Shelf-slope Break Research Articles

Deep-water fisheries at the Atlantic Frontier

The deep sea is often thought of as a cold, dark and uniform environment with a low-fish biomass, much of which is highly adapted for life in a food-poor environment. While this might be true of the pelagic fish living in the water column, it is certainly not true of the demersal fish which live on or close to the bottom on the continental slopes around the British Isles (the Atlantic Frontier). These fish are currently being commercially exploited. There is growing evidence to support the view that success of the demersal fish assemblages depends on the pelagic or benthopelagic food sources that impinge both vertically and horizontally onto the slope. There are several quite separate and distinct deep-water fisheries on the Atlantic Frontier. It is a physical barrier, the Wyville-Thomson Ridge, which results in the most significant division of the fisheries. The Ridge, which has a minimum depth of about 500 m, separates the warmer deep Atlantic waters from the much colder Norwegian Sea water and as a result, the deep-water fisheries to the west of the Hebrides and around the offshore banks are quite different from those of the Faroe-Shetland Channel (West of Shetland). The fisheries to the West of the Hebrides can be further divided by the fishing method used into bottom trawl, semipelagic trawl and longline. The bottom-trawl fisheries extend from the shelf-slope break down to about 1700 m and the target species varies with depth. The smallest vessels in the fleet fish on the upper slope, where an important target species is the anglerfish or monkfish ( Lophius spp.). On the mid-slope the main target species are blue ling ( Molva dypterygia) and roundnose grenadier ( Coryphaenoides rupestris), with bycatches of black scabbardfish ( Aphanopus carbo) and deep-water sharks. On the lower slope orange roughy ( Hoplostethus atlanticus) is an important target species. The major semipelagic trawl fishery is a seasonal fishery on spawning aggregations of blue whiting ( Micromesistius poutassou). The other semipelagic fishery is on spawning aggregations of the greater silver smelt or argentine ( Argentina silus). Spanish and UK vessels that target mainly hake ( Merluccius merluccius) and a Norwegian fleet that targets ling ( Molva molva), blue ling and tusk ( Brosme brosme) dominate the upper slope longline fishery. West of Shetland, the fishery on the upper slope has some similarities with that of the Hebridean slope, with anglerfish and blue ling being important target species. A quite different fishery occurs in the transition zone between the Atlantic and Norwegian Sea waters. Here the main target species is Greenland halibut ( Reinhardtius hippoglossoides). Below the transition zone biomass decreases rapidly and there is no fishery. It is generally agreed that many deep-water species have slow growth, a high age at first maturity and a low fecundity, which makes them vulnerable to over-exploitation. Other features of these fishes such as high mortality of discards and escapees will add to the problems. Despite this the only management procedures in place are general limitation of effort measures within the area of jurisdiction of the European Union.

Distribution and Abundance of Phytoplankton, Zooplankton, Ichthyoplankton, and Micronekton in the Deepwater Gulf of Mexico

Expeditions in the 1960s and 1970s are the basis for the general paradigm that standing stocks and productivity of phytoplankton are both low ( <0.1 mg chl·m-3; <150 mg C.m-2·d-1) seaward of the shelf-slope break in the Gulf of Mexico. The present review supports this description of the mean (stable) state but also shows "hot spots" in primary production (>2 g C·m-2·d-1) occur when/where nutrient availability is locally enhanced seaward of the shelf-slope break. Recent collections with Bongo and MOCNESS nets, midwater trawls, and bioacoustic surveys of the Loop Current and associated cyclonic and anticyclonic eddies in the Gulf of Mexico show that these deepwater "hot spots" have higher stocks of zooplankton and micronekton as well. The local aggregations ranged in size from coarse- to meso- spatial scales (10s to 100s of kilometers) though locations of such "oases" were spatially variable along the continental margin.

