Ordovician Units Research Articles

Middle to Late Ordovician rocky bottoms and rocky shores from the Manitoulin Island area, Ontario: Discussion

The Precambrian Lorrain Quartzite is exposed on Manitoulin, Great Cloche, and Birch islands in Lake Huron off the mainland of Ontario. Metamorphism created a massive unit, which later formed a resistant angular unconformity overlain by a succession of strata deposited from Middle to Late Ordovician times. At the start of Ordovician sedimentation, topographic relief was at least 125 m when the quartzite hills formed an archipelago of small, rocky islands. The Ordovician–Precambrian unconformity provides insight on a changing series of depositional environments over rocky bottoms and rocky shores. Four Ordovician units are well represented. The Swift Current Formation includes some red beds, while the younger Cloche Island and Cobourg formations are carbonates. The Collingwood Formation is a black shale. The first three include eroded quartzite clasts typical of a rocky-shore setting. The unconformity marked by the Collingwood Formation indicates a rapid transgression with little time for the accumulation o...

Middle to Late Ordovician rocky bottoms and rocky shores from the Manitoulin Island area, Ontario: Reply

The Precambrian Lorrain Quartzite is exposed on Manitoulin, Great Cloche, and Birch islands in Lake Huron off the mainland of Ontario. Metamorphism created a massive unit, which later formed a resistant angular unconformity overlain by a succession of strata deposited from Middle to Late Ordovician times. At the start of Ordovician sedimentation, topographic relief was at least 125 m when the quartzite hills formed an archipelago of small, rocky islands. The Ordovician–Precambrian unconformity provides insight on a changing series of depositional environments over rocky bottoms and rocky shores. Four Ordovician units are well represented. The Swift Current Formation includes some red beds, while the younger Cloche Island and Cobourg formations are carbonates. The Collingwood Formation is a black shale. The first three include eroded quartzite clasts typical of a rocky-shore setting. The unconformity marked by the Collingwood Formation indicates a rapid transgression with little time for the accumulation o...

Paleoecology of a well-preserved nautiloid assemblage from a Late Ordovician shale unit, southwestern Ohio

A diverse, well-preserved assemblage of nautiloid cephalopods was collected from the Treptoceras duseri shale, a 1.5-m-thick claystone within the Waynesville Formation (Late Ordovician, early Richmondian) exposed in southwest Ohio. The strata, the enclosed fauna, and its taphonomy indicate deposition in a low-energy, mud-bottom marine environment, in water depths of 20–25 m, below wave base but within the zone of storm-current reworking.Nautiloid specimens consist of complete conchs that have been replaced by calcite. Twelve species of nautiloids, belonging to eight genera, representative of four orders, have been collected from the shale in southwest Ohio. Longiconic orthocones are clearly the dominant nautiloid morphotype present, with the assemblage dominated by three species of the longiconic orthocerid Treptoceras and with fewer numbers of the endocerid Cameroceras and the slender orthocerid Isorthoceras?, the cyrtoconic oncocerids Oncoceras and Manitoulinoceras, and rare specimens of the orthocerid Gorbyoceras, the oncocerid Zittelloceras, and the ascocerid Schuchertoceras.Nautiloid taphonomy, the diversity of nautiloid taxa present, the lack of postmortem buoyancy in the shells of the more common taxa, the recurrent nature of this assemblage, and the restricted distribution of this Treptoceras–Cameroceras fauna to portions of eastern North America in the Late Ordovician suggest that this nautiloid assemblage represents an in-situ accumulation of nautiloids representative of a living assemblage. These nautiloids were important elements associated with benthic communities in these epeiric sea mud-bottom environments and not simply assemblages of drifted, necroplanktonic shells.

