Upper Cretaceous Mass Transport Systems Above the Wyandot Formation Chalk, Offshore Nova Scotia

B.M. Smith, M.E. Deptuck, and K.L. Kendell

Abstract Interpretations from 18 contiguous 3-D seismic surveys, covering more than 17,000 km2, show a wide variety of diagnostic geomorphic elements that indicate mass wasting was an important process in shaping the top surface of the Wyandot Formation, an Upper Cretaceous chalk unit deposited across wide areas off the coast of Nova Scotia. These geomorphic elements include a series of clearly defined ~80 m high head scarps extending more than 100 km across the margin, multiple 20 to 80 m high side scarps defining up to 40 km long failure corridors, and an irregular planform fabric above areas interpreted to correspond to bedding plane detachments, variably modified by overlying deformed strata. The latter morphology includes large >1 km wide slide blocks or erosional remnants. In addition, multiple intervals of chaotic seismic facies are recognized directly above the top Wyandot Formation seismic marker. These geomorphic features are attributed to a complex history of slope failure originating from a series of shelf-perched delta systems located 40 to 60 km landward from the paleo-continental slope break. These deltas prograded over the Wyandot Formation in the Late Cretaceous through early Paleogene. This study indicates that significant quantities of resedimented chalk may have been deposited on the outer shelf and upper slope. Such deposits could form important gas reservoirs, as demonstrated in the North Sea (e.g. Ekofisk field). However, the degree to which overlying prodelta muds were mixed with the Wyandot chalk during failure is unclear and likely has a direct bearing on the reservoir potential of resedimented chalks off Nova Scotia.

Keywords Wyandot Formation • chalk • mass wasting • head scarp • transport corridor • resedimented chalk

B.M. Smith (�), M.E. Deptuck, and K.L. KendellCanada-Nova Scotia Offshore Petroleum Board, 1791 Barrington St, Halifax, NS, Canada B2Y 4A2 e-mail: bmsmith@cnsopb.ns.ca

D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences, 619Advances in Natural and Technological Hazards Research, Vol 28,© Springer Science + Business Media B.V. 2010

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1 Introduction

Nova Scotia is located on the east coast of Canada with an offshore area of approximately 400,000 km2 (Fig. 1). The Scotian margin developed after North America rifted and separated from the African continent during the break-up of Pangea. Red beds and evaporites were the dominant deposits during the Late Triassic rift phase, whereas typical clastic progradational sequences with periods of carbonate deposition dominated the drift phase (Wade and MacLean 1990). Near the end of the Cretaceous, a period of generally high global sea level (Haq et al. 1987) resulted in the widespread deposition of chalks, marls and marine shales of the Dawson Canyon and Wyandot formations, which blanketed shelf and slope areas off Nova Scotia (MacIver 1972; Jansa and Wade 1975).

The Upper Cretaceous Wyandot Formation was deposited on a stable shallow-water, open marine continental shelf (Jansa and Wade 1975; Ings et al. 2005), with several wells also encountering the formation in probable upper slope settings, seaward of the paleo-continental shelf edge. Limestone of the Wyandot Formation generally consists of coccolithic-foraminiferal chalks and lime mudstones that are soft, fossiliferous, and argillaceous (Wade and MacLean 1990). They are commonly interbedded with marls, calcareous shales and marine mudstones. On gamma ray logs, the Wyandot Formation commonly passes up-section from a serrated to more blocky response, interpreted to reflect a decrease in clay content towards its top

Fig. 1 Location of the study area off Nova Scotia (NS), eastern Canada. NFLD = Newfoundland

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(Deptuck et al. 2003; Ings et al. 2005). Biostratigraphic age determinations indicate that across the Scotian Shelf the Wyandot Formation was deposited in the Coniacian to Maastrichtian (Wade and MacLean 1990) but in the study area, it is commonly Campanian or older (Fensome et al. 2008). The Wyandot Formation ranges in thickness from less than 50 m in the study area to greater than 250 m on the eastern Scotian shelf. The wide variation in thickness has been attributed to a combination of erosion and its proximity to prodeltaic shales associated with the influx of clastic depositional systems (i.e. in areas far removed from clastic depositional systems, like the eastern Scotian shelf, thicker intervals of chalk accumulated; MacRae et al. 2002; Fensome et al. 2008).

