Cambrian Sedimentary Rocks Research Articles

Denudation surfaces and tectonics in the southernmost part of the Baltic Shield

Abstract The sub-Cambrian peneplain is a fundamental landform in the scenery of the southernmost Baltic Shield. It had been shaped during an undefined time interval in the Late Precambrian and became conserved below a cover of Cambrian sedimentary rocks. After exhumation, the peneplain was subjected to further remodelling. In the study of the Phanerozoic denudational and tectonic history, the sub-Cambrian peneplain forms an important reference surface. Mesozoic and Tertiary denudation surfaces and sediments dominate in the south and reveal the sequence of events later in the Phanerozoic.

Complex regional fault history of the Badger Head region, northern Tasmania

Superimposed contraction, extension and oblique‐slip faulting has juxtaposed and segmented blocks and slices of Precambrian metasedimentary rocks, Cambrian marine sedimentary and ultramafic rocks, and Early‐Middle Palaeozoic strata near the boundary between the East and West Tasmania Terranes in the Badger Head region of northern Tasmania. Of greatest significance was post‐Early Devonian southwest‐directed thrusting which caused thin‐skinned style imbrication and led to juxtaposition of the East and West Tasmania Terranes. Polydeformed Precambrian meta‐turbiditic rocks of the Badger Head massif are part of a fault‐bounded allochthonous slice which was brought to the present structural level at this time. An inferred west‐directed Cambrian thrusting event emplaced ultramafic rocks (Andersons Creek ultramafic complex) and produced a tectonic melange (Port Sorell Formation), possibly within an east‐dipping accretionary wedge. The Cambrian event is considered to have involved an imbricated accretionary comple...

Do ages of authigenic K-feldspar date the formation of Mississippi Valley-type Pb-Zn deposits, central and southeastern United States?: Pb isotopic evidence

Pb concentrations and isotopic compositions have been determined for authigenic over-growths and detrital cores of K-feldspar from Cambrian sedimentary rocks in Texas, Tennessee, and Pennsylvania (referred to as group 1) and southeastern Missouri (referred to as group 2). Overgrowths and cores were separated by abrasion and analyzed separately. In three group 1 samples, cores have Pb isotopic ratios similar to initial ratios in K-feldspar from nearby Middle Proterozoic basement rocks ( 206 Pb/ 204 Pb =16.7-17.0). Pb ratios in overgrowths are radiogenic ( 206 Pb/ 204 Pb up to 19.3) and mostly are distinct from those in ore deposits in the southern Appalachians. Pb isotopic data for two samples of K-feldspar from southeastern Missouri are scattered; in one sample, cores and overgrowths contain radiogenic Pb, suggesting at least two periods of authigenic replacement, in agreement with petrographic evidence. Proterozoic material must still be present because 40 Ar/ 39 Ar stepwise analysis of a total (cores plus overgrowths) sample yields incremental ages ranging from about 500 to 750 Ma. The disparity in Pb isotopic ratios of group 1 overgrowths and Pb in nearby Mississippi Valley-type deposits implies that the regional authigenic K-feldspar event was not synchronous with ore deposition in the southeastern United States. In contrast, Pb isotopic ratios from group 2 authigenic K-feldspar are similar to ratios in ores of southeastern Missouri, suggesting a genetic relation in late Paleozoic time.

