The Upper Devonian-Permian in the Tarim Basin, composed of clastic rock and carbonate rock layers of alternate marine and continental facies, is one of the main target strata of oil and gas exploration and development in the Tarim Basin, and has a high degree of research. The coastal Donghe sandstone diachronic sediments from the end of Devonian to the Early Carboniferous and the shallow shelf mudstone and limestone sediments from the Early Carboniferous constitute the bottom transgressive reservoir-cap assemblage and hydrocarbon-bearing sequence. The stratigraphic structure of overlapping pinch-out is conducive to the development of stratigraphic lithologic traps. Wave-controlled littoral quartz sandstone in the Donghe sandstone member at the bottom of the transgression period is a set of high-quality reservoirs with high porosity and permeability. Large and medium-sized oil and gas fields have been discovered one after another in Donghetang, Tazhong, Hudson, Caohu and other areas, demonstrating the exploration and development potential of this formation is huge^[1].

Sequence stratigraphy theory and model proposed by predecessors were mainly based on exogenous clastic sediments in basins and shelf areas^[2,3,4]. The main sedimentary phenomena of this single sediment source model is the occurrence of inclined large-scale pre-deposition of clastic sediments in shelf and basin areas. In addition, important identifiable interfaces such as marine-flooding surface and condensed section also occur mainly in mudstone strata of shallow sea and basins. However, whether the theory and model of sequence stratigraphy are applicable needs to be studied further when the internal carbonate platform sediments appear in the shallow shelf area.

The Upper Devonian-Carboniferous in the Tarim Basin is a set of clastic rock intercalated with carbonate rock deposited in the mixed shelf area. Through detailed study of sequence stratigraphy of this set of strata, the development characteristics of sequence stratigraphy in mixed shelf area can be systematically summarized, and the development model of sequence stratigraphy in coastal-mixed shelf area can be established, in the hope to supplement and improve the theory and research methods of sequence stratigraphy.

The Tarim Basin is a superimposed composite basin with at least three prototype sedimentary basins developed in the Paleozoic. On the basis of the formation, filling, folding, structural uplift and denudation flattening of the underlying Silurian-Middle Devonian prototype sedimentary basins, a new prototype sedimentary basin was formed in the Late Devonian-Permian period superimposing on it, which underwent the whole process of basin formation, filling, evolution and extinction (Fig. 1). This period is also an important period for the structural development of the Tarim Basin^[5,6]. The tectonic pattern changes into the tectonic environment of north compression and south tension. The northern margin inherited the previous tectonic pattern and was still the active continental margin, which showed the arc formation and subtraction of strong subduction islands from the northern paleo-ocean to the south, resulting in the uplift of the northern part. The Paleo-Tethys Ocean was formed in the southwestern margin of the Tarim Plate, and the extensional pattern led to the subsidence of the southwestern margin, the transgression began thereafter. The Tarim Basin was a shallow sea basin on the paleopassive continental margin during the Late Devonian-Permian Period.

Late Devonian-Carboniferous is an important transgressive period in the Tarim Basin. The sea water intruded into the craton margin depression and inner depression of the Tarim craton from the direction of the Paleo-Tethyan Ocean. After filling up and leveling deposition of the littoral facies Donghe sandstone member, gravel sandstone member and shallow marine facies lower mudstone member, the sedimentary environment turned into the mixed shelf, with a thickness of 1200 m. During the Early Permian, large-scale regression occurred in the Tarim Basin. The regression was still from northeast to southwest, representing the "where to enter and where to leave" type of transgression and regression. Large-scale volcanic eruptions underwater and on land took place in the Middle Permian, leaving volcanic rocks covering most of the basin. The regression continued in the Late Permian, ending the sedimentary filling process of the prototype basin.