On seismic profiles, the top lithological boundary of the Wyandot Formation generates a high amplitude positive seismic signature that separates denser and faster velocity rocks below from less dense and lower velocity Upper Cretaceous and Paleogene clastics above. This boundary is distinctive across most of the Scotian shelf and the resulting seismic reflection is commonly used for determining the phase of seismic volumes and for calibrating well synthetic ties.

In the Sable Island area, abrupt variations in the thickness of the Wyandot Formation commonly coincide with an irregular reflection pattern on the top Wyandot seismic marker (MacRae et al. 2002). Using predominantly 2D reflection seismic profiles, previous workers interpreted the irregular top Wyandot surface as erosion resulting from ocean current scouring or submarine karst (Wielens et al. 2002; MacRae et al. 2002; Deptuck et al. 2003). The patchwork of individual 3D seismic programs used in this study, however, provides more constraints on the origin of some irregularities on the top Wyandot Formation marker across large areas of the Scotian shelf and slope. The objectives of this paper are to document the geomorphology of the top Wyandot Formation seismic marker mapped from 3D seismic data and to discuss additional processes, like mass wasting, that have strongly influenced its appearance.

2 Study Area, Data, and Methods

The study area, highlighted in the center of Fig. 1, encompasses most of the “Sable Subbasin”. With 67 hydrocarbon exploration wells, this subbasin is the most densely explored area off Nova Scotia. About half of the study area lies on the present-day continental shelf in water depths less than 200 m with the other half on the continental slope in water depths up to 3,000 m.

The data set consists of 18 contiguous time-migrated 3-D seismic surveys, covering an area >17,000 km2 above the present-day shelf and slope off Nova Scotia. The seismic surveys were acquired by several different companies between 1998 and 2002. All surveys were adjusted to zero phase and bulk shifted in time to provide a consistent data set, and the top of the Wyandot Formation was correlated to seismic at 18 well locations using synthetic seismograms.

The strong positive reflection from the top of the Wyandot Formation was mapped across the study area. The interpreted data was then gridded at spacing of

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25 by 25 m and displayed in time using Geoviz 3D visualization software, as shown in Fig. 2. In all figures, warm colors signify shorter two-way travel times (i.e. shallower areas) and cool colors represent longer two-way travel times (i.e. deeper areas). Dip magnitude maps, like the ones shown in Fig. 3, were generated using Schlumberger IESX software, and were derived from the gridded seismic data. Dark colours indicate steeper dips and lighter colours indicate shallower dips.

3 Results – Variation in Top Chalk Morphology

Meticulous mapping of the complex seismic reflection defining the top of the Wyandot Formation, combined with a fine 25 by 25 m gridding of the resulting surface, reveals a wide range of geomorphic features that range from smooth and

Fig. 2 Sun-illuminated 3D seismic render of the top Wyandot marker (two-way time) showing the wide variation in morphological features, including head scarps, side scarps, faults, canyons, and sediment transport corridors. The inset shows the same map with the two main Wyandot erosional zones highlighted in pink that are separated by the West Sable structural high (see text for details). The lettered boxes locate detailed dip map images shown in Fig. 3

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relatively featureless, to bumpy with a highly complex planform and cross-sectional appearance (e.g. Figs. 2–4). The top chalk morphologies in Figs. 3 and 4 are strikingly similar to features documented in the Petrel Member on the Grand Banks of Newfoundland, a Turonian chalk unit within the Dawson Canyon Formation (see Fig. 2.5 of Deptuck 2003).

Some of the most obvious geomorphic features preserved on the top Wyandot marker correspond to faults. Two varieties are apparent. The first are younger offsets associated with long-lived deep-seated down-to-the-basin listric growth faults (e.g. Fig. 3i) that form curved lineations with a dominant E-W or ENE-WSW orientation. Fault offset of the top Wyandot marker is commonly <50 to 120 m on the shelf, with >1 km of offset south of Fig. 2, where prominent salt withdrawal deformed the slope as sedimentation was focused on the hanging wall of mini-basin bounding faults. These faults are more numerous in areas interpreted to coincide with the paleo-continental shelf-edge (near the light blue shading in Fig. 2), where prominent growth took place in older stratigraphic intervals (below the top Wyandot marker). Most of these faults sole out above overpressured shale or allochthonous salt intervals. The second style of faulting corresponds to intra-formational or polygonal faults (e.g. Fig. 3h; Dewhurst et al. 1999; see also Hansen et al. 2004) that show offset is restricted to stratigraphic intervals immediately above and below the top Wyandot marker. These faults are most apparent in areas with the thickest in-situ chalk, for example near the Eagle D-21 well where the Wyandot Formation is about 190 m thick (see Ings et al. 2005). Offset along these faults is typically less than 30 m, forming a polygonal pattern of fault traces in plan view.

