Late Paleozoic Ouachita Orogeny Research Articles

Late Paleozoic Flexural Extension and Overprinting Shortening in the Southern Ozark Dome, Arkansas, USA: Evolving Fault Kinematics in the Foreland of the Ouachita Orogen

AbstractFaults and folds on the southern flank of the Ozark dome in northern Arkansas, USA, record flexural extension in a foreland area followed by shortening in response to the late Paleozoic Ouachita orogeny. Map‐scale structures and an analysis of fault‐slip data collected systematically during geologic mapping demonstrate that most deformation in the area accommodated north‐south extension as the southern margin of Laurentia was flexed beneath the thrust load of the Ouachita belt, probably during Middle Pennsylvanian. Extension was concentrated in northeast‐ and west‐northwest‐trending structural zones having sets of discontinuous, often en echelon normal and strike‐slip faults and associated monoclinal folds. Reactivation of basement weaknesses that underlie these zones is indicated by their close match to oblique‐rift models in which both the proportions of normal and strike‐slip faulting and the internal extension directions vary with orientation of the zones. Subsequent propagation of north‐south Ouachita shortening into the foreland formed small‐offset strike‐slip and sparse reverse faults that overprinted older extensional structures. Strike‐slip faults were concentrated in reactivated northeast‐trending structural zones. In two areas, reverse faults and local anticlines were developed in the footwalls of older normal faults, both near intersections of northeast‐ and west‐northwest‐trending structural zones. These are interpreted as areas of incipient inversion due to compressional stress concentrations at fault‐block corners. Spatial overlap of areas of north‐south shortening and fluid flux marked by silicification or lead‐zinc mineralization indicates that regional fluid flow of brines was coeval with and may have enhanced inversion during Late Pennsylvanian to early Permian.

Wilson Cycles, Tectonic Inheritance, and Rifting of the North American Gulf of Mexico Continental Margin

The tectonic evolution of the North American Gulf of Mexico continental margin is characterized by two Wilson cycles, i.e., repeated episodes of opening and closing of ocean basins along the same structural trend. This evolution includes (1) the Precambrian Grenville orogeny; (2) formation of a rift-transform margin during late Precambrian opening of the Iapetus Ocean; (3) the late Paleozoic Ouachita orogeny during assembly of Pangea; and (4) Mesozoic rifting during opening of the Gulf of Mexico. Unlike the Atlantic margins, where Wilson cycles were first recognized, breakup in the Gulf of Mexico did not initially focus within the orogen, but was instead accommodated within a diffuse region adjacent to the orogen. This variation in location of rifting is a consequence of variations in the prerift architecture of the orogens. The Appalachian-Caledonian orogeny involved substantial crustal shortening and formation of a thick crustal root. In contrast, the Ouachita orogeny resulted in minimal crustal shortening and thickening. In addition, rather than a crustal root, the Ouachita orogen was underlain by the lower plate of a relatively pristine Paleozoic subduction system that is characterized by a shallow mantle. A finite element model simulating extension on the margin demonstrates that this preexisting structure exerted fundamental controls on the style of Mesozoic rifting. The shallow mantle created a strong lithosphere beneath the orogen, causing extension to initiate adjacent to, rather than within, the orogen. On the Atlantic margins, the thick crustal root resulted in a weak lithosphere and initiation of extension within the interior of the orogen. Major features of the modern Gulf of Mexico margin, including the Interior Salt Basin, outboard unextended Wiggins arch, and an unusually broad region of extension beneath the coastal plain and continental shelf, are direct consequences of the prerift structure of the margin.

Mississippian Barnett Formation, Fort Worth Basin, Texas: Bulk geochemical inferences and Mo–TOC constraints on the severity of hydrographic restriction

