Geology Reports⌕ Search

Geology topics

C.H. Nelson

Publications and source records attributed to C.H. Nelson.

At least 19 recordsLinked to original sources
USGS2013

Liquefaction potential of the Yukon Prodelta, Bering Sea

The Yukon prodelta is exposed to large storm waves propagating northward from the southern Bering Sea. Shallow water depths of the prodelta enhance the transfer of energy from the surface waves to the bottom. As the bottom deposits are cyclically loaded by large storm waves, potential decrease in their resistance to shear could ultimately cause liquefaction. A preliminary assessment of the engineering properties of Yukon sandy silt suggests that the prodelta deposits may be susceptible to wave induced liquefaction during severe storm events. In addition, erosion and are suspension of sediment in the prodelta may be intensified because of the liquefaction process.

Conference Paper↗
USGS2003

Current controlled deposition on the Wilkes Land continental rise, Antarctica

Turbidite, contourite and hemipelagic deposition are the main components of Wilkes Land continental rise sedimentation above the regional unconformity WL2. On the continental shelf, unconformity WL2 marks the start of shelf progradation, which is interpreted to correspond with the onset of glacial conditions in this segment of the east Antarctic margin. Unusually large (i.e. up to 900 m relief and 18 km between levee crests) channel-levee deposits, and high relief (up to 490 m) mounded contourite-style deposits develop above unconformity WLlb. Unconformity WLlb overlies unconformity WL2 and is interpreted to have formed under a fully continental glacial regime where ice streams reached the palaeo-continental shelf edge. Based on an analysis of multichannel seismic profiles and sediment cores, we differentiate three phases in the development of the sedimentary unit between WLlb and the present seafloor. From older to younger these are: Phase 1, dominated by turbidite deposition; Phase 2, dominated by turbidite and contourite deposition with significant mound building; and Phase 3, dominated by turbidite and contourite deposition without active mound building. We hypothesize that building of the mounds during Phase 2 corresponded with times of expansion of the Antarctic ice-sheet when vast amounts of sediment were eroded from the continent and continental shelf. The large amount of unsorted glacial sediment supplied to the outer shelf apparently travelled down the slope canyons and rise channels as turbidity current flows to feed the usually large continental rise channel-levee complexes. The suspended fines of the turbidity flows were then entrained in a palaeo-nepheloid layer and carried by the westward flowing palaeo-contour currents until their deposition in the mounds. During Phase 3, sediment supply to the continental rise, although important in volume and capable of turbidite and contour-current deposition, was insufficient to support further building of the mounds. We believe the decrease in sediment supply to the continental rise from Phase 2 to Phase 3 could be the result of a change on sediment depocentres, with most of the sediment supplied to the margin during Phase 3 being trapped on the continental shelf. We believe that ultimately these changes are related to the stage of glacial evolution of the continent.

Geological Society, London, Memoirs↗
USGS2000

Morphology and acoustic character of the Antarctic Wilkes Land turbidite systems: Ice-sheet-sourced versus river-sourced fans

The Wilkes Land continental slope contains an intricate network of submarine canyons that on the continental rise develop into a series of channel and overbank deposits of turbidite systems. We can define upper-fan, middle-fan, and lower-fan provinces. The Wilkes Land upper fans are characterized by large channels with relief up to 900 m, distances between levee crests up to 18 km, and channel-floor widths up to 6 km. Middle-fan channels also have high relief (∼300 m), and locally, interchannel areas exhibit mounded contourite-style deposits with high relief (up to 490 m). Within middle-fan sediment mounds there are acoustic facies of channel-overbank deposits from turbidity currents, and of sediment waves from contour-current sedimentation. The lower rise is characterized by small, shallow channels (50-75 m relief) and by interchannel areas of low relief, both characteristic of a lower-fan environment. The Wilkes Land turbidite systems show the following significant morphological differences compared to most river-sourced fans: (1) multiple large tributary channels across the upper and middle fan, (2) channel relief several times greater (900 m) than typical relief (100-200 m) for channels on fans less than 300 km in diameter, and (3) steep middle-fan and lower-fan gradients. We interpret the differences in channel network patterns, channel size, and middle-fan and lower-fan gradients between the Wilkes Land fans and other fans to result from the continental ice sheet feeding glacial ice streams that reached the outer continental shelf at times of glacial maxima. The Wilkes Land canyon-channel network patterns are comparable with the high-latitude Laurentian Fan and Labrador Sea channels. The Laurentian Fan also has large upper-fan channels with larger relief (800 m) than typical relief (200-300 m) expected for fans that are 600 km in diameter. Both the Labrador Sea and Laurentian Fan are fed by continental ice sheets at the shelf edge.

