Geological Evolution and Volcanostratigraphy of the Wangfu Fault Depression: Insights from Structural and Volcano-Sedimentary Analysis in the Songliao Basin

Abstract

The Wangfu Fault Depression (WFD) is located in the southeastern uplift zone of the Songliao Basin and is an important geological site for studying tectonic evolution and volcanic stratigraphy. This study explores the complexity of the structure of the depression and the volcanic stratigraphy. The sedimentary sequence is divided into rift period and post-rift deposition, and the volcanic rocks are mainly concentrated in the Huoshiling Formation. Rhyolite deposits mark the bottom of the Yingcheng Formation. The volcanostratigraphic sequences are described by a detailed analysis of the seismic profiles, cutting samples, core data, geochemical, and well logging data, revealing the interaction between tectonic dynamics and volcanic activity. The volcanic facies are divided into vent breccia, pyroclastic, lava flow, and volcaniclastic sedimentary types, highlighting the diversity of depositional environments. In addition, the study identified key volcanic stratigraphic boundaries, such as eruptive and tectonic unconformities, which illustrate the alternation of intermittent volcanic activity with periods of inactivity and erosion. The study highlights the important role of faults in controlling the distribution and tectonic characteristics of volcanic rocks, and clearly distinguishes the western sag, middle slope, and eastern uplift zones. The chronostratigraphic framework supported by published U-Pb zircon dating elucidates the time course of volcanic and sedimentary processes, with volcanic activity peaking in the Early Cretaceous. Overall, the Wangfu Fault Depression is a dynamic geological entity formed by complex tectonic-volcanic interactions, providing valuable insights into the larger context of basin evolution and stratigraphic complexity.

1. Introduction

The Wangfu Fault Depression (WFD) is located in the northwest of the southeast uplift area of the Songliao Basin (Figure 1). It is adjacent to the Changling Fault Depression in the east, the Dehui Fault Depression in the north, and the Yushu Fault Depression in the west. It is a relatively small fault depression, with an area of more than 2500 km² in the Songliao Basin. The Wangfu Depression has gone through four stages of tectonic–sedimentary evolution, including mantle upwelling, rifting, post-rift thermal subsidence, and tectonic inversion [1], forming a double-layer stratigraphic framework with the lower part formed by syn-rifting and the upper part formed by post-rifting [1,2,3,4,5,6,7,8,9].

During the Mesozoic and Cenozoic Eras, the subduction of the Paleo-Pacific Plate beneath the vast Eurasian Plate led to significant stretching and thinning of the lithosphere in the Songliao area as well as the formation of a series of faults oriented predominantly in the north-northeast (NNE) direction, which has been widely documented in many academic works [3,5,8,13]. The collision of the Mongolian and Okhotsk plates triggered significant regional magmatic activity, which has been comprehensively studied and reported in the literature [3,14], and significant occurrence of such volcanism was also found in the specific study area considered [8]. This particular volcanic activity has been studied extensively, mainly that which occurred during the Huoshiling Period, and specifically in an area known as the Wangfu area, which is of great importance to both stratigraphers and volcanologists. The complex fault network, extending from north-northeast (NNE) to south (S), has played a key role in shaping and controlling the geological development of several tectonic belts (such as the Shandongtun, Xiaochengzi, and Wujiatun), which in turn extend from the western to the eastern part of the Wangfu Sag, as highlighted in the geological literature [4,15,16,17].

The sedimentary sequence of the syn-rift sedimentation period includes the Huoshiling Formation (J₃h), Shahezi Formation (K₁s), and Yingcheng Formation (K₁y), and the post-rift sequence includes the Denglouku Formation (K₁d) from bottom to top (Figure 2) [2,3,4,18] which is a set of clastic rocks interlayered by volcanic rocks. Volcaniclastic sedimentary rocks are widespread in volcanic basins as an important part of volcanic strata, such as the strata of the Songliao Basin (Huoshiling Formation, Shahezi Formation, and Yingcheng Formation), the Shahejie Formation of Liaohe depression, and the Stromboli volcano of Italy [19,20,21]. Horizontally, there are three distribution types, i.e., the lava-ignimbrite-volcaniclastic rock, volcaniclastic sedimentary rock, and volcaniclastic rock-volcaniclastic sedimentary rock [22,23,24]. Volcanic strata are quite different from sedimentary strata in terms of their genesis, provenance, and formation time. Despite extensive studies of the Songliao Basin, the detailed volcanic stratigraphy, tectonic evolution, and interaction of Early Cretaceous tectonics and volcanism in the Wangfu Fault Depression (WFD) remain relatively poorly understood. Existing studies often overlook the complexity of the distribution of volcanic facies and the role of unconformities in stratigraphic division. This study aims to address this shortcoming by constructing a high-resolution volcanic stratigraphic and chronostratigraphic framework by integrating seismic profiles, well logs, core samples, and zircon U-Pb age data. The goals are to delineate the volcanic facies architecture, recognize volcanostratigraphic boundaries, and reconstruct the structural evolution, chronostratigraphic framework, and sequence of the volcano-sedimentary processes that formed the WFD.

2. Data and Methodology

2.1. Data

Extensive seismic exploration has been conducted in the southern Songliao Basin over the past 50 years, providing valuable information for analyzing its deep structural and stratigraphic characteristics. The data used in this study mainly include drilling cores, geochemical analysis, thin sections, 2D seismic profiles, and well logs. The seismic data is presented on a vertical scale of two-way travel time (TWT).

To ensure a comprehensive coverage of the Wangfu Fault Depression, eleven wells were selected for detailed analysis and well-to-well correlation with the aid of 99.4 m cores, 1055 cutting samples, and 9680.17 m of Fullbore Formation Microimager (FMI) data These wells were chosen based on their spatial distribution across the depression and their inclusion of zircon U-Pb age data, which are crucial for establishing the absolute volcanostratigraphic framework. The well logs encompass sonic (AC), resistivity (RT), gamma ray (GR), and density datasets. For the purposes of this study, four stratigraphic formations, the Huoshiling, Shahezi, Yingcheng, and Denglouku were investigated. Furthermore, zircon U-Pb age data collected from previous research efforts were utilized to construct a robust absolute volcanostratigraphic framework for the Wangfu Fault Depression.

2.2. Research Methodology

2.2.1. Volcanostratigraphic Framework

The development of the volcanostratigraphic framework for the Wangfu Fault Depression is underpinned by several critical research parameters. These include the identification and classification of volcanostratigraphic boundaries, volcanic facies, well-to-well correlations, filling and compositional sequences, and a detailed chronostratigraphic analysis.

The major element compositions of the samples were analyzed at the Basic Geology Laboratory, College of Earth Sciences, Jilin University. Rock samples were first crushed to a coarse grain size, after which fresh fragments were selected for acid treatment, washing, and drying. The cleaned samples were then ground to 200 mesh. Major element concentrations were determined using X-ray fluorescence (XRF) spectrometry (ZSX Primus II) following the Borry frit method.

The classification of volcanic facies within the Wangfu Fault Depression is analyzed from drilling data and analysis of well logs [25,26,27,28].

The volcanostratigraphic boundaries were also delineated using core samples, well logs data, and seismic data. Information obtained from well log data enabled the identification of sedimentary rocks, weathered crust, and lithological variations in volcanic successions.

The analysis of volcanostratigraphic filling characteristics incorporates an integrated approach using well log and seismic data. Lithological data from well logs for Early Cretaceous formations were systematically compared across wells to understand the continuity and variability of volcanic and sedimentary units. Sedimentary rock lithology is grouped under a single category, whereas lithologies within the volcanostratigraphic sequence are assessed individually to evaluate eruption styles, periods of volcanic dormancy, and the evolutionary history of magmatic activity.

