Sandy Contourites Research Articles

Sedimentary processes and variations in slope-current activity during the last Glacial-Interglacial episode on the Hebrides Slope, northern Rockall Trough, North Atlantic Ocean

Shallow cores and high-resolution seismic reflection data collected from two areas along the Hebrides Slope indicate that a variety of depositional processes have been active since the last Glacial interval. Seismic reflection profiles from the most northerly slope area, immediately south of the Wyville-Thomson Ridge, reveal a slope apron and an embayed area with a relict mid-upper slope channel. Five cores from this locality consist of distal muddy turbidites in the area of the channel, with muddy contourites interbedded with glaciomarine ice-rafted sediments and thin debris flow deposits. Most of the cores are topped by sandy contourites. The preserved sequence suggests that lowered sea-levels during the last glaciation (> 10 ka) resulted in a predominance of downslope and glaciomarine sedimentation, with bottom-current activity greatly suppressed. At the onset of deglaciation, alongslope activity increased as bottom currents gained in strength, with climatic fluctuation and the retreat of ice-sheets from the shelf, producing muddy and sandy contourites. Present-day current activity is still much in evidence with two current-meter stations on the mid and lower slope recording a northward-flowing current following the bathymetric contours. Recorded peak velocities at 468 m and 403 m were 26–48 cm/s. Some elements of reverse flow at the deeper site may be tidal oscillation compounded by short-duration high-intensity benthic storms. Cores from the more southerly area, in the vicinity of the Geikie Escarpment, penetrated hemipelagites erosively overlain by sandy contourites. Dinoflagellate and nannofossil dating of these deposits indicate that the lower hemipelagites are of late Glacial age (> 13 ka). The sandy contourites are probably of early Holocene age. The lack of the Allerød-Bølling interstadial and Younger Dryas stadial may be indicative of a hiatus in sedimentation. A possible cause of this may be the increasing slope angles (up to 28° on the escarpment), concentrating the effects of a thermohaline slope current. Current flow was also strengthened during deglaciation, producing erosion or non-deposition of the lower units leaving the coarser “top only” sandy contourites.

Late Cenozoic sediment drift complex, northeast Rockall Trough, North Atlantic

Seismic reflection profiles and two shallow cores have revealed a sediment drift complex in the northeastern Rockall Trough.The drift complex consists of an elongate drift with associated sediment waves, a broad sheeted drift with a larger field of sediment waves, and smaller, moat‐related, isolated drifts. The internal reflection configuration of these features indicates consistent upslope migration throughout the late Cenozoic. The larger field of sediment waves has been described previously with a southerly migration; a reevalution of these waves suggests a migration direction to the east, conforming with the other drift complex features. The most vigorous current activity probably occurred during the Miocene, with a more reduced current flow prevailing during the Pliocene to Holocene interval. Core evidence from the elongate drift and the moat has revealed upper Pleistocene and lower Holocene glaciomarine sediments reworked to produce muddy‐silty and sandy contourites. Sedimentation rates for the early Holocene (pre‐7.5 ka) are up to 4‐times greater on the drift compared to the moat. It is suggested that the sediment drift complex has formed through the interaction of a northward flowing slope current, of North Atlantic Deep Water origin, with an area of complex bathymetry at the northeastern end of the Rockall Trough, where the Wyville‐Thomson Ridge intersects the Hebridean Margin.

Process Sedimentology and Reservoir Quality of Deep-Marine Bottom-Current Reworked Sands (Sandy Contourites): An Example from the Gulf of Mexico