The prototype sedimentary basins of Upper Devonian-Carboniferous-Permian in the Tarim Basin (abbreviated as D3CP hereafter) have a fairly complete set of strata with distinctive lithologic characteristics, stable lateral distribution, making it easy to classify, trace and correlate them, and their classification schemes are basically the same. Referring to the division scheme of the Research Institute of Tarim Oilfield Company, combined with the stratigraphic review and well connection correlation across the whole basin, the Upper Devonian-Carboniferous in the Tarim Basin is also divided into three series, three formations and ten lithologic members, and the Permian is divided into two series, two formations and four lithologic members (Figs. 2 and 3).

According to the latest research results of paleobiostratigraphy in the Bachu and Tazhong areas^[7], it is inferred that the stratigraphic boundary of the Upper Devonian-Lower Carboniferous is at a layer of 3 m thick limestone in the middle of the lower mudstone member in the Bachu Formation, and that the Late Devonian-Early Carboniferous deposited continuously.

The Late Devonian-Carboniferous is an important sedimentary period of marine strata in Tarim Basin. The bottom of Donghe sandstone member is basin-level unconformity. The Donghe sandstone member and the gravel-bearing sandstone member deposited from Late Devonian to Early Carboniferous are a typical diachronic lithologic member^[8,9,10]. The lower mudstone member and the bioclastic limestone member overlap and change facies from west to east. The standard limestone member and the Xiaohaizi Formation limestone at the Carboniferous top are important stratigraphic correlation markers and widely distributed in the basin.

Permian is an important period for the Tarim basin when it changed from marine facies to continental facies^[11]. The limestone member of lower Permian Nanzha Formation is only distributed in Bachu and southwestern Tarim, and in conformable-unconformable contact with the limestone member of upper Carboniferous Xiaohaizi Formation. It is marine carbonate platform deposit in the southwestern part of the early Permian sedimentary basin, with sedimentary range reducing significantly. In the late Early Permian, the basin retreated from east to west. In the Middle Permian, because of the closure of the South Tianshan Ocean and the subduction of the North Tianshan Ocean Basin, a large amount of volcanic rock developed in the basin. In Late Permian, most of the basin uplifted into land, only Tazhong and southwestern Tarim were back-arc basins, and marine-terrestrial interfacies deposits developed. At the end of Permian, the sea water completely withdrew from the Tarim basin, and the deposition of the prototype basin ended and a large part of the strata were denuded.

The Carboniferous standard limestone member in the Tarim Basin is a micrite limestone stratum deposited in a semi- closed platform environment with a thickness of 10-30 m. On logging curves, it features low gamma and high resistivity, making it distinct from the upper and lower strata. It corresponds to seismic Tg21 reflection phase axis and is isochronal sedimentary body, so it is a good marker for stratigraphic correlation.

In this study, a NE sedimentary facies profile of connected well strata across the Tarim Basin was made (Fig. 2). The direction of the section is basically perpendicular to the paleocoastline trend. The standard limestone member was taken as isochronal sedimentary stratum for stratigraphic leveling analysis.

The Upper Devonian-Carboniferous strata in Tarim Basin can be divided into one and a half sedimentary cycle stratigraphic sequence based on the transgression and regression cycles. The strata below the standard limestone member are basically transgressive sedimentary sequence, while those from the standard limestone member to the Xiaohaizi Formation are regression and re-transgression sedimentary sequence.

The Upper Devonian-Carboniferous in the Tarim Basin is dominated by marine carbonate rock and clastic rock, which appear as interbeds. Carbonate deposits predominate in the western region, coming in open platform, semi-limited platform, evaporative platform and other sedimentary facies types. In the eastern region, clastic rock takes the majority, appearing in many sedimentary facies types, including mixed shore, tidal flat, lagoon, delta, wave-controlled shore, river, and alluvial fan etc.

The profile shows that before the sedimentation of the Donghe sandstone member of the Upper Devonian, the paleogeomorphological characteristic of the D3CP prototype sedimentary basin was basically a wide and gentle slope with a height difference of 300 m between the East and west. The Donghe sandstone member and its upper gravel-bearing sandstone member have obvious diachronism. The sedimentary time of Donghe sandstone-gravel sandstone member west of Well Bd2 is Late Devonian. The sedimentary time of Donghe sandstone member east of Well Mx1 is obviously later than that west of Well Bd2. Its sedimentary time is approximately the same as that of the upper part of the lower mudstone member in the area west of Well Bd2. Further east to Well Hd4-well Jn4-2, the deposition time of Donghe sandstone is basically the same as that of bioclastic limestone, so the deposition time of Donghe sandstone in these areas is obviously the Early Carboniferous.

