Fluorspar Area Fault Complex Research Articles

Shear-Wave Birefringence Measurements in Faulted Near-Surface Sediment: Fluorspar Area Fault Complex, Central United States

Abstract The location and geometry of seismogenic faults are fundamental to understanding the regional seismic hazard; however, surface evidence for active faults in the seismically active central United States is rare, thus contributing to the uncertainty in the hazard estimation. Multicomponent S ‐wave reflection energy was acquired near the northern boundary of the New Madrid Seismic Zone across a well‐constrained N61°E‐oriented fault that extends above the top of Paleozoic bedrock and into near‐surface Quaternary sediment in order to evaluate the effectiveness of shear‐wave splitting measurements for determining azimuthal orientation of fault‐deformed unlithified sediment that lack a surface manifestation. The data were analyzed within the shear‐wave window (i.e., without wide‐angle distortion) using orthogonal source–receiver orientations. Normalized reflection energy spectra of the main and cross‐diagonal elements were rotated in 5° increments to determine fast (S1) and slow (S2) S ‐wave directions associated with the azimuthally anisotropic inclusion. The resultant natural coordinate system was N60°E and N30°W for S1 and S2, respectively. The S1 orientation correlated with the established N61°E fault strike. In addition, there was an average dynamic mistie of 11.3 ms that suggests 3.6% azimuthal anisotropy between the top of bedrock and ground surface. These results suggest that shear‐wave birefringence can be a useful tool for defining azimuthal orientation of neotectonic inclusions that lack surface or geomorphic expression, as well as for areas that may have limited accessibility (i.e., urban environments) for deploying multiple widespread geophysical surveys.

Site-specific Fault Rupture Hazard Assessment--Fluorspar Area Fault Complex, Western Kentucky

We evaluated post-Paleozoic sediments overlying a southerly projection of the Fluorspar area fault complex and coincident with an area of diffuse microseismicity for Quaternary deformation. Nearly 1 km of seismic reflection data were collected and interpreted for evidence of late Quaternary deformation. Five significant high-angle geophysical anomalies were interpreted to extend within approximately 7 m of the ground surface, near the upper limit of the seismic sampling. We subsequently collected 86 closely spaced, 9.1-m-deep, continuous cores above these anomalous features. We performed stratigraphic and chronological analyses on the cores to determine the presence or absence of structure above the geophysical anomalies and define the near-surface extent and age of deformation. Optical stimulated luminescence dates showed the sampled sediment age ranged between nearly 16 ka and greater than 125 ka. Interpretation of the resultant geologic cross-sections indicates identified stratigraphic anomalies were generally constrained to post-date a 53.6 to 75.5 ka loess deposit; however, no perceptible displacement was found at the base of younger loess dated between 16.6 and 23.5 ka.

Quaternary Fault Reactivation in the Fluorspar Area Fault Complex of Western Kentucky: Evidence from Shallow SH-wave Reflection Profiles

Shallow shear-wave seismic-reflection profiles were collected over the southwestern projection of the Fluorspar Area fault complex into the northern Jackson Purchase region of western Kentucky. The area lies at the northern end of the sediment-filled Mississippi embayment, where the Paleozoic carbonate rocks are masked by a relatively thin, approximately 100 m sequence of nonlithified Cretaceous, Tertiary, and Quaternary sediments. The interpreted profiles imaged clear evidence of fault and apparent fold propagation into the near-surface Quaternary units. The profiles also showed evidence of various structural styles associated with episodic movement. The exact timing of the latest tectonic episode exhibited on the profiles is not known because of the lack of more accurate stratigraphic detail coincident with the lines. However, physical evidence of Quaternary deformation less than 10 m below the ground surface, along with the instrumentally recorded seismic events located in the immediate vicinity of the study area, emphasizes the problematic nature of these fault segments for the design of critical or high-hazard structures.

