5:15 PM - 6:45 PM
[BCG05-P07] Late Jurassic radiolarians and sponge spicules from Bau Limestone in northwestern Borneo Island, Malaysia
Keywords:Bau Limestone, Late Jurassic, radiolaria, Vallupus, sponge spicule, rhaxella sorbyana
The Bau Limestone in northwestern Borneo Island, Malaysia, is one of the representative reef complexes deposited in shallow water near the equator during the Late Jurassic. The Bau Limestone is located at the gateway between the Paleo-Pacific Ocean and the Tethys Sea at that time, and is of great interest from the viewpoint of paleoceanography, which led to many paleontological studies (ex., foraminifers and brachiopods) during the 1960s and '70s. Based on the fossil occurrences, strontium and carbon isotopic stratigraphy (Kakizaki et al., 2013), and stratigraphic relationships with the strata above and below, the age of the Bau Limestone has been estimated to be roughly Late Jurassic to Early Cretaceous.
Four calcarenite samples collected from the reef slope sediments of ca. 15 m thick yield small amounts of radiolarians and abundant sponge spicules; A-66, Stichocapsa convexa (Yao, 1979); A-68, Cenosphaera spp., Gorgansium sp. Pantanellium spp., Vallupus sp.; A-76, Archaeodictyomitra apiarium (Rüst, 1885), Gongylothorax favosus (Dumitrica, 1970), Holocryptocanium barbui (Dumitrica, 1970), Pseudodictyomitra carpatica (Lozyniak, 1969), Zhamoidellum sp. aff. Z. ovum (Dumitrica, 1970), Zhamoidellum sp. A; A-77, badly-preserved and pyritized radiolarians. Based on the UAZone of Baumgartner et al. (1995), the occurrence range of radiolarians falls within UAZone 10-13, which is assigned to the Kimmeridgian-Tithonian interval of the Late Jurassic. Vallupus sp. from sample A-68 belongs to the Vallupinae, which shows a restricted occurrence range of the Late Jurassic and the acme in the Tithonian of latest Jurassic (Matsuoka, 1995). Calcarenite bearing radiolarians and sponge spicules in Bau Limestone probably indicates Tithonian in age.
Among the sponge spicules from four samples, a kidney-shaped those with a distinct hilum were abundant, which were identified as Rhaxella sorbyana (Blake). This morphotype from Jurassic and Cretaceous strata has been often described for rhaxes. A total of 226 individuals from the Bau Limestone showed a height of 89-196 μm and a width of 68-166 μm, with a strong positive correlation between height and width. Although the surfaces of most of the specimens were more or less corroded, 21 specimens among them showed surface ornamentation with numerous knobs connected by radially-extending bars. Rhaxella sorbyana is similar in geometry of its surface ornamentation and overall outline to oval to subspherical selenaster sponge spicules of recent Placospongiidae.
Four calcarenite samples collected from the reef slope sediments of ca. 15 m thick yield small amounts of radiolarians and abundant sponge spicules; A-66, Stichocapsa convexa (Yao, 1979); A-68, Cenosphaera spp., Gorgansium sp. Pantanellium spp., Vallupus sp.; A-76, Archaeodictyomitra apiarium (Rüst, 1885), Gongylothorax favosus (Dumitrica, 1970), Holocryptocanium barbui (Dumitrica, 1970), Pseudodictyomitra carpatica (Lozyniak, 1969), Zhamoidellum sp. aff. Z. ovum (Dumitrica, 1970), Zhamoidellum sp. A; A-77, badly-preserved and pyritized radiolarians. Based on the UAZone of Baumgartner et al. (1995), the occurrence range of radiolarians falls within UAZone 10-13, which is assigned to the Kimmeridgian-Tithonian interval of the Late Jurassic. Vallupus sp. from sample A-68 belongs to the Vallupinae, which shows a restricted occurrence range of the Late Jurassic and the acme in the Tithonian of latest Jurassic (Matsuoka, 1995). Calcarenite bearing radiolarians and sponge spicules in Bau Limestone probably indicates Tithonian in age.
Among the sponge spicules from four samples, a kidney-shaped those with a distinct hilum were abundant, which were identified as Rhaxella sorbyana (Blake). This morphotype from Jurassic and Cretaceous strata has been often described for rhaxes. A total of 226 individuals from the Bau Limestone showed a height of 89-196 μm and a width of 68-166 μm, with a strong positive correlation between height and width. Although the surfaces of most of the specimens were more or less corroded, 21 specimens among them showed surface ornamentation with numerous knobs connected by radially-extending bars. Rhaxella sorbyana is similar in geometry of its surface ornamentation and overall outline to oval to subspherical selenaster sponge spicules of recent Placospongiidae.