Abstract Abstract: Mass-transport deposits may act as barriers or baffles to fluid flow in the subsurface, or may conduct fluids via internal structures or lithological connectivity. Conventional seismic and borehole data present radically different scales of observation to assess the likely fluid-flow behavior of mass-transport deposits. Seismic-scale outcrops and high-resolution seismic data bridge this scale gap. Exceptional outcrops of large mass-transport deposits are used to develop strategies to relate core- and seismic-scale observations for the purposes of subsurface prediction of reservoir, baffle, or seal potential, and for prediction of fluid flow through mass-transport deposits in the subsurface. We present here an outline of our approach, and some preliminary results based on two systems of contrasting styles. One is a > 120-m-thick debrite of Carboniferous age in northwest Argentina; the other is an approximately 300-m-thick slide complex of Jurassic-Cretaceous age in Antarctica. Differences in these two systems are assessed by evaluating the internal structure and seismic expression of the deposits, based on forward modeling of the outcrop architecture. Topography on the top surface of mass-transport deposits is defined by very localized (a few meters wavelength and amplitude), localized (a few tens of meters wavelength, a few meters to ~ 10 m amplitude), and subregional (kilometers in wavelength, tens of meters in amplitude) “ponding” or partial confinement of turbidite beds immediately above the mass-transport deposits. Strain histories and strain distributions are complex and variable within deposits, implying that inferences based on limited well data are likely to yield incorrect conclusions regarding direction of movement and slope orientation. This observation is clearly illustrated by the non-coaxial deformation, which is visible in high-resolution seismic data.

Abstract This volume contains a selection of papers that summarize the state of knowledge of the effects of external controls on deepwater deposition systems, ranging from Holocene and Pleistocene systems at the sea floor to outcrops of ancient systems, and includes descriptions of attempts to model the response of depositional processes to these controls. This paper briefly reviews the main points of each of the papers included in the volume and attempts to draw some general conclusions. Despite the differences in type and resolution of data available for modern and ancient systems, and the major differences between the state of the Earth during the Quaternary and earlier periods, some comparisons can be drawn. A major conclusion is that stratigraphic models for the development of deepwater systems must take account of all these controls.

Abstract Deepwater depositional systems are in most cases ultimately fed by rivers that deliver sediment from the hinterland to the continental margin. Fluvial systems therefore provide the link between processes that control sediment supply to the margin and processes that control dispersal to deepwater. This paper reviews controls on fluvial sediment supply to the shelf-margin staging area over time scales of 10 6 yr or less. General controls on fluvial sediment discharge to the coastal oceans are reasonably well known: the newly published global database and empirical model of Syvitski and Milliman (2007) shows that geologic factors of drainage area, relief, and rock types explain more than 60% of between-river variance, whereas climatic factors of temperature and discharge explain 10% and 3%, respectively. Geologic factors vary little over time scales of 10 6 yr or less, whereas climatic factors are highly unsteady over those same time scales. Available literature suggests that unsteadiness due to Milankovitch-scale climate change likely results in variations in sediment yield that are less than ± 50%. The Syvitski and Milliman (2007) empirical model shows a positive correlation between temperature and sediment discharge: it follows that sediment discharge to the coastal oceans is predicted to have been less during glacial climates, relative to interglacials, for most unglaciated river systems of the tropics and lower midlatitudes. Routing of fluvial sediments to the shelf-margin staging area reflects a complicated suite of processes but varies with large-scale controls on drainage-basin size and shelf width. Moreover, in the Quaternary Icehouse world, climate changes are coupled to large-scale changes in global ice volume and resultant high-amplitude glacio-eustasy: river systems respond to sea-level changes of this magnitude by transit back and forth across the shelf, such that fluvial response to sea-level change plays a pivotal role in the modulation of sediment dispersal to the shelf-margin staging area. During highstand conditions, most large rivers discharge to the inner parts of broad shelves: direct dispersal to the shelf margin is favored by sea-level fall and transit of river mouths across the shelf to the staging area. By contrast, short, steep river systems discharge to narrow shelves, and can disperse sediment directly to the shelf margin during highstand, although rates are likely increased during lowstands. In some cases, sea-level fall and the transit of river mouths across the shelf results in the merger of river systems that, in a highstand world, discharge separately to the coastal oceans. Merging of drainage basins can significantly increase the magnitude of individual point-source inputs due to the addition of drainage area, but there will be fewer rivermouth point sources at the shelf margin than what might appear from an examination of present highstand conditions. Application of the Syvitski and Milliman (2007) empirical model to river systems of the Texas Gulf of Mexico coastal plain and shelf illustrates a number of key concepts. An estimated 5°C depression of basin-averaged temperatures during the last glacial maximum, relative to today, results in predicted sediment yields that were 25–30% less than today. The Colorado River extended across the shelf during the last glacial period, increasing drainage area by ˜ 45% because of merging with smaller streams; however, total sediment discharges are predicted to have been similar to today during the last glacial maximum sea-level lowstand. Nevertheless, significantly more sediment is predicted to have been delivered than is necessary to account for observed sediment volumes in the shelf-margin delta. In fact, 30% or more may be unaccounted for, and may represent the fraction of sediment dispersed to the slope and beyond. Relationships between fluvial sediment supply, climate and sea-level change, and dispersal to deepwater systems should be fundamentally different in “greenhouse” periods of Earth history when the amplitude of high-frequency sea-level changes is significantly less. Unsteadiness in fluvial sediment discharge due to climate change should not be modulated by the transit of river mouths and deltas across broad shelves during sea-level fall and rise, river systems should not merge to the same extent, and the number of point sources would not change over short time scales. As a result, sediment dispersal to deepwater should be directly coupled to Milankovitch-scale climate changes.

