Impact Geophysics

We report on the Azuara impact structure and its Rubielos de la Cérida companion crater, which establish the largest terrestrial doublet impact structure presently known. Both structures have diameters of roughly 35 - 40 km and they have been formed in a purely sedimentary target. From stratigraphic considerations
and palaeontologic dating, an Upper Eocene or Oligocene age is very probable. Geological mapping has established abundant geologic impact evidence in the form of monomictic and polymictic breccias and breccia dikes, megabreccias, dislocated
megablocks, remarkable structural features, extensive impact ejecta and impact signatures even in distant autochthonous deposits. The most striking impact evidence for both structures is given by strong shock metamorphism, including melt and diaplectic glass, planar deformation features (PDFs), different kinds of impact melt rocks (from former silicate melt, carbonate melt, carbonate-phosphate melt) and suevite breccias. Glassy amorphous carbon particles in a solid C-O compound may
be related with fullerenes and may originate from a quenched melt of extremely shocked coal or from extremely shocked limestones. It is assumed that the impact had considerable influence on the Mid-Tertiary regional geology of the Iberian System, and we suggest that respective geologic models which have so far not considered this peculiar and far-reaching event, need considerable revision.

DE LA BARDA NEGRA, NEUQUEN, ARGENTINA. A.C. Ocampo, A.C. Garrido, J. Rabassa, M.C.L. Rocca, J.C. Echaurren, and E. Mazzoni. NASA-Jet Propulsión Laboratory, Pasadena, CA, USA, adriana.c.ocampo@jpl.nasa.gov. Museo Carmen Funes, Av. Córdoba 55 (8318) Plaza Huincul, Neuquén, Argentina. CADIC, Ushuaia, Tierra del Fuego, Argentina. Mendoza 2779-16A, Ciudad de Buenos Aires, Argentina. Codelco, Chuquicamata, Chile. Universidad Nacional de la Patagonia Austral, Río Gallegos, Santa Cruz, Argentina.

The Azuara structure located some 50 km south of Zaragoza (NE Spain) has been identified a possible meteorite impact site (Emstson et al., 1985) with a diameter of more than 30 krn. The impact hypothesis originally based mainly on the occurrence
of shock-metamorphic effects within a small breccia outcrop is strengthened by recent observations. A gravity survey establishes a negative residual anomaly of about 100 µm/s2 , a mass deficiency of 1.24 x 1014 kg, and suggests the existence of an inner ring with a 15 km diameter. Measurements of the total intensity of the earth's magnetic field lack evidence of an endogenic origin of the Azuara structure. Typical dike breccias (breccia dikes) well-known from impact craters (Lambert, 1981) can be observed within most of the stratigraphic units. Giant in-situ breccias up to 80 m thick within the outer-ring region are interpreted as resulting from ejecting and rebound mechanisms. Melt products within paleozoic dike breccias confirm that
planar elements and cleavage in quartz grains as well as kinked micas are the result
of shock metamorphism.
The age of the postulated impact is estimated to be between Lower Creataceous
and Miocene.