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Progress in Natural Science 18 (2008) 1395–1402 www.elsevier.com/locate/pnsc

Constraints on the late Mesozoic regional angular unconformity in West –North by LA-ICP-MS dating

Hong Zhang a,b,*, Wenmin Guo a, Xiaoming Liu b

a College of Mechanical and Energy Engineering, Jiangsu Polytechnic University, Changzhou 213016, b Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China

Received 22 April 2008; received in revised form 7 May 2008; accepted 19 May 2008

Abstract

The angular unconformity between the Tuchengzi Formation and the Zhangjiakou Formation in the Chengde area is commonly regarded as the direct evidence of boundary between Jurassic and Cretaceous systems, and as the evidence of tectonic transformation interfaces in Mesozoic Era in the West Liaoning-North Hebei. Moreover, the lower boundary age of the Zhangjiakou Formation in the Chengde was to be thought about 135 Myr in the past. Based on the detailed study and systematic sampling in Yixian-, Lingy- uan, Chengde and Luanping areas in the field, the authors obtained the following isotopic ages by means of LA-ICP-MS dating: In the Lingyuan area, the Zhangjiakou Formation and the Yixian Formation are in angular unconformable contact; the age range of the Zhangjiakou Formation volcanic rock is from 133 to 130 Myr (seven samples), and that of the Yixian Formation volcanic rock is from 126 to 121 Myr (eleven samples). In the Luotuoshan of Chengde, the Tuchengzi Formation and the Zhangjiakou Formation are in angu- lar unconformable contact; the age at the lower Zhangjiakou Formation is about 124 Myr (one sample), which reflects that the age range of the lower Zhangjiakou Formation is from 129 to 124 Myr combined with the previous study. In the Luanping basin, there is the larg- est sediment gap in upper Mesozoic, whose lower boundary age is confined in 131 to 130 Myr. In the Yixian-Beipiao area, the second Member of the Yixian Formation, making up more than 3/4 thickness of the Yixian Formation, was formed in 126 to 119 Myr, and the age of the Yixian Formation at the bottom is about 132 Myr. Moreover, the upper boundary age of the Tuchengzi Formation is about 136 Myr in the West Liaoning-North Hebei region. Based on the above isotopic data combined with the geological characteristics in the field in the West Liaoning–North Hebei region, it reflects that there was a widespread tectonic event in the period of 136 to 126 Myr (mainly in 130–126 Myr) in the West Liaoning-North Hebei, and it formed the late Mesozoic regional angular unconformity. Moreover, the lower boundary ages of the late Mesozoic regional angular unconformity in different areas are different, and the tectonics and the creature assemblage are different in the strata at the top and bottom of the regional angular unconformity. It implies that the late Meso- zoic regional angular unconformity is a regional interface of tectonic transformation in Mesozoic Era in the West Liaoning-North Hebei. Ó 2008 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved.

Keywords: Dating; LA-ICP-MS; Angular unconformity; Interface of tectonic transformation; West Liaoning–North Hebei

1. Introduction ing the volcanic action, crust dismantling and biological evolvement in Mesozoic in eastern China, and had been The West Liaoning-North Hebei region, where the vol- focused in the past decade [1–12]. Some researchers pro- canic strata are very developed, is the key region in study- posed that the angular unconformity between the Zhang- jiakou and Tuchengzi formations in the Luotuoshan of Chengde is the direct evidence of tectonic transformation * Corresponding author. Tel./fax: +86 519 86191552. interface and boundary between Jurassic and Cretaceous E-mail address: [email protected] (H. Zhang). systems in the Mesozoic Era in eastern China, and that

