Abstract This paper documents normal fault sets observed in chalks exposed in widely separated localities in the UK and France. These faults are characterized by having a wide range of strikes at any one locality, are developed entirely within the chalk succession and do not seem to interconnect to deeper or shallower structures. These structures may result from two different mechanisms: (1) complex polyphase deformational histories involving contrasting stress states; or (2) a single deformational phase in which the faults develop to accommodate compactional strains. Evidence is presented from microstructural and petrographic data to support the latter interpretation. In particular, the association of calcite and marcasite mineralizations with fracture surfaces and fault zones and textural observations relating flint occurrence to early fault formation point towards fault propagation at a very early stage of burial and compaction of the chalky sediments. An analogy is drawn between these outcrop-scale structures and polygonal fault systems at a larger scale recognised from seismic observations of chalk sequences deposited at passive continental margins. The origin of these structures may be related to syneresis at an early stage of deformation followed by pressure solution phenomena that may reactivate this early-inherited polygonal fault pattern until the present day.

Abstract New multi-beam sonar and seismic data collected in Lake Superior document the widespread development of lake-floor rings in fine-grained lake-floor sediments. The multi-beam images reveal that the rings develop as connected clusters and that individual rings have an irregular polygonal appearance. High-resolution seismic data collected with a 28 kHz echo sounder reveal extensive fracturing and faulting in the glacio-lacustrine sediments below the lake-floor. Displacement on the faults is typically normal with throws of less than 50 cm. Three styles of faulting are recognized: (a) monoclinal flexure; (b) graben-like; and (c) conjugate. Zones of acoustic blanking below the faults may be associated with de-watering and mobilization of the sediments. Lateral thickness variation in some horizons suggests that fault and fracture development is linked with lateral movement of sediment. Piston cores collected near lake-floor rings show well-developed fractures and micro-faults, suggesting that fracturing and faulting occurs on a wide range of scales. The seismic and lithological characteristics of the glacio-lacustrine section are similar to those of sediments in which Polygonal Fault Systems (PFS) have been described. This suggests that the rings in Lake Superior may be the surface expression of PFS in the near-surface sediments.

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Abstract SILICA in fine particulate and mainly crystalline form has been studied by many different methods including both optical and electron microscopy and electron diffraction. Probably the most important reason for investigating silica in the micron and sub-micron range has been its importance as the causative factor in the lung disease known as silicosis. Optical methods have been used for identification, size measurement, and the study of surfaces. Some forms of amorphous silica have a recognizable shape and this may have some diagnostic value in identification, but crystalline silica appears to be either devoid of cleavage planes or to have insufficiently distinct weaknesses to break into anything other than randomly shaped particles. These cannot be identified by shape, and electron microscopy is of limited value in identification. This is partly due to the tendency for an amorphous or very disordered layer to form on particle surfaces, and to affect the structure to a depth of a similar order to that of the penetrating power of the electron beam. X-ray diffraction techniques for the quantitative determination of quartz must also take account of the effect of the amorphous layer on the quality and intensity of the diffraction pattern. Since crystalline silica particles are of random shape, reasonably reliable statistical correlation can be established between particle sizes as measured by electron microscopy and other parameters such as the surface and mass as measured by other techniques. This can be of importance in respirable particles of less than 2 μ m diameter, whose size can be reliably measured only by electron microscopy.