Irregular Basement Research Articles

Time-delay interpretation of seismic refraction data in the Caledonian front, Jämtland, central Scandinavian Caledonides

Abstract Seismic refraction studies have been carried out along several profiles through the Caledonian front in Jämtland in order to investigate the configuration of the Precambrian crystalline basement beneath the Caledonian sedimentary cover. The seismic sections were calculated according to the time-delay method and by assuming a linear increase of velocity with depth. Recordings from each of the 3 shotpoints along the profiles allowed the construction of continuous profiles. A method was applied whereby the basement velocity and the velocity gradients were varied until the best fit with respect to the calculated depths from different shotpoints was obtained. The seismic models are compared with drill-hole data, interpretation of aeromagnetic data and surface geology. Depths calculated from the seismic data compare well with the information from the few deep drillholes that reach down to the autochthonous basement. The magnetic and seismic models concur well along most of the profiles but differ greatly locally, thus demonstrating the importance of the application of various geophysical techniques, particularly when investigating complex stratigraphies, as in the Caledonides. Both the seismic and the magnetic models, suggest the existence of an undulating basement surface in the Caledonian front that coincides with some of the major folds in the sedimentary cover. East of the Offerdal synform the basement surface has previously been interpreted to dip gently (about 2°) westwards. The new seismic and magnetic data suggest a more irregular basement configuration than has previously been demonstrated.

Acoustic stratigraphy of the sedimentary succession west of Porcupine Bank, NE Atlantic Ocena: A preliminary account

Acoustic basement lies at an average of between 6.0 and 6.5 sec two-way time below sea level in the southern Rockall Trough and northern Porcupine Abyssal Plain. The overlying sedimentary succession reaches maximum thicknesses of at least 4.0 sec, and can be divided by 3 regionally-developed seismic reflecting horizons, which are used as a framework to establish an acoustic stratigraphy for the area by selecting three “type” seismic sections. These reflectors are named, in ascending order, Shackleton, Charcot and Challenger. The area is crossed by E—W basement high structures, the Clare Lineament (which may be an easterly extension of the Charlie Gibbs Fracture Zone), that separates the Porcupine Abyssal Plain from the eastern part of southern Rockall Trough. Under the latter, the post-Shackleton acoustic sequence is thickened, as if dammed to the north of the Clare Lineament, whilst a further thickening, above reflector Charcot, occurs along a NE—SW line somewhat farther north into the southern Rockall Trough. This can also be related to shallow-lying acoustic basement features. Pre-Shackleton sediments overlie a very irregular basement topography. The acoustic characters of the various sediment packages are described and it is speculated that major changes in the sedimentary environments took place across reflectors Shackleton and Challenger, the latter probably establishing the modern bottom current circulation patterns. No ages can be unequivocally assigned to the main reflectors, but previously published data suggest a late Eocene—Oligocene age for Challenger. Possible lavas or sills are identified in the succession between reflectors Shackleton and Charcot.

Columnar concretions in the Visakhapatnam red sediments on the east coast of India

Calcareous and siliceous columnar concretions occur in some of the Visakhapatnam red sediments on the east coast of India. The carbonate, externally derived, was rapidly precipitated in the intergranular space of the sediments by displacement of the fine material. Intense evaporation at the surface in the dry periods and the initial high permeability of the red sediments are considered to have produced the columnar forms. The fine-grained nature, low Mg-calcite and the negative δ 13C and δ 18O ratios of the carbonate cement suggest an evaporative precipitation in a fresh-water environment. Late-diagenetic changes and compaction of the red sediments caused the modern development of the concretions to cease. Irregular basement topography largely controlled the localized distribution of the concretions in the red sediments. Siliceous concretions are confined to a narrow zone along the coast. Amorphous silica, biogenous matter and silica microspheres constitute the cementing material of these concretions. The microspheres indicate a precipitation of silica from saturated solutions in a protected environment. The siliceous concretions are interpreted to have replaced the carbonate ones during the post-Pleistocene sea-level rise in the Visakhapatnam region.

Devonian lacustrine limestones at the margin of the Orcadian Basin, Scotland

A complex overlap of middle Old Red Sandstone occus in N. Caithness and Sutherland. The relief of the underlying surface of Moine metamorphics and intrusions is irregular. The facies variation of the sediments reflects lacustrine transgression and regression within the Orcadian Basin. When the lake level was high and the lake margin coincided with the basin margin, thick ( c. 4m) carbonates were deposited on the unconformity. Carbonate deposition was controlled by photosynthesizing algae. At such sites the depositional environment was oxygen-rich and the carbonates do not contain biotite, organic carbon or pyrite. The offshore facies was deposited in deeper stratified lakes, anoxic at depth, and comprises organically-enriched sediments (with pyrite and biotite). When the lake margin was not coincident with the basin margin alluvial sediments mantled the unconformity as rivers cut to a lower base level. At marginal sites compaction of the lacustrine sediments over the irregular basement during periods of low lake level, when no deposition occurred, produced local unconformities.