1002-0071/$ - see front matter Ó 2008 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved. doi:10.1016/j.pnsc.2008.05.013 1396 H. Zhang et al. / Progress in Natural Science 18 (2008) 1395–1402 the lower boundary age of the Zhangjiakou Formation is (Macquarie University), and the calculation of ages and about 135 Myr [10,11]. plotting of concordia diagram were made by using Isoplot Based on the systematic field work, the authors drew the (version 3.0). The correction of common lead was per- following conclusions by LA-ICP-MS dating results com- formed by the software of Andersen [17]. The detailed bined with the isotopic data from the previous researchers: description of zircon dating by LA-ICP-MS has been pub- There is a late Mesozoic regional angular unconformity in lished elsewhere [18,19]. the Lingyuan, Chengde and the Yixian-Beipiao areas in the West Liaoning-North Hebei, whose age range is from 136 4. Dating results and related analyses to 126 Myr. Due to the limitation of the article length, the great deal 2. Regional geological setting of original data and the related information cannot be shown in this article, but, if interested, can be obtained In the West Liaoning – North Hebei region, the Meso- from the author directly. zoic strata are developed. Among them, the Tiaojishan Formation, Tuchengzi Formation, Zhangjiakou Forma- 4.1. Dating results of the late Mesozoic regional angular tion, and the Yixian Formation are widespread, and their unconformity in the Lingyuan and Chengde areas mutual relationships are the problems that geological researchers have been focusing on [5–13]. 4.1.1. Lingyuan area In the North Hebei, the Tiaojishan Formation is in par- The Lingyuan area, located in the transitional region of allel unconformable contact with the overlying Tuchengzi NEN tectonic lines in West Liaoning and NEE tectonic Formation [14], and the stratum on the Tuchengzi Forma- lines in North Hebei, is a key area in studying tectonic tion is the Zhangjiakou Formation. In the field, there are transformation in Mesozoic in the West Liaoning-North two relationships between the Zhangjiakou and Tuchengzi Hebei. Therefore, the Mesozoic strata (see Fig. 1) in the formations: The first relationship is that the Zhangjiakou area have been studied [3,6,20–22]. In the area, the Zhang- Formation is in conformable contact with the Tuchengzi jiakou Formation is in angular unconformable contact Formation [9,15], which is prevalent in Luanping and fur- with the overlying Yixian Formation [20]. Moreover, the ther west. In other words, if only the two Formations exist tectonic lines in and beneath the Zhangjiakou Formation together in Fengning, Diao-e (Chicheng) or Luanping are about NEE orientation; however, the tectonic lines in areas, they conform. The second relationship is that the and above the Yixian Formation are about NE or NEN unfolded Zhangjiakou Formation is in angular unconform- orientation. Those phenomena are also clearly shown in able contact with the folded Tuchengzi Formation, which the Dawangzhangzi area. is only exposed at Luotuoshan, Jiguanshan of Chengde In the Dawangzhangzi area (Fig. 1), the Yixian Forma- and Shuangmiao of Pingquan. tion is a set of sediment-volcanic rocks. The Daw- In the West Liaoning, the Tiaojishan Formation is in angzhangzi bed with the thickness of more than 200 m at parallel unconformable contact with the overlying Tuc- the bottom of the Yixian Formation is a set of sediment hengzi Formation [16], the Tuchengzi Formation is in rocks formed in lake facies with a great deal of Jehol Biota, angular unconformable contact with the overlying Yixian and mainly consists of tuff-sandstone, silk sandstone, tuff Formation [6,16], and the Yixian Formation conforms and conglomerate. The rock above the Dawangzhangzi with the overlying Jiufotang Formation [16]. bed in the Yixian Formation is a set of acid volcanic lava and volcanic clastic rock with the thickness of more than 3. Analytical methods 200 m. The Zhangjiakou Formation consists of mainly acid volcanic rock, volcanic breccia with some intermediate vol- The in situ zircon analysis was conducted in the State canic rock and some sediment rock interbedded. Key Laboratory of Continental Dynamics, Northwest Uni- The systematic dating of the Zhangjiakou Formation versity, Xi’an. The ICP-MS type used is the Elan 6100DRC and the Yixian Formation in the Dawangzhangzi area or the Agilent 7500A. The laser-ablation system is a Geo- [20–22] reflects that the age range of the Zhangjiakou For- Las 200 M (MicroLas, Go¨ttingen, Germany) equipped mation volcanic rock is between 133 and 130 Myr (seven with a 193 nm ArF-excimer laser, a homogenizing and an samples), and the Yixian Formation volcanic rock is from imaging optical system. In analysis, the diameter of the 126 to 121 Myr (11 samples) (Fig. 2, Table 1). laser bean was usually set to be 30 lm. Helium was used Therefore, the upper boundary age of the late Mesozoic as a carrier gas to enhance transport efficiency of the aba- regional angular unconformity is about 126 Myr, and the ted material. The international standard zircon 91500 was lower boundary age of that is about 130 Myr in the Lingy- used as the external standard for the age calculation, and uan area. the NIST SRM 610 was used as the external standard for the concentration calculation, and 29Si (32.8% SiO2) was 4.1.2. Chengde area used as the internal standard. Isotopic ratios and elemental The Chengde area is located in the middle part of the concentrations were calculated by using GLITTER 4.0 Yanshan tectonic belt. In the area, the tectonic lines in and H. Zhang et al. / Progress in Natural Science 18 (2008) 1395–1402 1397