Middle to Late Ordovician rocky bottoms and rocky shores from the Manitoulin Island area, Ontario

The Precambrian Lorrain Quartzite is exposed on Manitoulin, Great Cloche, and Birch islands in Lake Huron off the mainland of Ontario. Metamorphism created a massive unit, which later formed a resistant angular unconformity overlain by a succession of strata deposited from Middle to Late Ordovician times. At the start of Ordovician sedimentation, topographic relief was at least 125 m when the quartzite hills formed an archipelago of small, rocky islands. The Ordovician–Precambrian unconformity provides insight on a changing series of depositional environments over rocky bottoms and rocky shores. Four Ordovician units are well represented. The Swift Current Formation includes some red beds, while the younger Cloche Island and Cobourg formations are carbonates. The Collingwood Formation is a black shale. The first three include eroded quartzite clasts typical of a rocky-shore setting. The unconformity marked by the Collingwood Formation indicates a rapid transgression with little time for the accumulation of eroded quartzite clasts. A rich trilobite and orthid brachiopod fauna was recovered from the Collingwood Formation immediately above the Lorrain Quartzite and it represents a rare association that lived in a rocky-bottom setting. This fauna includes the first-reported occurrence of the genus Triarthrus from Manitoulin Island. The Collingwood transgression is equated with a major rise in sea level widely recognized elsewhere in North America and Scandinavia.

The lithostratigraphy of the upper Ordovician of central Portugal

A lithostratigraphic scheme for the upper Ordovician of central Portugal is described incorporating previously-erected local lithostratigraphic terms and new units. Thirteen formations (10 new), including 11 members (eight new), with one named bed, are arranged into four groups (three new). A description of each unit is given, together with comments on its age and correlation. Deposition in a storm-dominated, shallow-shelf environment through Llandeilo and Caradoc time is indicated. Differential subsidence occurred to the NE and SW of the Dornes/Maçâo ‘rise’ particularly during the Caradoc. Lower to mid-Ashgill sequences in the north are dominated by volcanic rocks with subordinate carbonate deposition. Farther south a hiatus preceded the regressive deposits associated with the onset of the upper Ordovician glaciation. Hirnantian glaciomarine deposits occur across most of the area, except for the regions of carbonate and volcanic build-ups, and appear to represent the period of waning of the glaciation. The Ordovician/Silurian boundary is difficult to place, but probably lies close to or within a sheet sand-body which underlies a condensed Llandovery sequence. Lithostratigraphic correlation of the study area with W. Brittany, France, is generally good; the relationship of some of the upper Ordovician units in central Portugal aids the interpretation of the relationships of the equivalent deposits in Brittany.

Provenance Determination of Volcaniclastic Rocks: the Nature and Tectonic Significance of a Cambrian Conglomerate from the New England Fold Belt, Eastern Australia

ABSTRACT The Murrawong Creek and Pipeclay Creek Formations of the New England Fold Belt of eastern Australia provide one of the few records of Middle Cambrian-(?) Early Ordovician sedimentation along the eastern Gondwana margin. They comprise conglomerate, sandstone, siltstone, and laminated argillite that accumulated in an inner-submarine-fan environment. Sandstones are quartz-poor volcanolithic varieties. Conglomerate clasts are mainly andesite but range from basalt to rhyolite; all these rocks are porphyritic with phenocrysts of feldspar (albite after calcic plagioclase), augite, and, uncommonly, titanomagnetite. The augite is a low-TiO2 variety, and titanomagnetite has only a modest TiO2 content. Alteration has modified the magmatic composition of the clasts, but the ess mobile elements indicate they were derived from a low-K orogenic suite, in consonance with petrographic and mineralogical data. A western source and the presence of lower Paleozoic subduction-complex rocks to the east of the Cambrian-(?) Ordovician units indicates that the latter accumulated in a forearc setting. The distributive province may have been the precursor of the Macquarie Volcanic Belt of the eastern Lachlan Fold Belt in N.S.W., or one of the Cambrian oceanic island arcs preserved in the fragmented Gondwana margin sequences in southeastern Australia, northern Victoria Land (Antarctica), and New Zealand.