Ordovician impact crater at Kärdla, Hiiumaa Island, Estonia

The buried and well preserved Kärdla crater in Estonia (Southern Fennoscandia) (58°59Ń, 22°40É), with a diameter of approximately 4 km, was discovered in 1967. The subsurface structure of the crater has been studied by gravity anomaly measurements, aeromagnetics and by more than 300 boreholes in the crater, on the rim (“ring wall”) and in the surrounding area. Recent boreholes, reaching 800 m, have revealed that the crater is 540 m deep and has 100 m high central peak. Barely visible in the present topography, the crater is filled with Palaeozoic and Quaternary deposits and is mostly low marshy land surrounded by a low ridge along the ring wall. Quaternary landforms, such as eskers and post-glacial beach ridges, also complicate the topography. In the buried sub-Quaternary bedrock relief, the crater occurs as a roundish depression bordered with two, 10–30 m high horseshoe-shaped elevations. The bedrock topography was slightly drumlinized by glacial excavation. The crater formed 455 Ma in composite stratigraphy: middle and lower Ordovician (20 m) and Cambrian (120 m) sedimentary rocks overlying crystalline basement on the bottom of a shallow nearshore sea. The subsurface structure of the crater is well preserved. In vertical section from the top are: (1) post-impact cover of Ordovician sedimentary rocks, 15–100 m thick; (2) allochthonous breccias (filling the lower part of the crater) and beds of fall-out breccias and conglomerates, sandstones and sandy limestones consisting of debris of reworked fall-out breccia and wall rocks surrounding the crater; (3) a body of autochthonous and subautochthonous breccias forming the bottom and the central peak of the crater and also remnants of its rim. Shocked rocks and minerals from autochthonous and allochthonous breccias have been revealed by optical microscopy.

Thermochronologic constraints on the timing of Mississippi Valley-type ore formation from apatite fission track analysis

Apatite fission track studies have been initiated in three major Mississippi Valley-type (MVT) ore districts in order to place Pb-Zn mineralization within a regional thermal framework, and to place constraints on the timing and/or duration of ore formation. Data from Precambrian basement and Cambrian sedimentary rocks near the Viburnum Trend of southeast Missouri indicates that the mid-continent region has been heated in the Mesozoic to temperatures similar to those estimated for one formation, followed by final cooling in the Late Cretaceous. The Pine Point district, Canada, has been heated to temperatures comparable to those inferred for sulphide precipitation for an appreciable time only once since the Devonian, with cooling from these temperatures at some time after 100 Ma. The Lennard Shelf district, Australia, has probably experienced two major phases of regional heating following Devonian sedimentation. Peak temperatures estimated from well sequences for Late Paleozoic/Early Mesozoic burial are generally less than those estimated for ore formation. However, an earlier phase of heating to higher temperatures is predicted for the Late Devonian/Early Carboniferous. These studies indicate that major MVT ore districts have been regionally heated in the past to temperatures similar to those estimated for ore formation and suggest that MVT ore emplacement may be related to broad regional heating rather than to discrete fluid pulses.

Thrust-related metamorphism in basement windows of the central Scandinavian Caledonides

The late Caledonian metamorphic evolution of the basement beneath the Caledonian allochthons of the central Scandes has been determined by studying mafic schists and phyllonites of the crystalline basement, and the overlying Vendian and/or Cambrian sedimentary rocks. The studied rocks occur along the Caledonian front and in tectonic windows within the Scandinavian Caledonides. A gradual westwards increase in metamorphic grade is recorded, which varies from upper diagenetic conditions in the autochthonous rocks along the eastern thrust front of the Caledonides to middle amphibolite facies in the western parts of the Nasafjäll and Tømerås windows. Locally, strain-related deviations from this pattern occur. Further west the grade of Caledonian metamorphism of the basement increases and reaches the eclogite and granulite facies. The Scandian metamorphic conditions in the basement rocks studied prevailed in the mid- to late Silurian, at pressures of 8 ± 2 kbar and temperatures of 550 ± 50°C in the Tømmerås window, and 7 ± 2 kbar at 550 ± 50 °C in the Nasafjäll window. Uplift and cooling followed thereafter, continuing into the Devonian. The final retrograde metamorphism was characterized by temperatures of 450 ± 50°C at pressures of about 3 kbar in the Nasafjäll and Tømmerås windows. The occurence of syn-schistosity amphibolite-facies assemblages implies heating of the basement rocks during nappe emplacement. Available time constraints suggest that this was a relatively rapid process. In late Silurian to early Devonian, the prograde phase was followed by uplift and erosion. The metamorphic evolution also demonstrates that early Devonian thrust movements were associated with a decrease in the thickness of the nappe pile.

