Sedimentology and sequence stratigraphy of the mixed clastic-carbonate deposits in the Late Paleozoic icehouse period: A case study from the northern Qaidam Basin

1.   Introduction

The Upper Carboniferous Keluke and Zhabusagaxiu Formations in the northern Qaidam Basin are characterized by very distinctive sedimentary units—the whole set of strata is neither purely “muddy water sedimentary products”, clastic rocks, nor entirely “clear water environment products”, carbonates rocks, but a mixed of clastic-carbonate succession formed by frequent “muddy and clear alternation”. According to the regular changes of rock composition, lithological ratio, and thickness of fine-grained sediments, the mixed system formed different cyclic units of several meters, ten meters to hundreds of meters thick, showing a multi-level rhythmic deposition pattern. The multilevel alternation of clastic and carbonate rocks indicates frequent changes in environmental factors such as water properties, material supply and climate changes during deposition (Tucker ME, 2003; Coffey BP and Fred RJ, 2004; Chen SY et al., 2016; Seyedmehdi Z et al., 2016).

As a special recorder of changing depositional environment, the mixed clastic-carbonate succession is of significant academic significance in understanding the sedimentary dynamics and depositional process. The research on the sedimentary environment, sedimentary processes, and the main control factors of formation of the mixed clastic-carbonate succession has been a hot and key issue recently at home and abroad. Previous studies have shown that “mixed clastic-carbonate sequences” are generally the product of changing sea level and frequent wet and dry climatic conditions, and they were typical in the ice period when the sea level change was of high frequency and large amplitude due to glacial expansion and melting ( Li XH. 2008; Coffey BP and Fred RJ, 2004; Seyedmehdi Z et al., 2016).

The Upper Carboniferous mixed clastic-carbonate succession in the northern Qaidam Basin was formed during the Late Paleozoic icehouse period when there were no strong uplift and denudation events (Tang LJ et al., 2000). In this context, the formation and evolution of the mixed clastic-carbonate succession was mainly a response to the regional “climate-environment” change. Therefore, it was a recorder of the “Late Carboniferous Gondwana glacial event” and recorded the magnitude and duration of sea-level changes at different levels under glacial control, as it is generally accepted that the growth and extinction of global continental ice caps under relatively stable tectonic conditions are the main controlling factors of regional sea-level changes at three levels (Heckel PH, 1994; Isbell JL et al., 2003; Fielding RC et al., 2008). The Milankovitch astronomical drive and the environmental changes it affects then control the fluctuations of sea level from level IV to level V (Li XH et al., 1997; Heckel PH, 2008; Isbell JL et al., 2008).

Recently, most of the studies on the mixed clastic-carbonate succession are related to the depositional environment but the sequence stratigraphic characteristic of the mixed clastic-carbonate succession is rarely reported (Lubeseder S et al., 2009; Sun JP et al., 2014; Bai HQ et al., 2020). In this paper, we analyze the sedimentology and sequence stratigraphic characterization via the key section, well-preserved and continuous Shihuigou section, together with other sections, including the Silanggou section, Wanggaxiu section and Chengqianggou section, and the well ZK5-2 (Fig. 1), aim to provide an example of the sequence stratigraphic patterns under the icehouse period and the driving mechanisms of the formation of the mixed clastic-carbonate succession.

Figure  1.  The regional location map of the Northern Qaidam Basin (after Wei XJ et al., 2018).

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2.   Geological setting

The Qaidam Basin is located on the northern edge of the Tibetan Plateau and is a plateau intermountain basin surrounded by the Qilian Mountains in the northern, Aljinshan and Kunlun Mountains in the northwest and south. During the Late Paleozoic period, the Qaidam Basin belonged to the passive margin of the ancient Tethys Ocean and the post-arc basin. The northern Qaidam Basin refers to the vast area between the Lvliang Mountains and the Ulan Yak Mountains, which is bounded from north to south by the Zongulong Fault, the Eunan Fault, the Ebei Fault, and the Kunlun Fault, and is divided into five present-day tectonic units, the Delingha Depression, the Oronbrook Uplift, the Eunan Depression, the Emnik Uplift, and the Hobson Depression from north to south (Fig. 1). The Paleotethys Ocean on the southern side of the Qaidam Basin was in the process of continuous expansion during the Early-Middle Permian period and started to subduct northward until the Late Permian, thus leading to the continuous subsiding of the Qaidam Basin of various subsidence rates during the Late Paleozoic period. Besides, influenced by the Late Devonian rifting trough generated by the Hercynian movement, the Carboniferous basin presented a paleogeographic pattern of internal depression and uplift alternations.

