Chigan Fm

Partizansk depression of southern Primorye Basin (S-12; E of Vladivostok). The Chigan Formation is exposed in several sections located on Cape Otkrytyi, the station Strelok, the Promyslovka village, the Petrovka River, the Putyatin Island and the Litovka River (Sey and Kalacheva, 1995; Konovalov and Konovalova, 1999). The stratotype section of the Chigan Formation is located on Cape Otkrytyi (eastern coast of the Ussuri Bay)

In the stratotype section the Chigan Formation consists of basal conglomerates, sandstones and alternations of sandstones and siltstones with a total thickness of 266 m (Konovalov and Konovalova, 1999). The lower part of the formation (first 130 m) does not contain stratigraphically significant fauna. The most fossiliferous part is represented by a member of dark-grey fine- to medium-grained arkosic sandstones with a total thickness of 100 m (Member 6 of Konovalov and Konovalova, 1999) (interval 130-230 of the stratotype section).

The Chigan Formation is of particular interest because it possibly contains the Jurassic/ Cretaceous boundary and yields species of the Boreal genus Buchia together with Tethyan ammonites (Sey and Kalacheva, 1995, 1999; Konovalov and Konovalova, 1999). Therefore, the territory of Southern Primorye represented an ecotone region during the Berriasian.

In the most fossiliferous part, the lower part of this member is characterized by B. piochii, B. ex gr. fischeriana and B. ex gr. mosquensis (Konovalov and Konovalova, 1999). This assemblage is considered as middle Volgian because B. mosquensis is an index species of middle Volgian buchia zone and B. piochii occurs since uppermost middle Volgian (Zakharov, 1981; Zakharov and Rogov, 2020). The middle part of the member (20-50 m) is characterized by B. piochii and B. ex gr. fischeriana. Such an assemblage without B. mosquensis may suggest upper Volgian (Zakharov and Rogov, 2020). The interval 70-75 m is characterized by a typical upper Volgian assemblage of Buchia: B. piochii, B. fischeriana, B. terebratuloides and B. unschensis. The most abundant assemblage is documented from the overlying bed of the member (93-100 m). In this interval ammonites such as Pseudosubplanites cf. grandis and Berriasella ex gr. jacobi typical for the lower zone of lower Berriasian of the Tethyan Realm occur with B. piochii, B. tennuicollis, B. fischeriana, B. trigonioides, B. terebratuloides and B. unschensis. Following by recent proposal of the Berriasian GSSP (Wimbledon et al., 2020) the J/K boundary is assumed to be inside the Strambergella jacobi zone and, thus it is included in the buchiazone unschensis corresponding to the interval 70-100mof Member 6 and lower than the bed with Berriasian ammonites. Thus, the J/K boundary should be in the interval 70-93 m of Member 6.

Other interpretation of the stratotype section of the Chigan Formation suggests significantly higher thickness (more than 600 m) and correlation of the most (upper) part of the section characterized by macrofossils with the Berriasian (Sey and Kalacheva, 1999). Buchiid assemblages of the most part of the section includes Buchia piochii, B. terebratuloides, B. unschensis (Sey and Kalacheva, 1999, pl. 3). B. volgensis and B. fischeriana appeared above the first appearance of the latter three species in the section. Figures of ammonites Pseudosubplanites, Berriasella, Dalmasiceras described from the stratotype of the Chigan Formation are also given (Sey and Kalacheva, 1999, pls. 1, 2). Based on this, authors concludes the Berriasian range of the formation. However, in this interpretation, the fossils do not characterize the lower part of the section (about 200 m). Note that the interval characterized by B. terebratuloides and B. unschensis (below the first finds of B. volgensis) may be correlated with the Buchia unschensis zone in Northern Siberia (Zakharov and Rogov, 2020) containing the Jurassic/Cretaceous boundary. The overlying part of the section in this case corresponds to beds with B. volgenis considered as Berriasian (Zakharov and Rogov, 2020). The lower part of the Chigan Formation is weakly exposed near the Promyslovka Village and on the Putyatin Island (Sey and Kalacheva, 1995; Konovalov and Konovalova, 1999). On the Putyatin Island, it is represented by calcareous sandstones with Buchia rugosa and B. mosquensis and middle Tithonian ammonites (Sey and Kalacheva, 1995, figs. 3e5). The presence of the lower Tithonian in this region is debatable (Sey and Kalacheva, 1995; Konovalov and Konovalova, 1999). Therefore, the Chigan Formation is considered as middle Tithonian (possibly lower Tithonian) to lower Berriasian.