John S. Schlee

Shallow Structure and Geologic Development of the Southern Red Sea

A series of 34 shallow-penetration seismic-reflection profiles made across the Red Sea show that it developed in two main stages. Initially, an early or pre-Miocene uplift and lateral extension resulted in crustal thinning and eventual formation of the main Red Sea Basin. During Miocene time, the Red Sea was isolated from the Indian Ocean but possibly connected with the Mediterranean Sea, which, like the Red Sea, was an evaporite basin at that time. A distinct acoustic reflector (reflector S) in the Red Sea marks the top of the Miocene evaporite sequence and is correlative with reflector M in the Mediterranean, which is similarly identified with termination of evaporite conditions. In Pliocene time, connection with the Indian Ocean was re-established, the opening to the Mediterranean was closed, and normal marine conditions were resumed in the Red Sea. Sea-floor spreading first started in Pliocene-Pleistocene time, and resulted in the formation of the axial zone of the Red Sea.

UPLAND GRAVELS OF SOUTHERN MARYLAND

An upland capping of gravel and overlying silt (Pliocene ?) covers approximately 600 square miles of the Coastal Plain of southern Maryland. This sheetlike deposit, which successively overlaps older formations to the northwest, has a fairly constant thickness of 25–30 feet, and dips southeastward at approximately 5 feet per mile. In the northern part of the upland, size and composition of the gravel show orderly changes eastward, which is also the direction of sediment transport as indicated by cross-bedding and gravel fabric. In the southern part of the area, facies changes are less orderly, and pronounced anomalies appear. These anomalies characterize both scalar and vector properties and therefore record a modification in conditions of deposition, probably induced by early Pleistocene climatic changes. The upland deposits are fluvial in origin. They were probably deposited by a graded ancestral eastward-flowing Potomac River which, in the process of lateral corrasion, spread a veneer of channel gravel and flood-plain silt. Later side- and down-cutting combined with a gradual southeastward migration of the river, resulted in the present large, thin gravel sheet.

Structure of the Continental Margin of Liberia, West Africa

Geophysical surveys made by R/V Unitedgeo I (USGS–IDOE Cruise Leg 5), combined with earlier surveys and available geologic information, provide the basis for interpreting the structure of the continental margin of Liberia. This area lies at the junction of the Americas and Africa in published reconstructions of Gondwanaland prior to the opening of the North and South Atlantic in Jurassic and Cretaceous time, respectively. Three fracture zones (St. Paul, Cape Palmas, and Grand Cess) are inferred in the area southeast of 9°30′ W. on the basis of magnetic and gravity data, which is supported by bathymetric and seismic reflection data. The three fracture zones appear to exist as separate lineaments near the African coast. Farther seaward, they may be part of the same transform fault crossing the Atlantic (St. Paul fracture zone). The magnetic anomalies associated with these fracture zones, which may have originated in Cretaceous time at the opening of the South Atlantic, are continuous with magnetic anomalies over crust of Eburnean age (∼2,000 m.y.) in southeast Liberia and its continental shelf. This suggests that Eburnean age structures may have been zones of weakness that were reactivated in Cretaceous time. A positive gravity anomaly (∼50 mgal) along the coast and continental shelf of Liberia is attributed to deep crustal rocks that were uplifted and exposed in Pan-African time (∼550 m.y.). The land boundary of this anomaly coincides with a shear zone that marks the boundary between the Pan-African and the Liberian age province (∼2,700 m.y.); the shearing (in a thrust-fault sense) may be the result of compressive stress associated with the closing of a proto-Atlantic ocean. Liberian age magnetic anomalies in the area northwest of 9°30′ W. cross the Pan-African province (and the positive coastal gravity anomaly) and continue over the continental shelf and slope to about the 3,000-m bathymetric contour; the seaward limit of the anomalies is interpreted as representing the seaward limit of the old continental crust. This westward extension of the continental crust does not completely fill the gap in fit in various published reconstructions of Gondwanaland, and we suggest that the northern Florida block may have been located near the Liberian margin at one time. Magnetic data indicate a thick section of sedimentary rock, possibly as great as 8 km, on the continental slope. Comparison of gravity data over magnetically inferred basins in the shelf, slope, and rise suggests that low-density sedimentary rocks constitute a greater proportion of the section in basins beneath the shelf and beneath the slope and rise northwest of 9°309 W. than beneath the slope and rise in the area of the fracture zones. The gravitational attraction that corresponds to a crust-mantle boundary dipping 45° to 60° can be computed to fit observed data – as might be expected at a rifted continental margin. A shallow high-density block beneath the coast and continental shelf is required to fit the coastal positive anomaly; this block is represented by exposures on land of granulite-grade metamorphic rock of the Pan-African province.

