Transcript Margins

Chapter 2.
Origin and Morphology of Ocean Margins
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General features of continental margins
Margins are sediment traps
Atlantic-type (=passive) margins
Pacific-type (=active) margins
Shear margins and complex margins
The shelf areas
The shelf break
Continental slope and continental rise
Submarine canyons
Submarine fans
2.1 General features of Continental Margin
Fig.2.1 a-c Schematic isostatic block model for continent-ocean- transition.
a Cross-section through continent " floating" on mantle (Uyeda. 1978).
b Density profiles.
c Sketch of general nature of continental margin
Ocean margin = Continental margin
 Transition between continent and deep ocean
Differ greatly in their characteristics depending on whether
they occur
 In mid-plate area
 On the collision edge of a continent
 Along a shear zone
One thing in common; the occurrence of large masses of
sediment
Fig.2.2 a, b. Depth zones of the sea floor.
a Diagram defining the most common terms used
in connection with sea-floor depth and distance
from land. Profiles are usually strongly
exaggerated; in reality slopes are gentle. The inset
at the bottom shows this for a line off North West
Africa.
b Physiographic diagram of the continental margin
of NE America. a 1shelf; 2 continental slope; 3
continental rise; 4 abyssal plain; 5 submarine
canyon. [B. B. Heezen et at., 1959, Geol. Soc. Am.
Spec. Pap. 65.]
The term pelegic and neritic refer to marine
organisms as well as sediments.
Littoral down to hadal refer to water depths.
Littoral = intertidal
Supralittoral = supertidal
Sublittoral = subtidal
2.2 Margins Are Sediment Traps
Table.2.2 Land and ocean areas and drainage.
(After h. W. Menard and S. M. Smith, 1966, J Geophys Res 71 p4305, and other sources)
Margins are the dumping site for the debris coming from the continents, the terrigenous
sediments.
The margins are also …
 The most fertile parts of the ocean
 Productivity is high
 Much organic matter becomes buried
 If conditions are right … petroleum!
Which margins are likely to have thick
sediment wedges?
 An ocean basin draining a
large land area
 Margins of Atlantic Ocean
have thick sediments, up to
10 km or more.
 Largest proportions of slope
and continental rise areas
 Not only sediment supply, but
also old trailing edges
Two types of margins
Passive margin = Atlantic margins
 Steadily sinking regions
 Accumulating thick sediments in layer-cake
fashion
Active margin = Pacific margins
 Rising
 Associated with volcanism, folding, faulting, and
other mountain-building processes
2.3 Atlantic-Type (=Passive ) Margins
Fig.2.3 a-d. Evolution of Atlantic-type
continental margins. Uplift of Earth
mantle material
a expands the continental crust (CC)
causing graben structures. Volcanism
is common at thins stage. the
continental crust thins, subsides, and
b splits apart. Coarse terrigenous
sediments (dotted), volcanogenic
deposits (back) (and salt in some cases)
accumulate. Rifting is followed by
drifting, with further subsidence of
continental margins. Mantle material
forms new oceanic crust (OC) as
shown in c. This stage resembles
modern Red Sea conditions. d seafloor spreading widens newly formed
oceanic crust area. Sediments cover
older parts of sea floor, and build up
margin
C = Red Sea
Red Sea
 Mantle material pushes up and
tears the Arabian peninsula from
Africa
 Mechanisms of the sinking are not
fully understood.
 As the sea floor cools, it sinks, and
thus the margin next to it loses
support.
 Reefs can grow on the sinking
blocks, building up a carbonate
shelf, and further depressing the
crust with their weight.
 If the Red Sea were only slightly
less open, salt deposits would form
… there is evidence!
Fig.2.4 a Comparison of typical structural elements of "volcanic" (A) and "nonvolcanic" (B)
continental margins. 1 Normal thickness oceanic crust; 2 seaward dipping units (volcanic); 3 structural
high in continental crust, often occurring adjacent to 2; 6 thinned, subsided continental crust; 7
unstretched continental crust. Parallel signatures Sediments; double line Moho, Mantle underneath.[ J.
C. Mutter et al. 1987 and I. C. Siuet and Z. Mascle 1978 in European Science Foundation, Cosod Ⅱ
Report, Strasbourg, 1987:92]
Fig.2.4 b Rifted continental margins in the North
Atlantic "Volcanic" type (a, and black areas) and
“nonvolcanic" type (b). Iceland as hot spot on the
Mid-Atlantic Ridge. [R. S. Whis et al., 1987,
Nature, 330; 439.]
c Continuous seismic reflection profile across the
"volcanic" type of a passive continental margin,
with proposed drill sites for deep-sea drilling and
the final drill hole 642 (central vertical line).
(Voring plateau off Norway, see Fig b). Dipping
reflectors between horizons E and K. Top of
Lower Eocene basalt flows; K base of seawarddipping reflector sequences; N and P Tertiary
unconformities (M Middle/Upper Miocene, O
Middle Oligocene.) [Data from K. Hinz in O.