A number of studies on sequence stratigraphy of the Upper Devonian-Carboniferous in the Tarim Basin have been done. In the scheme of sequence division, each of them divided the sequence according to the basic sequence of the third-order sequence, but the results are different^{[12,13,14,15,16]}. The main reason for this problem is that there are significant differences in understanding the third-order sequence in the basin, and that the classification of the sequence of the prototype sedimentary basin hasn’t been correctly defined, understood and classified.

Exxon's five-level stratification scheme^[17] divides the sequence into five levels: megasequence, supersequence, sequence, parasequence group and parasequence. It is well defined and described the parasequence group and parasequence. However, the stratigraphic structures of the first-order sequence to the third-order sequence haven’t been defined and explained in detail. This is the most fundamental reason for the confusion of the division of third-order sequences in the application process.

In sequence stratigraphy, the sequence is defined as a set of genetically related strata bounded by unconformities and corresponding extended conformities at top and bottom. From the definition, the actual research goal of sequence stratigraphy is the prototype sedimentary basin, because the top and bottom of the prototype sedimentary basin are defined by the unconformity surface at the basin level. Sequence stratigraphy is a subject to study the sequence of stratigraphic filling assemblages in prototype sedimentary basins. It is an inevitable choice to place the five level sequence division scheme in the prototype sedimentary basin.

Prototype sedimentary basins refer to sedimentary basins with specific sedimentary strata formed in relatively single geodynamic system or single cycle tectonic evolution stage, whose top and bottom are basin-level unconformities and their corresponding extended conformities, with the marine flooding surface as isochronal scale, genetically related and cyclic stratigraphic filling sequences. It has undergone the whole process of basin formation, filling, evolution and extinction. Its time limit is a complete cycle of relative sea level rising from the lowest to the highest and falling back to the lowest.

In this study, the five-level sequence division scheme was fully applied to the D3CP prototype sedimentary basin in Tarim, and the results are as follows (Table 1 and Fig. 3).

(1) First-order megasequence: Named 1SQD3CP (expressed as a first-order megasequence, the Upper Devonian- Carboniferous-Permian), the named sequence of the prototype sedimentary basin, corresponds to the third tectonic layer. Its top and bottom are basin-level unconformities easy to identify. It represents the whole process of the formation, filling, evolution and extinction of the D3CP prototype sedimentary basin and the forced regression and erosion of the upper strata. Its time limit is about 118 Ma.

The division and tracing of the first-order megasequence is a comprehensive reflection of the research results of stratigraphic Paleontology and sedimentary facies, seismic sequence and tectonic development. Its distribution reflects not only the filling sequence of the prototype sedimentary basin, but also the overlapping relationship of different prototype sedimentary basins. The first-order megasequence is equivalent to one system or the combination of several systems of lithologic strata. For example, the Precambrian-Ordovician in Tarim Basin is a first-order megasequence, the Silurian-Middle Devonian is another first-order megasequence, and the Upper Devonian-Permian is a third first-order mega-sequence, which indicates that the Paleozoic in Tarim Basin is superimposed by three prototype sedimentary basins.

(2) Second-order supersequence: The top and bottom are defined by basin-level unconformities, and the middle is divided according to the maximum marine flooding surface within the prototype sedimentary basin into second-order supersequences. The Tarim D3CP prototype sedimentary basin can be divided into two second-order supersequences. The Upper Devonian-Carboniferous is a second-order supersequence 2SQD3C, the bottom is a basin-level unconformity, and the top limestone member of Xiaohaizi formation is the largest limestone deposit in the sea area, which constitutes a second-order transgressive system tract depositional sequence as a whole. The Permian forms a second-order supersequence 2SQP, which constitutes a second-order highstand system tract sedimentary sequence with a decreasing rate of sea level rise, a shrinking sea area and a complete forced regression. The whole sequence comprises a transgressive-high secondary cycle assemblage sequence.