Chemostratigraphic evidence from the Barnett Formation (Texas, USA) elucidates the restricted nature of the depositional environment in the Fort Worth Basin during the early progression of the late Paleozoic Ouachita Orogeny. In accord with recent lithostratigraphic and petrographic studies, stratigraphic bulk geochemical analyses reveal that the environment of deposition was anoxic to euxinic, sediment-starved, with relatively high rates of organic matter accumulation. Using an environmental proxy developed from the sediment geochemistry of modern anoxic silled basins, the stratigraphic concentrations of total organic carbon (TOC) and molybdenum (Mo) in the Barnett Formation reveal a high degree of subpycnoclinal water mass restriction and an extended timescale of deep-water renewal of at least 8 × 10 3 years and potentially as long as 2 × 10 4 years. An integrated assessment of elemental concentrations, the degree of pyritization, and TOC–S–Fe relationships reveals that severe Fe limitation controlled pyrite formation and the large excess of sulfide in the overlying water column. Mineralogical and elemental constraints suggest that, following sulfate reduction, less easily bio-extractable Fe(III) was bio-reduced under methanogenic conditions, consequently liberating Fe(II) which was subsequently incorporated into pore-water-formed dolomite (i.e., organogenic dolomite). The preserved Mo–TOC, TOC–S–Fe, and Fe-bearing mineral relationships collectively indicate that the protracted turnover rate and persistent sediment starvation resulted in a biologically inhospitable environment that limited the microbial consumption rate of organic carbon.

Early Paleozoic transform-margin structure beneath the Mississippi coastal plain, southeast United States

A geophysical transect across the central Gulf of Mexico coastal plain shows that the early Paleozoic continental margin of southern Laurentia is preserved in a nearly pristine state beneath younger strata that were emplaced during the late Paleozoic Ouachita orogeny and formation of the modern Gulf of Mexico coastal plain. The thickness of the crystalline crust decreases abruptly across the margin over a distance of ∼50 km, from 35 km beneath the Black Warrior foreland basin to 10 km beneath the Ouachita fold-and-thrust belt. This abrupt decrease in crustal thickness is similar to modern transform margins, but very different from most rifted margins, which display much more gradual transitions in crustal thickness. The geophysical data indicate an absence of synrift intrusive and volcanic rocks, underplated mafic rocks at the base of the crust, and abnormally thick oceanic crust adjacent to the margin. The lack of these features is also characteristic of modern transform margins. Combined with transects across the margin farther west, the data confirm previous suggestions that the central Gulf of Mexico coastal plain overlies an ∼800-km-long transform segment of the late Proterozoic–early Paleozoic southern Laurentian continental margin that extends continuously from western Arkansas to southeast Alabama.

Coordinated strike-slip and normal faulting in the southern Ozark dome of northern Arkansas: Deformation in a late Paleozoic foreland

Structures that formed on the southern flank of the Ozark dome, in the foreland of the late Paleozoic Ouachita orogeny, have received little modern study. New mapping of the western Buffalo River region of northern Arkansas identifies diversely oriented faults and monoclinal folds that displace the generally flat lying Mississippian Boone Formation over a 180 m elevation range. Kinematic measurements and spatial relations reveal the presence of both east-striking normal faults and broader northeast-striking dextral strike-slip fault zones that acted in a coordinated fashion to accommodate constrictional strain, in which north-south extension was balanced by vertical and east-directed shortening. North-south extension in the Buffalo River region probably reflects Pennsylvanian–Early Permian deformation within the flexural forebulge of the developing Ouachita orogeny, which closed progressively westward along the southern margin of the craton.

Tectonic history of the metamorphic basement rocks of the Sierra del Carmen, Coahuila, Mexico

The Ouachita interior zone, arching through central Texas into the Big Bend region of the Rio Grande, has been defined chiefly on the basis of well samples and its associated gravity high. The only known exposure of rocks of the interior zone lies at the base of the Sierra del Carmen normal fault escarpment under approximately 1100 m of Lower Cretaceous carbonates in northern Coahuila, Mexico. These lower greenschist facies metamorphic rocks are thinly interlayered graphitic muscovite schist, graphitic marble, and minor quartzite with abundant quartz veins. The dominant structures shown by the metamorphic rocks are isoclinal folds (F2) that have a well-defined axial planar foliation (S2). Earlier (F1) folds refolded by F2 are locally preserved. At the microscopic scale, S2 is a crenulation cleavage of the S1 foliation that has been enhanced by pressure solution. Feldspar porphyroblasts overgrow a folded graphitic S1 foliation. Throughout the area a second crenulation cleavage (S3) is present on the S2 foliation surface as an intersection lineation (L3). Subsequent Laramide regionalto outcrop-scale folds and associated thrusts overprint these ductile structures. Lower Cretaceous carbonates form a large, east-verging overturned to recumbent anticline-syncline fold pair with a northwest-trending fold axis. The S2 metamorphic foliations define an antiform that is nearly coincident in trend to that of the fold in the overlying carbonates, indicating that the metamorphic rocks were also folded by this regional structure. A Basin and Range normal fault is interpreted to have reactivated the Laramide thrust fault responsible for the contractional deformation. One to two normal faults cut the carbonates and metamorphic rocks and truncate the anticline-syncline fold pair. The maximum offset along the normal fault is 1600 m. Rb/Sr data from the muscovite schist gives a six-point, mineralÐwhole-rock isochron age of 277 ± 10 Ma, indicating that polyphase deformation and metamorphism occurred during the late Paleozoic Ouachita orogeny. An initial Sr ratio of 0.721 suggests an ancient source for these metasediments. Common Pb ratios of the metamorphic whole rock and galena from the Puerto Rico Mine are isotopically distinct from those of the North American craton, suggesting that they were derived from a nonÐNorth American source and thus may be equivalent to unmetamorphosed Ouachita flysch sediments. This indicates that the metamorphic rocks of the Ouachita interior zone were derived from an ancient allochthonous source that was deformed and metamorphosed during the late Paleozoic Ouachita orogeny.

