Early Ordovician Time Research Articles

The age of the Roberts Arm Group, north-central Newfoundland

Felsites from the Roberts Arm Group have yielded a Rb–Sr isochron age of 447 ± 7 Ma, with an initial 87Sr/86Sr of 0.7063, and a genetically related granite pluton has yielded an isochron age of 464 ± 13 Ma, with an initial ratio of 0.7064. The Mansfield Cove oceanic plagiogranite complex, which is in fault contact with the Roberts Arm Group, has yielded a minimum zircon 207Pb/206Pb age of 594 ± 10 Ma. This plagiogranite, and basaltic rocks of the Hall Hill complex which partly surround it, thus provide a basement of suitable age upon which the Roberts Arm Group may have been deposited. The age of this basement corresponds to that of the opening of the lapetus Ocean.The stratigraphic age of the Roberts Arm Group, indicated by these ages, is Middle or possibly Late Ordovician. The volcanics were deposited either before or slightly after the Caradocian black shales of the Notre Dame Bay region. In the former case, Ordovician volcanism around western Notre Dame Bay ceased at about the time of obduction of the allochthons of western Newfoundland. In the latter, volcanism of the Roberts Arm Group marks a north to westward shift of volcanism within the central mobile belt of Newfoundland that occurred in late Middle or early Late Ordovician time. If the Buchans Group is of Silurian age and correlative with the Roberts Arm Group then this lithostratigraphic unit is probably diachronous, younging to the south.

Paleozoic Disruptive Deformation in North American Continent and Its Relation to Formation and Development of Interior Basins and Deformation Within Orogenic Core: ABSTRACT

The two major disruptive tectonic events during the Paleozoic which affected the North American craton seem to be associated with Appalachian-Ouachita orogenic events. The first Paleozoic cratonic disruption was tensional rifting, which occurred during the Avalonian intrusive-metamorphic event (± 560 m.y.). Evidence continues to mount that these Cambrian rifts of the Appalachian-Ouachita foreland, such as the Rome trough of Kentucky and West Virginia, are not Cambrian aulacogens. By time and position, the rifts seem to be incipient basins along a developing back-arc trough. However, this disruptive deformation was not restricted to the developing arc trough, but extended far into the craton where it commonly involved reactivation of older rift zones. These zones form d the axial portion of the subsequently developed Paleozoic basins. The Paleozoic basins developed by epeirogenic movement after a period of relative quiescence during Late Cambrian through Early Ordovician (pre-Taconic) time. The second Paleozoic continental disruption created large upthrust blocks in the craton during the Pennsylvanian and early Permian, probably by compressional deformation. This event ties, both by time and position, to deformation within the Ouachita part of the orogenic core. Upthrust crustal blocks in the craton may be bounded by reactivated faults of precursor rifts. When they formed, the upthrusts often developed near the axial part on the middle Paleozoic basins to form the late Paleozoic yoked basins. The occurrence of axial rifts within interior and foreland basins, and of axial upthrusts in the craton-margin basins, suggests an interrelation among rifts, basin formation, and the late-forming yoked basins. The developing foreland trough (the Appalachian-Ouachita geosyncline) has a tectonic history similar to that of the cratonic basins but, along its trend, tensional bending of the basement predominated. End_of_Article - Last_Page 1588------------

Zinc Deposits Related to Diagenesis and Intrakarstic Sedimentation in the Lower Ordovician St. George Formation, Western Newfoundland