Integrated Geological and Geophysical Study of Neogene and Quaternary-age Deformation in the Northern Mississippi Embayment

A program of shallow drilling, trenching, outcrop mapping, and seismic-reflection acquisition in southern Illinois, just north of the New Madrid seismic zone, has produced the most comprehensive documentation to date of Quaternary-age faulting in the northern Mississippi embayment. Detailed structural cross-sections over five northeast-trending faults that continue from the Fluorspar Area fault complex southward into the embayment (Massac County, Illinois) indicate narrow grabens into which latest Cretaceous, Paleocene, and Eocene sediments were dropped and protected from erosion. The bounding faults were active during Neogene through middle Pleistocene time; in the deepest graben, the Late Miocene-Pleistocene-age Mounds Gravel is downthrown 150 m. Notably, definitive faulting of Wisconsinan loess or Holocene alluvium is not observed at any site, which would indicate that the faults have been inactive for at least 55 ka (basal loess ages) to 128 ka (youngest Illinoian age). Seismic profiles reveal that faults in Quaternary sediments penetrate Paleozoic bedrock, thus indicating that these are tectonic faults and not products of landsliding, solution collapse, or other merely near-surface processes. The faults exhibit vertical to steeply dipping normal and occasional reverse displacements that outline a variety of structures, including a series of narrow grabens. Tilting or folding of strata associated with faulting is also locally observed. At one site, observed faulting is superimposed over a prominent bedrock anticline, which may itself be a result of the Quaternary deformation that created the steep normal and reverse faults. Our results indicate that shallow sediment and bedrock faulting extends beyond the localized fault zones, implying that Quaternary deformation of the northern Mississippi embayment is more pervasive than formerly believed. The trend of Quaternary-age structures in southern Illinois is in line with the northeast-trending New Madrid seismic zone and is parallel to other geologically active features in the region, such as the Commerce geophysical lineament; the lack of observed Holocene movement, however, suggests that the faults in our study area are not currently active and thus may not contribute to seismic hazard. We propose a dynamic structural model that suggests a mechanism by which seismicity and active (Holocene) faulting have shifted within the central Mississippi Valley (away from the Fluorspar Area fault complex) over the last several 10,0009s of years.

Seismic Reflection Images of Shallow Faulting, Northernmost Mississippi Embayment, North of the New Madrid Seismic Zone

High-resolution seismic reflection surveys document tectonic faults that displace Pleistocene and older strata just beyond the northeast termination of the New Madrid seismic zone, at the northernmost extent of the Mississippi embayment. These faults, which are part of the Fluorspar Area fault complex in southeastern Illinois, are directly in line with the northeast-trending seismic zone. The reflection data were acquired using an elastic weight-drop source recorded to 500 msec by a 48-geophone array (24-fold) with a 10-ft (∼3.0 m) station interval. Recognizable reflections were recorded to about 200 msec (100–150 m). The effects of multiple reflections, numerous diffractions, low apparent velocity (i.e., steeply dipping) noise, and the relatively low-frequency content of the recorded signal provided challenges for data processing and interpreting subtle fault offsets. Data processing steps that were critical to the detection of faults included residual statics, post-stack migration, deconvolution, and noise-reduction filtering. Seismic migration was crucial for detecting and mitigating complex fault-related diffraction patterns, which produced an apparent `folding' of reflectors on unmigrated sections. Detected individual offsets of shallow reflectors range from 5 to 10 m for the top of Paleozoic bedrock and younger strata. The migrated sections generally indicate vertical to steeply dipping normal and reverse faults, which in places outline small horsts and/or grabens. Tilting or folding of stratal reflectors associated with faulting is also locally observed. At one site, the observed faulting is superimposed over a prominent antiformal structure, which may itself be a product of the Quaternary deformation that produced the steep normal and reverse faults. Our results suggest that faulting of the Paleozoic bedrock and younger sediments of the northern Mississippi embayment is more pervasive and less localized than previously thought.

