The Baisha Structure,with a rim-to-rim diameter of ~3.7 km,in the center of the Hainan Province,southern China has been considered to be an impact crater. Field investigation and petrological study are presented in this paper to investigate the impact hypothesis for this structure. The ~600-m-thick Lower Cretaceous feldspathic quartz sandstones from the Lumuwan Formation are the major outcrops both within and outside of the structure. The amphitheater-shaped rim of the structure is composed of granite porphyries that are intruded in the Lumuwan Formation. Previously interpreted impact breccia and impact melt rocks are actually granite porphyries different cooling rates and weathering status. Rocks from locations that most likely have recorded shock metamorphic signatures are sampled,but petrographic analyses reveal no indications of shock metamorphism. While subtle structural deformation occurs at the contact boundary between the granite porphyries and the feldspathic quartz sandstones,the feldspathic quartz sandstones exhibit uniformed dipping strata across the crater floor and walls. All the evidence suggests that the Baisha Structure was not formed by impact cratering. It most likely has been shaped by a combination of magmatic intrusion and long-term differential erosion.
The Xiuyan crater, 1.8 km in diameter, is the only confirmed Earth impact crater in China to date (Chen et al., 2010). The spatial distribution of discovered impact craters on Earth and the geologic evolution of China indicate that more Earth impact craters remain to be discovered in China (Xiao et al., 2018). Theoretical estimates based on the formation and removal rates of Earth impact craters suggest that ~20 craters with diameters ranging from 1 to 6 km should be preserved on the surface of China (Hergarten and Kenkmann, 2015). Considering that 65 of the 191 confirmed Earth impact craters have been buried after formation (http://www.passc.net/EarthImpactDatabase/), the number of undiscovered Earth impact craters in China is much larger. Indeed, many hypothesized Earth impact craters in China have been proposed based primarily on their circular morphology and topography (Xu et al., 2017).
We launched a program to search for possible impact craters in China in 2014. Those proposed by earlier researchers are our first stage targets. Six examples with high-priority have been investigated by field investigation and sample analyses, and four of them have been disproved to have an impact origin, e.g., the Duolun Basin (Xu et al., 2017). As another typical negative example, the Baisha Structure in the center of the Hainan Province (central coordinates 109.508°E, 19.183°N) has recently been investigated. The possible impact origin of the Baisha Structure has been widely advocated in Chinese media, and it is an important resource for the local tourism and agricultural economy (Yin and Wang, 2017b). Therefore, the Hainan Geological Survey has recently drilled two cores in the center of this structure to verify the hypothesized impact origin (Yin and Wang, 2017a).
The Baisha Structure, with a rim-to-rim diameter of ~3.7 km, was proposed to be an impact crater based on its circular morphology as seen in low-resolution satellite images (Wang and Li, 1993). Interpreted impact breccia, impact melt rocks, shock metamorphic features in quartz and feldspar, and remnant of meteorites from this structure have been reported (Wang et al., 1997; Wang, 1996, 1994; Wang and Wang, 1995; Wang and Li, 1993). Tektites found at Hainan, which are now recognized as part of the Australasian strewn field, were interpreted to be formed by the Baisha Structure, but no precise age constraint was provided (Wang et al., 1997). An international symposium and field trip were organized in October 1994 to verify the hypothesized impact origin of this structure, and the conference report suggested that the impact origin was confirmed (Wang et al., 1997). However, some of the above evidence was not described in detail with respect to petrography and geochemistry. For example, critical evidence supporting the interpreted meteoritic remnants were ambiguous, and the crystallographic orientations for the interpreted planar deformation features (i.e., PDFs) were not introduced. Moreover, the interpretations about the existence of melt sheet and melt- bearing rocks are not consistent with shock physics or inherent characteristics of similar-sized Earth impact craters. For example, the igneous rocks at the foot of the northwestern crater wall were believed to be impact melt sheet (Wang et al., 1997), but similar-sized Earth impact craters formed in sedimentary rocks are not capable to form such voluminous melt sheets, e.g., impact melt formed by the Goat Paddock crater (~5 km diameter) in western Australia occurs as suevites that overlie the crater floor and wall (Milton and MacDonald, 2005). Therefore, referring to the modern knowledge used in the search of Earth impact craters (French and Koeberl, 2010), the impact origin of the Baisha Structure is highly questionable.
