The development of the eastern Orpheus rift basin, offshore eastern Canada: A case study of the interplay between rift-related faulting and salt deposition and flow

The salt-rich Orpheus rift basin, part of the eastern North American (ENAM) rift system, formed during the Late Triassic to Early Jurassic prior to opening of the Atlantic Ocean. Using a dense grid of 2D seismic-reflection lines, data from nearby wells, and information from adjacent ENAM rift basins, we have established a tectonostratigraphic framework, identified key structural elements, and reconstructed the deformation history for the eastern part of the basin. Our work shows that a series of E-striking, S-dipping faults with normal separation bound the basin on the north. Deformation within the basin is complex with fault-propagation folds above deep-seated, intrabasin faults, detachment folds, detached thrust faults, and salt diapirs. The synrift salt consists of a lower massive unit that underlies a younger unit with two distinct interfingering facies. Facies A, consisting of salt and interbedded sedimentary rocks (likely shales), developed near the border-fault system and its relay ramps. Facies B, consisting of massive salt with few interbedded sedimentary rocks, developed within the basin center. The youngest synrift unit accumulated exclusively within minibasins near the northern border-fault system. Based on location, this unit likely consists of coarse-grained and poorly sorted alluvial-fan or talus-slope deposits shed from the footwall. All synrift units are intruded by igneous sheets likely associated with the Central Atlantic Magmatic Province and, thus, are mostly Late Triassic (or possibly older). The border-fault system profoundly affected deposition within the eastern Orpheus rift basin by providing pathways for clastic sediment input into the salt-rich basin. These depositional patterns subsequently influenced deformation associated with lateral salt flow during minibasin formation. In regions with interbedded salt, detachment folds and thrust faults developed, whereas salt walls and columns developed in regions with more massive salt. F-38 western H-84 well in Carson on biostratigraphic zonation by Barss BU: Breakup unconformity; ROU: Rift-onset unconformity. from 1988; and Wade, 1993).


INTRODUCTION
The eastern North American (ENAM) rift system that formed during the breakup of Pangea extends from the southeastern United States to southeastern Canada. It has three geographic segments with distinct tectonic histories (Fig. 1a). Rifting was underway in all segments by Late Outcrop data (together with limited borehole and seismic-reflection data) from the exposed onshore rift basins have supplied important information about the stratigraphy, structure, and tectonic history of the ENAM rift system (e.g., Olsen et al., 1996;Schlische et al., 2003;Withjack et al., 2009;Withjack et al., 2013). Critical questions, however, remain about the subsurface geology and timing of deformation for the ENAM rift system. The availability of high-quality, industry, borehole and seismic-reflection data from several of the offshore ENAM rift basins provide the opportunity to address these questions by better defining the subsurface stratigraphy and structure of the ENAM rift system and constraining the timing of deformation and, thus, the tectonic history of the ENAM margin during and after rifting.  Olsen, 1997 Figures 2 and 9. Red, labeled lines are seismic profiles presented in this paper.
The Orpheus rift basin, located offshore Nova Scotia and Newfoundland, Canada, is the northernmost rift basin in the central segment of the ENAM rift system (Fig. 1a). Previous studies, using primarily well data from the western Orpheus rift basin, have identified synrift clastic and evaporitic sedimentary rocks, including massive and interbedded salt (MacLean and Wade, 1993;Tanner and Brown, 2003). To complement these studies from the western Orpheus basin, we have used borehole data and a dense-grid of high-quality 2D seismic-reflection lines  Jurassic synrift rocks (Sinemurian) are exposed in the adjacent ENAM Fundy rift basin (Olsen, 1997). Therefore, rifting continued into the Early Jurassic in the region. The age of the oldest postrift strata in the Scotian basin, however, is poorly constrained. Researchers initially assigned an Early Jurassic age (i.e., late Sinemurian-early Pliensbachian) for the oldest postrift strata in the Scotian basin (Barss et al., 1979;Wade and MacLean, 1990 (Medina, 1995;Hafid, 2000), indicating that the cessation of rifting, continental breakup, and the onset of drifting occurred in Early Jurassic time (Withjack et al., 2012).
Information about the synrift rocks within the Orpheus rift basin is limited. Fluvial clastic sedimentary rocks of Late Triassic age crop out near the western end of the Orpheus rift basin (Tanner and Brown, 1999). Five wells in the western part of the basin reached synrift rocks. Of these, only the Eurydice P-36 and Argo F-38 wells penetrated a significant part of the synrift section; the other three wells (i.e., Adventure F-80, Hercules G-15, and Jason C-20 wells) drilled only the uppermost part (MacLean and Wade, 1992Wade, , 1993. In the Argo F-38 and Eurydice P-36 wells (Figs. 1b, 3), continental red sandstones and shales of the Eurydice Formation were present (MacLean and Wade, 1992;Wade et al., 1995;Tanner and Brown, 2003). These are dated as latest Triassic to earliest Jurassic (i.e., Rhaetian-Hettangian) (Bujak and Williams, 1977;Barss et al., 1979), but seismic-reflection data indicate that an estimated 2 km of older synrift rocks underlie the drilled section in the Eurydice P-36 well (Wade and MacLean, 1990;Tanner and Brown, 2003 . 3). In these wells, the Argo Formation consists of a lower massive salt unit and an upper unit of interbedded salt and shale. The amount of shale varies within the upper unit, ranging from shale-rich (shale-to-salt ratio of ~3:1) in the Eurydice P-36 well to relatively shale-poor   Barss et al. (1979). BU: Breakup unconformity; ROU: Rift-onset unconformity. (Adapted from Holser et al., 1988;MacLean and Wade, 1993).
(shale-to-salt ratio of ~1:10) in the Argo F-38 well (Fig. 3). The palynological age of the Argo H-84 well, Fig. 3). Others, however, refer to the entire salt package from the Scotian Shelf and Grand Banks regions as the Argo Formation (e.g., Sinclair, 1993;Weston et al., 2012). In this study, we refer to the synrift salt in the Orpheus rift basin, regardless of its age, as the Argo Formation.

