Implications for Danxia Surveying and Monitoring

Geoheritage (2015) 7:307–318 DOI 10.1007/s12371-015-0160-1

ORIGINAL ARTICLE

Petrology and Spectroscopy Studies on Danxia Geoheritage in Southeast Sichuan Area, China: Implications for Danxia Surveying and Monitoring

Jun-Ting Qiu1,2 & Peng-Ju Li 3 & Zhang-Fa Yu2 & Ping Li2

Received: 25 May 2015 /Accepted: 20 September 2015 /Published online: 12 October 2015 # The European Association for Conservation of the Geological Heritage 2015

Abstract This paper presents the achievements of petrology Keywords Danxia . Remote sensing . Petrology . and spectroscopy studies on rock samples collected from Spectroscopy . Surveying . Monitoring Danxia outcrops in southeast Sichuan area. The petrological study based on thin section observations was carried out using a polarizing microscope indicating that the Danxia rocks are Introduction mainly composed of quartz, plagioclase, and lithic grains, with hematite, limonite, and calcite cements. Some plagio- The Danxia landform (or Danxia geoheritage) named after the clase and lithic grains are altered to clay minerals, while quartz Danxia Mountain (Chen 1984) in Guangdong Province, grains are resistant to such alterations. The reflectance spectra China, is a type of petrographic geomorphology associated of Danxia rocks were determined with an ASD spectrometer. with erosional sandstone and conglomerates with a red color All the measured spectra share similar characteristics that are (Liu 1986;LiuandHuang1991; Peng 2000; Zhu et al. 2010). different from those of granitic rocks and other land covers, In 2010, the Mount Danxia, accompanied with other five typ- such as vegetation, water, and soil. The spectra also have ical Danxia landforms, was inscribed as a World Heritage Site absorption features at ~840, ~2210, and ~2339 nm that are with a general name of BChina Danxia^ (The World Heritage respectively related to hematite and limonite cements, altered Committee 2010). The grand and beautiful landscape of this clay minerals, and calcite cement. Application experiments landform makes it an important sightseeing resource and at- suggest that remote sensing can be a useful tool for Danxia tracts numerous tourists every year. In addition, the geological surveying and monitoring, but its capability relies on spectral phenomena, such as stratification, foliation, joint, and fault, in and spatial resolutions. Danxia, are valuable for geologists’ investigation on tectonic and sedimentary histories during the Cretaceous Period. Because of its high aesthetics, economic, educational, and scientific interests, a lot of efforts have been made since the Electronic supplementary material The online version of this article 1920s to explore this feature, and many national geological (doi:10.1007/s12371-015-0160-1) contains supplementary material, which is available to authorized users. parks have been established to develop and protect this kind of non-renewable tourism resource and study material (e.g., * Peng-Ju Li Jiang et al. 2009;Lietal.2013;Xuetal.2013;Zhang2013). [email protected] Traditional exploration and monitoring of Danxia is based mainly on manual work that is often inefficient and impracti- cal, as some regions, like mountainous areas, are usually in- 1 School of Earth Sciences and Resources, China University of Geosciences, 29 Xueyuan Road, Beijing 100083, China accessible. In contrast, remote sensing technology, which per- mits data on inaccessible regions to be obtained economically 2 National Key Laboratory of Science and Technology on Remote Sensing Information and Image Analysis, Beijing Research Institute and rapidly, may provide an alternative way to survey, moni- of Uranium Geology, Beijing 100029, China tor, and study Danxia. For example, digital elevation models 3 School of Economics and Management, Sichuan University of (DEMs) have been used to analyze the geomorphology of Science and Engineering, Zigong 643000, China Danxia landforms (Liu et al. 2007; Huang et al. 2010; 308 Geoheritage (2015) 7:307–318

