Richard Smosna

Silurian Evolution of Central Appalachian Basin

Isopach and lithofacies maps of eight Silurian formations illustrate that during Middle Silurian time a broad shelf emerged from southeast to northwest across southern West Virginia, separating an adjacent basin on the north from the rest of the Appalachian trough in Ohio, Kentucky, and Tennessee. This subdivision of the once-continuous Appalachian trough produced Middle and Late Silurian sedimentation patterns which are quite different from those of the Late Ordovician and Early Silurian. The Lower Silurian Tuscarora Sandstone (as well as the Upper Ordovician Juniata Formation) was deposited throughout an extensive, unbroken area from Tennessee to New York under fluvial to shallow-marine conditions--an onshore-offshore complex from east to west. Likewise, the Middle Silurian Rose Hill Formation was laid down in this same linear trend, with shallow-water ferruginous sandstones confined to the eastern margin. The Rose Hill isopach map indicates the beginning of differential subsidence with a major thickening of the formation in central West Virginia. The broad shelf and adjacent basin developed with sedimentation of the overlying Keefer Sandstone--high-energy sandstone on the shelf surrounding a central region of shale. During deposition of the Middle Silurian McKenzie F rmation and Upper Silurian Williamsport Sandstone, the same basin-shelf relation continued. Distinct members of the McKenzie indicate subaerial to shallow-subtidal conditions on shelves on the east, south, and west; greater subsidence identifies the enclosed basin. Clean Williamsport Sandstone accumulated on the southern shelf while immature sandstone was deposited in the northeast on mud flats associated with the Bloomsburg delta; simultaneously dolomite formed within the basin. The Wills Creek Formation represents uninterrupted deposition of fine clastic and carbonate materials in the basin and on mud flats in the northeast. Restriction of the sea in Late Silurian time is shown by the basin-centered Salina evaporites (anhydrite and halite) and the surrounding-shelf carbonate rocks (Ton loway Limestone). Shelves on three sides of this basin essentially isolated it from a Salina basin in Ohio. Higher energy environments were associated with the Silurian shelves during deposition of both sands and carbonate material, whereas shales, carbonate sediments, and evaporites accumulated in the adjoining basin under low-energy conditions. Known hydrocarbon reservoirs in the Keefer, McKenzie, Williamsport, and Salina are limited to the respective shelves, and future exploration for gas should be concentrated in those areas.

Paleokarst and reservoir porosity in the Ordovician Beekmantown Dolomite of the central Appalachian basin

A karst-unconformity play at the top of the Ordovician Beekmantown Dolomite is judged to have great petroleum potential in Ohio and adjacent states; wells have high ultimate reserves and large areas remain untested. To better understand the origin, development, and distribution of Beekmantown porosity, we conducted a petrologic-stratigraphic study of cores and thin sections from 15 oil and gas wells. The massive dolomite, characterized by a hypidiotopic-idiotopic texture, formed by the replacement of stacked peritidal carbonate cycles. Secondary porosity occurs at two scales: (1) mesoscopic—breccia porosity, solution-enlarged fractures, large vugs, and caverns, and (2) microscopic—intercrystalline, intracrystalline, molds, small vugs, and microfractures. Mesoscopic pores (providing the major storage capacity in this reservoir) were produced by intrastratal solution and collapse of carbonate layers, whereas microscopic pores (connecting the larger pores) generally formed by the leaching of individual carbonate grains and crystals. Most pore types developed during periods of subaerial exposure across the carbonate bank, tied to either the numerous, though brief falls of relative sea level during Beekmantown deposition or more importantly the prolonged Knox unconformity at the close of sedimentation. The distribution of reservoir-quality porosity is quite heterogeneous, being confined vertically to a zone immediately below the unconformity and best developed laterally beneath buried hills and noses of this erosion surface. The inferred, shallow flow of ground water in the Beckmantown karst, primarily below topographic highs and above a diagenetic base level close to the water table, led to this irregular distribution of porosity.

