Submarine Gullies Research Articles

Glossifungites suites and tubular tempestites in Devonian shallow-marine deposits from Paraná Basin

Abstract The Glossifungites ichnofacies can be a signature of allogenic processes that demark omission, exhumation and ravinement, or indicate autogenic processes impacting several depositional scenarios such as incised valleys, submarine gullies and canyons, erosive shorefaces, tidal channels and dewatered muddy substrates. Comparatively, autogenic surfaces demarked by the Glossifungites ichnofacies have been less studied than allogenic surfaces. In this study case, we interpret the examples from the transitional offshore to offshore deposits of the Devonian Ponta Grossa Formation (Paraná Basin, southern Brazil) as tubular tempestites. They are commonly associated with the initial phase of the transgressive systems tract when the shallow-marine environment becomes starved of sediment supply. Those occurrences are mostly characterized by Thalassinoides and less frequently by Palaeophycus , Skolithos and Rhizocorallium , with sharp to irregular walls, passive filling and descending from an erosive surface. In offshore deposits, the erosive surface can be obliterated, making the Glossifungites suite the unique evidence of storm processes (e.g. tubular tempestites). The results also highlight some differences between autogenic and allogenic Glossifungites from the Ponta Grossa Formation and demonstrate that the few levels with allogenic Glossifungites suites represent coplanar surfaces coupling sequence boundaries and transgressive surfaces developed in the early stages of transgressive systems tracts.

Are bright spots always hydrocarbons? A case study in the Taranaki Basin, New Zealand

In the shallower regions of the 3D Nimitz seismic survey, there exist multiple interesting bright seismic amplitude anomalies. These anomalies, or funny looking things, occur in a confined spatial and temporal region of the seismic. They have a concave-up seismic appearance along the cross section. Bright seismic amplitudes can be a direct hydrocarbon indicator, or they can be representative of strong lithologic contrasts and/or acquisition artifacts. We have set out to investigate misinterpreted seismic anomalies along cross-sectional lines. Therefore, we apply seismic attributes to indicate that these bright spot features, which we interpret to be submarine gullies looking along time-slice intersections, can possibly be mistaken for hydrocarbon anomalies in a cross-sectional view. However, we cannot fully rule out the presence of hydrocarbons because it is common for gas sands to create similar anomalies. Previously drilled wells within the survey (Korimako-1 and Tarapunga-1) point to a lack of hydrocarbon potential in the subsurface. Although it is possible that these bright spots are due to hydrocarbon presence, we develop a more likely hypothesis: The lithology of the interfluve sediments is similar to the gully-margin drapes but differs from the gully sediment fill. Funny looking thing (FLT): Submarine gullies Seismic appearance: High-amplitude spotted features Alternative interpretations: Lithologic anomalies, gas seeps, bright spots Features with a similar appearance: Gas accumulation, sediment fills in limestone paleocaves Formation: Giant Foresets Formation Age: Pleistocene Location: Taranaki Basin, New Zealand Seismic data: Nimitz 3D (cropped volume) Analysis tools: Curvature, instantaneous frequency, and sweetness attributes; well reports

Syn-sedimentary deformation as a mechanism for the initiation of submarine gullies on a carbonate platform to slope transition, Browse Basin, Australian North West Shelf

Syn-sedimentary deformation and submarine channel initiation are both important processes influencing the stratigraphic development and reservoir properties of carbonate platforms. Based on a high-resolution 3D seismic dataset our study shows that platform-margin parallel syn-sedimentary faulting in the vicinity of the clinoform breakpoint can lead to the indention of channel heads that subsequently grow and expand in headward and downslope direction. The carbonate-dominated Miocene succession of the Browse Basin on the Australian North West Shelf reveals a distinct submarine channel system during a time of overall platform progradation. The progradation occurs over a drowned Eocene to Lower Miocene carbonate ramp, which formed a large scale inflection. Previous studies investigated the architecture of the carbonate platform, containing a large Neogene barrier reef system. We present a high-resolution 3D seismic study of the contemporaneous platform to slope transition. Landward of first significant gullies, a distinct network of margin parallel, small scale arcuate faults is observed. Active faulting during appearance of the first gullies is exhibited by growth strata, which are located directly basinward of the faults. These steep (65°–85°), small-scale faults are characterized by throws of 5–25 ms TWT and show a tendency to merge laterally and vertically with adjacent faults. Syn-sedimentary deformation associated with these faults likely triggered slope failure and the generation of turbidity currents finally leading to the growth of the gullies by headward and downslope erosion.