Quaternary grabens in southernmost Illinois: deformation near an active intraplate seismic zone

Narrow grabens displace Quaternary sediments near the northern edge of the Mississippi Embayment in extreme southern Illinois, east-central United States. Grabens are part of the Fluorspar Area Fault Complex (FAFC), which has been recurrently active throughout Phanerozoic time. The FAFC strikes directly toward the New Madrid Seismic Zone (NMSZ), scene of some of the largest intra-plate earthquakes in history. The NMSZ and FAFC share origin in a failed Cambrian rift (Reelfoot Rift). Every major fault zone of the FAFC in Illinois exhibits Quaternary displacement. The structures appear to be strike-slip pull-apart grabens, but the magnitude and direction of horizontal slip and their relationship to the current stress field are unknown. Upper Tertiary strata are vertically displaced more than 100 m, Illinoian and older Pleistocene strata 10 to 30 m, and Wisconsinan deposits 1 m or less. No Holocene deformation has been observed. Average vertical slip rates are estimated at 0.01 to 0.03 mm/year, and recurrence intervals for earthquakes of magnitude 6 to 7 are on the order of 10,000s of years for any given fault. Previous authors remarked that the small amount of surface deformation in the New Madrid area implies that the NMSZ is a young feature. Our findings show that tectonic activity has shifted around throughout the Quaternary in the central Mississippi Valley. In addition to the NMSZ and southern Illinois, the Wabash Valley (Illinois–Indiana), Benton Hills (Missouri), Crowley's Ridge (Arkansas–Missouri), and possibly other sites have experienced Quaternary tectonism. The NMSZ may be only the latest manifestation of seismicity in an intensely fractured intra-plate region.

Tertiary and Quaternary tectonic faulting in southernmost Illinois

Tertiary and/or Quaternary tectonic faulting is documented in three areas of southernmost Illinois: the Fluorspar Area Fault Complex (FAFC) in Pope and Massac Counties, the Ste. Genevieve Fault Zone (SGFZ) in Alexander and Union Counties, and the Commerce Fault Zone (CFZ) in Alexander County. In the FAFC, faults that strike NE and NNE displace Mounds Gravel (late Miocene to early Pleistocene) and, locally, the Metropolis terrace gravel (Pleistocene; pre-Woodfordian). No Woodfordian or younger deposits are deformed. Faults typically outline narrow, linear grabens that formed under tension with a component of strike slip. North-south to NW-trending vertical faults near the southeast end of the SGFZ displace Eocene sediments. Again, faults outline narrow grabens and show indications of strike slip. Deformed Quaternary sediments have not been observed. The CFZ, which trends northeast, displaces Mounds Gravel in Illinois and units as young as Peoria Silt (Woodfordian) in Missouri. Quaternary movement has been interpreted as right-lateral strike-slip. The CFZ coincides with a subtle gravity and magnetic lineament and seems to reflect a major feature in the basement. Surface expression in Illinois is subtle, but mafic and ultramafic intrusions, hydrothermal alteration and small faults align with the Commerce geophysical lineament. Earthquake foci in Missouri and Illinois lie on or close to the CFZ; some focal mechanisms fit the fault trend. Among these structures, only the CFZ exhibits slip that conforms to the current stress field (principal compressive stress axis E-W to ENE-WSW). Possibly, the stress field changed during Neogene time. Alternatively, high fluid pressures or local stress concentrations may have induced slip on less favorably oriented fractures. Tighter constraints are needed on timing, magnitude, and direction of Neogene displacement.

