Shelf‐margin clinoforms and prediction of deepwater sands

Abstract

Early Eocene successions from Spitsbergen and o¡shore Ireland, showing well-developed shelfmargin 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 signi¢cant 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 di¡erent segments. Although this sedimentbudget partitioning is controlled by sediment type and £ux, shelf width 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 riverdominated deltas are the most e⁄cient delivery systems for dispersing sand into deep water beyond the shelf-slope break. In addition, low-angle or £at, channelled shelf-edge trajectories associate with co -eval deepwater slope and basin-£oor sands, whereas rising trajectories tend to associate with muddy slopes and basin £oors. Characteristic features of the shelf-edge, slope and basin-£oor segments of clinoforms for these trajectory types are documented. Seismic lines along the slope to basin-£oor 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-¢lled 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.

Towards the standardization of sequence stratigraphy

“…A key geometric element to be observed is the paleo-shelf break and its trajectory in a prograding package. The trajectory of the paleoshelf break (Steel and Olsen, 2002) is used to interpret changes in accommodation on the shelf (area located updip from the shelf break) (e.g., Johannessen and Steel, 2005;Carvajal and Steel, 2006). A sigmoid prograding package shows a parallelism of the upper stratal segments (topsets) indicating a continuous up-building (aggradation) of the topsets during progradation, and an upstepping shelf-break trajectory ( Figure 14).…”

Seismic expression of depositional elements associated with a strongly progradational shelf margin: northern Santos Basin, southeastern Brazil

“…Descending trajectories and truncated topsets indicate that their genesis is related to periods of relative sea-level fall. Variations of seismic amplitudes are associated to the transition from proximal sandy deposits to distal muddy deposits, while amplitude peaks in foreset areas suggest the occurrence of intra-slope (shingled) sand-rich turbidites (e.g., Johannessen & Steel, 2005).…”

“…In many cases, these high-relief clinoforms are contiguous to the shelf-margin deltas/shoreface deposits (seismic facies A), being interpreted as low-angle tangential (oblique) slope clinoforms (e.g., Porębski & Steel, 2003;Johannessen & Steel, 2005;Patruno et al, 2015). Descending trajectories and truncated topsets indicate that their genesis is related to periods of relative sea-level fall.…”

“…It is important to consider that clinoforms in seismic data are depositional forms with dimensions varying from few hundreds of meters to several kilometers (slope-accretion clinoforms; Steel & Olsen, 2002;Johannessen & Steel, 2005). Slope-accretion clinoforms are composed of topset, foreset and bottomset domains.…”

“…7D). Clinoforms from seismic facies C are interpreted as sigmoidal slope clinoforms (e.g., Porębski & Steel, 2003;Johannessen & Steel, 2005;Patruno et al, 2015), deposited during base-level falls. The external geometry of the clinoform sets is of high-relief slope-accretion prisms, limited on the top by subaerial unconformities (Fig.…”