Evolution of submarine gullies on a prograding slope: Insights from 3D seismic reflection data

Submarine gullies are ubiquitous on the modern seafloor of both passive and active continental margins. However, the processes dictating gully formation and the role of gullies in deep-water sediment transport are topics of debate. In this study, through 3D seismic reflection interpretation of Pliocene-Pleistocene continental slope strata in the Taranaki Basin, New Zealand, we present results constraining the evolution of gullies over a roughly 2 million year timespan. We document gullies ranging from 20–400m wide, ~10–60m deep, and 2–22km long, on slopes ranging from 2° to 5°. Gully width to depth ratios (3.5–5) are distinct from aspect ratios of submarine and fluvial channels (29.5 and 17), suggesting a process-based control on this morphometric relationship that could aid in predicting sediment transport styles from first order observations in datasets of modern or ancient seafloors. Gullies in this study commonly form nested complexes hundreds of meters thick, preserving patterns of aggradation, partial burial, and re-incision that highlight the long-lived nature of the gullies and their strong influence on subsequent turbidity flows. Gully longitudinal profiles are morphologically similar to slope interfluve profiles, suggesting that gullies are integral to slope sedimentation processes and that gully-forming flows are also the dominant drivers of continental slope sedimentation and outbuilding in this setting. We interpret these flows to be large, dilute, sheet-like turbidity currents, which produce net-aggradational, shallowly-incised gullies due to autogenic instabilities that promote regularly spaced variations in erosion and deposition. From the size and spacing of the gully arrays and the scale of sediment waves in the study area, we estimate these flows to have been at least tens of kilometers wide and up to 40m thick.

Submarine gullies and an axial channel in glacier-influenced Courtauld Fjord, East Greenland

Submarine gullies have been observed widely in swath-bathymetric imagery of the shelf edge and upper slope on high-latitude margins (e.g. Noormets et al. 2009; Gales et al. 2013), but less frequently in fjords. Gullies vary in distribution and dimensions depending on formation mechanisms, including submarine mass wasting, subglacially or proglacially derived turbid underflows and dense bottom-water currents linked to brine rejection during sea-ice formation (e.g. Noormets et al. 2009). Since recession of the Greenland Ice Sheet through Kangerlugssuaq Fjord (68° N) in East Greenland after the Last Glacial Maximum (Dowdeswell et al. 2010), significant seafloor erosion on the flanks of the inner tributary fjords has taken place to produce a series of submarine gullies and an axial channel (Fig. 1a–e). Fig. 1. Multibeam bathymetry and cross-profiles of submarine gullies in Courtauld Fjord, East Greenland. ( a ) Submarine gullies in the outer fjord and subglacial bedforms in the inner fjord. Courtauld Glacier drains into the head of the fjord and several glaciers are also present along the east flank of the fjord. Dashed white line and white arrows mark the lateral moraine on the western fjord wall produced when Courtauld Glacier last advanced to the outer fjord sill during the Little Ice …

Submarine gullies on the southern Weddell Sea slope, Antarctica

Submarine gullies are small-scale, confined channels on the order of tens of metres depth that form one of the most common morphological features of high-latitude continental slopes. Gully morphology varies in width, incision depth, length, sinuosity, branching order, shelf-incision, cross-sectional shape and gully spacing, with six distinct gully signatures recognized on high-latitude continental slopes (Gales et al. 2013 a , b ). Here we analyse the morphology of slope gullies off Halley and Filchner troughs in the southern Weddell Sea (Fig. 1a–f). Fig. 1. Multibeam bathymetry and along-slope profiles of gullies on the southern Weddell Sea continental margin. ( a ) Multibeam data showing V-shaped gullies at the mouth of Halley Trough. Acquisition system Kongsberg EM122. Frequency 12 kHz. Grid-cell size 30 m. ( b ) Sub-bottom profile of U-shaped gullies. VE×20. Location in (e). Acquisition system Kongsberg TOPAS PS 018. Frequency 18 kHz. Secondary beam frequency 1300–5000 Hz. ( c ) Along-slope profile through V-shaped gullies. Location in (a). VE×55. ( d ) Location of the study area (red boxes; map from IBCSO v. 3.0). FT, Filchner Trough; HT, Halley Trough. ( e ) Multibeam data showing U-shaped gullies at the mouth of Filchner Trough. Acquisition system Kongsberg EM120. Frequency 12 kHz. Grid-cell size 30 m. ( f ) Sub-bottom profile of U-shaped gullies. Location in (e). Acquisition …