Fig. 3 Dip magnitude images showing the variation in planform geomorphology of the Wyandot marker. Darker areas correspond to steeper dips. See Fig. 2 for location and text for discussion

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Fig. 4 Rendered time-structure perspective map showing a closer-up view of the top Wyandot marker. Seismic line AB crosses a head scarp associated with a slope failure that detached above a bedding plane located near the base of the Wyandot Formation. Although the younger chaotic mass transport deposit above the top Wyandot marker (yellow) could be associated with the same failure, it is probably associated with a younger failure composed predominantly of prodelta muds. The profile also crosses one of the isolated features interpreted as a slide block or an erosional remnant

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A wide range of other geomorphic elements on the top Wyandot marker correspond to erosional features. For example, the marker is truncated along the bases of deeply incised canyons of various ages (e.g. Fig. 3g). Several sinuous to linear canyon thalwegs are apparent in the western part of Fig. 2, but these are generally much younger features (Eocene to Miocene) that only affect the top Wyandot marker in areas of maximum incision depth. In some cases the slower velocity fill of younger canyons generates ‘velocity pushdown’ artifacts on the top Wyandot surface (e.g. western part of Fig. 2). Other erosional features, which form the primary focus of this paper, appear to be associated with an unconformity directly above the Wyandot Formation.

They are probably associated with a top chalk unconformity identified by Doeven (1983) and Fensome et al. (2008) in Onondaga E-84. Here, all of the Campanian is absent, and lower Campanian or Santonian chalks are overlain directly by Maastrichtian clastics.

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In the westernmost part of Fig. 2, the top Wyandot marker is smooth and continuous across elongated zones paralleling the orientation of prodelta clinoforms that overlie the marker (e.g. Fig. 5). In Fig. 5, subtle truncation of the smooth Wyandot marker is apparent below ‘delta 1’ (discussed later), and a much thinner Wyandot Formation was encountered in wells in these areas (e.g. 30.8 m thick at Cree E-35 and 11.5 m thick at Alma K-85). Such erosion has been attributed to scouring by shallow ocean currents that flowed parallel to the strike orientation of prodelta clinoforms (MacRae et al. 2002). Subtle 50 to 100 m wide slope-parallel linear scours above the top Wyandot marker (Fig. 2) and above younger ‘prodelta

Fig. 5 Three mapped surfaces D1, D2, and the top Wyandot marker (in dark blue). Location of seismic section AB is denoted by the solid black line. The top Wyandot marker is overlain by two younger progradational surfaces. The red mapped horizon (D2) defines the western map which is cropped at the black dashed line in order to display the older clinoform surface to the east (D1) defined by the top of the orange shading on the seismic section

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clinoform’ markers (e.g. the D2 marker in Fig. 5), appear to corroborate this idea. Deptuck (2003) documented similar erosional scours above prodelta clinoforms and the Wyandot Formation on the Grand Banks of Newfoundland to the north, and attributed them to erosion associated with the ancestral Gulf Stream. Such currents promoted oversteepening of clinoform sets, preconditioning them for failure.

Some of the most striking erosional features in the study area appear to be associated with slope failures that took place above the Wyandot Formation. Several sharp vertical offsets in the top Wyandot marker elevation are seen in Fig. 4. Some of these correspond to faults described earlier but others show no offset in underlying stratigraphic layers. Despite their similar plan view appearance, they do not correspond to faults. A seismic profile across the 27 km long NE oriented lineation in Figs. 3a and 4 indicates that the downward step in the top Wyandot marker corresponds to erosion of the upper parts of the Wyandot Formation. Erosion took place roughly parallel to the base of the Wyandot Formation. The sharp downward step therefore is interpreted as a head scarp associated with a mass failure that took place after the Wyandot Formation was deposited. South of the head scarp, the failure detached above a bedding plane located somewhere near the base of the Wyandot Formation, perhaps in a mechanically weaker, more argillaceous interval. A slab of Wyandot chalk, thus, appears to have detached and broke up as it was transported to the south.