Origin of the shelf break off Southeast Australia

Abstract Evidence from seismic reflection profiles across the southeast Australian continental margin indicates that in some areas eustatism has not influenced the shelf‐break zone. Where this is the case, insufficient sediment has accumulated along the rifted continental margin since the opening of the Tasman Sea to compensate for subsidence and to raise the sea floor to within reach of wave‐base erosive influences during low sea levels. In some instances there is no evidence of outbuilding, and the sediments are draped over basement structures; thus the change of slope at the shelf break is the surface expression of basement relief and changes in the depth of the shelf break are related to changes in depth of basement. Erosion during times of low sea level has played no part in the development of the shelf break and therefore it is not necessary to invoke neotectonism to explain variations in its depth. Other seismic sections reveal clear evidence of outbuilding of the wedge of sediments which accords with the generally accepted concept of continental shelf development. It is suggested that moulding of the major morphological elements of the outer shelf and development of the shelf break may have occurred as early as the late Miocene. The buried basement surface is commonly smooth at the crest of the continental slope, and irregular under the shelf. The smooth surface may be a marine abrasion platform formed during the initial transgression across the continental margin after rifting and opening of the Tasman Sea in the Late Cretaceous. The basal sediments resting on irregular basement under the outer shelf are probably continental in origin.

Geology and genesis of the Mexican Ridges: Western Gulf of Mexico

The western Gulf of Mexico sea floor contains numerous linear, parallel, anticlinal features that comprise the Mexican ridges province. The development of this ridge province essentially began during the Mesozoic with evaporite deposition in the gulf basin. These evaporites were subsequently buried by a thick sediment cover. Increasing overburden pressure and tectonic activity produced salt flowage on an irregular basement and thereby triggered salt anticline formation, the primary cause of ridge development. The ridges first developed during late Mesozoic or early Tertiary as salt anticlines and diapirs beneath the overburden. By Early Pliocene, salt tectonism had produced incipient surface ridges that effectively stemmed the seaward transport of terrigenous sediments. The ridges apparently are still undergoing uplift. Ridge trends are predominantly north-south but are not consistent and clearly have been influenced by basement uplifts. We present here a hypothesis for the development and trends of the Mexican ridges based on extensive geophysical data to show that the ridges are due to salt tectonism or flowage and the influence by basement highs.

The nephropathy of the nail-patella syndrome: Clinicopathologic analysis of 11 kindred

Seventy-two members of 11 kindred with the nail-patella syndrome were studied. Twelve patients who had electron microscopic examinations of renal tissue had abnormalities of the glomerular basement membrane. An unaffected family member did not. This lesion consisted of irregular basement membrane thickening, epithelial foot process fusion and the presence of fibrillar collagen-like material within the substance of the membrane. Many patients also showed areas of increased lucency giving the membrane a “moth-eaten” appearance. The presence of the ultrastructural lesion bore no relation to the presence or absence of abnormalities by light or immunofluorescent microscopy and also no relation to demonstrable alterations of renal function. The determinants of clinically significant nephropathy are unknown but in most patients the prognosis is favorable.

Structure and Stratigraphy of China Basin

Continuous seismic reflection profiles accompanied by total-field magnetic measurements were made in the China basin by civilian survey ships on contract to the U.S. Navy Oceanographic Office between 1967 and 1969. The results show the presence of stratigraphic units similar to those previously found in the adjacent East China Sea and South China Sea: (1) acoustic basement in the southern part of the basin that may be a continuation of the igneous and metamorphic rocks beneath the adjacent shelf which were peneplaned during Late Cretaceous-early Cenozoic time, and much more irregular basement in the northern part of the basin that may be oceanic basement (Layer 2); (2) predeformational sediment, conformably with the surface of much of the acoustic basement (probably Paleo ene); and (3) postdeformational sediment, largely turbidites deposited in deeper areas (probably Neogene to present) after the main episode of deformation. During the deformation a series of northeast-trending ridges was folded along the floor of the China basin. Similar ridges underlie the basin side slopes, and these served as dams to trap sediments brought to the ocean by streams from the adjacent land areas. Another ridge separates the Manila Trench and the West Luzon Trough, and extends northeastward as the Central Range of Taiwan. Oil potential appears to be greatest beneath the shelf between Taiwan and Hainan off mainland China, but the basin ridges that are surmounted by banks and islands also warrant further investigation.