Fig. 1. Simplified geological map and Mesozoic strata column in the studied area in the West Liaoning–North Hebei. 1, Coal bearing strata; 2, silk- sandstone, shale; 3, conglomerate; 4, acid volcanic rock; 5, intermediate volcanic rock; 6, basalt; 7, andesitic lava; 8, basaltic andesite; 9, working location;

10, parallel or angular unconformity; 11, Mesozoic basin. Ch, Changcheng System; J2–3t, Tiaojishan Formation; K1t, Tuchengzi Formation; K1z, Zhangjiakou Formation; K1y, Yixian Formation; K1d, Dabeigou Formation; K1dd, Dadianzi Formation; K1x, Xiguayuan Formation. beneath the Tuchengzi Formation are mainly of NEE orien- Jiguanshan area, an age of 135 ± 1 Myr without original tation, but the tectonic lines in and above the Zhangjiakou isotopic data from the bottom of the Zhangjiakou Forma- Formation are mainly of NEN orientation. Moreover, the tion was reported [10]; in the Luotuoshan area, a SHRIMP Tuchengzi Formation is in angular unconformable contact age of about 135 Myr without original isotopic data at lower with the overlying Zhangjiakou Formation, and the clear Zhangjiakou was reported [11], which was corrected later to exposed area is in Luotuoshan. In the Luotuoshan area, 128.8 ± 1.3 Myr with the original isotopic data [12]. we obtained the age of the lower Zhangjiakou Formation At present, only two isotopic ages with the original iso- which is 124.3 ± 4.5 Myr (CD02) (Fig. 2, Table 1). In topic data from the Zhangjiakou Formation in Chengde 1398 H. Zhang et al. / Progress in Natural Science 18 (2008) 1395–1402

Fig. 2. U-Pb concordia diagrams of zircons in the West Liaoning-North Hebei. The ICP-MS type used is the Elan 6100DRC or the Agilent 7500A, their precisions are different, thus, the precisions of the concordia diagrams of zircons are different. are believable: the SHRIMP age of 128.8 ± 1.3 Myr at the jiakou Formation at the north of the Luotuoshan area lower Zhangjiakou Formation in the Luotuoshan area and [12]. The combination of the above two ages and the dating the SHRIMP age of 125.9 ± 2.8 Myr at the upper Zhang- result by the authors (CD02: 124.3 ± 4.5 Myr) implies that H. Zhang et al. / Progress in Natural Science 18 (2008) 1395–1402 1399

