- Email: [email protected]
Coastal geomorphology and chronology of recent sediments. Example: the east coast of Rio de Janeiro (Brazil)
OLR (1988) 35 (6)
D. Submarine Geology and Geophysics
exact seismic response of an ocean and a n-layer configuration.' Geophys. Prospect, 35(9):10651069. A comment on Drijkoningen and Fokkema's (1987) use of the Cagniard-de Hoop technique for calculated synthetic seismograms for an n-layer model, calls attention to previous work on this approach by Cagniard (1939), Pekeris (1940), de Hoop (1960), and Abramovici (1978). In response, Drijkoningen and Fokkema acknowledge the contributions made by these researchers and list additional contributions made by other seismologists and physicists. (hbf) 88:3418 Okaya, D.A. and Zvi Ben-Avraham, 1987. Structure of the continental margin of southwestern Panarea. Geol. Soc. Am. Bull., 99(6):792-802.
The structure of the continental shelf off southwest Panama, which from Late Mesozoic to ,-~2 Mya lay adjacent to an active subduction zone, reflects the change from convergent to transform plate motion, which began ~ 3 - 5 Mya due to the passage of the Nazca-Cocos-Caribbean triple junction. A 24-fold multichannel seismic profile across the shelf in the Gulf of Chiriqui reveals this change in plate-margin tectonics and the presence of an acoustic basement, probably a Mid-Miocene unconformity, overlain by subhorizontal reflectors from marine shelf deposits, broken by faults which may extend for more than 80 km. A similar profile across the Gulf of Osa (southern Costa Rica), where subduction is still occurring northwest of the triple junction, indicates that bounding faults have not yet developed in that area. CALCRUST, Earth Sci. Div., Lawrence Berkeley Lab., Berkeley, CA 94720, USA. (hbf) 88:3419 Tesson, M., B. Gensous and M. Labraimi, 1987. Seismic analysis of the southern margin of the Aiboran Sea. d. Aft. Earth Sci., 6(6):813-821.
In the eastern area, recent tectonic movements have strongly affected deposits. The Alboran Ridge was in a complex antiform at the end of the Pliocene; its present disposition, in horst and subsiding trough structures with a thick infilling, is the result of Quaternary to Recent recurrent faulting. In the western area, near Gibraltar Strait, alternate stages of erosion and construction are apparent in the sequences of the marginal plateau, particularly in the northern part, which progrades toward the strait. Hydrodynamic patterns and glacio-eustatic variations of sea level have played a prominent role in the genesis of sedimentary bodies. Neither sedimentation nor tectonics indicate a general MiocenePlioeene boundary but the Pliocene-Quaternary
545
transition is marked, on the ridge and marginal areas, by a major discontinuity; it can be correlated with onshore neotectonic data. Lab. de Sedimentol. et Geochim. Mar., Univ. de Perpignan, 66000 Perpignan, France.
Dr0. Geomorphology (fans,
canyons, etc.)
88:3420 Caechione, D.A., M.E. Field, D.E. Drake and G.B. Tate, 1987. Crescentic dunes on the inner continental shelf off northern California. Geology, geol. Soc. Am., 15(12):1134-1137.
Side-scan sonar records (1981-1985) reveal the presence of crescentic barchan-like dunes within elongate scour depressions off the coast of northern California, the first such region to be reported on the continental shelf of western North America. Although the dunes appear to be migrating obliquely to the regional shelf slope, and there is ample evidence of sand transport, no significant change in dune position was noted over the 4-yr observation period. It is suggested that the dunes are active over a time scale measured in decades, and that dune migration may be an important mechanism for sand transport to the inner shelf. Branch of Pacific Mar. Geol., USGS, Menlo Park, CA 94025, USA. (hbf) 88:3421 O'Connell, S., W.B.F. Ryan and W.R. Normark, 1987. Modes of development of slope canyons and their relation to channel and levee features on the Ebro sediment apron, off-shore northeastern Spain. Mar. Petrol. Geol., 4(4):308-319. Ocean Drilling Prog., Dept. of Oceanogr., Texas A&M Univ., College Station, TX 77843, USA. 88:3422 Smith, D.K. and T.H. Jordan, 1987. The size distribution of Pacific seamounts. Geophys. Res. Lefts, 14(11):1119-1122.