Journal of Sedimentary Research↗
USGS1999

The Cadiz margin study off Spain: An introduction

The Cadiz continental margin of the northeastern Gulf of Cadiz off Spain was selected for a multidisciplinary project because of the interplay of complex tectonic history between the Iberian and African plates, sediment supply from multiple sources, and unique Mediterranean Gateway inflow and outflow currents. The nature of this complex margin, particularly during the last 5 million years, was investigated with emphasis on tectonic history, stratigraphic sequences, marine circulation, contourite depositional facies, geotechnical properties, geologic hazards, and human influences such as dispersal of river contaminants. This study provides an integrated view of the tectonic, sediment supply and oceanographic factors that control depositional processes and growth patterns of the Cadiz and similar modem and ancient continental margins.

Marine Geology↗
USGS1999

Influence of the Atlantic inflow and Mediterranean outflow currents on late Quaternary sedimentary facies of the Gulf of Cadiz continental margin

The late Quaternary pattern of sedimentary facies on the Spanish Gulf of Cadiz continental shelf results from an interaction between a number of controlling factors that are dominated by the Atlantic inflow currents flowing southeastward across the Cadiz shelf toward the Strait of Gibraltar. An inner shelf shoreface sand facies formed by shoaling waves is modified by the inflow currents to form a belt of sand dunes at 10-20 m that extends deeper and obliquely down paleo-valleys as a result of southward down-valley flow. A mid-shelf Holocene mud facies progrades offshore from river mouth sources, but Atlantic inflow currents cause extensive progradation along shelf toward the southeast. Increased inflow current speeds near the Strait of Gibraltar and the strong Mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf. At the outer shelf edge and underlying the mid-shelf mud and inner shelf sand facies is a late Pleistocene to Holocene transgressive sand sheet formed by the eustatic shoreline advance. The late Quaternary pattern of contourite deposits on the Spanish Gulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the Mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys between the ridges. Coincident with the northwestward decrease in outflow current speeds from the Strait there is the following northwestward gradation of contourite sediment facies: (1) upper slope sand to silt bed 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. By comparison, the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand. Because of the complex pattern of contour-parallel and valley-perpendicular flow paths of the Mediterranean outflow current, 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. Radiocarbon ages verify that the inner shelf shoreface sand facies (sedimentation rate 7.1 cm/kyr), mid-shelf mud facies (maximum rate 234 cm/kyr) and surface sandy contourite layer of 0.2-1.2 m thickness on the Cadiz slope (1-12 cm/kyr) have deposited during Holocene time when high sea level results in maximum water depth over the Gibraltar sill and full development of the Atlantic inflow and Mediterranean outflow currents. The transgressive sand sheet of the shelf, and the mud layer underlying the surface contourite sand sheet of the slope, correlate, respectively, with the late Pleistocene sea level lowstand and apparent weak Mediterranean outflow current.

Marine Geology↗
USGS1999

Potential geologic hazards on the eastern Gulf of Cadiz slope (SW Spain)

Geologic hazards resulting from sedimentary, oceanographic and tectonic processes affect more than one third of the offshore Gulf of Cadiz, and are identified by interpreting high-resolution seismic profiles and sonographs. Hazards of sedimentary origin include the occurrence of slope instability processes in the form of single or multiple slumps occupying up to 147 km2 mainly concentrated in the steeper, upper slope area. Besides the presence of steep slopes, the triggering of submarine landslides is probably due to seismic activity and favoured by the presence of biogenic gas within the sediment. Gassy sediments and associated seafloor pockmarks cover more than 240 km2 in the upper slope. Hazards from oceanographic processes result from the complex system of bottom currents created by the interaction of the strong Mediterranean Undercurrent and the rough seafloor physiography. The local intensification of bottom currents is responsible for erosive processes along more than 1900 km2 in the upper slope and in the canyons eroded in the central area of the slope, undermining slopes and causing instability. The strong bottom currents also create a mobile seafloor containing bedforms in an area of the Gulf that extends more than 2500 km2, mostly in the continental slope terraces. Hazards of tectonic origin are important because the Gulf of Cadiz straddles two major tectonic regions, the Azores-Gibraltar fracture zone and the Betic range, which results in diapir uplift over an area of more than 1000 km2, and in active seismicity with earthquakes of moderate magnitude. Also, tsunamis produced by strong earthquakes occur in the Gulf of Cadiz, and are related to the tectonic activity along the Azores-Gibraltar fracture zone.

