Stratigraphic implications of latest middle Miocene to earliest late Miocene diatoms in the Navidad Formation at Lo Abarca, central Chile (33° 30’S)

摘要

Neogene marine strata crop out at various localities along the Chilean coastline (see Encinas et al., 2008 and references therein). The best studied of these successions occurs in the Navidad area, between San Antonio and Punta Topocalma (33°30’- 34°30’S) (figure 1). Named and described by Darwin (1846) as the Navidad Formation, the geology and paleontology of this unit was subsequently studied by many other authors (e.g., Philippi, 1887; Bruggen, 1950; García, 1968; Tavera, 1979) and has been considered the stratigraphic reference for the marine Neogene of Chile (e.g., Cecioni, 1980; De Vries and Frassinetti, 2003). However, its age, stratigraphy and depositional environment were a matter of debate for several decades (e.g., Brüggen, 1950; García, 1968; Tavera, 1979; Encinas et al., 2006). Recent foraminiferal and sedimentologic studies indicate a late Miocene to early Pliocene age and a deep-marine setting for this unit (Finger et al., 2007; Encinas et al., 2008). Yet, the older latest Oligocene to middle Miocene age proposed for the molluscan fauna of this unit (De Vries and Frassinetti, 2003) still constitutes an unresolved problem (see below).