The previous study suggests that the tectonic activity in the northern margin Qaidam Basin during the Late Carboniferous period was relatively stable (Yang C et al., 2010; Sun JP et al., 2014; Chen SY et al., 2016). Accompanied by the massive development of the Gondwana continental ice cap in the Southern Hemisphere and the expansion and melting of glaciers, high-frequency and large-amplitude glacial sea-level changes occurred globally, with sea-level changes amplitude ranging from tens to hundreds of meters (Gonzalez-Bonorino G, 1992; Haq BU and Schutter SR, 2008; Rygel M et al., 2008). Therefore, the sea-level change caused by Gondwana glaciation was a prerequisite for the worldwide development of this mixed clastic-carbonate successions in the Qaidam Basin (Chen SY et al., 2016). In addition, paleomagnetic data indicate that the Late Carboniferous region was located at low latitudes in the Northern Hemisphere and was a relatively warm and humid tropical climate zone (Wang XL et al., 2002; Wang L et al., 2019), which led to the flourishing of biology and increased carbonate productivity and provided the material basis the development of carbonate rocks within the clastic-dominated strata of the Upper Carboniferous on the northern Qaidam Basin.

3.   Stratigraphical setting

The distribution of Carboniferous-Permian strata in the Qaidam Basin is controlled by multi-phase tectonic movements and is mainly exposed around the basin and in the foothills of the mountain front spreading in the NW-SE direction. The Carboniferous strata is constituted by the lower Carboniferous Chuanshangou Formation (Fm.) (C1ch), Chengqianggou Fm. (C1c), Huaitoutala Fm. (C1h), and the Upper Carboniferous Keluke Fm. (C2k) and the Lower part of Zhabusagaxiu Fm. (P1zh) from bottom to top (Fig. 2), and is mainly preserved in the Shihuigou, Chengqianggou, Wanggaxiu, and Oulongbluke Sections, etc. The target formation, Upper Carboniferous Keluke Formation is characterized by mixed clastic-carbonate succession, underlain by Lower Carboniferous Huaitoutala Formation as a large set of thick-bedded limestones, and overlain by Lower Permian Zhabusagaxu Formation, similar to Keluke Formation in terms of lithology but rich in amount of carbonate and differed by the presence of fossils of the spindle family (Ma LC et al., 2020). According to the lithological vertical superposition and variability, the Keluke Formation was further divided into four members, C2k₁, C2k₂, C2k₃, and C2k₄, from bottom to top (Chen SY et al., 2016). Herby, the C2k₁ member is dominated by coal-bearing intervals, composed by interbedded dark mudstone and siltstone with thin-bedded limestones; the C2k₂ member is relatively rich in clastic rocks, with interbedded limestone-mudstone-sandstone cycles; the C2k₃ member is thick-bedded clastic rocks alternated with thin-bedded limestones; and the C2k₄ member is mainly fine-grained clastic rocks interbedded with thin-bedded limestones (Wei XJ et al., 2018).

Figure  2.  Sedimentary construction within structural divisions in the northern Qaidam Basin.

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4.   Methods

Seven vertical sections and one drilling well of the Keluke Formation were measured across the present-day outcrop area at the northern Qaidam Basin, and among which, the Shihuigou section was selected as the key studied profile and the other were chosen as the reference profile. Lithology, grain size, sedimentary texture, sedimentary structures, bed or set thickness, and trace fossils were recorded from outcrops. Intensity and variations of burrows, tracks, and trace fossils were also registered. Thin section microscopic observation is mainly collected from carbonate rock samples of the Shihuigou section. Lithology, organic matter, biological characteristics were documented. A set of photographs were taken to aid in the facies association analysis and construction of the sequence stratigraphy framework.

5.   Results

5.1   Facies description and interpretation

The profiles selected for this study include the Shihuigou, Silanggou, and Wanggaxiu Sections and drilling wells CY2, QDD1, QDD2, and QDC1 in the northern Qaidam Basin. Based on the field outcrop description and thin section microscopic observation, three depositional systems were identified according to lithology, grain size, sedimentary structure, sedimentary architecture, fossils, and vertical depositional sequence.

