Palaeogeography of radiolarite and organic-rich deposits in Mesozoic Tethys

Abstract

Siliceous and marine organic-rich deposits are sometimes associated, sometimes separate in space and time; however, both are generally accepted to be the result of high planktonic productivity. Among the siliceous marine deposits, the phtanite family facies is distinguished from the radiolarite family facies by several characteristics: They contain organic material and as a result are blackish (vs red/green for radiolarite facies), their time of deposition corresponds with strong faunal modifications and they are deposited generally in shallower environments. A palaeogeographic analysis of locations of Tethyan biosiliceous and marine organic-rich rocks, both resulting from a high planktonic palaeoproductivity, for three Mesozoic high sea-level intervals, Toarcian, Kimmeridgian and Cenomanian, show: (a) during Jurassic times these Tethyan deposits were dissociated, the siliceous deposits being closer to open ocean waters than the organic-rich ones. This is a common disposition in modern upwelling systems and suggests a common process; (b) during Cretaceous times these Tethyan deposits were often associated, i.e. both occur at the same site, and are probably the result of a different process from that in the Jurassic.

Controls on the distribution of deep‐sea sediments

“…Although the majority of radiolarian oozes occur at depths of 4100-5100 m (Table 1) in abyssal plains, a large proportion occurs at much shallower depths of just a few hundred metres (Figures 2 and 3). This supports the assertion of Afanasieva et al [2005] and De Wever and Baudin [1996] that radiolarites are not necessarily indicative of deep water origin and that preserved species may not be representative of the initial living population [Afanasieva et al, 2005].…”

“…In the equatorial region, radiolarian oozes are located slightly outside of the high productivity zone in the Pacific Ocean (supporting information Figure S1). The majority of radiolarians preserved within oozes and ultimately within the geological record are siliceous polycystine variants [ Abelmann , ; Takahashi et al ., ] which live endosymbiotically with algae within the photic zone [ De Wever and Baudin , ]. Radiolarians are omnivorous and derive their nutrition from a wide variety of sources including algae, bacteria, crustacea and diatoms [ Anderson , ], which have been found within their food vacuoles [ Anderson et al ., ].…”

“…This supports the assertion of Afanasieva et al . [] and De Wever and Baudin [] that radiolarites are not necessarily indicative of deep water origin and that preserved species may not be representative of the initial living population [ Afanasieva et al ., ].…”

“…The study of living radiolarians, particularly their distribution patterns in the surface oceans, is lagging behind our understanding of their skeletal remains [De Wever et al, 2001]. However, it is generally accepted that they are especially abundant and diverse in the tropical climate belt and zones of upwelling, and their presence in sediments is considered to be a good indicator of high surface productivity [De Wever and Baudin, 1996;De Wever et al, 2001]. However, we find that the productivity values in surface water above radiolarian oozes are relatively low compared to values associated with calcareous oozes (Table 1) ranging from 166 to 229 mgC/m 2 /d in the Southern Hemisphere and 209-360 mgC/m 2 /d in the Northern Hemisphere (Table 1).…”

Stratigraphy and sedimentology of the chert unit of the Semanggol Formation

“…The chert is rich in radiolarians. It is considered as a radiolarian chert (radiolarite), which was deposited during a period of high plankton productivity associated with the supply of nutrient and silica by upwelling currents (De Wever and Baudin, 1996). The occurrence of radiolarian chert and the absence of pelagic limestone facies indicate that deposition occurred in a deep-water environment below the calcite compensation depth.…”