Quasi-Flexural Folding of Pseudo-Bedding
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30963 1st pages / 1 of 22 Quasi-fl exural folding of pseudo-bedding George H. Davis† Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA ABSTRACT These factors, plus the apparent ease with uncommon fold geometries and structural rela- which the limestone pseudo-beds deform by tionships in a limestone formation in the Pindos The disharmonic outcrop-scale folds and tectonically induced mass-transfer dissolu- fold-and-thrust belt in the Peloponnese, Greece. associated structures in a 300-m-thick Creta- tion creep, were responsible for the distinc- Particularly important is the infl uence on fold- ceous limestone formation in the Pindos Group tive structures observed in outcrop. These in- ing exerted by “pseudo-bedding” (Fig. 1), exposed on Mount Lykaion (Arcadia, Greece) clude indentation structures, where a part of which is created by pressure dissolution during can be understood only in the context of super- one fold limb indents into the other, or where diagenetic gravitational loading accompanying posed pre-tectonic and tectonic mass-transfer the hanging wall of a small thrust indents burial and primary compaction. This work calls dissolution creep. Though the structures de- into its footwall, or where the limbs of a com- particular attention to the kinematic limitations scribed may seem unusual, they are undoubt- mon anticline or syncline migrate toward one of pseudo-bedding stylolite surfaces in accom- edly abundant in strongly deformed pseudo- another by pressure dissolution removal of modating requisite fl exural slip on the limb of bedded limestone formations. In this example the axial-trace region. In terms of the overall a major anticline, and how fl exural-slip budget the Thick White Limestone Beds (TWLB) progressive deformation, tectonically driven defi cits are managed by tectonic pressure disso- formation displays pervasive “fl aggy” pseudo- layer-parallel shortening of pseudo-beds lution and certain uncommon mesoscopic-scale bedding, which is a diagenetic stylolitic layer- fi rst resulted in tectonic stylolites perpen- fold/fault mechanisms. Structural products that ing resulting from pressure dissolution. The dicular to pseudo-bedding, with teeth paral- result from this dissolution creep–dominated mass-transfer dissolution creep that produced lel to pseudo-bedding. This was superseded progressive deformation include quasi-fl exural the diagenetic stylolitic surfaces was achieved at the mesoscopic scale by a layer-parallel folding (Donath and Parker, 1964), an over- during gravitational loading accompanying shortening achieved by buckling of pseudo- looked category of folding which is likely com- burial, compaction, and lithifi cation, and was beds, with loci of buckling and incipient monplace in highly deformed pseudo-bedded driven by high stress concentrations at grain buckling geometries infl uenced primarily limestone formations in mountain belts of the contacts. The diagenetic stylolitic surfaces are by pseudo-bed thickness (and variations in world, such as within fold-and-thrust belts of the lined with insoluble residue and commonly same), wedge-outs of pseudo-beds, and the Appalachians, Arbuckles, Alps, Apennines, and are marked by interlocking stylolite teeth ori- mechanical properties of the pseudo-beds. the western Cordillera of North America. ented perpendicular to the pseudo-bedding. In some cases shortening and tightening of At the outcrop scale, individual pseudo-beds buckle folds involved fl exural-slip kinemat- MASS-TRANSFER DISSOLUTION display signifi cant thickness variability. More- ics, accommodated in part by slip or shear CREEP: AN OVERVIEW over, the individual pseudo-beds not uncom- along clayey insoluble residue at pseudo-bed monly wedge out and disappear laterally over contacts. Where diagenetic stylolite surfaces Stylolites very short distances. abruptly tipped out, or where friction along Along with all the Pindos Group strata, the them was excessively high, the slip defi cit Stylolites are surfaces of localized dissolution, TWLB formation was profoundly shortened (and thus shortening defi cit) was replaced by commonly displaying in cross-section a rough in the Late Cretaceous and early Tertiary as mass interpenetration of pseudo-beds, stylo- suture geometry lined with insoluble residue a result of inversion tectonics. The Pindos faulting, stylo-duplexing, and other forms of of clay, oxides, and/or organic matter (Fig. 2) Group limestone-dominated formations mass-transfer dissolution creep. Slip toward (Stockdale, 1922; Heald, 1955; Bayly, 1986; under lying and overlying TWLB readily ac- the tip lines of wedge-outs even may have Groshong, 1988; Aharonov and Katsman, 2009, commodated fl exural-slip folding, for these