Structural Traps in Duplexes Within the Pine Mountain Thrust System, Southern Appalachians: ABSTRACT

The Rose Hill and Ben Hur oil fields in the Pine Mountain thrust system are classic examples of structural traps formed by duplexes in the southern Appalachian thrust belt. Integration of surface, seismic, and well data has enabled the construction of eight balances cross sections and an interpretation of the three-dimensional geometry and kinematic development of structural traps in the area. In the southwestern and northeastern segments of the Pine Mountain thrust block, the structure consists of a simple ramp-related anticline, with the shortening decreasing from 70,000 to 10,000 ft toward the northeast. In the central segment, the Pine Mountain thrust is folded by one or more footwall thrusts, resulting in a system of overlapping ramp anticlines (class III duplex), with the total shortening varying between 54,000 and 28,000 ft. Structural traps in the Rose Hill and Ben Hur oil fields occur within Ordovician and Silurian units in the crest of the Bales I thrust sheet. These traps formed in areas with the maximum structural relief prior to refolding by subsequently formed footwall thrusts. The porosity and permeability of the reservoirs are enhanced by fold-related longitudinal and transverse fractures at high angles to bedding. Faults are typically characterized by amore » few inches of gouge surrounded by a brecciated and imbricated permeable zone several feet or tens of feet wide. This suggests that while the faults acted as impermeable barriers between different thrust sheets, they also provided effective pathways for communication within each sheet during the migration of hydrocarbons.« less

Wedge Failures along Tennessee Highways in the Appalachian Region: Their Occurrence and Correction

Of the different modes of rock slope failures encountered along highways in the Appalachian Region of Tennessee, the wedge failure has become the most dangerous to the motoring public. Nearly all of the wedge failures experienced by the Tennessee Department of Transportation have been located in eastern Tennessee both in the Ridge and Valley Province and the Blue Ridge Province. However, the majority of wedge failures experienced has been located in the mountainous topography of the Blue Ridge Province, particularly along Interstate 40 (1-40) in Cocke County. Wedge failures along Tennessee highways are usually associated with hard, well indurated shale or slate lithologies which have well developed planes of weakness. Most of the wedge failures are associated with pre-Cambrian rock units, although some Cambrian, Ordovician and Mississippian units are also involved. Rock slopes subject to wedge failures can be evaluated by steronet analysis techniques. These procedures are usually required for preventative and/or correction measures. The treatment of wedge failures in Tennessee involves both stabilization and protection methods. The concepts of drainage, removal, and restraint provide stabilization. Drainage methods involve de-watering with the use of horizontal drains. Removal concepts include mass grading operations, controlled blasting techniques and “toe” clean-out maintenance procedures. Restraint of rock wedges involves the use of buttressing concepts such as rock buttresses and concrete buttresses. Proposed, but yet untried in Tennessee, are buttressing concepts utilizing rock bolts. Protection methods include wire meshing, catchment fences, berms, and walls.

A transverse, thin-skinned, thrust-fault belt in the Paleozoic North Greenland Fold Belt

The southern part of the North Greenland Fold Belt in Peary Land has been affected by two widely different orogenic events: an Early to Middle Devonian, westward-directed, thrust-fault deformation and a Late Devonian to an early Carboniferous folding on east-west axes that folded the earlier thrust faults. The deformation in this part of the North Greenland Fold Belt affected an ∼5-km-thick pile of lower Paleozoic shales, turbidites, and conglomerates deposited in the Franklinian–North Greenland, clastic trough. The thrust-fault belt is separated from the northern part of the North Greenland Fold Belt by the east-west–trending transcurrent Harder Fjord fault zone, north of which no westward-directed thrust faults have been recognized. The thrust fault region covers an area of 3,000 km 2 , and the main transport direction was toward the west with a refraction toward southwest along the sideland to the south. The thrust fault region is rimmed by a frontal thrust fault and fold zone. Behind the frontal zone, the uppermost stratigraphical unit, the Silurian flysch, forms an allochthonous mega-cover, draping imbricate thrust structures in the Ordovician units below. The central part of the thrust-fault belt consists of a western, north-south–striking, imbricate, thrust-fault fan and an eastern and southern area characterized by flat-lying, coherent thrust sheets. The imbricate fan is formed by 20 steeply east-dipping thrust sheets with an individual thickness of 500 m. The flat-lying thrust sheets are 100–300 m thick, and the relative displacement along individual thrust faults is in the order of 1–10 km. The subhinterland or proximal thrust zone is situated in the eastern part of the thrust-fault region and is dominated by the Cambrian Frigg Fjord Mudstones. This zone is regarded as a peel-off area from which the Ordovician and Silurian sediments were removed and displaced toward the west. The Frigg Fjord Mudstones compose the lowermost unit exposed in the region and contain the main decollement zone for the thrust faulting. The cumulative effect of the thrusting is a total transport of ∼100 km from east to west. The thrust fault deformation is herein named the “Volvedal orogeny” and is interpreted as a foreland deformation related to the Caledonian Mobile Belt, which involved gravity gliding in the hinterland of the thrust-fault belt.

