Navidad Formation

Charles Darwin saw the formation in September 1834 during the second voyage of HMS Beagle. He became the first to describe it 1846 when he published his book Geological Observations on South America in 1846 and it was named by Darwin after the nearby town of Navidad. In this book Darwin calls the formation "Formation of Navidad" and "Sandstone Formation at Navidad". There are no signs of that Darwin would have attempted to make a formal definition of the formation. Juan Tavera further narrowed the age of the formation in 1968 and 1979 by proposing a Burdigalian (Lower Miocene) age for Navidad, Lincancheo and Rapel which were then the three subunits of Navidad Formation. Tavera's 1979 subdivision scheme for Navidad Formation remained popular until it was superseded in 2006 by a new one. == Outcrops and surface morphology ==

Navidad Formation is located in the Chilean Coast Range in Central Chile at the latitudes of 33°00' S–34°30' S. The coastal exposures extends from the vicinities of San Antonio in the north to Boca Pupuya in the south over a length of 16 km. Morphologically the coastal zone near the formation has been divided into four distinct zones: a coastal platform of rocky outcrops and sand beaches, marine terraces dipping gently towards the sea, dissected coastal plain at the mouth of Rapel River and fluvial terraces in the northern and southern shores of Rapel River. ==Stratigraphy and lithology==

The formation rests above both an Upper Cretaceous formation called Punta Tocopalma Formation and the Paleozoic granitic basement. The formation has a thickness of 100 to 200 meters. The sediments of the formation include detrital pyroxene, amphibole, garnet, zircon and other heavy minerals. Pyroxene is the most common heavy mineral in the lower sections of Navidad Formation. Part of the sediments are believed to have originated from the basement of the Chilean Coast Range based on the affinities of garnet with the coastal lithologies. Analysis of amphiboles and pyroxenes have led to the conclusion that they and other sediments originate from volcanic and subvolcanic rocks from the Central Valley and the Andean Cordillera. Over-all three erosion-and-deposition events have been distinguished in Navidad Formation. == Fossil record ==

The formation contains a great variety of fossils. Among the macrofossils there are remnants from sharks, crustaceans and gastropods. Among microfossils there are ostracodes and foraminifera. In addition there are fossils of leaves and pollen. Crustaceans Fossils of the marine crab genera Cancer, Hepatus, Pilumnus, Pinnotheres, Trichopeltarion, Callianassa, Pinnixa and Proterocarcinus have been reported from Navidad Formation. Navidad Formation host two of the first fossil crab species to be described from Chile: Cancer tyros and Pinnotheres promaucanus both described by Rodolfo Amando Philippi in 1887. Notably, the type specimen of Pinnotheres promaucanus has presumably been lost at the Chilean National Museum of Natural History. Molluscs Gastropod shells are the most common macrofossils of Navidad Formation. The mollusc fossil fauna of Navidad Formation is remarkably similar fossil faunas of the same age found in Peru. Some of the gastropod species found in Navidad Formation are Miltha vidali, Acanthina katzi, Olivancillaria claneophila, Testallium cepa, Ficus distans, Eucrassatella ponderosa, Glycymeris ibariformis and Glycymeris colchaguensis. Charcoal found together with pumice is thought to indicate that wildfires ignited by volcanic eruptions were common on land where Mediterranean climate prevailed in the Miocene as well as today. Trace fossils The trace fossils representing the ichnogenera of Zoophycos isp. and Chondrites isp. can be found in Navidad Formation. == Scientific controversy ==

Over the years different age estimates for Navidad Formation have been proposed and led to a substantial debate. In 2013 Gutiérrez et al. published an article in Andean Geology claiming an Early to Middle Miocene age for Navidad Formation. Further, Gutiérrez et al. (2013) disagreed with Encina et al.'s (2008b) suggestion of a deep marine depositional environment for Navidad Formation proposing a shallow marine environment instead. == See also ==