Vreeland Diamictites - Neoproterozoic glaciogenic slope deposits, Rocky Mountains, northeast British Columbia

Research Article| September 01, 2000 Vreeland Diamictites – Neoproterozoic glaciogenic slope deposits, Rocky Mountains, northeast British Columbia* M.E. McMechan M.E. McMechan Geological Survey of Canada, 3303 - 33rd St. NW, Calgary, AB, T2L 2A7 Search for other works by this author on: GSW Google Scholar Bulletin of Canadian Petroleum Geology (2000) 48 (3): 246–261. https://doi.org/10.2113/48.3.246 Article history received: 09 Dec 1999 accepted: 27 Apr 2000 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M.E. McMechan; Vreeland Diamictites – Neoproterozoic glaciogenic slope deposits, Rocky Mountains, northeast British Columbia. Bulletin of Canadian Petroleum Geology 2000;; 48 (3): 246–261. doi: https://doi.org/10.2113/48.3.246 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of Canadian Petroleum Geology Search Advanced Search Abstract Neoproterozoic diamictites of the Vreeland Formation exposed in the Snake Indian and Wapiti Pass thrust sheets of the central Rocky Mountains, northeast British Columbia, form a thick (350 to 2000 m) glaciogenic deposit within the passive margin succession of the Windermere Supergroup. They are the deposits of the younger worldwide episode of Neoproterozoic glaciation. Diamictites were deposited as laterally extensive sheets in a mid slope setting with minimal sediment reworking or transport by bottom currents. In the Wapiti Pass thrust sheet, the absence of dropstones and lone-stones, in facies-equivalent and interbedded fine-grained strata, indicates there was little, if any, ice rafting during sedimentation. Diamictites are the deposits of unconfined debris flows derived from the slumping of glacial deposits near the shelf-slope break. In the Snake Indian thrust sheet, dropstones and lonestones indicate ice rafting. Diamictites formed from both unconfined debris flows and the “rain-out” of ice-rafted and sediment plume material. Deposition of thick diamictites in the Mount Vreeland area was probably localized by syndepositional, west-side-down normal fault(s).Diamictites contain abundant extrabasinal (felsic and basic intrusive, volcanic, quartzite) and intrabasinal (argillite, sandstone, carbonate) clasts. They record a major source of material entering the Windermere depositional basin distinct from the basinal turbidite system of the southern Cordillera. The high volumes of glaciogenic sediment and large extrabasinal clasts indicate the transportation of massive amounts of material to the shelf-slope break by glaciers and glacial processes. Their deposition appears to be incompatible with the snowball Earth model for Neoproterozoic glaciation, with its thin patchy continental ice cover and the virtual elimination of the hydrological cycle.Locally, the diamictites are overlain by a thin deep-water limestone (cap carbonate). Olistoliths of plafformal carbonates in the immediately overlying argillite indicate deposition of postglacial (cap) carbonates on the shelf to the east, continued deposition in a slope environment in the study area, and continued instability and fault-related uplift of the shelf edge. Detailed mapping of the southwest flank of Ice Mountain shows that carbonate olistoliths, rather than a carbonate platform, overlie the diamictites. No evidence was found of penetrative deformation, uplift or erosion of diamictites during the Neoproterozoic to support a previous interpretation of the Neoproterozoic “Vreeland High”. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Shelfbreak gullies; products of sea-level lowstand and sediment failure; examples from Bowser Basin, northern British Columbia

ABSTRACT The transition from cyclothemic shelf to shale-dominated slope deposits (Callovian to Oxfordian) that accumulated along the northern convergent margin of Bowser Basin is delineated by spectacular, conglomerate-filled channels, or gullies. The gullies, representing the shelf-slope break, formed in two stages: (1) Slumping of upper slope-outer shelf sediment created topography, which determined (2) the course of fluvial, distributary-type channels and focused gravel-dominated sediment transport during relative sea-level lowstands. Gully fill consists predominantly of debris-flow deposits, locally incorporated into small (lowstand) fan deltas that prograded from gully margins. Lowstand fluvial channels overlie shelf cycles, incise the underlying highstand deposits, and are overlain by fossiliferous, transgressive sandstone. However, the shelfbreak gullies deposits are up to 10 times thicker than those of the associated lowstand fluvial channels. Although the lowstand fluvial channels likely acted as bypass conduits for gravel and sand during times of low sea level, the initiation and overdeepening of the shelfbreak gullies by slumping probably took place during both high and low relative sea levels. Deposition within gullies may also have continued during relative highstands, from sediment in storage on the outer shelf. The implications for sequence stratigraphic models are: shelf-break gullies can form at any stage of sea level fluctuation; gully thickness may not be related to the magnitude of lowstand incision on the adjacent shelf; and gullies formed by sediment gravity failure do not provide quantitative information about the magnitude of sea-level fall.

