J. Fred Read

Rapid onset of late Paleozoic glaciation on Gondwana: Evidence from Upper Mississippian strata of the Midcontinent, United States

Direct evidence of the late Paleozoic glaciation of Gondwana from glacial deposits suggests that geographically extensive continental glaciation began some time in the Namurian (Late Mississippian). However, the timing and characteristics of the onset of glaciation are poorly understood because of a lack of reliable paleontological control and reworking of initial glacial deposits by subsequent glacial advances. Indirect evidence of glaciation preserved in unconformity-bounded, low-latitude ramp sequences in the Illinois basin, United States, suggests that geographically extensive continental glaciation of Gondwana actually began in the late Visean. An abrupt change from carbonate-dominated sequences bounded by disconformities with little evidence of erosion to mixed carbonate-siliciclastic sequences bounded by unconformities with deep incised valleys was likely produced by a three-fold increase in the magnitude of eustatic sea-level fluctuations. The increase in the magnitude of sea-level fluctuations was likely driven by an equally abrupt increase in ice volume and marks the onset of the geographically extensive late Paleozoic glaciation of Gondwana. A possible explanation for the rapid onset of glaciation is the closing of the equatorial seaway between Laurussia and Gondwana. Closing of this seaway would have led to an abrupt change in oceanic and atmospheric circulation patterns that could have initiated major continental glaciation in the Southern Hemisphere.

Relation of eustasy to stacking patterns of meter-scale carbonate cycles, Late Cambrian, U.S.A.

ABSTRACT An interbasinal study of Late Cambrian cyclic carbonate successions in the Appalachian and Cordilleran passive margins suggests that superimposed orders of eustasy controlled the development of large-scale depositional sequences and the component peritidal to subtidal meter-scale cycles that comprise them. The focus of this paper is on the small-scale cyclicity, its probable control by Milankovitch-forced sea-level oscillations, and how stacking patterns of meter-scale cycles can be used to define internal components of larger-scale sequences and estimate variations in relative sea level. Fining-upward peritidal cycles showing evidence of episodic emergence grade seaward into coarsening-upward subtidal cycles which lack evidence of emergence and form a continuum across the Cambrian carbonate platforms. Eustasy appears to exert the dominant control on the simultaneous development of peritidal and subtidal cycles on Late Cambrian carbonate platforms. Evidence for Milankovitch forcing of glacio-eustatic sea-level oscillations is provided by a 4:1 bundling of fifth-order meter-scale cycles ( 96 ky) within fourth-order cycles spanning tens of meters ( 440 ky) within the Big Horse Member of the Orr Formation in the House Range of Utah. The 4:1 bundling may manifest the short e centricity to long eccentricity ratio of the Milankovitch astronomical rhythms. Systematic changes in the stacking patterns of meter-scale cycles can be used in conjunction with Fischer plots to define long-term sea-level cycles. On Fischer plots of peritidal cyclic successions, long-term relative sea-level rises are characterized by thick, subtidal-dominated cycles with thin laminite caps. Long-term relative sea-level falls are defined by stacks of thin, laminite-dominated cycles that show brecciated cycle caps and quartz sands toward the relative sea level lowstand. On Fischer plots of dominantly subtidal cyclic successions, long-term sea-level rise is characterized by storm-dominated, open marine carbonate cycles or thick, deep ramp, shale-based cycles. Falling segments of the Fischer plot are characterized by thin, shallow subtidal cycles compos d of restricted lithofacies. Cycle stacking patterns (parasequence sets) provide the crucial link between the meter-scale cycles (parasequences) and the larger scale sequences and their component systems tracts. One- and two-dimensional models of peritidal and subtidal cycle development indicate that, whereas peritidal cycle thickness is primarily controlled by accommodation space, deeper subtidal cycle thickness is primarily controlled by sedimentation rate. Lithofacies within peritidal cycles reflect the sedimentologic response to fluctuations in sea level; lithofacies within subtidal cycles may be related to fluctuations in the zones of fairweather and storm-wave reworking that oscillated in harmony with sea-level fluctuations and may have acted as a subtidal limit to upward aggradation. The 2-D modelling illustrates how stacked peritidal to deep subtidal carbonate cycles with thicknesses, compositions and stacking patterns similar to the Late Cambrian of North America can be generated by ilankovitch-driven composite eustasy.

