Unfortunately, they did not cite any published x-ray diffraction analyses, clay-mineral crystallinity indices, or more extensive documentation to support their claims. ( 1) argue that their single petrographic thin section of the above-mentioned sample does not show clay recrystallization or neoformation of metamorphic minerals. None of our ages have been questioned in Gaucher et al. This is one of the many examples we have previously illustrated ( 2) where our conclusions support that the igneous intrusion (our 585-million-year-old granite) is younger than the sedimentary rock (the 600-million-year-old trace-bearing strata). The same process is seen in the Tacuarí diamictites where clasts of granite have been deeply weathered when exposed to surface conditions. As in (A), feldspars in the granite have been partially decomposed to kaolinite due to recent weathering. The contact between the granite and the Tacuarí Formation is traced in red. Notice the irregular and sometimes knife-sharp contact oriented perpendicular to sedimentary strata and the deformation (concave-upwards) produced by the intrusion. (F) Well-exposed outcrop, not discussed in the comment ( 1), showing clear intrusive features of the granite into the Tacuarí rhythmites. (E) Microphotograph of the contact illustrating the development of chilled margins. Notice in all photos the sacaroid texture due to recrystallization and silicification, and the cross-cutting relationship with the rhythmites. (B to D) show detailed views of the contacts. (A) General view of the outcrop where the rhythmites have been intruded in two different and almost perpendicular directions. 1 Contact relationship between the intrusive granite and fossil-bearing strata (rhythmites) of the Tacuarí Formation. Furthermore, fossil site C is characterized by a discordant contact between the Tacuarí strata and the intrusive granite and not what is shown by Gaucher et al. We do not want to speculate on the origin of this sample, but we see no evidence that it comes from fossil site C it is not the ferruginized basal sandstone we previously documented. ( 1) take no notice of the outcrop-scale relationships and instead prefer to show five photographs from just one hand sample that they assigned to fossil site C to discredit the intrusive nature of the granite. The contact near fossil site C similarly shows the cross-cutting relationship. Indeed, in this outcrop, the rhythmites have actually been intruded in two different directions ( Fig. ( 1) provide a map at scale 1:500,000 to illustrate their point that what separates the Tacuarí Formation from the granite is a fault contact, while ignoring our figure S8 ( 2), which clearly shows an intrusive relationship. ( 2) (figure S1), at a scale of 1:20,000, is criticized for not being illustrative enough of the intrusive relationship between the granite and the Tacuarí strata. This is an idea that intuitively makes sense.First, the geological map presented in Pecoits et al. In the most common commonest expression of this concept, when we are digging in an archaeological site, we generally can assume that as we dig deeper, we encounter ever older layers. DIAGRAM OF UNDISTURBED STRATA YOUNGEST ROCK SERIESThe principles that he developed can be summarized as follows:ġ) In an undisturbed series of layers of sediment, the oldest layers are at the bottom, and the youngest are at the top.Ģ) Anything that is in an undisturbed layer cannot be older than the material that the layer is made from.ģ) Identifiable layers that are interrupted by a cut are assumed to be identical.Ĥ) Layers that intrude into other layers are younger.Īrchaeologists use these simple principles to interpret the sequence of events in archaeological sites. Using this information, he later produced the first known geological map. While working as a surveyor in coal mines and canals, he noticed that there appeared to be a predictable relationship between the various layers of rock, even when he moved from place to place across the country. In this article I will look a bit deeper into the subject to show how we do it.Īn amateur British geologist, William “Strata” Smith, revolutionized the study of stratigraphy in the late 18th-early 19th Century. Previous Dig It articles, including #8 (Stratigraphy), #20 (The Importance of Context), and #24 (Relative Dating), discussed how important the study of stratigraphy (the layering of sediments) is in interpreting archaeological sites.
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