Shatter cones and planar deformation features confirm Santa Marta in Piauí State, Brazil, as an impact structure

Abstract

A total of 184 confirmed impact structures are known on Earth to date, as registered by the Earth Impact Database. The discovery of new impact structures has progressed in recent years at a rather low rate of about two structures per year. Here, we introduce the discovery of the approximately 10 km diameter Santa Marta impact structure in Piauí State in northeastern Brazil. Santa Marta is a moderately sized complex crater structure, with a raised rim and an off‐center, approximately 3.2 km wide central elevated area interpreted to coincide with the central uplift of the impact structure. The Santa Marta structure was first recognized in remote sensing imagery and, later, by distinct gravity and magnetic anomalies. Here, we provide results obtained during the first detailed ground survey. The Bouguer anomaly map shows a transition from a positive to a negative anomaly within the structure along a NE–SW trend, which may be associated with the basement signature and in parts with the signature developed after the crater was formed. Macroscopic evidence for impact in the form of shatter cones has been found in situ at the base around the central elevated plateau, and also in the interior of fractured conglomerate boulders occurring on the floor of the surrounding annular basin. Planar deformation features (PDFs) are abundant in sandstones of the central elevated plateau and at scattered locations in the inner part of the ring syncline. Together, shatter cones and PDFs provide definitive shock evidence that confirms the impact origin of Santa Marta. Crystallographic orientations of PDFs occurring in multiple sets in quartz grains are indicative of peak shock pressures of 20–25 GPa in the rocks exposed at present in the interior of the crater. In contrast to recent studies that have used additional, and sometimes highly controversial, alleged shock recognition features, Santa Marta was identified based on well‐understood, traditional shock evidence.

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

“…), and Santa Marta (Oliveira et al. , ). However, it still has not been completely ascertained whether comparable features may be formed by other tectonic processes although the lack of such reports from both the impact and the tectonic communities strongly speaks out for FFs being characteristic impact deformation.…”

Estimating average shock pressures recorded by impactite samples based on universal stage investigations of planar deformation features in quartz—Sources of error and recommendations

“…; De Oliveira et al. ; Fazio et al. ), this program as well as the WIP program have not yet been fully evaluated, especially not in the case of a relatively high number of PDF sets per grain (i.e., more than three sets), and not compared to each other.…”

“…According to the authors of these two programs, an important reason for generating these automated indexing shortcuts was, aside from the time aspect and being error-prone, that the manual method for indexing PDFs allows a certain amount of unintentional (or intentional) "tweaking" of the data when manually plotting on the Wulff net and indexing using the SPT. Although results obtained using the ANIE program have been presented in several publications (e.g., Pittarello et al 2013;De Oliveira et al 2014;Fazio et al 2014), this program as well as the WIP program have not yet been fully evaluated, especially not in the case of a relatively high number of PDF sets per grain (i.e., more than three sets), and not compared to each other. Of special interest are samples that record a high average number of sets per grain because the two programs require that each PDF set is compared to all other PDF sets in a given quartz grain (i.e., as in the case of the manual graphical method using the SPT), as they must all fit into the allowed azimuthal angle difference "bins" (if not, then they are classified as unindexed).…”

3D gravimetric investigation of the Cerro do Jarau structure, Rio Grande do Sul, Brazil

“…These authors mention the occurrence of FF, in conjunction with other shock deformation features, in 26 terrestrial impact craters. FF were also identified in some Brazilian impact craters, such as Serra da Cangalha ), Vargeão (Cr osta et al 2011, and Santa Marta (Oliveira et al 2014). These new findings of FF in Cerro Jarau thus reinforce the impact nature of the structure.…”

“…), and Santa Marta (Oliveira et al. ). These new findings of FF in Cerro Jarau thus reinforce the impact nature of the structure.…”