Reply to Yin, J. Comment on “Zhang et al. New Age Constraints of the Bilong Co Oil Shale in the Qiangtang Basin, Northern Tibet: Evidence from In Situ U–Pb Dating and Palaeontology. Minerals 2024, 14, 246”

Abstract

The comment by Yin (2024) referred to the systematic morphology and identification of the newly recovered in situ ammonite assemblage Tiltoniceras sp. and Hildoceratidae sp. from the Bilong Co section in the Qiangtang Basin. Yin (2024) suggests that Tiltoniceras sp. should be assigned to Oxycerites aspidoides. Upon re-examination of our paper and the reply to the released comment, it is demonstrated that the morphologic characteristics of the recovered ammonites are more consistent with Tiltoniceras rather than Oxycerites aspidoides. These ammonites, along with in situ calcite U–Pb age (ca. 181 Ma), carbon isotope stratigraphy, and age-diagnostic palynomorphs, constrain the age of the Bilong Co oil shale to the Lower Jurassic (Toarcian) with plausible credentials.

1. Introduction

Recently, the newly discovered ammonite assemblage of Hildoceratidae–Tiltoniceras sp. at the top part of the Bilong Co oil shale, along with the new U–Pb age, confirmed the deposition during the Toarcian of the upper Lower Jurassic [1]. Therefore, the Bilong Co oil shale can be assigned to the Quse Formation, which is attributed to the Lower Jurassic rather than the Middle Jurassic. However, Yin [2] believed that there are inaccuracies in the identification of Tiltoniceras sp. (Zhang et al. [1], p. 6, Figure 5A,B,E) and Hildoceratidae sp. (Zhang et al. [1], p. 6, Figure 5C,D,F).

Firstly, Yin [2] referred that “those specimens of Zhang et al (2024 [1], p. 6, Figure 5A,B,E) belong to Oppellidae rather than Hildoceratidae”. Yin [2] considered that these specimens lack the diagnostic feature of Tiltoniceras: “…strong keel on the venter but no sulci of Tiltoniceras, and the ribs of the genus Tiltoniceras never form the falcoid or falcate ribbing pattern like that of the specimens shown by Zhang et al (2024 [1], p. 6, Figure 5A,B,E). The newly recovered in situ ammonites referred to as “Tiltoniceras sp.” by Zhang et al. 2024 (p. 6, Figure 5A,B,E) [1] should assign to Oxycerites aspidoides.”

Secondly, Yin [2] pointed out that “the use of Hildoceratidae sp. in the taxonomic article by Zhang et al. (2024) [1] is not standardized. Furthermore, classifying ammonite samples only as part of the Hildoceratidae family, without specifying their genus and species, may not significantly contribute to the determination of stratigraphic age”. In this reply, we aim to provide a detailed response to the accuracy of the proposed taxonomic identification of ammonite assemblages proposed by Zhang et al. [1] based on their morphologic characteristics.

2. Re-Examination of Ammonites from the Bilong Co Section

Based on the re-identification of the recovered ammonites from the Bilong Co section, the following conclusions have been reached.

Firstly, regarding the few small specimens, at least five whorls are visible based on their current state of preservation. These specimens, identified as compressed to oxyconic, sculptured Haploceratoidea within the Oppeliidae family, exhibit a variety of keel configurations, either unkeeled, unicarinate, bicarinate, or tricarinate, with diverse suture patterns, but ribbing usually more or less falcoid or falcate. Consequently, we posit that these specimens are more closely aligned with the Hildoceratidae rather than the Oppeliidae and agree with Yin that the preservation conditions do not provide significant stratigraphic age information.

Secondly, although the specimen recovered from the Qiangtang Basin lacks the typical keel on the venter, an examination of the compiled fossils provided in Figure 1 suggests its closer affinity with Tiltoniceras rather than Oppeliidae (Oxycerites exhibit more compressed, more involute whorls and a smaller umbilicus). The shell of the Xizang specimen is moderately involute, with fine ribs in the inner whorls that strengthen and form falcate ornamentation in the last whorl. The striae bend forward at the umbilical edge, become frankly retroverted, and gradually straighten out to finally curve forward at the whorl edges. In contrast, Oxycerites shells are involute, compressed, and generally smooth, with a sharply rounded venter on the outer rim, deeply impressed dorsum on the inner rim, and a small umbilicus. The living chamber occupies slightly more than half a whorl. Both Tiltoniceras and Oxycerites may exhibit falcoid or falcate ribs; however, the presence or absence of a ventral sulcus within the prominent keel distinguishes these genera. Unfortunately, due to the preservation condition of the recovered specimen, precise identification from the ventral sculpture is challenging. Nevertheless, based on characteristics such as a larger umbilicus and the nature of ribbing (fine to moderately strong, increasing from umbilicus to venter), the Tibetan specimens are more akin to Tiltoniceras (whereas Oxycerites, typically very involute, often obscure the inner whorls) [3,4,5].

3. Other Stratigraphic Evidence

The age of the Bilong Co oil shale is constrained by an in situ calcite U–Pb age and a zircon U–Pb age, respectively. A maximum depositional time of 184.4 ± 0.61 Ma for the base of the oil shale layer indicates that the depositional age of the Bilong Co oil shale is not earlier than the early Toarcian [7]. An in situ calcite U–Pb age of 181 ± 13 Ma, along with the ammonite Tiltoniceras sp., suggests an early Toarcian age of the oil shale [1]. In addition, Xia et al. [8] recovered a rich assemblage of dinoflagellate cysts from the oil shale interval of the Bilong Co section, indicative of a Toarcian age. This includes Nannoceratopsis gracilis, Mancodinium semitabulatum, Scriniocassis priscus, and Scriniocassis weberi. Moreover, other age-diagnostic palynomorphs were reported from the Bilong Co section, such as pollen species Callialasporites trilobatus, C. dampieri, and C. microvelatus, which spur further evidence of the Toarcian [8].

Regional correlation of negative carbon isotope excursion (N-CIE) events offers critical insights into the timing of major perturbations in the carbon cycle and associated oceanic anoxic events. The lower Toarcian oil shales of the Quse Formation show an abrupt N-CIE, with a peak excursion in δ¹³C_kerogen of 5.23‰ and averaging 3‰, aligning with the carbon cycle perturbation associated with the Toarcian Oceanic Anoxic Event [9]. This is in agreement with δ¹³C_org values from the same interval in the Bilong Co area with a N-CIE average magnitude of 2.8‰ [7,8]. The Quse N-CIE exhibits a comparable pattern with other regional sections not only from China, such as the Niandua and Wölong sections from the Tibetan Himalaya [10], the Anya section in the Ordos Basin [11], and the Suobucha section in the Southern Qiangtang Depression, but also from the western Tethys, including the Yorkshire section in the UK [12,13], the Peniche section in the Lusitanian Basin in Portugal [14,15], and the Rietheim Posidonia Shale in northern Switzerland [16]. This provides evidence of the Quse oil shale depositional age during the Toarcian. Thus, δ¹³C_kerogen and δ¹³C_org excursions reflect the early Toarcian global carbon perturbation and confirm the Toarcian age of the Bilong Co oil shales, in agreement with coeval records from the eastern Tethys and disparate sections worldwide.

4. Conclusions

In conclusion, the specimen of Qiangtang should belong to Tiltoniceras rather than Oxycerites, and along with calcite U–Pb age, carbon isotope stratigraphy, and diagnostic palynomorphs, they constrain the age of the Bilong Co oil shale in the Qiangtang Basin to the Early Jurassic with plausible credentialism.