For permission to copy, contact editing@geosociety.org © 2011 Geological Society of America Latest Cretaceous cone-in-cone structures and soft-sediment deformation (Basque-Cantabrian Basin, north Spain): A record of deep-marine paleoseismicity? B. Ábalos 1,† and J. Elorza 2 1 Departamento de Geodinámica, Universidad del País Vasco, P.O. Box 644, E-48080 Bilbao, Spain 2 Departamento de Mineralogía y Petrología, Universidad del País Vasco, P.O. Box 644, E-48080 Bilbao, Spain 427 GSA Bulletin; March/April 2011; v. 123; no. 3/4; p. 427–438; doi: 10.1130/B30047.1; 5 figures. † E-mail: benito.abalos@ehu.es. ABSTR ABSTRACT Unusual cone-in-cone structures are pre- served in fibrous celestite veins hosted by car- bonatic rocks. The veins are early diagenetic features formed near the water-sediment interface in a deep-marine environment of latest Cretaceous (early Maastrichtian) age. New field and microscopic evidence on the veins and their host rocks indicates that they predate sediment compaction and were related to synsedimentary, soft-sediment deformation structures. The mechanism of cone-in-cone formation has been related to fluid-pressure drops following sediment overpressurization, but instantaneous load- ing during emplacement of submarine slides or other mechanisms might be viable alterna- tives. We argue that this cone-in-cone struc- ture is an overprinting feature (penetrative conical fractures) that resulted from seismic wave propagation through a mechanically anisotropic medium. Submarine amplifi- cation of seismic surface waves (Rayleigh waves) induced shock wave formation in the contacts between veins and the bounding lithified calcarenite and unlithified marl sedi- ment layers. Fracture front wave propaga- tion (normal to the seafloor and the Rayleigh source) across the veins and along the celes- tite crystal fibers induced crack nucleation in heterogeneities and penetrative conical frac- turation. The cone-in-cone structure might thus be regarded as a new paleoseismological indicator in deep-sea sediments. INTRODUCTION Cone-in-cone structures have been known in veins (most often) and concretions from sedimentary rocks since the nineteenth century (Cole, 1893; Gresley, 1894). Comprehensive reviews of them were published by Selles- Martínez (1992, 1994). These structures are characterized at the microscopic scale by the dis- position of fibrous crystals (often calcite) with internal, geometrically organized discontinui- ties consisting of grain or subgrain boundaries, crystal exfoliations, fractures, aligned inclusions of minute mica/clay detrital films, and striations. In vein transversal sections, the discontinuities define families of nested cones sharing common axes normal to vein walls. In sections parallel to the veins, discontinuities usually exhibit subcir- cular, also nested arrangements. Cone-in-cone structures appear in “horizon- tal” (parallel to the stratification) calcite veins in specific horizons of shaly formations. Rare occurrences in calcite rims and concretions have been reported in the literature, too. Cone dimensions attain a few centimeters, and their apices in isolated layers often point “upward” (toward the sediment–water table interface), but they can as well point inward (up and down) in symmetrically coned veins (forming one or two mirror fringes; cf. Gilman and Metzger, 1967). Currently, the occurrence of these structures in sedimentary rocks is used as a guide to identify horizons that acted as seals and allowed the transient preservation of undercompacted units where fluids unable to escape during burial- induced compaction were subjected to excess pore pressures (Selles-Martínez, 1994). Other well-known tectonic features exist with geometries similar to cone-in-cone, though their origin is considered in a radically differ- ent context: “shear/shatter cones” and intrusive cone sheets. Shatter cones are rock discontinui- ties (similar or moderately larger in size than cone-in-cone) known only in sites of extrater- restrial impacts. They are assumed to have formed by impact-induced shock waves (Sagy et al., 2002, 2004). Intrusive cone sheets are various orders of magnitude larger than shat- ter cones and are the emplacement site of ring dikes and other magmatic intrusions (Phillips, 1974; Nicolaysen and Ferguson, 1990). The re- ported geometrical resemblance of crystal/rock discontinuities of such a different size might be due to the fact that they share a self-affine me- chanical origin (Turcotte, 1992). In this paper, we describe for the first time (to our knowledge) cone-in-cone structures preserved in celestite veins hosted by deep- marine carbonatic rocks. We relate them to soft-sediment deformation and diagenetic fea- tures recorded in the hosting turbidites of latest Cretaceous age. We discuss the possibility that this unique occurrence (both in the Basque Can- tabrian Basin and seemingly worldwide) was related to submarine propagation and amplifica- tion of seismic surface waves (Rayleigh waves) and that the structure might be used as a paleo- seismological gauge in those environments. GEOLOGICAL SETTING The Basque-Cantabrian Basin (Fig. 1A) con- nects the Pyrenees and the Cantabrian Mountains and contains a stratigraphically complex Creta- ceous sedimentary fill. The basin was inverted during the Paleogene (Martín Chivelet et al., 2002; Cámara, 1997; Cuevas et al., 1999; Ver- gés and García Senz, 2001; Gómez et al., 2002), and facies and thickness changes across several faults attest to protracted syntectonic sedimen- tation. The most intensely deformed portion of the Basque-Cantabrian Basin is the so-called Basque Arc (Feuillée and Rat, 1971). Here, sedimentation-related structures are obscured by the Paleogene deformations (N-vergent thrusts, strike-slip subvertical faults and major folds, some- times with associated foliations). The Biscay syn- clinorium (Fig. 1A) is one of the major structures