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Evidence for an Early-Middle Miocene Age of the Navidad Formation (Central Chile): Paleontological, Paleoclimatic and Tectonic Implications’ of Gutiérrez Et Al

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Evidence for an Early-Middle Miocene Age of the Navidad Formation (Central Chile): Paleontological, Paleoclimatic and Tectonic Implications’ of Gutiérrez Et Al Andean Geology ISSN: 0718-7092 [email protected] Servicio Nacional de Geología y Minería Chile Le Roux, Jacobus P.; Gutiérrez, Néstor M.; Hinojosa, Luis F.; Pedroza, Viviana; Becerra, Juan 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) Andean Geology, vol. 40, núm. 3, septiembre, 2013, pp. 580-588 Servicio Nacional de Geología y Minería Santiago, Chile Available in: http://www.redalyc.org/articulo.oa?id=173928393011 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative Andean Geology 40 (3): 580-588. September, 2013 Andean Geology doi: 10.5027/andgeoV40n3-a11 formerly Revista Geológica de Chile www.andeangeology.cl REPLY TO COMMENT 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) Jacobus P. Le Roux1*, Néstor M. Gutiérrez1, Luis F. Hinojosa2, Viviana Pedroza3, Juan Becerra1 1 Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile/Centro de Excelencia en Geotermia de los Andes (CEGA), Plaza Ercilla 803, Santiago [email protected]; [email protected]; [email protected] 2 Laboratorio de Paleoecología, Facultad de Ciencias-Instituto de Ecología y Biodiversidad (IEB), Universidad de Chile, Las Palmeras 3425, Santiago, Chile. [email protected] 3 Escuela de Ingeniería Geológica, Facultad Seccional Sogamoso, Universidad Pedagógica y Tecnológica de Colombia, Avenida Central del Norte 39-115, Boyacá, Colombia. [email protected] * Corresponding author: [email protected] We thank Finger et al. (2013) for their discussion Age of the Navidad Formation of our paper and appreciate their honesty in retracting the proposed Late Miocene-Pliocene age for the Finger et al. (2013) state that our assessment of Navidad Formation, due to misidentification of published data overwhelmingly favoring an Early to the foraminifer index species in question. In fact, Middle Miocene age ‘is misleading’, citing various although we suspected all along that these species may previous studies indicating the opposite. However, we have been misidentified, such an allegation seemed referred to the published data, not the interpretation improper to us in view of the recognized expertise of of this published data, and pointed out that almost Dr. Finger as a micro-paleontologist, whereas none all the species previously considered as occurring of us can claim such knowledge. That left us with only in Upper Miocene to Pliocene strata have in fact no other option but to suggest the earlier appearance been found elsewhere in Lower to Middle Miocene of these species in the southeastern Pacific Ocean successions. than elsewhere. Finger et al. (2013) summarize most of the pertinent information on the Navidad Molluscan biostratigraphy Formation mentioned in our paper, reiterating their interpretation of the depositional environment as Finger et al. (2013) contend that we ‘do not a middle bathyal (at least 1,500 m depth; Encinas, recognize that all benthic animals are controlled by 2006), continental slope in contrast to our proposal bottom facies that can be geographically diacronous’. of a continental shelf. While we agree on most of the Even though we are not invertebrate paleontologists, facts, it seems to us that Finger et al. (2013) have a we know enough about paleontology not to ignore tendency to turn a blind eye to the latter. this concept. Our Early to Middle Miocene age Le Roux et al. / Andean Geology 40 (3): 580-588, 2013 581 interpretation for the molluscs in the Navidad Formation was based mainly on Sr isotope data from various sources, including those of Encinas (2006), Nielsen and Glodny (2006), and our own (Gutiérrez, 2011), which all indicate this age range. The fact is that the latter agrees with that of the successions in Peru where the same species also occur. We do not agree that the molluscs have been transported from shallow to deeper water by turbidity currents, as indicated by their well-preserved, delicate ornamentation, articulated valves, the fact that the sediment matrix within their shells is the same as that of the surrounding host, and their clear lithofacies associations. In situ deposition is supported by the abundance of complete leaves and the presence of articulated crabs (Fig. 1). We are surprised that Finger et al. (2007) ignored such clear signals of in situ preservation and continue to do so in their discussion. In the sandstone package at Punta Perro, for example, many shallow water molluscs such as Turritella occur both dispersed throughout the succession and concentrated in lenses. At this locality and in the same sandstone package, the