Marine Geology↗
USGS1999

Interaction of tectonic and depositional processes that control the evolution of the Iberian Gulf of Cadiz margin

This study provides an integrated view of the growth patterns and factors that controlled the evolution of the Gulf of Cadiz continental margin based on studies of the tectonic, sedimentologic and oceanographic history of the area. Seven sedimentary regimes are identified, but there are more extensive descriptions of the late Cenozoic regimes because of the larger data base. The regimes of the Mesozoic passive margin include carbonate platforms, which become mixed calcareous-terrigenous deposits during the Late Cretaceous-early Tertiary. The Oligocene and Early Miocene terrigenous regimes developed, in contrast, over the active and transcurrent margins near the African-Iberian plate boundary. The top of the Gulf of Cadiz olistostrome, emplaced in the Late Miocene, is used as a key horizon to define the 'post-orogenic' depositional regimes. The Late Miocene progradational margin regime is characterized by a large terrigenous sediment supply to the margin and coincides with the closing of the Miocene Atlantic-Mediterranean gateways. The terrigenous drift depositional regime of the Early Pliocene resulted from the occurrence of high eustatic sea level and the characteristics of the Mediterranean outflow currents that developed after the opening of the Strait of Gibraltar. The Late Pliocene and Quaternary regimes are dominated by sequences of deposits related to cycles of high and low sea levels. Deposition of shelf-margin deltas and slope wedges correlate with regressive and low sea level regimes caused by eustasy and subsidence. During the highstand regimes of the Holocene, inner shelf prograding deltas and deep-water sediment drifts were developed under the influence of the Atlantic inflow and Mediterranean outflow currents, respectively. A modern human cultural regime began 2000 years ago with the Roman occupation of Iberia; human cultural effects on sedimentary regimes may have equalled natural factors such as climate change. Interplay of tectonic and oceanographic controls dominated the evolution of the Cadiz margin during the Cenozoic. Depositional sequences formed where the tectonic setting provided the accommodation space and the shape of the deposits has been greatly influenced by the strong unidirectional Atlantic inflow currents on the shelf and Mediterranean outflow currents on the slope. The entire cycle of the inflow and outflow deposition along the margin has been controlled first by the tectonic evolution of the Betic and Rif gateways, which become closed during the Late Miocene, and after the Messinian by the opening of the Strait of Gibraltar. Strong current development during eustatic sea level highstands of the Pliocene and Quaternary has controlled deposition because of maximum sill depths at Gibraltar for water circulation. Lowstand sea levels slowed circulation and resulted in mud drapes over the slope and regressive stratigraphic sequences over the shelf. More recently, the human industrial revolution has caused heavy metal contamination of sediment and water over the Cadiz margin. Human activity also has affected sedimentation rates because of deforestation that caused increased depositional rates near undammed rivers and decreased rates where rivers have been dammed. Future research efforts will need to focus on: (1) the effect of increased Mediterranean outflow caused by river damming plus global warming and the increased outflow as a potential trigger for new ice ages; (2) assessments of geologic hazards for planning man-made shoreline structures, developing offshore petroleum resources and maintaining undersea communications cables; and (3) confirmation of the general geologic history of the Cadiz margin.

Marine Geology↗
USGS1999

Tectonic and sediment supply control of deep rift lake turbidite systems: Lake Baikal, Russia

Tectonically influenced half-graben morphology controls the amount and type of sediment supply and consequent type of late Quaternary turbidite systems developed in the active rift basins of Lake Baikal, Russia. Steep border fault slopes (footwall) on the northwest sides of half-graben basins provide a limited supply of coarser grained clastic material to multiple small fan deltas. These multiple sediment sources in turn laterally feed small (<10 km diameter) unchannelized sublacustrine sand-rich aprons that are deposited at the slope base on the lake floor. Gradual slopes of the southeastern ramp margins (hanging wall) of the lake basins, conversely, feed finer grained sediment from larger drainages into two different types of channelized turbidite sublacustrine fan systems: (1) small (5–20 km) laterally fed sand-rich fans sourced by local rivers, often originating from glaciated valleys; and (2) large (>65 km) axially fed elongate mud-rich fans sourced by regional exterior drainage of the Selenga River that supplies large quantities of silt. Basin plain turbidites in the center of the linear basins and axial channels that are controlled by rift-parallel faults are fed from, and interfinger with, aprons and fans. The predictability of the turbidite systems in Lake Baikal provides the best example yet studied of how tectonics and sediment supply interact to control the development of a wide variety of coeval turbidite systems on a single basin floor.

