COMMENT Comment on Reply to Comment of Finger Et Al. (2013) On
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Andean Geology 41 (3): 639-656. September, 2014 Andean Geology doi: 10.5027/andgeoV40n2-a07 formerly Revista Geológica de Chile www.andeangeology.cl COMMENT Comment on Reply to Comment of Finger et al. (2013) on: ‘Evidence for an Early-Middle Miocene age of the Navidad Formation (central Chile): Paleontological, paleoclimatic and tectonic implications’ of Gutiérrez et al. (2013, Andean Geology 40 (1): 66-78) Alfonso Encinas1, Kenneth L. Finger2, Sven N. Nielsen3, Ximena Contardo4 1 Departamento de Ciencias de la Tierra, Universidad de Concepción, Casilla 160-C, Concepción, Chile. [email protected] 2 University of California Museum of Paleontology, 1101 Valley Life Sciences Building, Berkeley, CA 94720-4780, USA. [email protected] 3 Instituto de Ciencias Ambientales y Evolutivas, Universidad Austral de Chile, Casilla 567, Valdivia, Chile. [email protected] 4 Universidad Andrés Bello, Facultad de Ingeniería, Geología, Quillota 980, Viña del Mar, Chile. [email protected] In their answer to our Comment (Finger et al., discipline plays vital roles in geologic correlation 2013), Le Roux et al. (2013) misunderstand several and our understanding of earth history. of our remarks and present what we view as flawed We agree with some of the problems encoun- arguments, principally their case for a shallow-marine tered with biostratigraphy that were pointed out by environment for part of the Navidad Formation. Le Roux et al. (2013). Pitfalls of this science have We do not wish to see this exchange evolve into an been recognized for a long time, and they have endless discussion, but we feel obligated to clarify been described in detail in several texts (e.g., Miall, some points. We think this is necessary because of 1999). One of the problems is that a fossil species history and importance of the Navidad Formation is recognized according to its original description, as the reference for the marine Miocene of Chile. which is typically of the specimen designated and Here we also expound upon some concepts relevant illustrated as its holotype. Every paleontologist to the distinction between shallow-and deep-marine recognizes the subjectivity involved in identifying environments. a form that differs slightly from the holotype they most closely resemble. As with most uncertainties 1. Age of the Navidad Formation stemming from using a single approach to decipher a complex problem, they can often be recognized Finger et al. (2013) agrees with Le Roux et al. and possibly resolved when they are part of a mul- (2013) that at least part of the Navidad Formation tidisciplinary approach, and as relevant empirical was deposited during the Early Miocene, although data becomes available. For instance, imagining an we have some doubt about the reliability of younger example similar to that of the coelacanth presented radiometric ages derived from pumice clasts because by Le Roux et al. (2013), no geologist would give the isotopic signature could have been slightly altered credit to a Sr or K-Ar Miocene date obtained from a during transport (Finger et al., 2013). The negative sedimentary succession containing fossils of rudists in opinion of biostratigraphy expressed by Le Roux et living position, ammonites, and plesiosaurs because al. (2013), however, must be addressed because the an immense number of observations indicate that 640 COMMENT ON REPLY TO COMMENT OF FINGER ET AL. (2013 )... those taxa went extinct in the Cretaceous-Palogene (2013), principally the ages that defy the stratigraphic boundary. The results of other age-dating methods order of the samples (e.g., one of them being 3.3 My can orient the paleontologist toward published older than that from the underlying bed; figure 3 in faunas of the indicated age, which is particularly Gutiérrez et al., 2013). However, they contradict helpful in identifying species that may be difficult themselves when they admit that Gutiérrez et al. to distinguish among the collected specimens. In the (2013) did not mention the Sr date of 12.1±0.7 Ma (in case of the Navidad Formation, different methods Encinas, 2006), which was obtained from specimens produced inconsistent results that indicated Early, thought to be Neogloboquadrina acostaensis (Finger Middle, and Late Miocene or younger ages (Finger et al., 2007), because it was too old, even though the et al., 2013 and references therein), with all previous maximum global age for this species (10.9 Ma) is studies on the planktic foraminifera having identi- only 0.5 My younger. Their argument is even more fied species indicative of Late Miocene or younger illogical because they had considered that this and ages (Dremel, in Herm, 1969; Martínez-Pardo and other Navidad species of planktic foraminifers had Osorio, 1964; Cecioni, 1970; Ibaraki, 1992; Finger first appeared in the South East Pacific in the Early et al., 2007). Miocene (Gutiérrez et al., 2013). The global framework for planktic foraminiferal In summary, no geologic dating method is