Abstract

Our selected major landmarks for the past half-century of turbidite studies are based on more than 50 years of research in turbidite systems by each of us. Considering that our studies focused mainly on modern systems, we present here some major highlights for research on turbidite systems from our prospective. Time constraints limit the number of highlights we can cite and discuss and that we do not include experimental and modeling research. Prior to the past 50 years, Forel (1885, 1895) measured an underwater channel and described sediment-laden density currents that built the Rhone Channel and levee in Lake Geneva, Switzerland, in the late 1800s. Significant advances about earthquake triggering, turbidity-current flow and graded sand deposits were made in the 1950s by Arnold Bouma’s major professor Ph. H. Kuenen (e.g., Kuenen and Migliorini, 1950). In the 1950s and early 1960s, researchers such as Bruce C. Heezen, Maurice Ewing, and David Ericson confirmed the existence of turbidity currents in the modern ocean through core studies and documentation of submarine cable breaks (e.g., Heezen and Ewing, 1952; Ericson et al., 1952; Heezen et al., 1964).

In the 1960s, the Bouma (1962) sequence, which Arnold developed based on outcrops, provided immediate relevance for the characterization of turbidites in modern submarine environments (e.g., Astoria Fan a and b structures in proximal channels, Tc–Te in levees, Ta–Te in lobes, and d and e in basin plains), as well as for ancient outcrops, and industry boreholes (Figs. 1–3) (Nelson, 1968; Nelson and Nilsen, 1984; Nelson et al., 2009a). Bouma’s (1962) model of turbidite systems based on the Ta–Te sequence is still relevant today for base-of-slope sand-rich aprons that are not channelized (Fig. 4). Studies of modern turbidite systems in the late 1960s soon recognized the importance of channelized deposition in small-sized (5–100 km) and large-sized (100s of km) unconfined submarine fans (Fig. 5) (e.g., Nelson, 1968; Nelson et al., 1970; Normark, 1970). The Bouma (1962), Nelson (1968), and Normark (1970) models still provide the basic depositional patterns for unconfined turbidite systems.

In the early 1970s, Mutti and Ricchi Lucchi (1972) presented a submarine fan model based on outcrop studies. Their turbidite facies associations proposed in this model has continued to provide an excellent key to help researchers understand and compare modern and ancient turbidite systems (Fig. 6). In particular, their recognition of inner fan fining-upward channel fill and outer fan, prograding thickening-upward sequences still remains a standard approach to understanding outcrops and subsurface boreholes (Fig. 7) (Mutti and Ricchi Lucchi, 1975). In the late 1970s, the comparison of modern and ancient thin-bedded turbidites ended the debate that thin turbidites equaled distal turbidites and distinguished proximal, thin, highly structured levee turbidites from thin, more flat-laminated, truly distal turbidites (Mutti and Ricci Lucchi, 1972; Nelson et al., 1975, 1978).

In the early 1980s, the introduction of side-scan sonar and bathymetric swath mapping combined with high-resolution seismic studies revealed the complex morphology and architecture of modern submarine fans and their formation by turbidity-current processes. A GLORIA side-scan survey of the Amazon Fan revealed that turbidity currents can form highly meandering distributary channels and that these channels switched courses across the fan by avulsion (Fig. 8) (e.g., Damuth et al., 1982, 1988). Subsequently, these features have been documented to be ubiquitous on modern and ancient fans. Side-scan sonar and bathymetric swath mapping has also led to recognition of more detailed turbidite system depositional patterns in unconfined and confined basin settings and better definition of the main tectonic, sediment supply and climatic/sea level controlling factors (Figs. 9 and 10) (Nelson, 1983; Nelson and Maldonado, 1988).

In the late 1980s, Mutti and Normark (1987) introduced the hierarchy of turbidite system scales and the particularly important concept of the main turbidite system elements of erosive features, channels, levees, and lobes (Fig. 11) (Mutti and Normark, 1987). A significant step forward in ground truthing the lithology of these turbidite system elements was accomplished on Deep Sea Drilling Project (DSDP) Leg 96, which for the first time, drilled several holes on a modern fan, the Mississippi Fan. Led by Arnold Bouma, Leg 96 recovered sand and gravel from the most recently active channel on the middle fan and thick silt/sand turbidites from the lower fan lobes (Fig. 12) (Bouma et al., 1985).