Cyclic Ramp-to-Basin Carbonate Deposits, Lower Mississippian, Wyoming and Montana: A Combined Field and Computer Modeling Study

ABSTRACT The Lower Mississippian Lodgepole/lower Madison Formations (20-225 m thick) developed along a broad (> 700 km), storm-dominated pericratonic ramp. Three types of fifth-order upward-shallowing cycles are recognized across the ramp-to-basin transition. Peritidal cycles consist of very shallow subtidal facies overlain by algal-laminated tidal flat deposits, which are rarely capped by paleosol/breccia layers. Shallow subtidal cycles consist of stacked ooid grainstone shoal deposits, or deeper subtidal facies overlain by ooid-skeletal grainstone caps. Deep subtidal cycles located along the outer ramp consist of basal sub-storm wave base limestone-argillite, overlain by storm-deposited limestone, which are capped by hummocky stratified to massive skeletal-ooid grainstone. Deep subtidal c cles pass downslope into ramp-slope facies composed to rhythmically interbedded limestone and argillite, with local deep-water mud mounds; no upward-shallowing cycles occur within the ramp-slope facies. Average cycle durations calculated along the outer ramp are between 30-110 ky. The fifth-order cycles are stacked to form three third- to fourth-order depositional sequences, which are recognized by regional transgressive-regressive facies trends and cycles stacking patterns. Ramp-margin wedges (RMW) developed during long-term sea-level fall and lowstand and consist of cyclic crinoidal bank and oolitic shoal facies, which pass downdip into deep subtidal cycles. Transgressive systems tracts (TST), which onlapped the ramp during long-term sea-level rise, include thick shallow and deep subtidal cycles; peritidal cycles are restricted to the inner ramp. Highstand systems tracts (HST) developed during long-term sea-level highstand and fall, and along the ramp are composed of early HST shallow subtidal cycles overlain by late HST peritidal cycles; shallow through deep subtidal cycles characterize the HST along the ramp-slope. Two-dimensional computer modeling of the cyclic sequences suggests that for the assumed water depths of facies, fifth-order sea-level oscillations of at least 20-25 m were required to generate deep subtidal cycles along the ramp-slope. Synthetic sequences run with fifth-order sea-level amplitudes End_Page 1194----------------------- and TST deposition, a feature not observed in the actual sequences. Other factors, in addition to fifth-order sea-level oscillations, likely played a role in generating synchronous peritidal and deep subtidal cycles during HST deposition. These factors may include short-term climatic changes, which influenced the depths to storm-wave reworking. The moderate-amplitude sea-level oscillations suggested by the cyclic sequences may reflect the initial effects of Carboniferous glaciation that was occurring in Gondwana.

Late Middle to Late Ordovician seismites of Kentucky, southwest Ohio and Virginia: Sedimentary recorders of earthquakes in the Appalachian basin

Synsedimentary ball-and-pillow beds, breccias, and faults in late Middle to Late Ordovician foreland basin rocks of Kentucky, southwest Ohio, and Virginia indicate broad zones of seismicity near the Cincinnati arch and Taconic orogenic front during deposition. Earthquake-induced liquefaction formed seismites, that include ball-and-pillow beds and rare sedimentary breccias that are correlative over large areas (hundreds to thousands of square kilometers). Comparison of these features with liquefaction structures in Holocene sediments indicates that the Ordovician ball-and-pillow beds were probably generated by large earthquakes (magnitudes >6). The Ordovician seismites also provide information about epicenter location and the recurrence interval of large earthquakes in the Ordovician foreland basin. Some were produced by faulting in the foreland and accretionary prism. However, horizons of resedimented lithoclastic breccias in the Jeptha knob cryptoexplosive structure appear to correlate with several ball-and-pillow beds on Jessamine dome, along the Cincinnati arch, suggesting that some of the seismites may be genetically related to this enigmatic structure.

