Geology of the Scotian Margin - Salt deformation
The early stages of rifting saw the deposition of thick sequences of salt, which formed by evaporation of seawater as shallow seas covered the evolving margin. The salt has flowed extensively due to subsequent sediment loading and, possibly, to periodic reactivation of the rift fault system during later stages of continental breakup. Salt pillows, diapirs and canopies are common in areas of thick salt, particularly in a major zone of diapiric structures which trends beneath the continental slope from eastern Georges Bank to the western Grand Banks..
The following article by John Shimeld, published in the "Bedford Institute of Oceanography 2003 Review" and reprinted here with permission, summarizes recent studies of salt structures and their effects on sediment deposition and stratigraphy beneath the Scotian Slope.
The first offshore well drilled in search for oil and natural gas within the Scotian Basin was Tors Cove D-52 in 1966. The drilling location was chosen on the basis of seismic reflection data that revealed 1200 m of uplifted strata forming an anticline (a convex-upward fold) above an upright, roughly cylindrical mass of salt known as a diapir. Salt comprising the diapir originated from the more deeply buried Argo Formation which, in this region of the basin, is covered by about nine kilometers of sedimentary rocks. Since 1966, hundreds of salt diapirs, exhibiting a fascinating array of geometries, have been discovered and mapped throughout the Scotian Basin using seismic reflection data. Salt diapirs are also widespread beneath the Grand Banks of Newfoundland, the Gulf of Saint Lawrence, and regions of New Brunswick, Nova Scotia, and Prince Edward Island.
Figure 1. This is a map of the thickness (in km) of Mesozoic and Cenozoic sediments within the Scotian Basin. Shallowly-buried salt diapirs are indicated by purple polygons. Five distinct zones are defined beneath the deep-water region on the basis of diapiric styles and sedimentation patterns. The present-day 200 m isobath (blue line) marks the edge of the continental shelf. Well locations are indicated by the black dots. Tors Cove D-52 was the first exploratory well drilled in the Scotian Basin.
Salt diapirs create many favourable geometries for entrapment of oil and natural gas. Indeed, a significant proportion of the world's supply is tapped from geological structures directly associated with salt diapirs, so it is not surprising that they are considered attractive exploration targets in Atlantic Canada. It is surprising though that the mechanisms responsible for their creation and growth were discovered so recently, especially considering the importance of salt diapirs not just to the petroleum industry but also to the mining and chemical industries. Even the nuclear industry has given serious consideration to disposal of radioactive waste in salt diapirs, so it is important to understand how salt deforms and why diapirs exist. (Salt mines currently operate in Nova Scotia, New Brunswick, and the Magdalen Islands.)
Since salt that is buried deeper than 1.5 to 2.0 km is less dense than the overlying sedimentary rocks, the prevailing hypothesis, until the mid 1980s, was that buoyancy forces cause the salt to rise vertically, in a viscous manner, intruding and uplifting the overlying layers. By the late 1980s, serious challenges to this theory were posed by technological advancements in seismic imaging of the subsurface that clearly showed previously unsuspected relationships between diapirs and surrounding rocks. During the same time, exploration drilling in heavily explored regions like the Gulf of Mexico confirmed the existence of tabular, subhorizontal salt bodies that have been termed canopies. Some canopies cover thousands or even tens of thousands of km2, are completely detached from their source layer ten or more km below, and have moved laterally from 50 to 100 km.
Figure 2.
i. This is a seismic reflection image of the diapiric structure that, in 1966, prompted drilling of Tors Cove D-52, the first exploratory well in the Scotian Basin. Salt is indicated in purple. Only minor amounts of natural gas were detected but, three years later, gas was discovered in the same basin above the crest of a salt diapir, 20 km southwest of Sable Island, at the Onondaga field.
ii. Modern seismic reflection data clearly show the presence of a salt canopy that has moved both vertically and laterally seaward due to differential loading by sediments.