2.2.2. Chronostratigraphic Framework

The construction of a chronostratigraphic framework for the Wangfu Fault Depression integrates both relative and absolute geological age assessments. The relative geological chronological framework is established by analyzing the sequence of formation deposition within the Early Cretaceous stratigraphic units, focusing on the superimposed relationships among various layers. Through this analysis, a relative geological chronology is developed, which outlines the temporal sequence of the depositional events for each formation.

To create an absolute geological chronological framework, the zircon U–Pb dating method is employed as the primary tool due to its superior precision and resilience to external geological disturbances. Unlike the K–Ar method and other volcanic rock dating techniques previously used, zircon U–Pb [29] offers highly reliable age data owing to its elevated isotopic isolation temperature and resistance to weathering. By supplementing relative geological chronology with zircon U–Pb age data, an absolute age framework is constructed. This approach refines the stratigraphic divisions within the Wangfu Fault Depression and facilitates the determination of the formation era for each stratigraphic unit.

Using chronostratigraphic principles, this integrated framework utilizes high-resolution geological age maps derived from the relative and absolute dating results. These maps enhance the understanding of volcanostratigraphic sequences within the depression and provide a detailed temporal context for the stratigraphic units.

2.2.3. Structural Analysis

In addition to constructing a detailed volcanostratigraphic framework, this study also conducted a structural analysis of the Early Cretaceous sequence within the Wangfu Fault Depression to elucidate how tectonic processes affected the evolution and spatial arrangement of volcanic and sedimentary units. High-resolution seismic sections were interpreted using geological modeling software (MOVE 2013 and CorelDraw 2021) to identify faults, uplift zones, and stratigraphic boundaries. Three structural cross sections were analyzed to ensure complete spatial coverage of the depression, allowing a systematic assessment of deformation characteristics throughout the region. To reconstruct the tectonic history, the seismic data were digitized and processed using MOVE software, using decompaction and “move on fault” functions, and a simple shearing method due to the dominance of normal faults. The unfolding feature was then used to restore the stratigraphy to its pre-deformation configuration. CorelDraw was then used to create a clear, detailed cross-sectional drawings.

3. Results

3.1. Volcanostratigraphic Framework

Eleven wells and three seismic profiles in the Wangfu fault depression were selected to analyze the volcanic sedimentary sequence. By integrating well logging data, core data, thin sections, and seismic interpretation, the study examined the volcanic stratigraphic fill. The seismic profiles, which were almost perpendicular to the structural axis, were analyzed, providing information about fault-related subsidence and stratigraphic continuity.

3.1.1. Well-to-Well Stratigraphic Comparison

Huoshiling Formation

The Huoshiling Formation is typically characterized by the development of large areas of volcanic rock layers [1,30,31], and it is distributed throughout the study area (Figure 3, Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8). The basement of the Huoshiling Formation is located within the Wangfu Sag boundary in the southern Songliao Basin, and it consists of a thick layer of volcanic rocks.

On the seismic profile, the Huoshiling Formation mostly presents the characteristics of medium-amplitude, low-frequency, and medium-continuity (Figure 3, Figure 5, and Figure 7). The resistivity curve is mainly high-value, box-shaped for volcanic units, and the resistivity is low, showing box characteristics in interbedded clastic rock layers, (Figure 4, Figure 6, and Figure 8).

Drilling revealed that the thickest volcanic deposits were found in well CS7, at a depth of 3100 m and of about 1223 m thickness, though the well did not penetrate the entire unit. Core samples from wells WF1, CS11, and CS8 show that the volcanic deposits are mainly composed of trachyandesite and trachyte (Figure 4, Figure 8, Figure 9 and Figure 10), which were identified as the main components of these volcanic units and account for about 80% of the total volume [29].

In addition, there are clastic sedimentary rocks, with significantly thicker layers observed in wells CS12 and CS13 (Figure 4). The volcanostratigraphic filling sequences are divided into four units based on the thickness of the rock units and the characteristics of the well logging. The first unit is thick lava of trachyandesite, as seen in well CS8. Its maximum thickness is almost 300 m, and its gamma and density curves are low amplitude, blocky, and slightly pointed. The second unit is the interbedded type, represented by wells CS11, CS9, and WF1, which is characterized by the superposition of welded tuff and lava, with a single layer thickness of 20 to 50 m, a medium-amplitude blocky pattern, and a sharp peak at the center of the gamma and density curves. The third unit is the composite unit, found in wells CS9 and CS607, and is characterized by alternating thick and thin layers of lava and/or ignimbrite. Finally, the fourth unit is the interbedded clastic unit, presented in wells WF1, CS12, CS13, and CS2 (Figure 4, Figure 6, and Figure 8).

Among the eleven wells containing volcanic materials, trachyandesite accounts for about 27.4%, andesitic breccia accounts for about 15.2%, trachyandesite lapplistone accounts for about 13.3%, trachyandesitic ignimbrite accounts for about 9.7%, and clastic rock accounts for about 12.4% (Figure 11). In addition, there are andesite (7.9%), trachyte (4.7%), and tuff (3.2%). The Huoshiling Formation also contains rhyolitic welded breccias, reconstituted volcanic material, tuffs, and tuff breccias.

Shahezi Formation

The Shahezi Formation is not well developed in the study area, and the thickness of the formation is generally small. The seismic section is perpendicular to the tectonic axis, with thicker strata in the west and thinner strata in the east. The slope of the eastern region is mainly related to faults, with thicker strata in the north and thinner strata in the south along the tectonic axis.

The thickness of the formation is most evident in wells WF1, CS11, and CS606, ranging up to 300 m. In contrast, thinner formations are observed in wells CS12 and CS9 with a thickness of 20–50 m, while in wells CS13, CS2, and CS8, the formation is completely missing due to erosion (Figure 4, Figure 6, and Figure 8). From a lithological point of view, the formation can be divided into two types: one type is thick shale, generally distributed in the western area from well CS12 to well CS608; the other type gradually becomes narrower, thinner, or even completely absent toward the east.

The lithology of the second type includes shale mixed with tuff or tuffaceous sandstone (evidence from wells CS607 and CS606) (Figure 6), sandstone interbedded with shale, or juxtaposed conglomerate and shale (evidence from well WF1). The two types can be clearly distinguished from the resistivity log: the first type usually has a low resistivity and the resistivity of second type suddenly becomes high. The general seismic characteristics of the Shahezi Formation are moderate amplitude, medium to high frequency, strong continuity, and a parallel reflections line (Figure 3, Figure 5, and Figure 7).

Yingcheng Formation

The first section of the Yingcheng Formation is an important marker layer in the stratigraphic division of the Wangfu Fault within the scope of this study. Due to the relatively thin strata of volcanic rocks and the large difference between the upper and lower lithological units, the wave impedance contrast is obvious, the amplitude is strong, the continuity is good, the frequency in both directions is reduced, and the reflection parallel to the seismic axis is obvious. The lithology is mainly rhyolite and rhyolitic tuff with a maximum thickness of up to 50 m in the CS 9 well, and the resistivity curve is a high-value box-shaped one (Figure 4, Figure 6, Figure 8, and Figure 10).

