Leg 210, in conjunction with ODP Legs 149 and 173 and DSDP Site 398, was conceptualized as the first test of directly conjugate margins, with the goal of documenting basin architecture and evolution from rift-to-drift stages. Although only part of the drift history is represented by the sections successfully cored during Leg 210, this stratigraphy can be directly compared to that cored on the Iberian margin. Previously collected petrologic data sets from petroleum exploration wells on the Newfoundland margin and Iberian Legs 103 (Johnson, 1988) and 149 (Marsaglia et al., 1996) were combined with data collected in this study (Sites 1276, 398, 1065, and 1069) to compare the records of coarse clastic sedimentation on these margins and to synthesize a general model for rift-to-drift evolution as described below.
Petrofacies models for rift-to-drift successions along continental margins have not been adequately defined in the literature in part because these thick sedimentary successions often limit availability of the early rift facies (Marsaglia, 1991). Thus, the Iberian/Newfoundland conjugate pair was chosen because of their thin sedimentary cover as compared to other passive margins, such as the northern Gulf of Mexico. In a study of modern Rio Grande Rift sand petrofacies, Ingersoll (1990) showed that synrift petrofacies can be quite variable, depending on the distribution of prerift basement and sedimentary cover, as well as the distribution of synrift volcanic centers (Fig. F21). Garzanti et al. (2001) followed with an actualistic study of proto-oceanic basins, the Red Sea and Gulf of Aden, where there are distinct changes associated with the distribution and amount of synrift volcanism and degree of unroofing of crystalline basement. The undissected rift shoulders shed quartzose sand recycled from cratonic cover sequences, which when progressively removed expose basement terrains that yield quartzofeldspathic sand (Fig. F21). The margins characterized by volcanism first yield volcaniclastic successions, then when erosion is sufficient to remove the volcanic cover, they yield quartzofeldspathic basement-derived sand. Thus both volcanic and nonvolcanic rifted margins converge to basement-derived sand compositions in this young system. The geological record of Red Sea rifting is defined in the Gulf of Suez, where uplift associated with continental rifting results in the unroofing of crystalline basement rocks (Steckler and Omar, 1994). Petrologic evidence for this unroofing is recorded in the synrift sedimentary pile of the Gulf of Suez where clast populations show an inverted stratigraphy: clasts of younger formations in the oldest synrift section and older formations, including crystalline basement, in the younger synrift formations (Evans, 1990). Arribas et al. (2003) describe the fluvio-lacustrine fill of an intraplate rift basin in north central Spain and relate associated quartzose sand petrofacies to basement geology, paleogeography, and basin evolution. The range of compositions encountered in the synrift fill of the basin reflect local metamorphic, plutonic, and recycled sedimentary source rocks.
In his classic synthesis, Dickinson (1985) somewhat ignored rifted and passive margin settings but inferred that during the rift-to-drift transition in passive margin successions, sand would be first quartzofeldspathic (basement uplift during rifting) then more quartzose (reworking and larger river systems) upsection (Fig. F21). This model does not hold for the Iberian–Newfoundland system. On the Iberian margin there is a progression from lithic sandstones to more quartzofeldspathic sandstones that Marsaglia et al. (1996) attributed to unroofing or development of the fluvial systems that fed submarine fans. Prerift and rift data from Galicia Bank to the north (Johnson, 1988) also show the production of lithic-rich sandstones during rifting (Fig. F22), but quartzofeldspathic prerift sandstones suggest that basement terrains were already exposed, perhaps during an earlier rift phase. Lithic fragments in these examples were mainly sedimentary and metamorphic debris.
As stated earlier, synrift sandstones were not recovered at Site 1276, but the fill of the older Jeanne d'Arc rift basin and the postrift sections at Site 1276 suggest that this margin is characterized by more quartzolithic sandstones, both in the synrift and postrift rift units (Fig. F22). This may be a product of the fundamental differences between Iberian and Newfoundland Paleozoic geology. The Newfoundland sand compositions fall in the recycled orogen field of Dickinson (1985) and are consistent with having been recycled from older foreland basins sediments shed westward off the Hercynian orogen, whose dissected batholithic roots to the east, unroofed by rifting, now supply quartzofeldspathic sand to the Iberian passive margin.
Thus rift-to-drift transitions may have several evolutionary pathways depending on the importance of synrift volcanism and the nature of the continental successions (basement and cover sequences) within the rifted segment (Fig. F21). Lithic-rich synrift sandstones are the norm, except where the cover sequences are semiconsolidated quartzose sandstones. In that case, the recycled sediments are themselves quartzose (see also Arribas et al., 2003). If crystalline basement is minimally present or not exposed during rift or drift phases, such as in the case of the Newfoundland margin, then synrift and postrift sandstones may have very similar quartzolithic compositions (Fig. F21).