Geology↗
USGS1997

A120 yr record of widespread contamination from mining of the Iberian pyrite belt

A metal-enriched seawater plume entering the western Mediterranean Sea through the Strait of Gibraltar originates 300 km to the west in the Rio Tinto estuary of southwestern Spain. Mining of Rio Tinto ore, one of the largest metal-rich sulfide deposits in the world, started well before Roman times. Contemporary Rio Tinto waters draining the region are highly acidic (pH 2.5) with dissolved cadmium, zinc, and copper concentrations 10 5 −10 6 times higher than in uncontaminated surface water of the Gulf of Cadiz. Two dated sediment cores from the Spanish continental shelf show that metal inputs to the region increased with the onset of intensive mining activities during the second half of the 19th century. Although the impact of mining may have decreased over the past few decades, the Tinto river and estuary remain highly contaminated.

Geology↗
USGS1996

Morpho-structure and sedimentology of the Holocene Ebro prodelta mud belt (northwestern Mediterranean Sea)

The Ebro “mud belt” is a Holocene prodeltaic deposit which has developed around, and southwestward from, the present Ebro Delta plain, covering most of the inner and middle Ebro continental shelf. Seismic-reflection profiles of this mud belt exhibit a complex sigmoid—oblique configuration. Top-set strata dip gently seaward to the 20 m isobath, and overly the fore-set beds which are exposed in up to 40–60 m water depth. Top-set and fore-set beds have mostly parallel and high continuity reflectors. Thin, acoustically transparent bottom-set beds are present at the base of the fore-set beds and extend to the distal edge of the prodelta (60–80 m water depth), where they overly relict transgressive sand deposits. There is no evidence of mass movement. The suspended load discharged by the river is mainly transported alongshelf by advective processes. This dynamics produces thin clinoform deposits that extend alongshelf for tens of kilometres. Mud belt deposition began about 10,000–11,000 years BP. Accumulation rate ranges from less than 0.5 mm y −1 on the seaward and southern edges of the deposit to about 2.5 mm y −1 near the present river mouth.

Continental Shelf Research↗
USGS1996

Sediment mass-flow processes on a depositional lobe, outer Mississippi Fan

SeaMARC 1A sidescan-sonar imagery and cores from the distal reaches of a depositional lobe on the Mississippi Fan show that channelized mass flow was the dominant mechanism for transport of silt and sand during the formation of this part of the fan. Sediments in these flows were rapidly deposited once outside of their confining channels. The mass flows most likely originated from slope failure at the head of the Mississippi Canyon or on the outer continental shelf and flowed approximately 500 km to the distal reaches of the fan, with debris flow being the dominant flow type.

Mississippi↗
USGS1993

Heavy metal anomalies in the Tinto and Odiel River and estuary system, Spain

The Tinto and Odiel rivers drain 100 km from the Rio Tinto sulphide mining district, and join at a 20-km long estuary entering the Atlantic Ocean. A reconnaissance study of heavy metal anomalies in channel sand and overbank mud of the river and estuary by semi-quantitative emission dc-arc spectrographic analysis shows the following upstream to downstream ranges in ppm (??g g-1): As 3,000 to <200, Cd 30 to <0.1, Cu 1,500 to 10, Pb 2,000 to <10, Sb 3000 to <150, and Zn 3,000 to <200. Organic-rich (1.3-2.6% total organic carbon, TOC), sandysilty overbank clay has been analyzed to represent suspended load materials. The high content of heavy metals in the overbank clay throughout the river and estuary systems indicates the importance of suspended sediment transport for dispersing heavy metals from natural erosion and anthropogenic mining activities of the sulfide deposit. The organic-poor (0.21-0.37% TOC) river bed sand has been analyzed to represent bedload transport of naturally-occurring sulfide minerals. The sand has high concentrations of metals upstream but these decrease an order of magnitude in the lower estuary. Although heavy metal contamination of estuary mouth beach sand has been diluted to background levels estuary mud exhibits increased contamination apparently related to finer grain size, higher organic carbon content, precipitation of river-borne dissolved solids, and input of anthropogenic heavy metals from industrial sources. The contaminated estuary mud disperses to the inner shelf mud belt and offshore suspended sediment, which exhibit metal anomalies from natural erosion and mining of upstream Rio Tinto sulphide lode sources (Pb, Cu, Zn) and industrial activities within the estuary (Fe, Cr, Ti). Because heavy metal contamination of Tinto-Odiel river sediment reaches or exceeds the highest levels encountered in other river sediments of Spain and Europe, a detailed analysis of metals in water and suspended sediment throughout the system, and epidemiological analysis of heavy metal effects in humans is appropriate. ?? 1993 Estuarine Research Federation.