Slope gradient and lithology as controls on the initiation of submarine slope gullies; Insights from the North Carnarvon Basin, Offshore NW Australia

Slope-confined submarine gullies are present on many continental margins, yet the controls on their initiation and demise are poorly understood because modern or recently active systems are rarely if ever monitored, and exhumed systems, typically formed in very fine-grained successions, are poorly preserved at outcrop. We use 3D seismic reflection and borehole data from offshore NW Australia to investigate long-term (~40Myr) variations in the geomorphology of Eocene-to-Miocene gullies that developed in mixed carbonate–clastic clinothems. Through time, clinoform slope gradient increases from 1.6° to 3.2°, with gullies forming when the clinoform slope exceeds 2.5°. After their inception, gullies increase in width (from 350m to 770m) and depth (from 37m to 60m). Slope steepening appears to coincide with a change from poorly cemented, fine-grained carbonate to better-cemented, coarse-grained carbonate, implying a secondary, lithological control on slope dip and, ultimately, gully formation.

Characterizing slope morphology using multifractal technique: a study from the western continental margin of India

Here, we present the slope configuration of the submarine gullies, ridges and the adjacent slump zone off Goa, along the western continental margin of India utilizing multibeam bathymetric and single-channel seismic data. The fluid flow migration signature in the form of pockmark seepages, traces of mud volcanoes and enhanced reflectors is observed in the area. Altogether thirty-three depth profiles from the gully, ridge and slump areas depict downslope progression in gully incision and varying gradients in the gullies (1.19–4.07°) and ridges (2.13–3.70°), whereas the profiles of the slump zone are comparatively steady (2.25–2.51°). The scatter plot of the three slope characteristics, viz., gradient, mean depth and root mean square relief, characterizes the profiles of the gullies, ridges and slump zone into three distinct clusters. Principal component analysis as well corroborates the categorization. Furthermore, a stochastic multifractal technique has been employed to understand the nature of the fine-scale seafloor processes active in the slope region. The three estimated parameters of the depth profiles, i.e., the degree of multifractality (α), sparseness (C 1) and the degree of smoothness (H), substantiate a very high degree of multifractality for all the thirty-three bathymetric profiles. Except for the five adjacent profiles (four from the slump zone and one from the ridge), the remaining twenty-eight depth profiles of the gully, ridges and slump zones show negligible difference. Based on the multifractal study, we conclude that the observed discrimination might be due to the significant interaction between the bottom currents off Goa and the varied seafloor morphological aspects with seepages and faults.

Large-scale submarine landslides, channel and gully systems on the southern Weddell Sea margin, Antarctica

New multibeam bathymetric data from the southeastern Weddell Sea show significant differences in surface morphology of the outer continental shelf and slope between two adjacent cross-shelf troughs. These are the Filchner Trough and a smaller trough to the east which we refer to as the ‘Halley Trough’. Multibeam bathymetric data, acoustic sub-bottom profiler and seismic data show major differences in the incidence and morphologies of submarine gullies, channel systems, submarine slides and iceberg scours, and in sediment deposition. These large-scale differences suggest significant variation in slope and sedimentary processes and in the environmental setting between the two troughs, leading to much greater deposition at the mouth of the Filchner Trough. Bedforms, including a terminal moraine and scalloped embayments on the outer shelf of the Halley Trough, provide insight into the relative timing and extent of past ice-sheet grounding and point to grounded ice near to the shelf edge during the Late Quaternary. The new data reveal two large-scale submarine slides on the upper slope of the eastern Crary Fan, a trough mouth fan offshore from the Filchner Trough. Both slides head at the shelf edge (~ 500 m water depth), with the largest slide measuring 20 km wide and with an incision depth of 60 m. Multibeam and seismic data show elongate slabs on the seafloor surface of the mid-slope. The lack of a discernible sedimentary cover suggests that they were generated after the Last Glacial Maximum (LGM). This is unusual because post-LGM submarine slides are very rare on the Antarctic continental margin, and to our knowledge, no other post-LGM slides have been documented on an Antarctic trough mouth fan. Because the slides occur on a part of the continental slope where the deposition of glacial debris was greatest, we speculate that weaker, unconsolidated sedimentary layers within the subsurface are important for slide initiation here.

