Pho Han Fm

Upper formation of the Thuy Nguyen Gr (coastal to offshore sections of Quang Ninh Province). Holostratotype: Pho Han - Ben Beo section, situated from the Cat Co 3 Beach of the Pho Han Peninsula to Ben Beo, southeast Cat Ba Island, Cat Hai District, Hai Phong City (N = 20°43’30"; E = 107°02'48"). The Pho Han Fm was established and named by Ngo Quang Toan (1994) after Pho Han Townlet, SE Cat Ba Island.

Synonym: Hệ tằng Phố Hàn (part.): Ngô Quang Toàn et al.1994 (D₃-C₁). Hệ tầng Cát Bà: Ngô Quang Toàn et al. 1994 (C₁)

Siliceous limestone. Distinguished from the coeval Con Voi Fm by the cherty component. (1). Basal bed of black shale (with Dev/Carb boundary) grading upward into grey, thin-bedded organic limestone contains fragments of crinoids and brachiopods. Thin chert interbeds are observed upwards in the section, 25 m thick. (2). Dark grey, fine-grained, medium- to thick bedded limestone and cherty limestone with thin interbeds of black shale, 200 m thick. (3). Unclear banded clayey limestone and marl interedded with strongly weathered yellowish, red brown chety limestone and cherty shale, 60 m thick. (4). Thin-bedded limestone interbedded with chert, banded chert, 100 m thick. (5). Grey, dark grey, fine-grained limestone and lenses or thin interbeds of chert 30 m thick. The formation reaches a total thickness of about 470 m.

[Figure: Phố Hàn Fm: Chert with thin bedded structure (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

[Figure: Phố Hàn Fm: Limestone and chert interbeds (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

[Figure: Phố Hàn Fm: Graded bedding structure in the turbidite formation (Alternating bands of coarse and fine particles) (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

Fossils are abundant and in the basal part there are Latest Famennian foraminifera Uralinella bicamerata, Bisphaera malevkensis, Eoendothyra communis, Quasiendothyra kobeitusana, Q. konensis, and Septabrunsiina sp.; Conodonts Spathognathodus disparilis, Palmalolepis gracilis gracilis, and P. gracilis sigmoidalis; Algae Renalcis ex gr. nubiformis, and Girvanella problematica. In the upper part abundant Early Toumaisian conodonts were found, such as Siphonodella duplicata, S. quadruplicata, S. cooperi, S. sp., Polygnathus communis communis, Polygnathus purus purus, P. inornatus inornatus. Besides, in this member some Early Carboniferous corals were collected: Fuchungopora sp., Syringopora distans, Tetraporinus sp.

[Figure: Phố Hàn Fm: Lithology and D/C chronostratigraphical boundary (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

[Figure: Phố Hàn Fm: Brachiopod and Crinoid fossils (images courtesy of Prof. Ta Hoa Phuong, Vietnam Natl. Univ. Hanoi)]

These materials show that the Pho Han Fm originated in a deep marine, possibly continental slope depositional environment, but not in the shallow-shelf marine environment of the Con Voi Fm.