SEISMIC-REFLECTION AND WELL DATA
This study uses more than 13,500 km of time-migrated, 2D seismic-reflection profiles that cover >30,000 km 2 of offshore Nova Scotia and Newfoundland, Canada (Fig. 1b). These industry lines, acquired in the 1980s, 1990s, and 2000s, were processed using standard methods such as data resampling, bad-trace editing, deconvolution, velocity analysis, and the Kirchoff pre-stack time migration (Yilmaz, 1987). In 2006, the Geological Survey of Canada reprocessed some key seismic profiles (acquired in 1984-1985), suppressing multiples and improving seismic imaging. In some seismic profiles (Fig. 4), however, peg-leg multiples associated with the water column still occur below ~2 s two-way travel time (TWTT). These multiples, commonly associated with high-amplitude reflections, can obscure the primary seismic reflections at depth.
The western part of the Orpheus rift basin has relatively low-quality seismic-reflection profiles (acquired in the 1980s) with line spacing of 15-20 km, whereas the eastern part of the basin has high-quality data (acquired mostly from 1998 to 2002) with line spacing of 2-5 km. The 2D seismic-reflection data have a sampling interval of 2-4 milliseconds and a record length of 8-12 seconds two-way travel time (TWTT).
Ten boreholes in the Orpheus rift basin provide well data such as rock cuttings, gamma-ray logs, and check-shot velocities (MacLean and Wade, 1993) (Fig. 1b). As mentioned previously, five of the ten wells drilled into the Late Triassic-Early Jurassic synrift section.  Wade, 1992Wade, , 1993. To date the seismic horizons, we used the biostratigraphic data from seven wells (i.e., Argo F-38, Eurydice P-36 wells, Emerillon C-56, Hermine E-94, Sachem D-76, Hesper P-52, and Dauntless D-35 wells) (Barss et al., 1979;Ascoli, 1988). The sonic-log and check-shot velocity data in these wells permit well-to-seismic correlation using synthetic seismograms (MacLean and Wade, 1993). Using the check-shot velocity data from the Eurydice P-36 and Argo F-38 wells and the interval velocities from three seismic lines in the study area, we estimated that the synrift section in the study area has an average seismic velocity of ~4.5 km/s (Table 1). We used this value to display the seismic lines with approximately no vertical exaggeration at the level of the synrift section.