Arctic and Antarctic submarine gullies—A comparison of high latitude continental margins

Submarine gullies are common features of high latitude continental slopes and, over the last decade, have been shown to play a key role in continental margin evolution, submarine erosion, downslope sediment transport, slope deposits, and the architecture of petroleum reservoirs. However, the processes that form these gullies, the timescales over which they develop, and the environmental controls influencing their morphology remain poorly constrained. We present the first systematic and comparative analysis between Arctic and Antarctic gullies with the aim of identifying differences in slope character, from which we infer differences in processes operating in these environments.Quantitative analysis of multibeam echosounder data along 2441km of the continental shelf and upper slope and morphometric signatures of over 1450 gullies show that six geomorphically distinct gully types exist on high latitude continental margins. We identify distinct differences between Arctic and Antarctic gully morphologies. In the Arctic data sets, deep relief (>30m gully incision depth at 50m below the shelf edge) and shelf-incising gullies are lacking. These differences have implications for the timescales over which the gullies were formed and for the magnitude of the flows that formed them. We consider two hypotheses for these differences: (1) some Antarctic gullies developed through several glacial cycles; and (2) larger Antarctic gullies were formed since the Last Glacial Maximum as a result of erosive flows (i.e., sediment-laden subglacial meltwater) being more abundant on parts of the Antarctic margin over longer timescales.A second difference is that unique gully signatures are observed on Arctic and on Antarctic margins. Environmental controls, such as the oceanographic regime and geotechnical differences, may lead to particular styles of gully erosion observed on Arctic and Antarctic margins.

Geomorphic signature of Antarctic submarine gullies: Implications for continental slope processes

Five quantitatively distinct gully types are identified on the Antarctic continental margin from swath bathymetric data of over 1100 individual features. The gullies differ in terms of length, width, depth, width/depth ratio, cross-sectional shape, branching order, sinuosity and spatial density. Quantitative analysis suggests that Antarctic gully morphology varies with local slope character (i.e. slope geometry, gradient), regional factors (i.e. location of cross-shelf troughs, trough mouth fans, subglacial meltwater production rates, drainage basin size), sediment yield and ice-sheet history. In keeping with interpretations of previous researchers, most gullies are probably formed by hyperpycnal flows of sediment-laden subglacial meltwater released from beneath ice-sheets grounded at the continental shelf edge during glacial maxima. The limited down-slope extent of gullies on the western Antarctic Peninsula is explained by the steep gradient and slope geometry at the mouth of Marguerite Trough, which cause flows to accelerate and entrain seawater more quickly, resulting in a reduction of the negative buoyancy effect of the sediment load. Due to pressure gradients at the ice-sheet bed caused by variations in ice thickness inside and outside palaeo-ice stream troughs, subglacial meltwater flow was generally focussed towards trough margins. This has resulted in gullies with larger cross-sectional areas and higher sinuosities at the trough margins. A unique style of gullying is observed off one part of the western Antarctic Peninsula, corresponding to an area in which the ice-sheet grounding line is not thought to have reached the shelf edge during the Last Glacial Maximum. We interpret the features in this area as the cumulative result of slope processes that operated over a long period of time in the absence of hyperpycnal meltwater flows.