Tectonic setting of cambrian rifting, volcanism and ophiolite formation in western tasmania

In western Tasmania, small Precambrian regions are surrounded by a ramifying system of troughs filled with Cambrian sedimentary and volcanic rocks, and ophiolite complexes. The volcanic associations include a rift-related olivine tholeiite association, dacite-rhyolite and andesite associations, and a low-Ti, high-Mg andesite-tholeiite ophiolite association, and may have formed during a long-lived period of crustal thinning, punctuated by episodes of crustal rupturing, magmatism, and small scale rifting. Such extensional tectonism could occur in an active continental margin associated with strike-slip faulting of regional scale, and the volcanic associations may together constitute an igneous assemblage characteristic of magmatism in a transcurrent tectonic regime within an active continental margin undergoing break-up. The western Tasmanian Cambrian palaeogeography and volcanism preserve a transitional stage between the Late Proterozoic Kanmantoo regime of sedimentary basins with little volcanism developed at the rifting margin of the Proterozoic craton, and the tectonic regime of the Palaeozoic Lachlan Fold Belt where the Cambrian volcanic rocks are dominated by island-arc associations and the rift-related olivine tholeiite association is absent. Eastern Australian lithosphere may have grown by the insertion of newly-formed igneous complexes within the stretched and rifted continental margin, as well as by the accretion of “terrenes” and the addition of packets of subduction complexes which developed off-shore.

The bedrock of the Vaasa circular structure, Western Finland

The Vaasa circular structure (sometimes called the Soderfjarden basin) consists of roughly circular or polygonal flat cultivated ground with a diameter of 5–6 km and a surrounding bedrock ring rising 50 m above the centre of the basin and the sea. The maximum basin depth is about 300 m and is filled with Lower Cambrian sedimentary rocks with the uppermost levels being loose overburden of silts with a depth of 30 to 70 m.

Energy resources of Soviet arctic and subarctic regions

Energy resources of Soviet arctic and subarctic regions

The source of lead in the Osen sandstone lead deposit on the Baltic Shield, Norway

The sandstone-hosted lead deposits on the Baltic Shield occur in marine late Precambrian to Early Cambrian sedimentary rocks. A ground-water model in which the basement is postulated as the metal source has been proposed for these deposits. To test this model the Osen deposit was selected because it lies directly on a homogeneous granite basement.On the basis of U-Th-Pb whole-rock isotopic analyses for three samples of unweathered basement granite (Trysil granite), a basement age of 1,598 m.y. was established. The lead isotopic compositions of four out of five specimens of galena from the Osen deposit plot between isochrons calculated for the granite at 500 m.y. and 540 m.y. This is strong support for the hypothesis that the lead source was basement granite and suggests that mineralization took place soon after sedimentation. A basement source for the lead is further substantiated by the regional pattern of lead isotopic compositions of sandstone lead deposits, which reflects the character of adjacent basement terranes.

Ordovician cauldron subsidence of the Blue Hills Complex, eastern Massachusetts

The Blue Hills Complex, which lies south of the Boston Basin in eastern Massachusetts, is composed of Cambrian sedimentary rocks and Late Ordovician alkalic plutonic and volcanic rocks. On the north, the Complex is separated by a fault from the Boston Bay Group, which very recently has been shown to be late Precambrian. On the south, the Complex is overlain unconformably by the Pennsyvanian rocks of the Norfolk Basin. The bedding and volcanic flow structures of the Cambrian, Ordovician, and Pennsylvanian rocks are essentially parallel and dip steeply south. I believe that the Blue Hills Complex is a Late Ordovician cauldron subsidence that was tilted during the Alleghenian Orogeny and was thrust northward over the Boston Bay Group.

Revised stratigraphy of the Long Reach area, southern New Brunswick: evidence for major, northwestward-directed Acadian thrusting

In the Long Reach area of southern New Brunswick, a new stratigraphic succession has been delineated; it consists of Precambrian (?) volcanic rocks, Cambrian sedimentary, volcanic and hypabyssal rocks, Silurian sedimentary rocks, and Devonian plus Precambrian (?) heterogeneous, granitoid rocks. The northern boundary of this succession is postulated to be a northwestward-directed thrust fault of Acadian age. Other Acadian thrust faults are interpreted in the area and major reverse movement of the same age occurred along the Belleisle Fault.Mafic and felsic volcanic rocks that were previously thought to be either Precambrian or Silurian are demonstrably part of the Lower Cambrian section. Some of the granitoid rocks intrude Silurian strata and therefore cannot be basement to the Cambrian succession. Other granitoid rocks appear to be older and may be Precambrian in age.