5.1.1   Progradational coastal system

The progradational coastal system, either storm-wave or tide-dominated, are widely developed in the clastic intervals of the study area as tens of meters thick coarsening-upward package, mainly composed of conglomerates, coarse-grained, medium-grained, and fine-grained sandstones, and mudstones (Fig. 3). The whole set of a single sequence of progradational coastal systems is usually a few tens of meters (<30 m) thick, with a vertically compound rhythm that is coarsening upward from the mudstones towards the fine-to-medium-grained sandstones into conglomerates and then finning upward into fine-grained sandstone or mudstone. Hereby, the bottom purely mudstone interval, interpreted as either prodelta or marine shelf deposits, is ranging from several meters to a few tens of meters (<20 m), is usually intensively bioturbated. The middle sandy parts, are gradually overlying on the prodelta or marine shelf deposits as few meters thick sandy interval, and are primarily constituted by medium-grained sandstones that are generally planar and/or trough cross-stratified showing rare bioturbation, which is interpreted as lower or distal coastal deposits. The upper or proximal coastal deposits contain both fine-grained coastal plain deposits and coarse-grained deposits, such as erosively-based channels that are finning upward succession initiated from conglomerates towards coarse-grained sandstones into medium-grained and fine-grained sandstones or mudstones. The coal layers, coal chips, mud chips, and plant roots are present above the erosive base and the marine trace fossils are absent in the erosively-based finning-upward succession.

Figure  3.  Examples of progradational coastal system and the vertical succession in the Upper Carboniferous of the Qaidam Basin (Shihuigou section). a, b‒Hummocky and swaley cross strata, which is interpreted as storm-wave controlled distal coastal deposits; c‒interbedded sandstone and mudstone deposits within coastal plain; d‒stacked compound dunes probably deposited as tide-dominated distal coastal deposits.

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5.1.2   Incised valley system

The incised valley system in the study area, generally tens of meters thick well-sorted sandstone packages, exhibits as a distinctive three-unit sequence characterized by the bottom (1) fluvial channels, the middle (2) tide-dominated estuary, including fluvial-tidal influenced channels and tidal bars, and finally capped by the (3) marine offshore mudstone deposits, showing an overall upward finning trend (Fig. 4). The fluvial channels, approximately 1m thick, are erosively-based, poorly-sorted, medium-grained to coarse-grained sandstones, lack marine trace fossils, but show coal lags, conglomerates lag, plant roots at the erosion base. The primary sedimentary structures in the fluvial channels are trough and planar cross-stratification. The tide-dominated estuary is characterized by well-organized, stacked cross-stratified sandstones, approximately 10 m thick, that are relatively clean, well-sorted, medium-grained sandstones, showing abundant mud drapes and/or double mud drapes, suggesting strong tidal influence in restricted areas (Boyd R et al., 2006; Dalrymple RW and Choi K., 2007; Wei XJ et al., 2018).

Figure  4.  Sedimentary characteristics and vertical succession of incised valley deposits in the Upper Carboniferous strata of northern Qaidam Basin. a‒large-scale, well-organized, stacked compound cross strata; b‒poorly-sorted, conglomerates lined at erosive base; c‒example of incised valley system, with the red line representing the base of incised valley, and the yellow dot line suggesting the discontinuous limestone layer.

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5.1.3   Carbonate-dominated marine shelf

The Upper Carboniferous carbonate strata in the study area are usually interbedded with clastic strata in different stacking patterns (Fig. 5). It is easy to distinguish the thick massive limestones from the interbedded tens of meters thick sandstones in third-order sequence, but it is difficult to subdivide the thin set of limestones and mudstones which are frequently alternated at the meter scale in third-order sequence, therefore, we combine this type of alternations of mudstones and limestones in meter scale as a package, since they are both low-energy products. Hereby, the carbonates are usually characterized by interbedded lime mudstone, skeletal wackestone, packstone, grainstone, and calcareous shale, containing abundant brachiopods, crinoid, bivalves, and plant fossils/ phytolite, which are clean water products probably deposited in water depths greater than 100 m on an open shelf. The dark mudstones are mostly intensively bioturbated with marine trace fossils, suggesting deposition below the storm wave base as marine offshore products. The alternation of limestone and siltstone/mudstone is therefore interpreted as the products of open, clear-water offshore marine environments distal to sources of terrigenous clastic sediment.

