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INTRODUCTION The Tabernas Basin (Betic Cordillera) is filled with Neogene, Pliocene and Pleistocene sediments, discordant on a metamorphic basement (Fig. 1). During the Tortonian sedimentation an up to 70 m thick bed of breccia (the Gordo Megabed) was deposited. It consists of up to several tens of meters large, rotated sedimentary megablocks frequently capped and/or surrounded by a polymict breccia of mainly up to m-sized clasts of the crystalline (schist) basement. Previous workers have suggested the bed to be a seismite. The polymict composition of the studied breccias intercalated within the otherwise shallow-marine sediments, the presence of megablocks, plastically deformed megablocks and shock metamorphic features in quartzs of polymict breccias have led us to the description and reinterpretation of sediments and geomorphological features in the Tabernas Basin that could be related to an impact structure. MATERIALS AND METHODS Multiple field studies were carried out over the course of several years to study the geology and geomorphology of the Tabernas Basin and its surroundings with special focus on the Gordo Megabed. These studies were carried out in conjunction with geological mapping and the measurement of parameters of the observed geological structures. Thin sections (97 in total) from the different lithologies and locations were prepared and studied for shock metamorphic features under an optical microscope. Quartz grains displaying planar features were studied using the techniques described in Stöffler and Langenhorst (1994) and Ferrière et al. (2009). RESULTS Megablocks We define megablocks as coherent rock blocks that are tilted or rotated, lithologically homogeneous, and that extend in outcrops over at least 30 meters (i.e. having dimensions of several tens of meters). The material of the megablocks is often made up of sets of turbidites belonging to the Gordo Megabed and three zones have been identified and mapped: - Zone 1: Central part of the Neogene basin of Tabernas (Rambla de Tabernas). - Zone 2: Borders to zone 1 to the east and features such as Cerro Alfaro. - Zone 3: Neogene basin of Alhabia, on the northern flanks of Sierra de Gador. Plastically Deformed Megablocks Only three outcrops of these structures SB1, SB2 and SB3 (Fig. 2). They involve recumbent folds with reverse limb whose fold axes follow the direction: N25E, N275E and N210E. They represent plastic internal deformation of the within the slump unit mobilized stacks of Late Tortonian turbiditic marl and sandstones beds, inclined to the NE, W and SW and their directions do not match the main tectonic structures that formed these Neogene basins E-W to NE-SW (Martínez et al., 2017). Polymict breccias Most of the polymict breccias studied (28 in total) are located at the Rambla de Tabernas and present a single lithostratigraphic, mainly graphite-micaschist, quartzite and gneiss, as well as brecciated dolomite. These breccias are massives, unstratified, clast supported and lacks visible sedimentary structures. The angular shape of the fragments indicates short transport, but the basement clast lithologies of the breccia originates far from the present locations. Breccia 29 is in an anomalous stratigraphic position on Triassic phyllites in Sierra Alhamilla. This is a variable 10-50 cm thick deposit of Neogene material (loams, sands and limestones). Breccia 30 (sample 2) is located in Sierra de Gádor and constitutes an intermixture of different materials, from both the metamorphic substrate and the Neogene sedimentary sequence. These features in both breccias could be compatible with a proximal ejecta layer and may clearly be identified by the presence of shock-metamorphic features (Osinski et al., 2013). Shock-metamorphic features The systematic search for quartz grains with shock metamorphic features in breccias, resulted in six quartz grains, five grains displaying one set of planar features and one grain with two sets. The features are straight, parallel, sets of planes with a typical spacing between each plane of 1-5 μm. The planar features penetrate the entire grain in four cases, and in two cases they were only visible near the grain boundary in part of the host grain. The features are generally fresh, but occasionally decorated, i.e., lined by minute fluid inclusions (Fig. 3). Nevertheless, in light of the indicative, albeit not affirmative, presence of a shocked quartz grains in what we find as analogous lithologies to known impactites based on their composition, appearance, stratigraphic position and geographical distribution, as well as the other sedimentological, stratigraphic, petrographic and geomorphological observations, lead us to propose the existence of a possible impact crater in the Tabernas Basin of Almeria. In our model, the crater is still to large parts preserved, but covered by post-impact materials, which only leaves an already heavily eroded part of the structure visible on the eastern edge. CONCLUSIONS A comparison with established impact craters showing similar geological features suggests that the Tabernas Basin still holds a well-preserved, concentric impact structure with a mainly buried, 8-km wide nested crater in the basement surrounded by a 22-km wide, outer crater formed by extensive gravitative collapse of a tens of meters thick, upper target layer of semi-consolidated sedimentary strata, likely in a shallow aquatic paleoenvironment. REFERENCES Ferrière, L., Morrow, J. R., Amgaa, T. Koeberl, C. 2009. MAPS 44:925–940. Martínez, M., Galindo, J., Martínez, F. J. 2017. International Journal of Earth Sciences 106 (7), 2461-2471. Osinski, G. R., Grieve, R. A. F., Tornabane, L. L. 2013. In Impact Cratering: Processes and Products. Osinski G. R. and Pierazzo E. Eds. Wiley-Blackwell Publishing. Stöffler, D., Langenhorst, F. 1994. Meteoritics 29(2):155–181

Nakhlite meteorites are igneous rocks from Mars that were aqueously altered ~630 million years ago. Hydrothermal systems on Earth are known to provide microhabitats; knowledge of the extent and duration of these systems is crucial to establish whether they could sustain life elsewhere in the Solar System. Here, we explore the three-dimensional distribution of hydrous phases within the Miller Range 03346 nakhlite meteorite using nondestructive neutron and x-ray tomography to determine whether alteration is interconnected and pervasive. The results reveal discrete clusters of hydrous phases within and surrounding olivine grains, with limited interconnectivity between clusters. This implies that the fluid was localized and originated from the melting of local subsurface ice following an impact event. Consequently, the duration of the hydrous alteration was likely short, meaning that the martian crust sampled by the nakhlites could not have provided habitable environments that could harbor any life on Mars during the Amazonian.