We have recently conducted a field expedition to verify the hypothesized impact origin of the Baisha Structure. The regional stratigraphy and structural deformation were investigated. Previously interpreted impactities were revisited and samples from critical locations of this structure were collected for petrographic inspection of shock metamorphic features.
1.
GEOLOGICAL BACKGROUND AND RESEARCH STRATEGY
The Baisha Structure is located in the southeast of the Baisha County, central Hainan Province. In the Proterozoic, the Hainan Island was located at the southwest of the South China Plate. During the Jinning movement in the Late Proterozoic, the regional crust was uplifted and folded, forming the oldest crystalline basement that is sparsely outcropped in the Hainan Island. Several sets of west-to-east trending faults were formed in the Early Paleozoic, and their activity lasted to the Late Cretaceous. Intense vertical crust movements and volcanism occurred from the Early Paleozoic to the Early Cretaceous. In the Early Cretaceous, a series of NNE-trending shear faults were formed. These faults were subsequently used by crust uplift, forming three rift zones that trend NNE. In the Paleogene, the Hainan Island was separated from the South China Plate and drifted southeast, and it was finally relocated at the present location (Yichang Institute of Geology and Mineral et al., 1991a). During this time, the Qiongbei (i.e., northern Hainan) rift zone was formed, and widespread basaltic volcanic eruption occurred across the northern part of the Hainan Island. The Baisha Structure is located within Kangma Basin, which is part of the Lower Cretaceous rift zone in the central Hainan Province. Crust uplift has been the major tectonic activity in this region since the Early Cretaceous.
The Lower Cretaceous sandstones from the Lumuwan Formation are the dominant outcrops both within and outside of the Baisha Structure. Geological profile survey suggested that the Lumuwan Formation is ~1 200 m thick on average in the Kangma Basin (Yichang Institute of Geology and Mineral et al., 1991b), and this formation is ~600 m thick near the Baisha Structure because of the over 100 Ma crustal uplift and erosion (Wang et al., 1997).
The Baisha Structure was regarded to have circular rims based on low-resolution (1/100 000) satellite images (Wang and Li, 1993). However, high-resolution (1/54 000) satellite images and digital elevation model (DEM) show that this structure actually has an amphitheater-shaped plane morphology (Fig. 1a). The southwestern part of the structure has been heavily eroded by a fan-shaped drainage system, which is fed into the Nandu River to the southwest. The northern, northeastern, and eastern rims are composed by low-relief mountains that are ~100 to 500 m in elevation. The mountains continuously extend southward, which are connected with the Limu Mountain to the south (Fig. 1a). The amphitheater-shaped morphology is also manifested in the regional slope map, as the southern part of the crater is relatively flat (Fig. 1b). If the hypothesized impact event had occurred, the Baisha Structure should have fully developed circular rims even if the pre-impact topography is uneven, and the original rim-to-rim diameter should be much larger than 3.7 km considering that continuous crustal uplift has caused substantial post-formation erosion in the Kangma Basin. Notably, Earth impact craters formed in sedimentary rocks have a simple-to-complex transition diameter of ~2 km (Grieve et al., 1981). Therefore, if the Baisha Structure had an impact origin, it should be a complex crater. However, central peaks are not visible on the floor of the structure, and the north-to-south trending mountain on the crater floor is connected with the northern crater wall (Fig. 1a).
Figure
1.
