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Crescentic dunes on the inner continental shelf off northern California
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
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