Waite Creek Thrust Nappe—Western Ngalia Basin, Central Australia
A.T. Wells, F.J. Moss, 1983. "Waite Creek Thrust Nappe—Western Ngalia Basin, Central Australia", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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The seismic variable area cross section and interpretation presented (facing page) illustrate a typical profile of the thrusted northern margin of the Ngalia basin in central Australia. The seismic section is oriented approximately north to south, and is located at Waite Creek in the western part of the basin.
Apart from illustrating overthrust tectonics that are so characteristic of central Australian late Proterozoic to late Paleozoic intracratonic basins, the seismic record is unusual in showing a remarkably clear profile of the thrust fault.
The Ngalia basin underlies an area of 16,000 sq km (6,178 sq mi) between South lat. 22 to 23° and East long. 129 to 133°45'in the southern part of the Northern Territory, central Australia (Figure 1).
For the purposes of discussion and easy reference, the basin may be divided into three arbitrary zones which will be referred to in the text. They are a western zone, a central zone between East long. 131°25'and 132°, and an eastern zone.
The Ngalia basin is a structural feature comprising a wedge of predominantly arenaceous, late Proterozoic and Paleozoic sediments in an intracratonic downwarp in lower and middle Proterozoic igneous and metamorphic basement rocks, of the Arunta Block; in meridional cross section the basin structure is essentially a faulted asymmetrical syncline with the thickest sediments preserved toward the basins thrusted northern margin. The maximum known thicknesses of the sediments are: late Proterozoic 3,200 m (10,500 ft); Cambrian, 800 m (2,625 ft); Ordovician, 300 m (984 ft); Devono-Carboniferous, 3,100 m (10,171 ft). The thickest preserved sedimentary succession is 4,700 m (15,420 ft) in the northern part of the central basin zone. The sedimentary sequence is shown in Table 1.
Figures & Tables
Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.