ABSTRACT Sphalerite mineralization in the Lower Ordovician St. George Formation occurs in a white dolomite pseudobreccia confined to a unit of dolomite-mottled limestone and dolomite. This section is interbedded dolomite-mottled biointrapelsparite and dolomitized micrite deposited in two cyclically alternating epeiric environments. A transitional shallowing phase produced the overlying 50 ft (15 m) thick dolomitized biomicrite. An alternation between very shallow marine and sabkha depositional environments produced the next 200 ft (61 m) of preserved section -- a succession of incomplete cyclic units, bounded by diastems. A typical cyclic unit proceeds through breccias, dololaminite or argillaminite, and mottled bioturbated dolomite, to massive, fine-grained dolomite. This over-all progression of depositional environments, from the marine to the subaerial, culminated near the end of Early Ordovician time with the sedimentary pile being uplifted at least 300 ft (92 m) above baselevel. Because of this uplift, caves developed in the limestone into which carbonate residues and detrital, dolomite-rich pisolitic sediment overlain by collapse breccias were deposited. Adjacent to caves, the calcite among the interdolomite mottle in the dolomitic limestone was dissolved, leaving an open network of undissolved dolomite network. In these areas of greatest porosity, sphalerite was deposited in colloform fashion about the mottle nuclei. A return to over-all subsidence caused marine inundation of the karst drainage system and the remaining open spaces were filled with white, coarsely crystalline dolomite. When marine flooding reached the uppermost erosion surface, deposition of the earliest Middle Ordovician Table Head sediments began. Two models for Zn precipitation can be suggested for this paleohydrologic system: 1) mobilization of 10 to 50 ppm Zn from host-rock carbonates by meteoric waters, and precipitation downstream at the interface with formation fluids; 2) precipitation of Zn by meteoric water dilution of upward-welling, Zn-rich formational chloride brines. End_Page 393------------------------ Newfoundland Zinc Mines location map. End_Page 394------------------------

Occurrences and Origin of the Ophiolites of Southern Quebec, Northern Appalachians

The ophiolites of southern Quebec are thrust sheets. Prior to their folding with the country-rock. they were emplaced as solid masses into the thick metasedimentary and metavolcanic geosynclinal prism of Notre Dame Trough, in Early Ordovician time. The occurrence of the ophiolites is controlled by their structural and stratigraphic position. Complete ophiolites occur as stratified sheets lying structurally above the Cambrian Caldwell Group, and they are overlain by a mélange assumed to be of Early Ordovician age. Dismembered ophiolites consist of peridotite sheets and lenses which may have been attached originally to the basal peridotite of the larger stratified sheets; they are tectonically intruded into Cambrian rocks.The stratified sheets have a simple, regularly-layered structure with no well developed sheeted-dike complex. Other feature s include a thin upper unit of gabbroic and basaltic rocks with a relatively thick ultramafic cumulate at the base and a thicker lower unit of Alpine peridotite. There is apparently no transition zone between the upper and lower units. All these features suggest that the ophiolites of southern Quebec represent possible fragments of an oceanic crust formed on a rapidly spreading ridge. It is assumed that they have been emplaced on the Early Ordovician continental margin by obduction and contemporaneously with the development of a subduction zone, which may have caused the magmatism that generated the adamellitic granites intruded into the ophiolitic complexes, and the calcalkatline volcanic rocks of the Lower to Middle Ordovician Ascot and Weedon Groups.

Continuation of the Highland Boundary fault in Ireland

A major geophysical lineament in Ireland is correlated with the Highland Boundary fault in Scotland; it is shown to separate a southern block, deformed in Middle to Late Cambrian time, from a northern block that was deformed in early Ordovician time.

The Precambrian Shield and the Lower Paleozoic Shelf: The Unstable Provenance of the Lower Paleozoic Flysch Sandstones and Conglomerates of the Appalachians between Beaumont and Bic, Quebec

Three sections of lower Paleozoic flysch in the Quebec Appalachians were sampled at Beaumont, Bic, and Trois Pistoles in order to determine the position, composition, and relief of the source area. At Beaumont, the mean flow direction of sediment transport is east-southeasterly; at Trois Pistoles and Bic it is south-southeasterly. Locally there is strong dispersion of the data but no northerly directions have been observed.Albite is the common plagioclase in all Cambrian sandstones; grains generally show polysynthetic twins, but in a few beds only untwinned albite is present. Oligoclase and andesine are the dominant plagioclases in Lower Ordovician rocks. The accessory suite has few diagnostic species; pink garnet is present in all sections, but absent in the basal unit at Bic. Diopside, sphalerite, and barite are found only in the Lower Ordovician rocks at Beaumont. At the three localities, the feldspar content, grain size, and sand/shale ratio vary up-section.The original source area for the sandstones and conglomerates consisted of an early Paleozoic shelf and a Precambrian land-mass. The oldest sands were derived from Precambrian metasediments, Paleozoic shelf sediments, Precambrian sodic plutons, and to a minor degree from gneisses. In Late Cambrian time the major contributors were sedimentary rocks of the shelf, sodic plutons, and gneisses. By Early Ordovician time most of the sands were derived from Grenvillian gneisses and shelf sedimentary rocks.The relief of the Grenvillian Orogenic Belt was high and denudation rapid from Early Cambrian to Late Cambrian, suggesting continuous, but irregular uplift. Uplift of the shelf began early in Cambrian, with major movements occurring in Late Cambrian when the Grenvillian source was rejuvenated. The source area was stabilized by Early Ordovician time.