Tectonostratigraphic framework of eastern Orpheus rift basin
Using the dense grid of 2D seismic profiles and the available well data from the study area, we identified three tectonostratigraphic packages (i.e., prerift, synrift, and postrift) and three major angular unconformities (i.e., rift-onset, breakup, and near-base Cretaceous) in the eastern Orpheus rift basin (Fig. 5). Our tectonostratigraphic framework is similar to those established by Wade and Maclean (1990) and Maclean and Wade (1992). The deepest and oldest package in the study area is the prerift package. It is either seismically chaotic, likely representing crystalline basement, or it consists of folded reflections, likely representing the folded Paleozoic rocks as, 1 Fig. 1b Figure 5a give 13 locations of seismic sections enlarged in Figure 6. Seismic line is displayed 1:1 assuming a 14 velocity of 4.5 km/s. 15 16

Basement-involved structures in the eastern Orpheus rift basin 55
Basement-involved structures in the study area include the border-fault system of the 56 Orpheus rift basin and numerous intrabasin faults (Figs. 2, 7a-b). In the study area, the border-57 fault system consists of E-striking, right-stepping fault segments (Fig. 2a). Two large, 58 overlapping fault segments produce a major relay ramp (i.e., a monoclinal fold that connects the  near border-fault system and relay ramps. Salt is thin or absent in gray areas due to salt 123 evacuation caused by sediment loading (i.e., minibasin formation). Yellow lines in Figure 9a  124 show location of seismic lines described in text. 125 126 form ridges that are 5 to 10 km wide and more than 20 km long that are parallel or subparallel to 127 the strike of the border-fault segments (Fig. 9a). Internal deformation within the walls and 128 columns includes tightly folded and faulted beds (Fig. 10). The surrounding synrift and postrift 129 strata thicken and/or thin toward the walls or columns (Figs. 10, 11), indicating that their growth 130 occurred during and after rifting. Additionally, asymmetric synclines developed adjacent to the 131 walls and columns (Fig. 10). Most of these synclines developed directly to the south of the 132 border-fault system, creating depocenters that are 20 to 30 km long and 10 to 15 km wide (Fig.  133   9a). The thinning and/or thickening of strata within the synclines toward the adjacent walls and 134 columns indicate that the formation of these folds was coeval with that of the walls and columns. 135 Shallow faults with normal separation are also present in the study area. These faults 136 commonly offset the postrift strata and terminate at or offset the top of the synrift package (Fig.  137 8). Many of these faults formed as conjugate faults at the crest of anticlines that developed above 138 the massive walls or columns (Fig. 8). Most shallow faults with normal separation are planar. 139 However, some faults that penetrate deeper into the synrift package are listric (Fig. 12). None of 140 these faults directly connects at depth with the basement-involved faults except for a few faults 141 that terminate against the border-fault segments in the north (Fig. 8). The timing of shallow 142 faulting is poorly constrained because of the lack of associated growth beds. However, these 143 faults offset the near-base Cretaceous unconformity (NBCU) and strata above it; thus, they likely 144 developed in Cretaceous or later time (i.e., well after rifting). 145 24 146 Figure 10. Uninterpreted (a) and interpreted (b) versions of Line F (see Fig. 1b Figure 11. Uninterpreted (a) and interpreted (b) versions of Line E (see Fig. 1b

CAMP-related igneous activity in the eastern Orpheus rift basin 168
The synrift package of the Orpheus rift basin contains anomalous, high-amplitude 169 reflections with distinctive characteristics (Figs. 5, 6b). Many of these high-amplitude reflections 170 terminate abruptly within the synrift section (Figs. 6b, 6d, 12). Although commonly parallel to 171 subparallel to bedding, many locally cut through bedding, climbing to higher stratigraphic levels 172 (Fig. 6b). Some high-amplitude reflections are present along basement-involved faults (Figs. 10,  173 12), whereas others bifurcate or splay, forming a complex array of high-amplitude reflections 174

RIFT BASIN 200
As mentioned previously, reflections within the synrift package have highly variable 201 geometries (Fig. 2). We have subdivided the synrift package into four separate units with 202 distinctive characteristics. 203 204