Zhang et al. 2011;Fang2014), and several parameters, such as other structural-controlled landforms. Therefore, additional stream gradient index (SLK), hypsometric integral (HI), hyp- constrains on Danxia exploration are needed. sometric curve (HC), and slope index (SI), have been pro- Materials on the Earth’s surface that interact with the elec- posed to quantitatively describe the geomorphical characteris- tromagnetic waves emitted by the sun can be recorded by tics of Danxia (Liu et al. 2007; Zhang et al. 2011; Fang 2014). remote sensing sensors in the form of spectra. These spectra, Previous studies suggested that negative lineaments extracted like human fingerprints, can be used to search for materials from DEMs reflect the surface expression of Danxia valleys with a similar spectrum and to investigate the composition of and cliffs that may be associated with joints, faults, and shear the material (e.g., Nair and Mathew 2014). Unfortunately, due zones (Fang 2014). Additionally, many Danxia outcrops are to limited knowledge of the spectral characteristics of Danxia exposed in areas with a relatively high slope index (Liu et al. outcrop, the capability of remote sensing in surveying and 2007). Although DEMs have the advantage of revealing re- monitoring Danxia based using spectral feature was not been gional terrains, thus have been treated as candidates for well understood. Danxia exploration, they are usually problematic as the geo- The Sichuan Basin in southwest China is characterized morphic characteristics represented by SLK, HI, HC, and SI by widely distributed Cretaceous to Paleogene red sedi- are not unique to Danxia but may be also the characteristics of ments. Associated Danxia outcrops are well developed

Fig. 1 a Location of the study area in China. b Digital elevation map (ASTER DEM) of the study area with locations of the Tianxiandong, Huagaoxi, and Danshan parks Geoheritage (2015) 7:307–318 309

Fig. 2 Sedimentary sequence of the red strata in the study area (modified after Luo 2010)

and valuable for tourism and geological studies. Because bed^ in China) exposed in the study area include the many areas around the basin have low accessibility, sys- Cretaceous Woshantou (K1-2w)andSanhe(K2S)forma- tematic analysis of the spectral characteristics of Danxia tions and the Paleogene Liujia (E1l) Formation (Fig. 2) outcrops will be helpful for Danxia surveying for these (Luo 2010). The Woshantou Formation is a set of red- areas using remote sensing technology. Based on the fi- dish to deep-red conglomerate and sandstone with silt- nancial supports from Sichuan University of Science and stone and limestone interlayers, which rests Engineering and Tourism Bureau of Sichuan Province, disconformably upon the Jurassic Penglaizhen we carried out spectroscopy and petrology studies on Formation (Fig. 2), and contains the Early Cretaceous rocks from the Danxia in southeast Sichuan, in an at- charophyte fossils, Mesochara symmetrica.TheSanhe tempt to understand their spectral features and to evalu- Formation consists of deep-red fine-grained sandstone ate the capability of remote sensing in Danxia surveying with minor mudstone, which is characterized by rhyth- and monitoring. Additionally, the spectral and petrologi- mic layering, and contains ostracods (Cypridea sp. and cal features of Danxia in southeast Sichuan investigated Eucypris sp.) and conchostracans (Calestherites sp.). in this study provide good information for further com- The Liujia Formation is composed of reddish to deep- parative studies of Danxia elsewhere in China. red sandstone with minor mudstone and also contains ostracods (Eucypris sp. and Paracypris sp.). The disconformity between the Jurassic Penglaizhen Study Area Formation and the Cretaceous Woshantou Formation suggests that the terrigenous sediments, the main com- The study area is situated in the southeast of the position of the red strata, started to deposit in the south- Sichuan Basin, within the west of the Yangtze Block east of the Sichuan Basin during the Early Cretaceous. (Fig. 1a, b). The red sedimentary strata (called Bred The continuous erosion after Eocene uplift of the red 310 Geoheritage (2015) 7:307–318