Antipathetic Relation between Cambrian Trilobites and Stromatolites: GEOLOGIC NOTES

Recent studies describe an antipathetic relation between animals which graze or disturb the bottom sediment and the growth of algal mats. Evidence from tidal to shallow subtidal deposits in the Middle Cambrian of Nevada and Utah suggests that under normal salinity conditions trilobites were abundant and algal stromatolites absent. Along the margins of hypersaline pools within the carbonate bank stromatolites flourished. Salinity conditions there were unfavorable for trilobites and other bottom-dwelling invertebrates. The development of higher than normal salinities across the North American shelf was an important contributing factor in the reported low diversity of animal life in Cambrian through Early Ordovician times.

Cambrian and Lower Ordovician Stratigraphy in Bighorn Mountains and Associated Uplifts in Wyoming and Montana: ABSTRACT

New information concerning Cambrian and Lower Ordovician stratigraphy in Wyoming and Montana is derived from 26 measured sections in uplifts along a 430-mi trend from the Rawlins uplift on the south, through the Ferris Mountains, Rattlesnake Hills, Bighorn Mountains, Prior Mountains, Big Snowy Mountains, and Little Rocky Mountains on the north. Seas transgressed east-southeastward up a paleoslope of low regional relief but with several hundred feet of local relief. The sedimentary record consists of a series of lithotope belts each extending generally parallel with the shoreline. From the shore seaward the order is as follows: (1) sandstone, generally clean, medium- to coarse-grained, crossbedded and burrowed, deposited in beach, intertidal, and shallow subtidal zones; (2) sandstone, generally clean, fine-grained crossbedded, laminated and burrowed, deposited in the shallow subtidal zone; (3) sandstone, the same as (2) except a glauconitic greensand and may be interbedded with siltstone and shale; (4) siltstone, may be interbedded with fine-grained sandstone and shale; (5a) shale, usually burrowed, with micrite beds and nodu es, seaward from (4) in deeper water; (5b) shale and intraclast limestone conglomerates indicate shallower water and offshore intertidal zone environments; (6) laminated limestones, intraclast limestone conglomerates, and crinozoan calcarenites suggest an offshore shallow subtidal and intertidal environment. The faunal record begins with the Bathyuriscus-Elrathina Zone of medial Cambrian age and ends with the Bellefontia Zone of Early Ordovician age. Trilobites and brachiopods dominate the faunas but conodonts assume major importance in latest Cambrian and Early Ordovician time. Both faunal-zone boundaries and key beds are isochronous surfaces that form planes of correlation within lithotopes and across facies boundaries between lithotopes. The Bathyuriscus-Elrathina Zone and lower Bolaspidella Zone are of lithotopes (1) through (5). The upper Bolaspidella Zone reflects an abrupt east-southeastward shift in depositional environments and only lithotopes (3) through (5) are present. The Cedaria Zone is largely (4) and the Crepicephalus and Aphelaspis Zone are (5b). Seas withdrew after initial Aphelaspis Zone deposition and returned at about the beginning of the Elvinia Zone. Elvinia, Taenicephalus, Idahoia, and early Saukiella Zones are (5b) with the shale dominant at the base and limestone increasing upward. Late Saukiella, Mississquoia, and Bellefontia Zones are of lithotope (6). An unconformity variably truncates the section and younger strata lie on rocks ranging in age from Bellefontia Zone limestones in the Big Snowy Mountains to Bolaspidella Zone greensands in the Rawlins uplift. End_of_Article - Last_Page 914------------