High-resolution Seismic Evidence for Multiple Glaciation across the Southwest Iceland Shelf

Geophysical and lithological data collected from the southwest margin of Iceland provide clear evidence of multiple glaciations across the shelf, many of which reached the shelf-slope break. These paleo-ice sheets delivered much glacial sediment to the slope facing Denmark Strait. A large moraine some 20 to 100 m in height lies at the shelf break due west of Látra Bank. The moraine may mark the Late Weichselian extent of the western Iceland ice, although a single date from a 50-cm gravity core collected from the seaward side of the moraine provided an age of 35,650 ± 560 14C BP. Retreat of this ice sheet appears to have been rapid, except for a minor pause when the ice sheet terminus approached the coastline. A marine reservoir-corrected basal date of 12,705 ± 85 14C BP was obtained from a 17-m giant piston core collected in Jökuldjúp (a shelf trough) which sampled glacial marine sediments close to underlying ice-contact sediment. Because this date is very close to the age of shells near the marine limit in West Iceland, some 100 km to the east-northeast, it suggests that this margin of the Iceland Ice Sheet retreated extremely rapidly during the Bølling/Allerød interstadial (13,000 to 11,000 14C BP).

Variable rates of Late Quaternary surface uplift along the Banda Arc-Australian plate collision zone, eastern Indonesia

Abstract Radiometrically dated emergent coral terraces from southeastern Indonesia provide estimates of differential vertical strain in the Banda Arc-continent collision complex. At Semau island, two samples from the lowest emergent reef (5–7 m) yield 230 Th dates that correspond to the 5a ( c. 83 ka) sea-level highstand and a low surface uplift rate of 0.2–0.3 m per 1000 years. At Rote island, samples from the lowest emergent reef ( c. 1–2 m) on both north and south sides of the island yield late Holocene ages and an average short-term uplift rate of c. 1–1.5 m per 1000 years. Similarity of ages from different samples on the north coast of Rote suggests possible coseismic emergence. Survey data from nine emergent reefs and marine notches up to 170 m in altitude on the south side of Rote indicate that uplift rates may have been c. 1–1.5 m per 1000 years for c. 120 000–130 000 years. Combined with previous studies, these results indicate that late Quaternary surface uplift rates vary an order of magnitude along the strike of the Banda orogen. Vertical displacement rates are greatest in young parts of the orogen where the shelf-slope break recently has been underthrust beneath the orogenic wedge, as at Rote, and in older parts of the orogen where retroarc thrust faulting occurs, as at Alor island.

The South Westland Basin: seismic stratigraphy, basin geometry and evolution of a foreland basin within the Southern Alps collision zone, New Zealand

This paper develops further the case for a foreland basin origin of South Westland Basin, located adjacent to the Southern Alps mountain belt. Geohistory analyses show Middle Miocene initiation of subsidence in the basin, with marked increases at 5–6 Ma. Five seismic reflection horizons, including basement, Middle Miocene (top Awarua Limestone), top Miocene, mid-Pliocene (PPB) and mid-Pleistocene (PPA) have been mapped through the grid of seismic data. A series of five back-stripped structure contour maps taken together with five isopach maps show that prior to the Middle Miocene, subsidence and sedimentation occurred mainly along the rifted continental margin of the Challenger Plateau facing the Tasman Sea; subsequently it shifted to a foredeep trending parallel to the Southern Alps and located northwest of them. Through the Late Miocene–Recent this depocentre has progressively widened, and the loci of thickest sediment accumulation have moved northwestward, most prominently during the Late Pliocene and Pleistocene with the progradation of a shelf–slope complex. At the northern end of the basin the shelf–slope break is currently located over the forebulge, which appears not to have migrated significantly, probably because the mountain belt is not advancing significantly northwestwards. Modelling of the lithospheric flexure of the basement surface normal to the trend of the basin establishes values of 3.1 to 9.8×10 20 N m for the flexural rigidity of the Australia Plate. This is at the very low end of rigidities for plates, and 1–2 orders of magnitude less than for the Australia Plate beneath the Taranaki Basin. Maps of tectonic subsidence where the influence of sediment loading is removed also clearly identify the source of the loading as lying within or beneath the mountain belt. The basin fill shows a stratigraphic architecture typical of underfilled ancient peripheral foreland basins. This comprises transgressive (basal unconformity, thin limestone, slope-depth mudstone, flysch sequence) and regressive (prograding shelf–slope complex followed by molasse deposits) components. In addition the inner margin of the basin has been inverted as a result of becoming involved in the mountain building, as revealed earlier by fission track thermochronological data. The timing and degree of inversion fits well with the geometrical and stratigraphic development of the basin. That the inversion zone and the coastal plain underlain by molasse deposits are narrow, and most of the basin is beneath the sea, highlights this as an underfilled active foreland basin. The basin is geodynamically part of the Southern Alps collision zone.

