Stephen C. Hook

Rates of folding and faulting determined from growth strata

Many upper crustal folds on the scale of 1–10 km in compressive mountain belts grow by kink-band migration as a result of fault-bend, fault-propagation, or box folding. One or both kink-band boundaries sweep through the rock as the kink bands widen during fold growth. The kink bands typically have a constant width in pregrowth strata, which are strata that existed before deformation, whereas we predict — and observe on seismic lines — an upward decrease in kink-band width within the stratigraphic sequence deposited during fault slip and associated fold growth -here called growth strata. In fact this growth stratigraphic sequence provides a complete, decipherable record of the kinematics of deformation, much in the same way that sea-floor magnetic anomalies provide a decipherable record of plate kinematics. It is the continual addition of material that provides the detailed record of motion in both cases. In the simplest folds the kink band has a constant width within the pregrowth strata, narrows upward through the growth strata, and finally has a zero width at the top of the growth stratigraphy.

Structural Trend Analysis by Axial Surface Mapping

We present a new method of structural trend analysis in which axial surfaces (fold hinges) are mapped through grids of seismic reflection profiles and balanced forward models. Axial surface maps (1) define fold trends, lateral terminations, and closure in structure, (2) constrain underlying fault geometry and slip, and (3) highlight and connect regions that can be interpreted by a series of balanced models or cross sections. Maps of folds in southern California and northeastern Pennsylvania are compared with forward model solutions and used to construct balanced three-dimensional structural interpretations using fault-bend fold theory. Axial surface maps generated from balanced fault-bend fold models define theoretical relationships between map patterns and structural geo etry and fault slip. Kink-band widths between paired axial surfaces in map view are related to underlying fault slip. Lateral variations in fault slip, which can be identified on axial surface maps, may cause fold plunge and provide lateral closure. Lateral changes in subsurface fault geometry, which include lateral and oblique ramps, are also reflected by terminations, bends, or offsets of axial surfaces in map view. Based on these relationships between folding and faulting, we develop map-view interpretations and balanced cross sections across structures that are consistent with the geology along trend.

A Forward Modeling Strategy for Balancing Cross Sections (1)

A strategy to balance cross sections of complex structures is documented and illustrated by the interpretation of a compressional structure in the deep-water Gulf of Mexico. The strategy is applicable to structures formed in sedimentary rocks under low temperatures in both compressional and extensional environments, and involves the comparison of the observed structure with simple, balanced, forward models. Forward models generated using fault-related fold theory help in understanding the processes and kinematics involved in the deformation. Further, forward models are completely constrained and easy to balance, whereas it is difficult to balance data. Therefore, forward models are useful in evaluating ideas without completely solving the structure. Models are constructed assuming parallel behavior (preservation of layer thickness, no net distortion where layers are horizontal, and conservation of bed length). Unmetamorphosed, sedimentary rocks are generally observed to deform obeying the assumptions of parallel behavior in field, map, well, and seismic data.

Extensional Fault-Bend Folding and Synrift Deposition: An Example from the Central Sumatra Basin, Indonesia

We describe an analytical method for interpreting the geometry and structural history of asymmetric half grabens in rift basins with extensional fault-bend fold theory. Using seismic reflection profiles from the Central Sumatra basin and balanced forward models, we show how local variations in tectonic subsidence relative to deposition rates yield distinctive patterns of folded synrift strata and unconformities that record basin history. If the deposition rate exceeds the local subsidence rate, folded growth strata form upwardly narrowing kink bands that have been described previously as growth triangles. In contrast, if the deposition rate is less than or equals the local subsidence rate, growth strata are folded and truncated at the surface on half-graben margins. Subsequent increases in deposition rate relative to subsidence rate form angular unconformities near the half-graben margins. These unconformities develop without the necessity of erosion and are folded by continuing fault slip. Strata above and below the unconformities are concordant in the deeper parts of the half grabens. Thus, angular unconformities on half-graben margins are helpful for defining sequence boundaries that may reflect changes in deposition and tectonic subsidence rates. In addition, fault-bend fold interpretations yield fault geometry and measures of horizontal extension, both of which control three-dimensional half-graben geometry ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received April 4, 1995; revised manuscript received June 4, 1996; final acceptance October 23, 1996. 2Texaco Exploration and Production Technology Dept., 3901 Briarpark, Houston, Texas 77042. 3PT CALTEX Pacific Indonesia, Rumbai, Pekanbaru 28271, Indonesia. We thank Hongbin Xiao and John Suppe for helpful insights into their extensional fault-bend fold theory, which provided the foundation for this work. Exceptional reviews by M. Scott Wilkerson and Walter F. Dula, Jr., improved the manuscript. In addition, discussions with Karsani Aulia, Richard E. Bischke, Peter A. Brennan, Chris D. Connors, Paul W. Genovese, Tom L. Heidrick, and Elizabeth A. Lorenzetti provided insights into our structural interpretations and presentation of the theory. Seismic reflection data were provided by PT CALTEX Pacific Indonesia.

Upper Cretaceous Stratigraphy and Paleontology, Sevilleta Grant near La Joya, Socorro County, New Mexico: ABSTRACT

More than 1,300 ft (396 m) of Upper Cretaceous rocks overlies shales of the Upper Triassic Dockum Formation and are exposed on the Sevilleta Grant, near La Joya, Socorro County, New Mexico. The Upper Cretaceous sequence extends from the Dakota Sandstone to the Dilco Member of the Crevasse Canyon Formation and consists largely of shales and sandstones. The included Tres Hermanos Sandstone has been raised to formational status by Hook and Cobban and includes the lowermost Atarque Member, overlain by the Carthage Member and the Fite Ranch Member, respectively. The Atarque and Fite Ranch Members consist of sandstones; the Carthage Member consists of shales and sandstones and contains two coal beds. The Atarque Member is a moderately sorted, calcareous sandstone which coarsens upward with small- and medium- End_Page 554------------------------------ sized tangential, wedge and trough sets of cross-beds. The Atarque Member is about 11 ft (3.4 m) thick and contains fossil-rich lenses of poorly sorted, dark yellow-brown-weathering sands that are friable to moderately indurated and calcite cemented. Shale galls are present in the fossil-rich lenses and, with wood fragments, abundant turtle bone fragments, some crocodile tooth and scute fragments, amid vertebrae and teeth indicate a nearshore environment with a nearby source of fresh water. Selachian teeth and probable coprolites are richly varied and abundant. The following genera have been recognized: Hybodus, Lonchidion, Squalicorax, Squatina, Brachaelurus, Scapanorhynchus, Odontaspis, Cretoxyrhina, Cretolamna, Plicatolamna, Paranomotodon, Ischyrhiza, Ptychotrygon, Rhombodus, as we l as several as yet indeterminate genera. The dominant invertebrate genus is the oyster Crassostrea although at least two genera of gastropods are present. End_of_Article - Last_Page 555------------