The Fengcheng Formation demonstrates an unconformable contact with the P₁ j. The P₁ j is composed of pyroclastic rock, andesite, and sandstone at the bottom and sandstone at the top, respectively (Figure 2). The overlying strata of P₁ f is the P₂ x. P₁ f exhibits an unconformable contact with P₂ x. Next, the P₂ x is a set of gray–brown and brown–gray glutenite with a small amount of gray, brown–gray mudstone and sandstone.

The drilling of the Fengcheng Formation is distributed in the western margin of the Mahu sag, including the Xiazijie (wells X76, X88, X77, and X201), Fengcheng (wells Fn7, Fn4, Fn1, and Fn7), Baiquan (Bq1), and Mahu areas (wells B872 and Mh085) (Figure 1D).

The P₁ f ¹ is composed of a gray, dark gray thin layer of tuff, dolomitic mudstone, and argillaceous dolomite near the sedimentary center; whereas, near the orogenic belt, tuff and siltstone are interbedded between thin layers glutenite, volcanic breccia, and basalt. Moreover, their logging curve shows high natural gamma ray (GR) and amplitude values, wave growth, and medium frequency (Figure 3).

The P₁ f ² is composed of deep gray and gray–black thin layers of muddy dolomite and dolomitic mudstone near the sedimentary center. Near the orogenic belt, it contains thin sandstone and mudstone interbedded between tuff and gravel. The logging curve of these rocks displays the following features: high-to-low GR values, high amplitude, short wavelength, and high frequency (Figure 3).

Next, the tuffaceous and dolomitic components of the P₁ f ³ decrease; whereas, the sandy content increases. The bottom of the P₁ f ³ is composed of mudstone and siltstone interbedded between dolomitic mudstone, whereas, its top is coarse sandstone and sandy conglomerate. The logging curve shows a low GR value, small amplitude, and continuous stability (Figure 3).

During the sedimentation period of the Fengcheng Formation, the Mahu sag experienced complex tectonic activity, strong volcanic activity, and consequently formed various lithofacies. According to petrological characteristics and sedimentary structure, the sedimentary facies of the Fengcheng Formation are divided into two: lacustrine and fan–continental delta facies. Next, the lacustrine facies can be divided into semideep lake to deep lake and shore–shallow lake subfacies. The fan delta can be divided into fan–delta front subfacies and fan–delta plain subfacies.

Sections A and D–G are distributed from northwest to southeast, perpendicular to the extensional direction of the Mahu sag and the strike of the Kebai Fault (Figure 1D). Sections B and C are distributed in the northeast direction (Figure 1D).

The physical characteristics of sandy conglomerate, dolomitic sandstone, dolomitic mudstone, and volcanic rocks in the Fengcheng Formation are different, and they demonstrate apparent reflection characteristics in the seismic profiles (Figure 5). Different sedimentary facies correspond to different lithologic assemblages. Therefore, combined with the logging curve, we identify four kinds of sedimentary facies on the seismic profile (Figure 5). The reflection characteristics of subfacies’ seismic profiles in fan–delta plain are medium and weak amplitude, poor continuity, and disorderly reflection of medium and high frequencies (Figure 5). Next, the fan–delta front facies show low GR value, low true formation resistivity (RT) value, and straight curve in the logging facies. The seismic reflection of fan–delta front facies exhibits strong amplitude, good continuity, and medium- and low-frequency layered reflection (Figure 5). Volcanic rocks show good continuity, strong low-frequency reflection, and significant lateral differences in seismic profile (Figure 5).

Next, Section A is located north of the sag, extending from the Wuxia Fault to the entire Mahu sag up to the Xiayan uplift and extending 60 km. This section can identify the Permian strata in the study area and every member of the Fengcheng Formation. Figure 6 shows the whole profile shape of the Fengcheng Formation, which deepens from the orogenic belt to the depression. Moreover, currently, the strata of the Fengcheng Formation are deformed by faults. The three members of the Fengcheng Formation overlap and pinch out layer by layer to the west edge of the lake basin, from top to bottom. The distribution of P₁ f ³ is the most extensive, followed by P₁ f ² and P₁ f ¹, respectively.

Section B (Figure 7) extends south to north along the sag’s western margin and extends 80 km. This shows that the Fengcheng Formation in the north of the sag is thicker than that in the south. In addition, it also shows that the thickness of P₁ f ² is more significant than those of P₁ f ¹ and P₁ f ³.

