Sedimentary Geology 191 (2006) 115–133 www.elsevier.com/locate/sedgeo Influence of pre-existing plate-margin structures on foredeep filling: Insights from the Taconian (Blountian) clastic wedge, Southeastern USA ⁎ ⁎ Germán Bayona ,1, William A. Thomas Department of Geological Sciences, University of Kentucky, Lexington KY 40506, United States Received 26 June 2005; received in revised form 3 February 2006; accepted 9 February 2006 Abstract Break up of continents gives rise to conjugate margins with irregular shapes, and with thermal histories and sedimentary records that differ along strike. These elements need to be considered in the integrated study of a foreland-basin/orogenic-belt pair that formed during subsequent contractional events. This paper relates the promontory and embayment configuration of southeastern Laurentia (North American craton) to: (1) along-strike variations in width of the initial depositional profile of the Middle to Upper Ordovician Taconian (Blountian) foredeep; (2) northeastward (toward the embayment) along-strike thickening and migration of siliciclastic depocenters in the foredeep, that favored the resumption of carbonate-ramp deposition on the promontory during the Late Ordovician; and (3) variation in intensity of deformation of intrabasinal structures as recorded by reactivation of across-strike structures located on the transition zone between the promontory and embayment. Geometry and composition of the Blountian foreland strata indicate that the foredeep slope was narrower and steeper on the Alabama promontory than on the southwestern flank of the Tennessee embayment. Differential slope geometries were accommodated by reactivation of transverse basement faults, which controlled local conglomerate deposition in deep-water settings on the transition zone between the promontory and embayment. In contrast, conglomerate beds at the top of coarsening-upward successions on the southwestern flank of the Tennessee embayment record the cratonward progradation of deltaic depositional systems. The results presented here may have relevance to other peripheral foreland basins formed over rifted continental margins or retro-arc foreland basins formed adjacent to inverted intracratonic rifts. © 2006 Elsevier B.V. All rights reserved. Keywords: Foredeep filling; Fault reactivation; Collision; Blountian orogeny; Ordovician 1. Introduction Continent or arc collision along an irregularly shaped, rifted continental margin may incorporate ⁎ Corresponding authors. Bayona is to be contacted at fax: +1 57 1 significant along-strike variations in orogenic-belt 310 1736. Thomas, fax: +1 859 323 1938. deformation and foreland-basin evolution (e.g., Bradley, E-mail addresses: [email protected] (G. Bayona), [email protected] (W.A. Thomas). 1989). Geodynamic modeling of foreland basins has 1 Present address: Corporación Geológica ARES, Calle 57 N. 23-09 investigated complex spatial and temporal variations in Of. 202, Bogotá, Colombia. lithospheric strength in the evolution of foreland basins 0037-0738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2006.02.001 116 G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133 (e.g., Beaumont, 1981; Stockmal et al., 1986; Patton and Llanos foreland of Colombia and Venezuela; Cooper et O'Connor, 1988; Waschbusch and Royden, 1992; al., 1995). Lorenzo et al., 1998; Tandon et al., 2000; Cardozo and Promontories and embayments (plan view), as well Jordan, 2001), but only a few studies have considered as upper-plate and lower-plate geometries (cross-section the effects of the inherited structural/stratigraphic along- view) of rifted continental margins (Fig. 1), imply strike variations of rifted continental margins during stratigraphic/structural/thermal irregularities, which collision (e.g., Whiting and Thomas, 1994). The lack of have been defined by three-dimensional models of along-strike analysis of foreland basins is, in part, an rifted continental margins (see Lister et al., 1986; Buck artifact of the construction of two-dimensional strati- et al., 1988; Thomas, 1993; Thomas and Astini, 1999, graphic and structural models across thrust belts and for discussion and a more complete list of references on adjacent foreland basins perpendicular to the mountain the three-dimensional geometry and models of conti- belt. In this paper, we establish how the irregular plan- nental rifts). A three-dimensional array of extensional view shape and varying cross-section configurations faults limits the irregular geometry of rifted continental along the southeastern rifted margin of Laurentia margins and defines upper and lower plates of affected the patterns of filling, stratal architecture and extensional margins. The extensional structures include sediment