Figure  5.  Examples of shallow shelf sedimentary systems and sedimentary sequences of the Upper Carboniferous, northern Qaidam Basin (Shihuigou section). (a‒c)‒aggradational shallow marine shelf; (d‒f)‒progradational shallow marine shelf.

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5.2   Sequence stratigraphic framework

Based on the regional sedimentary and tectonic background, we apply Vail’s classical stratigraphic stratigraphy principle and recognize the key sequence boundaries and internal surfaces, including the regional unconformity interface, lithology transition interface, and flooding surfaces in the Upper Carboniferous section of the northern Qaidam Basin via outcrop and core observations. Correspondingly, four 3rd-order sequences were recognized in the studied Keluke interval, namely SQ1, SQ2, SQ3, and SQ4 from the bottom to the top. Each third-order sequence is basically composed of lowstand, transgressive, and highstand system tracts.

5.2.1   Sequence boundaries and transgressive surfaces

(i) Type I sequence boundary. Based on detailed observations of the Shihuigou, Wanggaxiu, and Silanggou sections as well as in the northern Qaidam Basin, an extensively developed Type I third-order sequence boundaries were identified regionally as a result of significant and rapid sea-level drops. It is characterized as undulated and discontinuous surfaces, with distinctly different stratigraphic color, lithology, and orientation above and below the interface, indicating the hiatus in sedimentation during this period.

The purple-red mudstone is present around the hiatus and dissolution pore layers filled with karst breccia inside are visible below the interface (Figs. 6, 7), representing exposed unconformities formed by regional tectonic uplift or sea-level fall. Besides, the presence of incised valley systems is considered to be induced by river rejuvenation with sea-level falls and consequently filled with the tide-dominated estuary as sea-level rise. Therefore, the base of the incised valley can be considered as a marker of the type I third-order sequence boundary.

Figure  6.  Characteristics of internal sequence boundary within C2k Formation of the northern Qaidam Basin. a‒discontinuity of the strata; b‒exposure of unconformity – paleosol.

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Figure  7.  The bottom surface of the incised valley in the Upper Carboniferous, northern Qaidam Basin – 3^rd order sequence boundary.

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(ii) First flooding surface. The first flooding surface is the transition between the relatively stable phase of sea level and the rapid sea level rising phase, and also the division interface between the lowstand system tract (LST) and the transgressive system tract (TST). The first flooding surface is the result of continuous landward migration in the high-energy, hydrodynamic conditions of the nearshore environment. Therefore, the accumulation patterns below and above the initial flooding surface are different, with the overlying strata displaying increasing finning and deepening upward trend, reflecting the process changing from relatively slow sea-level rise to rapidly rising period, and vice versa. The first flooding surface in the study area is identified between the fluvial channel and the tidal-fluvial channels/tidal sand bars, representing the decrease of terrestrial influence and the consequent increase of tidal influence as the sea-level rise when the rate of sea-level rise exceeded the rate of sediment supply.

(iii) Maximum flooding surfaces. Maximum flooding surfaces refer to the state when the sea level rises rapidly to the maximum, and are the interfaces between the transgressive system tracts (TST) and the highstand system tracts (HST). Due to the fast subsidence rate and insufficient supply of terrigenous supply, the basin is in a state of under compensated deposition, and the condensation section (CS section) is formed near the interface, which is formed in the late sea erosion and early high sea level stage. It is usually referred to as a very thin marine stratigraphic unit, consisting of deep and semi-deep sediments with very slow deposition rate, in a thin and continuous form, with biogenic subduction zones or low lithic consolidation or in the form of submarine hard substrate, which extends most widely in the region during the maximum seaward advance.

The study area was epicontinental during the Late Carboniferous period, and due to its relatively shallow sea level, it was not possible to form a deep or semi-deep marine environment with condensed layers. However, it is still reasonable to consider sediments formed during the rapid sea-level rise as condensate layers, thus expanding the scope of the meaning of condensate layers. When the sea level rises rapidly, the sea level inundates the basin rapidly, making the shoreline recede landward rapidly, limiting the terrigenous-derived clastic materials to enter the basin, and the endogenous sedimentation is mainly developed, and the carbonates and siliciclastic rocks formed in this background can constitute the condensed layer, which is recognized as intensively bioturbated, fine-grained sediments with high organic matter content.