Abstract The general tendency for sediment supply to deep water to be relatively high during eustatic fall and lowstand, and relatively low during rise and highstand, is recognized in the sequence-stratigraphy literature. Much less is known about the cumulative effect of repeated eustatic cycles on net deep-water sediment delivery. Here we investigate the net effect of offshore sediment delivery during a complete eustatic cycle, which we term sediment pumping , and the possibility of cumulative sediment pumping if repeated eustatic cycles increase the net delivery of sediment to deep water averaged over several cycles. We measure sediment pumping in terms of net offshore delivery after one or more complete eustatic and associated cycles relative to delivery in the absence of cycles. Combining data from a quasi-2D laboratory experiment and a 2D geometric model, we find that net sediment pumping over isolated and superimposed base-level cycles of variable period varies from somewhat negative to strongly positive, depending on (1) time period of imposed base-level cycle, (2) sense of rotation of the spatial subsidence pattern, and (3) the phase of sediment supply relative to eustatic variation. A relatively short-period base-level cycle (i.e., period less than the basin equilibrium time) increases net pumping (relative to the constant base-level reference case) whereas a relatively longperiod cycle yields no or even negative net pumping. Short-period base-level cycles superimposed on a long-period cycle produce a strong net offshore sediment pumping. Other factors being equal, base-level cycles with basin subsidence cause substantially greater net pumping in backtilted basins than in foretilted ones. When sediment supply varies over a base-level cycle, pumping is maximized when the sediment-supply maximum occurs during eustatic falling stage or lowstand. External Controls on Deep-Water Depositional Systems SEPM Special Publication No. 92 (CD version), Copyright © 2009 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-200-8, p. 41–56.