Comparison of Maturation Data and Fluid-Inclusion Homogenization Temperatures to Simple Thermal Models

Time-temperature index (TTI) modeling is used to establish a simple theoretical thermal maturity for Paleozoic strata in central Kansas. These thermal maturation calculations are based on estimates of likely geothermal gradients and best knowledge of the tectonic history of the region, as derived from stratigraphic thicknesses and estimates of erosion at unconformities. Major uncertainties in the data for the TTI modeling are burial during Cretaceous time and geothermal gradient, thus several models were calculated in which ranges of these two variables were considered. Results of the thermal modeling are then compared to available data on the thermal maturation. These data are principally derived from subsurface samples, on which vitrinite-reflectance, pyrolysis, and fluid-inclusion analyses have been performed.
 Vitrinite-reflectance and Rock-Eval maturation measurements indicate that Middle and Upper Ordovician strata (i.e., Simpson, Viola, and Maquoketa formations) in the study area are in initial phases of oil generation. Maturation modeling can match the results of the organic analyses, but geothermal gradients and burial during the Cretaceous have to be maximized.
 Although the TTI modeling utilizing very high geothermal gradients and near-excessive thicknesses of Cretaceous strata can match the observed maturation, the modeled results are probably not correct because fluid-inclusion data from saddle dolomites from the Upper Ordovician Viola Limestone indicate this unit reached temperatures 50° C higher than the maximum modeled temperature. A thermal event is inferred to account for the excess maturation and elevated fluid-inclusion homogenization temperatures. This thermal event may be manifested in the erratic increase of vitrinite-reflectance with depth for post-Devonian strata, as well as for pyrolysis measurements in wells for which maturation profiles are available. Flow of heated water onto the cratonic shelf out of the Anadarko basin during the late Paleozoic Ouachita orogeny may be responsible for the maturation anomalies.

The paleohydrology of Southeast Missouri mississippi valley-type deposits; interplay of faults, fluids, and adjoining lithologies

Observations and geochemical data provide insight into the paleohydrology influencing development of Mississippi Valley-type deposits in southeast Missouri and provide clarification and justification of flow paths in light of proposals of fault hydrodynamics versus a continuous aquifer model. Faults are recognized as pathways for, as well as horizontal barriers to, fluid flow in a hydrologically compartmentalized region. These relations also indicate that at least two geochemically distinct fluids mineralized Late Cambrian hosts and that these fluids had a similar history of progressive undersaturation with respect to galena.An overview of the synthesis indicates that one of the fluids may have originated in the Reelfoot rift, whereas the other evolved in a recharge area stretching along the Ouachita front and exploited the Reelfoot rift. Northwest-striking faults in the Precambrian basement acted as channels of discharge for flow out of or through the Reelfoot rift. Fluid movement along these pathways was rapid and allowed fluids to reach ore hosts during a migration interval related to the late Paleozoic Ouachita orogeny. Vertical circulation along faults and fracture systems introduced fluids into the existing Paleozoic section. Dispersion into the Lamotte Sandstone, resulting from relative permeability differences between discordant subvertical faults and the adjoining quartzarenite, was restricted to a northwest-trending, fault-bounded corridor and was concentrated at the base of the Bonneterre Formation. Continued vertical movement of fluids maintained fracture networks as vertical pathways, during the period that circulation localized Mississippi Valley-type deposits in the Late Cambrian section.

