Chemostratigraphic Recognition of a Disconformity in Mississippian Strata of the Northeast Appalachians, New Brunswick, Canada
Islam, Nazrul; Keighley, David
Mississippian strata host the only onshore gas field in Atlantic Canada at McCully, southern New Brunswick. Strata overlying the current tight-gas reservoir unit and shale-gas target (Albert Formation, Horton Group) consist of carbonate-evaporite rocks (Windsor Group) that are enclosed by ~ 2 km of redbeds assigned to a lower Sussex Group and upper Mabou Group. To date, this latter unit lacks significant marker beds and there is no significant biostratigraphic recovery, despite recent extraction of ~5 km of drill core from 8 wells. Research on this core relates to hydrogeological and stratigraphic issues with Mabou strata.
Broadly, mudstone, siltstone and sandstone at the base of the Mabou Group gradually coarsen up into conglomerate, and are considered the result of active alluvial fan progradation from the northeast. Also encountered in several of the cores was a single interval of localized, horizontally laminated to cross-stratified bluish grey sandstone, containing carbonaceous plant fragments and siltstone intraclasts. Although the interval can also be recognized in wireline logs, there does not appear to be a correlative reflector in seismic.
A total of 185 samples from four boreholes have so far been analyzed using ICP-MS and XRD. Chemostratigraphic analysis of various elemental ratios has revealed two packages bounded by an interval that correlates with the grey sandstone beds and intraclasts. Changes in the ratios are interpreted to mark a broader population of mineral species in the upper package. This further implies variation in the provenance and substrate environment of the alluvial-fan sediment either side of a disconformity represented by reduced horizons and rip-up clasts produced by sediment reworking along this boundary.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013