Deep-marine bottom-current reworked sands (sandy contourites) have been recognized in hydrocarbon-bearing sands of the Gulf of Mexico. A distinctive attribute of these sands is their traction bed forms, which occur in discrete units. Common sedimentary features of traction currents include cross-bedding, current ripples, horizontal lamination, sharp upper contacts, and inverse size grading. These sands also exhibit internal erosional surfaces and mud offshoots, indicating oscillating current energy conditions. The Pliocene-Pleistocene sequence cored in the Ewing Bank Block 826 field in the Gulf of Mexico provides an example of sand distribution and reservoir quality of deep-marine bottom-current reworked sands. Presumably, the Loop Current, a strong wind-driven surface current in the Gulf of Mexico, impinged on the sea bottom, as it does today, and resulted in bottom-current reworked sands. A depositional model based on the integration of well (core and log) and three-dimensional seismic data suggests that the reworked sediment package may be thick and continuous, but individual sand layers within the package may be thin and discontinuous. This unconventional model, which depicts the distribution of bottom-current reworked sands in interchannel slope areas as a distinctly different facies f om channel-levee facies, has the potential for general application to other slope plays outside the study area. In the Ewing Bank Block 826 field, the type 1 (L-1) reservoir with 80% sand exhibits higher permeability values (100-1800 md) than the type 2 (N-1) reservoir with 26% sand (50-800 md). The increased permeability in the type 1 sand has been attributed to high sand content, vigorous reworking, and microfractures. The clean, porous, and well-sorted type 1 sands with good communication between sand layers have produced at higher rates and recovery efficiencies than the type 2 sands with numerous interbedded mud layers.

Mediterranean undercurrent sandy contourites, Gulf of Cadiz, Spain

The Pliocene—Quaternary pattern of contourite deposits on the eastern Gulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are perpendicular to slope contours and the Mediterranean undercurrent that has flowed northwestward parallel to the slope contours and down valleys between the ridges since the late Miocene opening of the Strait of Gibraltar. Coincident with the northwestward decrease in undercurrent speeds from the Strait there is the following northwestward gradation of sediment facies associations: (1) upper slope facies, (2) sand dune facies on the upstream mid-slope terrace, (3) large mud wave facies on the lower slope, (4) sediment drift facies banked against the diapiric ridges, and (5) valley facies between the ridges. The southeastern sediment drift facies closest to Gibraltar contains medium-fine sand beds interbedded with mud. The adjacent valley floor facies is composed of gravelly, shelly coarse to medium sand lags and large sand dunes on the valley margins. Compared to this, the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand. Further northwestward, sediment drift grades to biogenous silt near the Faro Drift at the Portuguese border. Because of the complex pattern of contour-parallel and valley-perpendicular flow paths of the Mediterranean undercurrent, the larger-scale bedforms and coarser-grained sediment of valley facies trend perpendicular to the smaller-scale bedforms and finer-grained contourite deposits of adjacent sediment drift facies. The bottom-current deposits of valleys and the contourites of the Cadiz slope intervalley areas are distinct from turbidite systems. The valley sequences are not aggradational like turbidite channel—levee complexes, but typically exhibit bedrock walls against ridges, extensive scour and fill into adjacent contourites, transverse bedform fields and bioclastic lag deposits. Both valley and contourite deposits exhibit reverse graded bedding and sharp upper bed contacts in coarse-grained layers, low deposition rates, and a regional pattern of bedform zones, textural variation, and compositional gradation. The surface sandy contourite layer of 0.2–1.2 m thickness that covers the Gulf of Cadiz slope has formed during the present Holocene high sea level because high sea level results in maximum water depth over the Gibraltar sill and full development of the Mediterranean undercurrent. The late Pleistocene age of the mud underlying the surface sand sheet correlates with the age of the last sea-level lowstand and apparent weak Mediterranean undercurrent development. Thus, the cyclic deposition of sand or mud layers and contourite or drape sequences appear to be related to late Pliocene and Quaternary sea-level changes and Mediterranean water circulation patterns. Since its Pliocene origin, the contourite sequence has had low deposition rates of < 5 cm/1000y on the upper slope and < 13 cm/1000y in the middle slope sediment drift.

The Bottom Current Sediments (Contourites) in the Upper Cretaceus Deep-Water Deposits in the Northeeastern Part of Žumberak (Croatia)

The Upper Cretaceous deep marine deposits in NE parts of Žumberak contain sediments deposited by bottom currents - muddy, sandy and sandy-gravelly contourites. The complete (negative-positive) sequences and incomplete (negative-positive) sequences have been observed. Apart from the sediments that have been deposited by bottom currents, several turbidite layers reworked by bottom currents occur.