Estuaries↗
USGS1992

Characteristics of a sandy depositional lobe on the outer Mississippi fan from SeaMARC IA sidescan sonar images

SeaMARC IA sidescan sonar images of the distal reaches of a depositional lobe on the Mississippi Fan show that channelized rather than unconfined transport was the dominant transport mechanism for coarse-grained sediment during the formation of this part of the deep-sea fan. Overbank sheet flow of sands was not an important process in the transport and deposition of the sandy and silty sediment found on this fan. The dendritic distributary pattern and the high order of splaying of the channels, only one of which appears to have been active at a time, suggest that coarse-grained deposits on this fan are laterally discontinuous.

Mississippi↗
USGS1992

Upper Pleistocene turbidite sand beds and chaotic silt beds in the channelized, distal, outer-fan lobes of the Mississippi fan

Cores from a Mississippi outer-fan depositional lobe demonstrate that sublobes at the distal edge contain a complex local network of channelized-turbidite beds of graded sand and debris-flow beds of chaotic silt. Off-lobe basin plains lack siliciclastic coarse-grained beds. The basin-plain mud facies exhibit low acoustic backscatter on SeaMARC IA sidescan sonar images, whereas high acoustic backscatter characteristic of the lobe sand and silt facies. The depth of the first sand-silt layer correlates with relative backscatter intensity and stratigraphic age of the distal sublobes (i.e., shallowest sand = highest backscatter and youngest sublobe). The high proportion (>50%) of chaotic silt compared to graded sand in the distal, outer-fan sublobes may be related to the unstable, muddy, canyon-wall source areas of the extensive Mississippi delta-fed basin slope. A predominace of chaotic silt in cores or outcrops from outer-fan lobes thus may predict similar settings for ancient fans.

Mississippi Fan↗
USGS1990

Late Pleistocene and Holocene sedimentary facies on the Ebro continental shelf

Late Pleistocene—Holocene history of the Ebro continental shelf of northeastern Spain is recorded in two main sedimentary units: (1) a lower, transgressive unit that covers the shelf and is exposed on the outer shelf south of 40°40′N, and (2) an upper, progradational, prodeltaic unit that borders the Ebro Delta and extends southward along the inner shelf. The lower transgressive unit includes a large linear shoal found at a water depth of 90 m and hardground mounds at water depths of 70–80 m. Some patches of earlier Pleistocene prodelta mud remain also, exposed or covered by a thin veneer of transgressive sand on the northern outer shelf. This relict sand sheet is 2–3 m thick and contains 9000–12,500 yr old oyster and other shells at water depths of 78–88 m. The upper prodelta unit covers most of the inner shelf from water depths of 20–80 m and extends from the present Ebro River Delta to an area to the southwest where the unit progressively thins and narrows. Interpretation of high-resolution seismic reflection data shows the following facies occurring progressively offshore: (1) a thick stratified facies with thin progradational “foresets beds”, (2) a faintly laminated facies with sparse reflectors of low continuity, and (3) a thin transparent bottomset facies underlain by a prominent flat-lying reflector. Deposition in the northern half of the prodelta began as soon as the shoreline transgressed over the mid-shelf, but progradation of the southern half did not begin until about 1000–3000 yrs after the transgression. A classic deltaic progradational sequence is shown in the Ebro prodelta mud by (1) gradation of seismic facies away from the delta, (2) coarsening-upward sequences near the delta and fining-upward sequences in the distal mud belt deposits, and (3) thin storm-sand layers and shell lags in the nearshore stratified facies. The boundaries of the prodeltaic unit are controlled by increased current speeds on the outer shelf (where the shelf narrows) and by development of the shoreface sand body resulting from shoaling waves on the inner shelf.