composition of a peripheral foreland basin that low-angle detachment faults, synthetic and/or antithetic contains the sedimentary record of the Blountian phase normal faults, steep transform (transverse) faults that of the Taconic orogeny (hereinafter called the Blountian offset the planes of normal faults with consistent dip orogeny) (Rodgers, 1953). The relationship between directions but different offsets, and steep transform foreland evolution and the structural/stratigraphic con- (transfer) faults that separate domains of opposite dip figuration of rifted margins needs to be established directions of detachment faults. Structural domains of a because foreland basins are commonly superimposed on rifted margin can also be recognized from the synrift and earlier rifted margins (e.g., Papua New Guinea; see post-rift stratigraphy (Thomas, 1991, 1993). This paper examples in Sinclair, 1997) or to intraplate grabens (e.g., examines the response of lower and upper plates to Fig. 1. (A) Block diagram facing northwest showing the marginal and intraplate basement structural geometry of the southern Laurentian passive margin after Cambrian rifting and opening of the Iapetus Ocean (modified from Thomas, 1993). Curved lines show the shape of the upper-plate surface on the Alabama and Virginia promontories. The study area is shown as a shaded polygon southeast of the Birmingham graben (see Fig. 2A for location within the USA). (B, B′) Schematic structural profiles showing the contrasting configuration of basement structures and overlying rift and passive-margin sedimentary cover for a lower-plate margin in the Tennessee embayment and an upper-plate margin on the Alabama promontory, respectively, in Early Ordovician time. (C, C′) Schematic structural profiles across the foreland and orogenic belt in Late Ordovician time. Note the difference in deformation of the orogenic belt and thickness of foreland strata between the profile of the lower-plate margin (in the Tennessee embayment) and the profile of the upper-plate margin (on the Alabama promontory). G. Bayona, W.A. Thomas / Sedimentary Geology 191 (2006) 115–133 117 deformation related to compression and tectonic loading On the basis of palinspastic restoration and strati- of the rifted margin. graphic analyses of upper Precambrian to Cambrian The effects of reactivation of intraplate rift-related synrift and Cambrian to Lower Ordovician passive- structures have been considered in the evolution of margin deposits in the Appalachian and Ouachita foreland basins. Basement structures striking parallel to orogenic belts, Thomas (1977, 1991) proposed an the margin may be reactivated as reverse structures orthogonally zigzag geometry of the eastern Laurentian during early pulses of the orogeny (Gupta and Allen, rift margin (Fig. 1). The lower-plate configuration of the 2000; Bayona and Thomas, 2003) or as flexural normal Tennessee embayment and the upper-plate configuration faults (Bradley and Kidd, 1991; Lehmann et al., 1995). of the Alabama promontory resulted from the Blue Furthermore, offsets of foreland depocenters and Ridge rifting episode during late Precambrian time forebulges (Royden et al., 1987), and along-strike (Thomas, 1993). Although significant changes in synrift differences in subsidence and stratal architecture stratal architecture and basement structure are localized (Castle, 2001), are localized across reactivated basement in the transition between the embayment and promon- faults that strike nearly perpendicular to the mountain tory, the eastern margin of Laurentia was covered by a belt. The pre-existing structures that influence evolution shallow-water carbonate platform by Late Cambrian of a synorogenic foreland may be examined in the time (Thomas, 1991). Extension associated with the context of an earlier rifted margin. Ouachita rifting episode in Early Cambrian time reached This study makes along-strike comparisons of: (1) intracratonic areas of the Alabama promontory forming the initial geometry of the Blountian foredeep, as several graben structures, such as the Birmingham recorded by the termination of carbonate deposition basement graben (Figs. 1 and 2). The along-strike and correlation with basinal shale and sandstone configuration of the Birmingham graben changes across turbidite units (e.g., Sinclair, 1997); (2) stratal architec- several transverse basement faults (Bayona, 2003), ture of the synorogenic clastic wedge; and (3) the forming an accommodation zone that likely connects composition of conglomerate clasts and interbedded with the Georgia transform fault system (Fig. 1A) sandstones in the clastic wedge. These comparisons (Thomas, 1993). permit the establishment of the role of reactivation of rift-related continental-margin