Increasing Marine Influence Research Articles

Transformation of a subtropical river into a hyperhaline estuary: the Casamance River (Senegal)—paleogeographical implications

Located in the South of Senegal, in the transitional Guineo-Congolian/Soudanian zone, the Casamance system is composed of a small river, supplied by a narrow drainage basin and followed by a 250 km long estuary. The continuous rainfall decrease experienced by the region since the beginning of the century drastically affected the Casamance river. This climatic change suddenly accelerated during 1968–1970, which led to a four-stage succession: • - before 1968–1970 (stage 1, rainfall ⩾ 1300 mm), the Casamance estuary showed the customary negative upstream salinity gradient; • - between 1968–1970 and 1978 (stage 2, rainfall ⩽ 1000 mm), under decreasing rainfall, the Casamance estuary shifted to a hyperline “inverse estuary”. It showed well-defined physical, chemical and biological zonations. Fresh groundwater resources, supplying the upper reaches of the river during the dry season, were progressively depleted. • - between 1978 and 1986–1987 (stage 3), the salinity peak of the end of the dry season progressively rose and shifted upward. The ecological zonation showed the increasing marine influence until 1986; • - between 1986–1987 and 1990 (stage 4), the dry season fresh water input had weakened with the depletion of groundwater resources. A slight increase in rainfall was insufficient to repulse the hyperhaline waters and the restricted area extended upstream and downstream. The physical, chemical and biological zonations strongly widened and flattened. In the future, the return to stage 1 conditions, by an increase in average rainfall, would take decades. Inevitably it will be much longer than the transformation into a hyperhaline estuary. Thus a slight decrease of the average rainfall in the region provoked a drastic change of the hydrological and biological patterns of the ria. The result seems to be a major crisis in the region, and might be recorded in the fossil records. We discuss these observations in terms of possible misreadings of paleoenvironments, and in relation with long-term climate trends.

Holocene environments in the Hawkesbury valley, new South Wales: A comparison of terrestrial and marine records

Rapid sedimentation of sands, silts and muds associated with seaward prograding tidal delta and central basin deposits, characterise the early Holocene sediment infill of the Hawkesbury River's estuary. From about 6 ka BP fluvial muds and sands dominate the estuary and sedimentation rates are much reduced. Micro-fossil records contained in sediments from the middle Hawkesbury-Colo Rivers region support the sedimentological-lithostratigraphic interpretation of estuary development. Information from fossil diatoms indicates that for a period of about 300 years around 6 ka BP, there was an increased marine influence in this part of the present estuary. This may have resulted from the probable peak in the rate of relative sea-level rise, although there may also have been a contemporaneous increase in freshwater flow down the river. Pollen and charcoal records from Mill Creek, downstream from the Colo sequence, show that sclerophyll forest, occasionally with rainforest elements, became more dominant from 8 ka BP. In contrast, the upper tidal Casuarina glauca forest decreased in importance during the phases of sea-level rise and the subsequent ‘still-stand’. Together the palaeoenvironmental records show broad agreement in defining the patterns of regional environmental changes and linked phases of estuary development during the Holocene. Differences in records appear to relate to local site factors and to the limitations in provenance, persistence and resolving power of the data types. The rapid accumulation rates provide an opportunity to investigate high magnitude but low frequency events in estuarine environments, including floods and fire.

Aquifer Recharge and Exchangeable Cations in a Tertiary Clay Layer (Bartonian Clay, Flanders-Belgium)

The survey area comprises the northern part of Eastand West-Flanders in Belgium. The recharge of the Tertiary Ledo-Paniselian detrital aquifer is occurring by infiltration through the Bartonian clay in the highest regions in the south of the survey area. Both in the aquifer and in the overlying clay layer, marine conditions were prevailing until the last regression at the end of Tertiary times. The infiltration of fresh water has led to the gradual freshening of the aquifer. The present distribution of the groundwater types shows the increasing marine influence in the direction of groundwater flow to the north (Walraevens, 1990). In the westerly recharge area around Ursel, the hydraulic head above and below the Bartonian clay is accurately known, by the presence of 56 piezometers. The local thickness of the Bartonian being known as well (on the average l0 to 20 m), the hydraulic conductivity of the clay layer could be deduced. In the modeling of groundwater flow, an excellent agreement between observed and calculated heads is obtained with a value of l0 -9 m/s (Walraevens, 1987). Laboratory measurements mostly lead to values that are at least an order of magnitude lower. The discrepancy has been ascribed to the presence of preferential pathways in the clay, through which the flow is preferentially taking place.

