Quantitative characterisation of deltaic and subaqueous clinoforms

Abstract

Clinoforms are ubiquitous deltaic, shallow-marine and continental-margin depositional morphologies, occurring over a range of spatial scales (1-10 4 m in height). Up to four types of progressively larger-scale clinoforms may prograde synchronously along shoreline-to-abyssal plain transects, albeit at very different rates. Paired subaerial and subaqueous delta clinoforms (or 'delta-scale compound clinoforms'), in particular, constitute a hitherto overlooked depositional model for ancient shallow-marine sandbodies. The topset-to-foreset rollovers of subaqueous deltas are developed at up to 60 m water depths, such that ancient delta-scale clinoforms should not be assumed to record the position of ancient shorelines, even if they are sandstone-rich. This study analyses a large dataset of modern and ancient delta-scale, shelf-prism-and continental-margin-scale clinoforms, in order to characterise diagnostic features of different clinoform systems, and particularly of deltascale subaqueous clinoforms. Such diagnostic criteria allow different clinoform types and their dominant grain-size characteristics to be interpreted in seismic reflection and/or sedimentological data, and prove that all clinoforms are subject to similar physical laws. The examined dataset demonstrates that progressively larger scale clinoforms are deposited in increasingly deeper waters, over progressively larger time spans. Consequently, depositional flux, sedimentation and progradation rates of continental-margin clinoforms are up to 4-6 orders of magnitude lower than those of deltas. For all clinoform types, due to strong statistical correlations between these parameters, it is now possible to calculate clinoform paleobathymetries once clinoform heights, age spans or progradation rates have been constrained. Muddy and sandy delta-scale subaqueous clinoforms show many different features, but all share four characteristics.(1) They are formed during relative sea-level stillstands (e.g., Late Holocene); (2) their stratigraphic architecture and facies character are dominated by basinal processes, and are quite uniform; (3) their plan-view morphology is shore-parallel and laterally extensive; (4) their sigmoidal cross-sectional geometry contrasts with the oblique profiles of most subaerial deltas. Holocene-age, delta-scale, sand-prone subaqueous clinoforms occur on steep (≥0.26°) and narrow (5-32 km) shelves, at typical distances of 0.6-7.2 km from the shoreline break. That contrasts with mud-prone subaqueous deltas, which form clinoforms on gently-sloping (0.01-0.38°), wide (23-376 km) shelves, at usual distances of 7.5-125 km from the shoreline. Delta-scale sand-prone subaqueous clinoforms have diagnostically steep foresets (0.7-23°). Similarly steep gradients were observed in much larger shelf-prism-and continental-margin-scale clinoforms. Gentler foreset gradients are shown by sand-prone subaerial deltas (0.1-2.7°), and mud-prone subaqueous and subaerial deltas (0.03-1.50°). Due to the lack of connections with river mouths, Holocene delta-scale sa...

The early Quaternary North Sea Basin

“…9). Each clinoform represents a suite of paleo-environmental conditions, from the shallow shelf to the bathyal environment, with associated variations in sediment grain-size distribution (Posamentier & Allen 1993;Stuart & Huuse 2012;Patruno et al 2014Patruno et al , 2015.…”

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

“…Slope-accretion clinoforms show important geometric changes, which are a consequence of variations on depositional conditions (e.g., Saller & Dharmasamadhi, 2012;Patruno et al, 2015; Fig. 7).…”

“…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.…”

“…This facies is interpreted as shelf-margin deltas (e.g., Porębski & Steel, 2003;Moreira & Carminatti, 2004;Patruno et al, 2015) or shoreface deposits (e.g., Bourget et al, 2014) formed when sand-rich progradational systems advanced beyond the original shelf edge. In seismic data, both deposits appear as smaller-scale prograding clinoforms, and in well data both show a sand-rich composition.…”

“…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.…”