Contourite facies of the Faro Drift, Gulf of Cadiz

Summary The Faro Drift is an elongate sediment body (50 km long, 300 m thick) that parallels the northern margin of the Gulf of Cadiz south of Portugal. On the basis of location, morphology and seismic character of the drift together with bottom photographs, sediment distribution and the known regional oceanography, we can be certain that the Faro Drift was constructed by bottom currents related to the deep outflow of Mediterranean water. Detailed study of a closely-spaced suite of cores has therefore allowed characterization of the sediments into three contourite facies: sands and silts, mottled silts and muds, and homogeneous muds. The first is equivalent to the sandy contourites and the two others to the muddy contourites of earlier studies. These three facies are arranged in irregular vertical ‘sequences’ that reflect long-term variations in bottom current velocity. They are distinctly different from typical fine-grained turbidite and hemipelagite facies.

Identification of ancient sandy contourites

Research Article| August 01, 1981 Identification of ancient sandy contourites J.P.B. Lovell; J.P.B. Lovell 1Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland Search for other works by this author on: GSW Google Scholar D.A.V. Stow D.A.V. Stow 1Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland Search for other works by this author on: GSW Google Scholar Geology (1981) 9 (8): 347–349. https://doi.org/10.1130/0091-7613(1981)9<347:IOASC>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J.P.B. Lovell, D.A.V. Stow; Identification of ancient sandy contourites. Geology 1981;; 9 (8): 347–349. doi: https://doi.org/10.1130/0091-7613(1981)9<347:IOASC>2.0.CO;2 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 SocietyGeology Search Advanced Search Abstract Proposed criteria for the identification of ancient sandy contourites include occurrence (thin beds, reworked sandy turbidites, coarse lag), structure (bioturbated, no regular vertical sequence, orientation of cross-lamination perpendicular to regional downslope paleocurrents), grain size (silt or sand, little mud, low or negative skewness), fabric (grain orientation), and composition (downslope and along-slope provenance, concentrations of heavy minerals). A suggested three-stage approach to recognition of sandy contourites in the field involves work on a progressively increasing scale, with particular emphasis on paleocurrent data. The various deep-water settings in which disparate current directions may occur are summarized, and those in which sandy contourites may be formed and recognizably preserved are indicated. A broad definition of the term “contourite” is now possible because of the widespread documentation of contourites even in freshwater environments. Along-slope flow that is persistent in time and space is a requirement. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Contourites: Their recognition in modern and ancient sediments

We suggest that publication of evidence that the continental rise of the western North Atlantic has been shaped by bottom currents flowing parallel to bathymetric contours (Heezen, Hollister and Ruddiman, 1966) marked the beginning of a revolution in sedimentology comparable to the turbidite revolution launched by Kuenen and Migliorini in their classical 1950 paper. Intensification of slow, thermohaline circulation on the western margins of the ocean basins leads to high-velocity, deep, boundary currents, capable of eroding, transporting and depositing fine-grained sediment. Long-period, direct current measurements suggest a complex, periodic flow for these currents, while bottom photographs indicate their influence on the sediment surface. Sediment ridges in the North Atlantic can be closely related to the deep-water circulation pattern. Other morphological features (ripples, furrows, waves), echogram characteristics, and the presence of well-developed nepheloid layers cannot be uniquely attributed to the action of bottom currents. Critical review of marine-based investigations reveals a lack of generally accepted criteria for the recognition of contourites on the basis of sediment character. We discuss the problems in establishing such criteria and recognize that: (a) a continuum may exist between dilute turbidity flows, bottom currents and hemipelagic settling; (b) interbedded turbidites, contourites and hemipelagites are common, especially in a rise environment; and (c) composition and other criteria may be only locally applicable. However, we can identify two main contourite groups, muddy contourites and sandy contourites, and have proposed new criteria for their recognition. Muddy contourites are generally bioturbated, have poorly defined bedding, and contain biogenic sand often concentrated into irregular layers. They may be texturally and compositionally distinct from interbedded turbidites, and have relatively high CaCO 3 and organic carbon contents. Sandy contourites occur as thin, bioturbated, irregular lag-deposits, or as reworked tops of sandy turbidites. In the latter case they may be clean, well sorted, parallel- or cross-laminated, but show no offshore trends or vertical structural sequence. Grain orientation shows the bottom current direction, often superimposed upon the original turbidite fabric. Reveiw of land-based work shows that there is growing recognition of the need for a new concept to complement turbidity-current theory, but that there have been relatively few claims of firm contourite identification. Recognition of ancient contourites has been based either on the application of previous sedimentological criteria, or on an interpretation of the broader environmental framework. It is suggested that it is lack of suitable criteria for the identification of contourites rather than a true scarcity of these rocks that has led to such a restricted literature. Mindful of problems created by diagenesis, tectonic activity and the limited preservation potential of many diagnostic features of marine contourites, we do suggest criteria, and a procedure, for the recognition of contourites in land-based work. Sandy contourites of the reworked-turbidite variety may be the most easily recognised; the presence of bimodal palaeocurrent directions at about 90° is an important indicator of this type. The geological significance of contourites in palaeo-oceanographical, palaeogeographical and tectonic reconstructions is emphasised, especially in relation to work on passive (Atlantic-type) continental margins. We also refer to the possible economic significance of contourites as exploration for hydrocarbons moves into deeper waters.