Table 1 Isotopic ages related with the regional angular unconformity in the West Liaoning–North Hebei Area Samp no. GPS coordinates Location Age (Myr) Ref. Luanping YX285 N40°52.2010, E117°16.9340 Top of Dadianzi Fm. 131.4 ± 3.7 [15] YX282 N40°54.8430, E117°21.9530 Top of Dadianzi Fm. 130.2 ± 3.0 Chengde CD02 N40°52.5300,E 118°01.7850 Lower Zhangjiakou Fm. 124.3 ± 4.5 Author’s unpublished data Lingyaun 1018-1 N41°03.0940, E119°08.7410 Yixian Fm. 121.9 ± 1.8 1018-2 N41°03.1160, E119°08.4270 Yixian Fm. 122.7 ± 1.5 1018-3 N41°06.9400, E119°08.3970 Yixian Fm. 120.6 ± 3.0 YX286 N41°01.3740, E119°12.0090 Yixian Fm. 126.4 ± 2.1 YX288 N41°09.4630, E119°15.8520 Yixian Fm. 126.1 ± 2.3 402-2 N41°03.8420, E119°12.5380 Yixian Fm. 122.4 ± 2.9 402-6 N41°08.8610, E119°12.5330 Lower of Zhangjiakou Fm. 130.5 ± 2.6 402-1 N41°01.5410, E119°11.8270 Top of Zhangjiakou Fm. 130.8 ± 1.5 1017-1 N41°01.7870, E119°11.9220 Upper of Zhangjiakou Fm. 130.2 ± 1.5 [20] YX214 Upper of Zhangjiakou Fm. 132.4 ± 1.4 YX277 N41°01.4660, E119°11.7790 Bottom of Yixian Fm. 125.4 ± 0.9 [21] YX278 N41°02.5400, E119°12.5540 Middle of Yixian Fm. 125.3 ± 0.9 YX279 N41°02.5010, E119°12.3710 Middle of Yixian Fm. 124.4 ± 1.1 41017-2 N41°03.1860, E119°11.9070 Middle-Lower of Yixian Fm. 122.3 ± 2.1 YX281 N41°09.1060, E119°16.9010 Middle-upper of Yixian Fm. 123.0 ± 1.7 129-3 N41°01.2640, E119°08.6250 Bottom of Zhangjiakou Fm. 129.6 ± 0.9 [22] 129-4 N41°01.2280, E119°09.1560 Middle of Zhangjiakou Fm. 129.5 ± 0.8 41017-4 N41°02.8470,E 119°12.6840 Zhangjiakou Fm. 133.4 ± 2.6

the age at the lower part of the Zhangjiakou Formation is found in the Luanping basin of North Hebei and the Yix- from 129 to 124 Myr, not 135 Myr. ian-Beipiao area of West Liaoning.