Shelf-to-basin facies distributions and sequence stratigraphy of a steep-rimmed carbonate margin; Capitan depositional system, McKittrick Canyon, New Mexico and Texas

ABSTRACT Shelf-to-basin outcrop studies in steep-rimmed, shelf-margin settings are uncommon because continuous shelf-to-basin transects are rarely exposed in a single outcrop. Discontinuous or absent stratigraphic marker beds across the shelf margin further complicate outcrop studies in the shelf-margin setting. This paper discusses the results of a high-resolution investigation of the shelf-to-basin profile along the north wall of North McKittrick Canyon, New Mexico and Texas. In McKittrick Canyon, carbonate-dominated sedimentary rocks associated with the steep-rimmed, Upper Permian Capitan depositional system are exposed along a continuous 5-km outcrop face. Measured sections, lateral transects, scintillometer readings, and geochemical data were synthesized into a digital database and interpreted in conjunction with a digital photomosaic of the entire canyon wall. Results of this work include a shelf-to-basin facies map and sedimentologic interpretation of the north wall of North McKittrick Canyon, and indicate that the dominant bathymetric profile during Capitan deposition was a marginal mound. In this model, the Capitan reef facies was deposited at the shelf-slope break in water depths ranging from 15 to 75 m, but always downdip from the topographically higher shelf crest. This model is supported by the following observations and interpretations: (1) a facies progression from the shelf crest to the shelf margin interpreted to represent a shallow-to-deeper-water succession; (2) proportional expansion of beds in a downdip direction; (3) presence of oriented (transported) fusulinid grainstones downdip from in situ fusulinid wackestones and packstones updip; (4) siltstones that thin and pinch out towards the shelf margin; (5) a decrease in dolomite from the shelf crest to the shelf margin; and (6) the absence of true toplap stratal geometries. In reality, a static paleobathymetric model cannot characterize the depositional system, because the facies distributions, facies proportions, stratal geometries, and quantified depositional parameters vary systematically from the Seven Rivers through the Tansill. In order to understand the observed variations, emphasis was placed on quantifying key depositional parameters such as progradation, aggradation, offlap angle, outer-shelf dip, water depth, distance to the shelf margin and toe of slope, and facies-tract width. The systematic variations in these parameters, in conjunction with the facies distribution map and stratal geometries, helped to define the sequence-stratigraphic framework, and allowed for comparative evaluation of such things as sediment accumulation rates and sites, and stratigraphic evolution. The Capitan depositional system is represented by three composite sequences, each containing four high-frequency sequences. Two and one half of these composite sequences are exposed in McKittrick Canyon. The overall depositional system is interpreted to have evolved predictably from a deeper-water margin in the Seven Rivers composite sequence, to a shallow-water margin in the Tansill composite sequence. The subtidal outer-shelf and shelf-margin facies tracts were sites of major sediment production. Accumulation rates across the shelf margin indicate a relatively continuous growth history, with periods of nondeposition or erosion limited to the terminal phase of each composite sequence. As a result, the preserved sedimentary record of high-frequency and composite sequences in the outer-shelf to upper-slope position is equally proportioned between transgressive and highstand systems tracts. This symmetric outer-shelf to upper-slope record of carbonate accumulation is significantly different from the asymmetric, highstand-dominated middle-shelf accumulation record reported previously for this and many other carbonate shelves.

