Impact cratering: The South American record – Part 1

The circa 14 km diameter Pantasma circular structure in Oligocene volcanic rocks in Nicaragua is here studied for the first time to understand its origin. Geomorphology, field mapping, and petrographic and geochemical investigations all are consistent with an impact origin for the Pantasma structure. Observations supporting an impact origin include outward‐dipping volcanic flows, the presence of former melt‐bearing polymict breccia, impact glass (with lechatelierite and low H2O, <300 ppm), and also a possible ejecta layer containing Paleozoic rocks which originated from hundreds of meters below the surface. Diagnostic evidence for impact is provided by detection in impact glass of the former presence of reidite in granular zircon as well as coesite, and extraterrestrial ε54Cr value in polymict breccia. Two 40Ar/39Ar plateau ages with a combined weighted mean age of 815 ± 11 ka (2 σ; P = 0.17) were obtained on impact glass. This age is consistent with geomorphological data and erosion modeling, which all suggest a rather young crater. Pantasma is only the fourth exposed crater >10 km found in the Americas south of N30 latitude, and provides further evidence that a significant number of impact craters may remain to be discovered in Central and South America.

Section: Introductionmentioning

confidence: 99%

Pantasma: Evidence for a Pleistocene circa 14 km diameter impact crater in Nicaragua

Rochette

Alac

Beck

et al. 2019

“…In addition, Cr osta et al (2010) pointed to similarities between these deformed rocks and the equivalent, shocked lithologies that occur at the Vargeão impact structure, located some 500 km to the north. Vargeão is also formed in basalt and sandstone of the same lithological units (Cr osta et al 2019).…”

Section: Introductionmentioning

confidence: 99%

Shock deformation confirms the impact origin for the Cerro do Jarau, Rio Grande do Sul, Brazil, structure

Reimold

Crósta

Hasch

et al. 2018

Cerro do Jarau is a conspicuous, circular morpho‐structural feature in Rio Grande do Sul State (Brazil), with a central elevated core in the otherwise flat “Pampas” terrain typical for the border regions between Brazil and Uruguay. The structure has a diameter of approximately 13.5 km. It is centered at 30o12′S and 56o32′W and was formed on basaltic flows of the Cretaceous Serra Geral Formation, which is part of the Paraná‐Etendeka Large Igneous Province (LIP), and in sandstones of the Botucatu and Guará formations. The structure was first spotted on aerial photographs in the 1960s. Ever since, its origin has been debated, sometimes in terms of an endogenous (igneous) origin, sometimes as the result of an exogenous (meteorite impact) event. In recent years, a number of studies have been conducted in order to investigate its nature and origin. Although the results have indicated a possible impact origin, no conclusive evidence could be produced. The interpretation of an impact origin was mostly based on the morphological characteristics of the structure; geophysical data; as well as the occurrence of different breccia types; extensive deformation/silicification of the rocks within the structure, in particular the sandstones; and also on the widespread occurrence of low‐pressure deformation features, including some planar fractures (PFs). A detailed optical microscopic analysis of samples collected during a number of field campaigns since 2007 resulted in the disclosure of a large number of quartz grains from sandstone and monomict arenite breccia from the central part of the structure with PFs and feather features (FFs), as well as a number of quartz grains exhibiting planar deformation features (PDFs). While most of these latter grains only carry a single set of PDFs, we have observed several with two sets, and one grain with three sets of PDFs. Consequently, we here propose Cerro do Jarau as the seventh confirmed impact structure in Brazil. Cerro do Jarau, together with Vargeão Dome (Santa Catalina state) and Vista Alegre (Paraná State), is one of very few impact structures on Earth formed in basaltic rocks.

“…Its impact origin was first proposed by Cr osta et al (2004), based on the occurrence of polymict lithic breccia bearing shatter cone fragments in the interior of the structure. This initial suggestion was then enhanced by Cr osta et al (2010a) with the discovery of abundant aggregates of shatter cones and microscopic shock features (see also Cr osta et al 2019). The structure was formed in Cretaceous basalts of the Serra Geral Formation, within the Phanerozoic Paran a Basin.…”

Section: Introductionmentioning

confidence: 93%

“…Vista Alegre was described by Cr osta et al (2010aCr osta et al ( , 2012Cr osta et al ( , 2019 as a complex impact structure, although a central elevation or peak, as commonly seen in complex structures, is not evident. This interpretation was based on the anomalous presence of sandstones in the innermost 3 km wide portion of the structure.…”

Section: Introductionmentioning

confidence: 99%

“…The structure was formed in Cretaceous basalts of the Serra Geral Formation, within the Phanerozoic Paran a Basin. Two other impact structures were formed in the same stratigraphic unit in southern Brazil: Vargeão (Cr osta et al 2006(Cr osta et al , 2019 and Cerro do Jarau (Cr osta et al 2010b(Cr osta et al , 2019Reimold et al 2019). Impact craters formed in basaltic targets are rare on Earth and include, together with these structures in Brazil, only one other example: Lonar crater in India (e.g., Fredriksson et al 1973;Fudali et al 1980;Osae et al 2005;Maloof et al 2010).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Insights about the formation of a complex impact structure formed in basalt from numerical modeling: The Vista Alegre structure, southern Brazil

Vasconcelos

Rocha

Crósta

et al. 2019

We present the outcomes of simulations of the formation of the Vista Alegre impact structure, Paraná Basin, Brazil. The target comprised a thick sequence of volcanic rocks of predominantly basaltic composition of the Serra Geral Formation that had been deposited on top of sedimentary rocks (sandstones) of the Pirambóia/Botucatu formations. The cratering process was modeled using the iSALE shock physics code. Our best‐fit model suggests that (1) the crater was originally ~10 km in size; (2) it was formed in ~115 s by a stony projectile of 1000 m in diameter, for an assumed impact velocity of 12 km s−1; (3) target rocks underwent a peak pressure of ~20 GPa, in agreement with previous petrographic investigations of shock deformation. Furthermore, the model points out that the sedimentary strata below the layer of volcanic rocks were raised by ~650 meters at the central part of the crater, which resulted in the current partial exposure of the sandstones at the surface. The outcomes of our modeling suggest that parameters like cohesion and strength of the target rocks, after shock compression, determined the final morphology of the crater, especially the absence of a topographically prominent central peak. Finally, the results of the numerical modeling are roughly in agreement with gravity data over the structure, in particular with respect to the presence of the uplifted sedimentary strata, which are responsible for a low gravity signature at the center of the structure.