Palaeomagnetic Studies in the British Caledonides?VI Regional and Global Implications

Summary Palaeozoic palaeomagnetic data from the British Isles are reviewed and are found to be consistent with erratic polar shift from the vicinity of 10° N, 180° E in the Ordovician to 0°, 145° E in the Siluro-Devonian and to 25° N, 160° E in the early Carboniferous. The Cambrian pole is less well established near 25° N, 170° E. The rates and times of shifts are discussed. World-wide palaeomagnetic data suggest the existence of three groups of continents in the Early and Middle Palaeozoic—the Gondwanan, Euramerican and Siberian groups—separated by large oceans which closed in the Late Palaeozoic along the lines of the Hercynides and Uralides. The data from Gondwana and Siberia are internally consistent and each may have been a single plate to which only peripheral slices were added subsequently. Data from Euramerica are less coherent. The most probable explanation seems to be that this group of continents was loosely-knit and consisted of the North American and the Baltic/Russian plates (which joined in the Silurian on the site of the Scandinavian Caledonides) and the British Isles sub-plate which joined in the Devonian (on previously unrecognized sutures lying in the present North Atlantic and North Sea respectively). This interpretation takes all the data at face value and assumes that time-coverage is adequate in all parts of the region. Alternatively, if time-coverage was not adequate, Britain might have been part of the North American plate, but further work is required to discriminate between these interpretations. Palaeomagnetic evidence from the British Isles also indicates that little or no closure has taken place across the British Caledonides since Early Ordovician time. If so the British Isles sub-plate may have been rotated in azimuth as well as being transported laterally before joining the Euramerican plate. Examples of rotation and remagnetization on a more local scale are cited, and are ascribed to tectonic and thermal effects of orogeny respectively. The Euramerica and Gondwana groups were already close together by early Carboniferous times, which is distinctly earlier than the Hercynian-Alleghenian orogenic belt which may mark their juncture. It may be generally true that large scale crustal drift, involving consumption of a large area of oceanic crust between two continents, significantly predates the orogenic belt which forms when the two continents have met. If this is so, large scale closure across the British Caledonides might have occurred in Precambrian or Cambrian time, but palaeomagnetic data are not yet adequate to evaluate this.

Large-scale Horizontal Displacement within Australo-Antarctica in the Ordovician

Crawford and Campbell have postulated1 (page 11) that the southern continental margin of Australia between longitude 131° ? and 143° ? “is the relic of a major shear which in Early Ordovician time extended still farther across what is now north-eastern Tasmania and that dextral movement of at least 300 km occurred”. We believe that this hypothesis is without foundation and that the curvature of the southern portion of the Adelaide Orogen and the curvilinear pattern of the Mount Read Volcanics in Tasmania, which were cited to support their arguments, can be explained without invoking1 (page 12) “fault-drag on an immense scale” on opposite sides of the postulated shear (see line AB in Fig. 1).

Remnant Mountains in Early Ordovician Seas of the El Paso Region, Texas and New Mexico

The Thunderbird mass of Precambrian rhyolite porphyry in the Franklin Mountains of westernmost Texas is a low remnant mountain that protruded above the floor of the Early Ordovician seas. This extrusive rhyolite, dated at 950 m.y., is surrounded and overlapped by limestone of the Early Ordovician El Paso Group. Sparite-cemented masses of rhyolite talus border the mountain, grading into dolomitized biointrasparrudite containing many large angular clasts of rhyolite. This rhyolite mountain and other Precambrian-rock islands in the region provided detritus to the Early Ordovician seas of southern New Mexico, west Texas, and northern Mexico. Preservation of such highlands for much of Early Ordovician time appears to be extraordinary. The El Paso limestone of the Franklin Mountains, however, reflects in entirety an environment ranging from supratidal to shallow-water marine, as evidenced by the occurrence of such features as polygonally cracked micrites, algal stromatolites, digitate algae, and archaeoscyphid mounds. Protection of the remnant mountains from high-energy waves can be attributed to the regional, virtually intertidal, mudflatlike environment; the occurrence of archaeoscyphid biohermal barriers; periods of submergence below wave base; or a combination of these factors. The sedimentary features imply that a vast region of remarkably uniform shallow-water to supratidal environmentals existed during Early Ordovician time.