Fig. 3 a Tabular cross-bed with essentially planar bounding surfaces; b trough cross-bed with curved bounding surfaces; c air- raid shelter; d watch tower; e 3D topography of the Huagaoxi Park (source from Google Earth), from which the half-graben and normal faults can be identified; f paralleled normal faults; g Cyathea spinulosa; h exposed Danxia outcrop in the Danshan Park

strata that eventually created the Danxia landforms, an important geophenomenon that can tell geologists many which offer both scenic beauty and geological signifi- things about ancient environments, such as the paleocurrent cance, is crucial to the understanding of the sedimentary and the environmental conditions at the time of deposition. history of the Sichuan Basin. Cross-beds in the Tianxiandong Park can be divided into the Over the last decade, many Danxia parks in the southeast tabular cross-beds (Fig. 3a) that have essentially planar SichuanareahavebeenestablishedfortourismandDanxia bounding surfaces and the trough cross-beds (Fig. 3b) that conservation. The Tianxiandong Park is located to the south of are featured by curved or scoop-shaped bounding surfaces. the Luzhou city (Fig. 1b). It is characterized by cross-bedding, The individual beds of tabular and trough cross-beds range Geoheritage (2015) 7:307–318 311

Fig. 4 Photographs of the Danxia outcrops in Danshan, Tianxiandong, and Huagaoxi parks, from which samples Danxia-1, Danxia-2, and Danxia- 3werecollected

in thickness from 10 cm to a few tens of centimeters, and the cross-stratification layers are a few millimeters thick only. In addition to cross-bedding, the Tianxiandong Park is also famous for historic relics. During the World War II, the Chinese government built a secret weapon factory in this site. Although many tools and devices used for weapon manufac- ture were lost or ruined during the Civil War of China between 1945 and 1950, some relics, such as air-raid shelter (Fig. 3c) and watch tower (Fig. 3d), are well preserved and are now open to the public. The Tianxiandong Park is an excellent example of syncronicity between human augmentation of a landscape and geological heritage conservation, as the heritage values are mutually beneficial as visitors go to the area site for multiple reasons. The Huagaoxi Park in the Xuyong County (Fig. 1b)lies along a normal fault of a half-graben (Fig. 3e). Since the fault is vertical to the axis of the sedimentary beds, the crossing- bedding is not easy to be observed in this park. In contrast, parallel normal faults were well developed (Fig. 3f). The faults are distributed in an E-W direction and extend for hundreds of meters to several kilometers, which provides a good opportu- nity for visitors to understand geological faults and their rela- tionship to the half-graben. In addition, the Huagaoxi Park is also a national natural reserve for Cyathea spinulosa (Fig. 3g)—a species of tree fern with single tall stem which is a rare plant in China, with origins in the Mesozoic. The Danshan Park is also located in the Xuyong County. It has well-exposed Danxia outcrops generally tens of meters in height (Fig. 3h), which are suitable materials for remote sensing studies.

Methods

Sampling

Three rock samples, including Danxia-1 from Danshan Park, Danxia-2 from Tianxiandong Park, and Danxia-3 from Fig. 5 Reflectance spectra for samples Danxia-1, Danxia-2, and Danxia- 3. Each rock sample was measured ten times using ASD Fieldspec. The Huagaoxi Park, were collected during our field work. The lo- original spectral data can be found in Appendix Table 1 cations of the three parks are shown by Fig. 1b.Allsamplesare 312 Geoheritage (2015) 7:307–318

