Research on geology, geophysics, and
petrology of impact structures (meteorite impact craters)
A FIELD GUIDE TO THE AZUARA IMPACT STRUCTURE
(and its RUBIELOS DE LA CÉRIDA companion crater)
IMPACT Workshop on "Impact Markers in the Stratigraphic Record"
Granada (Spain), May 19-May 25, 2001.
By Kord Ernstson, Ferran Claudin & Michael R. Rampino
The 35 - 40 km-diameter Azuara impact structure is located in northeast Spain roughly 50 km south of Zaragoza (Fig.1). From stratigraphic considerations, its age is estimated to be Upper Eocene or Oligocene, thus in some respect conflicting with the Alpidic orogeny of the Iberian Chain.
Disregarding very small magmatic intrusions, the target was purely sedimentary, which is a rare case with very large impact structures. The target was composed of the Paleozoic core of the Iberian chain and the Mesozoic cover, overlain by Tertiary molasse sediments of probably 1000 - 2000 m thickness (Fig.2). This prominent thickness of more or less loosely bound sediments as well as the dominant carbonate rocks in the sedimentary sequence are related to many peculiarities of the Azuara structure such as relics of carbonate melt, a special facies of the impact ejecta, and distinct breccia dikes.
Young post-impact sediments widely hamper the investigation of the central part of the Azuara structure. Therefore, most outcrops concentrate on the rim region, where they are very abundant enabling excellent insight into the geology of the impact process.
The impact hypothesis for the Azuara structure was first established by Ernstson et al. in 1985. Until then, the structure attracted practically no interest except for a paper by Carls (1983) who mentioned a tectonic Azuara block. Later, the Azuara structure is regarded by some geologists as a basin which formed by syn-tectonic sedimentation.
The model of a giant meteorite impact is based on a very detailed geological mapping, geophysical measurements, photo-lineation studies, and petrographic analyses of about 1000 thin sections. From these investigations (Ernstson & Fiebag 1992, and references therein), the impact is documented by gravity negative anomalies, abundant monomict and polymict breccias and breccia dikes (frequently displaying shock-metamorphic effects such as diaplectic glass, melt particles, all kinds of planar deformation features [PDFs]), extended and thick megabreccias, a large quantity of dislocated megablocks, hydrothermal signature (Siegert 1997), and extended and thick impact ejecta (Ernstson & Claudin 1990).
The Rubielos de la Cérida impact structure is located some 50 km south-southwest of the Azuara structure (Fig.1) and is defined by an approximately circular uplift of Mesozoic rocks surrounded by a semi-circular to elliptical depression and a related conspicuous drainage pattern. The diameter of the uplift is roughly 15 km, and the west-east diameter of the depression amounts to about 40 km. The geological map shows that the existence of the central uplift is structurally controlled with Muschelkalk as the oldest rocks in the core. The stratigraphic uplift amounts to roughly 500 m. The depression surrounding the uplift is characterized by Quaternary and post-tectonic Neogene deposits. "Post-tectonic" means that the flat-lying sediments are neither affected by Alpidic tectonics nor by impact deformation. In the western and northern part of the structure, its rim is defined by Mesozoic and Paleozoic rocks belonging to the Western Iberian Chain. Few hills of Mesozoic rocks are emerging from the Quaternary in the western part of the depression. In the south and east, the Rubielos de la Cérida structure is less well defined. In the literature on the regional geology, no special account is given of the Rubielos de la Cérida structure.
The impact nature of the structure is established by the occurence of shock metamorphism including impact melt rocks. As in the Azuara structure, all kinds of breccias (monomict and polymict breccias, megabreccias, breccia dikes), dislocated megablocks, and extended impact ejecta are observed.
The neighbourhood to the Azuara structure and the stratigraphic age (Lower to Mid-Tertiary) of both structures make a synchronous impact of a paired projectile very probable, hence constituting the largest presently known terrestrial doublet impact structure.
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