FLUID ENTRAPMENT AND RELEASE FROM SEDIMENTS INTRUDED BY VOLCANIC SILLS, NEWFOUNDLAND MARGIN

Ocean Drilling Program Hole 1276A, at the toe of the SE facing Newfoundland Margin, provided access to more than 600 m of mid-Cretaceous, deep-marine, organic-rich mudstones with two intercalated volcanic sills near the base of the hole. A 10 m basaltic sill, dated as 105 Ma, was recovered at 1612 m below seafloor. Another sill, at least 17 m thick and recovered some 100 m deeper in the section, is 8 M.y. younger than the upper sill. Undercompacted, plastic, high-porosity sediments recovered from between the two sills yielded elevated concentrations of gaseous hydrocarbons.

Metamorphic minerals and textures characterise the narrow contact zones between sills and host sediment. Further away from the narrow metamorphic contact zones, the thermal aureole of the sills is recorded by unusual gradients in the alteration of organic and mineral components. In the poorly compacted, gas-bearing interval, however, organic maturity indicators reach a minimum where clay mineral assemblages indicate substantial alteration This discrepancy may be related to the unusually elevated porosity: Whereas overlying sediment was compacted to a porosity around 20%, porosity values up to 44% were retained in the gas-bearing interval. Pore-water overpressure may have delayed the maturation of organic matter whereas mineral transformation was probably favoured by ion supply from alteration near the underlying sill through the open pore framework.

Elevated concentrations of gaseous hydrocarbons were likely generated at the lower sill contact or in the underlying sedimentary section. Numerical simulation suggests that pore pressure build-up beneath the upper sill is strong enough to overcome its tensile strength. Mineralised veins within the metamorphic contact zone are thought to have precipitated from fluids that escaped from the compartment between the sills when pressure exceeded the strength of the overlying seal.

The intercalation of undercompacted sediments with sills creates a high-amplitude seismic reflector that covers much of the proximal Newfoundland Basin. We expect that sills and adjacent, normally compacted shales, created extensive, vertically stacked fluid barriers. Where pressure build-up was sufficient, fracturing of the sills allowed for intermittent fluid release, wheras other sills may remain unbreached.