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The palaeoenvironmental changes of Upper Jurassic (Middle Tithonian) to Lower Cretaceous (Lower Albian) sediments are here analysed along a North-South transect following the southwestern margin of Gondwana (central Chile, Patagonia, and Antarctica). The focus of this high-resolution multidisciplinary study is paid to climatic perturbations, which are evident in palynofacies and lithofacies records as well as by presence/absence of the cold-water indicative mineral glendonite. In central Chile two sections corresponding to the Tithonian–Hauterivian Lo Valdés Formation were analysed from the Mendoza-Neuquén backarc basin. An uppermost Tithonian unit was identified which corresponds to a relatively low sea level and deposition in proximal to inner platform environments. This interval is followed by a transgressive interval in the Berriasian, leading to deep shelf to basin depositional settings. The early–Middle Valanginian shows again a relatively low sea level with inner platform to slope deposits, followed by a second transgressive phase in the Upper Valanginian to Hauterivian at the top of the sections, represented by deep shelf to basin deposits. Sedimentary organic matter (OM) in the sections is intensely altered by thermal degradation and mostly consists of terrestrial OM with only very minor amounts of marine OM. Calpionellids are found in the Berriasian interval, indicating warm, oxygen-rich conditions at least for the water column. Corresponding to both intervals of low sea level (uppermost Tithonian, early–Middle Valanginian) microscopic glendonite was identified in the insoluble sample residue. The occurrence of glendonites in these shallow marine shelf environments indicates cold water temperatures and thus provides evidence for significant climatic cooling in the latest Tithonian and early–Middle Valanginian. In Patagonia (Región de Magallanes y de la Antártica Chilena) drilled core cuttings of two wells were analysed reaching from the Tithonian to Berriasian and Hauterivian to Aptian (Sofia 1 core – Tobífera Formation, lower and middle Zapata Formation, Austral Basin, backarc) and from the Upper Barremian to Lower Aptian (Nevenka 1 core – Springhill/Río Mayer Formation, Austral Basin, backarc). Samples of the Sofia 1 core show highly altered organic matter but information about the depositional setting could still be obtained by using the opaque:translucent ratio of phytoclasts and the shape and size of organic particles. Organic matter is better preserved in the Nevenka 1 core and the AOM-Phytoclast-Palynomorph relationship was used to identify depositional environments. Both Patagonian sections present a predominance of siliciclastic sedimentation while increased carbonate content was confined to short intervals. Microscopic glendonites were detected in the Upper Barremian and Lower Aptian section and are accompanied by proximal depositional environments, thus pointing to low sea level reflecting glacioeustasy. In Antarctica, the sampled interval ranges from the Middle Tithonian to the Lower Albian. Analysed sections include the Anchorage-, President Beaches- and Chester Cone Formation of the Byers Group (Byers Peninsula, intra-arc), the Himalia Ridge-, Spartan Glacier- and Pluto Glacier Formation of the Fossil Bluff Group (Alexander Island, forearc), the Nordenskjöld Formation and the Kotick Point Formation of the Gustav Group (James Ross Island, backarc). In the Byers- and Fossil Bluff Groups, a few samples show increased carbonate contents while most are composed of siliciclastic material. In contrast, carbonate content in the Nordenskjöld- and Kotick Point Formation is generally higher reaching up to 44 %. Again, occurrences of microscopic glendonite are majorly observed in intervals of proximal deposition in the Upper Tithonian, lowermost and uppermost Berriasian, lower and Upper Valanginian, Lower Barremian and throughout the Aptian. Presence of micro-glendonite and majorly concurrent proximal deposition points to intervals of low sea level, which is interpreted as a result of glacioeustasy. Scarce occurrences of micro-glendonite in intervals with distal depositional settings are considered to reflect cool bottom water conditions in times of higher sea level. Micrometre-sized glendonite pseudomorphs after ikaite are therefore recorded from all studied areas along the North-South transect (central Chile, Patagonia, Antarctica), but their restriction to specific levels of the Tithonian to Aptian sediment succession provides strong evidence for episodic cooling. Until now glendonites of Late Jurassic–Early Cretaceous ages have only been known from northern high latitudes. The new data from Chile and Antarctica provide strong evidence for repeated small scale glaciations in the southern hemisphere. These cold episodes in the Upper Tithonian, lowermost Berriasian, Valanginian, lowermost Hauterivian, Lower Barremian, Aptian and Lower Albian punctuated the Early Cretaceous greenhouse climate in both the northern and southern hemispheres and must thus have been global events. |