Marine geophysical survey in the southwestern Barents Sea

A marine geophysical survey in the southwestern Barents Sea reveals an over-all sequence of layered sediments. In the main part of this area relatively high seismic velocities (2.5–3.0 km/sec) occur near the sea floor, but a low-velocity (1.85–2.2 km/sec) sedimentary wedge exists near the shelf edge. The low-velocity section is believed to consist of material deposited during the Cenozoic when the Barents shelf was uplifted. However, the main portion of the sediments seem to be consolidated, of Mesozoic and Paleozoic age. Magnetic and gravity data suggest an irregular basement relief in the west and a smoother and probably deeper basement toward the east. The latter is associated with the Riphean structural complex, whereas the western area may reflect a continuation of the Caledonides.

Continental Rise off Eastern North America

During mid-1967 two cruises of the Woods Hole Oceanographic Institution's R/V Chain provided nearly continuous seismic, geomagnetic, and gravity measurements along 8,000 km of ship track. These measurements supplement earlier ones from various sources to provide a comprehensive picture of the composition and geologic history of the continental margin off eastern North America, an area that is much larger than all of the United States east of the Mississippi River. The geomagnetic profiles portray a systematic pattern of positive and negative anomalies that are in accord with the concept of sea-floor spreading, whereby North America separated from Europe and Africa at the beginning of the Permian Period, and drifted westward from the site of rifting (the Mid-Atlantic Ridge) at average rates of 0.8-1.4 cm/year. During all this time the continent has been coupled firmly with the adjacent sea floor, as though both continent and sea floor were on the same conveyor belt. Gravity information suggests that a relict structure of the original rift is preserved in the same general area as the geomagnetic slope anomaly, beneath the seaward part of the continental shelf, the continental slope, or the upper continental rise. It has the form of a complex linear ridge of crystalline rocks that rises above the zone of sharpest landward slope of the Mohorovicic discontinuity. Seismic refraction measurements support the presence of such a ridge, bordered on both sides by linear trenches. The continuous seismic reflection profiles measured during the cruises reveal shallow acoustic basement in the form of a ridge complex that is shallower, but in the same general area, and probably is related to the deep ridge. The ridge and associated trenches served as dams and b sin sinks to trap land-derived sediments during the Mesozoic Era, so that only pelagic silts and clays could reach and be deposited on the irregular oceanic basement seaward of the barrier. During Late Cretaceous to middle Eocene time one or more thick deposits of probably chemical origin formed blankets of deep-sea chert throughout broad abyssal plains, which produce the acoustic reflector known as Horizon A. About middle Eocene time the land-derived sediments filled the trap west of the ridge and prograded eastward over the ridge top and built the present continental rise atop the Mesozoic abyssal plain. Continuous seismic reflection profiles show that the rise is a huge prism of generally seaward-dipping, interbedded pelagic sediments and turbidites that contain many masses of sediment displaced from higher on the continental rise and from the continental slope. Such slides continue to occur, a large one having occurred in 1929. The volume of the Cenozoic continental rise in the study region is nearly 3 million km3, about half the volume of all sediments deposited on basement during Mesozoic time. The interbedding of sandy turbidites with organic-rich silts and clays displaced from the continental slope may constitute a thick sequence of oil reservoir and oil source beds, but no exploratory drilling into them has been done.

Reflection Techniques Suggest Nature of Deep-Ocean Sediments: ABSTRACT

Conventional reflection profiling reveals a deep-ocean sedimentary column up to 3,500 m. thick. Sedimentary layers are grouped into an upper acoustically transparent zone, 20-510 m. thick, and a lower acoustically responsive zone up to 3,000 m. thick. The former, exhibiting a minimum of internal structure, has an average Vi = 1.70 km./sec. determined from X2, T2 analyses, an average thickness of 200-250 m., and is interpreted as unconsolidated, water-saturated red clay or pelagic ooze. The responsive zone is strongly layered, has an average Vi = 3.0 km./sec., and is interpreted to be semi-consolidated to consolidated mixed red clay and ooze, turbidites, or volcanic products. Low-velocity sediments blanket ocean-bottom topography and exhibit relief (up to 100 m.) largely at the water-sediment interface suggesting bottom scour and transport. Higher-velocity, stratified sediments lie on an irregular basement with pronounced discordance; thickness is dependent on basement topography and suggests differing depositional processes and rates of diagenesis. The uniform, plane surface interface between the two sedimentary units and extraordinary smoothness of layers within the stratified zone may be explained by the leveling effect of turbidity currents in rapidly filling sediment basins. End_of_Article - Last_Page 1757------------