Geography and Faunal Provinces in Tremadoc Epoch: ABSTRACT

A major change took place in trilobite faunas during Late Cambrian and Early Ordovician time, that is, during the Tremadoc Epoch. The majority of preexisting families died out shortly before or during this epoch. They were replaced by several short-lived, new groups and the earliest members of other families which dominated succeeding Ordovician trilobite faunas. The dissimilarities between samples of Tremadoc trilobite faunas from 18 areas in the world were analyzed by a nonmetric multidimensional scaling technique. Groupings obtained by such analyses reveal faunal provinces of the Ordovician, and for the early part of the period 4 provinces were recognized. The analyses of Tremadoc faunas should reveal whether or not changes in provinces occurred during this epoch. The ssumption that a faunal province originally extended over the shallow seas surrounding a single continental mass is used, with palaeomagnetic data, to propose a model for Tremadoc paleogeography. End_of_Article - Last_Page 662------------

Problems on the origin of ore deposits in the lower Ordovician formations of east Tennessee

Most recent workers in the East Tennessee zinc and barite districts are in general agreement that the host breccias were formed by solution-collapse processes in early Middle Ordovician time, probably in an ancient carbonate aquifer system of regional extent. There is little agreement, however, on the source and nature of the ore fluids from which the epigenetic minerals were formed. In this symposium Hanshaw and his colleagues have presented evidence in support of the concept that much secondary dolomite may have been formed by ground water. On the other hand, Roedder has shown that fluid inclusion studies indicate hot saline brines (75 degrees -150 degrees C) as the source fluids for the ore minerals, and he suggests that they may have been deeply circulating connate brines. Other investigators in the past have proposed a variety of source fluids, including juvenile waters, meteoric waters that have become enriched by the leaching of particular elements from the surrounding rock, and sea water.In the zinc deposits, sphalerite is accompanied locally by minor amounts of galena and pyrite, and occasionally, traces of chalcopyrite. Dolomite is the chief gangue mineral and is associated with lesser amounts of quartz, calcite, barite, fluorite, anhydrite, gypsum, and bitumen. Some deposits containing barite as the dominant ore mineral have various amounts of fluorite and pyrite; sphalerite and galena may be present locally along with the other gangue minerals typical of the zinc deposits. The paragenetic sequences of the minerals have been determined for most of the mines and many of the prospects, and, except for minor refinements, will be little changed by additional studies. The real problem, though, is the correlation of these depositional sequences from mine to mine and from district to district. Perhaps the major difficulty in the correlation is the seemingly uncomplicated mineralogy that disguises a rather complex regional paragenesis.Questions regarding the routes the ore fluids travelled to reach the sites of deposition, the reasons why the ores are concentrated in some areas more than others, and the reasons why the mineralogy varies from area to area are questions that are unanswered or at best only partly answered. Studies to date have produced convincing evidence concerning the nature of the structures and the physical conditions responsible for the localization of ore in the Kingsport Formation (Lower Ordovician). On the other hand, the relation of the Kingsport ores to the smaller deposits in the underlying Lower Ordovician and Cambrian rocks is but little understood, and no final answer to the origin of these deposits can be reached until this relationship has been satisfactorily explained.Fluid inclusion studies, trace element analysis, and isotopic studies of all sorts are needed to further our understanding of these seemingly simple ores. Continued detailed studies of field relations, both regional and local, are of major importance, not only for what they will reveal in themselves, but also to perfect the frames of reference in which to place the growing mass of laboratory data.