Carbonate ramps and the Pleistocene-Recent depositional systems of the Arabian Gulf

Abstract The term ramp was devised to be helpful. It was to provide an interpretative model with genetic significance for a specific category of carbonate body and one with predictive value in oil exploration. The present-day southern Arabian Gulf is commonly cited as a ramp but it fails to provide a good example. In contrast with most of its pre-Miocene history, the Arabian Gulf is a foreland basin with a tectonically, climatically and glacioeustatically controlled mixed carbonate-clastic fill. Because of glacioeustasy and its relatively shallow depth, the Gulf has spent much of the last 2.5 Ma emergent. Three separate and equally important sedimentary systems currently contribute to its development: the Arabian Gulf marine carbonate system, the Mesopotamian fluviodeltaic system and the Arabian continental aeolian system. The Arabian Gulf carbonate system is a ‘high stress’ regime that is in tectonic, eustatic and depositional disequilibrium. Its structural, morphological and sedimentological diversity discount it as a Recent analogue for the thick carbonate bodies in the geological record that show persistent accumulation of depth-related facies developed across gentle slopes lacking a marked shelf-slope break. The Arabian Gulf fails some fundamental tests for whether it should be regarded as a stable carbonate ramp depositional system and its use as an example of a ramp is not helpful.

Abstract: Sequence Stratigraphic Framework and Depositional Variations of Early Miocene Lowstand Systems Tracts, Southwestern Louisiana

Abstract Facies of Miocene lowstand systems tracts comprise a large portion of the remaining exploration potential for deep untested objectives I southwest Louisiana. This complex and highly variable section is contained within 22 major depositional sequences deposited from 21.9 to 7 million years ago; each sequence ranges in duration from 0.5 to 1.5 million years. Regional well-log, seismic, and biostratigraphic correlation and mapping of the sequence stratigraphic framework defines the occurrence and extent of lowstand facies that filled intra-slope basins across south Louisiana's coastal zone. From oldest to youngest, the sequence encompass the Cristellaria R through Cristellaria K biozones. Within onshore and state water areas, 14 sequences contain regionally correlative lowstand systems tract components. Almagamated, massive turbidite facies of basin floor fan complexes are not recognized within intra-slope basins in association with rapid depositional cycles. Deep-water fan and channeled levee facies of slope fan complexes and deltaic facies of prograding complexes are well-developed throughout the early and middle Miocene section. Late Miocene lowstand deposits are best developed in the offshore trend. Syndepositional fault movement and salt withdrawal controlled thickness trends, facies patterns, sandstone content, and the position and trend of the shelf-slope break. Sandstone-rich basin floor fans, slope fans, and prograding deltaic complexes are best developed in intraslope basins where paleobathymetric confinement and accommodation were greatest. End_of_Record - Last_Page 349-------

Three-dimensional propagation effects near the Mid-Atlantic Bight

A recent set of experiments sponsored by the Office of Naval Research has been conducted in the vicinity of the Mid-Atlantic Bight shelf break region off the coast of New Jersey. The environment in this region is highly variable in both range and bearing due to the shelf break slope and its associated front. One of the goals of this Shelf Break Primer project is a determination of the influence of azimuthal coupling on the 3-D propagation. By using a fully 3-D PE model and comparing with results from the associated 2-DxN PE approximation, a quantitative assessment of the influence of azimuthal coupling can be made. Some sample benchmark cases are performed to illustrate this method. Using this approach, the significance of both the sloping bathymetry and the shelf break front will be considered separately and combined. In addition, the influence on the arrival time structure of a broadband pulse, similar to a signal employed during the experiments, will be determined. Characteristic features of such 3-D effects in the pulse propagation will then be explored in the available data sets. [This work is supported by the Office of Naval Research, Code 321OA.]