Proterozoic Structure, Cambrian Rifting, and Younger Faulting as Revealed by a Regional Seismic Reflection Network in the Southern Illinois Basin

Four high-quality seismic reflection profiles through the southern Illinois Basin, totaling 245 km in length, provide an excellent regional subsurface stratigraphic and structural framework for evaluation of seismic risk, hydrocarbon occurrence, and other regional geologic studies. These data provide extensive subsurface information on the geometry of the intersection of the Cambrian Reelfoot and Rough Creek rifts, on extensive Proterozoic reflection sequences, and on structures (including the Fluorspar Area Fault Complex and Hicks Dome) that underlie a transitional area between the well-defined New Madrid seismic zone (to the southwest) and a more diffuse area of seismicity in the southern Illinois Basin. Our principal interpretations from these data are listed here in order of geologic age, from oldest to youngest: 1. Prominent Proterozoic layering, possibly equivalent to Proterozoic (∼1 Ga) Middle Run Formation clastic strata and underlying (1.3–1.5 Ga) volcanic rocks of the East Continent rift basin, has been strongly deformed, probably as part of the Grenville foreland fold and thrust belt. 2. A well-defined angular unconformity is seen in many places between Proterozoic and Cambrian strata; a post-Grenville Proterozoic sequence is also apparent locally, directly beneath the base of the Cambrian. 3. We infer a major reversal in Cambrian rift polarity (accommodation zone) in the Rough Creek Graben in western Kentucky. 4. Seismic facies analysis suggests the presence of basin-floor fan complexes at and near the base of the Cambrian interval and within parts of a Proterozoic post-Grenville sequence in several parts of the Rough Creek Graben. 5. There is an abrupt pinchout of the Mount Simon Sandstone against crystalline basement beneath the Dale Dome (near the Texaco no. 1 Cuppy well, Hamilton County) in southeastern Illinois, and a more gradual Mount Simon pinchout to the southeast. 6. Where crossed by the seismic reflection line in southeast Illinois, some faults in the Wabash Valley Fault System produce discrete offset in Ordovician and younger strata only; one of the Wabash Valley faults cuts the top of the Precambrian on this seismic profile. 7. The data show clear evidence of late Paleozoic reverse faulting along both boundaries of the Rough Creek Graben in western Kentucky, although significant unreactivated Cambrian rift-bounding faults are also preserved. 8. Chaotic reflection patterns in the lower and middle Paleozoic strata near Hicks Dome, southern Illinois, are related to a combination of intrusive brecciation, intense faulting, and alteration of carbonate strata by acidic mineralizing fluids, all of which occurred in the Permian. 9. Late Paleozoic(?) reverse faulting is interpreted on one flank of the Rock Creek Graben, southern Illinois. 10. Permian and Mesozoic(?) extensional faulting is clearly imaged in the Fluorspar Area Fault Complex; neotectonic studies suggest that these structures were reactivated in the Quaternary.

Boundary Separating the Seismically Active Reelfoot Rift from the Sparsely Seismic Rough Creek Graben, Kentucky and Illinois

The Reelfoot rift is the most active of six Iapetan rifts and grabens in central and eastern North America. In contrast, the Rough Creek graben is one of the least active, being seismically indistinguishable from the central craton of North America. Yet the rift and graben adjoin. Hazard assessment in the rift and graben would be aided by identification of a boundary between them. Changes in the strikes of single large faults, the location of a Cambrian transfer zone, and the geographic extent of alkaline igneous rocks provide three independent estimates of the location of a structural boundary between the rift and the graben. The boundary trends north-northwest through the northeastern part of the Fluorspar Area Fault Complex of Kentucky and Illinois, and has no obvious surface expression. The boundary involves the largest faults, which are the most likely to penetrate to hypocentral depths, and the boundary coincides with the geographic change from abundant seismicity in the rift to sparse seismicity in the graben. Because the structural boundary was defined by geologic variables that are expected to be causally associated with seismicity, it may continue to bound the Reelfoot rift seismicity in the future.

