Navidad Formation

Charles Darwin saw the formation in September 1834 during the second voyage of HMS Beagle.[3] 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.[2] 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.[4]

...the next point at which I landed was at Navidad, 160 miles north of Concepcion, and 60 miles south of Valparaiso. The cliffs here are about 800 feet in height : they consist, wherever I could examine them, of fine-grained, yellowish, earthy sandstones, with ferruginous veins, and with concretions of hard calcareous sandstone. [...] The sandstone contains fragments of wood, either in the state of lignite or partially silicified, shark's teeth, and shells in great abundance...

— Charles Darwin[5]

Early fossil descriptions from Navidad Formation were those of George Sowerby in Geological Observations on South America (1846) and by Rodolfo Amando Philippi (1887).[4]

Gustav Steinmann redefined the Navidad Formation in 1895, then called Piso Navidad, by giving it a Lower Tertiary age and spanning much of south-central Chile. In 1934 Juan Brüggen separated Piso Concepción from Steinmanns Piso Navidad after showing there was a discordance between them.[4] Humberto Fuenzalida published research on fossils of the formation in 1950–1951.[6]

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 Tavera's 1979 subdivision scheme for Navidad Formation remained popular until it was superseded in 2006 by a new one.[4]

Southward view towards the mouth of Rapel River. Navidad outcrops can be seen above the beach in the right of the image.

Navidad Formation is located in the Chilean Coast Range in Central Chile at the latitudes of 33°00' S–34°30' S.[7] The formation crops out more-less continuously along coastal bluffs displaying well-preserved exposures.[1] 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.[7] 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.[8]

The formation rests above both an Upper Cretaceous formation called Punta Tocopalma Formation and the Paleozoic granitic basement.[9] It lies below Licancheu Formation to which it has a concordant contact to.[4] The formation has a thickness of 100 to 200 meters.[9] The sedimentary rocks that make up the formation includes a basal conglomerate stratigraphically followed by intercalated sandstone and siltstone with smaller occurrences of conglomerate and coquina.[7]

There are more than one subdivision scheme for the stratigraphy of the formation. A scheme made by Juan Tavera in 1979 that was by 2005 the most used compromises tree members Navidad, Licancheo and Rapel.[7] A new scheme proposed in 2006 elevated Tavera's sub-units Licancheo (renamed Licancheu) and Rapel to formations leaving the new definition of Navidad Formation as the former sub-unit Navidad. The rationale for subdividing Tavera's Navidad Formation was that the sub-units were separated by regional discontinuities.[4]

The stratotype of the formation is the coastal bluff west of Punta Perro. The stratopype does not coincide with the original description by Darwin.[10]

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.[11]

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.[2] The teeth of the shark Odontaspis ferox have been identified in the formation.[12]

Over the years different age estimates for Navidad Formation have been proposed and led to a substantial debate.[2][17]

Based on a biostratrigraphic analysis Encinas et al. (2008b) suggested a Tortonian to Zanclean (Late Miocene–Early Pliocene) age for Navidad Formation. They further suggested that Oligocene–Early Miocene shark teeth that occur in basal conglomerate are reworked material originating at another formation that re–sedimented in Navidad Formation. The formation was interpreted as including both deep marine and shallow marine depositional environments.[9]

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.[2] This prompted a response from Encina and his associates (Finger et al., 2013) contesting elements of these claims but stressing it is true that some of the formation is of shallow marine origin and that at least part of the formation is of Early Miocene origin.[17][18] In their comment Finger et al. (2013) revealed that previous identification of foraminera was erroneous as was also the Late Miocene–Early Pleistocene age estimate based on incorrect identification of foraminifera.[18] Gutiérrez and his associates defended their findings in a reply in 2013[19] and the exchange continued in 2014.[18][20]