Initiation of ∼2.45-2.1 Ga intracratonic basin sedimentation of the Hurwitz Group, Keewatin Hinterland, Northwest territories, Canada

Continental to shallow-marine rocks of the Hurwitz Group, exposed as outliers across the Hearne Province in northern Canada, were deposited between ∼2.45 and 2.11 Ga on a 3.3-2.6 Ga granite-greenstone basement. Initiation of the Hurwitz Basin was characterized by low relief and rates of subsidence that never progressed beyond the intensity of regional sagging. During stage 1, polymictic conglomerate (lower Noomut Formation) was deposited in palaeovalleys cut into Archaean basement, and subarkose to quartz arenite ± quartz pebble conglomerate (upper Noomut Formation) was deposited on a low-relief sand plain. Abundant first-cycle quartz-rich sandstones are attributed to intense humid climate weathering in low-relief source areas and at sites of temporary storage, as well as to repeated fluvial and eolian reworking. Interbedding of pyritic palaeoplacers and continental redbeds denotes fluctuating levels of atmospheric oxygen; disappearance of pyritic palaeoplacers upsection signifies ultimate changeover to an atmosphere with sustained free oxygen. Stage 2 (Padlei Formation) was marked by a climatic shift toward cold (glacial?) conditions as indicated by dropstones, greywacke (till?) pellets and possible ikaite pseudomorphs in lacustrine rhythmites. Stage 3 (Kinga Formation) represents a return to wet/warm conditions with fluvial (± eolian) deposition of subarkose and quartz arenite on a low-relief sand plain (Maguse Member) and deposition of supermature quartz arenite in a vast (100,000 km 2 minimum), shallow-water lake or series of lakes (Whiterock Member). Local chert beds at the top of the Kinga Formation (Hawk Hill Member) formed by spring discharge due to regional, gravity-driven, convective groundwater flow. An intracratonic setting is implied by: (1) a symmetric, basin-centred fill; (2) absence of features indicating a shelf-slope break; (3) absence of rift rocks; (4) a prolonged slow-subsidence history; (5) likely correlation with other continental to shallow-marine deposits scattered across the Keewatin Hinterland; and (6) positioning 1000–2000 km away from closest coeval passive margin sequences. We speculate that Hurwitz Basin formed due to lithospheric stretching related to a large-scale mantle upwelling during the protracted breakup of an earliest Palaeoproterozoic supercontinent (‘Kenorland’).

Numerical modelling of stratal architectures resulting from differential loading of a mobile substrate

Abstract A numerical model is presented in which a viscous layer flows in response to differential loading by deltaic sedimentation. No density effects are cconsidered; the mobile layer and overlying sediments deform solely in response to differential thicknesses of sedimentary overburden. Modelling results include the following key features: (1) sedimentary depocentres are asymmetric and spoon-shaped, with flat-lying shelf strata grading laterally into foreset slope strata at a prograding shelf-slope break; (2) the locus of deformation is controlled by the position of the shelf-slope break; and (3) a displaced bulge of mobile substrate migrates seaward faster than the shelf-slope break progrades. Using viscosities characterisitic of natural salt (η = 10 16 to 10 19 Pa s), the sensitivity of the model to several parameters is examined. Increasing the mobile layer thickness increases the rate of substrate withdrawal in a manner similar to decreasing its viscosity. A shorter sediment transport distance results in steeper depositional slopes and faster substrate withdrawal. Comparison with a Miocene example from offshore Texas demonstrates that structures qualitatively attributed to differential loading can be successfully modelled using reasonable values for sediment supply, relative sea-level rise, and substrate thickness.

Small scale spatial structure in zooplankton from high-frequency acoustical measurements

Volume backscattering was measured at multiple discrete depths at frequencies of 0.165 and 1.1 MHz from a mooring on the shelf-slope break at the northern edge of the San Pedro Basin near Los Angeles, CA. Attempts to understand temporal variability and vertical structure in the data from the mooring led us to make additional measurements along an onshore–offshore transect at this location at frequencies of 0.265, 1.1, 1.8, and 3.0 MHz. These measurements, when processed with an inverse algorithm to determine plankton size and abundance, revealed both an onshore–offshore trend and small scale structure in this environment. Results of sampling the same environment with a multiple opening–closing net sensor system (MOCNESS) are consistent with the acoustical measurements, but at the same time highlight the difficulty of sampling a heterogeneous spatial distribution with a net system. [Work supported by ONR, NSF, and Tracor Applied Sciences.]