Structure and tectonics of the Hudson Valley fold-thrust belt, eastern New York State

Analysis of structural features displayed in Silurian and Devonian strata of the Hudson Valley region indicate that these rocks form the western edge of a now largely eroded, west-verging, fold-thrust belt (the Hudson Valley fold-thrust belt, or “HVB”). West of the Hudson River, remnants of this fold-thrust belt define a miniature valley and ridge province. Faults of the HVB die out to the west as blind thrusts in the basal units of the Catskill clastic wedge. Structural geometries displayed in the HVB vary rapidly along strike and are locally complicated by internal deformation of thrust sheets and by movement on out-of-the-syncline faults. It is proposed that exposed faults of the HVB west of the river arise from a basal detachment, here called the “Rondout detachment,” that lies above the Taconic unconformity. The Rondout detachment and the overlying thrust system have been bent into large-amplitude folds; Ordovician units beneath the Rondout detachment must have shortened during development of these folds. Area balancing of cross-sectional models suggests that the shortening affecting Ordovician units could be restricted to rock above a lower blind detachment (the “Austin Glen detachment”) positioned within Ordovician strata at a depth of ∼400 m below ground surface. The age of deformation and the tectonic setting of the HVB is enigmatic; its spatial relation to the Catskill clastic wedge and to Acadian metamorphic terranes suggests that the belt represents foreland shortening developed during the Acadian Orogeny.

Depositional History and Petroleum Potential of Middle and Upper Ordovician of Alabama Appalachians: ABSTRACT

Middle and Upper Ordovician deposits occupy a significant position in the Paleozoic sequence in the southern Appalachians, since they represent a transition from passive margin carbonate to active margin clastic deposition. In the Alabama Valley and Ridge these Middle and Upper Ordovician deposits are exposed in two northeast-southwest trending outcrop belts separated by the Helena fault. Units west of the fault are essentially autochthonous, while those east of the Helena have been displaced some distance to the west by late Paleozoic thrusting. Middle Ordovician units show a transition from shallow-water deposits in the west to deeper water basinal deposits in the east. West of the Helena fault the Middle Ordovician is represented by peritidal to shallow subtidal lithologic characteristics of the Chickamauga Limestone. East of the Helena these shallow-water deposits are replaced by deeper water carbonates of the Lenoir and Little Oak Limestones and graptolitic shales of the Athens Formation. As this deep-water basin filled during the late Middle Ordovician, tectonic uplift generated clastic sediments which prograded into the basin from the east. Red-green mudrocks of the Greensport Formation were deposited in shallow-shelf to tidal-flat environments and were in turn overlain by quartz arenites of the Colvin Mountain Sandston , deposited as part of a shallow-barrier system. With continued uplift during the Late Ordovician, additional clastics prograded westward over the filled basin. Early Late Ordovician shallow-shelf to tidal-flat mudrocks of the Sequatchie Formation grade westward into shallow-water carbonates of the Inman and Leipers Formations. With continued input, Sequatchie clastics prograded westward and overrode the westerly carbonates. A relative sea-level rise during the late Late Ordovician was accompanied by deposition of open-marine shelf, bio-clastic End_Page 1461------------------------------ limestones of the Fernvale facies of the Sequatchie throughout much of the western Valley and Ridge. The petroleum potential of the Middle and Upper Ordovician sequence in the Alabama Appalachians appears to range from marginal to moderate. West of the Helena fault the Chickamauga Limestone appears to have the best potential, though both source rock and reservoir potential are limited. Source rock potential is better east of the Helena, particularly in the Athens Formation, but reservoir potential is limited again. The existence of significant reservoirs in this area appears dependent upon the development of fracture porosity associated with Appalachian structures. End_of_Article - Last_Page 1462------------