(3) Third-order sequence: It is the third-order transgressive-high sedimentary cycle combination sequence studied by sequence stratigraphy. It is a set of genetically related stratum units showing as transgression-high cycle combination bounded by the unconformity surfaces and its corresponding conformity-unconformity surfaces at bottom and top, and separated by the maximum or secondary marine flooding surfaces inside. The third-order sequence is divided into type I sequence and type II sequence, in which type I sequence is bounded by SB1 unconformity and consists of lowstand system tract, transgressive system tract and highstand system tract from bottom to top. Type II sequence is bounded by SB2 unconformity and consists of transgressive system tract and highstand system tract from bottom to top. These two types of sequences are typical third-order sequences.

The D3CP prototype sedimentary basin in Tarim was a basin developed on the passive continental margin, the lowstand system tract is presumed to develop in the south of southwestern Tarim, and there is no sedimentary sequence of the lowstand system tract found in the basin. It has three third-order sequences, 3SQD3C1, 3SQC2P1 and 3SQP1P2. The lower third-order sequence is a type I sequence, while the middle and upper third-order sequences are all type II sequences. These three third-order sequences are usually composed of transgressive system tracts in the lower part and highstand system tracts in the upper part of the shelf, and separated by the largest or secondary marine flooding surfaces in the middle part. Therefore, the most intuitive way to distinguish the third-order sequence is to establish sequence or lithologic correlation maps and see whether there are huge mudstone and limestone wedges in the profile from the sea to the land.

(4) Fourth-order parasequence group: The fourth-order parasequence group is composed of a series of genetically related parasequence with specific overlapping or cyclic assemblage patterns. The center of the parasequence group is an important marine flooding surface or a comparable surface, which is basically composed of more than two parasequence. The parasequence group is generally equivalent to a formation with complex lithology and stratigraphy member, but may not correspond to each other. The D3CP prototype sedimentary basin in Tarim can be divided into eight parasequence groups.

(5) Fifth-order parasequence: The fifth-order parasequence is the basic unit of sequence stratigraphy. It is a group of relatively conformable lithologic section or lithologic group sequence bounded by the lowest marine flooding surfaces or equivalent conformity or unconformity surfaces, and is a single-cycle sedimentary sequence. In the D3CP prototype sedimentary basin of Tarim, there are 18 parasequence, including the Donghe Sandstone Member.

The fifth-order parasequence generally corresponds to the lithologic member or the formation with single lithology, because the lithologic member or the formation with single lithology is basically the same in sediment characteristics, and is a set of strata deposited in a single sedimentary environment. For example, the sedimentary environment of Donghe Sandstone Member in Tarim Basin is wave-controlled littoral facies. Vertical facies sequence shows that it consists of back shore, coastal beach and front shore from bottom to top. It is a set of littoral quartz sandstone strata deposited under standard transgressive background, so it is a single transgressive cycle fifth-order parasequence. The gravel-bearing sandstone member above is a fluvial delta facies sedimentary stratum. The vertical facies sequence is composed of sand-mud interbeds of fluvial facies. The lateral lithology from continental to marine facies turns from conglomerate facies to conglomerate-bearing sandstone and then to fine sandstone. Therefore, it is recognized as a highstand single-cycle fifth-order parasequence. Further cyclic assemblages of single-cycle fifth-order parasequences constitute the fourth-order parasequence group. For example, transgressive fifth-order parasequence of Donghe sandstone member and highstand fifth-order parasequence of gravel-bearing sandstone member constitute a fourth-order parasequence group cyclic sequence.