Significance of the fine drainage pattern for submarine canyon evolution: The Foix Canyon System, Northwestern Mediterranean Sea

Submarine gullies are relatively small valleys that occur in a variety of submarine slopes. They are very common in continental slopes and in submarine canyon heads and flanks, where they often form tributary networks. Gullies are smaller than submarine canyons, though there is no standardised size criterion to distinguish between them. Gullies and gully networks have been often viewed as initial steps in the development of larger submarine canyons and more mature drainage networks. The shelf-incising submarine Foix Canyon System (FCS) is located in the northwestern Mediterranean Sea. Numerous, well-developed and morphologically diverse gullies carve its two heads and flanks. The objective of this study is to analyse the drainage network of the FCS and decipher the role of gullies in its evolution. Submarine gully and canyon networks were extracted from swath bathymetry data of 50m grid size using Geographic Information Systems (GIS). A systematic morphometric analysis was carried out on drainage network of the FCS by using the Horton–Strahler method. Our results show that the FCS is formed by 1660 streams, 1000km in total length, which can be classified to six stream orders. To detect relevant morphological changes along valley sections, the drainage density, the stream frequency and the drainage area relief parameters were applied. Furthermore, a branching index (Ib) was developed to characterise the geometry of the submarine drainage network. In the canyon heads Ib values are ~1.7, which correspond to a dendritic network, whereas Ib in the canyon branches displays values of ~2.2 corresponding to a pinnate one. At a finer scale, we have identified two types of canyon flank gullies, namely “rim gullies” and “toe gullies”: (1) rim gullies form large, dendritic networks that extend from the canyon thalweg up to the canyon rim, and (2) toe gullies form smaller pinnate networks restricted to the lower part of the canyon flanks. The formation and development of rim gullies are interpreted as the main process responsible for canyon head growth and across-flank transport of material from the continental shelf. Toe gullies, on the other hand, are the morphological expression of the rejuvenation of rim gullies and the canyon itself. Axial incision and toe gully formation are interpreted as the result of bottom-to-top erosive processes acting at different scales, in response to regional and local changes of the factors controlling the system, respectively. This study helps to identify a variety of gullies and the processes they result from and provides a morphometric methodology to characterise submarine drainage networks at fine resolution scales.

Southern Weddell Sea shelf edge geomorphology: Implications for gully formation by the overflow of high‐salinity water

Submarine gullies are the most common morphological features observed on Antarctic continental slopes. The processes forming these gullies, however, remain poorly constrained. In some areas, gully heads incise the continental shelf edge, and one hypothesis proposed is erosion by overflow of cold, dense water masses formed on the continental shelf. We examined new multibeam echo sounder bathymetric data from the Weddell Sea continental slope, the region that has the highest rate of cold, dense water overflow in Antarctica. Ice Shelf Water (ISW) cascades downslope with an average transport rate of 1.6 Sverdrups (Sv) in the southern Weddell Sea. Our new data show that within this region, ISW overflow does not deeply incise the shelf edge. The absence of gullies extending deeply into the glacial sediments at the shelf edge implies that cold, high salinity water overflow is unlikely to have caused the extensive shelf edge erosion observed on other parts of the Antarctic continental margin. Instead, the gullies observed in the southern Weddell Sea are relatively small and their characteristics indicative of small‐scale slides, probably resulting from the rapid accumulation and subsequent failure of proglacial sediment during glacial maxima.

Submarine gullies on Italian upper slopes and their relationship with volcanic activity revisited 20 years after Bill Normark’s pioneering work