(a) The Baisha Structure shown by a DEM overlying a Google Earth image (~19 m/pixel, Mercator projection). The DEM is obtained by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER; https://gdex.cr.usgs.gov/gdex/; Mercator projection; horizontal resolution 30 m/pixel; vertical accuracy 10-25 m). The red rectangle denotes the survey area shown in (c). The green lines are the designed investigation routes; and the black dashed curve marks the interpreted apparent rim (Wang and Li, 1993). Note that the plane morphology is not circular. (b) Slope map of the Baisha Structure calculated based on the ASTER DEM shown in (a). (c) Google Earth image showing the sampling sites (Google Earth image; ~1 m/pixel, Mercator projection).
Unequivocal evidence proving the impact origin of Earth impact craters includes shatter cones (Baratoux and Reimold, 2016), planar deformation features (Grieve et al., 1996), planar fractures (PFs) (French and Koeberl, 2010), diaplectic glass, high-pressure polymorphs (e.g., coesite, stishovite and diamond), and meteorites in particular settings (Langenhorst and Deutsch, 2012). Higher abundances of siderophile and platinum group elements have also been used to determine the impact origin for a few Earth impact structures (Koeberl et al., 2012).
Referring to the theoretical distribution of shocked materials during impact cratering (French, 1998) and those produced by similar-sized Earth impact craters that are formed in sedimentary rocks (Milton and MacDonald, 2005), the center of the Baisha Structure has a high potential of finding shatter cones and melt-bearing breccia (e.g., suevites), which most likely contain shocked features (French, 1998). Therefore, during the field investigation, the following works have been done: (1) reviewed positions where impacties had been reported (Wang et al., 1997); (2) investigated critical locations where rocks most likely have recorded shock metamorphic effects; (3) investigated the structural deformation of rocks in the crater.
We have collected samples from 16 sites from the structure (Fig. 1c). Twenty-seven standard thin sections are prepared to perform petrographic analyses. Possible shocked features (e.g., PDFs and PFs) are searched utilizing polarizing microscopes in a transmission light mode (French and Koeberl, 2010).
2.
RESULTS
Our field investigation and petrographic analyses confirm that the feldspathic quartz sandstones from the Lumuwan Formation and the granite porphyries are the major outcrops within this structure (Fig. 2). The NW-trending granite vein that transects the southern part of the structure was not encountered during the field trip. Previously interpreted impact breccias and impact melt rocks are actually granite porphyries that have various weathering status. Crosscutting relationship shows that the granite porphyries are intrusions within the feldspathic quartz sandstones. Except for minor structural deformations that occur at the contact boundary between the granite porphyries and the feldspathic quartz sandstones, the feldspathic quartz sandstones exhibit uniformed dipping strata all across the crater floor and walls (Fig. 2).
Figure
2.
(a) A geologic sketch map of the Baisha Structure that is overhauled from Wang and Li (1993) and Yin and Wang (2017a) based on our field survey. 1. Lower Cretaceous feldspathic quartz sandstones; 2. granite porphyries; 3. monzonite porphyry vein, which was not encountered during the field survey; 4. interpreted apparent rim of the Baisha Structure; 5. location of the geological profile. (b) The geological profile along line a1-b1. This profile is modified from Wang and Li (1993) and Yin and Wang (2017a) based on our field investigation. 1. Lower Cretaceous feldspathic quartz sandstones; 2. granite porphyries; 3. monzonite porphyry vein. (c) The geological profile at the D11 sampling site shown in Fig. 1c. Note the minor structural deformation at the contact between the feldspathic quartz sandstones and granite porphyries. The same legends are shared with both (b) and (c).
Rocks on the northern rim of the Baisha Structure were interpreted to be impact ejecta deposits (Wang and Li, 1993), e.g., the Nos. D0 and D1 sampling sites (Fig. 1c). PDFs in quartz and feldspar were claimed to have been discovered in these rocks, but statistical data of the crystallographic orientations of these fractures were not described (Wang et al., 1997). The interpreted PDFs in quartz, as shown in the IV-3, 4, 5 of Wang et al. (1997), have non-uniform spaces and they frequently cross grain boundaries, and the widths of the fractures are 1-10 μm, which are much larger than that of real PDFs (< 1 μm) (French and Koeberl, 2010). These characteristics are not observed in shocked PDFs (French, 1998).