At the same time of sequence classification, system tracts must be described by classification. System tracts refer to a series of sedimentary systems or assemblages formed at the same time. According to their three-dimensional spatial distribution characteristics, the type and sedimentary background of the system tract can be determined. There are only two types of system tracts in the D3CP prototype sedimentary basin of Tarim, namely transgressive system tract and highstand system tract. The sedimentary cycles and assemblages of these two systems tracts constitute the basis for the division of the four orders of sequences except the first-order sequence. The transition interface between transgressive system tract and highstand system tract often corresponds to different levels of marine flooding surface.

Based on the analysis of sequence stratigraphic characteristics of the D3CP prototype sedimentary basin in Tarim and the basic principles and models of sequence stratigraphy, a solid model of sequence development and evolution of the Upper Devonian-Lower Carboniferous from west to east coast- mixed shelf in Tarim Basin has been established (Fig. 4).

The Upper Devonian-Lower Carboniferous in the Tarim Basin was deposited in the coastal-shallow shelf area. Its stratigraphic distribution is characterized by the existence of a large wedge-shaped body composed of mudstone and carbonate rocks from sea to land. Therefore, it can be concluded that the Upper Devonian-Lower Carboniferous is a third-order sequence with a sedimentary time limit of about 49 Ma.

The bottom of the third-order sequence is the basin-level type I unconformity surface (SB1) at the bottom of Donghe sandstone member, and the top of S2 layer sandstone- mudstone member is the conformity-unconformity type II sequence boundary (SB2) formed by the decrease of relative sea-level rise rate. A complete third-order sequence is developed between them. The standard limestone member is a secondary marine flooding sedimentary stratum, which separates the sedimentary sequence of the third-order transgressive system tract in the lower part and the third-order highstand system tract in the upper part, and constitutes the third-order transgressive-highstand cycle assemblage sequence as a whole. Above the type I unconformity, the fifth-order parasequence of transgressive system tract in Donghe sandstone member and the fourth-order parasequence group of Donghe sandstone-gravel-bearing sandstone member develop successively.

Unlike the classical sequence stratigraphic sedimentary model with only exogenous sediments, the Upper Devonian-Lower Carboniferous in the Tarim Basin is a third-order sequence developed in the coastal-mixed shelf area sourcing by both exogenous clastic sediments and endogenous carbonate sediments. Some important conclusions and understandings can be drawn from the analysis and research of the principle of sequence stratigraphy.

4.2.1. Carbonate thin interlayer, isochronal deposits and marine flooding indication strata

Sequence stratigraphy is based on the fact that only exogenous clastic sediments exist in sedimentary basins. Therefore, isochronal sediment is also defined and identified by marine flooding surface and condensed section, but it is difficult to identify them. In the Lower Carboniferous mixed shelf sediments of the Tarim Basin, there are two carbonate thin sediment bioclastic limestone members ^[18] and standard limestone member which are characterized by stable distribution, isochronal sedimentation and marine flooding. Therefore, it is relatively easy to identify isochronal sediments and marine flooding surfaces. Thin limestone layers sandwiched in mudstone can be used as markers of marine flooding. In fact, marine flooding refers to the sudden enlargement of sea area after sea level crosses the slope break zone formed by the unconformity in front of it. After the expansion of sea area, the muddy water must brought about mudstone deposit first, and then carbonate rock layer would occur when the sea water becomes clear and meets the conditions for carbonate precipitation in the later stage. Therefore, if there is no reliable marine flooding surface in mudstone, it is acceptable to use the bottom or top of limestone as the marine flooding surface.

The D3CP prototype sedimentary basin in Tarim had three periods of very obvious marine flooding sedimentary events, namely, the sedimentary periods of clastic limestone, standard limestone and Xiaohaizi Formation limestone, and each period has a wider sedimentary range than the previous period. In addition, there is a secondary marine flooding surface between Donghe sandstone and gravel-bearing sandstone. The existence and identification of these marine flooding strata provide an important basis for the classification of sequence stratigraphy and the determination of system tracts.