Evidence on the age of barite, zinc, and iron mineralization in the lower Paleozoic rocks of east Tennessee

Based on a study of minor occurrences of zinc, barite, and iron in East Tennessee, certain deposits are interpreted to have formed during early Middle Ordovician time. Principal lines of evidence supporting this age are: (1) the occurrence of barite, sphalerite, and pyrite associated with synsedimentary collapse breccias which span the pre-Middle Ordovician unconformity at the Lost Creek barite mine in Union County; (2) barite in basal Middle Ordovician beds (Athens Shale and Lenoir Limestone) in northeastern Tennessee and southwestern Virginia; (3) sphalerite and pyrite in dolomite filling paleocaves in the uppermost part of the Lower Ordovician Mascot Dolomite at the Trotter prospect near Douglas Dam in Sevier County; and (4) oolitic hematite and pyritiferous beds near the base of the Athens Shale and subjacent pyritiferous collapse breccias in the Mascot and Kingsport Formations at several localities along the eastern side of the Appalachian Valley. Thus, it is concluded that ascending solutions formed epigenetic deposits in pre-Middle Ordovician carbonate strata nearly contemporaneously with syngenetic deposits in early Middle Ordovician sediments.These findings are in general accord with those of recent workers in the Mascot-Jefferson City and Copper Ridge districts who have proposed that mineralization in these areas occurred during the time interval of pre-Middle Ordovician erosion. The long-held theory which associates mineralization with late Paleozoic diastrophism is not supported.

An early middle Ordovician age for collapse breccias in the east Tennessee zinc districts as indicated by compaction and porosity features

The carbonate strata of the Lower Ordovician formations in the southern Appalachian Valley were probably considerably thicker, less compacted, and more porous and permeable in early Middle Ordovician time than they are now. Analysis of selected compaction features of rocks associated with the breccias of the East Tennessee zinc districts permits quantitative estimation of the relative compaction (reduction in thickness) these sediments have undergone before and after brecciation.Total reduction in thickness of certain Lower Ordovician marker beds because of compaction during burial under younger Paleozoic sedimentary rocks and imbricate Appalachian thrust sheets appears to be at least some 40 to 50 percent. Furthermore, it is also estimated that about half of this compaction took place after brecciation in the solution-collapse structures of the zinc deposits. In comparison with laboratory compaction tests on carbonate muds it is apparent that the Lower Ordovician rocks had been buried to a depth of at least several thousand feet by the end of the Paleozoic Era. On the other hand the zones of collapse breccias probably formed at depths of generally less than a thousand feet, thus indicating an early Middle Ordovician age for their development. The general character of the rock during this time probably was similar to that of Tertiary limestone, which acts as the principal aquifer in the Coastal Plain region of southeastern United States.

Appalachian strata-bound deposits; their essential features, genesis and the exploration problem

Zinc deposits of Appalachian type were formed in nearly horizontal strata and were subsequently folded and faulted by Appalachian orogeny. In their original nearly horizontal position they closely resemble deposits of the Mississippi Valley with which they are usually classified. With the exception of the Austinville-type deposits, which will not be considered herein, the subject deposits are related to a Lower Ordovician paleoaquifer. The classic examples are Friedensville, Pennsylvania; Timberville (Shenandoah Valley), Virginia; Mascot-Jefferson City district, Tennessee; and Copper Ridge district, Tennessee. Representatives of deposits of this type hereinafter referred to as Tennessee sub-type extend the length of the Appalachian-Ouachita chain from New-foundland to Oklahoma. Except for the superimposed structural variations, these deposits are remarkably similar in their essential characteristics which include Lower Ordovician carbonate sedimentary host, low-iron sphalerite, minor to insignificant galena and pyrite, pervasive dolomitization, sulfide deposition at shallow depth in an essentially lithified section, and sulfide mineralization restricted to a fairly narrow vertical range of about 400 feet, more or less.The recently discovered zinc deposits in the Lower Ordovician of Middle Tennessee are similar in all essential respects to the Tennessee sub-type as described above. Because the Middle Tennessee deposits in the Mid-Continent structural province have not been subjected to deformation of the Appalachian orogeny, they furnish dramatic new evidence of the similarity between Mississippi Valley and Appalachian type deposits.The preorogenic structures of the Lower Ordovician carbonate sediments as modified by the paleoaquifer in the mining districts of East Tennessee appear to be analogous to those of the contemporary aquifer in Tertiary carbonate sediments of the Florida and Yucatan Peninsulas. The Lower Ordovician was characterized by a widespread permeable karst system developed by dissolution of carbonate rocks by meteoric water. Probably in early Middle Ordovician time deposition of sulfide ore began from concentrated saline water which probably contained lime, magnesia, chlorine, silica, sulfur, and ammonia, as well as zinc. The pregnant saline solutions moved into and through the paleoaquifer towards areas of lower hydrostatic potential capturing the discharge areas originally established by the karst system and displacing the fresh water upward.The deposition of zinc sulfide was controlled by a complex of physical and chemical factors which are obscure. Factors which may have been significant are the availability of hydrogen sulfide, the interface of fresh and saline water, and a critical pressure-temperature zone at which a complexing agent would volatilize or which would otherwise trigger sulfide deposition. It appears that the factors controlling deposition were quite sensitive to depth below surface. Probably because of this sensitivity to elevation, host rocks generally considered favorable are mineralized only in local areas. Subtle structures, now obliterated by later tectonism, may have been instrumental in localizing the deposition in the permeable zones only within the critical elevation or pressure-temperature range. In any case, pregnant saline solutions traveled extensively through the aquifer causing widespread dolomitization which with other ore-associated alteration phenomena is much more extensive than the ore itself. Exploration for these deposits should take into account the above factors.The source of the zinc is elusive, and there appears to be nothing in the deposits, their history or environment that is not ambiguous relative to this question. Nevertheless, the diagenesis of regionally adjacent basins of thick argillaceous accumulations is considered to be the probable source of zinc-bearing brines from which the ore was deposited.