In well WF1, the first section (33.6 m thick) is a volcanic rock section, with light gray rhyolite and gray and green-gray tuff and a relatively high resistivity curve that is box-shaped. The second section (179.4 m thick) is a clastic coal-bearing section, mainly fine clastic rocks, containing a thick coal seam (32.4 m thick). It has a relatively low resistivity curve that gradually thins to the east. A thick shale layer with a thickness of about 100 m has developed at the bottom of Well CS608, and the lithology is composed of conglomerate, sandstone, and shale as cemented units. The seismic attributes of this section are mainly medium-weak amplitude, medium-high frequency, good continuity, and parallel-subparallel reflection mode, and the amplitude peaks of some points in the wellbore are significantly enhanced (Figure 3, Figure 5, and Figure 7). The resistivity curve is mainly low magnitude, box-shaped, and has stable lithology.

The Tertiary strata of the Yingcheng Formation are widely distributed, and drilling operations are carried out throughout the area. The rock composition is mainly conglomerate interbedded with sandstone and shale.

Denglouku Formation

The Denglouku Formation in the Wangfu fault depression is also an important stratigraphic unit that deserves thorough study and analysis. The spatial distribution of this particular formation is characterized by a pronounced thickness that is particularly large in the western region, ranging up to 300 m while it decreases toward the eastern margin, ranging up to 50 m before eventually leading to a complete attenuation of the formation in this direction.

The geological formation is mainly composed of a variety of sedimentary lithology, including but not limited to mudstone, sandstone, conglomerate, siltstone, and silty mudstone, each of which contributes to the complex stratigraphy of the area (Figure 4, Figure 6, and Figure 8). Formation thickness measurements indicate that the formation volume is relatively large in specific wells with more prominent geological features, such as WF1, CS607, and CS12. In stark contrast, in wells CS13, CS11, and CS608, the formation thickness is relatively small and the formation integrity is compromised (Figure 4, Figure 6, and Figure 8).

Furthermore, it is noteworthy that the eastern wells (particularly CS9, CS2, and CS8) are completely devoid of the Denglouku Formation, highlighting the geological discontinuity of the region (Figure 4, Figure 6, and Figure 8). The seismic characteristics of this formation reveal an interesting pattern exhibiting parallel to subparallel seismic reflections characterized by moderate to high continuity, weak amplitude, and moderate to high frequency, indicating the complexity of geological processes (Figure 3, Figure 5, and Figure 7). These seismic attributes provide valuable information about the depositional environment and tectonic history of the formation over geological time scales. In conclusion, the Denglouku Formation is an important research object for basin analysis and stratigraphic research in the Songliao Basin.

3.1.2. Volcanic Facies of WFD

Volcanic facies classification frameworks developed by various researchers have laid the foundation for interpreting volcanic deposits based on eruption dynamics and depositional processes [33,34,35,36]. Regionally, Wang [34] proposed a detailed classification for the Songliao Basin, dividing volcanic facies into multiple facies and subfacies, which has been widely applied in fault depressions. Subsequent studies in adjacent basins have refined facies identification using well log and petrographic data [19,35,36]. However, previous classification schemes are not without limitations. In particular, they do not fully differentiate the diagnostic features of acidic and intermediate-basic volcanic lithologies based on petrophysical responses. As volcanic facies classification is highly sensitive to magma composition, eruption style, and post-emplacement alteration, we have supplemented previous facies’ model by incorporating key log and petrographic criteria that help resolve these ambiguities. For instance, acidic pyroclastic units, such as rhyolitic tuff and ignimbrite tend to produce higher gamma ray peaks, moderate resistivity values, and well-developed layered textures, while intermediate-basic lavas such as trachyandesite typically display blocky, lower gamma ray curves and higher resistivity values due to their dense, low-porosity nature. These distinctions have been carefully integrated into our facies identification process.

Volcanic lithofacies in the Wangfu Fault Depression are classified into vent breccia facies, pyroclastic facies (flow and fall subfacies), lava flow facies, and volcaniclastic sedimentary facies [33,36].

Vent Breccia Facies

Volcanic conduits are pathways through which magma can erupt, creating a connection between the magma chamber and the crater roof. The conduit may have developed throughout the volcanic event; however, it primarily represents a product of subsequent activity. The vent facies is found beneath and near the central part of the volcanic edifice and consists of pyroclastic rocks and/or lava that was retained or filled in after magma ascended to the surface [37]. Lava, pyroclastic rocks, and welded pyroclastic rocks are the types of deposits associated with volcanic vents. These deposits are typically characterized by their angularity, lack of sorting, and the occurrence of hydrothermal alteration [31,38].

Integrated analysis of core, thin section, and wireline log data from the Wangfu Fault Depression reveals the presence of vent breccia facies within the Huoshiling Formation, observed exclusively in Well CS2 at depths of 1950–1980 m and 2160–2200 m (Figure 6, Figure 9, and Figure 11). These facies are characterized by coarse, angular, mottled, and greenish-gray trachytic and andesitic volcanic breccias, displaying clast-supported textures with sharp, irregular fragment boundaries. The log response is characterized by low to medium gamma ray values, medium to high resistivity, and moderate acoustic and density readings, which are consistent with coarse, heterogeneous breccias deposited near the volcanic vent. These petrophysical signatures, together with the lithological assemblages, support the interpretation of a vent breccia facies formed by short-distance fragmentation and emplacement. The vent breccia facies accounts for only 0.6% of the total volume in the Wangfu fault depression (WFD). (Figure 12).

Pyroclastic Facies

The pyroclastic facies encompass a range of pyroclastic deposits derived from explosive eruptions, typically associated with high-viscosity, volatile-rich magmas. These eruptions may occur throughout the volcanic cycle. However, they are mainly generated during the initial and peak stages of an eruption. Pyroclasts can vary greatly in size, depending on their distance from the volcanic vent. In general, the closer the volcanic vent, the coarser the pyroclasts that are deposited. Based on eruption dynamics, magma rheology, and depositional processes, the pyroclastic deposits are divided into pyroclastic flows and pyroclastic fall subfacies.

Pyroclastic Flows Subfacies

These subfacies are specifically identified in wells WF1, CS11, CS12, CS607, CS608, CS2, and CS13, respectively (Figure 4, Figure 6, and Figure 8). The lithological features are andesite breccia, tuff breccia, trachyandesite-ignimbrite, andesite ignimbrite, rhyolite tuff, and rhyolite ignimbrite (Figure 4, Figure 6, Figure 8, Figure 9, and Figure 11).

This subfacies is mainly found in the Huoshiling Formation, with thinner horizons observed in the Yingcheng Formation (Figure 12). These deposits exhibit features such as flow-aligned pyroclasts, glassy or vitric textures, massive structure, and crystal-fragmental textures. The matrix is typically felsic in composition (Figure 9). The well log responses for this subfacies show distinctive characteristics. The gamma ray (GR) curve exhibits low to medium values, reflecting the relatively low clay content and felsic composition of the ignimbrites and tuffs. Resistivity logs (RLLD) show medium to high values, indicating strong lithification and limited fluid content, likely due to welded textures and low porosity. The density (DEN) curve is moderate and relatively stable, consistent with consolidated volcanic deposits. Acoustic (AC) logs show low to medium values, consistent with the compact and welded nature of the pyroclastic flow deposits, with a limited development of primary porosity. The pyroclastic flow subfacies account for 13.5% of the total volume of the Wangfu fault depression (WFD) (Figure 12).