Multibeam bathymetry and high-resolution seismic profiles depict in detail the characteristics of submarine gullies present in the upper continental slope offshore of the mouths of the Tiber and Volturno Rivers in the Tyrrhenian Sea and the Simeto River in the Ionian Sea. Upper slope gullies are interpreted as depositional features, growing because of faster aggradation on intergully areas with respect to their axes. The dispersal of river-flood sediment through plunging of hyperpycnal flows has been interpreted to be the limiting factor in sedimentation in gully axes. However, the generation of hyperpycnal flows requires a sediment concentration of several tens of kilograms per cubic meter in the river flows to overcome the higher density of seawater. This threshold is difficult to reach in medium-sized rivers (i.e., catchment basin of 500–5000 km 2 ), such as the Tiber and Volturno. Two alternative scenarios of enhanced sediment availability for these rivers have been proposed. The first scenario is related to sea-level fall and lowstand stages, when the climate conditions may have been different and a huge amount of unconsolidated, fine-grained sediment deposited during the previous highstand stage may have been eroded from the river valleys, coastal plain valleys and shelf delta, as suggested in pioneering work of one of the authors (Chiocci) and Bill Normark. This scenario might explain the formation of some of the gullies offshore of the Tiber River mouth, which developed during the last glacial maximum; however, it is unable to explain the gullies within deposits related to transgressive and highstand system tracts, such as offshore of the Simeto and Volturno River mouths. An alternative scenario is thus proposed on the basis of the relationship observed between the studied rivers and the presence of large volcanic edifices in their catchment basins. Explosive volcanic activity could have drastically modified their catchment basins and caused a sudden and large supply of loose tephras, increasing the sediment load in watercourses and consequently favoring the development of gully-forming hyperpycnal flows. The timing of volcanic activity on the three study areas matches, in fact, the presence or absence of gullies in upper slope depositional sequences.

A topographic signature of a hydrodynamic origin for submarine gullies

Submarine gullies—small-scale, straight, shallow channels formed in relatively high seafloor-slope settings—are ubiquitous features that play an important role in the general evolution of continental margin morphology. The mechanisms associated with the origin and evolution of submarine gullies are, however, still poorly defined. In this paper, we present evidence of a topographic signature of gully erosion in the Cook Strait sector of the Hikurangi subduction margin, New Zealand. This signature indicates that submarine gully initiation is a threshold process driven by unconfined, directionally stable fluid or sediment gravity flows accelerating downslope. We propose cascading dense water, a type of current that is driven by seawater density contrast, as the source of these flows. The sensitivity of such ephemeral hydrodynamic events to climate change raises questions regarding implications for future variation of the distribution and magnitude of a significant seafloor erosion process.

Laboratory experiments on the formation of subaqueous depositional gullies by turbidity currents

Gullies and furrows are shallow, sub-parallel features widespread on the seafloor and some lacustrine environments. Vast fields of linear channels of erosional origin called furrows, with spacings of meters and up to hundred of meters, are common in settings with relatively low seafloor slopes and have been previously ascribed to helical motion in benthic boundary layers. Submarine gullies are linear, donwslope-trending features of erosional, depositional or mixed nature, commonly observed in the upper continental slope, submarine canyon walls, and other relatively steep submarine environments. While gully fields have been associated to the action of unconfined turbidity currents and other cascading dense flows, their relatively large spacings (ranging from several hundreds of meters to 1–2 km) suggest that these features might be the result of other mechanisms different than boundary layer helical motion. In this paper we propose an alternative mechanism for the initiation of gullies observed in relatively steep seabed gradients, downslope of a seabed slope change such as those observed in continental margins (self-slope break) or submarine canyon walls. Our working hypothesis is that gullies could be the result of a centrifugal instability that sets in interfaces of gravity currents as they flow through a bedslope transition. Since for these cases the centrifugal force excess is of the same order of magnitude of the driving force, and therefore a potential source of flow instability, we propose that gully spacing might be controlled by the magnitude of the centrifugal force, scaling roughly with the radius of curvature of the transition (slope break). We also present experimental evidence on the formation of longitudinal features resembling slope gullies that were created in a laboratory tank after running successive supercritical, depositional turbidity currents through a slope change similar to that observed in continental margins. Gullies in the tank developed through differential deposition only, and showed spacing an order of magnitude larger that that expected from a boundary-layer helical motion origin. These experimental results along with the observation that gullies were not formed in the tank without a bed slope change (low to high) support the hypothesis of a centrifugal instability origin for longitudinal features in relatively steeper submarine settings. Our experimental results also support our first-order approximate gully spacing rule, which is in general agreement with field observations of gully spacing in upper continental slopes and submarine canyon walls.