Wave-induced bottom currents in the Celtic Sea : Hadley M.L., 1964. Mar. Geology, 2 (1/2): 164–167

We suggest that publication of evidence that the continental rise of the western North Atlantic has been shaped by bottom currents flowing parallel to bathymetric contours (Heezen, Hollister and Ruddiman, 1966) marked the beginning of a revolution in sedimentology comparable to the turbidite revolution launched by Kuenen and Migliorini in their classical 1950 paper.Intensification of slow, thermohaline circulation on the western margins of the ocean basins leads to high-velocity, deep, boundary currents, capable of eroding, transporting and depositing fine-grained sediment. Long-period, direct current measurements suggest a complex, periodic flow for these currents, while bottom photographs indicate their influence on the sediment surface. Sediment ridges in the North Atlantic can be closely related to the deep-water circulation pattern. Other morphological features (ripples, furrows, waves), echogram characteristics, and the presence of well-developed nepheloid layers cannot be uniquely attributed to the action of bottom currents.Critical review of marine-based investigations reveals a lack of generally accepted criteria for the recognition of contourites on the basis of sediment character. We discuss the problems in establishing such criteria and recognize that: (a) a continuum may exist between dilute turbidity flows, bottom currents and hemipelagic settling; (b) interbedded turbidites, contourites and hemipelagites are common, especially in a rise environment; and (c) composition and other criteria may be only locally applicable. However, we can identify two main contourite groups, muddy contourites and sandy contourites, and have proposed new criteria for their recognition. Muddy contourites are generally bioturbated, have poorly defined bedding, and contain biogenic sand often concentrated into irregular layers. They may be texturally and compositionally distinct from interbedded turbidites, and have relatively high CaCO3 and organic carbon contents. Sandy contourites occur as thin, bioturbated, irregular lag-deposits, or as reworked tops of sandy turbidites. In the latter case they may be clean, well sorted, parallel- or cross-laminated, but show no offshore trends or vertical structural sequence. Grain orientation shows the bottom current direction, often superimposed upon the original turbidite fabric.Reveiw of land-based work shows that there is growing recognition of the need for a new concept to complement turbidity-current theory, but that there have been relatively few claims of firm contourite identification. Recognition of ancient contourites has been based either on the application of previous sedimentological criteria, or on an interpretation of the broader environmental framework. It is suggested that it is lack of suitable criteria for the identification of contourites rather than a true scarcity of these rocks that has led to such a restricted literature.Mindful of problems created by diagenesis, tectonic activity and the limited preservation potential of many diagnostic features of marine contourites, we do suggest criteria, and a procedure, for the recognition of contourites in land-based work. Sandy contourites of the reworked-turbidite variety may be the most easily recognised; the presence of bimodal palaeocurrent directions at about 90° is an important indicator of this type.The geological significance of contourites in palaeo-oceanographical, palaeogeographical and tectonic reconstructions is emphasised, especially in relation to work on passive (Atlantic-type) continental margins. We also refer to the possible economic significance of contourites as exploration for hydrocarbons moves into deeper waters.