--> Abstract: Why Does the Naskapi Mudstone "Blanket" Prograde the Paleo-Shelf Edge Offshore Nova Scotia? by Donald I. Cummings, Robert W. Dalrymple, and R.W.C. (Bill) Arnott; #90039 (2005)

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Why Does the Naskapi Mudstone "Blanket" Prograde the Paleo-Shelf Edge Offshore Nova Scotia?

Donald I. Cummings1, Robert W. Dalrymple1, and R.W.C. (Bill) Arnott2
1 Queen's University, Kingston, ON
2 University of Ottawa, Ottawa, ON

The Naskapi Member of the Logan Canyon Formation (Aptian) forms a regionally extensive terrigenous mudstone “blanket” (<300 m thick and >1000 km parallel to depositional strike) over hydrocarbon-rich sandstones of the Missisauga Formation in the passive margin basin offshore Nova Scotia. Although a substantial regional landward translation of the shoreline might conventionally be interpreted at the Missisauga-Naskapi contact, evidence for a major transgression/regression is in fact lacking in seismic data from the Sable Subbasin (i.e., there is no obvious downlap surface). Rather, Naskapi reflections are parallel and, where the paleoshelf-slope transition can be resolved, correlate with progradational reflections at the paleoshelf-edge. Core and well-log data from the Panuke Field, located ~25 km updip of the paleoshelf-edge, show that the Naskapi, despite being mudstone-rich, is composed of stacked, upward-coarsening units (“parasequences”) that are capped by wave-ravinement lags and contain rare hummocky cross-stratified fine sandstone interbeds. The occurrence of upward-coarsening units with wave-eroded tops, in addition to the lack of seismic downlap, suggests that transport of mud to the paleoshelf-edge during deposition of the Naskapi Member was facilitated by multiple high-frequency falls of relative sea-level that forced shorelines repeatedly back to, or at least close to, the shelf edge, with parallel reflections representing transgressive surfaces, not passive mudstone drapes. The Missisauga-Naskapi contact in the Sable Subbasin may therefore reflect, at least in part, a change in sediment supply, not major deepening.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005