Isabel P. Montanez

Late Diagenetic Dolomitization of Lower Ordovician, Upper Knox Carbonates: A Record of the Hydrodynamic Evolution of the Southern Appalachian Basin

Late diagenetic dolomitization of the Lower Ordovician, Upper Knox Group in the southern Appalachian basin was closely associated with widespread secondary porosity development, hydrocarbon migration, and local Mississippi Valley-type mineralization. Regionally extensive (^sim70,000 km2), late diagenetic dolomites consist of replacement dolomites and zoned dolomite cements. Late diagenetic replacement dolomites comprise 15 to 50% of all Knox matrix dolomites. The ^dgr18O (-11.9 to -5.3^pmil), ^dgr13C (-3.8 to +0.9^pmil), and 87Sr/86Sr (0.70895 to 0.70918) values of late diagenetic replacement dolomites overlap with those of the first zone of dolomite cements (zone 2), early replacement dolomites, and Lower Ordovician arine calcites, reflecting rock buffering of initial dolomitizing fluids and extensive neomorphism of replacement dolomites by subsequent late diagenetic fluids. Nonporous to sucrosic, late diagenetic dolomites have porosities (1 to 16%) and permeabilities (0 to 1030 md) significantly greater than those of early diagenetic replacement dolomites and host limestones ( 165°C), saline (13 to 22 wt.% NaCl equivalent) basinal brines that underwent extensive fluid-rock interaction with clastics. Precipitation temperatures of late diagenetic dolomites estimated from fluid inclusion homogenization temperatures and systematic trends in ^dgr18O va ues record a regionally developed, prograde-to-retrograde thermal history. Knox late diagenetic dolomites are interpreted to record the spatial and temporal evolution of large-scale fluid flow systems that developed in response to different burial and tectonic stages of the southern Appalachian basin. The occurrence of zoned dolomite cements in tectonic fractures and breccias, and their close association with noncarbonate diagenetic minerals of Pennsylvanian to Early Permian ages, suggest that most Knox late diagenetic dolomites record deep subsurface (2 to >5 km) fluid migration in response to late Paleozoic Alleghenian tectonism (330 to 265 Ma). Late diagenetic matrix dolomites served as long-lived conduits that focused and channeled diagenetic fluids in the deep subsurface. The occurrence of bitumen in secondary porosity within late diagenetic dolomite indicates that they likely were the most viable reservoirs during hydrocarbon migration in the late Paleozoic.

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.

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.