Marine Geology↗
USGS1990

Estimated post-Messinian sediment supply and sedimentation rates on the Ebro continental margin, Spain

Because of the extensive data base of seismic profiles, radiometric ages, and stratigraphic time markers such as the subaerial Messinian surface, sedimentation rates and Ebro River sediment discharge can be estimated for different periods and environments of the Ebro continental margin. New values for sediment discharge (i.e., 6.2 versus previous estimates of 2–3.5 million t/yr) for the Holocene highstand are more reliable but remain minimum estimates because a small proportion of Ebro sediment advected to the Balearic Rise and Abyssal Plain cannot be accounted for, especially during lowstands. The general highstand conditions of the Pliocene, which were similar to those of the Holocene, resulted in a low discharge of Ebro River sediment (ca. 6.5 million t/yr) and an even thickness of sediment across the margin that deposited at rates of about 24–40 cm/ky. In contrast, sediment supply increased two-three times during the Pleistocene, the margin prograded rapidly and deposition occurred at rates of 101–165 cm/ky on the outer shelf and slope, but basin floor rates remained anomalously low (21–26 cm/ky) because sediment was drained and broadly dispersed eastward in Valencia Trough. During the late Pleistocene rise of sea level, the main depocenters progressively shifted shoreward and sedimentation rates greatly decreased from 175 cm/ky on the upper slope during the early transgression to 106 cm/ky on the outer shelf and then to 63 cm/ky on the mid-shelf during the late transgression as the river sediment discharge dropped to half by Holocene time. Maximal sedimentation rates occurred in active depocenters of sediment dispersal such as the Holocene delta (370 cm/ky) or the youngest Pleistocene Oropesa channel-levee complex (705 cm/ky) where deposition rates increased by an order of magnitude or more compared to average Ebro shelf (38 cm/ky) or base-of-slope rates in the Pleistocene (21 cm/ky). The sedimentation rates verify the importance of sea-level control on the progressive change in location of depocenters and amount of sediment supply, but Pleistocene climatic change and deforestation alone can be observed to double river sediment discharge. The latter observation helps explain the anomalously high deposition rates in Pleistocene turbidite systems compared with older systems that may be controlled more by tectonic and sea-level changes alone. During the past 2000 years, in contrast, man has controlled deposition in the Ebro margin system, first by deforestation that more than doubled river sediment discharge and shelf deposition rates to equal those of Pleistocene time; and second by dam contruction that reduced sediment discharge to less than 5% of the normal Holocene discharge. Similar recent discharge reductions from the Nile and Rhone Rivers suggest that loss of the majority of the river sediment supply in the Mediterranean Sea may result in significant erosion of biologically and agriculturally important lobate delta areas.

Marine Geology↗
USGS1990

Factors controlling late Cenozoic continental margin growth from the Ebro Delta to the western Mediterranean deep sea

The Ebro continental margin sedimentation system originated with a Messinian fluvial system. This system eroded both a major subaerial canyon cutting the margin southeastward from the present Ebro Delta and an axial valley that drained northeastward down Valencia Trough. Post-Messinian submergence of this topography and the Pliocene regime of high sea levels resulted in a marine hemipelagic drape over the margin. Late Pliocene to Pleistocene glacial climatic cycles, drainagebasin deforestation, and sea-level lowstands combined to increase sediment supply, cause the margin to prograde, and create a regime of lowstand sediment-gravity flows in the deeper margin. The depositional patterns of regressive, transgressive and highstand sea-level regimes suggest that location of the sediment source near the present Ebro Delta throughout the late Cenozoic, southward current advection of sediment, and greater subsidence in the southern margin combined to cause generally asymmetric progradation of the margin to the southeast. Thicker, less stable deposits filling the Messinian subaerial canyon underwent multiple retrograde failures, eroded wide gullied canyons and formed unchanneled base-of-slope sediment aprons in the central margin area; other margin areas to the north and south developed a series of channel-levee complexes. On the basin floor, the formation of Valencia Valley over the Messinian subaerial valley and earlier faults led to draining of about 20% of the Ebro Pleistocene sediment from channel-levee complexes through the valley to prograde Valencia Fan as much as 500 km northeast of the margin. Thus, the Ebro margin has two growth directions, mainly southeastward during higher sea levels, and eastward to northeastward during lower sea levels. The northeastward draining of turbidity currents has produced unusually thin and widely dispersed turbidite systems compared to those on ponded basin floors. During the past few centuries, man's impact has exceeded natural controls on Ebro margin growth. Deforestation of the drainage basin more than doubled the normal Holocene sediment supply, and construction of dams then reduced the supply by 95%. This reduction of the past 50 years has caused erosion of the delta and contamination of bottom sediment because normal Holocene sediment discharge is not available to prograde the delta or help dilute pollutants.

Marine Geology↗