Paleomagnetism of Lower and Middle Cambrian sedimentary rocks from the Desert Range, Nevada

Over 500 oriented samples were collected from a homoclinal sequence of Lower and Middle Cambrian terrigenous and carbonate miogeoclinal strata in the Desert Range, Nevada. Extensive thermal demagnetization was successful in isolating similar characteristic magnetization directions in red‐purple mudstones of the Wood Canyon Formation (Lower(?) and Lower Cambrian) and in gray limestones of the Carrara and Bonanza King Formations (Lower and Middle Cambrian). Lithologic and magnetic evidence suggest that these magnetizations were acquired penecontemporaneously with deposition. The similarity of the characteristic magnetization directions in these strata implies that little apparent polar wander occurred with respect to North America from early Early through middle Middle Cambrian time. The divergence of these directions from those from the partly coeval Tapeats Sandstone of the Colorado Plateau probably resulted from a net 36° clockwise rotation of the Desert Range section about a vertical axis. This rotation is probably due to mid‐Tertiary oroflexural bending but may in part have been caused by Mesozoic thrusting. The polarity of the geomagnetic field in early Middle Cambrian time appears to have been continuously reversed. Stratigraphic intervals of mixed polarity, possibly correlative with those found in the Tapeats, may be present in the Lower Cambrian Zabriskie Quartzite.

Similarities between the thick-skinned Blue Ridge anticlinorium and the thin-skinned Powell Valley anticline

The Blue Ridge anticlinorium in northern Virginia is a part of an integrated deformational system spanning the area from the Piedmont to the Appalachian Plateaus. Deformation intensity within the system decreases from east to west. Differences of opinion have emerged concerning the central Appalachians as to whether the basement rocks exposed in the core of the Blue Ridge anticlinorium are rooted or are allochthonous. Available surface and subsurface stratigraphic and structural data suggest that the anticlinorium may be a rootless thick-skinned analogue to the rootless thin-skinned Powell Valley anticline in the Valley and Ridge. Both structures were produced during the Alleghenian orogeny by similar deformational processes. The form of the Powell Valley anticline is attributed to duplication of about 4,575 m (15,000 ft) of sedimentary rock during approximately 16 km (10 mi) of northwest movement above a subhorizontal decollement. Similarly, the form of the Blue Ridge anticlinorium is attributed to duplication of about 9,150 m (30,000 ft) of igneous, metamorphic, and sedimentary rock during a minimum of 59 km (37 mi) of northwest movement above an eastward continuation of a subhorizontal decollement within Cambrian sedimentary rocks beneath the Valley and Ridge. Thus, in northern Virginia there is a mixing of structural styles: the thick-skinned rootless Blue Ridge anticlinorium sits above a thin-skinned detachment. This relationship implies that thin- and thick-skinned styles are simply end members of a complex deformational process that includes a transition zone, where characteristics of both styles commingle.

Papoose Flat pluton: A granitic blister in the Inyo Mountains, California

The Papoose Flat pluton (75 to 81 m.y. old) is one of several Mesozoic granitic bodies in the White-Inyo Range which are regarded as satellites of the Sierra Nevada batholith. The pluton, composed chiefly of quartz monzonite with K-feldspar megacrysts, crops out as an elongated east-trending dome, 16 km long and 8 km wide. It was emplaced within the southwest limb of the major southeast-plunging Inyo anticline, consisting of virtually unmetamorphosed late Precambrian and Cambrian sedimentary rocks. Stratigraphic units around the western half of the pluton have a regionally metamorphosed aspect and are tectonically and concordantly thinned to as little as 10% of their regional thicknesses without loss of Stratigraphic identity or continuity. Around the discordant eastern contact, however, wall-rock textures are hornfelsic, and there is little or no attenuation of the stratigraphic succession. Foliation, lineation, and preferred orientation of minerals are well developed in the western half of the pluton and its wall rocks and, together with the boudinaged wall rocks, are consistent with the geometry of strain required for attenuation of the stratigraphic succession. The structural evidence indicates that the granite penetrated discordantly through lower formations in the anticline and formed a “blister” in the fold limb beneath the stretched higher formations. Metamorphic mineral assemblages of the wall rocks suggest a maximum temperature of metamorphism of less than 600 °C and a pressure of less than 2.5 kb.