In fact, the interpreted impact breccias on the northern rim of Baisha Structure are weathered granite porphyries (Fig. 3a). The granite porphyries are composed of quartz, K-feldspar, plagioclase and mica. Quartz phenocrysts that are up to 500 µm are frequently visible, but no PDFs or PFs are discovered (Figs. 3d-3f). Notably, with the magnification scale shown in Figs. 3d-3f, PDFs or PFs should be visible if they exist (French and Koeberl, 2010; French, 1998). Moreover, there is no mixture of granite porphyries and feldspathic quartz sandstones on the northern crater rim. This is in contradiction with the impact hypothesis if it occurred on the mixed target (Fig. 2).
Figure
3.
Granite porphyries at the northern rim of the Baisha Structure. (a) Granite porphyry that has breccia structure in morphology shaped by small internal fractures (red arrows). The specimen has been polished, and the coin to the right is 2.5 cm in diameter. (b)-(f) are thin section images for the granite porphyries under crossed polarizers. Quartz phenocrysts are shown in (c)-(f). Melting corrosion structures are visible around the quartz (Q), K-feldspar (Kf) and plagioclase (Pl). No PFs or PDFs are visible in these thin sections.
The ~20 m thick igneous rocks at the foot of the northwestern wall (the D6 sampling site in Fig. 1c; Fig. 4c) were interpreted as impact melt sheet, but neither geochemical nor petrological evidence was given (Wang et al., 1997). Well- developed columnar joints are visible in these rocks (Fig. 4c). The igneous rocks are actually clast-free granite porphyries that have intruded into the feldspathic quartz sandstones from the Lumuwan Formation (Fig. 4b). The granite porphyries are not formed in an impact melt body, considering that wall of complex craters on planetary bodies are composed of target rocks that are deformed under stresses well below the Hugoniot stress limit. Moreover, no clasts are visible within the outcrops of specimen, even at the contact boundaries with the feldspathic quartz sandstones (Fig. 4b). This is in contrast with the turbulent emplacement of impact melt during impact cratering, and abundant clasts are frequently visible within impact melt bodies when they are in contact with solid debris (French, 1998). Thin section examinations reveal no shock metamorphism features in these granite porphyries (Figs. 4e-4f).
Figure
4.
Granite porphyries with well-developed columnar joints at the northwestern foot within the Baisha Structure. (a) A ~20 m high cliff that is composed of granite porphyries. Columnar joints are visible on the outcrops (red arrow). The photo is taken at the No. D6 sampling site shown in Fig. 1c, and the camera azimuth is 280°NW. (b) The contact (red dashed line) between the purple-reddish feldspathic quartz sandstones (beneath the water) and the granite porphyries. The photo is taken at the No. D6 sampling site shown in Fig. 1c, and the camera azimuth is 320°NW. The length of the hammer is about 30 cm. (c) Columnar joints developed in the granite porphyries. This photo is taken at the No. D7 sampling site shown in Fig. 1c, and the length of the hammer is about 30 cm. Quartz (Q), K-feldspar (Kf), plagioclase (Pl) and mica (Mi) phenocrysts in granite porphyries are shown in (d)-(f) under crossed polarizers. No shock metamorphic features are visible in the thin sections.
The granite porphyries here contain fewer phenocrysts and smaller mineral sizes than those observed on the northern crater rim (Figs. 3 and 4), indicating a higher cooling rate so that well-organized columnar joints and smaller crystals can form (Xiao et al., 2014). The granite porphyries at the two locations are both intrusions that have different intrusive depths.
2.2
Structural Deformation
The topographic rims of the Baisha Structure (Fig. 1) are composed of granite porphyries, suggesting that formation of this structure postdated the intrusions. Rocks older than sandstones from the Lumuwan Formation are not exposed within or adjacent to the structure (Fig. 2), indicating that if the impact hypothesis had been correct, the impact event must have occurred during or after the Early Cretaceous. In this case, the feldspathic quartz sandstones and/or the granite porphyries should have been heavily deformed, especially those within the center (French, 1998). Therefore, we have carefully investigated the structural deformation of the feldspathic quartz sandstones in the crater.