Pb isotopes of ore deposits in Trans-Pecos Texas and northeastern Chihuahua, Mexico; basement, igneous, and sedimentary sources of metals

Tertiary ore deposits and prospects in Trans-Pecos Texas and adjacent Mexico have Pb isotope ratios that indicate probable sources of metals. Sulfides from these deposits have a wide range of Pb isotope ratios: 206 Pb/ 204 Pb, 17.74 to 20.04; 207 Pb/ 204 Pb, 15.46 to 15.69; 208 Pb/ 204 Pb, 37.52 to 40.37. In part, this variability reflects derivation of Pb from contrasting basement and Tertiary igneous rocks across a major lithospheric boundary between the North American craton on the northwest and terranes accreted during the late Paleozoic Ouachita orogeny to the southeast. Precambrian basement and Tertiary igneous rocks northwest of a northeast-trending zone through Trans-Pecos Texas and Chihuahua, Mexico, are less radiogenic than Phanerozoic and Precambrian basement and Tertiary igneous rocks to the southeast.Although isotopic ratios from ore deposits and prospects in the region generally reflect basement type, steep arrays on 206 Pb/ 204 Pb versus 207 Pb/ 204 Pb diagrams from some deposits in the northwest basement province indicate mixing with additional, more radiogenic, sources of Pb. We infer that these sources are local sedimentary rocks or basinal fluids. Vein- and porphyry-type prospects hosted by igneous rocks show the least influence from these radiogenic sources. They have Pb isotope ratios that fall within the range of Pb isotope ratios of local Tertiary igneous rocks. In contrast, red-bed-hosted Ag-Cu-Pb deposits in the northwest, with no apparent igneous source of fluids, generally have relatively high 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios, which are apparently inherited from their host sediments. Veins hosted by sedimentary rocks and high-temperature carbonate-hosted Ag-Pb-Zn deposits (manto, chimney, and skarn) have intermediate Pb isotope ratios that range between the lower 207 Pb/ 204 Pb igneous-hosted deposits and the higher 207 Pb/ 204 Pb sedimentary-hosted deposits. They appear to contain variable mixtures of Pb from mid-Tertiary igneous sources and sedimentary rocks. In the northwest basement province Ag-Pb-Zn deposits at Shafter, Texas, have low Pb isotope ratios derived from associated igneous rocks and indicative of underlying Precambrian crust with low 206 Pb/ 204 Pb and 207 Pb/ 204 Pb. Similar, but larger, deposits at Santa Eulalia, Chihuahua, contain a mixture of Pb from at least two sources. It appears that a low 207 Pb/ 204 Pb component associated with Tertiary igneous rocks and ultimately derived from Precambrian basement has mixed with a higher 207 Pb/ 204 Pb component derived from the sedimentary wall rock. Published fluid inclusion and S isotope studies also suggest that mixing of magmatic and sedimentary components is common in the genesis of high-temperature carbonate-hosted Ag-Pb-Zn deposits.

Contrasting styles of Pre‐Cenozoic and Mid‐Tertiary crustal evolution in northern Mexico: Evidence from deep crustal xenoliths from La olivina