An analysis of 157,000 km of wide beam SeaBeam profiles from the eastern and southern Pacific supports the conclusion of Jordan et al. (1983) that the size distribution of east Pacific seamounts is approximately exponential over a large range of summit heights, which in the present study ranged from <300 m to >3500 m. The results indicate an average of 5400_+ 650 seamounts per million square km with 170_+ 17 of heights > 1 kin. If it is assumed that volcano height is controlled by an hydraulic head, which is a function of the depth of the underlying magma, a volcano with a height of 285 m, a characteristic value, would have source depths
546
D. Submarine Geology and Geophysics
typical of the thickness of the crust. Dept. of Geol. and Geophys., WHOI, Woods Hole, MA 02543, USA. (hbf)
D70. Coasts, beaches, marshes 88:3423 Coe Neto, R., J.-M. Froidefond and Bruno Turcq, 1986. Coastal geomorphology and chronology of recent sediments. Example: the east coast of Rio de Janeiro (Brazil). Bull. Inst. G~ol. Bassin Aquitaine, 40:67-83. (In French, English abstract.) Univ. Fed. de Fluminense, Inst. de Geochim., Niteroi R.J., Brazil. 88:3424 Liu, Cangzi, Licheng Wu and Min Cao, 1987. Sedimentary characteristics of eheniers in southern Changjlang Delta and their origin and age determination. Acta oceanol, sin. (English version), 6(3):405-412. Inst. of Estuarine and Coastal Res., East China Normal Univ., Shanghai, People's Republic of China. 88:3425 Maa, P.-Y. and A.J. Mehta, 1987. Mud erosion by waves: a laboratory study. Continent. Shelf Res., 7(11-12): 1269-1284.
Two types of cohesive sediment, a commercial kaolinite and an estuarial mud, were used to study the mechanism by which soft muds erode under progressive waves in a flume. A multi-layered hydrodynamic model which considers the mud to be viscoelastic was used to evaluate bed shear stress at the oscillating mud-water interface. The viscoelastic property of the mud was confirmed by rheological measurements, and model results on velocity, pressure and wave attenuation verified against flume data. Concentration profiles indicate an evolutionary pattern resulting in a highly stratified suspension. Just above the bed, a thin layer of fluid mud is generated; above this layer, the suspension concentration is significantly lower. This two-layered feature is related to the oscillatory response of the mud and water layers, and the associated momentum exchange and mass diffusion characteristics. An expression relating rate of erosion to bed shear stress in excess of bed shear resistance was developed. VIMS, Coll. of William and Mary, Gloucester Point, VA 23062, USA. 88:3426 Parker, W.R., 1987. Observations on fine sediment transport phenomena in turbid coastal environments. Continent. Shelf Res., 7(I 1-12): 1285-1293.
OLR (1988) 35 (6)
Measurements using new technology highlight unsteady behaviour and the importance of the cohesive boundary--a time-dependent transition between moving fluid and stationary bed. Discrepancies between various techniques for describing the properties and behaviour of the cohesive boundary become clear when the purpose of the description is considered. Although transport processes are described in physical terms, the impact of biological processes is now widely recognized. Non-intrusive measurement techniques offer a route to more realistic quantification of fine sediment transport phenomena in these environments. Sci. and Engng Res. Council, c/o Blackdown Consultants Ltd., 8, Fore St., Taunton, Somerset TA3 75H, UK. 88:3427 Sahl, L.E. and M.A.H. Marsden, 1987. Shelf sediment dispersal during the dry season, Princess Charlotte Bay, Great Barrier Reef, Australia. Continent. Shelf Res., 7(10): 1139-1159. 68 Bellewood Ave., South Setauket, NY 11720, USA. 88:3428 Salomons, W. and W.G. Mook, 1987. Natural tracers for sediment transport studies. Continent. Shelf Res., 7(11-12): 1333-1343.
Differences in chemical, mineralogical and isotope geochemical composition of marine and fluvial sediments can be used as tracers, provided that three criteria are met: conservative behaviour during transport, with time, and after deposition. Examples of the utility of natural tracers are presented, including their use for the interpretation of pollutant patterns in estuaries. The application of natural tracers shows marine sediments may be transported past the freshwater boundary in estuaries, and thus contribute to the sedimentation in freshwater tidal areas. Delft Hydraulics Lab., c/o Inst. for Soil Fertility, P.O. Box 30003, 9750 RA Haren, Gr, Netherlands. 88:3429 Schauer, Ursula, 1987. Determination of bottom boundary layer parameters at two shallow sea sites using the profile method. Continent. Shelf Res., 7(10):1211-1230.