Ordovician metre-scale cycles: implications for climate and eustatic fluctuations in the central Appalachians during a global greenhouse, non-glacial to glacial transition

Abstract Metre-scale shallowing-upward cycles in Ordovician carbonates of the central Appalachian Basin record climate and eustatic fluctuations during a transition from Early Ordovician global greenhouse to Late Ordovician icehouse conditions. Peritidal facies and shale abundance suggest a long-term trend in this area from semi-arid (Early Ordovician) to more humid (Middle to early Late Ordovician) conditions, with a return to semi-arid conditions during the Late Ordovician. The climatic fluctuations were most likely produced by tectonics (uplift and erosion) related to Taconic orogenesis, plate motion of North America and the areal extent of water covering the shelf. Peritidal cyclic facies indicate that high-frequency relative sea level fluctuations were of small amplitude ( 20 m) relative sea-level fluctuations that decreased into the later Ordovician. If these sea-level changes are eustatic, then the increased amplitude may mark the early initiation of continental glaciation on Gondwana in the late Middle Ordovician, followed by waning of ice sheets prior to the latest Ordovician glaciation.

Eustatic control on early dolomitization of cyclic peritidal carbonates: Evidence from the Early Ordovician Upper Knox Group, Appalachians

The Early Ordovician Upper Knox Group is characterized by stacked meter-scale peritidal cycles that repeat at high frequencies (10 4 to 10 5 yr). Stacking patterns (stratigraphic trends in lithofacies and cycle thickness) of meter-scale cycles define five depositional sequences that, in conjunction with Fischer plots, delineate five long-term relative sea-level fluctuations during Upper Knox deposition. Intrabasinal and interbasinal correlation of Upper Knox Fischer plots suggests that the third-order sea-level events were eustatic. Meter-scale peritidal cycles likely formed in response to high-frequency, fourth-and fifth-order, eustatic sea-level fluctuations superimposed on these third-order sea-level events. Upper Knox cyclic carbonates are extensively dolomitized; as much as 85% of all dolomite is stratiform and consists of early dolomite exhibiting minor to extensive modification by burial dolomite. Synsedimentary dolomitization likely occurred in modified sea water during tidal-flat progradation governed by high-frequency sea-level events. This is suggested by the common association of dolomite with mud-cracked laminites and silicified evaporite nodules, the systematic decrease in dolomite abundance below laminite cycle caps, and the presence of dolomite clasts in regoliths veneering high-frequency cycle tops or in transgressive limestones of the overlying cycle. Dolomite distribution within depositional sequences shows a strong relationship to third- and fourth-order eustatic sea-level events, indicating that long-term eustasy also strongly controlled early dolomitization of Upper Knox carbonates. Mass-balance calculations show that the proposed sabkha model of dolomitization in concert with composite eustasy could generate stratiform dolomite of considerable vertical and lateral extent in peritidal cyclic carbonates. This reflects the duration of progradation and supratidal exposure (10 4 to 10 5 yr) available for dolomitization to proceed, and the broad zone of active dolomitization that would develop during continued progradation throughout each cycle period.

Fluid-Rock Interaction History During Stabilization of Early Dolomites, Upper Knox Group (Lower Ordovician), U.S. Appalachians