Abstract Three case studies are used to exemplify the wide variety of controlling factors that combine to influence the development of modern turbidite systems, and how these vary with location and time. For example, Cascadia Basin in the Pacific Ocean off western North America, which is underlain by the Cascadia Subduction Zone, exhibits the dominant tectonic control of earthquake triggering for turbidity currents, the increased sediment-supply effects of the Mt. Mazama catastrophic volcanic eruption in 7626 yr B.P., the glacial climatic and sea-level lowstand control on rapid turbidite–system growth rates, and the recent anthropogenic control that reduces sediment supply rates. Lake Baikal in Russia shows how the rift-basin tectonic setting controls the number and type of sediment input points, the amount of sediment supply, and the consequent types of turbidite systems developed along different margins of the Baikal basin. Pleistocene glacial climatic changes, without changes in lake base level, causes increased sediment input and the rapid growth rate of Baikal turbidite systems that is three to five times greater than that during the Holocene interglacial climate. The Ebro turbidite systems in the northwest Mediterranean Sea exhibit control of system types by the Messinian salinity-crisis lowstand, of channel locations by oceanographic current patterns, and of sediment-supply increase by glacial climatic changes as well as recent decrease by anthropogenic changes. Both active-margin and passive-margin settings have some common controls such as climatic and sea-level changes, and develop similar types of turbidite systems such as base-of-slope aprons, submarine fans, and deep-sea or axial channels. Each margin also has specific local controlling factors, for example the volcanic events in Cascadia Basin, glacial climatic without erosional base-level control in Lake Baikal, and the Messinian extreme lowstand in the Mediterranean Sea. Comparison of modern turbidite systems points out new insights on external controls such as importance of: (1) earthquakes for triggering turbidity currents on active tectonic margins, (2) equal or greater Pleistocene climatic control compared to lowered base level for sediment supply, (3) direct glacial sediment input that results in doubled proximal channel size, (4) greatly reduced deposition rates in drained compared to ponded turbidite basins, (5) importance of ocean currents on location of turbidite systems and channel development, and (6) anthropogenic effects from river damming during the last century that sometimes reduces present sediment supply to turbidite systems by orders of magnitude. External Controls on Deep-Water Depositional Systems SEPM Special Publication No. 92 (CD version), Copyright © 2009 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-200-8, p. 57–76.

Abstract A wide variety of proxies has been applied to study the terrestrial input in the Congo deep-sea fan area, the composition of its overlying waters, and the land–ocean interactions of the past 1 to 2 million years. These proxies include stable isotopes of foraminifers, total organic carbon (TOC), alkenone-derived sea-surface temperatures (SST), biomarker content and compound-specific stable isotopes, element composition, clay minerals, pollen and spores, dinoflagellate cysts, diatom valves, and opal. Not only the sedimentation in the deep-sea fan but also the productivity of the overlying waters is strongly influenced by the Congo River discharge and its fluctuations depending on the strength of the monsoon. SST and marine productivity are further affected by wind- and river-induced upwelling. A direct relation between SST, precipitation in the Congo Basin, vegetation cover, chemical weathering, and runoff could be established for the past 200 thousand years. Increase of mean global ice volume between 1000 and 500 ka suppressed the precession forcing of trade-wind zonality and monsoonal river runoff, leading to a higher production of nonsilica marine organisms compared to diatoms, and increased eolian transport of terrigenous material.

Abstract The presently active Zaire (Congo) turbidite system reveals a well-organized Quaternary architecture, with depocenters that partly overlap each other in response to avulsion. Based on previous work, more than 76 channels are organized into three individual fans (Northern, Southern, and Axial Fan, from the oldest to the youngest). A statistical analysis of both longitudinal and lateral migration of depocenters was conducted. The longitudinal shifts were studied through the temporal evolution of the channel lengths and the distances to the bifurcation points from a common reference point arbitrarily positioned on the canyon course, up-dip from the most proximal bifurcation point. The number of bifurcation points on a channel was also calculated. These three architectural parameters show a cyclic organization through time, better expressed in the Axial Fan, with cycles of down-fan and up-fan movements reflecting prograding–retrograding cycles. Based on a previous study of the kaolinite/smectite (K/S) ratio in the hemipelagic drape covering the Southern Fan, i.e., contemporaneous with the building of the Axial Fan, the prograding peaks of the Axial cycles are correlated to peaks in K/S, which reflect phases of intense Zaire River water discharge, during warm and humid interglacial periods. These correlations suggest that both the channel lengths and the avulsion process are controlled by climate changes that appear as a major forcing factor throughout the Quaternary. The effects of climate control can be modified by the interplay of other internal and/or external factors. Study of the lateral migration revealed that topographic compensation is the major autogenic control, and that external factors such as tectonic evolution in the drainage basin of the Zaire River or halokinesis at the Angola base of slope can locally play a significant role in the location of depocenters.