presence of wave ripples clearly indicates shallow water. Furthermore, to suggest that turbidity currents are ‘non-abrasive flows’ negates all we know about this phenomenon-velocities exceeding 120 km per hour and extreme turbulence capable of breaking telegraph cables (e.g., Fine et al., 2005) can hardly be described as ‘non-abrasive’. It would be hard to imagine delicate insect body parts (Fig. 2) surviving such a process. The occurrence of rocky shoreline mollusc species (Nielsen et al., 2004) in ‘deep FIG. 1. Unidentified, articulated crabs in fine-grained sandstones water sandstones’ would also require some turbidity of the Navidad Formation. currents to have originated at the shoreline and to have transported them across the continental shelf, which in view of the typical low gradient of the latter were displaced into deeper water. Neither do we (0.1°) is unlikely. see why their preservation in fossil-rich lenses must A ‘somewhat chalky surface texture’ in some indicate long distances of transport, because even molluscs is a very weak argument against their in small, local channels, common for example at Punta situ occurrence, as it completely ignores modern Perro, can reconcentrate organisms. weathering processes. The proposal of Finger et al. (2007) that these molluscs were reworked from older Strontium isotope stratigraphy rocks is no longer valid, and if they were eroded from contemporaneous sediments on the continental shelf We cited Nielsen and Glodny (2006) on p. 69 of and transported by turbidity currents into deeper water our paper, in which they reported 21 Sr ages ranging (within at most a few hours), this would definitely not between 24.3-16.1 Ma. Their 2009 publication did change a fresh surface into a chalky texture. It would not contribute any new, contradicting data. As therefore make no difference whether they occurred concerns the problem with the stratigraphic order, in situ on the shelf (which is what we maintain) or we acknowledge that small discrepancies are present 582 REPLY TO COMMENT OF FINGER ET AL. (2013) FIG. 2. Delicate insect body parts, including head, thorax, and wings, preserved in the Navidad Formation. between individual samples, as also shown in our and transported to be deposited as discrete beds Table 3. However, it should be kept in mind that in the Navidad Formation, thus reflecting the true after-death recrystallization from aragonite to calcite, age of the latter. Finger et al. (2013) seem to have especially prominent in gastropod shells, might misunderstood this point, as they originally argued alter the obtained Sr ages somewhat. Nevertheless, that the volcanic material was eroded from older, all the Sr ages in our lower member are within the Middle Miocene strata, not we. range 22.1-16 Ma and there is a general decrease in age up the column, which is consistent with a Ostracods and pollen decrease in the 40Ar/39Ar ages from 26.1-18.9 Ma. The upper member also yielded a stratigraphically With the exception of one ostracod, Bradleya consistent 40Ar/39Ar age of 12 Ma. Although the Sr normani, all the identified diatoms and ostracods have dates may not be precise due to neomorphism, we also been found in Lower and/or Middle Miocene do not consider this to be a major problem in the strata. Bradleya normani is a very rare species, so light of the overwhelmingly Early Miocene ages for it is not surprising that it had not yet been found in the lower member and Middle Miocene age for the older strata. In fact, the Navidad Formation is the upper member, especially now that this has been first such occurrence, as even Finger et al. (2013) confirmed by a reinterpretation of the foraminifer now concede to its Early to Middle Miocene age. ranges. Planktic foraminifers Radiometric dating Finger et al. (2007) dismiss the hypothesis that Our argument concerning Middle Miocene five index species of foraminifers occurred in the scoria ages from the Navidad Formation was that southeastern part of the Pacific before elsewhere in this material reflects contemporaneous volcanism, the global ocean ‘without hesitation’. Before 1938, where ash covering the landscape around volcanoes they would have said the same if we had discovered after an eruption would be immediately eroded a coelacanth at Punta Perro. Le Roux et al. / Andean Geology 40 (3): 580-588, 2013 583 We all know very well that micropaleontological deposition contemporary with volcanic activity. dating largely stands upon its own shoulders. A On the other hand, if marine deposition started look at classic biostratigraphic charts (e.g., that during the late Miocene, the provenance results reproduced in Encinas, 2006) shows that 3 index show a retrograde erosive response to landscape species, including Neogloboquadrina acostaensis, for a regional uplift event proposed for that period appeared simultaneously and globally at 10.9 Ma.
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