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Panthalassan radiolarite versus pelagic carbonate in Intra-Pangaean basins during the Late Jurassic- Early Cretaceous transition – paleofertility and ocean circulation
Peter O. Baumgartner, Špela Goričan, Luis O'Dogherty, Atsushi Matsuoka

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-07 02:20 PM – 02:50 PM
Last modified: 2018-12-15


The transition between the Jurassic and Cretaceous systems is gradual without any distinct change in stable isotope values, typical of other system boundaries. Accordingly, continuous radiolarite sections observed throughout Eastern Tethys and Panthalassa (>80% of the world ocean) do not show any marked change suggestive of a "natural" Jurassic/Cretaceous boundary.

Intra-Pangaean, i. e. Proto-Caribbean, Central Atlantic and Western Tethyan sedimentary records across the Jurassic-Cretaceous transition differ from the radiolarite-dominated series known from Panthalassa: 1. During the Jurassic, radiolarite did not form in the Proto-Caribbean, Gulf of Mexico and the Central Atlantic. The first sediment in these basins is pelagic, often Corg-rich, claystone (DSDP Site 534) or pelagic cherty limestone since the Oxfordian (Guaniguanico Terrane, NW-Cuba, Taman Formation, E-Mexico). 2. In Western Tethys, Middle to lower Upper Jurassic ribbon bedded radiolarite progressively grades during the Oxfordian-Kimmeridgian (depending on paleogeography) into cherty pelagic limestone and passes during the middle-late Tithonian into pure nannofossil limestone with occasional radiolarian-rich chert layers (Maiolica or Biancone Formation).

During the Late Jurassic, calcareous nannofossils became for the first time an important component of the Intra-Pangaean sediments: Watznaueria spp. appear first, while Nannoconus spp. became dominant during the late Tithonian-early Valanginian and declined again in favor of Watznaueria during the late Valanginian-early Hauterivian (Erba & Tremolada 2004), Nannoconids came back during late Hauterivian and finally declined during late Barremian. When nannoconids thrived, Watznaueria declined in numbers and size (Bornemann et al. 2003). While nannoconids became rock forming in Intra-Pangaean basins and epicontinental seas, this genus has been reported from Panthalassa only in small abundances, mainly from shallow DSDP/ODP sites (e.g. Ongtong-Java, Mid-Pacific Mountains, Erba 1994).

Hyaline calpionellids (Crassicollaria, Calpionella, Tintinopsella , etc.) have a very similar paleogeographic and stratigraphic distribution, in that they appeared and became common in Intra-Pangaean basins during the late Tithonian, evolved quickly and disappeared with the first nannoconid crisis at the end of the early Valanginian. Isolated calpionellid occurrences have been reported form E-Tethys (Kiogar Nappes of Tibet, Heim & Gansser 1939), but this group is unknown from Panthalassa.

In the UAZ95 zonation (Baumgartner et al. 1995, now in revision) we recognized an important radiolarian faunal turnover within UAZ13, which is defined by 39 species appearances at its base and 9 disappearances at its top (see also Bartolini et al. 1999). UAZ 13 approximately corresponds with the Crassicollaria Zone, the first occurrence of hyaline calpionellids, and includes the first occurrence of Nannoconus. If the radiolarian faunal turnover, largely defined in Intra-Pangaean sections, can be confirmed in Panthalassan sections, it may serve as a global correlation of the rather local first occurrences of hyaline calpionellids and nannoconids. Currently, the Crassicollaria Zone is placed in the Late Tithonian. Its base, in the upper part of Magnetic Anomaly M20n, coincides with the FAD of Nannoconus infans and as mentioned above, with the FAD of many radiolarian species and genera. This event may be the only chance for a global correlation. A slight change of redox conditions, materialized by the change from pink to grey chert, may be suspected near this event in many Tethyan sections, such as Bosso and Torre di Busi, but is absent in Panthalassan sections.

The paleo-ecological affinities of nannoconids have been discussed controversially, including the following interpretations: warm waters; low nutrients; low CO2 concentrations, low terrigenous input and high carbonate productivity. J. Mutterlose (written communication) currently interprets nannoconid blooms as indicating clear waters, barren of mud, reflecting relative warm, arid and low nutrient conditions.

These conditions clearly contrast with conditions generally advocated for the formation of radiolarite, namely increased surface fertility and/or bottom water conditions that favored carbonate dissolution and silica preservation (Baumgartner 2013). On the other hand, the high radiolarian diversity observed during the Beriasian-early Valanginian, before the extinction event related to the Weissert-Event (E1 of O'Dogherty & Guex 2002) may reflect the well-known inverse correlation of trophic level and plankton diversity. The distribution of pelagic carbonate vs. radiolarite is critical to the paleo-oceanographic interpretation of the Trans-Pangaean Seaway: It does not support an east-west directed current (often conceived as a circum-global equatorial system) across this seaway, whether it is open or intermittently closed during the Middle Jurassic-Early Cretaceous (Brunetti et al. 2015). The Intra-Pangaean basins were "Mediterranean" seas, characterized by a sluggish, lagoonal circulation, with stratified water masses and relatively low surface fertility, only poorly connected to the world ocean. In the jaw-like opening of Western Tethys during the Middle-Late Jurassic, the boundaries between silica and carbonate realms depended on local and regional paleogeography. Radiolarite facies reached nevertheless to the W into Subbetic and the Rif realms adjacent to the Central Atlantic, while the nannoconid carbonate facies reached eastwards to the drowned Arabian Platform, but not to the deep water Hawasina Realm.