Origin and Morphology of Ocean Margins

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

Chapter 2.

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

2.1 General features 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

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 = intertidalSupralittoral = supertidalSublittoral = subtidal

Fig.2.2 a, b. Depth zones of the sea floor.

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)

2.2 Margins Are Sediment Traps

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

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 sea-floor spreading widens newly formed oceanic crust area. Sediments cover older parts of sea floor, and build up margin

2.3 Atlantic-Type (=Passive ) Margins

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 Report, Strasbourg, 1987:Ⅱ92]

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 seaward-dipping 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.]

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.]

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.2.6. Passive of Atlantic-type Continental Margins. Different types off Africa. A Nonmarine; B marine sediments.[K. T. Pickering et al., 1989: 252, Deep-marine Environments, Unwin Hyman, London.]

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.

Unsolved Questions

2.4 부분을 번역할 것 ( 숙제 )

J. T. Wilson 의 가설은 무엇인가 ?

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.]

2.5 Pacific-Type (=Active) Margins

Two types of collision margins1) 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!

Characteristics

Folding and shearing of sediments Addition of volcanic and plutonic material from t

he 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

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.)

obduction

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 amp

hibolites 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

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.]

2.8 The Shelf Break

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.]

2.9 Continental Slope and Continental Rise

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 si 후민 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.]

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.)

2.10 Submarine Canyons

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.]

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.]

2.11 Deep-Sea Fans

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)