Fresh-water and salt-water distribution in passive margin sediments: Insights from Integrated Ocean Drilling Program Expedition 313 on the New Jersey Margin

Abstract

International audienceOn the New Jersey shelf (offshore North America), the presence of pore water fresher than seawater is known from a series of boreholes completed during the 1970s and 1980s. To account for this fresh water, a fi rst hypothesis involves possible present-day active dynamic connections with onshore aquifers, while a second involves meteoritic and/or sub- ice-sheet waters during periods of lowered sea level. Expedition 313 drilled three boreholes on the middle shelf, offering a unique opportunity for the internal structure of the siliciclastic system to be accessed, at scales ranging from the depositional matrix to the continental margin. This enables the stratigraphic architecture to be correlated with the spatial distribution and salinity of saturating fl uids. Expedition 313 revealed both very low salinities (<3 g/L) at depths exceeding 400 m below the seafl oor and evidence for a multilayered reservoir organization, with freshand/ or brackish-water intervals alternating vertically with salty intervals. In this study we present a revised distribution of the salinity beneath the middle shelf. Our observations suggest that the processes controlling salinity are strongly infl uenced by lithology, porosity, and permeability. Saltier pore waters generally occur in coarse-grained intervals and fresher pore waters occur in fi ne-grained intervals. The transition from fresher to saltier intervals is often marked by cemented horizons that probably act as permeability barriers. In the lowermost parts of two holes, the salinity varies independently of lithology, suggesting different mechanisms and/or sources of salinity. We present an interpretation of the sedimentary facies distribution, derived from core, logs, and seismic profi le analyses, that is used to discuss the marginscale two-dimensional reservoir geometry and permeability distribution. These proposed geometries are of primary importance when considering the possible pathways and emplacement mechanisms for the fresh and salty water below the New Jersey shelf

3D characterisation and quantification of an offshore freshened groundwater system in the Canterbury Bight

“…5b). These permeabilities are on the middle to upper end of the range of permeability measurements made for coastal plain and marine sediments 22,76,84 . Measured permeability anisotropy (k x /k z ) from sediment cores is typically around 3-10 for clastic materials 85 .…”

“…This stems from the fact that our understanding of OFG systems is predominantly based on offshore borehole data from legacy drilling campaigns and incidental discoveries in petroleum wells 20 . The coverage of these borehole data is limited 1 , and direct observation of the aquifer structure and geochemical characteristics of OFG remain rare 21,22 . In addition, most measurements and research efforts related to OFG have focused on the nearshore zone 23 .…”

“…In addition, most measurements and research efforts related to OFG have focused on the nearshore zone 23 . The control of the geological environment on the spatial distribution and flow of OFG is also poorly constrained 22,24 . Contrasting results have been reported, with freshwater being preferentially stored in coarse-grained sandy deposits 20 or in fine-grained clay intervals 22 , and with faults acting as both barriers and conduits 25 .…”

Scientific Drilling Across the Shoreline

“…Expedition 313 confirmed the assumption that sequence boundaries are the primary source of impedance contrasts, hence, seismic reflections (Miller et al, 2013a), tested sequence stratigraphic models with core-log-seismic integration (Miller et al, 2013b;Proust et al, 2018), and provided amplitudes of Miocene sea level change, including the influence of mantle dynamic topography (Kominz et al, 2016). Drilling in this New Jersey nearshore setting also identified three groundwater sources: marine seawater, deep-sourced brines, and meteoric freshwater that represents a potential resource for future generations (Lofi et al, 2013;Van Geldern et al, 2013). Integration of nearshore drilling by Expedition 313 with previous onshore and deeper water offshore drilling has established the mid-Atlantic US margin as a natural laboratory for understanding the cause, history, and consequence of sea level change on the sedimentary record and the nearshore distribution of groundwater resources.…”

The development of ocean test beds for ocean technology adaptation and integration into the emerging U.S. offshore wind energy industry

“…In the New England area, there is evidence suggesting that fresh water forced into the subsurface beneath glaciers during lowstands (Siegel et al, 2012;2014a), and now buoyant relative to seawater, forms localized regions of overpressure in the subsurface (Siegel et al, 2014b) that may present a hazard to large structures. Offshore New Jersey, scientific drilling has confirmed the presence of fresh groundwater within confined units beneath the shelf that may be in hydrologic equilibrium with the terrestrial groundwater system (Lofi et al, 2013). This water is also gravitationally unstable and thus prone to produce seepage sites if confining units are breached.…”