Synrift Units 1-3 205
Unit 1, the oldest, directly overlies the prerift strata (Fig. 5). In the study area, its maximum 206 thickness is ~0.4 second TWTT (~ 0.9 km). It consists of subparallel reflections that are gently  Unit 2 overlies Unit 1, lacks internal seismic reflections, and has significant thickness 214 variations ranging from less than 0.1 s TWTT (~0.2 km) to ~1.5 s TWTT (~3.4 km) (Figs. 5, 12). 215 For example, on Line C (Fig. 5), Unit 2 thins northward, becoming thin or absent near the 216 border-fault system. Unit 2 also thickens within the cores of anticlines and thins beneath 217 synclines (Fig. 8). In some parts of the basin, Unit 2 mixes with the overlying Unit 3 forming 218 massive walls and columns (Fig. 10). Unit 2 commonly serves as a major detachment level 219 within the synrift package that decouples the deep and shallow deformation (Fig. 8), further 220 indicating a highly ductile behavior. 221 Unit 3 overlies Unit 2 and has two distinct interfingering seismic facies (i.e., 3A and 3B). 222 Facies 3A consists of numerous coherent and parallel-to-subparallel reflections, whereas Facies 223 3B is acoustically more transparent with few internal reflections (e.g., Fig. 5). Commonly, these 224 internal reflections are discontinuous, chaotic, and moderately to intensely folded (Fig. 10). where deformed, forms large walls and columns and exhibits highly ductile behavior (Figs. 7g,  234 10). The parallel internal reflections in Unit 3 show that its deposition was widespread in a 235 broad, subsiding basin and that most deformation occurred after deposition. Locally, growth 236 30 237 Figure 13. Uninterpreted (a) and interpreted (b) versions of Line G (see Fig. 1b (Figs. 5, 6b). Here, the internal reflections of Facies 3A converge and thin 245 toward the footwall of the border-fault system, indicating that the border-fault system was active 246 during the deposition of Unit 3. 247 248

Unit 4 249
Unit 4 is the synrift component of the fill within asymmetric synclines in the hanging-wall 250 of the border-fault system (Figs. 6d, 8, 10). Most internal reflections within Unit 4 either thin or 251 thicken toward the adjacent wall or columns (Fig. 10) or detachment folds (Fig. 8). In the study 252 area, the vertical thickness of Unit 4 can reach up to 2.5 second TWTT (~5.6 km). 253 254

INTERPRETATION OF THE SYNRIFT UNITS IN THE EASTERN ORPHEUS RIFT 255
BASIN 256

Lithologies and ages of synrift units 1 to 3 257
Unit 1, the oldest synrift unit in the Orpheus rift basin, is gently folded and cut by basement-258 involved faults (i.e., it has a brittle behavior), whereas overlying Unit 2 exhibits a highly ductile 259 behavior, acting as a major detachment level and having significant thickness variations. The 260 overlying Unit 3 has two coeval facies, each with distinct characteristics and behaviors. Facies 261 3A has pronounced internal layering that is folded and faulted, whereas Facies 3B lacks 262 significant internal layering and commonly exhibits a highly ductile behavior. Facies 3B, 263 together with the underlying Unit 2, commonly mix to produce large walls and columns in the 264 study area. Using well data from the western Orpheus rift basin, Bujak and Williams (1977) and Barss 294 et al. (1979) determined the palynological age of the Argo Formation as Early Jurassic (i.e., 295 Hettangian-Sinemurian). As explained below, this palynological age is not consistent with our 296 seismic observations from the eastern Orpheus rift basin. CAMP was a short-lived magmatic 297 event (< 1 my) that began during the latest Triassic and ended during the earliest Jurassic (see 298 Section 2). As discussed previously, igneous sheets, likely related to CAMP, intrude the entire 299 synrift package in the eastern Orpheus rift basin (e.g., Figs. 11, 12). If these are CAMP-related 300 igneous sheets, then the deposition of the preserved synrift section in the eastern Orpheus rift 301 basin would predate the CAMP-related igneous activity, and the thick salt-rich section in the 302 eastern Orpheus rift basin (Units 2 and 3) would have a Late Triassic age (Fig. 14). Thus, the underlying Unit 1, representing the Eurydice Formation, is Late Triassic or older (Fig. 14). Despite limited information about these minibasins, we propose that sedimentary rocks of 334 Unit 4 are likely synrift rocks of latest Triassic to Early Jurassic age (Fig. 14). Unit 4 335 stratigraphically overlies the synrift salt of Unit 3 and is intruded by likely CAMP-related 336 igneous sheets (Figs. 6d, 11), indicating that its deposition would have begun after the deposition 337 of the Late Triassic salt and before and/or during CAMP-related magmatic activity in the latest 338 Triassic to earliest Jurassic. The formation of some minibasins may have continued after CAMP-339 The presence of CAMP-related igneous sheets is critical to constrain the timing of 362 shortening. As discussed above, the growth strata in Unit 4 indicate that most shortening was 363 coeval with minibasin formation. In our study area, likely CAMP-related igneous rocks intruded