4.2.2. Equilibrium sedimentary characteristics of mixed shelf

Mixed shelf is an area where carbonate and clastic rocks deposit in balance. Therefore, the most remarkable difference from the sequence sedimentary model of clastic rock is the weakening and disappearance of large-scale foreset deposits inclined from land-to-sea sediments. Because the mixed shelf has mixed deposits of carbonate and clastic rocks, there exist the phenomena of one retreating and the other advancing and finger-crossing depositions of endogenous carbonate rock and exogenous clastic rock, forming a balanced sedimentary profile. The most prominent feature of this balanced sedimentation is the phenomenon of isochronal sedimentation with equal thickness. For example, the limestone strata of standard limestone member and bioclastic limestone member are basically equal thickness in shallow sea shelf area, moreover, there is no "condensed section".

4.2.3. The upper mudstone member and the S2-S3 bed of sand-mudstone member is a highstand system tract.

The standard limestone member is a set of micrite limestone formation with stable thickness and wide distribution. It can be regarded as a marine isochronal sedimentary stratum in the middle of Lower Carboniferous in Tarim Basin. After this set of formation deposited, the shelf area transferred to highstand system tract deposits. The upper mudstone member and the lower and middle sand-mudstone member are highstand system tract deposits. The tidal flat and delta sand bodies prograded toward the sea, forming sandstone and mudstone interbedded deposits from land to shallow sea.

S3 bed of sand-mudstone member is an important oil-producing sandstone formation of Carboniferous in Lunnan. It is a set of mudstone interbedded with thin sandstone layers. Its relationship with the upper mudstone member is upper and lower strata in vertical direction, but laterally, it is a phase transition relationship, the inner thin sandstone thins and pinch out towards the sea area.

4.2.4. Diachronism of Donghe sandstone member caused by stepped paleogeomorphology

The paleogeomorphological analysis in this study adopted the thickness contour map of Bachu Formation of Upper Devonian-Lower Carboniferous in Tarim Basin, and a sketch map of paleogeomorphology was drawn based on the discrimination of contour geomorphology (Fig. 5).

Paleogeomorphological analysis shows that the overall pattern of paleogeomorphology before the deposition of Carboniferous was low in the West and high in the east, and showed ladder-like elevation from the first terrace - slope break zone-second terrace-slope break zone-third terrace-Lunnan Peninsula. This kind of paleogeomorphology directly made the transgression from west to East periodic and cyclic.

When the sea level rose, and the coastline reached the bottom of the slope break zone, the coastline would show the characteristics of slow transgressive sedimentation. When sediments deposited below the slope break zone, according to the adequacy and inadequacy of the accommodation space of the littoral zone, the transgressive-highstand fourth-order parasequence group sedimentary cycle came up, and the sedimentary strata of Donghe sandstone member and gravel-bearing sandstone member built up under the slope break zone.

Once the sea level was higher than the slope break zone, the characteristics of rapid transgression and marine flooding would occur. For example, in the third terrace of Mangar area, because the paleotopography was very flat, the transgression speed was very fast, and the coastline advanced very fast too, resulting in no Donghe sandstone deposits in this area, and the mudstone of lower mudstone member even directly overlaps the unconformity surface. The coastline rapidly advanced to the other slope break zone of Lunnan Peninsula and Caohu area to start another fourth-order parasequence group sedimentary cycle in the coastal zone.

4.2.5. The Donghe sandstone member and the gravel- bearing sandstone member making up a local fourth-order parasequence group.

The contact relationship between Donghe sandstone member and upper gravel-bearing sandstone member or breccia member has been controversial. Some researchers^[19] believed that the top interface of Donghe sandstone was consistent with the type of paleo-weathering exposed surface and was a parallel unconformity of weathering and denudation origin. After a long-term study, the authors found that alluvial fan and river delta sandstone-conglomerate-mudstone strata directly covered the top of Donghe sandstone in Bachu, Qunkuqiak, Tazhong and Tabei areas^[20,21], and there was erosion phe-nomenon. Therefore, it is judged that this is a common sedimentary phenomenon.

According to the viewpoint of sequence stratigraphy, the transgression-highstand, re-transgression-highstand sedimentary cycles caused by the change of accommodation space in coastal zone can well explain the phenomenon that the delta channel sandstone and conglomerate overlay the littoral quartz sandstone (Fig. 6).