Aborted Proterozoic rifting in eastern Newfoundland

Recent geological mapping and geochemical studies of the late Proterozoic rocks of the Burin Peninsula in the southwestern part of the Avalon Zone of Newfoundland show the following sequence: (1) The basal Rock Harbour Group, a sedimentary assemblage ranging from conglomerates to turbidites, was derived from a subaerial volcanic–plutonic source region. (2) The overlying Burin Group, a belt of mafic volcanic and intrusive rocks that have alkalic affinities at the base but are dominated by oceanic tholeiite. (3) The Marystown Group, a bimodal sequence of alkalic basalts and rhyolites deposited in a subaerial environment unconformably above the Burin Group, and apparently conformably overlain by Infracambrian to Cambrian sedimentary rocks. Stromatolites occur within resedimented limestone breccias and conglomerates at the top of the Rock Harbour Group and throughout the Burin Group.These assemblages are interpreted to indicate that after a period of continental type bimodal volcanism, the Avalon terrain was rifted in Burin Group times to form some kind of small ocean basin, which was abruptly closed with a reversion to continental type bimodal volcanism of the Marystown Group.

Determination d'un nouveau pole paleomagnetique africain sur des formations cambriennes du Maroc

Thermal and alternating field cleaning treatments of Cambrian sedimentary and volcanic rocks from the Moroccan Anti-Atlas yield three directions of magnetization. The oldest, of high thermal stability, gives a new Cambrian pole 47°N, 42°E with an associated radiometric age of 530 m.y. A secondary magnetization, of lower thermal stability, has been superimposed during a thermal event which occurred in Permo-Triassic time, according to the palaeomagnetic evidence (pole 47°S, 88°E). A third component was acquired viscously in the present-day magnetic field direction. This study emphasises the possibility of thermal demagnetization measurements, particularly on hematite-bearing rocks, and of comparing superimposed magnetizations with radiometric resetting ages.

Geology and geochemistry of the St. Lawrence peralkaline granite and associated fluorite deposits, southeast Newfoundland

The southern part of the Burin Peninsula of southeast Newfoundland is marked by extensive Carboniferous granite magmatism, intruding Late Precambrian to Middle Cambrian volcanic and sedimentary rocks. The St. Lawrence pluton, representative of this Carboniferous magmatism, ranges from equigranular to porphyritic in texture, displays much evidence of gas brecciation, and contains no pegmatites or schlieren, all features of high-level intrusion. The bulk chemistry corresponds to cotectic compositions in the synthetic Q–Ab–Or–H2O system for 500 bar [Formula: see text], likewise suggesting shallow levels of intrusion and crystallization. The granite is alkaline to peralkaline and of commenditic affinity, with agpaitic indices around 1 and modal aegirine and riebeckite. Its chemical affinity with less silicic alkaline granites of the same age in southeast Newfoundland suggests an origin by differentiation of such granites. A high initial Sr87/Sr86 ratio of 0.722 suggests substantial interaction with or derivation from continental crust. The St. Lawrence fluorite deposits occur as veins confined to and derived from the granite during the final stages of cooling.

