| dc.description.abstract | The Early Carboniferous (Tournaisian) Grantmire Formation belongs to the Horton
Group and is ~800 m thick, based on exposures and drill core in the northern part of the Sydney
Basin onshore. The 503 m measured section ofthe Grantmire Formation in drillcore PE 83-1 is
dominantly pebble conglomerate with interbeds of siltstone and minor beds of sandstone. The
conglomerate (facies 1) is light to medium red, polymictic, poorly sorted, and clast supported
with subangular to subrounded clasts. Conglomerate beds reach 15 m thickness with a maximum
recorded clast size of 22 em. They are divided into three subfacies: interbedded pebble
conglomerate/sandstone, pebble to cobble conglomerate, and small boulder conglomerate. The
other facies are sandstone (facies 2), siltstone with multiple sandy layers (facies 3), coarse
siltstone (facies 4), and fine siltstone (facies 5). Siltstones are medium reddish brown and in two
facies have calcareous nodules with green reduction patches and/or envelopes suggesting paleosol
or shallow groundwater origin. Macroscale patterns suggest coarsening upward sequences on the
10-50 m scale and a rare 100 m scale are the result of fan progradation as indicated by thickening
upward trends and increasing clast size. Siltstone-rich intervals suggest distal fan or interfan
conditions. Mesoscale (<5m) coarsening upward sequences may represent small lobe or levee
progradation whereas large-scale fining upward sequences (5-10 m) are channel fills.
The Grantmire Formation has been interpreted as the clastic fill of fault-bounded basins
within the region of the Sydney Basin. Currently, the Grantmire Formation is the only mapped
unit of the Horton Group in the Sydney Basin. The presence of black shales in the Horton Group
is important for hydrocarbon potential regionally; they are not presently identified in the Sydney
Basin. The main clast types in the Grantmire Formation are chert, sedimentary lithoclasts,
quartzite, volcanic clasts, and granitic clasts. Chert is derived from an older sedimentary source
than the siltstone and sandstone clasts. Volcanic clasts are dominantly rhyolite with minor basalt
t}lat could have primary or reworked origins. Acidic plutons are the origin of granitic clasts and
likely provide a significant proportion of sand-sized quartz, feldspar, and mica.
Grantmire paragenesis begins with deposition of sand- and gravel-sized clasts with ironrich
clay. The clays were oxidized at the surface or in the shallow subsurface early in the
depositional history forming hematite grain rims. Calcite nodules with fine mosaic textures in
siltstone, are linked to shallow groundwaters. A locally pervasive poikilotopic calcite cement was
t~mplaced prior to significant burial. Calcite commonly partially replaces potassium feldspar
grains, possibly around the same time interval or subsequently. Dissolution of some grains, clays
and calcite cement post-dates consolidation and has generated secondary porosity.
Porosity of sandstones and conglomerates averages 9.6% and ranges from 4.2 to 15.7%
and permeability averages 2.26 md and ranges from 0.06 to 7.72 md. Reservoir quality ranging
from poor to good is likely controlled by variable amount of detrital clay, authigenic minerals,
carbonate cement, paleosol development, and irregular laminae of finer material. | en_US |
