Sedimentology and paleontology of the Oligocene-Miocene transition in the Lago Cardiel area, Santa Cruz

RODRIGUEZ RAISING, MARTÍN; GRIFFIN, MIGUEL; CASADIO, SILVIO

Córdoba

Congreso; Congreso Argentino de Paleontología y Bioestratigrafía; 2006

Academia Nacional de Ciencias-Cipal

The Río Leona and Centinela formations are a continuous sequence of fluvial and marine rocks of late Oligocene-early Miocene age. These rocks are exposed at many localities in western Santa Cruz. The aim of this contribution is to describe the sedimentary paleoenvironments and the marine invertebrates recorded in outcrops of these units located West of Lago Cardiel (S 49º 04’ 41.8’’; W 71º 03’ 22.1’’). The base of the section includes coarse sandstone and massive coal-bearing siltstones at the top of the Río Leona Formation. Facies analysis allowed recognition of floodplains and marshplains predominating over fluvial channels. This would be related to an increase of the accommodation rate and the onset of the Patagonian sea transgression. The deposits of the Río Leona Formation gradually give way to the marine siltstones of the Centinela Formation. The latter carry the following invertebrate fauna: Chaceon peruvianus (d’Orbigny 1842); Terebratulidae indet.; Dentalium sp.;  Iheringinucula tricesima (Ihering, 1897), Australoneilo sp.; Scaeoleda? ortmanni (Ihering, 1907); Arca patagonica Ihering, 1897; Glycymeris sp; Pteromyrtea crucialis (Ihering, 1907); Elliptotellina patagonica (Ihering, 1897); Valdesia n. sp. ; “Turritella” sp.; Calyptraea sp.; Naticidae indet.; Proscaphella dorbignyana (Philippi, 1887), and Kaitoa patagonica (Ihering, 1897). Overlying this facies are beds with heterolithic lamination followed by reefs of Crassostrea? hatcheri (Ortmann, 1897) developed in an inner shelf environment. At the top of the section there is evidence of shallowing conditions given by the presence of deposits of bedform migration such as dunes in a sandy subtidal plain. These deposits are overlain by medium- to fine-grained sandstones with an abundant marine mollusc fauna that suggests a new deepening event. The invertebrates recorded in these beds include: Flabellum sp.; Terebratulidae indet.; Solariella dautzenbergi Cossmann, 1899; Valdesia dalli (Ihering, 1897); Juliania sp.; Bittium sp.; “Cerithiopsis” juliana Ihering, 1907; Seila? sp.;  “Turritella” sp.; Cirsotrema sp.; Struthiolarella patagoniensis Camacho y Zinsmeister, 1989; Polinices santacruzensis Ihering, 1907; Oamaruia sp.; Sveltella? gracilis (Ihering, 1897); Brocchinia sp.; Zeadmete ameghinoi (Ihering, 1897); Austroimbricaria quemadensis (Ihering, 1897); Psudofax sp.; Xymenella dautzenbergi (Ihering, 1897); Fuegotrophon sp.; Crassilabrum pyriforme (Ihering, 1897); Terebra quemadensis Ihering, 1897; Inquisitor sp.; Gemmula sp.; Splendrillia santacruzensis (Ortmann, 1900); Austrotoma quemadensis (Ihering, 1897); Neoguraleus sp.; Acteon sp.; Zygochlamys geminata (Sowerby, 1846); Jorgechlamys proxima (Sowerby, 1846); Pleuromeris cannada (Ihering, 1907); Pholadidea patagonica (Philippi, 1887); Balanidae indet.; and spines of Echinoidea indet. This assemblage reveals clear affinities with the fauna recorded in the Monte León Formation, whether in its type area (mouth of the Santa Cruz River and Monte León) or in other localities of southern Patagonia (i.e. Cañadón de los Artilleros).