Compaction law for Cretaceous sandstones of Alaska's North Slope

Lithic arenites and wackes of the National Petroleum Reserve in Alaska contain a significant volume of ductile grains and have suffered substantial physical compaction with burial. As determined by stratigraphic studies, these rocks were buried to maximum depths of 500 to 4,400 m. Initially the grain fraction (grain volume as a percent of total rock volume) must have been near 59% but increased to over 90% at depth. Statistical analysis reveals that the grain fraction (GF) is primarily a function of burial depth in meters (d), percent ductile grains such as phyllite rock fragments (P), and matrix content (M); multiple regression suggests the following linear equation: GF = 90 + 0.23P - 0.72M + 0.0018d. Simple grain rearrangement operates to a depth of 550-800 m and accounts for a 25% increase in grain fraction. At greater depths compaction is by plastic deformation. However, the increase in grain fraction appears to fall off considerably below 1,000 m, and compaction due to plastic deformation increases grain fraction on average by only 7%. Early quartz cementation never progressed beyond a beginning stage and thus was ineffective at preventing compaction. On the other hand, clay matrix drastically impedes compaction. Grain fraction for lithic wackes with an abundant matrix never exceeds 83%. The compaction law derived in this study may prove useful in further exploration for petroleum reservoirs. For example, lithic arenites with a low ductile content should retain an intergranular porosity of 6% to a burial depth of 2,000 m.

Compositional maturity of limestones - a review

Abstract Compositional maturity, as defined for carbonates, is the extent to which a sediment approaches the constituent end-member (intraclasts, ooids, fossils, peloids, micrite matrix, and terrigenous minerals) to which it is driven by the environmental processes operating upon it. An immature lime sediment is produced in an environment where many biological, physical, and chemical processes are operating simultaneously. The resulting sediment is a mixture of several of the six major constituents. As the number of competing processes decreases in an environment, the sediment progresses through advanced stages of compositional maturity. A supermature sediment is produced in an environment where a single formative process has operated to completion; consequently, only one constituent dominates the sediment. The index of compositional maturity provides a clue to the complexity of the depositional setting, reflecting the number of environmental processes, their interactions, and rates of sediment production. There is a natural tendency for many different carbonate constituents to be produced on the shallow-water shelf, and most lime sediments of this deposystem are mature and immature. On the other hand, the number of environmental processes is generally low and the compositional maturity high on mobile oolite shoals, beaches, tidal flats and sabkhas, and in deep-water basins. Application of compositional maturity in carbonate-facies analysis, therefore, focuses attention on sedimentary processes in the depositional environment.

A Scheme for Multivariate Analysis in Carbonate Petrology with an Example from the Silurian Tonoloway Limestone

ABSTRACT Two methods of exploratory data analysis--a numerical classification system applied in conjunction with an ordination technique--were used to facilitate environmental interpretation of 61 Tonoloway Limestone samples from Pinto, Maryland. We suggest that the scheme of multivariate analysis presented here, because it involves nonparametric methods, is more applicable to typical geologic data than other strategies. In the Tonoloway Limestone 7 facies (clusters) were identified by Q-mode cluster analysis: (1) micrite, (2) micrite-dolomite, and (3) dolomite microfacies from the supratidal zone; (4) a micrite-peloid microfacies of the intertidal mud flat; (5) intraclast and (6) ooid-calcite cement microfacies representing a shallow-subtidal environment; and (7) a peloid-calcite cement micro acies from the deeper-subtidal environment. Multidimensional scaling illustrated interrelationships among the clusters and the gradational nature of samples; neither factor was observed by cluster analysis alone. Recognition of the gradational nature of samples then led to interpretations of three environmental gradients that were normal to the paleoshoreline. Hydrodynamic energy was low on both the tidal flat and offshore near wave base and systematically increased from both directions toward the shallow-subtidal environment. Petrographic fossil diversity was highest near wave base with a second, lesser peak on the intertidal mud flat; this variable decreased into the shallow-subtidal high-energy belt and also onto the supratidal mud flat. Dolomitization, apparently of two modes, was mo t pronounced in rocks of the schizohaline supratidal zone.

Stratigraphic-Tectonic Relations in Spain's Cantabrian Mountains: Fan Delta Meets Carbonate Shelf

