Shelf-margin clinoforms and prediction of deepwater sands
E.P. Johannessen and R.J. Steel
Journal name: Basin Research
Issue: Vol 17, No 4, December 2005 pp. 521 - 550
Info: Article, PDF ( 25.16Mb )
Early Eocene successions from Spitsbergen and offshore Ireland, showing well-developed shelf margin clinoforms and a variety of deepwater sands, are used to develop models to predict the presence or absence of turbidite sands in clinoform strata without significant slope disturbance/ ponding by salt or mud diapers. The studied clinoforms formed in front of narrow to moderate width (10-60 km) shelves and have slopes, 2-41, that are typical of accreting shelf margins. The clinoforms are evaluated in terms of both shelf-transiting sediment-delivery systems and the resultant partitioning of the sand and mud budget along their different segments. Although this sediment budget partitioning is controlled by sediment type and flux, shelfwidth and gradient, process regime on the shelf and relative sea-level behaviour, the most tell-tale or predictive signs in the stratigraphic record appear to be (1) sediment-delivery system type, (2) degree of shelf- edge channelling and (3) character of shelf- edge trajectory through time. The clinoform data sets from the Porcupine Basin (wells and 3-D seismic) and from the Central Basin on Spitsbergen (outcrops) suggest that river dominated deltas are the most efficient delivery systems for dispersing sand into deep water beyond the shelf- slope break. In addition, low-angle or flat, channelled shelf-edge trajectories associate with co-eval deepwater slope and basin- floor sands, whereas rising trajectories tend to associate with muddy slopes and basin floors. Characteristic features of the shelf-edge, slope and basin- floor segments of clinoforms for these trajectory types are documented. Seismic lines along the slope to basin- floor transects tend to show apparent up-dip sandstone pinchouts, but most of these are likely to be simply sidelap features. Dip lines aligned along the axes of sandy fairways show that stratigraphic traps are unlikely, unless slope channels become mud- filled or are structurally partitioned. Another feature that is prominent in the data sets examined is the lack of slope onlap. During the relative rise of sea level back up to the shelf, the clinoform slopes are generally mud-prone and they are characteristically aggradational.