Next, Section C is located in the Fengcheng area of the sag. Glutenite accumulates near the fault zone, and the range of lacustrine facies decreases from the P₁ f ¹ to P₁ f ³ (Figure 8), showing the process of lake regression.

Then, Sections D, E, and F are located in the Huangyangquan fan in the central Mahu sag (Figure 1D) and identify the Fengcheng Formation fan–delta inner–front sand conglomerate, fan–delta outer–front sand gravel, and shallow lake–semideep lacustrine dolomitic shale (Figure 9). Section G is located in the south of the sag, which can identify the glutenite in the inner front of the fan delta, glutenite in the outer front of the fan delta, and dolomitic mudstone of the shore–shallow lake (Figure 10). The sedimentary thickness of the south of the sag is thinner than that of the north, and the sand–conglomerate range of the inner front of the fan delta in the P₁ f ³ expands in the later stage. In addition, it shows that the outer-front dolomitic sandstone is covered with lacustrine mudstone (Figure 10).

Through complete lithofacies’ description and interpretation, stratigraphic sequence framework, and 2D seismic–reflection profile identification, we draw new lateral and vertical sedimentary-facies evolution maps of P₁ f ¹, P₁ f ², and P₁ f ³ in the Mahu sag (Figure 14). In addition, the potential provenances and transport routes of the Mahu sag were traced through previous provenance studies (Tang et al., 2020). These provide necessary sedimentary face variations for the sedimentary-filling process described in this study.

During the sedimentary period of P₁ f in the Xiazijie area, the lithology into pyroclastic deposits. P₁ f ¹ is composed of tuff, followed by tuffaceous mudstone and siltstone. These sedimentary characteristics indicate that the Xiazijie area was in an evaporated shore–shallow lacustrine environment during the P₁ f ¹. Next, volcanic activity was reduced as reflected in the decreased amount of tuff. At the beginning of P₁ f ², the sedimentary grain size was reduced, and the content of tuffaceous rocks increased, indicating that the lake level rose, and volcanic activity increased. In P₁ f ³, little tuff and more sandy conglomerate is found, indicating that volcanic activity almost stopped, and the fan delta expanded. In the Xiazijie area, the provenance appears to be the Hala’alate mountains and volcanic materials in the Luliang uplift (Lu, 2018).

In the Fengcheng area, the bottom of P₁ f ¹ contains limestone, mudstone, and sandstone, and the top of P₁ f ¹ contains interbedded dolomitic mudstone and sandstone. This reflects that the P₁ f ¹ is a semideep lake–deep lake environment with weak hydrodynamics and weak volcanic activity. In P₁ f ², alkaline lacustrine deposits and salt-bearing mudstones reflect a dry climate, with low-lake levels and high salinity that encouraged the formation of unique alkaline minerals. These alkaline minerals may be affected by deep hydrothermal fluids (Chang et al., 2016; Zhang et al., 2018; Tang et al., 2020). P₁ f ³ consists of lacustrine dolomite in the early stage and terrigenous clastic rock in the late stage, reflecting the decrease of lacustrine facies and the development of fan delta. It can be seen from Sections B (Figure 7) and C (Figure 8) that the Fengcheng area is the sedimentation center.

The Baiquan area is close to the Kebai fault zone and is located at the edge of the sedimentary lacustrine basin. P₁ f ¹ is composed of dolomitic sandstone and a small amount of dolomitic mudstone. At the bottom of P₁ f ², interbeds of cloud sand and mudstone are found, and the top is tuffaceous coarse sandstone. P₁ f ³ is a thick sandy conglomerate. Therefore, in P₁ f ¹, the Baikouquan area is shore–shallow lacustrine facies. P₁ f ² volcanic activity developed, and fan deltas began to develop. P₁ f ³ fan delta developed further. This is consistent with the change in sedimentary facies interpreted using the seismic profile. In Sections C–G (Figures 8–10), lacustrine deposits are deposited at the bottom of P₁ f ¹, and a large area of lacustrine facies is transformed into fan–delta front facies within a short time. From P₁ f ² to P₁ f ³, the fan–delta outer-front developed into the fan–delta inner front. In P₁ f ³, Sections D and E (Figure 9) develop fan–delta plain facies near the fault zone. Moreover, the provenance of the Baiquan area likely originated from the West Junggar block and Zaire mountains.