A series of lineations also trend perpendicular to the orientation of normal faults (Fig. 4). Like the interpreted head scarps, they show no offset in underlying seismic markers, and are interpreted as side scarps oriented parallel to the direction of sediment transport. Two such side scarps define a 5 km wide north–south oriented ‘failure corridor’ (Fig. 3b) that extends towards the paleo continental shelf edge. The distance from the head scarp to its distal end is approximately 60 km. Several north–south oriented linear scours are present on the floor of the failure corridor (see Fig. 4). A more prominent side scarp is also present east of the failure corridor. It is 37 km long, has a north–south orientation, and a somewhat irregular shape in plan view. The side scarp sharply separates the more intact polygonally faulted chalk to the east from highly complex ‘deformed’ top chalk surface to the west.

A wide range of morphologies are preserved within failed areas defined by head scarps and side scarps. The seismic profile in Fig. 4 crosses a 1 km wide and 2 km long angular ‘block’ and a more irregular fabric above the bedding plane detach-ment (Fig. 3c). The seismic character of the block is identical to intact intervals of chalk north of the head scarp, and this feature is interpreted as a slide block composed of chalk left behind during slab break-up. Several additional blocks are recognized to the east of the largest block (Fig. 4). It is also possible that some of these features are erosional remnants of intact chalk, though this is not our preferred interpretation. NE of the largest slide block, several positive relief features are elongated parallel to the head scarp and may be comprised of a cluster of rotated or folded slide blocks transported normal to a failure scarp located just north of the 3D seismic coverage.

In addition to the angular slide blocks interpreted in Fig. 4, some morphologies on the failed top chalk surface are of unknown origin. For example, a series of

Upper Cretaceous Mass Transport Systems 627

elongated morphological features are present in Fig. 3e. They are locally v-shaped in cross section and could represent elongated scours. Alternately, they could also represent a series of rotated blocks associated with shallower failures that locally modified the top Wyandot marker. In addition, a cluster of sub-circular depressions in Fig. 3f could correspond to areas of irregular chalk dissolution (associated with submarine karst?) or could represent fluid escape structures.

3.1 Slope Failures Above the Wyandot Formation

Collectively, the head scarps, side scarps, and zones of irregular reflection character define the boundaries of a large complex area of erosion and transport across the top of the Wyandot Formation. The ‘Wyandot erosion and transport zone’ (WETZ for short) encompasses an area of about 1,700 km2 and can be subdivided into two sub-areas (Eastern WETZ and Western WETZ) shown on the inset map in Fig. 2.

The Western WETZ covers an area of 700 km2. This area is bound to the north by a 27 km long and 80 m high head scarp, which continues beyond 3D seismic coverage to the west. The western boundary of the Western WETZ is located within this data gap as shown in Fig. 2.The eastern boundary of the Western WETZ is defined by a structural high, trending south from the West Sable structure. There appears to be less Wyandot erosion above this high and the overlying MTD thins. No side scarp was identified along this eastern boundary, and instead the transition to in situ chalk is more gradual.

The Eastern WETZ covers an area of 1,000 km2 and is more clearly defined on seismic than the Western WETZ. The 80 m high head scarp shown in Fig. 3a continues to the east beyond 3D seismic coverage, but can be traced further on 2D seismic profiles. The head scarp merges into a prominent side scarp that sharply separates the Eastern WETZ from an area of more intact, polygonally faulted chalk to the east. This prominent side scarp forms the eastern boundary of the Eastern WETZ. Its southern limit transitions gradually into a surface with less obvious erosive character, except along the prominent failure corridor that indicates failed material locally was transported to and probably beyond the paleo continental slope break. The western extent of the head scarp merges into another side scarp forming the western boundary of the Eastern WETZ. This boundary continues south along the south-trending structural high characterized by thicker in-situ chalk (south of the West Sable structure shown in Fig. 2) and eventually forms the western edge of the failure corridor.

Areas where the top Wyandot chalk surface is highly complex are commonly overlain by low amplitude chaotic mass transport deposits derived from mud-prone prodelta clinoforms in overlying Upper Cretaceous and lowermost Paleogene strata. As such, the complex fabric observed above the Wyandot Formation could be closely associated with failures derived from variably oriented clinoforms.

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3.2 Relationship of Failures to Adjacent and Overlying Prograding Clastic Systems

During and after deposition of the Wyandot Formation, Campanian(?) to Maastrichtian clastics prograded from the west as a succession of delta lobes (Fensome et al. 2008). At least two delta lobes, delta 1 and delta 2 (Fig. 5) prograded from the northwest downlapping onto the chalks of the Wyandot Formation or the chaotic deposits that overlie it. These appear to be ‘shelf-perched’ delta systems, and within the study area the seaward most offlap break at the delta front stopped 40 to 60 km short of the paleo-continental slope break. Many of the mass transport features recorded in the top Wyandot marker are believed to be associated with failures that were triggered in slightly younger intervals corresponding to these shelf-perched deltas.