Ore controls and related sedimentary features at the Flat Gap Mine, Treadway, Tennessee

The Flat Gap ore body is a strata-bound, low-grade sphalerite deposit in Lower Ordovician carbonate beds that have a 37 degree dip. The mineralization is localized in breccia bodies formed in the Mascot and Kingsport Formations when the beds were flat lying. Uplift of the sediments at the end of Early Ordovician time resulted in the development of an unconformity of major significance. Limestone beds, located in the upper part of the Kingsport, served as a paleoaquifer of regional extent. Meteoric waters, circulating along faults and joints in the aquifer, dissolved porous limestone beds to the extent that interbedded and overlying dolomite beds collapsed forming long, narrow breccia bodies. These locally termed fine, rock-matrix breccias, are older than the ore-bearing breccias and appear to be the framework upon which the main ore trends developed.Warm metal-bearing brines, introduced into the aquifer shortly after the initial periods of dissolution and collapse, extensively enlarged, extended, and mineralized the older breccias as well as developed new breccias along joints essentially normal to the main trend. Both chemical and mechanical erosion of the solution channels resulted in the accumulation of debris at the bottom of the breccias. These accumulations, locally termed consist of a poorly-sorted sediment of dolomitized limestone, fine-grained dolomite, and bleached chert fragments as well as ubiquitous sand-size accumulations. Above the trash zones, the breccias are cemented with white crystalline dolomite and sulfides. The sulfides are not restricted to the later breccias but are present in the trash zone as well, occurring as breccia fragments, open-space filling, and possibly replacement. Mineralization appears to have been essentially completed before the deposition of Middle Ordovician Chickamauga sediments.The Flat Gap ore body has two distinct types of ore; one is older and contains galena-bearing, dark reddish-brown sphalerite, the other is galena-free and contains yellow sphalerite. The dark ore is confined to a single, faulted, eastward-plunging, narrow shoot, that is at least 1 1/2 miles in length. Radiating from the dark ore shoot in a net-like pattern are numerous yellow sphalerite shoots. The entire ore body is cut by several large post-ore faults that have horizontal displacements of as much as 1,500 feet.