Our field survey reveals that the feldspathic quartz sandstones at different locations of this structure generally occur as undisturbed and parallel beddings that have uniformed dipping strata, except for those at contact boundaries with the granite porphyries where minor deformation is visible (Fig. 5c). No shatter cones have been discovered in the field. Sandstones at the center of this structure also exhibit a same attitude (Fig. 5c). Besides strong weathering in sandstone layers that have a higher content of silts (Figs. 5a and 5b), no cataclysmic deformation is visible (Figs. 5b-5c). Moreover, the feldspathic quartz sandstones exhibit the same attitudes down to more than 200 m beneath the crater floor as revealed by the recent drilling projects carried out by the Hainan Geological Survey (Yin and Wang, 2017a). Therefore, together with the falsely interpreted impactites (Section 2.1), the possible stratigraphic age for the formation of the structure and the uniform dipping strata of the feldspathic quartz sandstones provide the conclusive evidence that the Baisha Structure was not formed by impact cratering.
Figure
5.
Feldspathic quartz sandstones close to the center of the Baisha Structure have the same attitude (220°-245°∠10°-35°) with those elsewhere in this structure. (a) Feldspathic quartz sandstones at the No. D5 sampling site. Thin layers of sandstones are interbedded with siltstones, which are more heavily eroded (red arrow). The camera azimuth is 350°NW. The length of the hammer is about 30 cm. (b) Thin layers (~8 cm) of argillaceous siltstone are intercalated with the feldspathic quartz sandstones at the No. D10 sampling site. The siltstones are more prone to weathering (red arrow). The camera azimuth is 190°NW. The length of the hammer is about 30 cm. (c) Feldspathic quartz sandstones at the No. D9 sampling site have three sets of joints. The camera azimuth is 190°NW. (d) Thin section of the feldspathic quartz sandstones under crossed polarizers. The grains are composed of quartz (Q), K-feldspar (Kf), plagioclase (Pl) and mica (Mi). (e)-(h) show brittle fractures within the quartz under crossed polarizers. These examples represent the most-heavily deformed quartz in all the samples, but shock metamorphic features are absent.
We carried out a geological survey and performed sample analyses to investigate the impact hypothesis for the Baisha Structure in the center of the Hainan Province. The occurrence of different-aged rocks in the structure indicates that this structure was formed during or after the Early Cretaceous. However, the Lower Cretaceous feldspathic quartz sandstones in the structure are not structurally disturbed. The rims of this structure are composed of competent granite porphyries, which are intrusions within the feldspathic quartz sandstones, not impact breccia or impact melt rocks as previously interpreted. No diagnostic shocked features were identified at the critical locations that were sampled. The above evidence suggests that the Baisha Structure was not formed by impact cratering. Instead, the spatial distribution and embayment relationship between the feldspathic quartz sandstones and the granite porphyries suggest that the amphitheater-shaped morphology of this structure was caused by a combination of magmatic intrusion and long-term differential erosion, since the granite porphyries are more competent against weathering than the feldspathic quartz sandstones.
ACKNOWLEDGMENTS
The Hainan Geological Survey kindly provided support during the field trip. This study was supported by the National Natural Science Foundation of China (Nos. 41773063 and 41403053) to Zhiyong Xiao and Jiang Pu, the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (Nos. CUG180601 and CUG2018JM10), the National Natural Science Foundation of China (No. 41772050) to Long Xiao, and the Natural Science Foundation of Jiangxi Province (No. 20171BAB213027) to Cheng Huang. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0887-0.
