Ferran Claudin 1, Kord Ernstson 2, Michael R. Rampino 3, and Francisco Anguita 4

1 IES Giola, fercmam@retemail.es; 2 Fakultät für Geowissenschaften, Universität Würzburg, kord@ernstson.de; 3 New York University & NASA Goddard Institute for Space Studies, mrr1@nyu.edu; 4 Departamento de Petrología y Geoquímica, Universidad Complutense, Madrid, anguita@max.sim.ucm.es

Puerto Mínguez (Fig.) was first described as the location of an "enigmatic" deposit of Paleozoic quartzite clasts [1]. The originally small outcrop was compared by some authors with the Pelarda Fm., which had been interpreted as of fluvial origin [2]. Later studies [3] suggested that the Pelarda Fm. consisted of ejecta from the Azuara impact structure, and again pointed to the similarities with the Puerto Mínguez deposits.

Stratigraphically, the Puerto Minguez deposits (now continuously exposed over 5 km length) consist of uncemented and frequently intermixed Paleozoic, Mesozoic, and Lower Tertiary material.

The mixed cobble and block components show variable morphology (well-rounded, sub-rounded, and angular), and are supported by a sandy and marly matrix. Megablocks with preserved stratification and original characteristics are also intermixed within the deposit.

Most of the pebbles, cobbles, and blocks are strongly deformed. They show striations all around (including nailhead striae), penetration marks, deep grooves, rotated fractures, irregular fractures with complex bifurcations, and mirror polish. While some of these features occur in components independent of lithology, most are especially well-displayed in the limestone components.

Along the outcrop, components of different lithology (e.g., slates, schists, limestones) display distinct striae, similar to that seen in the Pelarda Fm. [3], which may occur as multiple sets, sometimes showing irregular orientations. The grooves, penetration marks, and imprints are preferentially associated with the limestone components. The polish is restricted to carbonate components. The distinct penetration marks resemble the effect of a knife spread over soft butter, showing plasticity during deformation. Occasionally, penetration marks in contact result in a faceted sculpture of the cobbles and blocks. Similar features are reported for the impact ejecta of the Ries and Chicxulub structures [4, 5, 6].

The irregular fractures and their complex bifurcations are well known from other ejecta deposits [3]. Fractured components resemble bread cut into slices, but the clasts remain coherent, and are not broken into pieces. Very similar features are well known from carbonate concretions and belemnites in impact ejecta of the Ries crater (Bunte Breccia). The intense internal torsion of many cobbles and blocks is documented by hinges and rotated fractures that penetrate the entire clast. The rotated fractures (first described for the Pelarda Fm. ejecta [3]), may result from pure torsion, or from a superposition of torsion and tension.

From the analysis of the lithologic characteristics, surface features, and intense deformation of the components and the matrix of Puerto Minguez deposits, we conclude that the origin of these features is connected with simultaneous plastic and brittle deformation, acting rapidly, and under high contact pressure between the matrix and larger components. The rotated fractures are regarded as typical of dynamic, rapid deformation under high confining pressure in an impact event [6]. The mesoscopic characteristics observed in the Puerto Mínguez deposits are compatible with an impact origin, and are similar to other impact ejecta [3, 4, 6, 7]. They are not compatible with an origin from fluvial deposition, quasi-static tectonics, the action of glaciers, or syn-tectonic sedimentation [8]. The confirmed impact origin sheds additional light on the Rubielos de la Cérida structure newly proposed as a companion to the Azuara impact structure [9, 10]. The observations and analyses presented here may serve as an additional diagnostic tool for impact-related diamictites.

References: [1] Moissenet, E. et al. (1972) Melanges casa Velazquez, 8: 1-47; [2] Carls & Monninger (1974) N. Jb. Geol. Paläont. Abh., 145: 1-16; [3] Ernstson, K. & Claudin, F. (1990) N. Jb. Geol. Paläont. Mh, 10: 581-599; [4] Rampino, M.R. et al. (1997) Abstracts Conference on Large Meteorite Impacts & Planetary Evolution, Sudbury, Ontario, Canada, 4; [5] Rampino, M.R. et al. (1997) Abstracts w. Prog Geological Society of America, 29; [6] Chao, E.C.T. (1976) Science, 194: 615-618; [7] Chao, E.C.T. (1977) Geol. Jb., A43: 1-81; [8] Casas, R. et al. (2000) Geodinamica Acta , nº , 1-17 [9] Ernstson, K. et al., this volume; [10] Hradil. K. et al., this volume.