Evolution and stratigraphic architecture of marine slope gully complexes: Monterey Formation (Miocene), Gaviota Beach, California

Three small headlands in the sea cliffs west of Gaviota Beach, California, are the remnant fill of three discrete submarine gullies incised into the late Miocene submarine slope environment. These promontories provide excellent, three-dimensional exposure of the gully fill in outcrop, permitting documentation of their complex internal stratigraphic architecture. Detailed study of these exposures elucidates the sedimentologic processes that occur in the filling of slope gullies, guides interpretation of the acoustic records of otherwise unsampled modern gully systems on continental slopes, and provides insight into the heterogeneity that may characterize slope gully petroleum reservoirs. We develop a comprehensive facies scheme to describe the variability within these intercalated coarse- and fine-grained deposits and use two-dimensional photopans to interpret the overall depositional system. Defined by internal bedforms, sedimentation units, and sediment size, each facies records sedimentation under different hydrodynamic conditions and can be genetically related to a discrete depositional mechanism. We differentiate channel axis, margin, and overbank sub-environments within the gully fill, which together define overall crudely braided, low-sinuosity channels within the gullies. Like many modern gully systems, the Gaviota gullies probably initiated through local slope oversteepening that led to slope failure, slumping, and initiation of sediment flows. Erosion and sedimentation from these high-density turbidity currents formed the primary depositional process for the Gaviota conglomerate units. Once initiated, the gullies acted as sediment conduits from the shelf and possibly onshore regions. Despite their prevalence on modern upper slopes and their pivotal role in shaping shelf margins and transporting sediment to deeper water, the details of submarine slope gully formation and filling remain obscure. The Gaviota gully complexes provide valuable insights into long-term gully fill attributes not easily obtained from modern slope gully systems and rarely preserved in the rock record.

The formation of submarine gullies by turbidity currents

A mathematical model of the formation of submarine gullies due to turbidity currents is presented in this study. A linear stability analysis is performed with the use of layer‐averaged momentum equations of turbidity currents, a dispersion equation of suspended sediment, and the Exner equation extended for the seabed subject to erosion and deposition due to turbidity currents. The analysis shows that the dominant gully spacing is of the order of 1500–8000 times the Richardson‐critical layer thickness of turbidity currents. Evaluating the Richardson‐critical layer thickness to be of the order of 0.1–1.0 m, the spacing of submarine gullies is of the order of 150–8000 m. According to observations, the spacing of submarine gullies ranges from a few hundred meters to a few kilometers. The theoretical results are found to be consistent with the observations.

Lower Miocene Nukhul Formation, Gebel el Zeit, Egypt: Model for structural control on early synrift strata and reservoirs, Gulf of Suez

The Aquitanian-early Burdigalian (lower Miocene) Nukhul Formation at Gebel el Zeit, Egypt, was deposited during early stages of Gulf of Suez rifting. The unit dips 8-15° less than underlying prerift strata, indicating that significant rotation and extension preceded subsidence of the Gebel el Zeit fault block. The Nukhul Formation at Gebel el Zeit is up to 75 m thick in outcrop and consists of a lower sandstone and an upper carbonate unit. The formation varies considerably along strike because of syndepositional differential movement of small fault-bounded blocks. The lower clastic unit at South Gebel el Zeit contains poorly sorted, conglomeratic, marly sandstone that commonly displays grading and Bouma sequences. Beds were deposited below storm base by sediment gravity flows. Thicker intervals are inferred to fill small, structurally controlled, submarine gullies that funneled sand and gravel southwestward to a half-graben basin. In contrast, an inferred correlative, thin, basal conglomeratic unit in North Gebel el Zeit was deposited in a shallow-marine setting. The presence of basement clasts in Nukhul strata indicates early syndepositional uplift due to structural tilting. The upper carbonate unit consists of bioclast, peloid, and intraclast packstone, wackestone, and grainstone with minor floatstone, rudstone, and coral-algal boundstone. Carbonate strata were deposited variously in deep-marine, low-energy peritidal and subtidal, and reefal environments. Deeper submerged blocks were the site of carbonate resedimentation or deeper shelf deposition. Reefs and shallow-marine bioclast shoals formed on higher submerged blocks. Nukhul strata show that synrift reservoir prediction in the Gulf of Suez, the Red Sea, and presumably in other rifts requires mapping of synrift cross faults and fault block by fault block facies analysis. Robert Winn is a consulting geologist. He received a Ph.D. from the University of Wisconsin-Madison in 1975 and joined Marathon Oil Company as a research geologist in 1977. He left Marathon in 1994 for the University of Papua New Guinea, where he was senior lecturer and then associate professor. He was head of the Geology Department from 1996 to 2000. His primary interests are in clastic sedimentology and sequence stratigraphy.Paul Crevello is a consulting geologist and technical director of PetrexAsia, which he formed in 1997. He received an M.S. degree from the University of Miami (1978) and a Ph.D. from Colorado School of Mines (1989). He was employed by Marathon Oil as a research geologist from 1978 to 1994 and by the University of Brunei as senior lecturer from 1994 to 1997. His specialties are in sequence stratigraphy and sedimentology of carbonate and turbidite systems and the integrated calibration of 3-D reservoir models. William Bosworth is employed by Marathon Oil Company. He joined Marathon in 1984 and has worked principally on international exploration and development projects. From 1980 to 1984 he taught structural geology and tectonics at Colgate University in Hamilton, New York. His research interests are principally in extensional tectonics, continental stress field evolution, and the paleogeodynamics of the African plate.

