Celyphus rallus Batten en depocentros del Cretácico Temprano, flanco norte de la Cuenca San Jorge: significado estratigráfico y oleogenético

Viviana Barreda, Eduardo Bellosi, Guillermo Jalfin


The filamentous cyanobacteria Celyphus rallus (rivulariacean?) is reported from samples of D-129 well, type locality of D-129 Formation (interval 3059-3570 m), the main source rock of the basin. Palynologic assemblages suggest a Hauterivian to Barremian age for that section. However, based on greater changes in frequencies and minor ones in composition, two different associations could be identified. The Lower Association (3335-3570 m) dominated by C. rallus with subordinate Cyclusphaera psilata Volkheimer & Sepúlveda, Balmeiopsis limbatus (Balme) Archangelsky, Classopollis spp., Foraminisporis wonthaggiensis (Cookson & Dettmann) Dettmann and F. variornatus Archangelsky has probably developed in the Hauterivian. The high percentages of C. rallus allowed to correlate this interval with those of the Las Heras Group ("Neocomian") of the Western Flank of the basin. The Upper Association (3059-3320 m), with scarse C. rallus, relatively abundant Classopollis spp., C. psilata and B. limbatus is more probably Barremian in age and surely belongs to the D-129 Formation. Thus, the section which includes the palynologic Lower Association in the D-129 well, previously referred to the D-129 Formation, should be reassigned to the Las Heras Group. These deposits correspond to syntectonic fill of extensional half-grabens developed during the late rift stage of the basin. C. rallus is the dominant palynomorph in organic rich lacustrine mudstones. This facies was considered as a secondary source rock in the western flank of the basin. Recent findings extend their hydrocarbon generation capability to the eastern flank as well, raising the exploratory potential particularly in marginal areas where the D-129 Formation has low-quality organic facies or does not attain oil window maturities.

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