Fig. 6 Micrographs of the samples Danxia-1 (a, d, e, f), Danxia-2 (b),andDanxia-3(c) of red color and were directly collected from Danxia outcrops. remove marks from the saw blade on their surfaces, then Photographs of the sampling outcrops are shown in Fig. 4. glued to glass slides with epoxy. The rock chips epoxied to glass slides were cut off and grinded to 30 μmthick, then covered with thin glass slips to protect them from Spectral Measurement damage. The rock sections were observed using a Germany Leica DM4500P polarizing microscope. The Spectral measurement of the three rock samples collected micrographs of the sections were taken by the digital from the three Danxia outcrops were carried out using an photographic system of Leica Application Suit V4. ASD Fieldspec Pro spectrometer. The spectral range for this instrument is from 350 to 2500 nm. The spectral resolutions are 3 nm between 350 and 1000 nm and 10 nm between 1000 and 2500 nm. The instrument was set up following the guide- Results lines described in Labspec4 user manual. An accessory light Spectrum source was employed to measure spectrum without sunlight. During the measurement, the detector was attached to a rela- The original spectral data for three Danxia samples obtained tively flat surface of the rock sample to avoid possible influ- using ASD spectrometer are listed in Appendix Table 1 and ence from fluorescence lighting in the environment. A white displayed in Fig. 5. All spectra share similar characteristics. reference was introduced in the measuring procedure to opti- They show low reflectance between 350 and 550 nm, then mize and calibrate the instrument. For each rock sample, ten 3 increase with increasing wavelength between 550 and spectra were recorded using the RS software. The spectra 1850 nm, and decrease with increasing wavelength between were converted into ASCII files with ViewSpecPro software. 1850 and 2500 nm. The spectra have absorption features at ~840, ~1425, ~1895, ~2210, and ~2339 nm. Petrological Study

The petrological study was based on thin section obser- Petrology vations. Three samples, including Danxia-1, Danxia-2, and Danxia-3, were cut using a slab saw, then reduced The results of mechanical analyses of the three Danxia sam- in size to slightly smaller than a thin section using a trim ples based on thin section observation are shown in Fig. 6 and saw. The cut and reduced rock chips were polished to briefly described in the following. Geoheritage (2015) 7:307–318 313

Fig. 7 Reflectance spectra for a ferric iron minerals, b clay minerals, c 2003) that has been included in the ENVI5.0 software. The spectra of land carbonate minerals, d quartz grains, e red granitic rock, and f land covers. covers source from Bhttp://www.seos-project.eu/modules/classification/ The spectra of Danxia-1 were drawn in the figure for comparison. The classification-c00-p05.html^ spectra of minerals source from the spectral library of USGS (Clark et al.

All three samples (Danxia-1, Danxia-2, and Danxia-3) are are low spherical, subangular to rounded, and generally moderately sorted (Fig. 6a, b, c). The framework grains are 50×100 μm to 100×150 μm in size. They can also be composed largely of quartz (80 to 90 %) (Fig. 6a, b, c), with divided into two groups (Fig. 6d). The first group show some plagioclase (5 to 10 %) (Fig. 6c,d,f) and lithic clear polysynthetic twins, while the second group is dirty materials (5 to 10 %) (Fig. 6a, b). Quartz grains are low on their surfaces due to possible clay mineral alteration. spherical, angular to subangular, and variable in size, The lithic grains are low spherical, subrounded to rounded generally ranging from 100×150 μmto20×50μm, with (Fig. 6a). The grain sizes range from 50×100 μmto100× some elongated grains 100×250 mm (Fig. 6c). The grains 200 μm. They are severely altered to clay minerals. can be divided into a homogeneous group (Fig. 6a), which Muscovite can be found in samples Danxia-1 and Danxia- may source from earlier volcanic or intrusive rocks, and an 2 as accessory (Fig. 6b, d, e). The cements include hematite, inhomogeneous group that is mainly composed of flints limonite, and calcite. Some calcite cementations are present (Fig. 6a, b, c), which may have been generated from the in pore spaces (Fig. 6d, f), suggesting that the rock samples weathering of previous volcanic rocks. Plagioclase grains were once poorly cemented. 314 Geoheritage (2015) 7:307–318