Sedimentation in the Chile Trench: Depositional morphologies, lithofacies, and stratigraphy

The depositional bodies of the Chile Trench (trench fans, the axial channel, sheeted basins, ponded basins, and axial sediment lobes) control the spatial distribution of modem lithofacies in the basin. Sheeted basins (south of 41°S) are presumably fed by closely spaced submarine gullies that approximate a line of source of sediment supply along the base of the slope. Trench fans (41°S−33°S) are built at the mouths of major submarine canyon systems which act as point sources of sediment supply. The axial channel follows the northward gravitational gradient, draining the distributary networks of trench fans into the longitudinal transport system. Down-gradient (northern) fan lobes are severely dissected by erosional processes; evidently, periods of proximal deposition alternate with periods of massive sediment remobilization and progradation of the axial dispersal system into more distal environments. Channelized basins in the canyon-mouth areas yield to sheeted basins (depositional surface maintains an axial gradient) or ponded basins (depositional surface is strictly flat) in inter-canyon areas. Tectonic disruption of the oceanic basement can locally augment the axial gradient and stimulate flow channelization, or reverse the gradient and induce sediment ponding. A large, margin-parallel sediment lobe is built at the base of a high axial escarpment near 33°S, where the axial channel crosses a transverse discontinuity at the convergent plate boundary. Five lithofacies are defined by Q-mode factor analysis of sediment textures and hydro-dynamic structures in 27 cores from the Chile Trench. The Channel facies (thick, amalgamated sand, massive to laminated or cross-bedded) is deposited by high-energy processes within the coarse-grained bedload of turbidity currents; it forms in distributary and axial channels. The Levee facies (rhythmically bedded, internally structureless, graded sand and graded silt) is deposited from concentrated sediment suspensions that quickly lose momentum during channel spillover; it forms on channel flanks, although constructional levees are not always present. The Basin-1 facies (more complete Bouma sequences, both upper and lower flow regime structures) forms in ponded basins where flows are confined by a high-relief, seaward trench wall. The Basin-2 facies (graded and laminated silt, lower flow regime structures) forms in low-energy environments, such as interchannel areas, distal basins, trench walls, and elevated topographic features. The Contourite facies (silt and sand laminations winnowed from hemipelagic muds and distal turbidites) is best developed in sediment-starved basins where geostrophic currents are constricted and accelerated between the steep inner and outer trench walls. The trench wedge records a coarsening-upward sequence as the oceanic plate migrates toward and into the trench during plate convergence, and becomes more proximal to sediment sources along the base of the continental margin. Near canyon mouths, prograding trench fans drive the axial channel seaward into the trench wedge, and the coarsening-upward sequence is truncated by a time-transgressive erosional unconformity. Abandoned axial channel deposits are carried landward beneath prograding fans to record a fining-upward sequence above the basal unconformity. Channel migration and lobe aggradation may produce fining- and coarsening-upward sequences on depositional fan lobes, but sequences on the erosional lobes are fragmented by numerous truncation surfaces.