Abstract
Distinctive sedimentological and micropalaeontological responses characterise the basal interval of the Agrio Formation (Valanginian) in central-western Argentina, which is associated to a transgressive event. The effect of the sea-level rise is interpreted from the record of abundant radiolarians and the widespread marlstone and limestone deposits associated with eutrophication of surface waters and development of low-oxygen conditions on the sea-floor. Such a situation accords with pyrite and organic matter rich laminated sediments, as well as with a monotypic foraminiferal assemblage of Epistomina. Bi/plano-convex trochospiral test, epifaunal/semi-epifaunal microhabitat, and deposit-feeding trophic strategy are favourable features for adaptation of this genus to oxygen depressed sea-bottom conditions. The large size of the test could be related to chemolithotrophic bacterial symbionts. Epistomina loncochensis nov. sp. BALLENT is described.
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