Glaciation on the Continental Margin off New England

The Pleistocene glacial limit in the marine environment off New England can be traced by plotting the seaward limit of abundant sandy gravel and the position of shoals. Maximum limit of the last glaciation was probably along an irregular line extending through Nantucket Shoals, across Great South Channel, northern Georges Bank, and at least to the edge of the Scotian Shelf. If, as we assume, glaciers lowered sea level approximately 130 m, the ice margin was probably a subaerial one on Nantucket Shoals and Georges Bank, and it was bordered by outwash and meltwater channels leading away from the ice front. On the Scotian Shelf, the margin may have bordered directly on the ocean, to judge by the lack of shoals and the widespread dispersion of gravel out to the shelf edge. The glaciofluvial nature of the original deposits and marine reworking during the eustatic rise in sea level have made it difficult to recognize ice-contact deposits near the limit of maximum glacial advance. The gravel on shallow banks and ledges is in a bimodal mixture with sand. Association of coarse gravel and sand suggests postdepositional reworking of till by marine processes and removal of silt and clay. Gravel in the Gulf of Maine is mixed with sand, silt, and clay, a mixture characteristic of till.

Submarine processes of the middle Atlantic continental rise based on GLORIA imagery

Approximately 6,100 km of 3.5-kHz echo-sounding profiles was correlated with a GLORIA side-scan sonar image of the mid-Atlantic United States (34°N, 70°W) lower slope-upper continental rise. The image allows us to map the major erosional and depositional features and to identify major processes that have shaped the area. The GLORIA imagery shows three approximately triangular-shaped sediment-gather areas that cover the upper-rise and slope transition areas near Wilmington, Baltimore, and Norfolk Canyons. The gather areas, which are interspersed with hemipelagic drape areas, are composed of dendritic networks of channels that extend from the base of the slope toward major channel systems on the middle rise, such as Wilmington Valley. Seaward of Cape Hatteras, a tongue-shaped area of mottled high back scatter on the GLORIA imagery denotes the Albemarle-Currituck mass-movement complex, a large (60 km wide by >190 km long) area of the sea floor that is marked by the disruption of upper-rise sedimentary strata and by mass-flow deposits. Interpretation of GLORIA imagery and echo-sounding profiles indicates that mass movement is the predominant process affecting sediment on the United States east coast mid-Atlantic slope and upper rise and that isobath-parallel sediment movement by geostrophic currents is restricted mainly to the lower continental rise. The mass-movement processes evident on the rise probably were most active during the Pleistocene, when sea level was lower and sediment input more active.

Early Cretaceous shelf-edge deltas of the Baltimore Canyon Trough: principal sources for sediment gravity deposits of the northern Hatteras Basin

We present evidence that the principal sources for Early Cretaceous (Berriasian-Valanginian) gravity-flow deposits of the northern Hatteras Basin were three large shelf-edge deltas located along the outer margin of the Baltimore Canyon Trough, ∼ 100 km southeast of Cape Charles, Virginia, Ocean City, Maryland, and Long Branch, New Jersey. Sedimentary detritus from the central Appalachian highlands and the Maryland-Virginia coastal plain was transported across the Early Cretaceous continental shelf to form the Cape Charles and Ocean City deltas, whereas deposits of the Long Branch delta came chiefly from the Adirondack and New England highlands. Each delta supplied sediment gravity flows to large slope aprons and submarine-fan complexes on the Early Cretaceous continental slope and rise. The most conspicuous distributary of sediment on the Early Cretaceous continental rise extends 500 km basinward from the Ocean City delta, where its distal deposits were cored at Deep Sea Drilling Project Site 603.