Middle Ordovician Knox Unconformity, Virginia Appalachians: Transition from Passive to Convergent Margin: ABSTRACT

The Knox unconformity, central and southern Appalachians, is one of the major targets for petroleum exploration in the Appalachians, as well as being closely associated with base metal mineralization. It developed on Lower to Middle Ordovician Knox/Beekmantown carbonates, and marks transition from passive margin carbonate deposition to deposition in a foreland basin (convergent margin), possibly during global sea level lowering. Erosional relief on the unconformity is over 140 m (460 ft) in southwest Virginia, decreasing to 20 m (66 ft) or less in northern Virginia. Decrease in relief is accompanied by rapid depositional thickening of Lower Ordovician and earliest Middle Ordovician units into the Pennsylvania depocenter. Paleokarst features include topographic highs (tens of meters relief), breccia, and mud-filled sinkholes and caves that extend to 65 m (215 ft) below the unconformity, and sub-unconformity intraformational dolomite breccias that formed after dissolution of limestone interbeds. Coarse detritus on the unconformity surface formed thin to thick veneers of regolith that were locally reworked by fluvial and marine processes. Dolomite detritus also was deposited in alluvial fan and playa mud flats i lows on the unconformity surface. Under the luminoscope, the detrital dolomite is evidenced by corroded nonluminescent detrital cores overgrown by luminescent dolomite. The unconformity influenced the regional distribution, composition, and thickness of some post-unconformity peritidal carbonates and may have localized some Middle Ordovician downslope buildups. The unconformity also localized deposition of Pb-Zn ores, which commonly are associated with unconformity related intraformational breccias that remained open during deeper burial diagenesis. Development of regional unconformities on shelf sequences of passive margins immediately beneath foreland basin sequences is common in other orogens, reflecting gentle warping of the shelf prior to foundering and burial beneath synorogenic clastics. Such unconformities may localize hydrocarbons and base metal deposits (Pb-Zn) by developing permeable zones adjacent to the unconformity. End_of_Article - Last_Page 523------------

Could Middle Ordovician Carbonate Shelf Depositional Patterns in Appalachian Orogene Indicate Collision Along an Irregular Continental Margin?: ABSTRACT

Analysis of Lower to Middle Ordovician carbonate successions and the conodonts they contain from Tennessee to New York show that (1) in the northern Great Valley, strike-parallel disconformable contacts occur between Lower and Middle Ordovician and within Middle Ordovician carbonate units, (2) apparent maximum extent of unconformities along tectonic promontories, and (3) relatively rapid subsidence and transgression at promontories. Diachronous early Paleozoic collisional events at an irregular continental margin might explain these observations. Initial Taconic collisions may have occurred at the Virginia promontory resulting in uplift and erosion of the Knox/Beekmantown shelf in Whiterockian time followed by rapid subsidence and transgression. Uplift and erosion, possib y related to continued convergence migrated southwest and at least as far northeast as Lexington, Virginia; beyond Lexington, shelf deposition continued relatively uninterrupted on the east side of the Great Valley. Collision at the New York Promontory could have produced the two pre-Blackriverian and pre-Rocklandian Middle Ordovician intervals of uplift and erosion on the Beekmantown shelf. These unconformities are greatest near Newburgh, New York, and decrease in magnitude northeastward and southwestward. West of Reading, Pennsylvania, in the Great Valley, carbonate shelf deposition remained virtually continuous. Thus between Reading and Lexington, in the Pennsylvania reentrant, continuous deposition during early Paleozoic collisions suggests that an irregular outline of the continenta margin may have controlled patterns of sedimentation during collision tectonics. End_of_Article - Last_Page 1171------------