Eldholm et al., Proc. ODP Initial Repots, 104, 12.]
Large petroleum reserves may be associated
with the salt domes, because the South
Atlantic was the site for deposition of
organic-rich sediments during middle
Cretaceous.
Fig.2.5 a-c. Evaporite deposition in the early
Atlantic.
a Geographic distribution of Mesozoic evaporites.
[K. O. Emery. 1977, AAPG continuing
Education Course Notes Ser 5: B-1]
b Salt diapir structures (S) as seen on air gun
profile of Meteor Cruise 39, off Morocco (near
30° N). Water depth at triangle is approximately
1800 m.[E. Seibold et al., 1976.]
c Relationship of salt diapirs to margin structure
off Angola (SW Africa). The Aptian salt is
underlain by nonmarine clastic deposits which
fill graben-like depression within pre-Cambrian
basement [ R. H. Beck and P. Lehner, 1974,
AAPG Bull. 58, 376.]
The kind of material accumulating depends on the
geologic settings of the region.
• In the tropics and where no large
rivers bring sediment or freshwater
… reef carbonates
• mixtures of lagoonal and riverine
sediments … offshore deposits …
hemipelagic mud, rich in the shells
of planktonic and benthic
organisms
• thick sediments (10~15 km) from
off the Niger, Mississippi and
other deltas.
Fig.2.6. Passive of Atlantic-type
Continental Margins. Different types off
Africa. A Nonmarine; B marine sediments.
[K. T. Pickering et al., 1989: 252, Deepmarine Environments, Unwin Hyman,
London.]
Unsolved Questions
 2.4 부분을 번역할 것 (숙제)
 J. T. Wilson의 가설은 무엇인가?
2.5 Pacific-Type (=Active) Margins
Two types of collision margins
1) Those produced by continent-ocean
collision, as at the Peru-Chile
Trench
2) Those where the subduction takes
place along island arcs, as along
Marianas … East Sea!
Fig.2.7 a, b. Sketch of collision margin, in profile (note to
scale).
a Peru-type collision (ocean-continent). Slope sediments
are being tectonically deformed. Igneous activity
including volcanism derives from melts generated within
the subduction zone. Complicated areal distribution of
extension and compression.[J. Aubouin 1984, Bull. Geol.
Soc. France, 3.]
b Island-arc situation (ocean-ocean). volcanic islands
build up over subduction zone. Back-arc basin with
spreading center.[ Sources; J. R. Curray, D. G. Moore, in
C. A. Burk and C. L. Drake 1974, ref. p. 250; and D. R,
Seely, W. R. Dickinson 1977 Amer. Assoc. Petrol. Geol.
Continuing Educ. Notes Ser. 5.]
Characteristics
 Folding and shearing of sediments
 Addition of volcanic and plutonic material from
the active vents sitting on top of the down-going
lithosphere
 Fractionational processes associated with partial
melting on the descending slab and with
hydrothermal reactions can lead to enrichment of
melts with heavy metals … ore deposits
Types of rocks; varied
 Basaltic rocks, serpentinite, gabbro, peridotite
which were derived from the mantle and altered
by hydrothermal reactions under various
conditions of pressure and temperature
 Various kinds of pelagic sediments ; deep sea
clay, shelf carbonates, biogenic silica
 Ophiolites
obduction
Fig.2.8 a Reflection seismic
profile from the subduction zone
at the Nankai trough southeast of
southern Japan. TWS two-way
travel tiome in seconds; BSR
bottom simulating reflector. Note
the downgoing oceanic crust of
the Philippine plate with the
decollement zone (in Miocene
sediments). The accretionary
prism above it consists of
turbidites and hemipelagic
sediments and is intensively
deforned, thus opening paths for
fluids. [A Taira and Y. Ogawa.
1991, Episodes 14, 3: 209.] b
Diagram showing paths for fluids
in a sandy accretionary prism. [J.
C. Moore et al., 1991,GSA Today,
1, 12: 269.] Where these paths
reach the surface, seepage-related
biological communities may occur,
as observed by submersibles in
the Nankai trough(see Chap. 6. 9.)
 Steep slopes leading into the trench.
 Large-scale gravitational transport of rock
masses from the land slide into the subduction
zone
 The jumbled masses (melange) are sheared and
metamorphosed.
 Blue schists, green schists and subsequently
amphibolites can form.
Geosyncline = 지향사
 Passive margin = miogeosyncline
 Active margin = eugeosyncline
Shear Margins and Complex Margins
 East-west-running margins of North Brazil and
the African Guinea Coast
 They parallel to the many fracture zones near the
Equator
 A third type, with narrow shelves
The Shelf Areas
 Grand scale; tectonics (active vs. passive) and the
recent rise of sea level
 Regional scale; climatic conditions and
sedimentary supply are of importance.