Seismic Stratigraphy and Facies of Continental Slope and Rise Seaward of Baltimore Canyon Trough

As part of a survey of the United States continental rise seaward of the northern Baltimore Canyon Trough, four major depositional sequences were mapped on a grid of 2,350 km of multichannel seismic reflection profiles. The sequences, which range in age from Jurassic (?) to Quaternary, record a gradual sedimentary buildup of fine-grained onlapping and slope-front fill. A broad wedge of Jurassic-age (?) sediment up to 5 km thick was deposited seaward of a conspicuous platform. During the Cretaceous, the slope-rise transition became much gentler, and sequences are more blanket-like because the declivity seaward of the platform was smoothed and filled by fine-grained clastic sediments and thin-bedded limestones. The main constructional phase for the continental rise was during the Cenozoic, when a thick (0.1-2.4 km) wedge formed seaward of the shelf edge in response to major fluctuations in sea level and erosion of the gentle, ancestral continental slope. The Cenozoic rise section can be subdivided into two main sequences separated by a conspicuous unconformity. The lower sequence is mostly a blanket (0.2-0.8 km thick) of Paleogene hemipelagic ooze and claystone. The sequence above the unconformity is a complex association of Neogene slump deposits, turbidites, hemipelagic clays, and channel fill that thickens seaward to 2.2 km under the middle continental rise. The final phase of rise construction was caused by widespread fluctuations in coastal onlap. These regressions resulted in deltaic outward-building on the shelf, extensive Pleistocene landward erosion of the slope, and the accumulation of a broad sedimentary apron on the rise.

Shallow Structure and Stratigraphy of Liberian Continental Margin

The rifting of Africa from North and South America has affected the structural framework off Liberia in two episodes. As shown by bathymetry, seismic-reflection profiles, magnetic data, and stratigraphy, the southeastern third of the margin is cut by west-southwest-trending fracture zones which we interpret as the extension of the St. Pauls fracture zone. These fracture zones intersect the continental margin off Cape Palmas to give rise to a blockfaulted and slump topography, similar to that in the area where the Romanche fracture zone intersects the African continent off Cape Three Points, Ghana. The fracture zones are covered by a prograded wedge of presumed Tertiary and Cretaceous sedimentary rock off central Liberia. Adjacent to the northwestern third of the Liberian margin, northwest-trending basins filled mainly with Lower Cretaceous paralic sediments are under the continental shelf; they extend to the upper slope, where they are downdropped along a northwest-striking fault zone that separates the shelf deposits from a thick prism of sediment beneath the continental rise. The southeastern third of the margin appears to have formed during the separation of Africa and South America in the Late Jurassic-Early Cretaceous. The rest of the margin seems, more strongly influenced by the tensional forces created during the rifting of Africa and North America; volcanic rocks are Late Triassic to Early Jurassic in age, and shelf sedimentation occurred mainly after the continents broke apart.

Geophysical evidence for the intersection of the St Paul, Cape Palmas and Grand Cess fracture zones with the continental margin of Liberia, West Africa

PUBLISHED reconstructions of Gondwana continent1 (Fig. la) show a gap in fit near the junction of the Americas and Africa. To study this critical area, the Unitedgeo I made geophysical measurements and collected rock samples across the continental margin of Liberia (USGS-IDOE cruise leg 5) in November 1971. Figure Ib indicates the location of the 5,400 km of ship track on a generalised bathymetric map2. We shall discuss the data in detail elsewhere. Here we present the evidence for the existence of three fracture zones, two of which have not been reported previously, intersecting the continental margin at the north end of the South Atlantic, which remained closed probably until Cretaceous time. We suggest that Precambrian structures on the African continent controlled the location of these fracture zones. Figure Ic compares gravity and magnetic profiles and interpretations of the seismic profiles for three selected lines (27, 30 and 34) crossing the Grand Cess, Cape Palmas and St Paul fracture zones, respectively.

Sediments and fossiliferous rocks from the eastern side of the Tongue of the Ocean, Bahamas

Abstract In August 1966, two dives were made with the deep-diving submersible Alvin along the eastern side of the Tongue of the Ocean to sample the rock and sediment. Physiographically, the area is marked by steep slopes of silty carbonate sediment and precipitous rock cliffs dusted by carbonate debris. Three rocks, obtained from the lower and middle side of the canyon (914–1676 m depth), are late Miocene-early Pliocene to late Pleistocene-Recent in age; all are deep-water pelagic limestones. They show (i) that the Tongue of the Ocean has been a deep-water area at least back into the Miocene, and (ii) that much shallow-water detritus has been swept off neighbouring banks to be incorporated with the deep-water fauna in the sediment.