Revision and correlation of late Middle Ordovician stratigraphy northeast of Quebec City

Autochthonous late Middle Ordovician rock units between Neuville (near Quebec City) and La Malbaie (160 km to the northeast) have been revised lithologically and correlated by means of graptolites. The following rock units have either had a slight modification of their lithologic definition or have been found to extend into a region where they were not previously recognized: Cap-à-l'Aigle Formation, Saint-Irénée Formation, Utica Shale, and Lotbinière Formation. In addition, two new rock units have been named: Beaupré Formation and Moulin River facies (an informal term) of the Trenton Group.The depositional environments of each of the late Middle Ordovician rock units have been briefly identified (although the bulk of evidence is reserved for a later report). Thus, when the various facies are correlated, some important facts emerge about the southern margin of the Laurentian Platform and the rise of the Taconic Orogen to the southeast of Logan's Line. The Deschambault limestone and underlying Cap-à-l'Aigle sandstone were deposited during a marine transgression, after which a structural feature (Montmorency Promontory) developed in the Quebec City to Beaupré area. The promontory was asymmetrical, with a gentle gradient to the southwest, and a steep gradient and a basin to the northeast. Sedimentary thicknesses increased three- to fourfold in the basin northeast of the promontory, and the facies within the basin are markedly different from those over the promontory and southwest. Within the basin there developed deep-water limestone (Moulin River facies), and submarine-fan facies (Saint-Irénée Formation and 'new facies') with interfan microflysch (Lotbinière Formation). Trenton limestone facies on the promontory were deposited in a carbonate bank environment. No Utica Shale is found northeast of the Montmorency Promontory. In contrast, the 'typical' succession of Trenton–Utica–Lotbinière–Lorraine was developed in the region of the promontory and south-west in the St. Lawrence Lowland. An exception is the Beaupré submarine fan that developed over the promontory. All three submarine fans were shed by the rising Taconic Orogen which can be shown by graptolites and the presence of Mg-rich chromite to have been deeply eroded as early as ruedemanni zone time. No Lorraine Group facies are known from the area of our investigation.

Welling Formation, New Name for Upper Ordovician Unit in Eastern Oklahoma (Formerly Called Fernvale"): GEOLOGIC NOTES"

Welling Formation, New Name for Upper Ordovician Unit in Eastern Oklahoma (Formerly Called Fernvale"): GEOLOGIC NOTES"

Caledonian Geology and Uranium-bearing Strata in the Tåsjö Lake Area, Sweden

Abstract Uranium indications in the Tasjo Lake area, Vasternorrland County, discovered in 1957 by the Swedish Atomic Energy Company, have been the subject of comprehensive geological studies by this company from 1960 through 1965. Attention has been directed mainly to the basal Ordovician units within the Cambro-Ordovician of the area. Electromagnetic, magnetic and radiometric aerial prospecting has been carried out over the entire concession area, with a line spacing of 125 m at an altitude of 30 m. By means of field studies, including 68 boreholes comprising in all 2131 m of core, geological, tectonic, radiometric and geochemical ground studies and investigations of the aerial anomalies, it has been possible to map the extension of the uranium-bearing unit within an area 20–30 km long and 10 km wide. Palaeontological studies in the area have shown that the uranium-bearing unit is Early Ordovician in age. The structural geology of the area is characterised by short overthrusts of the Schuppen type, altho...