4.2. Re-understand the mesozoic volcanic rock in Luanping 4.1.3. Apocalypse of the dating results from the Chengde and basin and Yixian-Beipiao area Lingyuan areas The dating results reflect that the age range of the lower 4.2.1. Luanping basin Zhangjiakou Formation volcanic rock which is on the The Luanping basin is located in the middle part of the regional angular unconformity, is from 129 to 124 Myr in Yanshan tectonic belt. The Mesozoic strata exposed in the the Chengde area; the age range of the Yixian Formation area can be seen in Fig. 1. volcanic rock, which is on the regional angular unconfor- In the basin, there is a special bed of basaltic andesite, mity is from 126 to 121 Myr in the Lingyuan area. In the which is located at the top of the Dadianzi Formation with two areas, the lithology of the volcanic rock on the regional the thickness of about 50 m to 100 m. It is noteworthy that angular unconformity is similar, and mainly consists of the Tuchengzi Formation, Zhangjiakou Formation and the acid volcanic rock. It also indicates that there was a similar Dabeigou Formation, all are beneath the basaltic andesite volcanic action in the two areas in the period of 129 to bed, are conformable [5,14,15]; and the volcanic rock in the 121 Myr, and there had been a similar violent tectonic strata is mainly acid (Fig. 1). The stratum on the basaltic event which had formed the late Mesozoic regional angular andesite bed is a set of polymictic conglomerates located unconformity in the two areas. at the bottom of the Xiguayuan Formation–the Lahailiang On the other hand, the regional angular unconformity in conglomerate [5]; moreover, the sediment gap between the the two areas is different: In the Chengde area, the stratum basaltic andesite and the conglomerate beds is the largest under the angular unconformity is the clastic rock of the one in the basin in the late Mesozoic Era [5]. Those suggest Tuchengzi Formation. However, in the Lingyuan area, that there was a tectonic movement at the end of the Dad- the stratum under the angular unconformity is the acid vol- ianzi period, which made the basic-intermediate magma canic rock of the Zhangjiakou Formation; moreover the erupt, and made the sediment action stop for a long time, upper boundary age of the Zhangjiakou Formation volca- and formed the polymictic conglomerate bed at the bottom nic rock — the lower boundary age of the angular uncon- of the Xiguayuan Formation in the end (Fig. 1). formity – is about 130 Myr. So, the question is whether the The above geological information indicates that the angular unconformity with the lower boundary age of basaltic andesite is the reflection of an intense tectonic about 130 Myr in Lingyuan area is a common example movement. Moreover, the age of the basaltic andesite is or not in West Liaoning-North Hebei. Further research from 131.4 ± 3.7 Myr (YX285) to 130.2 ± 3.0 Myr indicates that the period of about 130 Myr is an important (YX282) [15] (Table 1, Fig. 2), implying that the Luanping tectonic, magmatic ‘‘window”, and some evidence has been basin was affected by an intense tectonic movement in the 1400 H. Zhang et al. / Progress in Natural Science 18 (2008) 1395–1402 period of 131 to 130Myr. Moreover, this age is close to the ond Member is about 2500 m, making up more than 3/4 lower boundary age (about 130 Myr) of the regional angu- thickness of the Yixian Formation (Fig. 3). Moreover, lar unconformity in Lingyuan. Therefore, we infer that the the ages of the second Member is from 126 Myr to tectonic movement forming the late Mesozoic regional 119 Myr. The first Member mainly consists of basic-inter- angular unconformity in Lingyuan has affected the Luan- mediate volcanic rocks with the thickness of about 400 m ping basin in North Hebei. [6,23], whose ages are mainly from 132 to 126 Myr [23] (Fig. 3). 4.2.2. Yixian - Beipiao area It should be emphasized that the Yixian Formation is The Mesozoic strata in Yixian and Sihetun of Beipiao distributed in West Liaoning in large scale, that is, the Yix- are different (Fig. 1), the Yixian Formation in the two areas ian Formation is distributed not only in the Yixian, Bei- is comparable [6,23]. In the Sihetun of Beipiao, the Tuc- piao and the Lingyuan areas, but also in Jianchang, hengzi Formation is in angular unconformable contact and Zhangwu and so on. But the first Member of with the overlying Yixian Formation; in the Yixian basin, the Yixian Formation accounts for only a small part of the Tuchengzi Formation does not exist; but in Lingyuan, the Yixian Formation, especially in Jianchang and Jiangchang, Fuxin and Zhangwu areas of West Liaoning, Zhangwu areas, where there is hardly the first member. the angular unconformity of the Tuchengzi Formation In other words, the Yixian Formation in West Liaoning and the Yixian Formation is common [6,16]. These reflect is mainly the second Member with the age of mainly in that the angular unconformity between the Tuchengzi For- 126–119 Myr. Thus, the angular unconformity between mation and the Yixian Formation in West Liaoning is the Tuchengzi Formation and Yixian Formation in West regional and widespread. Liaoning is actually the angular unconformity between The Yixian Formation in the area is a major concern [1– the Tuchengzi Formation and the second Member of the 3,24–28]. In the Yixian Formation, there is a key sediment Yixian Formation. interbed–the Jianshangou (Zhuanchengzi) bed, and the interbed divides the Yixian Formation into two Members 4.3. Determination of the age of the regional angular (Fig. 3): the first Member is the Yixian Formation beneath unconformity in West Liaoning-North Hebei the Jianshangou (Zhuanchengzi) bed, and the second Member is the Yixian Formation above the Jianshangou (1) There is a late Mesozoic regional angular unconfor- (Zhuanchengzi) bed (including the Jianshangou bed). The mity in both the Chengde and the Lingyuan. It is notewor- second Member mainly consists of intermediate-acid volca- thy that the top bed of the regional angular unconformity nic rock with a large thickness, whose ages are mainly from in Lingyuan is the Dawangzhangzi bed at the bottom of the 126 to 119 Myr (Fig. 3). For example, in the Mashenmiao- Yixian Formation, and its fossil assemblage is identical to Songbahu vertical section in Yixian area, the thickness of that in the Jianshangou bed of the Yixian Formation in the the Yixian Formation is 2890 m [6,23], and that of the sec- Yixian-Beipiao area [3,6,20]. Moreover, the age range of

