to shore and shallow marine environments. To the north of the Tazhong paleouplift was a shallow marine gentle shelf ramp, which extended from the eastern Guchengxu low rise to the northwest Shuntuoguole low rise during the late Middle to Late Ordovician. This period of uplifting, companied with sea level falling, resulted in extensive exposure of the northern interior of the basin. The northwest Shuntuoguole low rise, in fact, was a low nose uplift protruded northward into the north depression. The shallow marine shelf ramp to the southeast of the Tazhong paleouplift was relatively narrow and extended southeastward to the semi-deep marine environment of the Tanggubasi depression. The shelf slope break zone was an abrupt slope belt transitional from shelf to the submarine basin, and usually controlled by syndepositonal faults. The reconstructed paleogeomorphic map shows a paleoslope break zone between the northern shelf interior and the northeastern Manjiaer deep depression (Figure 5). Along the slope break zone, the environment changed abruptly from the shelf marine environment to the abysmal plain, and the drop of the slope is up to 1500－2200 m meas-


Figure 5 Paleostructural geomorphic feature of the central uplift and surrounding area during the Middle to early Late Ordovician (a) and its constraint on the distribution of sedimentary facies (b).

ured from the shelf edge clinoforms observed on seismic profiles. This shelf slope break zone began to exist from the Late Cambrian or Early Ordovician and shifted from west to east with time, and elongated southeastward to the northeast marginal slope of the Tazhong paleouplift. It has been realized that the northern marginal slope of the Tazhong paleouplift was different from that of the southeastern or the southwestern; the former formed a shelf slope break zone near the deep marine basin. The abyssal basin below the slope break zone received thick turbidite, silicolite and calcareous mudstone deposits. Several thousands meters thick of turbidite deposits of the Middle and Upper Ordovician, for instance, have been found in the borehole Tazhong 1 in the northern Manjiaer depression and the Kuluketage outcrop profiles in the eastern part of the Kuqa depression. (2) Paleostructural geomorphic control on the development of carbonate facies and reservoirs. The paleogeomorphic features of the central paleouplift and surrounding slopes significantly influenced the develop-

ment and distribution of the Middle to Late Ordovician carbonate facies and reservoirs. The uplift highs and the marginal slopes in the central part of the uplift belt had suffered repeated erosion and weathering. The seriously faulted, crushed or densely fractured zones and those strain easily concentrated deformation belts were the most favorable districts for karstification and formation of karst-type reservoirs. Some paleouplift highs do not keep their original topographic feature at present and just display as a composite unconformity belts. The relatively large-scale faults in the paleouplift highs could serve as important geofluid migration pathways and then were advantageous of dolomitization. The sags formed in the top of the faulted uplift highs have proven to be the most favorable parts for the formation of high quality reservoirs in the paleouplift belt. The marginal slope or slope break zone of the paleouplift was an abrupt paleogeomorphic zone from shallow water environments to deep-water basin. Systematic investigation of sedimentary facies in the study


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area indicates that the upper marginal slopes were the most favorable zones for the development of high energy carbonate facies such as reefal, beach or shoal facies, while the lower slope or the base of the slope commonly deposited marginal slump breccias or fan systems. High-energy facies of the Yijianfang and the Lianglitage formations of the Middle and Late Ordovician, consisting mainly of biaclastic limestone, granule limestone, intraspararenite and oolitic limestone, were distributed likely along the platform marginal slope break belts of the uplift. The high-energy facies seem to develop mainly in the highstand systems tracts underneath the unconformities or the exposed sequence boundaries and the overlying early transgressive systems tracts. When the sea level fell, the marginal slopes were sensitive to synchronous karstification or dissolution. The petroleum exploration has indicated that the platform marginal slope developed along the No.1 fault belt was an important petroleum accumulation zone, where the reefal and shoal facies, combined with dissolution and fracturing, generated the so-called “connected permeable bodies”, which comprise the major proven reservoirs of the stratigraphic traps in the area. The southwestern and southeastern margins of the central paleouplift belts were of similar paleogeomorphic features to that of the No.1 fault belt, and should have favorable conditions or environments for the development of high-energy facies. Small-scale reef, oolitic and intraspararenite shoal deposits may develop along the southwestern and southeastern margins, comprising the major reservoirs of potential stratigraphic traps. 3.2 Paleostructural geomorphology of Early Silurian and Late Devonian