Tertiary limestone aquifer system in the southeastern states

The hydrogeologic history of the Tertiary limestone system of the Southeastern States is reconstructed, especially as it relates to circulation of ground water and the development of solution cavities. The development of these solution cavities resembles in many respects the development of cavities in carbonates of the Knox Group of Tennessee during Middle Ordovician time, the cavities in the Knox having since been filled with collapse breccia that has been recemented. Some general principles of the circulation of water in limestone terranes and the related development of solution openings are reviewed so that a generic basis for comparison can be made of the modern southeast carbonate setting with the Ordovician carbonate setting in Tennessee.The major requirements for solutional development as cavities--(1) presence of highly soluble material, (2) a fracture system or some other form of incipient permeability, (3) water undersaturated with respect to soluble rocks, such as recharge from precipitation, and (4) hydraulic gradient--are found in the Tertiary limestone terrane of the southeast; much of the limestone has been elevated above sea level as a homoclinal seaward-dipping unit. Such a broad homoclinal setting, which also existed in the Knox at the close of early Ordovician time, facilitates extensive solutional development in the upper part of the zone of saturation. Circulation of water great enough to form a significant cavern network requires concentrated discharge areas, commonly as entrenched permanent streams or near-shore springs and seepage. This condition prevails where the Tertiary limestone is fairly close to land surface.Reconstruction of the geologic and hydrogeologic history of a carbonate region generally reveals the extent of early solution and karst development in relation to current karstification. Caverns, partly filled in some cases with loose or poorly cemented rock fragments that have fallen from cavern roofs, give evidence of karstification that is either current or that probably developed since the last marine inundation of the carbonate terrane. On the other hand, filling of caverns with overlying rock debris and reconstituting the debris into breccia are conditions that require evaluation of paleohydrology.

Early Ordovician Sedimentary History of Marathon Geosyncline, Trans-Pecos, Texas

The Lower Ordovician Marathon Formation, exposed in the Trans-Pecos part of the Ouachita-Marathon fold belt, consists of cyclic alternations of pelagic shale and gravity-displaced beds of siltstone, sandstone, calcarenite, calcisiltite, and conglomerate. Gravity displacement is inferred from the numerous examples of the Bouma turbidite cycle, from displaced faunas, slump structures, and boulder beds present in the formation. The Marathon Formation is a slope facies whose sediments, derived from mixed terrigenous and shallow-water sedimentary, igneous, and metamorphic sources, were deposited in relatively deep (30-300 m) water by turbidity currents, debris flows, slumps, and submarine landslides. The existence of this slope facies during Early Ordovician time implies tha a trough (Marathon geosyncline) with a well-developed shelf-slope-basin geometry already was functioning as a depositional basin. Three intraformational exotic boulder beds are evidence of locally intense tectonic movements along the northwest or cratonal side of the geosyncline. These movements produced large-scale (up to 21 cu km), isochronous downslope displacements (olistostromes) of shelf and upper-slope sediments. Volcanism occurred prior to, and probably during, this tectonic activity. Volcanic boulders are present in the boulder beds, and volcanic fragments are common in sandstone and calcarenite beds throughout the formation. The submarine landslides and the volcanic fragments show that cratonal areas marginal to the Marathon geosyncline were tectonically active even during very early stages of the geosynclinal cycle.

XI.—The Evolution of the Scottish Caledonides in Relation to their Isotopic Age Pattern

SynopsisThree main stages are recognized in the evolution of the Scottish Caledonian orogen. Firstly, the development of the depositional framework from late Pre-Cambrian to early Ordovician times is outlined, The thick Moine and Dalradian sediments, accumulating on a continental rise, are shown to be equivalent to the shelf sequence of the foreland, and likely correlations are suggested. Secondly, the events comprising the Caledonian deformation and metamorphism of these sediments are reviewed and related to the development of a Benioff zone and coupled oceanic trench along the southern margin of the orogen through the Ballantrae complex. Comparison of stratigraphic and isotopic evidence for the age of these events leads to the conclusion that all major deformation and metamorphism occurred during a relatively short climactic episode 480–510 m.y. ago, within the Arenigian stage of the Ordovician. The third factor in the evolution of the orogen is post-climactic uplift and erosion continuing throughout Upper Ordovician, Silurian and Lower Devonian times. Contours of K–Ar mica ages are presented and related to the geological evidence for this prolonged period of isostatic recovery and thermal adjustment. The style and timing of granitic plutonism, which is closely associated with this third stage, may be indicative of crustal behaviour during uplift rather than continued metamorphism at depth.