Factors that influence deformation style in salt-rich rift basins 384
Structures vary considerably throughout the eastern Orpheus rift basin, ranging from 385 basement-involved faults, to fault-propagation folds above basement-involved faults, to 386 detachment folds and detached thrust faults, to intensely folded intrasalt stringers within massive 387 salt walls and columns (Fig. 7). We propose that two factors profoundly impacted the The presence of interbedded shales strongly influenced the bulk behavior of the synrift salt 420 of the Argo Formation, which ultimately controlled the deformation style of the eastern Orpheus 421 rift basin (Fig. 16). Detachment folds and thrust faults affected Facies 3A with its abundant 422 interbedded shales and accommodated the shortening associated with lateral salt movement 423 during minibasin formation (Fig. 16). In contrast, Facies 3B with few interbedded shales 424 exhibited a highly ductile behavior, combining with Unit 2 to form massive salt walls and 425 columns adjacent to the minibasins (Fig. 16). These walls and columns had complex internal 426 deformation with tightly folded and faulted intrasalt stringers. Formation) to accumulate within a wide rift basin (Fig. 17a). As rifting continued, major relay 450 ramps developed within the border-fault system, influencing depositional patterns during the 451 accumulation of the upper Argo Formation (Unit 3) (Fig. 17b). The amount of interbedded shale 452 within the upper Argo Formation varied laterally, depending on proximity to the relay ramps 453 (Fig. 17b). Facies 3A with many interbedded shale layers formed within and near the relay 454 ramps, whereas Facies 3B with few interbedded shale layers formed far from the relay ramps.  Fig. 1b (Fig. 17c). In latest Triassic/earliest Jurassic time, widespread igneous activity 482 associated with the Central Atlantic Magmatic Province (CAMP) affected the eastern Orpheus 483 rift basin (Fig. 17d) with numerous igneous sheets intruding the entire synrift section (Units 1-4). 484 During the transition from rifting to drifting in the Early Jurassic, some of the basement-involved 485 faults may have been reactivated with left-lateral and reverse components of slip, producing the 486 steeply dipping strata and tight folds directly above basement-involved faults in the southern part 487 of the basin far from the border-fault system and minibasins (Figs. 8, 17e). shale layers. Unit 4, the youngest synrift unit, accumulated exclusively within minibasins 508 near the border-fault system. The lithology of Unit 4 is unclear. Its proximity to the 509 northern border-fault system, however, suggests that it might consist, at least in part, of 510 coarse-grained and poorly sorted alluvial-fan or talus-slope deposits. We would like to thank our colleague, Zulfitriadi Syamsir, for his many valuable geologic 543 and geophysical contributions and insights during this study. We thank TGS, ConocoPhillips, the 544 Canadian Department of Natural resources, the Canada-Nova Scotia Offshore Petroleum Board 545 (CNSOPB), and Suncor Energy for providing the 2D seismic-reflection data used in this study. 546 We acknowledge the generosity of Schlumberger for providing Petrel, the software tool used to 547 interpret the seismic data. We also thank Husky Energy for providing general support of the 548 Rutgers Structure Group. Finally, we thank Rutgers University for providing support during this 549 study. 550 551