Fig. 3. Dating results in Yixian –Sihetun Beipiao of West Liaoning. 1, Basic volcanic rock; 2, intermediate volcanic rock; 3, acid-intermediate volcanic rock; 4, rhyolite; 5, sedimental rock (interbed); 6, Yixian Formation; 7, Tuchengzi Formation; 8, Pre-Mesozoic Era; 9, fault; 10, location of Mashenmiao –

Songbahu vertical section. 1 M, first member; 2 M, second member; J2–3t, Tiaojishan Formation; K1t, Tuchengzi Formation; K1y, Yixian Formation; K1, Jiufotang–Fuxin Formation; Q, Quaternary. H. Zhang et al. / Progress in Natural Science 18 (2008) 1395–1402 1401 the Jianshangou bed and the above volcanic rock in the boundary age of the late Mesozoic regional angular uncon- Yixian Formation is from 126 to 119 Myr, which is identi- formity in the region should be no more than 136 Myr. cal to that of the Yixian Formation in Lingyuan. The In Lingyuan, the lower boundary age of the late Meso- above geological information reflects that there were simi- zoic regional angular unconformity is about 130 Myr; in lar tectonic–magmatic settings from 126 to 120 Myr in the Yixian-Beipiao area, the lower boundary age of the Lingyuan and the Yixian-Beipiao areas, and similar tec- Yixian Formation is about 132 Myr; in Luanping of North tonic–magmatic settings between 129 to 126 Myr and Hebei, the lower boundary age of the largest sediment gap 120 Myr in Lingyuan, Yixian-Beipiao and Chengde areas. in the late Mesozoic is from 131 to 130 Myr. The above iso- In the Lingyuan and the Sihetun areas of Beipiao, the topic ages are all near 130 Myr and reflect a violent tec- Yixian Formation does not go all the way in rock proper- tonic-magma event in the West Liaoning-North Hebei ties and ages formed. In the Lingyuan, the ages of the region in the period of about 130 Myr. Of course, the sig- regional angular unconformity is from 130 to 126 Myr, nificance of the age in different areas is different. In Lingy- and no strata have been formed. In the Sihetun area, the uan, the age of 130 Myr represents the age of the end of an age of the first Member of the Yixian Formation is from early magmatic action. In the Yixian–Beipiao area, the age 132 to 126 Myr. The authors believe that the above geolog- of 132 Myr represents the age of the beginning of a new ical phenomena in the two areas do not conflict because, tectonic-magmatic action. It also indicates that the age of generally, a large tectonic event can form the obvious angu- 132–130 Myr is the main lower boundary age of the late lar unconformities in some areas and can also form mag- Mesozoic regional angular unconformity, though the cor- matic rock near large, deep faults or magma sources responding lower boundary ages in different areas are not related to the tectonic event. In West Liaoning, the regional completely identical. angular unconformity between the Yixian Formation and The above analyses show that the age range of the late Tuchengzi Formation reflects the existence of a large tec- Mesozoic regional angular unconformity in the West Lia- tonic movement; the first Member of the Yixian Formation oning-North Hebei region is from 136 to 126 Myr, and (the high-Mg basic-intermediate volcanic rock) indicates the main age range is between 132–130 Myr and 126 Myr. the arrival of the main phase of a new large tectonic event. Moreover, the age of the main erupting period of the Yix- 5. Conclusions and discussion ian Formation is from 