Time series of mean current profiles within 3.5 m of the bottom were obtained in two hydrographically different regions. The shear velocity reached maximum values of u. = 6 cm/s. Values of z 0 are very high (0[cm]) and show a systematic variability with the velocity. The profiles are slightly concave, thus z 0 and u. decrease with the profile height considered. Both results indicate differences between observations and the Prandtl theory. Factors influencing the
D. Submarine Geology and Geophysics
exact seismic response of an ocean and a n-layer configuration.' Geophys. Prospect, 35(9):10651069. A comment on Drijkoningen and Fokkema's (1987) use of the Cagniard-de Hoop technique for calculated synthetic seismograms for an n-layer model, calls attention to previous work on this approach by Cagniard (1939), Pekeris (1940), de Hoop (1960), and Abramovici (1978). In response, Drijkoningen and Fokkema acknowledge the contributions made by these researchers and list additional contributions made by other seismologists and physicists. (hbf) 88:3418 Okaya, D.A. and Zvi Ben-Avraham, 1987. Structure of the continental margin of southwestern Panarea. Geol. Soc. Am. Bull., 99(6):792-802.
The structure of the continental shelf off southwest Panama, which from Late Mesozoic to ,-~2 Mya lay adjacent to an active subduction zone, reflects the change from convergent to transform plate motion, which began ~ 3 - 5 Mya due to the passage of the Nazca-Cocos-Caribbean triple junction. A 24-fold multichannel seismic profile across the shelf in the Gulf of Chiriqui reveals this change in plate-margin tectonics and the presence of an acoustic basement, probably a Mid-Miocene unconformity, overlain by subhorizontal reflectors from marine shelf deposits, broken by faults which may extend for more than 80 km. A similar profile across the Gulf of Osa (southern Costa Rica), where subduction is still occurring northwest of the triple junction, indicates that bounding faults have not yet developed in that area. CALCRUST, Earth Sci. Div., Lawrence Berkeley Lab., Berkeley, CA 94720, USA. (hbf) 88:3419 Tesson, M., B. Gensous and M. Labraimi, 1987. Seismic analysis of the southern margin of the Aiboran Sea. d. Aft. Earth Sci., 6(6):813-821.
In the eastern area, recent tectonic movements have strongly affected deposits. The Alboran Ridge was in a complex antiform at the end of the Pliocene; its present disposition, in horst and subsiding trough structures with a thick infilling, is the result of Quaternary to Recent recurrent faulting. In the western area, near Gibraltar Strait, alternate stages of erosion and construction are apparent in the sequences of the marginal plateau, particularly in the northern part, which progrades toward the strait. Hydrodynamic patterns and glacio-eustatic variations of sea level have played a prominent role in the genesis of sedimentary bodies. Neither sedimentation nor tectonics indicate a general MiocenePlioeene boundary but the Pliocene-Quaternary
545
transition is marked, on the ridge and marginal areas, by a major discontinuity; it can be correlated with onshore neotectonic data. Lab. de Sedimentol. et Geochim. Mar., Univ. de Perpignan, 66000 Perpignan, France.
Dr0. Geomorphology (fans,
canyons, etc.)
88:3420 Caechione, D.A., M.E. Field, D.E. Drake and G.B. Tate, 1987. Crescentic dunes on the inner continental shelf off northern California. Geology, geol. Soc. Am., 15(12):1134-1137.
Side-scan sonar records (1981-1985) reveal the presence of crescentic barchan-like dunes within elongate scour depressions off the coast of northern California, the first such region to be reported on the continental shelf of western North America. Although the dunes appear to be migrating obliquely to the regional shelf slope, and there is ample evidence of sand transport, no significant change in dune position was noted over the 4-yr observation period. It is suggested that the dunes are active over a time scale measured in decades, and that dune migration may be an important mechanism for sand transport to the inner shelf. Branch of Pacific Mar. Geol., USGS, Menlo Park, CA 94025, USA. (hbf) 88:3421 O'Connell, S., W.B.F. Ryan and W.R. Normark, 1987. Modes of development of slope canyons and their relation to channel and levee features on the Ebro sediment apron, off-shore northeastern Spain. Mar. Petrol. Geol., 4(4):308-319. Ocean Drilling Prog., Dept. of Oceanogr., Texas A&M Univ., College Station, TX 77843, USA. 88:3422 Smith, D.K. and T.H. Jordan, 1987. The size distribution of Pacific seamounts. Geophys. Res. Lefts, 14(11):1119-1122.
