John L. Sexton

Tectonic development of the New Madrid rift complex, Mississippi embayment, North America

Abstract Geological and geophysical studies of the New Madrid Seismic Zone have revealed a buried late Precambrian rift beneath the upper Mississippi Embayment area. The rift has influenced the tectonics and geologic history of the area since late Precambrian time and is presently associated with the contemporary earthquake activity of the New Madrid Seismic Zone. The rift formed during late Precambrian to earliest Cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions. The configuration of the buried rift is interpreted from gravity, magnetic, seismic refraction, seismic reflection and stratigraphic studies. The increased mass of the crust in the rift zone, which is reflected by regional positive gravity anomalies over the upper Mississippi Embayment area, has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex. The correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures. The slippage is due to reactivation of the structure by the contemporary, nearly E-W regional compressive stress which is the result of plate motions.

Seismic reflection profiling studies of a buried Precambrian rift beneath the Wabash Valley fault zone

Sixty‐eight kilometers of 12-fold seismic reflection data were collected in the Wabash River Valley of southwestern Indiana and southeastern Illinois to investigate the configuration of a basement structure inferred from regional gravity and magnetic anomaly data. The seismic profiles were also positioned to cross faults of the Wabash Valley fault system in a number of locations. Interpretation of the seismic reflection profiles and detailed gravity and magnetic profile data provides evidence for a series of northeasterly trending grabens in the basement. These grabens are filled with pre‐Mt. Simon layered rocks and are overlain by Paleozoic sedimentary rocks of the Illinois basin. Beneath the Wabash River near Grayville, Illinois, an interpreted graben (the Grayville graben) is approximately 15 km wide and contains about 3 km of fill. Individual boundary faults for the graben cut prominent reflectors within pre‐Mt. Simon rocks and display offsets of up to 500 m. The interpreted configuration of basement ...

Premine study of shallow coal seams using high-resolution seismic reflection methods

Geological investigations in the Illinois Basin coalfields have shown that significant differences in safe and economical exploitation of coal depends directly on accurate mapping of the roof rock overlying the seam, as well as on geological structures in the coal measures. In roof rock transition zones above the Herrin (No. 6) coal where the nonmarine Energy shale changes to the Anna shale, a change often occurs from low to high sulfur coal and from low to high stability roof rocks. In many instances, use of borehole data alone is inadequate to locate these features in advance of mining. High‐resolution seismic reflection data collected near Harco, Illinois were used as part of premine planning to help predict roof instability, areas of low sulfur coal, and geologic disturbances. Several faults, channels, and facies changes affecting the Herrin (No. 6) and the Springfield (No. 5) coal seams at depths of 137 m and 167 m, respectively, were interpreted and modeled. One‐ and two‐dimensional synthetic seismo...

Age estimation of the Cottage Grove Fault System (Illinois, U.S.A.) from high-resolution seismic reflection studies of coal measures

Abstract Geologic and high-resolution seismic reflection studies of Pennsylvanian coal deposits have provided new insights concerning the Cottage Grove Fault System (CGFS), a major east-west oriented wrench fault system in southern Illinois. The age of faulting has previously been estimated to be late Pennsylvanian to early Permian, based upon KAr dating of igneous intrusive rocks associated with subsidiary faults. Although these dates are accurate for intrusions and faulting at certain locations, the duration of Cottage Grove master fault zone or subsidiary faulting is still unclear. High-resolution seismic reflection profiles were collected approximately 6.4 km north of the master fault zone in the vicinity of mapped subsidiary faulting to study coal deposits. Several small-offset, high-angle normal and reverse faults were interpreted from the seismic records. Two small-offset fault zones observed on the seismic records, interpreted to be subsidiary faults of the CGFS, bound a gentle asymmetric anticline. Several small sandstone channels located on the down-thrown sides of these faults are interpreted from the seismic data. Vertical stacking of these sandstone channels and other sands within the coal measures is also observable in the correlated well data. Location of channel deposits suggests depositional control by these faults, indicating that subsidiary faulting was contemporaneous with sand deposition. Thus, subsidiary faulting of the CGFS occurred as early as middle Pennsylvanian time.