ABSTRACT The Lower Ordovician Upper Knox Group is characterized by shallowing-upward peritidal cycles that are extensively dolomitized to form massive dolomite. Most (85%) replacive dolomite is stratigraphically controlled, composed of very fine to medium crystalline, planar to nonplanar (early) dolomite composed of Zones 1, 2 and 3 dolomites. Remaining dolomite occurs as medium to coarsely crystalline, nonplanar (late) replacive dolomite (Zones 2 and 3 late dolomites) that selectively replaces limestone associated with stylolites and fractures or forms massive dolomite fronts. Complexly zoned (Zone 2 to 6) saddle dolomite cements make up less than 5% of all dolomite and primarily fill solution voids and fractures. Early dolomites are near stoichiometric and, relative to Early Ordovician marin carbonates, are similar to or significantly depleted in 18O and Sr, slightly to significantly enriched in Mn and Fe, and have similar 87Sr/86Sr values. Relative to early dolomites, late (Zones 2 and 3) dolomites are depleted in 18O and St, enriched in Mn and Fe, and have similar 87Sr/86Sr values. Early dolomites are interpreted to have formed from modified seawater during seaward progradation of tidal flats within each high-frequency (fifth- and fourth-order) cycle duration. Late dolomites, based on their texture and geochemistry, formed as a result of recrystallization of early dolomites, as direct replacement of limestone at elevated temperatures, and as cement in fractures and secondary solution voids. Covariant trends between textures and geochemical compositions for early dolomite indicate that presentday compositions of Knox early dolomite record a history of progressive diagenetic modification (stabilization or recrystallization), involving varying degrees of changes of textural and geochemical compositions during multiple episodes of dolomitization. Early dolomites wit the most extensively altered textures are more stoichiometric, more depleted in 18O and Sr, and more enriched in Mn and Fe relative to texturally less-altered early dolomite. Initial stabilization may have occurred syndepositionally in modified seawater. Continued stabilization of early dolomite likely occurred in fresh or mixed waters of a Middle Ordovician meteoric aquifer associated with Knox unconformity development. Final stabilization occurred by deep burial brines at elevated temperatures as indicated by 1) extensive replacement and overgrowth of early dolomite by late dolomites: 2) a progressive increase in crystal size and abundance of nonplanar crystal boundaries with increase in replacement by late dolomites; and 3) the similarity of the geochemical compositions of texturally most-extensively altered early dolomites and late dolomites.

Shallow Marine Record of Orbitally Forced Cyclicity in a Late Triassic Carbonate Platform, Hungary

ABSTRACT Well preserved examples of Milankovitch-driven cyclicity in the Triassic sedimentary record include Triassic-Jurassic lake sedimentary cycles, marine cyclic carbonates on small Middle Triassic platforms in the Dolomite Alps, and off-shelf facies equivalent to Late Triassic large passive-margin platforms. In contrast, the Late Triassic Hungarian shallow marine carbonate platform that formed on the southern margin of Tethys records a far from perfect Milankovitch eustatic signal. These carbonate cycles contain subtidal skeletal wackestone/packstone, tidal laminites, and paleosols. The cycles include not only transgressive laminites (the classic Lofer cycles): some cycles contain regressive laminites, whereas other cycles have both transgressive and regressive laminites. The stratigraphic successions do not show the clear bundling of five precessional cycles into larger-eccentricity cycles. The poor record of Milankovitch sea-level changes is interpreted to be due to many missed beats in the platform stratigraphy. Missed beats are evidenced by (1) caliches and paleosols and (2) thick amalgamated subtidal carbonates, and may result from precessional sea-level fluctuations either not flooding the platform or flooding it too deeply to allow shallowing up to sea le el in one precessional beat. Fourth-order bundling of the cycles is weak, with fewer than the typical 5 cycles/100 k.y. bundle and far less than 20 cycles for inferred 400 k.y. eccentricity bundles. Fischer plots of the Hungarian cycles show third-order bundling that matches Aigners (1992) sea-level curve from the German Muschelkalk basin but is less similar to the Haq et al. (1987) global sea-level curve.

Ordovician Knox Paleokarst Unconformity, Appalachians

The Ordovician Knox unconformity in the Appalachians developed in less than 10 m.y. during a time of initial collision of the passive margin and of eu- static sealevel lowering. It formed on cyclic limestones and dolomites of the 200- to 1200-m-thick Upper Knox-Beekmantown Group, and provides an example of the effects of long-term exposure on a carbonate shelf and the subsequent diagenesis related to karsting followed by deep burial. Erosional relief is over 100 m in the south. It increases over synclepositional structures and bevels down to Upper Cambrian rocks on the craton. The disconformity is virtually absent in the Pennsylvania depocenter.