iii. Continual interaction between sedimentation and salt deformation has created these structures which are highly attractive for hydrocarbon exploration
Vertical buoyancy forces do not explain lateral movement of salt. It is now understood that salt deforms in a plastic manner.like silly putty over geological timespans.under the influence of even relatively small differential loads. This happened within the Scotian Basin, during the Jurassic and Early Cretaceous (from about 200 to 100 million years ago), as rivers carried sediment from the eroding Appalachian mountains in the west and northwest, and deposited it in a seaward tapering wedge along the margin of the nascent North Atlantic ocean. The ocean had formed in response to rifting of Pangea, the supercontinent that split to eventually become Africa and North America. During the rifting process, which had begun some 20 million years earlier in the Late Triassic, shallow saline lakes and inland seas evaporated vigourously under the influence of a hot and arid climate, and that left behind extensive deposits of salt, perhaps as much as one to two km thick in some regions (the general setting was similar to the modern rift system that extends southeastward from Jordan, beneath the Red Sea, and southward along the African Rift Valley between Ethiopia and Mozambique). Thus, once continental rifting had finished and the Atlantic Ocean began to form in the Jurassic, the tapering wedge of sediment was deposited on top of the salt. At a regional scale, this created a differential load that caused the salt to be expelled both vertically and laterally.
Figure 3. These are three frames from a numerical model created to simulate the interaction between sedimentation and salt deformation. The model is being developed by Ph.D. candidate Steven Ings, with guidance from GSC-Atlantic researchers and members of the Dalhousie University Geodynamics Group headed by Dr. Chris Beaumont. Notice that the salt, shown in purple, begins deforming as soon as it experiences differential loading. Many of the geometries seen in this model closely resemble structures that have been mapped in the Scotian Basin using modern 2-D seismic data.
Myriad diapir and canopy geometries are the result of variations in the pattern of sedimentation and differential loading. Almost as soon as sediments accumulate on top of it, a salt layer will begin to deform. Loci of deposition form naturally in seafloor lows and, as sediments accumulate there, the differential load affecting the underlying salt increases. This forces salt from beneath the depositional centres and into nearby diapirs. Growth of the diapirs keeps pace with and directly influences the sedimentation, at least until the source layer of the salt is depleted. Such revelations, during the late 1980s and early 1990s, significantly altered hydrocarbon exploration concepts and strategies for the many sedimentary basins around the world where salt diapirs exist.
While these concepts were being developed elsewhere, exploration activity within the Scotian Basin had come to a near standstill. In 1998, a new cycle of exploration began, focussed primarily on the continental slopes of eastern Canada in water depths ranging between 200 and 3500 m. In the Nova Scotian jurisdiction, for example, companies have committed $1.5 billion to explore 70,000 km2 of licensed holdings over the continental slope. Since 1998, several hundred thousand km of modern seismic reflection data have been acquired in the deep-water region which, not coincidentally, overlies the highest concentration of salt diapirs within the Scotian Basin.
The deep-water is truly a frontier region: only 11 exploratory wells have been drilled (compared with a total of 134 exploratory wells for the entire basin). So little is known about the geology that publicly available maps, like those published in 1990 by the Geological Survey of Canada, are highly conceptual or even blank for the deep-water region of the Scotian Basin. To fill the gaps, GSC-Atlantic researchers are interpreting 34,000 km of high quality seismic reflection data acquired during 1998/9 by a company named TGS-Nopec. These data allow, for the first time, an assessment of salt-sediment interaction in the basin using the modern insights gained elsewhere. As a result, five distinct zones have been defined beneath the deep-water region, among which there are significant differences in the morphology of the diapirs and the pattern of sedimentation. The differences are linked to regional variations in sediment flux to the basin through time and have important implications for oil and gas exploration. The growth history of individual diapirs also provides valuable clues that can help to unravel the constant interplay between regional subsidence or uplift and global fluctuations in sea level and climate throughout the evolution of the Scotian Basin. These insights will underpin future geological models of the nature and distribution of hydrocarbon reservoirs and seals in the deep-water region.
Interpretations produced by GSC-Atlantic researchers provide regional context for companies that focus on site-specific assessments of hydrocarbon potential within their licence holdings. The Minerals Management Service of the U.S. Department of the Interior is also using the latest GSC-Atlantic interpretations for their comparative basin studies and global resource assessments. Federal/provincial regulations, enacted through the Canada-Nova Scotia Offshore Petroleum Board, stipulate that the seismic data are confidential until 2008/9. However, through a research agreement signed in 2001 with the data owner (TGS-Nopec), GSC researchers have received permission to publish numerous aspects of their research. Half a dozen oral and poster presentations have been given at various local, national, and international conferences, and a manuscript has been accepted for a special publication of the 24th annual Bob F. Perkins Research Conference of Salt-Sediment Interactions, to be held in Houston in December 2004. The data continue to be valuable for GSC projects on basin assessment, geo-hazards, and gas hydrates, and have helped to create important links with academic researchers.