PALYNOLOGY AND MICROPALEONTOLOGY OF THE SAN JULIAN AND MONTE LEON FORMATIONS (OLIGOCENE - EARLY MIOCENE), CABO CURIOSO SUBSURFACE, SANTA CRUZ PROVINCE, ARGENTINA

摘要

PALYNOLOGY AND MICROPALEONTOLOGY OF THE SAN JULIAN AND MONTE LEON FORMATIONS (OLIGOCENE - EARLY MIOCENE), CABO CURIOS SUBSURFACE, SANTA CRUZ PROVINCE, ARGENTINA. The CC-3 and CC-4 boreholes drilled a Cenozoic sedimentary column integrated by the Monte Le On and San Julian formations, similar to that cropping out in the Gran Bajo de San Julian. The palynology and micropaleontology of both boreholes are analyzed, particularly focused on the "basal muddy section" of the San Julian Formation. The palynomorphs of this section are of continental origin, reflecting a regional forest dominated by Nothofagaceae, Myrtaceae, Podocarpaceae and Palmae, developed under a temperate to warm-temperate and humid climate. The palynomorphs from the carbonaceous claystones of the upper part of this section, dominated by Poaceae, some Podocarpaceae and the presence of fresh-water algae, mainly reflect the local vegetation associated with fresh to brackish-water swamps. The foraminifera, recovered from only two horizons from the middle part of the "basal muddy section", scarce and poorly preserved, suggest a marginal marine paleoenvironment, and resemble those described from the Gran Bajo Member of the San Julian Formation; the occurrence of Boltovskoyella patagonica Malumian and Masiuk, suggests affinity with the assemblage of the late middle Eocene Man Aike Formation as well, but specimens are rare and tentatively identified. An Oligocene age and a paralic paleoenvironment is estimated for the "basal muddy section" in the CC-3 and CC-4 boreholes. The upper section of the San Julian Formation (late Oligocene) yielded an inner shelf foraminiferal assemblage, similar to that described for the Meseta Chica Member of this Formation. The foraminiferal assemblage from the base of the overlying Monte Leon Formation (early Miocene), mainly composed of agglutinated species, and the presence of dinoflagellate cysts suggest more open marine conditions, probably a middle to outer shelf setting, reflecting the transgressive character of this Formation. The abrupt change and dissimilitude between the foraminiferal assemblages of the San Julian and Monte Leon formations suggest an unconformable relationship between these two units.