produced end loading that, in turn, generated p. 607). They occur at mesoscopic and micro- formations are marked by optimum mechan- adjacent localized buckle folds. The proper- scopic scales. Lengths of stylolite surfaces typi- ical stratigraphy, ubiquitous thin bedding, ties of the folds and associated structures in cally range from centimeters to many meters, and pervasive bedding-plane slip surfaces. the TWLB formation are introduced here as though trace lengths of individual stylolite sur- For the TWLB formation, however, strain a new category of quasi-fl exural folds in the faces of 1 km and more have been observed and accommodation by fl exural-slip kinematics Donath and Parker (1964) classifi cation. measured by Ben-Itzhak et al. (2012). Lengths was spotty and ineffi cient, primarily because of stylolite surfaces generally increase according individual diagenetic stylolite surfaces are by INTRODUCTION to the amount of contractional strain accommo- no means planar or smoothly curviplanar; dated by the stylolite (Benedicto and Schultz, they tip out over short distances; and their The purpose of this paper is to describe and 2010; Ben-Itzhak et al., 2012). interlocking stylolite teeth create elevated interpret the infl uence of mass-transfer dis- Stylolites that are sutured resemble brain frictional resistance to layer-parallel slip. solution creep, i.e., both pre-tectonic and tec- sutures and are distinguished by serrated sur- tonic, as a dominant deformation mechanism faces marked by abundant teeth (Geiser, 1974; †E-mail: [email protected]. that contributed to generating some apparently Geiser and Sansone, 1981; Ben-Itzhak et al., GSA Bulletin; Month/Month 2014; v. 1xx; no. X/X; p. 1–22; doi:10.1130/B30963.1; 30 fi gures. For permission to copy, contact [email protected] 1 © 2014 Geological Society of America 30963 1st pages / 2 of 22 Davis Diagenetic Stylolites Diagenetic stylolites tend to be parallel or subparallel to bedding. They form in the late stages of diagenesis (Groshong, 1988; Smith, 2000; Larbi, 2003) within rock, not unconsoli- dated sediments (Merino et al., 1983; Merino, 1992). Sutured diagenetic stylolites display teeth oriented perpendicular to layering (see Fig. 1), consistent with the horizontality of original bed- ding and the verticality of gravitational loading at the time of their genesis (Larbi, 2003). During sediment burial, compaction, and diagenesis, the ever-increasing gravitational loading raises the magnitudes of compres- sive tractions at grain contacts, which bear the load. Under these conditions of contractional strain calcite grains may rotate, break, and/or twin, and additionally they may dissolve and go into solution as a response to pressure dis- solution (Groshong, 1988; Aharonov and Kats- man, 2009). Merino et al. (1983) and Aharonov Figure 1. Pseudo-bedding in building blocks, Temple of Apollo Epicurius in Bassai, Greece. and Katsman (2009) have elaborated on the Note thickness variations of individual pseudo-beds. Middle block is ~80 cm long. Diagenetic kinematics and mechanics of such dissolu- stylolites are lined with insoluble clayey residue (black), now etched by weathering. Light tion in calcite limestones. This deformational gray steeply dipping fabric elements are calcite veins, products of pressure dissolution. mechanism is a mass-transfer creep involving material dissolution, diffusion, and reprecipita- tion (Guzzetta, 1984; Bayly, 1986; Karcz and 2012). Stylo lite teeth are commonly quite Scholz, 2003). Grains are brought into grain- evident in outcrop and thin section because to-grain contact, thus producing elevated stress they are lined with dark insoluble residue (see concentrations (Railsback, 1993; Aharonov and Fig. 2). Amplitudes of teeth vary in size, but Katsman, 2009). Dissolution creep takes place generally range from 1 µm to 1 cm (Sheppard, at the locations of high stress concentration, 2002; Aharo nov and Katsman, 2009), though high surface energy, and high chemical potential Merino et al. (1983) have reported stylolite teeth (Koehn, 2011). Calcium carbonate is removed amplitudes in the range of tens of centimeters. by diffusion to sites of lower stress concentra- Groshong (1975, p. 1367) defi ned “stylolite” in tion and chemical potential. Dissolution at grain a way that eliminated any requirement for the contacts eases strain energy (Aharonov and presence of suture geometry. He considered a Katsman, 2009). Insoluble residue accumulates stylolite to be “any zone of relatively insoluble at faceted contacts. Where such pressure dis- residue against which the original fabric ele- solution operates, there is a slow interpenetra- ments of the rock are truncated by removal tion of grains and rock elements (Merino et al., rather than offset.” This broader defi nition of 1983; Aharonov and