Discussion and Application Table 1 Technical specifications of ETM+ bands Band number Range (nm) Spatial resolution (m) Property Spectral Characteristics of Danxia in Southeast Sichuan Area 1450–515 30 Multi 2525–605 30 Multi Reflectance spectroscopy can serve as a diagnostic tool for 3630–690 30 Multi rock composition, as different minerals in a rock display 4750–900 30 Multi unique spectral characteristics (Clark et al. 2003). For exam- 51550–1750 30 Multi ple, Fig. 7 displays the spectra of ferric iron minerals (hematite 6 10,400–12,500 60 TIR and limonite; Fig. 7a), clay minerals (muscovite, montmoril- 72090–2350 30 Multi lonite, illite, and kaolinite; Fig. 7b), carbonate minerals (cal- 8520–900 15 PAN cite and dolomite; Fig. 7c), and quartz (Fig. 7d). The absorption features at ~840 nm for Danxia rocks in this The original data source from http://landsat.gsfc.nasa.gov/?p=3225 study are probably related to ferric iron minerals (Fig. 7a), Multi multi-spectral band, TIR thermal infrared band, PAN panchromatic band while those at ~2210 nm (Fig. 7b) and ~2339 nm (Fig. 7c) are associated with clay minerals and calcite, respectively. The above mineral assemblage (ferric iron + calcite + clay min- ~2339 nm (Fig. 7e), suggesting that the rock is depleted erals) diagnosed based on rock spectra are consistent with in carbonate minerals. those confirmed with microscope observation. For example, Some land covers, such as vegetation, soil, and water, also limonite and calcite can be found in all Danxia samples as have their unique spectral characteristics that are different cement (Fig. 6a, b, c), while clay minerals can be found on from those of Danxia rocks (Fig. 7f). These differences con- surfaces of some altered lithic and plagioclase grains (Fig. 6a, tribute to the discrimination of Danxia rock. d). In contrast to ferric iron, carbonate, and clay minerals, quartz grains show no absorption feature on a 350 to 2500 nm spectrum (Fig. 7d), thus cannot be diagnosed spec- Surveying and Monitoring Danxia troscopically. Therefore, cements and altered minerals, rather than quartz grains, significantly control the spectral character- In order to evaluate the capability of remote sensing in sur- istics of Danxia outcrops. veying and monitoring Danxia, we carried out application Different types of rocks, due to their different mineral experiments based on ETM+ image—a kind of satellite assemblages, have different spectral characteristics, even multi-spectral remote sensing data widely used in geological though they may be similar in color. For example, a red studies (Table 1). The data is free of charge and can be easily granitic rock usually has negligible absorption at downloaded from the EarthExplorer Platform of the US ~840 nm and weak absorption at ~2210 nm (Fig. 7e), Geological Survey (USGS). It contains one panchromatic which indicates that few ferric iron and minor clay min- band, six multi-spectral bands, and one thermal infrared band. erals may be contained in the rock. Compared with The image size is 183×170 km, and the temporal resolution is Danxia rocks, the granitic rock has no absorption at 16 days.

Fig. 8 Comparison between continuous spectra (a) and ETM spectra (b) Geoheritage (2015) 7:307–318 315

Fig. 9 Regional terrain slope of the study area

The original ETM+ image covering the southeast Sichuan except for soil that has relatively similar spectrum with area and its adjacent regions was initially acquired during dry Danxia, suggesting that ETM+ image is a possible candidate season on 22 May 2001 by Landsat 7 satellite (path=128; for Danxia discrimination but that the influence from soil row=40) and then processed to generate standard product by should be treated with great caution. Liu et al. (2007)argued USGS on 4 December 2002. The cloud content of the product that Danxia are usually exposed in the areas with dip slopes is less than 1 %. As band 6 is a thermal infrared band and may greater than 45°, which could be employed to discriminate be influenced by temperature diversity caused by elevation, it Danxia from soil that is usually found in bench terraces with was excluded from the image dataset. The image was calibrat- slopes smaller than 25°. The ASTER DEM with 30 m spatial ed with ENVI 5.0 software using BLandsat Calibration^ func- resolution covering the study area was used to reveal regional tion. Atmosphere correction was carried out using the BQUick terrain slope of the study area (Fig. 9). Atmosphere Correction^ function. The calibrated and To date, many methods and algorithms have been proposed corrected image was normalized using Band Math function. for lithology discrimination, among which the Spectral Angle Since ETM+ is a multi-spectral remote sensing image, it Mapper (SAM) is most common (Kruse et al. 1993;Sadeghi cannot yield a continuous reflectance spectrum as shown in et al. 2013). In order to avoid the influence from soil, the Fig. 8a. However, as displayed in Fig. 8b, the spectra obtained minimum slope angle was conservatively set to 35°. The map- from the ETM+ image are generally similar in shape with ping results for Danxia outcrops using SAM method and the those displayed Fig. 8a, even though some absorption fea- areas with slope angles greater than 35° are shown in Fig. 10. tures, such as those at 2210 and 2239 nm, are missing due to For the Huagaoxi Park, the Danxia outcrops were not well low spectral resolution of ETM+ image (some commercial recognized, which may be attributed to the mixed pixel effect remote sensing images, such as ASTER, can provide better induced by the dense vegetation in the park (see Fig. 4). This recognitions of these absorption features but will not be problem may be addressed by using remote sensing images discussed in this study). with higher spatial resolution. The Danxia outcrops with large Regardless of the disadvantage of low spectral resolution, it exposed areas in the Danshan and Tianxiandong parks were is can be noticed that the spectra of some land covers identified (Fig. 11a, b), while those with small exposed areas, displayed in Fig. 8b are different from those of Danxia rock, such as the Shibadongtian in the Tianxiandong Park, were not 316 Geoheritage (2015) 7:307–318