Pyroclastic Fall Subfacies

The pyroclastic fall subfacies consists of tephra deposited from volcanic plumes settling through the atmosphere under gravity. These deposits are typically well-sorted, layered, and exhibit grain-size grading [31,38]. The pyroclastic fall subfacies have been found in wells WF1, CS13, CS607, and CS9. The lithological compositions of these subfacies include trachyandesitic lapplistone, tuff, brecciated tuff, lapplistone, and trachyandesitic tuff (Figure 4, Figure 6, Figure 8, Figure 9, and Figure 11). The maximum thickness of up to 490 m of this subfacies is recorded in well CS9 in the Huoshiling Formation, while the subfacies are thinnest in the Shahezi Formation, ranging up to 10 m in well CS9 and WF1. Well log responses are characterized by low to medium gamma ray values with minor peaks and medium to high resistivity and density, likely reflecting compaction, cementation, or interbedded denser layers. Acoustic travel times are low to medium, indicating variable consolidation and suggesting that these deposits are more compacted or partially reworked than typical unconsolidated fallout material. The pyroclastic fall subfacies account for 11.5% of the total volume of the Wangfu fault depression (WFD).

Lava Flow Facies

The lava flow facies form during effusive eruptive phases, characterized by the surface outflow and subsequent cooling of magma. These facies can be subdivided into simple and compound lava flow subfacies, based on internal architecture, cooling behavior, and eruptive dynamics. This classification reflects the differences in how lava bodies were emplaced and solidified. Analysis reveals that the lava flow facies in the study area are dominantly of the simple lava flow subfacies.

Simple lava flows are characterized by dense, massive interiors and autobrecciated margins. Despite compositional variability, from rhyolite and trachyte to andesite and trachyandesite, they exhibit uniform internal architecture and structural coherence, consistent with typical simple lava flow subfacies (Figure 9 and Figure 11). Most of these flows are concentrated in the Huoshiling Formation, along with some thin-bedded flows observed in the first member of the Yingcheng Formation, particularly in wells WF1, CS12, and CS9. The well log responses for the simple lava flow subfacies show distinctive characteristics. The gamma ray (GR) curve exhibits medium values, reflecting the intermediate to felsic volcanic composition and low clay content. Resistivity logs (RLLD) display consistently high values, indicative of dense, well-lithified flows with minimal fluid saturation. The density (DEN) curve is elevated and stable, consistent with compact volcanic rocks of rhyolite, andesite, trachyandesite, and trachyte. Acoustic (AC) logs show low readings, corresponding to the competent and consolidated nature of these volcanic units. The simple lava flow subfacies accounts for 21.7% of the total volume in the Wangfu fault depression (WFD) (Figure 12).

Volcaniclastic Sedimentary Facies

This facies represents sedimentary intervals dominated by volcaniclastic materials, formed during periods of waning or paused volcanic activity. These deposits result from the erosion, transport, and redeposition of pyroclastic and volcanic debris in subaqueous or subaerial environments [31,38]. By examining cutting sections and well log data, it has been determined that the sedimentary succession includes volcaniclastic components, primarily represented by interbedded sedimentary tuffs along with conglomerates, sandstones, and shales. This lithofacies occurs interlayered with volcanic rocks and reflects intervals of reduced or paused volcanic activity (Figure 9 and Figure 12). It is mainly found in wells WF1, CS12, CS13, CS607, CS608, CS9, CS11, and CS2 (Figure 4, Figure 6, and Figure 8). In the study area, sedimentary facies are predominant. Sedimentary facies exist in the three geological strata of the Shahezi Formation, Yingcheng Formation, and Denglouku Formation, while volcanic facies predominate in the Huoshiling Formation (Figure 12).

The response characteristics of conventional log curves are as follows: the gamma curve (GR) has large amplitude fluctuations, a slightly bell-shaped profile, and an obvious peak; the resistivity laterolog curve (RLLD) has moderate amplitude, is generally bell-shaped, and does not change much; and the alternating current (AC) curve is slightly jagged and finger-shaped, with large amplitude fluctuations. The density (DEN) curve exhibits a relatively linear trend, punctuated by peak fluctuations.

3.1.3. Volcanostratigraphic Boundary

Volcanic stratigraphic boundaries can be classified according to the time span and dynamics of their formation. These include eruptive conformity boundaries (ECBs), eruptive unconformity boundaries (EUBs), eruptive interval unconformity boundaries (EIUBs), tectonic unconformity boundaries (TUBs), and intrusive contacts (ICs) [22,23,39,40]. EIUBs and EUBs align with minor unconformities, whereas a TUB is generally associated with moderate to major unconformities [41]. Sedimentary and weathered crustal rocks signify intervals between volcanic eruptions and are classified as EIUBs or TUBs [39,42,43,44,45]. Conversely, lithological variations induced by eruptive styles and magma evolution are interpreted as ECBs or EUBs.

Eruptive Conformity Boundary (ECB) and Eruptive Unconformity Boundary (EUB):

Boundaries exist between volcanic phenomena, such as lava flows, pyroclastic flows, airborne sediments, and lahars, which occur during intermittent eruptive activity at intervals ranging from seconds to years [34,46,47,48]. ECBs can occur through repeated eruptions characterized by a short lifespan. Properties of lava flows can vary greatly, depending on the velocity and viscosity properties of the erupting magma [49]. In addition, the presence of flow units above and below the boundary constitutes the main evidence of an unconformity or conformity.

In well log analysis, EUBs were found in well WF1, where trachyandesite is interbedded with trachyandesitic ignimbrite and trachyandesitic lapillistone (Figure 4 and Figure 8). In addition, several EUBs can be observed in well CS11, where there are trachyandesite and trachyandesite-ignimbrite intercalations. ECBs were found in wells CS13 and CS606 (Figure 4 and Figure 6), where andesite is interbedded with trachyandesite. In addition, ECBs were also found in well CS9, where tuffs were interbedded with trachyandesitic lapplistone (Figure 6).

Eruptive Interval Unconformity Boundary (EIUB)

EIUBs are boundaries located within or as part of a volcano, formed during eruptive phases or periods, ranging from a few years to thousands of years [34,39]. EUIBs are identified by characteristic layers composed of reworked volcanic rocks or weathered crusts, which serve as distinctive markers [42,43,44]. Depending on the EIUB, two different filling sequences can be distinguished. The first is the volcanic stratigraphy of two wells, which show a single EUIB located between the overlying volcanic and sedimentary stratigraphy in wells CS606 and CS8 (Figure 6 and Figure 8). The absence of interbedded sedimentary rocks indicates that the volcanic formation was in a state of continuous development, suggesting that it formed in a relatively short time span. Second, infill sequences with multiple EUIBs were documented in nine wells.

The EUIB interface or eruptive unit constitutes the upper boundary of the volcanic beds and is also the dividing line separating the interbedded sedimentary rocks from the volcanic beds beneath them, as shown in Figure 4, Figure 6, and Figure 8.

Tectonic Unconformity Boundary (TUB)

Tectonic unconformity boundaries (TUBs) are important markers that separate volcanic sequences and represent time intervals ranging from tens of thousands to several million years [39]. Two main types of boundary relationships can be identified. The first type is characterized by the contact between the overlying sedimentary rock layers and the underlying volcanic rock layers, which can be hundreds of meters thick. The second type is characterized by the boundary formed by long-term exposure and erosion of the underlying volcanic rocks, as pointed out by [50].

According to the results of well log curve analysis, TUBs were found in many wells, including WF1, CS607, CS606, CS608, CS11, CS8, CS12, and CS13 (Figure 4, Figure 6, and Figure 8). The relationship between TUBs in the study area is shown as the upper sedimentary rocks in contact with the lower volcanic rocks, indicating the existence of obvious stratigraphic relationships. A TUB can present at least two different forms. The first is a strong amplitude surface, which is closely related to the original topography of the volcanic field. The second is a weak amplitude surface, which is the result of the action of the plain surface on the relief form.