Another important step forward in the late 1970s and into the 1980s was the development by industry of a depositional model for deepwater sedimentation based on seismic-sequence stratigraphy. The Vail-Exxon sea-level model helped define an entire new concept of continental-margin stratigraphy based on sea-level changes (Fig. 13) (Mutti, 1985; Vail et al., 1977, 1991). Architectural features of deep-sea fans (channel-levee systems, lobes, etc.) were integrated into the Vail-Exxon model (e.g., channelized slope fan, basin-floor fan sheet sands) (Mitchum, 1985; Vail et al., 1991). However, the conceptual models of sequence stratigraphy, which separated earlier basin floor fan deposition from later channelized slope development, were at odds with earlier models (Figs., 5, 6, and 9), and all studies of modern turbidite systems that showed feeding canyons and channels were always coeval with basin floor deposits (Figs. 5 and 14).

Beginning in the late 1980s and into the early 1990s, deep-tow side-scan sonar together with precise bottom transponder navigation revealed the complexity of channel and lobe morphologies and lithologies (Fig. 14) (Nelson et al., 1992; Twichell et al., 1992). By the late 1990s, industry exploration, using time slices from 3D seismic data, revealed detailed resolution of various architectural elements (e.g., bypass channels, ponded turbidites, and mass transport deposits [MTDs]) of deeply buried productive fans and turbidite systems at resolutions that finally permitted comparison with modern systems (Fig. 15) (Prather et al., 1998; Damuth et al., 2006). The 3D studies confirmed the relevance of using present-day turbidite systems as a key to evaluating ancient outcrop and subsurface systems.

In the new millennium, 3D seismic geomorphology and detailed outcrop studies have improved on prior modern/ancient source to sink comparisons, as well as provided new insights into the complex interplay of MTD, bottom-current, and turbidite deposits. Industry and academic studies have outlined the detailed seismic and lithologic facies of mini-basins, backstop basins, and bypass-channel facies. Reservoirs in minibasins include ponded turbidites, perched fans and bypass channel deposits that are intermixed with chaotic extrabasinal and intrabasinal mass-transport deposits (Figs. 16 and 17) (e.g. Beaubouef and Friedman, 2000, Beaubouef et al., 2003, Nelson et al., 2009b: Bohn et al., 2012; Prather et al., 2012). These detailed seismic geomorphology studies also have been crucial for petroleum reservoir development in complex channel fills of continental margins such as the Gulf of Mexico and West Africa (Fig. 18) (e.g., Posamentier and Walker, 2006).

To define better petroleum reservoir architectures, industry and academic (Deep Sea Drililng Project [DSDP], Ocean Drilling Program [ODP], and Integrated Ocean Drilling Program [IODP]) drilling, have outlined differences in turbidite-system stratigraphy between active and passive continental margins as well as identified new varieties of complex turbidite lithologies, such as linked debrites and seismo-turbidites (Figs. 19–22). Combined studies of swath bathymetry, high-resolution sides-can sonar, detailed coring, deep-sea drill holes and industry boreholes have been particularly important for understanding lobe facies, which are not simple sheet-like deposits (Fig. 20) (Nelson et al., 1992, 2011; Twichell et al., 1992, 2009; Klaucke et al., 2004; Gervais et al., 2006). New complex petroleum plays in lobes, such as those in the Wilcox in the Gulf of Mexico, appear to contain hybrid beds of linked debrites and HARP deposits (high-amplitude reflections packet) of avulsed channels (Figs. 19 and 21) (Flood et al., 1991; Normark et al., 1997; Damuth, 2002; Kane et al., 2012; Powers et al., 2013). Seismo-turbidite studies show promise for defining the complex lithologies and predicting sand prone facies distribution in confined basins of active tectonic margins, which will greatly enhance reservoir development (Fig. 22) (Van Daele et al., 2013).

In summary, the turbidite studies which began with a handful of papers 50 years ago, now have ballooned to several hundred papers a year that contribute to our understanding of resources (Figs. 15 and 16), geologic hazards (Fig. 22), and environmental change (Fig. 23). These results have addressed societal needs to provide increased petroleum reserves, show climatic history changes, and develop worldwide paleo-seismic records to assess earthquake hazards.