ABSTRACT Stratigraphic patterns indicate that the Hercynian orogenic belt of northwestern Spain underwent an important reorganization in the latest Carboniferous Period. The passive margin of the Cantabrian foreland basin, initially far removed from southwestern mountains, was the early site of a major carbonate platform. This distant margin, however, experienced a late phase of deformation directed from the north. Limestones of the passive platform then became the thrust-faulted source area for three successive fan deltas, which prograded across the remaining carbonate shelf. The fan deltas are comprised of conglomerates and coarse sandstones of several areally restricted Upper Carboniferous formations. Their distribution identifies a local subbasin in the Picos de Europa Province, and paleocurrent data confirm a new northern source of sediment. These coarse-grained deposits are interpreted to have formed in direct response to tectonic uplift along a nearby thrust-fault system. The stratigraphic succession spans a 7 Ma interval from latest Moscovian to Late Kasimovian time and defines three separate tectonic episodes: late Myachkovskian (304 Ma), early Chamovnicheskian (300 Ma), and early Dorogomilovskian (298 Ma). Rather than advancing basinward, though, the fan deltas shifted laterally during this period. They become younger in an eastward direction, that is, parallel to the structural grain. The underlying subregional unconformity, likewise related to thrust faulting, also becomes younger to the east. The thrust-fault system apparently propagated laterally by means of three consecutive tectonic episodes, which led in turn to the sideward displacement of sediment source area and fan-delta depocenter.

Diagenesis of a stromatoporoid patch reef

ABSTRACT At Mustoe, Virginia, four biofacies of the Lower Devonian Helderberg Group, including a 13-m patch reef, are well exposed. A detailed petrographic analysis indicates that these lime sediments were lithified and altered in two subsurface environments. After growth ceased, the reef was blanketed by nearshore and shoreline facies; deposition at this time was regressive, terminating in regional subaerial exposure. Fresh water then invaded the near-surface sediments, and diagenesis in the meteoric-phreatic zone included rapid neomorphism of some aragonitic fossils, leaching of others, and precipitation of calcite cement with three crystal forms. Overlying limestone facies reflect subsequent subsidence, and the reef was buried progressively deeper. Diagenesis continued in the deep subsurfac and included pressure solution, probable smectite-illite conversion, precipitation of ferroan saddle dolomite and coarse calcite cements, and replacement of argillaceous matrix by dolomite. In both subsurface environments, several interrelated diagenetic processes were operating, and the final crystal textures, fabrics, and mineralogies were a consequence of this interplay.

On minimum spanning trees and the intergradation of clusters

Cluster analysis groups samples, but does not generally show gradations between clusters. To illustrate these relationships, principal-coordinate analysis and multidimensional scaling can be employed, but these methods may not be appropriate due to structural distortion. A minimum spanning tree (MST) computes a point-to-point path through the original matrix, and clusters are mapped on the MST diagram. The MST linkages, therefore, provide a natural continuum between the clusters, without distortion. Forty-six Silurian Tonoloway Limestone samples were analyzed for 16 sedimentary features indicative of subaerial exposure. Cluster analysis identified nine clusters which were reduced to five final, ordered groups by MST.

Environmental aspects of Middle Ordovician limestones in the central Appalachians

Abstract Black River and Trenton limestones of the outcrop belts in West Virginia and Maryland were deposited on a gentle carbonate ramp that sloped eastward into a deep-water shale basin. The overwhelming sediment type on the ramp was lime mud, laid down below wave base. Water turbidity and circulation fluctuated, which precluded many epifauna. Burrowing infauna, however, were common. The consistency of the mud was generally soft, but hardgrounds developed locally. Another common sediment type, fossiliferous lime mud, represents muddy substrate patches more abundantly inhabited by organisms. These communities, dominated by echinoderms, trilobites, and brachiopods, had both low densities and diversities. Such patches were initially established by large, flat brachiopod pioneers but did not greatly expand because of the high physiological stress and the soft consistency of adjacent substrate. Occasionally, bioclastic sands were produced by storms reworking skeletal grains of the patches. These storm deposits cut into underlying sediments, and the bioclastic debris was clearly locally derived. Other skeletal sands, containing abundant calcareous algae and Tetradium corals as well as peloids and intraclasts, were deposited above wave base on shallower portions of the ramp. Rare cross-laminated peloid sands were confined to small lenses and channels at various depths, and intermittent storm currents were probably responsible for their deposition. Into progressively deeper water on the ramp, skeletal sediments decreased in abundance, storm- and current-laid sediments also decreased, and shale increased. Carbonate sedimentation eventually ended when the ramp facies were overstepped by basinal shales.

Environmental Impact Statements—Worthwhile or worthless?

A geological evaluation of Environmental Impact Statements (EIS) reveals that many contain erroneous, irrelevant, misleading, and grammatically poor geological statements. We stress the need for use of pertinent geological information, such as general bedrock and soil conditions, geologic hazards, mineral and water resources, and regional seismicity, as well as the need for suggestions for remedial actions.