Sedimentologic Data on Ordovician-Silurian Boundary in Central Appalachians: ABSTRACT

The Ordovician-Silurian boundary in the western Valley and Ridge province is in an unfossiliferous, continental sequence, and has been traditionally placed to separate red Juniata Formation from the overlying white Tuscarora Formation. The systemic boundary, thus based on color differences, has been assumed to represent a time-parallel surface of considerable paleogeographic significance. Detailed field and mineralogic data now indicate that this assumption leads to ambiguities in correlations, and that a new means of defining the boundary is necessary. Sedimentologic aspects of rocks adjoining the color boundary provide primary control on diachroneity. Westward-prograding and eastward-transgressing fluvial regimes generated a sequence of distinct conglomerate, sandstone, and shale lithofacies recognizable over wide areas and occupying the 3,500-ft interval between datable Upper Ordovician and Middle Silurian fossiliferous marine units. Boundaries based on lithofacies approach more closely than other kinds of boundaries to time-parallel surfaces. The color boundary, for example, fluctuates as much as 400 ft vertically, both parallel with and across depositional strike, irregularly and unpredictably crossing lithofacies boundaries. Zones of constant clay, heavy, and opaque mineralogy within lithofacies are likewise randomly truncated y the color boundary. These data strongly suggest that redefinition of the Ordovician-Silurian systemic boundary in terms of the newly recognized lithofacies sequence is in order. Such revision depends fundamentally on precise time correlations of specific lithofacies with age-established marine units outside the Valley and Ridge area. In the absence of such rock units and of meaningful ichno-fossil data, exact location of the boundary may not be possible. End_of_Article - Last_Page 658------------

Clay-Mineral Diagenesis in Redbed Sequence, Juniata Formation, Central Pennsylvania: ABSTRACT

The Bald Eagle and Juniata Formations are thick Upper Ordovician units which comprise much of the Taconic clastic wedge in central Pennsylvania. Large parts of these formations consist of a homogeneous subgraywacke-sandstone and conglomerate lithofacies of fluvial origin. The Bald Eagle (lower) part of this lithofacies is greenish gray (drab), whereas the Juniata (upper) part is dark red, with thin drab layers. Because drab and red parts of this lithofacies have identical sedimentologic histories, X-ray diffraction investigations of sandstone-matrix clay minerals were undertaken to establish possible differences in abundance and in octahedral-layer cation content of clay-mineral species between the drab and red beds. The major clay phases present are illite and chlorite. Consistent variations in the ratios illite/chlorite (determined by peak area) and Fe+2/Mg in chlorite (determined by structure factors) occur between adjacent drab and red rocks. The illite/chlorite ratio is lower in drab than in red beds, and the Fe/Mg ratio is higher in drab-bed than in red-bed chlorite. Statistically, these ratios are inversely correlative, and suggest that the present clay-mineral distribution is not of depositional origin but is a result of diagenetic modifications of a detrital clay suite. Drab-bed chlorite commonly occurs as coatings between two generations of silica cement, which suggests diagenetic generation of chlorite in drab beds rather than secondary destruction of preexisting End_Page 745------------------------------ chlorite in red beds. Reduction and dissolution of hematite pigment in red beds may have supplied the necessary iron. End_of_Article - Last_Page 746------------

Upper Ordovician Stratigraphic Relations Between Covington and Clays Ferry, Kentucky: GEOLOGICAL NOTES

Upper Ordovician rock-stratigraphic units previously described in southwest Ohio and the central Bluegrass area of Kentucky are correlated by comparing lithologic characteristics and clastic percentage curves derived from detailed measured sections in northern Kentucky. The results presented here suggest a southern source area in this region during Late Ordovician time, and a gradual northward shift of concentrated centers of biota.

Relations of Middle Ordovician Time and Rock Units in Basin Ranges, Western United States

Stratigraphic studies across southern Nevada have resulted in reappraisal and suggested revision of the zones of the Whiterock Stage of Cooper (1956), the Rhysostrophia zone being reassigned to his Marmor Stage. The Palliseria zone is revealed as an assemblage closely allied to lithic facies. The zone transgresses other zones based on brachiopods and is younger on the west than on the east. Dating based on these revised zones indicates upward migration of several lithic facies in a westerly direction. In addition, sands which later comprised the Eureka Quartzite may have moved southward along the miogeosyncline, seemingly from a northern rather than an eastern source.