An analysis of 157,000 km of wide beam SeaBeam profiles from the eastern and southern Pacific supports the conclusion of Jordan et al. (1983) that the size distribution of east Pacific seamounts is approximately exponential over a large range of summit heights, which in the present study ranged from <300 m to >3500 m. The results indicate an average of 5400_+ 650 seamounts per million square km with 170_+ 17 of heights > 1 kin. If it is assumed that volcano height is controlled by an hydraulic head, which is a function of the depth of the underlying magma, a volcano with a height of 285 m, a characteristic value, would have source depths
546
D. Submarine Geology and Geophysics
typical of the thickness of the crust. Dept. of Geol. and Geophys., WHOI, Woods Hole, MA 02543, USA. (hbf)
D70. Coasts, beaches, marshes 88:3423 Coe Neto, R., J.-M. Froidefond and Bruno Turcq, 1986. Coastal geomorphology and chronology of recent sediments. Example: the east coast of Rio de Janeiro (Brazil). Bull. Inst. G~ol. Bassin Aquitaine, 40:67-83. (In French, English abstract.) Univ. Fed. de Fluminense, Inst. de Geochim., Niteroi R.J., Brazil. 88:3424 Liu, Cangzi, Licheng Wu and Min Cao, 1987. Sedimentary characteristics of eheniers in southern Changjlang Delta and their origin and age determination. Acta oceanol, sin. (English version), 6(3):405-412. Inst. of Estuarine and Coastal Res., East China Normal Univ., Shanghai, People's Republic of China. 88:3425 Maa, P.-Y. and A.J. Mehta, 1987. Mud erosion by waves: a laboratory study. Continent. Shelf Res., 7(11-12): 1269-1284.
Two types of cohesive sediment, a commercial kaolinite and an estuarial mud, were used to study the mechanism by which soft muds erode under progressive waves in a flume. A multi-layered hydrodynamic model which considers the mud to be viscoelastic was used to evaluate bed shear stress at the oscillating mud-water interface. The viscoelastic property of the mud was confirmed by rheological measurements, and model results on velocity, pressure and wave attenuation verified against flume data. Concentration profiles indicate an evolutionary pattern resulting in a highly stratified suspension. Just above the bed, a thin layer of fluid mud is generated; above this layer, the suspension concentration is significantly lower. This two-layered feature is related to the oscillatory response of the mud and water layers, and the associated momentum exchange and mass diffusion characteristics. An expression relating rate of erosion to bed shear stress in excess of bed shear resistance was developed. VIMS, Coll. of William and Mary, Gloucester Point, VA 23062, USA. 88:3426 Parker, W.R., 1987. Observations on fine sediment transport phenomena in turbid coastal environments. Continent. Shelf Res., 7(I 1-12): 1285-1293.
OLR (1988) 35 (6)
Measurements using new technology highlight unsteady behaviour and the importance of the cohesive boundary--a time-dependent transition between moving fluid and stationary bed. Discrepancies between various techniques for describing the properties and behaviour of the cohesive boundary become clear when the purpose of the description is considered. Although transport processes are described in physical terms, the impact of biological processes is now widely recognized. Non-intrusive measurement techniques offer a route to more realistic quantification of fine sediment transport phenomena in these environments. Sci. and Engng Res. Council, c/o Blackdown Consultants Ltd., 8, Fore St., Taunton, Somerset TA3 75H, UK. 88:3427 Sahl, L.E. and M.A.H. Marsden, 1987. Shelf sediment dispersal during the dry season, Princess Charlotte Bay, Great Barrier Reef, Australia. Continent. Shelf Res., 7(10): 1139-1159. 68 Bellewood Ave., South Setauket, NY 11720, USA. 88:3428 Salomons, W. and W.G. Mook, 1987. Natural tracers for sediment transport studies. Continent. Shelf Res., 7(11-12): 1333-1343.
Differences in chemical, mineralogical and isotope geochemical composition of marine and fluvial sediments can be used as tracers, provided that three criteria are met: conservative behaviour during transport, with time, and after deposition. Examples of the utility of natural tracers are presented, including their use for the interpretation of pollutant patterns in estuaries. The application of natural tracers shows marine sediments may be transported past the freshwater boundary in estuaries, and thus contribute to the sedimentation in freshwater tidal areas. Delft Hydraulics Lab., c/o Inst. for Soil Fertility, P.O. Box 30003, 9750 RA Haren, Gr, Netherlands. 88:3429 Schauer, Ursula, 1987. Determination of bottom boundary layer parameters at two shallow sea sites using the profile method. Continent. Shelf Res., 7(10):1211-1230.
Time series of mean current profiles within 3.5 m of the bottom were obtained in two hydrographically different regions. The shear velocity reached maximum values of u. = 6 cm/s. Values of z 0 are very high (0[cm]) and show a systematic variability with the velocity. The profiles are slightly concave, thus z 0 and u. decrease with the profile height considered. Both results indicate differences between observations and the Prandtl theory. Factors influencing the