Transitional and Composite Flow Deposits: Character and Distribution in the Maastrichtian Springar Fm., Vøring Basin, Norwegian Sea
Southern, Sarah J.; Kane, Ian A.; Warchol, Michal; Porten, Kristen W.; McCaffrey, William D.; Mountney, Nigel
Transitional and composite flow deposits (e.g. beds dominated by compositionally-banded sandstone and clean sand with overlying, potentially linked, highly argillaceous sand, respectively) record deposition from flows with spatially and temporally variable turbulent-laminar rheologies, and commonly exhibit strongly contrasting reservoir quality at an intra-bed scale. Despite their importance in sediment transport and deposition, and their increased recognition, predictive concepts for their distribution are in their relative infancy. The Hvithval Member of the Maastrichtian Springar Fm. comprises a south-westward dispersing deep-water system in the Vøring Basin, Norwegian Sea. Five cores have been cut along a c.150 km downstream transect; graphic logs totaling ~240 m were collected and analyzed alongside thin-sections from key facies, conventional core plugs, continuous mini-permeametry data and wireline data in order to characterize deposit distribution.
A range of flow types is inferred (e.g. high- and low-density turbidity currents, transitional flows and debris flows) from a variety of bed types characterized by differing relative proportions of discrete depositional facies, each contrasting in their grain size, sorting, clay type and distribution, and thus porosity and permeability characteristics. Downstream there are trends of increasing ratio of composite and transitional flow deposits to turbidite deposits, grain size decrease, clay content increase and a corresponding reduction in reservoir quality. Such variation in the longitudinal distribution of bed types within the system demonstrates downstream flow transformation from relatively turbulent and well mixed flow to progressively less turbulent, higher concentration flow containing segregated distinct rheological divisions. Transformation is considered to have arisen from relative concentration increase via one or more of the following: 1) entrainment of fine-grained material; 2) deceleration associated with flow expansion; 3) longitudinal segregation of sediment types. Stacking of bed types into cleaning- (e.g. reducing composite flow deposit proportions), coarsening- and thickening-upward trends, in some cases followed by the reverse trend (albeit thinner), suggest stages of system progradation and retreat. These observations provide a means for interpreting spatial and stratigraphic distributions of "non-classical' deep-water deposits and offer broader insight into system development.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013