5.2.2   Recognition of system tracts

System tract refers to a series of depositional system assemblages formed in the same period. Within a sequence, three system tracts can be divided according to the relative sea-level position and its ascending and descending trend. Based on the characteristics of sequence boundaries, first flooding surface, maximum flooding surface, lithology and stacking patterns of parasequence, four third-order sequences, involving lowstand system tract (LST), transgressive system tract (TST), and highstand system tract (HST), are identified in the Keluke Formation of the northern Qaidam Basin.

Lowstand system tract (LST) is formed when the rate of sea-level fall drops to the lowest point, and is confined by the third-order surface and the first flooding surface, representing the product of the relative sea-level fall process. The lowstand system tract in the study area is dominated by the fluvial channel sandstone filling at the bottom of the incised valley, as a shallowing and coarsening upward succession. In the stage of relative sea-level fall, the fluvial system extends the fluvial channels to the basin and cuts into the underlying strata through the incision action. In the continental shelf, the bottom boundary of the incised valley can usually be used as the sequence boundary.

Transgressive system tract (TST) is the interval between the first flooding surface and the maximum flooding surface and formed in the middle stage of sequence development. In the study area, the transgressive system tract is mainly developing the estuarine deposits and the retrogradation offshore marine shelf that is dominated by fine-grained mixed carbonate and mudstone succession. The TST is composed of a set of regressive parasequence sets, which thickens toward the shelf and then thins to the overlying of the bottom surface. The transgressive system tract is formed under the condition of an accelerated sea-level rise period.

Highstand system tract (HST) refers to the interval between the maximum flooding surface and the third-order sequence boundary as the product of late-stage sea level rise and the early stage of sea-level fall. The highstand system tract in this study is mainly accumulating the progradational coastal system, including deltaic deposits and/or wave-dominated shoreface, and accretional mixed carbonate-dominated marine shelf deposition that is a series of superimposed, upward coarsening, shallowing and thickening depositional sequences.

5.2.3   Sequence stratigraphy framework

Detailed observation and interpretation of the well-preserved, continuous Shihuigou section, and Wanggaxiu section as well as Silanggou section and wells CY2, QDD1, QDD2, and QDC1 were carried out and five distinct types I third-order sequence boundaries were recognized in the Late Carboniferous Keluke Formation strata on the basis of the recognition of sequence boundaries, first flooding surfaces, maximum flooding surfaces, and system tracts. Accordingly, four Type I third-order sequences (SQ) were identified, namely Sequence 1 (SQ1), Sequence 2 (SQ2), Sequence 3 (SQ3), and Sequence 4 (SQ4) from bottom to top (Fig. 8). Among them, the bottom boundary of SQ1 is identified as a set of lithologic transition interface between the large set of limestones and the interbedded coal-bearing mudstone and thin-bedded limestones, representing seaward shoreline migration. The other sequence boundaries are all recognized as regional erosion surface or exposed unconformity.

Figure  8.  Well correlation of the 3^rd-order sequence and sedimentary characteristics in the Upper Carboniferous, northern Qaidam Basin.

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SQ1 is the lowest sequence unit in the Keluke Formation (Fig. 8), which mainly corresponds to section 1 of the Keluke Formation (C2k1), only developing TST and HST, without significant LST. SQ2 and SQ3 correspond to formations 2–4 of the Keluke Formation (Fig. 8), and SQ4 corresponds to the upper section of the Keluke Formation and the lower section of the Zhabusagaxiu Formation (Fig. 8), in which the transgressive system tract mainly develops receding shallow shelf deposits and the high system tract consists of accretionary shallow shelf deposits and wave-controlled shoreline deposits. LST is fluvial sediment-filled at the bottom of the undercut valley, which is a set of sedimentary sequences with gravelly coarse sandstone at the bottom gradually thinning upward and gradually thinning, representing the rapid decline of sea level, the terrigenous shed partly exposing the surface, the river advancing to the terrigenous shed and undercutting the terrigenous shed, forming the river deep-cut valley of the river rejuvenation. TST consists of (1) sandstones of medium and coarse-grained thick-layered tidal phase, which change upward to sandy mudstone and mudstone interstratification, forming one or more receding accumulation-type parasequences, and this superposition pattern represents the deposition of the estuarine bay filled during the sea-level rise; (2) multiple sets of interstratification of limestones and mudstones, which generally present the receding accumulation-type parasequence group superposition, representing the shallow sea deposition of terrigenous shed formed by long time and multiple sea erosion Sedimentation. HST consists of (1) multiple sets of interbedded limestones and mudstones, with a parasequence stratigraphic superposition of accretion and accretion, representing the shallow marine sedimentation of the shelf formed from the slow sea invasion to the beginning of recession; and (2) sandy mudstones of the delta with upward accretion, and the parasequence is characterized by accretion, while the accretion is not obvious, reflecting the characteristics of a slow sea invasion and local water recession.