Shelf-Slope Break Sedimentation and Erosion During Deposition of the Claiborne Group in Colorado and Wharton Counties, Texas: ABSTRACT

ABSTRACT Geological and geophysical data over a 6,500 square kilometer area across Wharton and Colorado Counties, Texas, are utilized to understand sedimentary processes that occurred along the shelf-slope break of the Eocene Claiborne Group. Sedimentation models based on seismic profile data and well log data are critical to the oil and gas industry. It can be demonstrated that Claiborne deposition generally took place in middle neritic water depths on the shelf, however, some sediment did bypass the shelf and was deposited in deeper waters downdip. Sands of the Yegua Fm. were transported basinward from the deltaic and fluvial depositional environments in the updip area of Colorado County, bypassing shelf deposition. In general, there was little sedimentation taking place along the shelf except during short periods of transgression. Downdip at the shelf-edge, subsidence rates were higher and sedimentation became more aggradational. Possible drops in sea-level and/or changes in depocenter locations brought about downdip deposition of sediment beyond the shelf edge during most periods of Yegua-age deposition. It appears that even during low accumulation times on the shelf, sediment was being deposited downdip beyond the shelf-edge. Slumping along the over-steepened shelf margin was common during Claiborne deposition. The style of Claiborne age slumping was controlled by many factors including the placement of the Wilcox shelf. During high sedimentation times, erosion of the shelf margin from slump related carving appears to have taken place.

Applications of High Resolution Sequence Stratigraphy in North Sea Syn-Rift Reservoir Correlation and Development: ABSTRACT

Tectonically active basins may host a spectrum of sequence stratigraphic expressions previously considered to be spatially mutually exclusive. In low accommodation areas with high sediment supply, fourth order eustatic cyclicity results in high frequency sequence sets while within rapidly subsiding areas, time-equivalent Type-2 sequences are expressed by highly asymmetrical coarsening upward successions, resembling large parasequences. In the shallow marine Fulmar Formation, of the U.K. North Sea Central Graben a sequence boundary and overlying lowstand deposits, which illustrate the effects of laterally variable subsidence rate and intrabasinal topography on the expression of a eustatic sea-level fall, lie between the Glosense and Serratum (J54a and J54b) maximum flooding surfaces. The syn-rift physiography comprises major tilted fault blocks, with the Central Graben dipping parallel to the major faults, simulating a ramp setting. Where the throw of the faults were greatest (SE), the structure acted as a local shelf-slope break. Adjacent to the basin margin, incised valley were cut at fluvial input points (structural transfer zones) and laterally, interfluvial sequence boundaries developed. During early lowstand, sand bypassed the footwall shelf and was deposited as lowstand fan sediments within the deepest part of the hangingwall, with the fault zone acting as a local shelf slopemore » break. Within the shallower water areas of the hangingwall a localised ramp geometry existed parallel to the fault zone. Forced regression deposits developed here were coeval but not physically related to the deep water lowstand turbidite fan deposits.« less

Fluxes of particulate matter on the slope of the southern Middle Atlantic Bight: SEEP-II

An array of 17 sediment traps was moored in two cross-isobath transects during the SEEP-II experiment, one north of the Washington Canyon, and the other south of the Norfolk Canyon off the Delmarva Peninsula in the southern Middle Atlantic Bight. The overall duration of the experiment was 15 months, during which the moorings were deployed three times, and the traps collected the vertical flux of particles during a total of 30 sampling intervals of average duration 12.8 days. The 483 particulate samples successfully recovered were each analyzed for total mass flux and the abundance of major biogenic phases (organic carbon, nitrogen, calcium carbonate and opal) and for 210Pb. A smaller number were also analyzed for 234Th. The distribution of fluxes in space requires transport of particles both downslope from the shelf-slope break and out into the water column, and alongslope transport. The region of maximum in particulate-matter flux to the slope, the depocenter, is from 400 to 1000 m, and the position of this maximum coincides with the maximum in alongslope current activity as expressed by the average total kinetic energy density. The total mass flux and the flux of all components to the slope depocenter increases between the northern Middle Atlantic Bight region, studied during the SEEP-I experiment, and SEEP-II sites in the southern Middle Atlantic Bight by about a factor of three to four. The temporal variability of the fluxes measured is dominated by storm-driven resuspension events on the shelf and shelf-slope break, but this extreme, short-term variability is not seen in the depocenter traps, the fluxes to which are very constant in magnitude and in composition. Canyons are suggested to serve both as conduits for transport of sediment from the shelf to the slope, and as a reservoir that feeds shelf-derived particles to the slope depocenter on a regular basis, even when sediment resuspension on the shelf is subdued or absent.