3.1.4. Chronostratigraphic Characteristics of WFD

The chronostratigraphic features associated with the WFD indicate that it is the most complete basin with well-developed Cretaceous strata in northern China and is geographically very close to the Early Cretaceous strata in western Liaoning Province. The stratigraphic framework of the Songliao Basin, especially the discussion surrounding the period of deep faulting, has been of great interest to many geologists who have attempted to unravel the complexities of this geological narrative. Tectonic activity and faulting significantly influence stratigraphic architecture. Isotopic dating plays a critical role in constraining the timing of these events. This study integrates previously published isotopic data to clarify the temporal framework of volcanic and tectonic evolution in the region. This study provides a detailed quantification and comprehensive analysis of the chronology of fault-tectonic-volcanic rocks recorded in several academic papers published in previous years. The empirical data obtained show considerable differences in chronology, indicating that the volcanic rocks associated with the Huoshiling Formation have an age range of approximately 169 million to 124 million years ago, while the volcanic rocks of the Yingcheng Formation have an age range of 192 million to 41 million years ago [3,8,29,31,51,52]. It is generally believed among petrological researchers that the main reason for these large chronological differences is the methods used, especially the potassium–argon (K–Ar) dating technique and the argon–argon (Ar–Ar) dating method, especially the K–Ar system. It should be noted that the K–Ar method is characterized by a low closure temperature of minerals and is easily affected by hydrothermal alteration occurring in geological basins.

In contrast, zircon crystals are thought to have significantly higher closure temperatures, typically above 900 °C [53,54], and are therefore a more reliable means for accurate dating. Based on this basic knowledge, we carefully selected all samples and analyzed the zircons using U–Pb dating technology.

Based on the zircon U–Pb dating data for the Wangfu Fault (Tables S2 and S3), the age distribution of zircons within this geological feature can be directly determined. The box plot (Figure 13) effectively illustrates that the comprehensive age distribution of the Yingcheng Formation located in the Wangfu Depression ranges from 106 million to 118 million years ago, while the comprehensive age distribution of the Huoshiling Formation in the same geological depression ranges from 121 million to 129 million years ago (Figure 13) (Tables S2 and S3).

3.2. Structural Analysis of Wangfu Fault Depression

The Songliao Basin in the Mesozoic-Cenozoic era was formed into a large superimposed basin through multiple periods of compression uplift and extensional faulting, with nineteen fault depressions developed. The Wangfu fault depression is one of them.

3.2.1. Major Faults in WFD

There are three main faults which are associated with the Wangfu fault depression. These are Wangfu West Fault, Wangfu East 1 Fault, and Chengshen East 2 Fault. The Wangfu West Fault is a shovel-shaped fault, steep at the top and gentle at the bottom. It is a large boundary fault that controls the formation and development of the depression. The Wangfu (WF) 1 East Fault and the Chengshen (CS) 2 East Fault control the formation of the two secondary structural belts of the Shandongtun and Xiaochengzi troughs.

Wangfu West Fault

The Wangfu West Fault is the boundary fault of the Wangfu dustpan-shaped fault depression, and it controls the formation and sedimentary evolution of the fault depression. Its strike is consistent with the regional structural strike, which is nearly north–south, with a plane extension length of more than 120 km [15,16,17]. It is a shovel-shaped normal fault with a steep top and gentle bottom and a large fault throw.

The fault was developed in the early syn-rift stage of the basin during the Early Cretaceous time, controlling the formation and distribution of the Huoshiling Formation and the Yingcheng Formation and affecting the development of the Denglouku Formation. The Wangfu West Fault disappeared during the deposition of the Quantou Formation and ended before the basin depression-type deposition.

The Mesozoic fault deformation of the Wangfu fault depression is different in the north–south direction (Figure 1). The Wangfu West Boundary Fault has a small dip angle and a gentle shape in the northern section, and a larger dip angle and a steeper shape toward the south [15,16,17]. The fault plane shows an “upward convex” shape in some locations, and the footwall of the fault is a dense and hard volcanic rock basement. On the other hand, the fault throw increases from north to south, indicating greater vertical displacement, represented by thickening of the strata in the subsidence center, with obvious syntectonic sedimentary characteristics. The subsidence center gradually approaches the Wangfu West Boundary Fault toward the south, indicating that the activity of the Wangfu West Boundary Fault and the tectonic subsidence gradually increase toward the south.

WF1 East and CS2 East Faults

The Wangfu 1 East Fault and the Chengshen 2 East Fault are the main controlling factors of structural formation. A rolling anticline has developed close to one side of the fault; the strike is consistent with the controlling fault, and it is distributed in a nearly north–south and north-northeast direction. Nearly north–south slip fault steps have also developed on both sides of the fault, and a series of closed structures formed at the same time.

The structural settlement-deposition center is distributed along the boundary (Wangfu West) fault, and the stratum thickness in the trough is the largest. In the eastern part of the area far from the boundary fault, secondary faults with smaller fault throws are mainly developed, the fault throw and fault activity are relatively small, and the stratum thickness thins eastward.

After the uplift during the fault depression period and the subsequent superposition and transformation, there are many small-scale faults, the stratum is broken, and the continuity is poor; the structural deformation is mainly controlled by the activity of faults at all levels [15,16,17]. The structural balance profile can also reflect the structural evolution process and the main period of fault formation. The structural balance profile (Figure 14) shows that during the deposition of the faulted structural layer, the fault activity was intense; the strata were wedge-shaped and controlled by the main fault. During the deposition of the depression structural layer, the intensity of fault activity was significantly weakened and the control of faults on sedimentary strata was weakened.

3.2.2. Structural Cross Section Along A-A’

This structural transect covers the northern area of the Wangfu Depression (Figure S1), which is an important area of interest for geological research. In this detailed assessment, the section was systematically divided into three distinct units, each of which displays unique geological features. These units constitute the eastern uplift zone, which is characterized by significant changes in elevation; the middle slope zone, where normal faults are well developed and have a large impact; and the western depression zone, where significant subsidence occurs, which is a key aspect of the overall structure. It should be noted that the area with the greatest thickness of geological structures appears in the western depression zone, which is a direct result of tectonic processes. Moving to the eastern uplift zone, the thickness of geological structures decreases significantly, indicating that the geological features show a gradient change. Multiple normal faults run parallel through the region, forming a complex network that is crucial to understanding the structural dynamics of the area. These normal faults can be divided into three temporal stages based on their formation and activity. Early faults were formed during the syn-rift of basin development, have long-term inherited activity, and last throughout the geological history of the area. These early faults mainly served as boundaries for depressions and became controls on subsequent subsidence within the basin, thus playing an integral role in shaping the tectonic landscape, as highlighted in the work of [15,16,17]. The importance of these faults lies in their ability to extend from basement through younger geological structures to the surface, revealing the complex history of tectonic activity in the region. As for the intermediate faults, they are considered to be derivatives or offsets of the early faults, and they display a unique multilayered structure that may present a “Y”-shaped or reverse “Y”-shaped configuration with respect to the early faults as shown in Figure 14. This intermediate-stage fault is characterized by relatively short fault displacements that do not extend far, which further complicates the fault patterns observed in the region. In contrast, the late faults represent the most recent tectonic activity and are confined to the upper part of the geological section, specifically within the Yingcheng and Denglouku Formations.