The strata of the Upper Carboniferous in the northern Qaidam basin deposited in the epicontinental sea that has the characteristics of the extremely gentle slope of the seabed and sudden transgression and can be spread over thousands of kilometers in a short period. Therefore, the sedimentary facies sequence has an obvious discontinuity, and the marine limestone can directly cover the shallow water or exposed sediments in a large area.

During the Late Carboniferous period, the tectonic activity is infrequent and the tectonic subsidence is not obvious in the northern Qaidam Basin. With high-frequency and high-amplitude sea-level change induced by the Gondwana glacier formation and ablation, the sedimentary energy changed significantly and the sedimentary facies zone migrated correspondingly, and the sedimentary filling showed different characteristics.

The vertical evolution of the sedimentary facies of the Keluke Formation consisted of multiple cycles of shallowing-upward to deepening-upward sequences, with the shallowing-upward succession typically represented by wave-dominated shoreface/delta system, and the deepening-upward succession typically represented by the tide-dominated estuary and the retrogradation shallow marine shelf deposits. In these four third-order sequences, the LST is relatively thin, approximately 1 to 2 m, reflecting the gentle slope of the seabed, and the TST and HST are obviously thick and have a large thickness ratio, displaying multiple and frequent sea-level fall features. From the perspective of the long period, during the period of the late stage of early Carboniferous to the early Permian, the northern Qaidam Basin experienced a remarkable and long-term sea-level fall and rise, and the Late Carboniferous period witnessed a long-term and overall sea-level fall process.

6.   Discussion

6.1   Depositional process of the cyclical mixed clastic-carbonate succession

The mixed clastic-carbonate succession in the Qaidam Basin is composed of several repeating cycles, and each cycle can be summarized into a certain pattern that is -initiated from the base of the incised valley evolving into the shallow marine shelf and finally capped by progradational coastal deposits. These cycles are repeated and stacked into hundreds of meters thick packages constituting the Upper Carboniferous strata, which are further assigned into different hierarchy sequences and system tracts. The development pattern and depositional process of each system tract and a typical cycle is described as follows (Fig. 9).

Figure  9.  Depositional model of mixed clastic-carbonate successions of the Upper Carboniferous strata in the northern Qaidam Basin under the control of the 3^rd-order sequence stratigraphic framework.

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The initial of the cycle, characterized by lowstand system tract, develops during the period when sea level significantly and rapidly decreases to cut the continental shelf into the incised valley above the sequence boundary. At the beginning, due to the rapid reduction of accommodation space, the depositional base level is shifted down significantly, so that the earlier deposited strata are exposed above the erosional surface, which is recognized as third-order sequence boundary. At this point, sedimentation ceases and is replaced by increasing erosional scouring, with clastic material being transported through incised valleys to the interior of the basin below the sedimentary base level. The basal incised valley deposits are poorly sorted and rounded, with high sand/mud ratios and low mineral structure maturity and compositional maturity.

The middle part of the cycle expressed as the transgressive system tract is developed in the period between the rapid increase of the accommodation space and the maximum growth rate. As the accommodation space increases, the sediment supply inside the basin gradually decreases and is “starved”, and almost all the sediment is preserved in the vicinity of the continuously overtopping shoreline. The parasequence is superimposed in a regressive mode, and the tide-dominated estuary is mainly developed in the early stage. With the gradual rise of sea level, the terrigenous supply is gradually weakened, and the regressive shallow marine shelf deposits accumulated as alternation of carbonate and mudstone layers begins to develop.

The end of the cycle, accumulated as a highstand system tract, is developed when the growth rate of the accommodation space is reaching to the maximum. As the growth rate of accommodation space decreases, the parasequence will change from a regressive to a progradational pattern, and the sediments will continue to progradation into the basin. The highstand system tract mainly develops the sedimentation of the shallow marine shelf, and as the base level decreases, the terrigenous supply is gradually sufficient, and deltaic or wave-dominated shoreface begins to develop.