Geomorphic Thresholds and Complex Response of Fluvial Systems--Some Implications for Sequence Stratigraphy

First-generation sequence stratigraphic models have dealt in a very rudimentary fashion with the response of fluvial systems to eustasy. A major element of presently accepted models is that rivers incise when sea level falls and aggrade during the ensuing rise. Well-established geomorphic principles state that fluvial systems are complex, process-response systems that can adjust to internal and external changes in other ways besides incision and aggradation by modifying their stream patterns and channel geometries. Application of geomorphic principles to sequence stratigraphic models results in the following observations. During eustatic lowstands, rivers may adjust to lowered base levels and changes in slope by modifying channel patterns. Therefore, not all lowstands produce type 1 sequence boundaries. Type 1 sequence boundaries characterized by fluvial-valley incision are more likely to develop when sea level drops below the shelf-slope break, resulting in topological relief near the strandline in which headwardly eroding knickpoints form. Rates of eustatic change, although rapid in a geological context, are sufficiently low that geomorphic systems can easily maintain their equilibrium during eustatic changes and migrate back and forth across the shelf without major modifications. Finally, und r conditions of relatively static sea level, sequences and parasequences of the same scale in time and space as those which form during eustatic change can be deposited as the result of purely intrinsic causes and responses of a fluvial system. In general, eustasy controls the location of deposition and erosion, but the resultant stratal geometry is controlled by sediment supply and processes acting on the sediments as the shoreline moves across the shelf in response to eustasy.

Sequential Biostratigraphy of the San Jose-1, Puerto Estrella-1, Santa Ana-1, Uashir-1, and Jarara-1 Offshore Wells, Guajira Peninsula, Colombia

Sequence stratigraphical analysis of the San Jose-1, Puerto Estrella-1, Santa Ana-1, Santa Ana-1, Uashir-1, and Jarara-1 offshore wells located in the Portete area, Guajira Peninsula, provide insight into the development and distribution of reservoirs and seals and allows for a finer stratigraphic resolution and correlation than previous available. The sequence stratigraphical interpretation is based on the interpretation of detailed biostratigraphical analysis, the parasequence stacking patterns from the wireline logs, sedimentological data, and seismic stratigraphical interpretation. The Eocene to Pliocene sediments of the Portete wells were deposited for the most part in outer shelf (shelf-slope break) to upper bathyal, disoxic to anoxic environments. Important reservoir development is formed by lowstand slope and basin floor fans interpreted to correspond to the Early Miocene TB1, 1.4 and 1.5, the Middle Miocene TB2 2.3 and 2.4 sequences, and to the Late Eocene to Early Oligocen TA4 supercycle.

Sediment Pathways of the Northwest Green Canyon Area, Offshore Louisiana

ABSTRACT Physiography of the upper slope in the northwest Green Canyon area is strongly influenced by the occurrence of shallow salt features. During sea level lowstands, sediments are transported from the shelf through feeder channels in pathways between bathymetric highs controlled by salt tectonics. Five seismic sequences have been mapped in the northwest Green Canyon area. Each sequence is bounded by condensed sections, which commonly display high-amplitude, continuous reflections overlain by thin highstand/transgressive systems tracts with typically high- to moderately-high amplitude, continuous reflections on seismic profiles. Comparatively thick lowstand systems tracts overlie highstand/transgressive tracts. The hummocky-to-chaotic lowstand systems tracts in this area consist almost entirely of mass-transport complexes. These features develop from slumping and mass-wasting along the shelf-slope break and the over-steepened margins along the sediment pathways. The oldest mapped horizon corresponds to the highest occurrence of Trimosina 'A' (0.65 Ma, Shaffer, 1990). Time-thickness maps of five lowstand sequences were used to map sediment pathways. A strong correlation exists between present-day bathymetry and positions of Late Pleistocene to Recent sediment pathways.