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Figure 1. (a) The Baisha Structure shown by a DEM overlying a Google Earth image (~19 m/pixel, Mercator projection). The DEM is obtained by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER; https://gdex.cr.usgs.gov/gdex/; Mercator projection; horizontal resolution 30 m/pixel; vertical accuracy 10-25 m). The red rectangle denotes the survey area shown in (c). The green lines are the designed investigation routes; and the black dashed curve marks the interpreted apparent rim (Wang and Li, 1993). Note that the plane morphology is not circular. (b) Slope map of the Baisha Structure calculated based on the ASTER DEM shown in (a). (c) Google Earth image showing the sampling sites (Google Earth image; ~1 m/pixel, Mercator projection).
Figure 2. (a) A geologic sketch map of the Baisha Structure that is overhauled from Wang and Li (1993) and Yin and Wang (2017a) based on our field survey. 1. Lower Cretaceous feldspathic quartz sandstones; 2. granite porphyries; 3. monzonite porphyry vein, which was not encountered during the field survey; 4. interpreted apparent rim of the Baisha Structure; 5. location of the geological profile. (b) The geological profile along line a1-b1. This profile is modified from Wang and Li (1993) and Yin and Wang (2017a) based on our field investigation. 1. Lower Cretaceous feldspathic quartz sandstones; 2. granite porphyries; 3. monzonite porphyry vein. (c) The geological profile at the D11 sampling site shown in Fig. 1c. Note the minor structural deformation at the contact between the feldspathic quartz sandstones and granite porphyries. The same legends are shared with both (b) and (c).
Figure 3. Granite porphyries at the northern rim of the Baisha Structure. (a) Granite porphyry that has breccia structure in morphology shaped by small internal fractures (red arrows). The specimen has been polished, and the coin to the right is 2.5 cm in diameter. (b)-(f) are thin section images for the granite porphyries under crossed polarizers. Quartz phenocrysts are shown in (c)-(f). Melting corrosion structures are visible around the quartz (Q), K-feldspar (Kf) and plagioclase (Pl). No PFs or PDFs are visible in these thin sections.
Figure 4. Granite porphyries with well-developed columnar joints at the northwestern foot within the Baisha Structure. (a) A ~20 m high cliff that is composed of granite porphyries. Columnar joints are visible on the outcrops (red arrow). The photo is taken at the No. D6 sampling site shown in Fig. 1c, and the camera azimuth is 280°NW. (b) The contact (red dashed line) between the purple-reddish feldspathic quartz sandstones (beneath the water) and the granite porphyries. The photo is taken at the No. D6 sampling site shown in Fig. 1c, and the camera azimuth is 320°NW. The length of the hammer is about 30 cm. (c) Columnar joints developed in the granite porphyries. This photo is taken at the No. D7 sampling site shown in Fig. 1c, and the length of the hammer is about 30 cm. Quartz (Q), K-feldspar (Kf), plagioclase (Pl) and mica (Mi) phenocrysts in granite porphyries are shown in (d)-(f) under crossed polarizers. No shock metamorphic features are visible in the thin sections.
Figure 5. Feldspathic quartz sandstones close to the center of the Baisha Structure have the same attitude (220°-245°∠10°-35°) with those elsewhere in this structure. (a) Feldspathic quartz sandstones at the No. D5 sampling site. Thin layers of sandstones are interbedded with siltstones, which are more heavily eroded (red arrow). The camera azimuth is 350°NW. The length of the hammer is about 30 cm. (b) Thin layers (~8 cm) of argillaceous siltstone are intercalated with the feldspathic quartz sandstones at the No. D10 sampling site. The siltstones are more prone to weathering (red arrow). The camera azimuth is 190°NW. The length of the hammer is about 30 cm. (c) Feldspathic quartz sandstones at the No. D9 sampling site have three sets of joints. The camera azimuth is 190°NW. (d) Thin section of the feldspathic quartz sandstones under crossed polarizers. The grains are composed of quartz (Q), K-feldspar (Kf), plagioclase (Pl) and mica (Mi). (e)-(h) show brittle fractures within the quartz under crossed polarizers. These examples represent the most-heavily deformed quartz in all the samples, but shock metamorphic features are absent.
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