Geology topics
Terri L. Purdy
Publications and source records attributed to Terri L. Purdy.
Disturbed zones: Indicators of deep-seated subsurface faults in the Valley and Ridge and Appalachian structural front of Pennsylvania
Field studies of geologic structures in the Valley and Ridge and adjacent parts of the Appalachian Plateau provinces in Pennsylvania have shown a new type of structure, formerly poorly understood and frequently unmapped, is a significant indicator of deep-seated subsurface faulting. These structures, herein called disturbed zones, are formed by movement between closely spaced pairs of thrust faults. Disturbed zones are characterized at the surface by long, narrow, intensely folded and faulted zones of rocks in a relatively undisturbed stratigraphic sequence. These zones are frequently kilometers to tens of kilometers long and tens to hundreds of meters wide. Although disturbed zones generally occur in sequences of alternating siltstone and shale beds, they can also occur in other lithologies including massively-bedded sandstones and carbonates. Disturbed zones are not only easily recognized in outcrop but their presence can also be inferred on geologic maps by disharmonic fold patterns, which necessitates a detachment between adjacent units that show the disharmony. A number of geologic problems can be clarified by understanding the principles of the sequence of formation and the method of location of disturbed zones, including the interpretation of some published geologic cross sections and maps. The intense folding and faulting which accompanies the formation of a typical disturbed zone produces a region of fracture porosity which, if sealed off from the surface, might well serve as a commercially-exploitable hydrocarbon trap. We believe that the careful mapping of concentrations of disturbed zones can serve as an important exploration method which is much less expensive than speculation seismic lines.
Generalized map of occurrences of limonitic rocks in the Walker Lake 1° by 2° quadrangle, Nevada-California
No abstract available.
Decollement in the southern Allegheny Plateau of Pennsylvania
Recent field investigations in the Allegheny Plateau north of Williamsport, Pennsylvania have shown the presence of both southward dipping, low-angle thrust faults translated to the north and northward dipping back or antithectic thrusts translated to the south at the leading (north) edge of Barclay syncline. The presence of these thrusts, along with reported anomalous dips along the leading edge of the next syncline to the south (the Bernice or Barbours syncline) may indicate that the Allegheny Plateau of Pennsylvania is underlain by master decollement and that this decollement gives rise to a series of splay thrusts. In some cases such as the Barclay syncline, the combination of these forward and back thrust faults may serve as fracture porosity traps.
Thrust fault zones in the Allegheny Plateau of north-central Pennsylvania
Field investigations in the Williamsport Valley identify lineaments found on Landsat III images, have shown the presence of six discrete fault zones that strike subparallel to the trend of the Appalachian folds. These zones range from 0.5 to 1.75 km in width and from at least 10 km to more than 50 km in length. The individual thrust faults within each zone occur in 'staircase-type' folds and are at a low angle to bedding. Although each individual fault may have 0nly centimeters to displacement, many of these individual faults appear to exist within the six zones. We believe that the stress that produced that Valley and Ridge folds to the south was largely dissipated in faulting in the Williamsport Valley. This dissipation of the stress would explain the presence of only broad open folds to the north on the Allegheny Plateau. The extreme faulting in the Williamsport Valley along with the unique 'staircase' and 'reverse staircase' structures may result in fracture porosity traps at depth.