The geological features of the area are characterized by local upward bulges. The faults have a consistent overall trend and are closely connected in a north–south direction, forming Y-shaped and flower-shaped structures, which reflect complex geological processes. The inclinations of these faults vary, with steeper inclinations at the top and gentler slopes at the bottom, demonstrating the complex stratification and structural dynamics of the area. The overall structure of the Wangfu Fault Depression (WFD) has a significant impact on the thickness of various geological structures, especially in the context of the Cretaceous sequence. If one examines the thickness of these formations, one will find that the greatest thickness is concentrated in the western depression, where all Cretaceous rocks are well represented. However, as one moves eastward, the thickness decreases significantly, eventually resulting in a pinch-off effect due to a combination of uplift and erosion, which have gradually shaped the landform over time.

3.2.3. Structural Cross Section Along B-B’

This section covers the middle part of the Wangfu Depression, and like section A-A’ discussed above, its eastern side shows significant uplift, with Cretaceous and later geological formations either not deposited or severely eroded by the uplift processes that dominated the area. In stark contrast, the western side of this section shows thick sediments from the Cretaceous sequence, highlighting the clear differences in the geological development of the entire fault depression. The Huoshiling Formation is a persistent feature of this section and is the thickest formation compared to other existing geological formations in the area, further emphasizing its importance in the broader geological context. A large part of the area is underlain by Paleozoic crystalline basement rocks that extend from east to west with a narrowing trend that becomes more pronounced toward the eastern boundary of the study area. In addition, a series of normal faults, similar to those found in section A-A’, are present in this section, but it is noteworthy that the strength of these faults is relatively weak compared to other sections of the fault depression. Three types of faults also appear in this central section: early, middle, and late, illustrating the complexity and variability of tectonic activity in this geological landscape. The early faults extended from the basement rocks into the Cretaceous sequence, effectively cutting through all the intervening strata and having an important influence on their thickness and sedimentation patterns. In addition, the intermediate faults are characterized by short projections, originating in the Huoshiling Formation and extending upward to the Shahezi and Yingcheng Formations, exacerbating the intricate fault dynamics in the region. Finally, later or younger faults also exist, but they are limited to the upper parts of the Yingcheng and Denglouku Formations, where they have the lowest throw and therefore contribute less to the overall structural complexity. Along the section, the faults show a general trend that is closely related to the near north–south direction, accompanied by different dips ranging from steep to gentle, reflecting the dynamic geological processes that shaped the region over geological time.

3.2.4. Structural Cross Section Along A-C

This structural transect extends from near the north to the southeast part of the Wangfu Fault depression (Figure S2). Similar to the sections discussed above, this area is systematically divided into three distinct structural sections, namely the Western Depression, the Middle Slope, and the Eastern Uplift. The Eastern Uplift, although relatively small in size within the region, is characterized by the Huoshiling Formation lying on an underlying basement, while all of the other formations that once occupied this area have been severely eroded such that their deposition and thickness have been greatly affected by the ongoing uplift process. It is noteworthy that the Cretaceous sequence can still be observed on the western side of the uplifted area, which is bounded by a distinctive, long, steep-dipping normal fault originating from the basement and extending through all the geological structures to the surface. Moreover, the Huoshiling Formation has the greatest thickness in this particular section compared to other nearby geological formations. Furthermore, a number of normal faults can be detected within this section. However, it is noteworthy that these faults have significantly higher dips and slopes and extend from the basement to the surface, intersecting and affecting all geological structures encountered along their path. This section also reveals the presence of intermediate faults that are visible and extend from the Huoshiling Formation to the surface. In addition, younger faults were encountered in this section, which were formed in later geological stages and exist only within the Yingcheng Formation and extend to the Denglouku Formation along with other younger geological structures. The general orientation of the faults observed in this section is consistent with the trends observed in other sections, showing a predominantly near north–south orientation, and the fault patterns include various types, including domino, Y-shaped, and reverse Y-shaped faults. In certain places, the area presents an upward convex profile, which further increases the complexity of the geological structure of the area.

4. Discussion

4.1. Volcanostratigraphic Framework

Volcanic stratigraphy is an important aspect of all basins, as volcanic rocks can account for up to 25% of a basin’s volume [56,57,58,59]. The volcanic stratigraphic framework observed within the study area is primarily located within a geological structure characterized by a half-graben basin, which occurred during the early stages of rifting associated with the Songliao Basin [9], as shown in (Figure 1). Interpretation of the seismic data indicates that the maximum thickness of the basin fill is about 1800 m, indicating a large volume of sediments over tens of millions of years [12]. Overall, the volcanic stratigraphy is estimated to represent about 58% of the fault segments of the basin, with an overall representativeness of about 28% when the entire basin is considered as a single entity [12]. Based on the observed thickness of the volcanic stratigraphy, it can be inferred with considerable confidence that the volcanic center is strategically located in the central part of the Wangfu Fault Depression (WFD). The seismic data also indicate that the lithofacies associated with the volcanic stratigraphy are primarily tabular with some sections of mound-like internal reflections that show relatively weak amplitude signals and are characterized by low-frequency components. However, it must be recognized that the inherent limitations of seismic data resolution have resulted in many overlying geological relationships being largely unknown and difficult to understand.

The volcanic formations located near the western depression zone underwent a major subsidence geological process, which began with the basin filling and continued until the middle Campanian. In contrast, the volcanic formations in the middle slope area underwent a long period of denudation, which lasted until the early Berriasian, followed by a subsidence phase from the early Baronian to the middle Campanian. In sharp contrast, the volcanic formations in the eastern uplift zone experienced a complex series of geological events, including extensive uplift and denudation from the initial basin filling to the Albian, followed by a period of subsidence from the Cenomanian to the middle Campanian, and then uplift-related denudation in the late Campanian, culminating in the final burial phase in the Neogene [12]. It is important to note that this secondary burial did not exceed the maximum paleo burial depth previously experienced [12].

4.1.1. Well-to-Well Stratigraphic Comparison

Through the analysis of the seismic profiles that are almost perpendicular to the structural axis, it is found that the Wangfu fault depression has obvious structural characteristics of deep depression in the west and uplift in the east. In addition, the overall distribution of the strata during the depression period is characterized by a significant increase in thickness in the west and a significant narrowing toward the east of the basin.

It is worth noting that no bedrock has been found in the various boreholes collected during this investigation. On the seismic profile, the basement characterized by the Wangfu fault depression shows medium to strong amplitude characteristics, as well as low-frequency signals with poor continuity and reflectivity.

Volcanic rocks in the Wangfu fault depression are mainly concentrated in the Huoshiling Formation and the Yingcheng Formation, corresponding to the syn-rift stage of basin development. Geochemical analysis, core description, logging response, and thin section analysis reveal a range of volcanic lithologies, including trachyandesite, andesite breccia, rhyolite, tuff, and ignimbrite (Figure 9 and Figure 10) (Table S1). These results were further supported by the previous published studies of the Wangfu fault depression by [12,29,32] (Figure 10). These rocks show typical volcanic features such as porphyritic texture, vesicular texture, flow banding, and pyroclastic components (e.g., lapilli and ash), indicating that the eruption mode was both effusive and explosive. The presence of vent breccia and pyroclastic flow deposits, as well as rhyolitic lavas at the base of the Yingcheng Formation, supports its interpretation as a syn-rift volcanic unit. This interpretation is supported by the U–Pb zircon age data [29], which place the volcanic activity in the Early Cretaceous rift stage of the Songliao’s Basin evolution.