126 to 119 Myr; therefore, the first Member of the Yixian Formation aged from 132 to (1) In the West Liaoning–North Hebei region, the tec- 126 Myr in the Yixian-Beipiao area is evidence of the regio- tonics and fossil assemblage are quite different in the strata nal angular unconformity in the Lingyuan. above and beneath the late Mesozoic regional angular In summary, the upper boundary age of the regional unconformity. Regarding the tectonics, the tectonics in angular unconformity in the West Liaoning-North Hebei the strata above the unconformity are of NE orientation, is about 126 Myr. and the tectonics in the strata beneath the unconformity (2) In the Chengde and Sihetun areas, the bottom strata are mainly of NEE orientation in North Hebei; the tecton- of the late Mesozoic regional angular unconformity is the ics in the strata above the unconformity are of ENE orien- Tuchengzi Formation; however, the top strata of that is tation, and the tectonics in the strata beneath the different. The top strata of the angular unconformity in unconformity are mainly are of NE orientation in West the Sihetun area is the Yixian Formation, while it is the Liaoning. Regarding the fossil assemblage, in the top beds Zhangjiakou Formation in the Chengde area. The tradi- of the unconformity (i.e. the Jianshangou bed in the tional interpretation is that the late Mesozoic regional Yixian-Beipiao area and the Daxinfangzi bed in Lingyuan), angular unconformity in the two areas was the result of the Jehol biota had experienced a firestorm; in the bottom the same tectonic event [6,14,16]; the tectonic movement strata of the unconformity, most fossils were of Tuchengzi formed the angular unconformity between the Tuchengzi biota [6]; in the formation period of the unconformity Formation and Zhangjiakou Formation, and the reason (136–126 Myr) (i.e. the Yixian Formation beneath the Jian- that the Yixian Formation is in direct angular unconform- shangou bed in West Liaoning and the Dabeigou Forma- able contact with the Tuchengzi Formation in West tion in Luanping), the Jehol biota was in a burgeoned Liaoning is because of the absence of the Zhangjiakou For- stage [6]. Therefore, the late Mesozoic regional angular mation [6,14,16]. However, new dating results reflect that unconformity in the West Liaoning-North Hebei region the age (129–124 Myr) at the lower Zhangjiakou Forma- is an important interface of the tectonic frame transforma- tion in Chengde is younger than that (about 132– tion and creature stratum unit. 126 Myr) at the lower Yixian Formation in the Yixian- (2) This study shows that the formation of the late Beipiao area, indicating that the Yixian Formation in the Mesozoic regional angular unconformity in the West Lia- Yixian–Beipiao area is nearer to the lower boundary age oning-North Hebei region is approximately dated from of the late Mesozoic regional angular unconformity. 136 to 126 Myr, and the age of the main period is between A recent research has shown that the upper boundary 132 to 130 Myr and 126 Myr. However, the traditional age of the Tuchengzi Formation is 136 Myr [29] in the West interpretation is that the upper boundary age of the late Liaoning-North Hebei region, indicating that the lower Mesozoic regional angular unconformity is close to 1402 H. Zhang et al. / Progress in Natural Science 18 (2008) 1395–1402

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