Dolomitization of a Carbonate Platform During Late Burial: Lower to Middle Cambrian Shady Dolomite, Virginia Appalachians

ABSTRACT Outer platform carbonates of the Lower to Middle Cambrian Shady Dolomite, Virginia Appalachians, record a lengthy and complex diagenetic history, ranging from marine cementation to burial diagenesis during late Paleozoic overthrusting. Analysis of petrographically well-preserved marine cements has provided an estimate of the isotopic composition of Lower Cambrian marine carbonate (18O = -7.5 to -6.1 PDB; 13C = +0.2 to +0.8 PDB; 87Sr/86Sr = 0.70869 to 0.70975), establishing a base line with which to assess later diagenetic carbonate . Meteoric dissolution and early calcite cement postdate marine cementation and formed during subaerial exposure. The platform margin carbonates were extensively dolomitized; dolomitization postdates calcite cementation and the initial development of stylolites and tectonic fractures. Several episodes of dolomite replacement and cementation are evident. Zone 1 dolomite consists of rare relict dolomite cores which were replaced and overgrown by zone 2A dolomite, the dominant replacement phase. Zone 2A dolomite is depleted in 18O (18O = -10.2 to -7.0) and enriched in 13C (13C = +1.0 to +1.6) relative to marine carbonate, reflecting precipitation from warm formation fluids buffered by the host carbonate. Uniformly low Sr contents (25 to 65 ppm) and nonradiogenic 87Sr/86Sr (0.70900 to 0.70971) also attest to pore water equilibration with the host rock. Replacement dolomitization was followed by widespread carbonate dissolution, which formed vuggy porosity and local solution breccias. The host dolomite locally is encrusted by Mississippi Valley-type (MVT) occurrences of ore mineralization consisting of sphalerite, galena, and pyrite. This major generation of ore minerals is overgrown by three generations of dolomite cement. The first dolomite cement, zone 2B, has similar 18O composition (13O = -10.3 to -7.4) but slightly depleted 13C values (13C = +0.4 to +1.1) relative to zone 2A dolomite and has similar Sr contents (30 to 45 ppm) and 87Sr/86Sr ratios (0.70933 to 0.70966). Zone 2B dolomite cement exhibits petrographic, isotopic, and geochemical affinities with zone 2A replacement dolomite, implying that the two dolomite types formed from fluids of similar composition and temperature. Primary fluid inclusions in zone 2B dolomite indicate precipitation from warm (minimum 120 to 150°C) brines (23-26 wt. % NaCl equiv.). These temperatures imply that the dolomitizing fluid was enriched in 18O due to extensive water-rock interaction with host carbonate, which also buffered the fluid 13C, Sr, and 87Sr/86Sr content. Zones 3 and 4 dolomite cement are depleted in 18O (13O = -13.8 to -11.3) relative to the previous dolomites, implying a temperature increase to 150 to 210°C. The late dolomite cements have 13C compositions (13C = -0.7 to +0.9) ranging from those similar to the earlier dolomites to more depleted values. Zones 3 and 4 dolomite cement are enriched in Sr (60 to 135 ppm) which is radiogenic (87Sr/86Sr = 0.71025 to 0.71445). These later cements thus record fluids that were no longer buffered by the host carbonate. Dolomite cements locally were overgrown by sphalerite, followed by authigenic quartz and calcite cement. Secondary three phase fluid inclusions in quartz and calcite indicate hot (175 to 225°C) saline (30 to 33 wt. % NaCl equiv.) fluids. Temperatures and pressures interpreted from the fluid inclusions are compatible with estimated burial depths in excess of 5 km. The dolomites formed during deep burial, coeval with late Paleozoic, Alleghanian-age thrusting. The Mg required for burial dolomitization was derived from pressure solution of structurally lower, platform interior dolomites that were overridden by the Shady Dolomite thrust sheet. Tectonic uplift and deformation resulted in regional fluid flow, which transported this Mg to the Shady Dolomite outer platform during burial dolomitization.