Abstract High resolution seismic data collected on four profiles located on the Bengal Fan at 8° N, 11° N, 14° N, and 17° N were analyzed. Numerous channel–levee systems were identified as main architectural elements, all characterized by erosional incision into underlying sediments. On the upper middle fan (17° N), the channel–levee systems are grouped into four complexes. They are separated by regional unconformities, which were partly caused by generation of nonchannelized turbidity currents or switching of the feeding canyon. Succession of the systems and dating of two surface channels ( Weber et al., 1997 ; Weber et al., 2003 ) indicate activity of the Bengal Fan even during sea-level rises and highstands. In all three profiles from the lower fan, two regional unconformities were found. At 8° N, the unconformities could be dated at DSDP Site 218 to have Pliocene and Pleistocene ages, and were interpreted to be the equivalents of unconformities found in the central Indian Ocean, which are caused by deformation events of the oceanic crust ( Krishna et al., 2001a ). Faults terminating within Pleistocene sediments suggest tectonic activity at least within the Pleistocene at 8° N. The unconformities identified at 11° N and 14° N may also result from tectonic events. Besides these unconformities, variations of sedimentation rates in time and space determined at 8° N and the onset of channel–levee systems simultaneously with lithological changes at ODP Leg 116 sites suggest that tectonic events at the Bengal Fan as well as changes of sediment supply and transport occurred partly concurrently. The sediment supply in turn depends on the erosional regime in the Himalayas, which is controlled by tectonic or climate or an interaction of both. Therefore we propose that a tectonic link may exist between source and sink areas of Bengal Fan turbidites, i.e., between uplift of the Himalayas and deformation events of the Indian Ocean lithosphere.

Abstract The late Pleistocene Amazon deep-sea fan provides a “modern” analogue to ancient fan systems containing sandy hydrocarbon reservoirs. Extensive deposits of sand-rich material are found in the Amazon Fan mass-transport deposits (MTDs) and the base-of-channel avulsed sand bodies which underlie the channel–levee systems. These deposits were drilled as part of ODP Leg 155, the results of which form the basis of this review. The hemipelagic sediment above the MTDs and avulsed sand bodies were dated using primarily AMS radiocarbon dating. The dating provides support for the interpretation that the MTDs and avulsed sand bodies were triggered by relatively small, millennial-scale changes in relative global sea level (± 5–20 m). Equally controversial has been the suggestion that changes in sea level also control the architecture of the channel–levee distributive systems within the Amazon Fan. For example, Maslin et al. (2006) proposed that prior to 22,000 calendar years BP a tripartite channel system existed while afterwards only one active channel–levee system existed. This switch may have been due to the fall in sea level below the shelf break, providing direct access between the canyon and the sediment supplied to and eroded from the shelf-edge delta front. This would have significantly increased the sediment supply to the Amazon Fan at 22 ka, contributing to channel entrenchment involving channel-floor erosion and the growth of levees within the canyon–channel transition area, promoting the development of a single deep, incised channel. If future work confirms that Amazon deep-sea Fan sedimentation is sensitive to relatively small changes in sea level, this will provide support for the central assumption of the theory of sequence stratigraphy, namely that changes in sea level control basin sedimentation and the emplacement of sand-rich, potential hydrocarbon-bearing, deposits. It is hoped that these controversial suggestions reviewed here will stimulate more investigations into the Amazon Fan and other deep-sea fans.

Abstract The Mississippi Fan is a Plio-Pleistocene deposit that occupies much of the eastern Gulf of Mexico Basin. Sidescan sonar imagery, high-resolution seismic profiles, and short cores indicate a three-stage progression in the evolution of the fan surface since the last lowstand of sea level: (1) deposition of turbidites and debrites on the distal fan sourced from the shelf edge during the initial stage of the Holocene transgression, (2) failures in the Mississippi Canyon and along the adjacent continental slope that buried large areas of the surface of the proximal Mississippi Fan, including the channel that had previously supplied sediment to the distal fan, and (3) continued reworking of the surface of the fan and the overlying hemipelagic cover by bottom currents up to the present. Because not all of this progression in depositional styles is recorded in any one place on the fan, both an accurate chronology and a regional synthesis of stratigraphy and processes are needed to decipher these three stages that have shaped the fan surface and to link them to the external conditions that contributed to their formation.