Geological Characterization of the Idalia Hill Fault Zone and Its Structural Association with the Commerce Geophysical Lineament, Idalia, Missouri

The Commerce geophysical lineament (cgl) is a 600-km-long, 5- to 10-km-wide, northeast-trending magnetic and gravity anomaly that extends from northeastern Arkansas to central Indiana. The seismogenic potential of the cgl is poorly known. However, recent geologic and geophysical studies along faults overlying the cgl support the notion that this prominent geophysical anomaly may be a source of future large-magnitude earthquakes in the central United States. Our geomorphic mapping, acquisition of seismic reflection, and ground-penetrating radar data, coupled with paleoseismic trenching and borehole information, collected at the South Holly Ridge site, near Idalia, Missouri, provide evidence of late Pleistocene to early Holocene deformation on the Idalia Hill fault zone. The Idalia Hill fault is a 0.5-km-wide zone of northeast-striking, near-vertical faults that directly overlie the cgl and coincide with the southeastern margin of the Bloomfield Hills. At the South Holly Ridge site, fault-related geomorphic features include a prominent, northeast- trending linear trough bounded on the southeast by a 2- to 3-m-high linear, north- facing escarpment and an apparent right-laterally deflected creek. These features, in turn, are aligned with regional northeast-trending linear valleys, escarpments, linear and deflected drainages, springs and bedrock notches mapped northeast and southwest of the site. Seismic reflection data across the Idalia Hill fault zone image near- vertical faults that displace Tertiary/Cretaceous and Tertiary/Quaternary reflectors and project upward to faults interpreted from ground-penetrating radar (gpr) surveys, and trench exposures. Together, these results suggest that deformation at the site is coincident with the cgl and is of primary tectonic origin. Trenches excavated across the linear escarpment at the South Holly Ridge site expose faulted and warped Tertiary deposits, Pleistocene gravel (reworked Mounds Gravel), the Sangamon Geosol, late Pleistocene Peoria Loess, and a late Pleistocene to early Holocene paleosol. Analyses of trench, borehole, and gpr data constrain the timing of the most recent event on the Idalia Hill fault to between the latest Pleistocene (about 18 ka) and early Holocene (pre-7.7 ka). Evidence for a penultimate event predates 23 to 18 ka. The timing of these events overlaps with late Pleistocene to early Holocene faulting and paleoliquefaction events evaluated elsewhere on northeast-striking faults overlying the cgl in southeastern Missouri. The earthquake- timing data from these sites in southeast Missouri indicate that the Commerce section of the cgl acted as a seismogenic source into at least the early Holocene and thus should be considered in future seismic-hazard analysis of the central United States.

A microprocessor interactive graphics engineering seismic and computing system

Abstract A general purpose microprocessor graphics computing system has been interfaced to a single channel signal enhancement seismograph to provide a seismic system useful for waveform analysis and geophysical engineering studies. A description is given of the system components, computer processing system organization and use of the system for seismic refraction data analysis. The system represents one application of the new microprocessor technology to the field of geophysics and geotechnical engineering.

Georadar Investigation of Karst in a Limestone Quarry near Anna, Illinois

SUMMARY Clay-filled karst was encountered during normal operations of a limestone quarry near Anna, Illinois. Attempts to determine karst extent with a drill rig became too dangerous. Consequently, a ground-penetrating radar (GPR) system was used to map karst extent and to determine any possible relationship to fractures observable along quarry walls. Georadar profiles were located adjacent to major fractures to intersect exposed portions of the karst feature. Reflector velocity information was obtained using the commonmidpoint (CMP) method at selected locations along the radar profiles. Radar profiles were collected using 50 and 100 MHz antennae. Karst features filled with sediment and air were imaged remarkably well on georadar data using both antennae. Stratigraphy associated with the karst includes 4 m of finegrained limestone overlain by 5-6 m of interbedded chert and limestone. This vertical transition in stratigraphy is observable on the processed georadar data, as are several fractures which intersect the main cavity. Fractures served as conduits for local groundwater, which resulted in carbonate dissolution, and for sediment which later filled the void. Karst extent was determined quickly and efficiently using simple ground-penetrating radar methods, and quarry operators were provided with information necessary to continue production of limestone materials.