6.2   Controlling factors of the formation of the Upper Carboniferous strata in northern Qaidam Basin

It is suggested that the climatic conditions, the paleomorphological characteristics, and the sea-level changes caused by glacier eustatic changes and regional tectonic movements were the main controlling factors for the development of the Late Carboniferous-Early Permian strata. Considering the tectonic subsidence and the regional paleoclimatic and paleomorphological characteristics, the vertical evolution pattern of the late stage of Early Carboniferous to the Early Permian strata of the northern Qaidam Basin and their controlling factors are analyzed and discussed based on the outcrop, core, and thin section microscopic observation and the comprehensive analysis of the sedimentology and stratigraphic stratigraphy.

The Early Carboniferous strata in the northeast Qaidam Basin was formed under the stable regional tectonic environment and the warm and humid climatic environment at the middle and low latitudes.

By contrast, the high-frequency and high-amplitude sea-level changes caused by the influence of the global ice age and the paleomorphological features were the main controlling factors of the development of the mixed clastic-carbonate succession during the Late Carboniferous and Early Permian stage. The magnitude and frequency of sea-level rise and fall in different periods caused the difference in lithologic thickness of the corresponding longitudinal sedimentary sequences.

In every single cycle or sequence, the sea-level fall stage is dominated by the development of fluvial channels filling at the bottom of the incised valley at the region of the tectonic high. Immediately after the rising sea-level, a tide-dominated estuary was formed and filled in the incised valley. As the sea-level rises to the maximum, the alternations of limestones and mudstones are dominant, the proportion of limestones decreases, and the proportion of mudstone increases. As the sea level begins to fall again in one cycle, progradational coastal systems, such as deltas and wave-dominated shoreface, begin to develop. The high-frequency sea-level advances and retreats in repeated cycles lead to the wide deposition of the mixed clastic-carbonate succession within an ice age context.

7.   Conclusions

Based on the regional background of the Qaidam Basin, we analyze the detailed sedimentology and sequence stratigraphy characteristics of the well-preserved and continuous Upper Carboniferous Shighuigou section in the northern Qaidam Basin, comprehensive analysis of the Late Carboniferous Keluke Formation stratigraphy in the northern Qaidam Basin, a detailed study of the depositional environment and vertical evolution history of the Late Carboniferous Keluke Formation stratigraphy in the northern Qaidam Basin was carried out based on sedimentological and stratigraphic means via outcrop, core, and thin-section observation, and the main results obtained are as follows.

(i) The depositional environment of the Upper Carboniferous strata in the northern Qaidam Basin belongs to the marine-continental interaction, and three depositional systems have been identified as paorgradational coastal systems, incised valley system, and shallow marine shelf.

(ii) Using the principle of stratigraphic stratigraphy, five third-order sequence boundaries (SB) and correspondingly four third-order sequences are identified in the Upper Carboniferous Keluke Formation named as SQ1, SQ2, SQ3, and SQ4, and these third-order sequences are composed by lowstand system tract, transgressive system tract and highstand system tract, which shows high-frequency and high-amplitude than usual third-order sequences. The overall sedimentary environment shows an evolutionary pattern from the marine environment towards the terrestrial-marine transitional environment and then back into the marine environment again in the long term.

(iii) Comprehensive analysis of the sedimentary environment and stratigraphic stratigraphy suggests that the formation of the Late Paleozoic sedimentary strata in the northern Qaidam Basin is mainly controlled by the frequent sea-level change caused by the short-term high-frequency and large-amplitude global eustatic changes and the long-term sea-level rise in the Late Carboniferous.

CRediT authorship contribution statement

Xiao-jie Wei and Zong-xing Li conceived the present idea and wrote the manuscript with input from all authors. Zong-xing Li, Yin-sheng Ma, and Xin-xin Fang supervised the project. Yi-fan Li and Jun-jie Hu helped revise the manuscript. Jun-jie Hu and Kui-Liu assisted with fieldwork and core observation. Yi-fan Li helped in thin-section observation.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (41702124, 41772272) and the China Geological Survey Program (DD20190094). The authors would like to thank the editor-in-chief and the anonymous reviewers for the critical reviews of the paper.