Abstract Little is known about relative importance of climate as one of the external controls on deep-water depositional systems compared with eustasy and tectonics, due to sparse evidence. This study provides evidence of climatic control on turbidite deposition for the last 70 kyr along the Toyama Deep-Sea Channel (TDSC) in the central Japan Sea. The study is based on stratigraphic variations of coarse fraction, and frequency and thickness of surge-type and hyperpycnal-flow turbidites in three well-correlated cores collected along the TDSC. Four depositional phases have been identified in terms of temporal variations of turbidites; (1) from 72 to 22 ka, turbidite deposition fluctuated, with two peaks at 72–52 ka and 29–22 ka. The peaks are attributed to increase in sediment input to the TDSC induced by a large flux of ablation meltwater from glaciers and paraglacial processes during glaciation and deglaciation in the sediment source area. As a result, both hyperpycnites and surge-type turbidites increased along the TDSC. During 52–29 ka, turbidites deposition decreased, with a majority of surge-type turbidites, probably because fluctuating sea level induced collapses of delta slopes. (2) during 22–18 ka, turbidite deposition was reduced by decreased sediment input due to a cold and dry climate in the sediment source area. Consequent deposits were rich in finegrained hyperpycnites. (3) during 18–7 ka, turbidite deposition was most intensified due to increased sediment input from the sediment sources. Coarse debris trapped in the mountains during the previous phase began to be removed into coastal lowlands because of intensified rainfall and destabilized mountain slopes. Millennial-scale turbidite fluctuation has also been recorded in response to rapid climatic changes around the Younger Dryas period. Surge-type turbidites increased as a result of increased flow volume and efficiency. (4) During 7 ka-present, turbidite deposition was reduced as the sediment sources were gradually depleted in transportable debris. Resultant deposits are mostly fine-grained hyperpycnites in the distal reaches. The present study also sheds lights on difference of characteristics between surge-type and hyperpycnal-flow deposits. Hyperpycnites are generally finer grained than surge-type turbidites and contain abundant organic carbon derived from terrestrial plants. Hyperpycnal flows are considered to be slower and long-lived flows with estimated flow velocity of ~ 2 m/s and flow duration of the order of several days or more. Fines-rich hyperpycnal flows are high-efficiency flows that can transport sediments for long distances, whereas low-efficiency surge-type flows require large flow volumes to travel long distances. The difference in characteristics between hyperpycnal flows and surge-type flows would have resulted in different spatial and temporal distributions of deposits observed in the present cores. The study thus provides implications for assessment of past climate changes and sequence stratigraphic models, and also for prediction of hydrocarbon reservoir and source potential.

Abstract This paper analyzes recurrence times of Holocene turbidites as proxies for earthquakes on the Cascadia and northern California active margins of western Northern America. We compare the age, frequency, and recurrence time intervals of turbidites using two methods: (1) radiometric dating ( 14 C method), and (2) relative dating, using hemipelagic sediment thickness and sedimentation rates (H method). The two approaches complement each other, and when used together provide a better age framework than 14 C ages alone. Comparison of hemipelagic sediment thickness in several cores from the same site is used to evaluate the erosiveness of turbidity currents and improve the correlation of turbidites and consequent paleoseismic history based only on less complete and unrefined data sets of 14 C turbidite ages along the continental margin. Chronology of hemipelagic sediment thickness provides (1) the best estimate of minimum recurrence times, which are the most important for seismic hazards risk analysis, and (2) the most complete dataset of recurrence times, which shows a normal distribution pattern for paleoseismic turbidite frequencies. We observe that on these tectonically active continental margins, during the sea-level highstand of Holocene time, triggering of turbidity currents is controlled dominantly by earthquakes, and paleoseismic turbidites have an average recurrence time of ~ 550 yr in northern Cascadia Basin and ~ 200 yr along northern California margin. This difference in frequency of turbidites in a subduction zone compared to a transform-fault margin suggests significant differences in earthquake activity that compare favorably with independent paleoseismic indicators.