Structural Geology of Shawneetown Fault Zone, Southeastern Illinois: ABSTRACT

Vertical movements of crustal blocks along the narrow east-west-trending Shawneetown fault zone in southeastern Illinois occurred between Early Permian and Late Cretaceous. The main blocks moved vertically and returned to roughly their original positions so that strata now show little relative offset across the fault zone. However, individual faults with displacements up to 3,500 ft (1,070 m) bound narrow slices of steeply tilted or overturned strata resulting in a juxtaposition of Kinderhookian (Lower Mississippian) and Upper Devonian strata with Lower Pennsylvanian strata. The bedrock is intensely fractured, commonly brecciated, and cemented with either silica or calcite. Slickensides and mullion display various orientations within the zone and on individual outcrops. T e dominant movement, however, appears to be vertical with no evidence for significant strike-slip movements. Pleistocene deposits do not exhibit offsets across the fault zone, indicating that no tectonic activity has occurred since the beginning of the epoch. The trend of the fault zone changes abruptly from east-west in southern Gallatin and easternmost Saline Counties to south-southwest in southern Saline and northeastern Pope Counties, where it joins the Fluorspar area fault complex. Here the zone widens and develops a braided pattern as the amount of displacement along individual faults decreases. The Shawneetown fault zone and Fluorspar area fault complex in part are younger than the Cottage Grove fault system to the northwest and the Wabash Valley fault system to the north. The hope of finding structural traps near the junctions of the fault systems has spurred recent oil exploration in the area. End_of_Article - Last_Page 1457------------

New Burnside Anticline"--Part of Fluorspar Area Fault Complex?: ABSTRACT"

Field mapping in the Abbott Formation and examination of topographic lineaments in the Creal Springs, Stonefort, Eddyville, and Harrisburg Quadrangles (southeastern Illinois) reveal the New Burnside anticline and its northeastern extension, the Stonefort anticline to be a single, extensively faulted structure. Interpretation of this evidence also leads to the conclusion that this is a fault-block structure rather than an anticline. Trending notheast-southwest, the structure seems to be the northwesternmost extent of the Fluorspar Area fault complex. The authors found evidence for two episodes of faulting. The first involved northeast-trending, high-angle faults similar to those in the known Fluorspar complex to the southeast. Faults on the northeast (Stonefort antilcine) step down toward the center of the structure, forming a graben. Vertical movement also occurred to the southwest (New Burnside anticline), but the structure in this vicinity is a horst with some blocks tilted. As with other faults in the Fluorspar complex, horizontal slickensides are present locally. The second episode of movement occurred along northwest-southeast-trending strike-slip faults that offset the northeast-trending high-angle faults. This second phase of faulting may correspond with previously reported reactivation of northwest-trending faults elsewhere in the Fluorspar Area fault complex.

Tectonic History of Southeastern Illinois: ABSTRACT

Recurrent movements on the northeast-trending Reelfoot rift and west-trending Rough Creek fault zone dominated southeastern Illinois tectonic history. Early Cambrian rifting along both zones created deep trenches that began to fill with sediments. Intermittent movements continued, but faults were quiescent by the Mississippian. Then renewed extension on the Reelfoot rift in the Early Permian produced high-angle normal faults in the Wabash Valley fault system and Fluorspar area fault complex, and the right-lateral Cottage Grove fault system. Igneous intrusions accompanied this action: upwelling magma formed Omaha dome; Hicks dome and associated concentric and radial faults appear to have been formed by explosive igneous activity. After the Early Permian, recurrent up-and-down movements of several thousand feet reactivated the fluorspar area fault complex and created the present day Rough Creek and Shawneetown fault zones. Blocks bordering faults returned roughly to their original positions by the Late Cretaceous, leaving narrow slices of rock upthrown and downthrown along faults. Faults in Illinois probably have been inactive since the Cretaceous Period, although the Reelfoot rift south of Cairo has been reactivated. Earthquakes in Illinois today apparently are caused by local east-west horizontal compressional stresses not related to known bedrock faults. End_of_Article - Last_Page 1458------------