4.1.2. EIUBs of Volcanostratigraphy

The presence of these interbedded units is an important indicator that the volcanic stratigraphic sequence associated with this particular type of EUIB is characterized by discontinuities, suggesting that the temporal extent of the volcanic stratigraphy may cover a long period of tens of thousands of years, reflecting complex geological processes [39,45,60]. In general, the presence of one or two EUIBs in the geological setting implies that the extensive volcanic stratigraphy observed in the WFD or volcanic field in question has gone through a multifaceted and complex eruptive history characterized by a complex interplay of volcanic activity, dormant periods, and severe erosive events [12,45]. This complex volcanic narrative is characterized by eruptive peaks alternating with quiescent phases, during which the geological record shows gaps caused by the cessation of volcanic activity or the subsequent disappearance of any evidence related to such activity due to erosive processes [8,46,60].

The discovery of multiple unconformities in this geological setting strongly suggests that volcanic activity is episodic rather than a continuous phenomenon, indicating that eruptive events are not evenly distributed over time [12,45,60]. Each unconformity is a key marker, marking a significant transformation in the volcanic system itself, which can be attributed to various factors, such as changes in magma composition, changes in eruption style, or changes in the tectonic environment. This particular pattern of intermittent volcanic activity is a common feature of persistent volcanic fields or areas subject to complex tectonic dynamics, further emphasizing the intricate relationship between geological processes and landscape evolution [12,39].

4.1.3. Chronostratigraphic Characteristics of WFD

Focusing specifically on the Huoshiling Formation, two distinct periods of volcanic eruptions can be identified: the first eruption occurred approximately 121 million years ago, while the second eruption occurred 129 million years ago [29] (Figure 12 and Figure 15).

Furthermore, the recorded intervals between eruptions occurred between 122 and 128 million years ago. The volcanic activity of the Yingcheng Formation lasted from 114 million to 119 million years ago, with a brief eruption between 106 million and 107 million years ago, followed by a gap in volcanic activity between 107 million and 114 million years ago.

The peak of volcanic activity occurred between 114 and 116 million years ago, which corresponds to the formation period of the Yingcheng Formation. It is noteworthy that during the above-mentioned eruptions, sedimentary rocks began to develop above the volcanic rocks, forming an unconformity boundary related to the eruption period (Figure 12 and Figure 15).

Zircon U–Pb ages obtained from the volcanic units of the Wangfu fault depression are well consistent with the major tectonic–magmatic phases recorded in northeast China. The oldest volcanic activity in Huoshiling (129–121 million years ago) coincides with extensive rifting and peak magmatism in the Songliao Basin during the Early Cretaceous, driven by slab retreat of the Paleo-Pacific Plate [61,62]. This phase reflects significant lithospheric extension, asthenospheric upwelling, and intense magmatism in eastern China. In contrast, the youngest volcanic activity recorded in the Yingcheng Formation (118–106 Ma) represents a waning rift phase characterized by more localized magmatism and increasing post-rift subsidence, synchronous with the early stages of basin subsidence development. These results are well consistent with a regional tectonic model in which magmatism evolved from widespread rift-related eruptions to thermal subsidence and minor extensional reactivation before the onset of tectonic inversion in the Late Cretaceous. Therefore, the chronostratigraphic records of the Wangfu Fault provide an important record of the transition from intense rift volcanism to post-rift sedimentary basin evolution in the broader tectonic evolution of the Songliao Basin.

4.2. Structural Analysis of Wangfu Fault Depression

Since the Mesozoic, a fault lake basin has been developed, and its tectonic movement is consistent with the basin’s evolution [9,63]. The western side of the fault depression is bounded by the Wangfu West Fault. The structural subsidence and stratigraphic deposition caused by the fault activity have a significant impact on the fault depression. Affected by the second episode of the Yanshan Movement, the Songliao area entered the syn-rift stage in the Late Jurassic [16] Magma in the deep Wangfu fault depression surged, the crust uplifted, and large-scale, multi-stage volcanic eruptions formed a thick volcanic basement with alternating uplifts and depressions [9]. In the Late Jurassic–Early Cretaceous, extensional faulting began in the shallow crust under a regional extensional regime [6], forming the Wangfu West Fault on the western boundary, while the eastern strata were uplifted and the sediments formed by erosion filled the Wangfu West Boundary Fault [15,16,17].

The Xiaochengzi trough controlled by the rift system formed the Lower Cretaceous Shahezi Formation (K1sh), which is mainly composed of terrigenous clastic strata [64]. The volcanic basement and sedimentary cover of the fault depression constitute an obvious “binary stratigraphic combination” [65]. The volcanic basement consists predominantly of lithological units such as andesite and rhyolite along with pyroclastic deposits, which are mainly distributed in the northern part of the Xiaochengzi Trough. The overlying Lower Cretaceous Shahezi Formation (K1sh) is a stratum developed under intense faulting activity. It formed in a sedimentary environment dominated by a fan delta and lake [66]. Due to the uneven basement topography and structural settlement, the Shahezi Formation is thickest in the western Xiaochengzi trough and is generally thinned by erosion in the central and eastern regions. The upper part of the formation is interbedded with gray conglomerate and dark gray mudstone, the middle part is dark gray to black mudstone interbedded with conglomerate, and a set of coal seams has developed in the lower part, which is in conformity–angular unconformity contact with the underlying volcanic basement [15,16,17].

The tectonic and volcanostratigraphic evolution of the Wangfu Fault reflects the broader tectonic system of the Songliao Basin during the Mesozoic, which was primarily controlled by the subduction of the Paleo-Pacific Plate onto the Eurasian Plate. The tectonic–sedimentary evolution is divided into four stages: mantle upwelling, rifting, post-rift thermal subsidence, and tectonic inversion, which correspond closely to regional extensional tectonics triggered by slab rollback and back-arc basin formation. The rift-related volcanism (Huoshiling Formation) and subsequent syn-rift to post-rift sedimentation (Shahezi Formation, Yingcheng Formation, and Denglouku Formation) recorded in the Wangfu Fault are consistent with a general model of basin-scale rift evolution in Northeast Asia during the Early and Late Cretaceous. Specifically, the dominance of normal faults, the westward shift of depocenters, and the intermittent volcanism observed in this study are consistent with rollback-driven extensional and subsequent compressional events recognized in the broader tectonic model of the Songliao Basin.

The geological evolution of the Wangfu Fault Depression (WFD) cannot be fully understood in isolation, as it is part of a network of fault-controlled depressions on the southeastern margin of the Songliao Basin. The WFD shares similar tectonic-stratigraphic relationships with adjacent depressions, such as the Changling Fault Depression to the east, the Dehui Fault Depression to the north, and the Yushu Fault Depression to the west. Together, these depressions reflect the dynamic response of the basin to the Mesozoic-Cenozoic lithospheric extension driven by Paleo-Pacific subduction. Comparative analysis shows that while the WFD experienced significant volcanic and tectonic activity during the Early Cretaceous, concentrated in the Huoshiling and Yingcheng Formations, similar rift-related volcano-sedimentary sequences are found in its adjacent areas, albeit with different facies structures, fault intensity, and thermal subsidence patterns. The contrast and continuity of these areas suggest a multiphase and structurally segmented basin evolution, in which local fault geometry and magma flows modulated stratigraphic development.

