Fault Throw Research Articles

Depiction of Faults on Stratigraphic Isopach Maps: GEOLOGICAL NOTE

Isopach maps generally are more difficult to draw than structure contour maps. Many geologists have problems depicting the patterns of faults on such maps; this paper demonstrates proper technique. Expressions of faults are of much greater width on isopach maps than on accompanying structure maps. Moreover, isopach lines do not stop at a fault trace, but continue across it in a distinctive fashion which is quite different from that of structure contours. Where faults cross or bifurcate, or die out within a formation, they also exhibit characteristic patterns on isopach maps. Isopach lines near the ends of faults, and in situations where bed thickness equals or exceeds fault throw show typical attitudes, as do isopach-map features produced by high angle reverse faults.

Geology of South Bosco-Duson-Ridge Area, Acadia and Lafayette Parishes, Louisiana: ABSTRACT

The South Bosco-Duson-Ridge fields area is in the Oligocene and Miocene oil- and gas-producing trends of Acadia and Lafayette Parishes, Louisiana. In wells drilled to sufficient depth, three facies were found: End_Page 2319------------------------------ (1) a Pliocene and Miocene nearshore to continental massive sandstone facies; (2) an early Miocene and Oligocene continental-shelf facies of alternating sandstone and shale; and (3) a thick bathyal shale facies of Oligocene age. Oil and gas production in the area is confined almost entirely to sandstone beds in the continental-shelf facies. South Bosco-Duson-Ridge fields are on a faulted, elongated north-south anticline that trends normal to the regional stratigraphic strike. All faults are normal and are either down-to-the-basin or up-to-the-basin. The down-to-the-basin faults are regional and are parallel with the regional stratigraphic strike. The up-to-the-basin faults are compensating faults confined to the South Bosco-Duson-Ridge complex. The fault throw generally increases with depth. All the faults die out upward in the section in or before reaching early Miocene sedimentary rocks. Strata thicken into the downthrown sides of the faults and thin toward the crest and over the highest parts of the structure. The amounts of thickening or thinning generally increase with depth. Thus, fault movement and anticlinal folding were contemporaneous with sedimentation, continuously from Oligocene into Miocene time, and were most intense during Oligocene time. The South Bosco-Duson-Ridge structure probably originated from faulting and anticlinal folding on the continental slope during Oligocene time. Structural growth was greatest where the structure was in the unstable environment of the hinge line (or shelf edge). Structural activity continued during the deposition of the neritic sediments of the Oligocene and early Miocene but at a steadily reduced rate. The structure was buried by a great thickness of nearshore and continental massive sandstone beds during Miocene and Pliocene time. End_of_Article - Last_Page 2320------------

Contributions to the Geology of the Mere Fault and the Vale of Wardour Anticline

Detailed mapping of the western end of the northern limb of the asymmetrical Tertiary Wardour Anticline around East Knoyle and the Mere Fault from West Knoyle to Bayford, a distance of nine miles, is described. The Mere Fault downthrows north bringing Jurassic rocks on the upthrown side against Cretaceous Beds on the downthrown side. The fault throw ranges from 600 feet at West Knoyle to 400 feet at Mere and 150 feet at Penselwood and Bayford. It maps as a steep reversed fault and as it is closely associated with the Tertiary Wardour fold it is likely to be of similar date. In this respect the Mere Fault has close analogues with the Poxwell-Ridgeway faults in south Dorset and with the Hog's Back structures in Surrey.

Role of Concurrent Deposition and Deformation of Sediments in Development of Salt-Dome Graben Structures

The mechanics of graben development over deep-seated salt domes are described and illustrated by results of sectional-model experiments and study of the Heidelberg field, Mississippi. Relative vertical uplift of the salt produces graben structures in the overlying sediments. Within these structures downward movement of rock material occurs by wedge-shaped blocks. Fault movement at one boundary of a block is accompanied by similar movement along one or more fault planes at the opposing margin of the wedge. Increase of fault throw with depth and abnormally thick sections in graben blocks constitute effects impressed on the geometry of graben structures by deposition of sediments during deformation of the underlying strata. The results of two sectional-model experiments originally recorded on motion-picture film are illustrated by eight photographs taken from individual frames. These experiments demonstrate the concentration of faulting in the upper beds over the crest of the salt uplift as deformation continues, relative age of faults, upward termination with consequent inactivity of the outer faults located at the margin of graben structures, and changes in fault dip as structural development proceeds.