In the late stage of the deposition of the transgressive system tract of Donghe sandstone parasequence, due to the long-term stagnation of the coastline beneath the slope break zone, the accommodation space near shore might be filled up, then the river delta carrying a large amount of sediment would cross the coastal zone into the offshore-offshore shelf sea area with accommodating space, forming the river delta facies sand-mudstone interbedded deposits in the highstand system tract. The bottom of the highstand system tract in the coastal zone is channel erosion deposit, while the bottom of the channel is usually stagnant conglomerate deposit, resulting in erosion and deposition of gravel directly on Donghe sandstone. Therefore, there is no unconformity between Donghe sandstone member and gravel-bearing sandstone member, but a transition interface between system tracts.

When the sea level overflew the top of the slope break zone, rapid transgression and marine flooding would occur (attention should be paid to the sedimentary phenomenon that the lower mudstone section in Tz28 well area directly covers the unconformity surface), and the coastline would rapidly advance to the next slope break zone and repeat the transgressive-highstand sedimentary cycle. This is the reason why the fourth-order parasequence group transgressive-highstand cycle sequence occurred at different sedimentary times in different areas, such as the upper and lower Donghe sandstone member in Qunkuqiak area, the Donghe sandstone member and the gravel-bearing sandstone member in Tazhong area, and the Donghe sandstone member and the breccia member in Tabei area.

Mixed shelf has the characteristics of balanced deposition of exogenous clastic and endogenous carbonate sediments, and there is no "condensed section" stratum. Therefore, the most significant difference between the sequence sedimentary model of clastic rock and the mixed shelf is the weakening and disappearance of large-scale foreset of sediments from land to sea. Thin carbonate layers are also characteristic isochronal markers and marine flooding markers.

Sequence stratigraphy can be used to study the sequence of sedimentary filling assemblages in prototype sedimentary basins. According to the theory of transgression-highstand, re-transgression-highstand sedimentary cycle, the sequence of the D3CP prototype sedimentary basin in Tarim is divided into five levels. The first-order megasequence 1SQD3CP is the sequence that the prototype sedimentary basin is named after. With the top and bottom defined by basin-level unconformity surfaces, the megasequence corresponds to the tectonic sequence. Secondary supersequence divides the prototype sedimentary basin into two supersequence 2SQD3C and 2SQP, bounded by the top limestone member of Xiaohaizi Formation in the maximum marine flooding sedimentary stratum. The third-order sequence 3SQD3C1, 3SQC2P1 and 3SQP1P2 constitute the classical sequences of sequence stratigraphy, i.e. type I sequence and type II sequence. Their common feature is that there exists a huge mudstone and limestone wedge in the sequence from sea to land. The fourth-order parasequence group is a transgressive-highstand sedimentary cycle combination of the fifth-order parasequence. The fifth-order parasequence is the basic unit of sequence stratigraphy, which consists of a single system tract, corresponding to a single lithologic member or a single lithologic formation.

The Upper Devonian-Lower Carboniferous in Tarim Basin is a complete third-order sequence in the mixed shelf area, and it is also a type I sequence (no lowstand systems tract). The Donghe sandstone member-standard limestone member is a transgressive system tract depositional stratum in the lower part of the third-order sequence, while the standard limestone member-S2 bed sand-mudstone member is a highstand system tract depositional stratum in the upper part of the third-order sequence, while lowstand system tract doesn’t exist.

The analysis of paleogeomorphology before sedimentation of D3CP prototype sedimentary basin shows that the whole sedimentary basin at that time was low in the west and high in the east, and showed the distribution characteristics of terrace-slope break zone. This kind of ancient landscape directly caused the uneven and periodic transgression speed from west to East, the diachronism of transgression parasequence of Donghe sandstone member, and the cyclicity of transgressive-highstand fourth order sequence made up of Donghe sandstone member and the gravel-bearing sandstone member depositing below the slope break zone.