im- biostratigraphy, on the other hand, is based on ex- mune to pitfalls that can result in inaccurate age tensive data gathered over several decades from a determinations. Sr dates must be taken with caution countless number of outcrop and borehole sections, as diagenesis can modify the original 87Sr/86Sr ratio many millions of specimens, and integration with (DePaolo, 1986), which is a likely explanation for other microfossil biostratigraphies (e.g., calcareous the aberrant ages of 12.1±0.7 and 31.5±0.6 reported nannoplankton), magnetostratigraphy, and the absolute by Encinas (2006). The use of a scanning electron time scale. Those sequences, especially deep-sea cores, microscopy (SEM) to recognize diagenetic alteration, are often rich in planktic foraminifers, continuous, and additional means of age-dating are advisable. and span multiple ages, which is in stark contrast to the Navidad material. For these reasons, Finger 2. Sedimentary environment of the Navidad et al. (2013) dismissed the proposition by Gutiérrez Formation et al. (2013) that the planktic foraminifera reported by Finger et al. (2007) appeared 10 to 20 Myr Le Roux et al. (2013) discuss old and new data earlier in the Southeast Pacific than elsewhere in in favor of a shallow-marine environment for at least the global ocean. part of the Navidad Formation. Their arguments are Chronostratigraphic dating based on isotopic ratios based principally on the presence of shallow-marine is not immune to problematic results. For example, invertebrates and taphonomy. They also provide Wyss et al. (1996) emphasizes the problems of some new sedimentological data to support their overlapping radiometric ages between the Abanico interpretation. and Farellones formations, whereas Flynn et al. (2008) noted that ‘the Cura-Mallín and Trapa Trapa 2.1. Taphonomy formations radiometrically dated at Laguna del Laja are substantially younger (by more than ~10 million The most important arguments employed by and ~5-10 Myr, respectively) than the same formations Le Roux et al. (2013) to support a shallow marine in Argentina’. Le Roux et al. (2004) dated the marine environment for the Navidad Formation are derived Coquimbo Formation as Miocene to Pliocene and from taphonomic analysis, most notably the good noted some problems derived from the Sr ages. For preservation of some of the marine invertebrates and example, they obtained Sr dates of 7.3, 5.4, and 5.2 terrestrial leaves and insects. They interpret those Ma from the same bed and concluded that the older specimens as in situ and counter our interpretation age (7.3 Ma) must belong to a reworked fossil, which of displacement by turbidity currents by claiming also exemplifies that age-related assumptions are not that they are not conductive to good preservation exclusive to biostratigraphic studies. In the case of the because they have velocities ‘exceeding 120 km per Navidad Formation, Le Roux et al. (2013) recognize hour and extreme turbulence capable of breaking the problems with Sr dating indicated by Finger et al. telegraph cables’. Encinas et al. / Andean Geology 41 (3): 639-656, 2014 641 Not all of the Navidad Formation was deposited figure 3; this work, figure 1) all show abundant in deep water, as this unit includes some shallow- and well-preserved leaves but the Gutiérrez et al. marine facies (Encinas et al., 2008) and we have (2013) scheme has the Matanzas flora at the base doubts on our previous interpretation of some of of the Navidad Formation, whereas the other two the sections of this unit as deep-water sedimentary correspond to the upper part of this unit, which ac- facies (Finger et al., 2013). Regardless, we consider cording to these authors was deposited at the top of the taphonomic arguments of Le Roux et al. (2013) a deepening-upward succession. More importantly, ungrounded, as studies have reported well-preserved as noted by Finger et al. (2013) but not mentioned displaced fossils in gravity-flow deposits, such as by Le Roux et al. (2013), the leaf-rich successions molluscs (Walker, 2001), and even delicate leaves of the Navidad Formation include sedimentary fa- (Zavala et al., 2012), insects (Gaudant and Busquets, cies, ichnofacies, and microfossils typical of deep- 1996), and pterosaur wing bones (Hilton, 2003). Also, marine environments (Figs. 1 and 2). We therefore relatively slow turbidity current velocities of 3.6 to interpret the Navidad Formation leaf-bearing strata 10.8 km/h have been recorded, such as on the Amazon as deposited by E turbidity currents (sensu Zavala et Fan (Pirmez et al., 2000), and high-speed flows that al., 2012) that originated from hyperpycnical flows. can snap submarine cables are exceptional (e.g., Fine Decapod crustaceans inhabit all ocean depths et al., 2005). Sediment can also be transported to (Feldmann et al., 1991); thus, the presence of un- deep-water by other kinds of gravity flows, such as identified, articulated crabs in strata of the Navidad debris or sandy debris flows where grain movement Formation is not necessarily indicative of shallow- is minimum (e.g., Shanmungan, 2000).