(1) Paleostructural geomorphic feature and the framework of uplifts and depressions. The strong tectonic deformations occurring at the end of the Late Ordovician and the end of Middle Devonian generated two basin-wide angular unconformities (Tg5 and Tg3), and remarkably reformed the paleostructural framework of the Tarim Basin. During this period, the northern Manjiaer depression gradually uplifted and meanwhile the southwest of the basin subsided, which resulted in a significant variation in paleogeomorphology from the initial low in east and high in west to high in northeast and low in southwest. The eastern part of the Tazhong paleouplift belt hinged up along with the uplifting of the southeastern basin margin, and then the uplift inclined 832

westward as a whole. Toward the late of the Middle Devonian, the Tadong, Tazhong and Taxinan paleouplifts were connected to form a giant paleouplift belt along the southeastern basin margin. Based on the reconstructed paleogeomorphic map and facies associations, several distinctive paleostructural geomorphic units have been identified from the central paleouplift belt to the north depression, including mainly the strong eroded uplift high, uplift marginal slope belt, gentle ramp of the depression margin and the depression (Figure 6). Accordingly, the unconformity distribution styles varied from the composite high angular unconformity belt, minor angular unconformity belt to parallel and conformity area. The similar tendency of the paleogeomorphic variation existed from the north Tabei paleouplift belt southward to the depression. This paleostructural geomorphic setting controlled generally the distribution of the siliciclastic depositional facies of the Early Silurian, and the Late Devonian to Early Carboniferous. (2) Constraints of paleostructural geomorphology at the end of tectonic uplifting on the development of the lowstand to early transgressive systems tracts. In this study, we have recognized two kinds of important clastic systems tracts according to the interplay of tectonic uplifting and sea level change, and their distribution was obviously constrained by the paleostructural geomorphic feature of the basin (Figure 6(b) and (c)). One is the lowstand systems tracts formed during the latest uplifting period along the relic basin margins when the paleouplifts and surrounding paleoslopes were eroded. The other is the transgressive systems tracts usually deposited along the paleoslopes when the paleouplifts gradually subsided after the latest uplifting stage and following regional transgression occurred. The paleouplifts were gradually onlaped and most parts of or even the entire paleouplifts submerged and finally were capped with marine mudstone or lime mudstone. The lowstand systems tracts formed during the latest uplifting stages mainly develop along the depression marginal ramps below the initially truncated point. The tectonic uplifting increased the erosional area and produced large amount of clastic sediment inputs resulted in widely developed lowstand fan or deltaic systems. We can delineate the distribution of the relic basin and the marginal lowstand systems tracts through tracing the initially truncated point based on networked seismic


Figure 6 Reconstructed paleostructural geomorphic feature of the northern Tarim Basin during the Early Silurian (a) and the depositional models of the lowstand and transgressive systems tracts during or after the latest uplifting stage ((b)and (c)).

profiles. During the Early Silurian the deltaic and estuarine or tide flat systems of lowstand and transgressive system tracts were broadly developed along the gentle marginal slopes of the Tazhong and the Tabei paleouplifts to the north depression. The deltaic progradational clinoforms of the lowstand systems tracts can be observed on seismic profiles along the northern paleoslopes of the Tazhong paleouplift and the southern paleoslopes of the Tabei paleouplift. The lowstand and early transgressive systems tracts formed during or after the latest uplifting period of the Late Devonian to Early

Carboniferous comprise plenty of beach and deltaic sandy deposits, and particularly the widely spread beach and near-shore sandy bodies with higher textural and compositional maturity overlapping on the major unconformity formed the most prolific reservoir in the area. These sandstones combined with the overlying marine mudstone or muddy limestone form ideal assemblage of seals and reservoirs in the basin. The Hadexun oil field recently found in the southern marginal slope of the Tabei paleouplift has been proven to be this type of sandstone stratigraphic trap.


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4 Conclusions (1) By using the paleogeomorphic reconstruction technique flattening the overlying sedimentary horizon of the paleouplifts, together with the analysis of the contact relationship and distribution styles of unconformities, we can rebuild the palaeostructural geomorphic features of paleouplifts, and reveal their constraints on the development and distribution of depositional facies. This study has provided important evidence for the prediction of stratigraphic traps formed along the paleouplifts and the surrounding paleoslopes. (2) The central paleouplift belt of the Tarim Basin initiated to develop during the Middle Ordovician, and experienced four major phases of uplifting. Consisting of the Hetianhe, Hetianhedong, Tazhong paleouplifts and Bachu paleoslope, the central paleouplift belt was a large composite paleouplift and paleoslope belt with complicated paleogeomorphic features. It was relatively high in the west and low in the east in topography, and is inclined as a whole eastward. The tectonism occurred during the Late Ordovician resulted in the uplifting of the southeastern basin margin, and as a result, the eastern part of the Tazhong paleouplift hinged up and inclined westward in reverse toward the end of the Late Ordovician. (3) The distribution styles of the unconformities in the Tarim Basin are closely related to the evolution of the paleostructural geomorphology of the paleouplift belt. There are several unconformity belts identified from paleouplifts to depressions: the composite unconformity belts on the paleouplift high, the truncated unconformity triangle belts or the onlap unconformity belts along the surrounding paleoslopes, the minor angular or parallel 1