Developments in East-Central States in 1973

Total oil production in Illinois, Indiana, Kentucky, and Tennessee in 1973 was 45 million bbl, down 11.9% from 1972. Tennessee had a very slight increase; all the other states had decreases. Gas production in Kentucky was 69.7 Bcf in 1973, down 7%. Gas production in the other states is not significant. A total of 1,524 oil and gas tests was drilled in 1973, down 11.4% from 1972. Exploratory tests totaled 565, only 4 tests fewer than in 1972. Of these, 85 (15%) were successful. Tennessee had 98 oil and gas tests in 1973, up 4.6% from 1972. The number of exploratory tests was down 1.5%; development wells were up 18.5%. Total footage for all tests was 141,950, of which 97,282 ft was exploratory. Much of the activity was concentrated in Scott, Morgan, and Fentress Counties. Three oil fields, 4 gas fields, 2 oil pools, and 5 extensions (2 oil, 3 gas) were discovered in 1973. A dry 4,875-ft basement test was drilled in Macon County. A basement test is drilling in DeKalb County, and a projected Copper Ridge test in Smith County is shut down at 3,380 ft. Leasing activity increased markedly on the east flank of the Nashville dome. Primary deep objectives in Tennessee are probably the lower Knox and pre-Knox Cambrian sedimentary rocks, and shallower objectives are th upper Knox, Trenton, and Mississippian carbonate rocks. Crude-oil production in Tennessee in 1973 was 201,000 bbl, up 1.5%. Illinois had 556 oil and gas tests in 1973, down 7.8% from 1972. Exploratory tests were up 29.8%, from 151 in 1972 to 196 in 1973. Development wells were down 20.4%. Total footage drilled in oil and gas tests and service wells was 1,446,083, up 3.3% from 1972. Three oil fields, 11 extensions, 4 new-pool wildcats, and 7 new-pay zones in existing fields were discovered. The main focus of exploration was in southwestern Illinois in the Marion and Washington Counties area. Silurian reef production was the principal objective there, but several Trenton tests also were drilled. Nashville field in Washington County was the most significant discovery in 1973. Production is from a Silurian reef. Reserves in the field are estimated at more than 1 million bbl. Silurian reefs and Cambrian-Ordov cian strata are expected to be targets of exploration in 1974. Crude-oil production in Illinois in 1973 was 30,669,000 bbl, down 12.1%. Indiana had 139 exploratory wells (8 more than in 1972) and 100 development wells in 1973. Exploratory tests resulted in 14 (10.1%) successes. Seven oil fields, 3 gas fields, and 6 oil pools were discovered (2 of the new fields resulted from workovers, not included in AAPG-CSD statistics). The discovery well of Westphalia South, one of the most promising of the new fields, was completed in December for an initial daily production of 384 bbl. Production is from the Ste. Genevieve Limestone. The trap appears to be stratigraphic rather than structural. Exploratory activity was concentrated in the belt of known Silurian reefs. Three counties within this area (Daviess, Greene, and Knox) accounted for more than half of the exploratory tests in the state. Exploration in 1974 is expected to f llow the trend of recent years. Silurian reefs, Knox erosional remnants at the Lower Ordovician-Middle Ordovician unconformity in eastern Indiana, and the Middle Ordovician (Trenton) of northern Indiana are expected to be targets. Estimated production in Indiana in 1973 was 5,400,000 bbl, down 730,000 bbl from 1972. A total of 631 oil and gas tests was drilled in Kentucky in 1973. Of 164 exploratory tests drilled, 34 were successful (20.7%). The exploratory successes included 8 fields, 20 deeper or shallower pools, and 6 extensions. Intensive leasing in many parts of Kentucky reflected an interest in deeper Paleozoic strata. Geophysical activity may have reached an all-time high in all parts of the Commonwealth. In Johnson County, a Precambrian test reached a new depth record of 14,566 ft. Other deep wells in 1973 included a 10,000-ft Tomstown test and a 5,877-ft Knox test, both in Johnson County, and a 4,584-ft Copper Ridge test in Morgan County. A Knox test was nearing completion in Logan County. Crude-oil production in Kentucky in 1973 was 8,686,854 bbl, down about 1 million bbl from 1972. Gas production was 69,737,007 Mcf, down about 5 Bcf.

Chaos Structure and Turtleback Dome, Mineral Ridge, Esmeralda County, Nevada

Blocks of late Precambrian and Lower Cambrian sedimentary rocks are in fault contact with the underlying complexly deformed Pre-cambrian units of Mineral Ridge in Nevada. A structure contour map of the contact between the metamorphic and sedimentary rocks was constructed using the topography of low-relief areas on the metamorphic rocks. This map shows the structure to be a dome, with smaller-scale elevations and depressions superimposed on the dome shape. The disordered and yet unfolded aspect of the fault blocks and the age relationships of the rocks involved are strikingly similar to the characteristics of the Amargosa Chaos of the Death Valley area; moreover, the form of the fault contact resembles the turtleback domes associated with the chaos at Death Valley. Similarities to areas of gravity sliding, including a possible erosion surface overriden by the thrust blocks, indicate the chaotic structures of Mineral Ridge may be the results of near-surface gravity movements rather than the secondary features of strike-slip faults.