The Wangfu fault depression has experienced multiple phases of large-scale tectonic movements, and the fault activity is relatively strong, forming a complex fault system. The overall characteristics of this fault system are that the main controlling fault strikes the same direction as the regional tectonic strike, which is nearly north–south, followed by north-northeast [4,15,16,17,64] (Figure 3, Figure 5, and Figure 7). The main controlling fault has the characteristics of early development, large fault throws, and long activity period.

4.2.1. Lithological Affects in Fault Movement

The basement in the north of Xiaochengzi trough is mainly composed of volcanic clastic rocks, while the basement in the south is mainly developed with andesite and rhyolite [29]. The different basement lithology from north to south leads to differences in mechanical properties including internal friction angles, which ultimately affect the fault morphology (Figure 14).

4.2.2. Syntectonic Sedimentation

Syntectonic sedimentation is widely present in various extensional fault basins and reacts to the tectonic deformation process of the same period [67,68]. In Wangfu Fault Depression, the continuous profile shows that the sedimentary strata in the south are the thickest, and the syntectonic sedimentation is greater than that in the north where the strata are thinner. The thicknesses of the strata in the depression increase southward, indicating that the syntectonic sedimentation gradually intensifies southward (Figure 14).

The sedimentary strata in the southern part of Wangfu Fault Depression are the thickest, so strong syntectonic sedimentation may be an important reason for the steepening of the Wangfu western boundary fault.

4.2.3. Basement Uplift

The basin basement may affect the geometry and kinematic characteristics of late structures. During the Jurassic period, large-scale and multi-stage magmatic activities in the Wangfu Depression caused the volcanic basement to have an uneven topography. The top of the Wangfu fault depression basement is composed of relatively dense and hard volcanic rocks such as rhyolite superimposed on each other. The top surface of the volcanic rock body is uneven [29,30,69], especially the top of the volcanic channel, which has a hemispherical uplift. The overlying sedimentary strata and late structures developed on many dense and hard volcanic uplifts. The paleo-geomorphology also shows that there are a large number of basement uplift structures on the bottom of the Xiaochengzi depression. Mesozoic tectonic activities caused the fault to propagate upward through the basement (Figure 14). The geological profile shows that the section of the Wangfu West Fault presents an “upward convex” shape in some locations.

4.2.4. Faults Controlling Volcanic Rocks and Structures

Volcanic rocks are the products of magma eruption, overflow, or intrusion along weak zones deep underground. Weak zones are usually long-term inherited active basement faults. The statistical analysis of the drilling results of wells WF1, CS12, CS13, CS 607, CS 606, CS 608, CS9, CS2, CS11, and CS 8 in the Wangfu fault depression shows that the volcanic rocks that have mainly developed in the Huoshiling Formation (Figure 11) in the vertical direction are basaltic andesite and rhyolite. The volcanic rocks of the Yingcheng Formation are distributed at the bottom of the Yingcheng Formation and are mainly rhyolite. Volcanic rocks are widely distributed in the depression. From the analysis of different sections and fault profile positions in the Wangfu fault depression (Figure 14), it can be seen that the distribution of volcanic rocks is obviously controlled by basement faults. The volcanic rocks in the Wangfu fault depression are distributed along the fault development zone as a whole, showing the characteristics of fissure-type volcanic eruptions.

4.2.5. Regional Unconformities

There are important structural sequence interfaces or unconformity surfaces in the syn-rift strata/sedimentary sequence, mainly distributed at the seismic reflection interfaces in Wangfu fault depression. From bottom to top, they are the bottom of the Huoshiling Formation (T₅), the bottom of the Shahezi Formation (T₄₂), the bottom of the Yingcheng Formation (T₄₁), the bottom of the Denglouku Formation (T₄), and the bottom of the Sifangtai Formation (T₀₃) (Figure 3, Figure 4, and Figure 7). All of the interfaces are regional unconformities and are important interfaces for changes in regional stress fields and basin tectonic evolution [1,15,16,17,70,71,72].

Nonconformity and disconformity are the two main types of unconformity surfaces observed in the geological setting of the Wangfu Fault Depression, which is an important research area for understanding the stratigraphic complexity of sedimentary basins. The T₅ seismic reflection interface is an important boundary, defining the relationship between the basin basement and the overlying Huoshiling Formation, as shown in Figure 3, Figure 5, and Figure 7, and it has been fully analyzed and interpreted, with results indicating that it has nonconformity characteristics. The T₄₂ seismic reflection provided key insights into the stratigraphic relationship between the Huoshiling and Shahezi Formations, indicating that the contact between the two formations was mainly interpreted as nonconformity in most of the drilled wells, with the exception of well CS9, where the geological contact was classified as a disconformity. In addition, the T₄₁ seismic reflection interface between the Shahezi Formation and the Yingcheng Formation is mainly nonconformity, except for wells CS606, and CS11, which are discordant (disconformity) interfaces. Finally, the interface between the Yingcheng Formation and the Denglouku Formation is represented by seismic reflection T₅. From the data of each well, the conformity surfaces are mainly discordant (disconformity), except for the geological contact surface in well CS13, where the contact shows nonconformity with the underlying Huoshiling Formation.

According to structural sequence interfaces, the Mesozoic and Cenozoic strata in the Wangfu fault depression are also divided into three structural layers [73], namely, the Huoshiling Formation (K₁h) to the Yingcheng Formation (K₁yc) are fault structural layers, the Denglouku Formation (K1d) to Nenjiang Formation (K₂n) are depression structural layers, and the Sifangtai Formation (K₂s) to the Quaternary (Q) strata are inversion structural layers. Among them, the inversion structural layers are largely eroded or missing in the study area. From bottom to top, three sets of fault systems are developed in Wangfu fault depression, namely, the extension faults during rifting (Figure 14), the depression (tension) period fault system, and the inversion period (compression) fault system. The inversion period fault system is not obvious due to the large amount of erosion or absence of the inversion structural layer.

Extensional Faults During Rifting

Extension faults in the rift period are faults developed between the reflection layers of T₅ (the bottom of the Huoshiling Formation) and T₄ (the bottom of the Denglouku Formation) (Figure 14). They mainly formed during the deposition of the Huoshiling Formation and control the settlement and sedimentary filling of the half-graben or graben structures in the rift period. There are two causes of extension faults in the rift period. One is the extension of deep faults, the arching of the middle and lower crust, and the extension of the upper crust faults. The other is the differential uplift of deep faults or the upwelling of mantle materials, the flexure of the middle and lower crust, and the extension of the upper crust faults.

5. Conclusions

This study presents a comprehensive analysis of the Wangfu Fault Depression in the southeastern Songliao Basin, elucidating its complex tectono-volcanic-sedimentary evolution through integrated seismic interpretation, well log correlation, core analysis, and zircon U–Pb geochronology. The depression exhibits a distinct stratigraphic framework shaped by episodic tectonism, with fault-controlled subsidence governing sedimentation and volcanic deposition, primarily concentrated in the Huoshiling and Yingcheng Formations. Volcanic facies are categorized into vent breccia, pyroclastic, lava flow, and volcaniclastic sedimentary types, revealing diverse eruption styles and depositional settings across the basin. Key volcanostratigraphic boundaries, including eruptive and tectonic unconformities, mark shifts between volcanic activity and sedimentary hiatuses, reflecting the intermittent nature of eruptions. Zircon U–Pb dating constrains the main volcanic episodes to between ~130 and ~106 Ma, placing peak activity in the Early Cretaceous. Structurally, the depression is segmented into western sag, central slope, and eastern uplift zones, shaped by NNE-trending normal faults such as the Wangfu West Fault. These structures controlled not only the distribution of volcanic rocks but also the overall basin geometry and fill patterns.