Fig. 10 Mapping results of Danxia for Huagaoxi (a), Danshan (b), Tianxiandong (c), and Chishui (d)parks

recognized (Fig. 11c). In addition, red soil in the study area great diversity of types of human alteration of nature, such indeed influenced the mapping results of Danxia outcrops as urbanization, agriculture, and road construction, or may (Fig. 11d) but can be avoided effectively by considering the be associated with some natural processes, such as earthquake, terrain slope. landslide, and bio-weathering. Although remote sensing can- The Chishui National Geological Park in Guizhou not directly produce Danxia from these threats, the changes Province, one of the World Heritage Sites for China Danxia, induced by human and natural processes, which may be harm- was also identified during the mapping process (Fig. 10d). ful to Danxia, will be observed, with time, by using this Although this park is well known to the public, it is the first technology. time that the park has been successfully recognized using As shown by Fig. 12, some exposed Danxia outcrops in the remote sensing technology, suggesting that remote sensing is Chishui National Geological Park, as seen in 2001, were sub- valuable for Danxia exploration. However, as mentioned merged and covered by water and vegetation in 2002. The above, this exploration ability relies greatly on spectral and rising water level was related to the construction of a down- spatial resolutions. A remote sensing image with higher spec- stream water reservoir and, as a result, has lowered the scien- tral and spatial resolutions will provide better discrimination tific interests of Danxia here, and some other geological phe- and identification of Danxia but will consume more money nomena in the area may be covered by water as well. The and time during acquisition and processing procedures. recognition of these changes in the Chishui National With respect to regions where Danxia parks are well Geological Park is a good example of Danxia monitoring established, Danxia conservation is more important than ex- and demonstrates that remote sensing can serve as an observer ploration. Possible threats to Danxia may be related to the and provides valuable information for Danxia conservation. Geoheritage (2015) 7:307–318 317

Fig. 11 Some photographs of field inspection of mapping results (a,b and c)Danxiaoutcrops;d Red soil

Conclusion grains do not contribute to the spectral absorptions of Danxia rocks. The rocks from Danxia outcrops in southeast Sichuan The Danxia in southeast Sichuan Province have spectra Province are mainly composed of the quartz grains with some that are different from those of granitic rocks and some land plagioclase and lithic grains. The cements include hematite, covers, including vegetation, soil, and water, suggesting that limonite, and calcite. they can be discriminated spectroscopically. The spectra of Danxia rocks have absorption features at Application examples based on ETM+ image demonstrat- ~840, ~2210, and ~2339 nm that are, respectively, associated ed that remote sensing technology is valuable for surveying with ferric iron, clay minerals, and calcite. In contrast, quartz and monitoring Danxia. However, its spectral and spatial

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