In the Mahu area, P₁ f ¹ is deposited in shallow lacustrine facies. The tuffaceous sandy conglomerate is developed during the early stage, and tuffaceous mudstone is developed in a low-energy environment. During the P₁ f ² period, volcanic activity was frequent, and pyroclastic rocks were abundant. Volcanic rocks were developed at the top of P₁ f ² in wells Mh085 (Figure 3). Therefore, the volcanic activity was intense during the deposition of upper P₁ f ². During the deposition of P₁ f ³, the volcanic activity ceased, and the lake level dropped. The deposition is characterized by the fan–delta facies. Section G (Figure 10) shows that the sedimentary process of the Mahu area is similar to that of the Baiquan area. Thus, the fan delta was developed in the southern sag during the deposition of P₁ f ² and P₁ f ³.

In summary, the P₁ f ¹ deposition center was between the Baiquan and Fengcheng areas, and the lake reached its maximum depth. During this period, volcanic activity was frequent, and volcanic material provided part of the source. The fluvial delta deposits on the eastern slope originated from the Luliang uplift and flew through the river toward the center of the Mahu sag (Tang et al., 2021a). Next, during the deposition of P₁ f ² and P₁ f ³, provenance increased in the southern Mahu and Baiquan areas, and fan deltas with a considerable thickness were developed (Figure 11). This provenance may come from the Zaire mountains.

Sections A and B (Figures 6 and 7) reflect the architecture of the Fengcheng Formation in the Mahu sag, which is deposited from the West Junggar terrane to the lake (Figure 1D). The distribution of the Fengcheng Formation in the north is controlled by paleogeomorphology and faults, and the depocenter is in the Fengcheng area of the Mahu sag. Moreover, the east–west seismic profile reveals that the characteristics of multiple half-graben structures in the lower Permian strata are pronounced (Figure 6). Unidirectional and single boundary faults control these structures. Next, the activity of the northwest-trending marginal–fault system in the late Permian led to the tectonic inversion of rift-related depressions in the early Permian (Liang et al., 2020). The half-graben structure and faulting style revealed by the seismic profiles (Figure 6), the paleogeographic reconstruction in the stratigraphic sequence framework (Figure 11), and the identification of soft-sediment deformation structures indicate that the early Permian Mahu sag was a rift basin (Yu et al., 2016; Liang et al., 2020; Tang et al., 2020, Tang et al., 2021a).

Figure 11 shows that P₁ f ¹ is a set of lacustrine deposits with its depocenter in the Fengcheng area, and only a small part of P₁ f ¹ was deposited in a fan–delta environment in the southern Mahu sag (Figure 15A). In P₁ f ², the range of the lacustrine facies continues to expand (Figure 15B), and the area of the lake basin reaches the maximum (Figure 11). Tang et al. (2020) proposed that most of the soft-sediment deformation structures in P₁ f ² are caused by frequent earthquakes, indicating basin-bounding normal fault activities in the depocenter, creating maximal accommodation. Next, geochemical data and alkaline minerals show that the P₁ f ² member of the Mahu sag is an alkaline lake basin with high salinity (Gong et al., 2014; Cao et al., 2015; Zhang et al., 2018). At the beginning of the deposition of P₁ f ³, the range of lacustrine facies shrinks, and the Xiazijie and Fengcheng areas are shore–shallow lacustrine deposits. The Baiquan and Mahu areas were the fan–delta facies’ deposits, and the lake basin began to shrink (Figure 15C).

During the syn-rift stage of a rift basin, due to gravity isostasy, the continuous subsidence of the half-graben will lead to the uplift of the horst, increasing the sedimentary supply from the proximal horst sources (Chao et al., 2021). Based on the detrital zircon U–Pb chronology of the Fengcheng Formation, Tang et al. (2021c) demonstrated a transition from distant axial provenance areas to proximal lateral horsts in West Junggar during the syn-rifting processes in the Mahu sag. Next, Sections C–F (Figures 8 and 9) in the Baikouquan area and Section G (Figure 10) in the Mahu area show different filling processes in the margin of the basin. In the Baiquan area, fan–delta sequences accumulate toward the depocenter in the Mahu sag (Figures 8 and 9). The rock grain size became coarser and evolved from lacustrine facies to fan–delta front facies in P₁ f ¹ and P₁ f ² and finally to fan–delta plain facies in P₁ f ³, suggesting that the rate of sedimentary supply is higher than subsidence, which is consistent with the filling processes in the margin of a rift basin.

Therefore, the sediment-filling process of the Fengcheng Formation is controlled by fault activity. The Fengcheng Formation experienced a lake-basin expansion during the deposition of P₁ f ¹ and P₁ f ² and a shrinkage stage during the deposition of P₁ f ³ (Figure 15).