Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021 Confined turbidite systems SIMON A. LOMAS 1 & PHILIPPE JOSEPH 2 1Baker Atlas Geoscience, Stoneywood Park North, Aberdeen AB21 7EA, UK (e-mail." [email protected]) 2IFP School/Geology-Geochemistry Division, Institut Franfais du P~trole, 228-232, avenue Napoleon Bonaparte, 92852 Rueil Malmaison Cedex, France (e-mail." [email protected]) Abstract: Deep-water sedimentation is currently a major focus of both academic research and industrial interest. Recent studies have emphasized the fundamental influence of sea- floor topography on the growth and morphology of submarine 'fans': in many turbidite systems and turbidite hydrocarbon reservoirs, depositional system development has been moderately to strongly confined by pre-existing bounding slopes. This publication examines aspects of sediment dispersal and accumulation in deepwater systems where basin-floor topo- graphy has profoundly affected deposition, and the associated controls on hydrocarbon reservoir architecture and heterogeneity. The papers herein offer a global perspective which is wide-ranging in terms of both approach and location, including contrasting case studies of outcrop, subsurface, modern and experimental systems. Confined turbidite systems are those deepwater are applied equally to process (ponded turbidity clastic depositional systems whose development current), depositional product (ponded turbidite) has been fundamentally constrained by pro- and basin/stratigraphic behaviour (ponded depo- nounced basin-floor topography. Many early centre). We favour the more general term con- conceptual and experimental models of turbidity finement to describe situations where sediment current deposition and submarine fan develop- gravity flows and their deposits are appreciably ment envisaged essentially unconfined radial affected by the presence of significant basin- development of lobe and fan-shaped deposi- floor topography, but without the connotation tional bodies. However, a wealth of case studies of complete containment. now available from many contrasting turbidite Closely associated with the concept of confined systems worldwide makes it clear that, in many deposition are the notions of flow reflection and basins, both sediment dispersal patterns and the deflection. In their study of ponded Quaternary geometries of depositional bodies have been pro- sediments in valleys on the flanks of the foundly affected by pre-existing or developing Mid-Atlantic Ridge, Van Andel & Komar (1969) basin-floor relief. It seems that perhaps most speculated that the rebound of turbidity currents large natural turbidity currents, particularly in off the bounding slopes was responsible for intracontinental basins, are not free to spread repeated grading profiles seen in their piston radially across a uniform basin floor. cores. Picketing & Hiscott (1985) recognized con- Van Andel & Komar (1969) used the term tainment of turbidity currents in the Cloridorme ponding to refer to a situation in which turbidity Formation (Ordovician of Qu6bec) from reversals currents of sufficiently large volume are fully of palaeocurrent indicators, which they inter- contained by an area of enclosed bathymetry. preted as the result of deflection and reflection of Similarly, Picketing & Hiscott (1985) used the part of large-volume turbidity currents from con- term contained turbidites to describe beds depos- fining slopes. Similar evidence of divergent palaeo- ited from turbidity currents that were confined flow indicators has subsequently been recognized within a basin too small to permit sustained at outcrop in various other successions represent- unidirectional flow. The terms ponding and, to a ing depositional systems where turbidity currents lesser extent, containment, are now generally have interacted with basin-bounding or intra- used to indicate a situation in which sediment basinal slopes (e.g. Kneller et al 1991; Smith, this gravity flows fill the floor of an enclosed depres- volume), and the likely effects of such interaction sion and are unable to surmount the bounding have been widely investigated through physical slopes (or 'sills') which bound that depositional experiments (e.g. Simpson 1987; Muck & Under- area ('mini-basin', 'silled sub-basin' or 'ponded wood 1990; Alexander & Morris 1994; Kneller depocentre'). Thus, essentially all of the sediment 1995; AI Ja'Aidi et al., this volume). carried into the depositional area by that flow will Implicit in the concept of confinement is the be trapped and deposited there. The same terms relationship of flow magnitude to size of From: LOMAS, S. A. & JosEen, P. (eds) 2004. Confined Turbidite Systems. Geological Society, London, Special Publications, 222, 1-7. 