Introduction Geochemical evidence for a collision between a large (~10 km) extraterrestrial body and Earth at the end of the Cretaceous, ca. 66 million years ago, and the conclusion that this event was the cause for the global Cretaceous-Paleogene boundary extinction event [1], triggered an intense worldwide search for evidence of other impact events that could be temporally correlated to other extinction events [2, 3]. To date, no convincing links to any other extinction events have been presented [2]. Connecting extinction events with a specific impact event/crater is complicated by the fact that the inventory of terrestrial impact craters remains largely incomplete, due to, e.g., erosion and sedimentary cover. In addition, our understanding of impact diagnostic markers in sedimentary strata is far from complete, especially in the case of target rocks that lack quartz or when dealing with extraterrestrial projectiles with “not so distinctive” geochemical signature, such as achondrites. On top of these considerations, the ages of a large number of the known terrestrial impact structures are poorly constrained. A precise, and most importantly accurate, age for an impact event is crucial when attempting to tie a specific impact event to a biosphere episode recorded in the stratigraphic record. The 40-km-diameter Puchezh-Katunki impact structure [4], located ~400 km northeast of Moscow (Russia), with an estimated age ranging from ~164 to 203 Ma [5, 6], has been discussed as a possible trigger for four different biotic crises (Fig. 1; [7-12]). Here we present new 40Ar/39Ar data on impact melt rock samples from the Vorotilovo Deep Well drill core, retrieved from the center of the Puchezh-Katunki impact structure [4,6]. The new age obtained for the structure provides valuable data regarding relationships between impact events and extinction events in general and specifically the proposed relationship with some Mesozoic extinction/biotic events, and also contributes to the improvement of the age database of terrestrial impact structures. Results and Discussion Laser step-heating of four whole-rock samples of impact melt rock from the Puchezh-Katunki impact structure were run in duplicate (Fig. 1). The samples (V1018, V1144, and V1251) come from the upper parts of the drill core. The new 40Ar/39Ar data allow us to precisely and accurately date the formation of the Puchezh-Katunki impact structure to 195.8 ± 1.0 Ma (2σ; mean square of weighted deviates (MSWD) = 2.0; P = 0.11). This new age makes Puchezh-Katunki one of the most precisely dated impact structures on Earth (2σ relative error: 0.51%). As previously noted, only one global mass extinction has been confirmed to be the direct result of an impact event. The possible relation between impact events and global mass extinctions has been extensively discussed in the literature [2, 3], and lower limits on crater sizes that cause global effects have been debated as well (op.cit.). For example, there is little evidence that the late Eocene impact events (e.g., the Popigai and Chesapeake Bay impact events) have caused any global biotic effects. It is of course possible that less severe, perhaps regional, biotic events have also played an important role in the evolution of life (e.g., [13]). Understanding these events can yield important insights into e.g., the relationship between environmental changes and extinctions (i.e., causality), and how life recovers after such sudden events. Despite not causing global mass extinction, impact events of significant magnitude to cause regional havoc (e.g., resulting in impact craters equivalent to the size of Puchezh-Katunki) will have an impact on life (e.g., [14]) at least on a regional to continent-wide scale, and could possibly have caused both yet undetected biotic events as well as some of the (minor) extinction events that are recognized in the stratigraphic succession. However, since the geological timescale is continuously refined and ages updated, connecting even a well-dated impact structure with an event in the stratigraphic record is by no means straight-forward. Our new age of 195.8 ± 1.0 Ma for the formation of Puchezh-Katunki excludes any relationship with the previously mentioned biotic/extinction events, and clearly illustrates that connecting poorly constrained crater ages with extinction events is troublesome. Therefore, we stress that using poorly constrained or outdated ages leads to erroneous or problematic conclusions (see also e.g., [15, 16]). We also want to emphasize that proving synchronicity between an impact event and a biotic event is not the same thing as proving causality. Fig. 1. Comparison of previous age data with our results from 40Ar/39Ar analyses of Puchezh-Katunki impact melt rocks, with plateau ages and mini-plateau “ages” (2σ errors, all ages in million years). For references to previous age estimates see [5, 6]. For references to environmental/biotic events that have been suggested to possibly be synchronous with the Puchezh-Katunki impact event, see text. Abbreviations: Baj/Bat = Bajocian/Bathonian, Pl/To = Pliensbachian/Toarcian, Si/Pl = Sinemurian/Pliensbachian, Tr = Triassic. Acknowledgements We are very grateful to our late friend and colleague Dr. Svetlana Bogdanova for providing the samples investigated here. SHA is supported by VR (2017-06388). 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