Abstract Late Pleistocene and Holocene depositional patterns of the Almeria Turbidite Channel (Alboran Sea, Western Mediterranean) were investigated using multi-proxy analyses of the sediment properties in seven cores from the proximal, medial, and distal channel depositional environments. Turbidites interbedded with hemipelagic facies during the Pleistocene and hemipelagic facies during the Holocene were found, with a recent turbidite event taking place later than 1.3 ka. In the Almeria region, Pleistocene turbidite emplacement was controlled by sea-level position and by the canyon–channel connection to the sediment loads provided by the Andarax River and three tributary valley systems. During the Holocene, the Almeria Channel gradually became inactive as sea level reached its present position. The most recent turbidite deposition is related to local tectonics and morphological factors at the canyon head.

Abstract The Tyrrhenian Sea was formed through rifting due to back-arc extension above the subducting Ionian oceanic slab. Within the basin and the surrounding regions, extensional processes were diachronous, first affecting the Sardinian margin and then migrating southeastward toward the Sicilian margin and the Calabrian margin and the western side of the Italian peninsula. Thus, at the Sardinia passive margin, tectonic activity has been quiescent since the Early Pliocene, whereas extensional processes are ongoing at the Sicilian margin. As a result, the different geological setting of the associated hinterland areas has had a large impact on the present-day depositional systems along the two margins. At the Sardinian margin, a relatively large spacing of river entry points results in widely spaced submarine slope canyons that feed isolated submarine fans in the intraslope basins. At the Sicilian margin, as a consequence of smaller river drainage basins, canyons are very close together along the slope and feed base-of-slope coalescing sedimentary bodies, creating an apron consisting of channel–levee deposits and channel-mouth lobes. Hinterland local tectonic regime also influences the nature of the deep-sea depositional systems along each of the margins. Generally, the small, radial fans of the Sardinian margin lie basinward of narrow shelf regions, where a direct fluvial input of coarse-grained sediments to the canyon heads can be inferred. The large elongated Caprera fan, in contrast, forms in an area where the shelf is wider and can efficiently trap much of the coarse-grained fraction of throughput sediments. At the Sicilian apron, in the areas facing the depressed regions, with smaller continental uplift rates, the Gioia basin channel–levee system is actively aggrading. On the other hand, destructive processes, resulting in widespread masswasting deposits, are affecting the Sicilian apron, where high uplift rates are affecting the adjacent land areas.

Abstract The 3500-m-deep Vavilov basin plain lies in the center of the Tyrrhenian Sea, and is subdivided by intrabasinal highs into the Magnaghi, the western Gortani, the eastern Gortani Basins, and the Marsili basin plain. Geophysical data allow the recent depositional character of the basins to be interpreted. The Sardinia Valley crosses the Sardinian passive margin, where intraslope fans act as sediment storage areas, and is the source of a low supply of mainly fine-grained sediments to the Magnaghi Basin. Here, a low-relief channel feeds a single, terminal lobate area that rapidly passes to basinwide sheetlike deposits. The Ischia Valley enters the eastern Gortani basin, and is mainly supplied through instability processes affecting the slope of the Latium–Campanian margin, where sediment is trapped in largely underfilled intraslope basins and subsiding coastal basins. Due to the intermittent nature of the feeding system, small, short-lived channels develop in the eastern Gortani basin plain and are rapidly replaced basinward by sheet turbidites and thick acoustically transparent layers deposited by large-volume slope-failure-induced flows that spread also over the western Gortani basin. The Marsili Basin is flanked by the tectonically active and uplifting Calabrian and Sicilian margins and is fed by the Stromboli Valley, which receives coarse-grained sediments from the presently active Aeolian volcanic arc. As a consequence, a coarse-grained fan with proximal distributary channels, small channel mouth lobes, intrachannel and interchannel longitudinal bars, and a distal, large unchannelized lobate area forms in the Marsili Basin. The present study highlights that, even in areas characterized by wide, topographically complex slopes, the regional geology of the hinterland areas is a primary external control on the style of basin-plain deposition.