Figure 5 illustrates the aerial relationship between the younger deltas and the mass wasting features expressed on the top Wyandot marker. The top of each delta lobe has been mapped and are denoted by the orange (D1) and the red (D2) horizons on seismic section AB (Fig. 5). Delta 1 aggraded to a height of approxi-mately 300 m prior to failure of mud rich clinoforms and to some extent sandier delta top-sets. The resulting mass transport deposit (MTD) is shaded in yellow on the seismic section. The MTD overlies the head scarp on the top Wyandot marker associated with the Western WETZ. It is unclear whether this mass transport system exploited a plane of weakness within the Wyandot Formation and eroded the upper portion of the chalks creating the scarp or whether this mass transport deposit is a later feature that simply draped a pre-existing scarp above the top Wyandot surface. The similarity in orientation of the two head scarps may imply that the southern scarp formed during an initial prodelta failure, with retrograda-tion producing the head scarp that truncates the seaward edge of Delta 1. If the erosion above the Wyandot Formation is a result of prodelta failure, then the resulting MTD should consist of a mixture of Wyandot carbonates and delta front to prodelta sediments. A series of younger clinoforms can also be seen downlap-ping onto this MTD and many additional failures appear to have been initiated as clastic systems continued to prograde. These failure deposits commonly merge with and erode into older MTDs, resulting in complicated stratigraphy in the central area of the WETZ.

A second failure scarp is evident on the younger D2 surface to the west. This younger approximately 10 km wide failure was initiated after the Wyandot sur-face was buried by a combination of clinoform bottom-sets and chaotic deposits associated with earlier slope failures. Therefore the Wyandot surface in this location was protected by a thicker clastic cover and was less affected by the failed delta front at the D2 surface. A secondary scarp is also evident south of the failure scarp, and may record the position of the initial failure, with retrogradational failure defining the ultimate head scarp location.

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3.3 Potential Triggering Mechanisms

Numerous triggering mechanisms for these failures are possible. Norris et al. (2000) suggested that the 65 Ma Chicxulub bolide impact at the K/T boundary was responsible for slope instability and mass failure of chalks across extensive areas of the western North Atlantic. However, the presence of Maastrichtian age clastic deposits associated with prodelta clinoforms above the unconformity (Doeven 1983; Fensome et al. 2008) indicates erosion above the Wyandot Formation in the study area predates the K/T bolide impact. In addition, erosion above the Wyandot Formation is diachronous, and hence is unlikely to be associated with any single event. It is possible that oversteepening of the prodelta clinoforms, which are locally steeper than 5°, could have preconditioned them for failure. The steep gradient of these clinoforms may be associated with the passage of ocean currents that locally undercut clinoform bottom-sets, as observed in the Jeanne d’Arc Basin to the north (where time-equivalent clinoforms are as steep as 9°; see Figure 4.11 of Deptuck 2003). Ground shaking by earthquakes or short-term drops in eustatic sea level, could equally have initiated these failures.

4 Conclusions

1. Interpretations from 18 contiguous 3-D surveys, covering more than 17,000 km2, show a wide variety of geomorphic elements that indicate mass wasting was an important process in shaping the top surface of the Wyandot Formation.

2. Head scarps, side scarps, bedding plane detachments, slide blocks or erosional remnants, scours and chaotic mass transport deposits are all recognized along or above the top Wyandot marker. They record a complex history of Late Cretaceous slope failure above the Wyandot chalk.

3. Athough most chaotic MTDs come to rest on the relatively flat-lying shelf, at least one transport corridor reached the paleo continental slope break, indicating that some resedimented chalks may have been transported into deeper water where the slope morphology is complex.

4. Erosional features associated with failures above the Wyandot Formation appear to be closely tied to prodelta slope failures shed from overlying shelf perched deltas. This close association could mean that resedimented chalks are heavily mixed with prodelta shales, making them less desirable reservoir targets.

Acknowledgments We wish to thank Andrew MacRae for his detailed review of our paper and his generous contribution of ideas about Upper Cretaceous depositional systems on the Scotian Margin. A review from Grant Wach also helped sharpen our ideas.

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