0305-8719/04/$15.00 9The Geological Society of London 2004. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021 2 S. A. LOMAS & P. JOSEPH depocentre. The general concept of flow efficiency in varied modern and ancient examples. The (e.g. Mutti 1979) is helpful here, describing a author emphasizes the contrasting behaviour flow's ability to deliver sediment, especially and architectural characteristics of coarse- sand, in a basinward direction. Thus, small grained/sand-rich and fine-grained/mud-rich volume, low-efficiency sediment gravity flows end-member systems but reminds us of the may behave in an effectively unconfined manner inadequacy of general models in capturing the even in small basins. Conversely, very large mag- variability of these real cases. The practical lim- nitude flows may interact with marginal confining itations of idealised models arise either from slopes even in large basins. Of course, if a con- emphasis on only one or two aspects or from fined depocentre is filled more quickly than the over-simplification (being too general to display topographic relief grows, the surface area of the architectural details). Bouma advocates the con- basin floor will increase with time. Hence, if struction of case-specific models based on a sediment supply characteristics remain similar, reconstruction of the specific natural controls successive depositional systems would be and their interactions in the generation and deliv- expected to :reflect progressively decreasing con- ery of sedimentary particles, the evolving basin finement. The reverse scenario could develop in configuration and processes of deposition and certain compressional settings. possible redeposition in that basin. In most of the studies detailed in this volume, Smith takes an alternative look at patterns of the origin of the confining topography is tectonic: sediment dispersal on topographically complex large-scale structural features, local fault scarps, slopes, distinguishing two conceptual end- folds or more subtle perturbations of basin-floor member systems, which he refers to as 'cascades gradients with associated tilting and faulting. In of silled sub-basins' and 'connected tortuous other instances, basin-floor relief is the result of corridors'. The first scenario is essentially a refine- slope collapse masses, halokinesis, mud diapirism ment of the 'fill-and-spill' model that emerged and, to a limited extent, differential compaction. from studies of the northern Gulf of Mexico The collection of papers hosted in this volume slope (e.g. Winker 1996; Prather et al 1998). In is not intended to provide an exhaustive treat- the 'connected tortuous corridors' scenario, ment of the subject of confined deepwater flows partly circumvent bathymetric obstacles, sedimentation, but rather to bring together a but follow a (laterally confined) continuous tortu- diverse assortment of approaches and observa- ous path down the slope. From two-dimensional tions which have a bearing on this topic. We sections, these two scenarios may be difficult to have ordered the contributions to reflect the distinguish (i.e. apparently separate sub-basins varying approaches of the researchers. Here may be directly connected out of the plane of we present an overview of those contributions section), although recognition of stratal onlap and the new perspectives they afford, and draw against a confining frontal slope should be diag- attention to promising channels (and lobes?) for nostic of the fill of a true silled sub-basin. future research. We are pleased that several of the systems described in the following papers are not widely known. It is hoped that the juxta- Experimental studies position of conceptual reviews, experimental work and case-studies of outcrop, subsurface Laboratory experiments have offered crucial and modern systems will stimulate new insights insights into the potential effects of basin-floor and connections and highlight key general ques- topography on sedimentation (e.g. Simpson tions which surmount the usual bounding slopes 1987; Kneller et al., 1991; Alexander & Morris of these scientific sub-communities. 1994). A! Ja'Aidi et al. outline a series of scaled experiments (using ballotini and silica flour to model sand and mud) to investigate how flow Overviews of confined turbidite systems volume, density and grain-size distribution affect the transport efficiency of turbidity cur- Two review papers introduce the broad-scale rents (i.e. their ability to carry 'sand' basinward) context of controls on, and patterns of, confined and the geometric characteristics of the resulting turbidite system development from contrasting deposits. In general, these experiments reaffirm viewpoints. intuitive expectations