The principal deep crustal rock types found at the La Olivina xenolith locality in southeastern Chihuahua, Mexico, are mafic granulites, paragneisses, and intermediate‐ to silicic‐composition orthogneisses. These granulite facies xenoliths are interpreted in terms of two age groups, pre‐Cenozoic and mid‐Tertiary, based on previous ion probe dating of zircons from the xenoliths and on isotopic comparisons of the xenoliths to rocks of known age. The mafic granulites have Pb, Nd, and Sr isotopic compositions identical to those of Oligocene volcanic rocks from the La Olivina region. Compositionally, they are olivine‐normative gabbroic cumulates, and they precipitated from two or more mid‐Tertiary basalt to dacite or rhyolite assimilation/fractional crystallization series. Mineral assemblages in the xenoliths record pressures of ≤7.2 kbar or depths of <25 km. If these are the maximum pressures the rocks experienced and if the crust was >35 km thick in Oligocene time as inferred from regional tectonic considerations, then the mafic granulites cannot be samples of basaltic magmas underplated near the crust‐mantle boundary. The cumulate protoliths for the mafic granulites probably formed in magma chambers well above the Moho. The mafic granulites are plausibly representative of kilometers of new crust that formed in mid‐Tertiary time beneath the extensive ignimbrite fields of Mexico. Most orthogneiss xenoliths are pre‐Cenozoic, and they are rocks associated with the late Paleozoic Ouachita Orogeny and older events (e.g., Proterozoic basement and Paleozoic arc rocks). The Ouachita Orogeny was a collision event involving North America and a continental plate to the south, and the results of this study indicate that La Olivina is located above the southern plate. The paragneiss xenoliths overlap in isotopic composition with Carboniferous flysch units exposed in the Marathon uplift of west Texas. These sedimentary rocks and the sedimentary protoliths of the paragneiss xenoliths were derived from the southern plate and deposited before the orogeny in the ocean basin that separated North America and the southern plate. The paragneisses were not metamorphosed to granulite facies until mid‐Tertiary time. Pre‐Cenozoic and mid‐Tertiary crustal evolution followed very different paths in northern Mexico. For example, Nd isotopic evidence for crustal recycling is much more evident in rocks associated with the Paleozoic convergence than in rocks produced during mid‐Tertiary magmatism. Furthermore, mafic rocks are very rare in the pre‐Cenozoic xenolith suite, but they dominate the mid‐Tertiary one. These contrasting characteristics of the pre‐Cenozoic and mid‐Tertiary xenolith suites are interpreted to reflect differences in tectonic environment and crustal thickness. Preceding the collision event, the southern plate had an Andean‐type margin, and the abundant evidence for crustal recycling during this time implies that the crust was thick. The rarity of pre‐Cenozoic mafic xenoliths suggests that Proterozoic and Paleozoic lower crust may have delaminated in response to crustal overthickening associated with the Ouachita collision event.

Deep Exploration Possibilities along the Ouachita Trend

The late Paleozoic Ouachita orogeny produced a sinuous deformed belt that extends from Alabama through Mississippi, Arkansas, Oklahoma, Texas, and finally southward into Mexico. Recent geophysical studies have provided important new data that make it possible to reevaluate exploration possibilities along the Ouachita trend. A deep seismic study alo profile extending from the Ouachita Mountains in southwestern Arkansas into northwestern Louisiana has provided a surprisingly clear image of the early Paleozoic continental margin in the area. This margin is largely undeformed and suggests that the Ouachita orogeny involved little if any shortening of continental crust. Along with gravity data, this result implies that Ouachita thrusts are thin-skinned structures that include features such as the Benton and Broken Bow uplifts. Using these results as a base from which to build, the author has been able to trace this margin southeastward into Mississippi and westward through Texas and into Mexico. Although some production has been established in Ouachita facies rocks, gravity anomalies and limited amounts of seismic data suggest that two subthrusts plays have potential. In addition, there are areas where completely new subthrusts basins are possible. The second play is the early Paleozoic continental margin and rift basins that formed duringmore » the early stages of its development. His results to data indicate that these features are not greatly deformed and are at economically drillable depths in many areas. However, the area south of the Paleozoic continental margin should be avoided because the thickness of Ouachita facies rocks exceeds 10 km.« less

Regional Gravity and Tectonic Study of the Ouachita System: ABSTRACT

A regional study of the Ouachita system has been undertaken using gravity and deep-drilling data. An integrated analysis of gravity maps, computer models, and geologic data suggests that the crust beneath the Gulf coastal plain is variably attenuated continental crust. The transition zone between this crust and the craton, which is marked by a steep gravity gradient that lies along the trend of the Ouachita system, may have been created by Mesozoic reactivation of a crustal zone of weakness inherited from a plate collision during the late Paleozoic Ouachita orogeny. Gravity minima along the frontal zone of the Ouachita system are due to a thick sedimentary rock pile in conjunction with a gulfward dipping intracrustal or crust/mantle boundary in some areas. The arcuate Oua hita gravity maximum is the result of denser (metamorphic) rocks of the interior zone, with uplifts and upper crustal mafic intrusions making contributions in some areas. Gravity anomalies in the Gulf coastal plain are a combined effect of variable crustal attenuation, subsidence, and densification of the upper crust. Maxima in the southern Oklahoma aulacogen area are the result of uplifts and upper crustal mafic intrusions and/or lower crustal upwarps. End_of_Article - Last_Page 497------------