Sedimentology of a Mudstone Source Rock in a Tectonically Dissected Carbonate Platform — The Black Cove Formation, Newfoundland, Canada
The Ordovician succession in Newfoundland represents one of the numerous hydrocarbon habitats related to Appalachian foreland basin evolution exhibiting shallow to deep shelf and adjacent slope to basinal sediments. While reservoir rocks are common within the Paleozoic carbonate platform succession, reservoir rocks are much less explored. One potential candidate for sourcing carbonate plays in western Newfoundland is the Black Cove Formation (BCF) of Darriwilian (Middle Ordovician) age. This study focuses on (1) understanding sedimentary processes that led to the formation of this high TOC-bearing unit, in order to (2) reconstructing the BCF depositional environment. The resulting model can then be used to predict the occurrence of this potential source rock in the subsurface.
The BCF is only about 4 meters thick in outcrop and consists entirely of black organic-rich mudstones with one intercalated decimeter-thick bed of pellet-rich carbonate pack to grainstone. The mudstones are massive throughout and characterized by mostly brownish to black clay-sized matrix containing sub-millimeter flakes of algal-derived organic material and carbonate silt grains. The silt is generally randomly distributed in the matrix but occasionally arranged in remnant sub-millimeter thick laminae. All BCF sediments are heavily bioturbated by vertical burrows penetrating up to several millimeters into the sediment, and in places also by horizontal Planolites trace fossils.
The abundance of burrows suggests that the BCF depositional environment was well oxygenated to allow for benthic life despite its elevated TOC content. Even though the unit is dominated by clay the silt laminae reflect occasional high-energy input of coarser material, likely by bed-load transport. The intense bioturbation, however, has destroyed all sedimentary structures and therefore it remains unclear how much of the clay and organic material was deposited by suspension or current activity. Nevertheless, the intercalated carbonate beds contain shallow-shelf grain size assemblages that do not show turbidite characteristics. It is therefore likely that the BCF was deposited in a deep shelf environment in close proximity to a shallow-water carbonate platform rather than in a deep oceanic basin. Its distribution should therefore follow the mid-Darriwilian carbonate platform environment rather than being directly associated with the lower slope deposits of the time-equivalent Cape Cormorant Formation.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California