 Tsunamis
2.8 The Shelf Break
Fig. 2.9. Shelf break at the entrance of the Persian Gulf, subsurface echo profile by the research vessel Meteor
(1965). Note the accumulation of soft, layered sediment behind the rugged reef structure. Upper slope collects
reefal debris. The reef is dead. Shelf break is somewhat above 100m depth. [ E. Seibold, Der Meeresboden
(1974), 15, Springer, Berlin, Heidelberg, New York.]
2.9 Continental Slope and Continental Rise
Fig.2.10. Physiographic diagram of the ocean margin off California. Note the narrow shelves bounded by sea cliffs toward the land
(uplift!). The Continental Borderland in the south is a submerged basin-and-range province. The continental slope essentially consists
of enormous coalescing fans which transgress over the abyssal hills province.[Sketch based on physiographic diagram of H. W.
Menard, 1964.]
Fig.2.11. Submarine mass movements off Dakar (NW Africa). Air gun record of Meteor Cruise 25/1971. Shelf edge upper right. Slide
starts at 1050 m. depth (return time for outging sound pulse: 1.4s). Thickness of slide ~ 200m. Material cane to rest below about
2300m depth (=3.6s) at the foot of the continental slope. Insert left Air gun system with air gun as sound source. Acoustic si후민 are
reflected by the sea fleer and by subbottom layers and are recorded by hydrophones in the streamer. [E. seibold, Der Meeresboden
(1974)m 17, Springer, Berlin, Heidelberg, New York.]
Fig.2.12 Compilation of exogenic processes shaping (passive) continental margins.
[G. Einsele et al. (eds.). Cycles and events in stratigraphy, Springer, Heidlberg,
1991:318.]
2.10 Submarine Canyons
Fig.2.13. Profile of Monterey Canyon compared with that of the Grand Canyon of
Arizona [ F. P. Sheard and R. F. Dill, 1966.]. The resemblance is coincidental, but
illustrates the enormous size of the Monterey Canyon(see Fig 2.10.)
Fig.2.14 Illustration of morphological similarities of subaerial canyon (Grand Canyon left) and
submarine canyon(La Jolla Canyon right). Note steepness and overhang in both cases [photo left, E.
S; underwater photo courtesy R. F. Dill.]
Fig.2.15 a, b. Origin of graded layers. a Experiment of Ph. H. Kuenen. 1 Turbid, sediment-laden water is introduced
into the tank; 2 water in tank remains still and clear over the bottom, where the denser muddy water rushes
downslope; 3 turbulent front of the turbidity current. Depending on its strength, a turbidity current can erode and
redeposit enormous amounts of sediment. [J. Giluly et al., 1968, Principles of geology. W. H. Freeman. San
Francisco, after photo by H. S. Bell, Cal. Tech.] b Standard sequence of divisions in a turbidity layer, as proposed by
A. H. Bouma. The lower part is the graded bed, produced by a turbidity current.
The upper part results from "normal" sedimentation; it contains almost all the geologic time represented. Sudden
loading of these pelagic clays may produce "load casts". High velocities of the currents and indicated by drag and
flute marks at the base of the turbidite. [ G. V. Middleton, M. A. Hampton. 1976, in D. J. Staley, D. J. P. Swift,
Marine sediment transport and environmental management, John Wiley, New York.]
Fig.2.16. Grand banks 1929 earthquake. the
cable break sequence (later conbined with the
stratigraphic record in the cores) was
interpreted by B. C. Heezen and M. Ewing
(1952. Am J Sci 250:849) as evidence for high
velocity turbidity currents. [ B. C. Heezen, in
M. N. Hill, 1963, The Sea, 3: 744.]
2.11 Deep-Sea Fans
Fig.2.17. Build op of deep-sea fans. 1 Canyon-cutting through shelf and upper slope traps and funnels sediment to the fan; 2
upper fan valley, walls with slum[p features (U), bottom with debris flows(V) and mostly graded coarse grained beds,
forming conglomerates in fossil examples. Levees with thin-bedded turbidities (X). Levees can be breached (note dead
channels);3 active suprafan with distributary channel, filled with pebbly of massive sands (Y); 4 outer fan with classical
turbidites (Z); 5 abyssal hill region beyond fan Valleys between fills may have distal fan material. [ Based on a sketch by W.
R. Normark, 1970, Am Assoc Pet Geol Bull 54;2170 and on R. Walker, 1978, Am. Assoc. Petrol Geol. Bull. 63.932.]
Fig.2.18. Abyssal Plains. Seismic echo profile across a stretch of abyssal plain. (Courtesy C. D. Hollister).
Note that the sediment surface is perfectly horizontal regardless of the underlying basement topography. (2800
fathoms = 5100 m; 3600 fathoms = 6600 m)