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Zeng H, Ambrose W A, Villalta E. Seismic sedimentology and regional depositional systems in the Mioceno Norte Area, Lake Maracaibo, Venezuela. Leading Edge, 2001, 20: 1260―1269 Posamentier H, Kolla V. Seismic palaeogeomorphology and stratigraphy of depositional elements in deep-water settings. J Sediment Res, 2003, 73: 367―388 Derek E S, Peter B, Flemings R, et al. Seismic palaeogeomorphology, lithology, and evolution of the late Pleistocene Mars-Ursa turbidite region, Mississippi Canyon area, northern Gulf of Mexico. AAPG Bull, 2003, 91(2): 215―234 Carter D C. 3-D seismic palaeogeomorphology: Insights into fluvial reservoir deposition and performance, Widuri field, Java Sea. AAPG Bull, 2003, 87: 909―934 Li D S, Liang D G, Jia C Z, et al. Hydrocarbon accumulation in the Tarim Basin, China. AAPG Bull, 1996, 80(10): 1587―1603 Jia C Z. The Tectonic Characteristic of the Tarim Basin in China (in Chinese). Beijing: Petroleum Industry Press, 1997

unconformity belts transitional from the paleouplift marginal slope to depression. Their development and distribution obviously determined the formation and distribution of the stratigraphic traps in the basin. (4) The paleostructural geomorphic units recognized from the paleouplifts to the depressions during the Middle to Late Ordovician in the Tarim Basin include the paleouplift high, marginal paleoslopes and paleoslope break zone, shelf slope and relief platform, shelf slope break and abyssal basin. They significantly influenced the configuration of geography and the formation and distribution of the favorable reservoirs and stratigraphic traps. The paleouplift highs and surrounding paleoslopes, particularly the major composite unconformity belts were the advantageous zones for the formation of the weathering karst and structural fracture reservoirs. The paleouplift marginal slopes or slope break belts were the suitable zones to the development of reefs and oolitic beach or shoal facies. Along the zone also commonly occurred the synchronous dissolution, advantageous of generating ideal reefal and shoal carbonate reservoirs. The reconstruction of palaeostructural geomorphic features of paleouplift belts can provide an important base for the prediction of stratigraphic traps. (5) During the Early Silurian and early Late Devonian, the transitional zones from the paleouplift marginal slopes to the marginal ramp of the relic depressions below the initially truncated points were favorable to the deposition of the lowstand and the early transgressive systems tracts during or after the latest uplifting stages. These siliciclastic depositional systems combined with overlying marine fine-grained deposits formed ideal assemblage of seals and reservoirs, favorable to the formation of large-scale stratigraphic traps. 7 8

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Kang Y Z, Kang Z H. The tectonic evolution and petroleum of the Tarim Basin (in Chinese). Sci Geol Sin, 1994, 3(4): 180―191 Jin Z J, Wang Q C. Recent developments in study of the typical superimposed basins and petroleum accumulation in China: Exemplified by the Tarim Basin. Sci China Ser D-Earth Sci, 2004, 47(Suppl. 2): 1―15 He D F, Xie X A. The central uplifts and petroleum exploration in Clatonic basins in China (in Chinese). Petrol Expl, 1997, 2(2): 11―19 Lin C S, Zhang Y M, Li S T, et al. Quantitatively modeling of multiple stretching of lithosphere and deep thermal history of some Tertiary rift basins In East China. Acta Geol Sin, 2002, 76(3): 324―330 Lin C S. Tectonostratigraphic analysis of sedimentary basins― Case study on the tectonic active basins in China (in Chinese). Geosciences, 2006, 20(2): 185―194 Lin C S, Pan Y L, Xiao J X, et al. structural slope-break zone: key concept for sequence stratigraphic analysis and petroleum prediction in fault basins (in Chinese). Earth Sci―J China Univ Geosci, 2000, 25(3): 260―265