Pre-Mesozoic geology of the western and north-western Irish continental shelf

Exploration wells drilled offshore west and north-west of Ireland penetrated a c . 3.7 m cumulative thickness of Permo-Carboniferous sediments overlying Dalradian metasediments intruded by Caledonian granitic plutons. In the north, late Tournaisian to early Visean clastics were deposited coeval with the Rheic transgression upon an emergent craton in a continental to marginal marine environment. They record an increasing marine influence upwards with the establishment of a carbonate shelf in the Asbian. Clastic/carbonate sedimentation, which became cyclical in late Visean to early Namurian, temporally correlates with the Yoredale cyclothems. Younger Namurian sediments are not preserved. Sedimentation in early Westphalian progressed from prodelta to coal-enriched delta-top and abandonment facies, and is conformably succeeded by brackish to freshwater sediments that contain evidence for local marine incursions of Stephanian age. An Autuno-Silesian section, that is preserved offshore, may also have been deposited onshore prior to erosion. Seismic data enable partial delineation of N-S and NE-SW early sub-basins marginal to the Main Porcupine Basin. These sub-basins which contain Silesian sediments, may have styled successive extensional episodes in the basin and augment support for rift models of Carboniferous basin development in the North Atlantic/NW European domain. BIRPS data provide supporting evidence for the offshore prolonga­tion of pre-Mesozoic basement fault-zones.

Palaeoecology and event stratigraphy of the Wealden-Lower Greensand transition in the Isle of Wight

Event stratigraphy combines sedimentological, trace fossil and palaeoecological criteria, in identifying six fades associations in the Vectis Formation (Weald Shales) of the Isle of Wight. This sequence suggests that prior to deposition of the marine Lower Greensand (Aptian-Albian), the Vectis Formation underwent a gradual increase in marine influence. Storm/flood event beds are common, and black shales with a lack of benthos are ascribed to limited oxygenation in the Vectis Formation lagoon. As far as eustatic models are concerned it is suggested that the Upper Barremian to Aptian sea-level rise was gradual, the Perna Beds (lowest Aptian) forming at the restricted to normal marine transition during a period of base-level rise.

Depositional Environments of Lower Cretaceous McMurray Formation, Athabasca Oil Sands, Alberta

Most of the bitumen resources in the Athabasca oil sands deposit are contained in upward-fining channel sands of the Lower Cretaceous McMurray Formation. The channels were part of a northward-flowing fluvial drainage system that was being encroached upon by the rising Clearwater Sea. The lower member of the formation, present only in the deepest valleys on the underlying Devonian limestone surface, is dominantly fluvial in character. Rising sea level during middle member deposition resulted in a decreased gradient and a change from the shallow (5-10 m or 15-30 ft), commonly coarse-grained channels of the lower member to the deep (20-30 m or 65-100 ft), narrow, sinuous, high suspended-load channels of the middle member. Although there is evidence for some marine influence in the middle member channels, the processes of channel formation appear to be dominantly fluvial. In off-channel areas of the middle member, rising sea level resulted in the development of widespread subaqueous conditions and rapid vertical aggradation. Thick sequences of mud accumulated in lacustrine and brackish-bay environments that existed contemporaneously with the channels. Coal beds and rooted zones, common in the lower and upper members, are notably absent from the middle member. The upper member of the McMurray Formation was deposited during a time of slower aggradation and increased marine influence. It is characterized by smaller scale sedimentation units, commonly small upward-coarsening sequences interpreted as splay deposits of nearby large channels. In northern and western parts of the study area, open-marine conditions existed in upper member time, and upward-coarsening, offshore marine bar sands developed at the top of the formation.

Detailed sedimentary velocities from seismic refraction profiles in the Western North Atlantic : Houtz, R. E. and J. I. Ewing, 1963. J. Geophys. Res., 68 (18): 5233–5258

Recent studies of continental margins suggest that small, high-yield rivers are capable of generating shelf sediment-gravity flows, an idea that fundamentally alters our understanding of material flux from the continents to the ocean. Discharge measurements indicate that the Waiapu River, North Island, New Zealand reaches hyperpycnal concentrations (> 36 kg m− 2) on a yearly basis. This study contrasts shelf-edge basins with a broad trough along the shelf-edge off the Waiapu River, testing whether there is evidence that shelf sediment-gravity flows propagate to topographic lows. Observations and measurements through geochemical and sedimentological analyses of sediment cores, EM1002 swath bathymetry, and Chirp sub-bottom profiles suggest differing transport modalities on the outer shelf. In general a southern trough-shaped region exhibits high terrigenous inputs and non-steady-state 210Pb profiles, whereas the northern basins contain steady-state 210Pb profiles and increased marine influence. Sediment-gravity flows dominate accumulation in the southern region, whereas within the northern portion, surface plume sedimentation is indicated. Overall this study suggests that sediment-gravity flows could be bypassing the northern basins, perhaps a result of oceanographic influences and bathymetric steering as they seek a more direct route across the shelf.