Palaeozoic Carbonate Research Articles

Metamorphic and tectonic history of the Pennsylvania Piedmont

The Piedmont Province of SE Pennsylvania consists of gneisses overlain by, or in fault contact with, metasedimentary phyllites and schists. Correlation of these crystalline rocks with similar units in Maryland and isotopic ages of the metamorphic events suggest ages from pre-Grenville (1000 Ma) to Ordovician; Cambrian and Ordovician fossil-bearing units occupy a narrow band across the central part of the region. A 1000 Ma regional granulite facies episode is documented in the basement gneiss units. A second younger and lower pressure granulite facies metamorphism centres around an intrusive anorthosite-norite complex. A third metamorphism ranging from greenschist to upper amphibolite facies affects the entire terrain; this is probably close to 440 Ma old. Major folds and faults formed during or after the last metamorphic episode; earlier structural features are either obliterated or not exposed. The proposed tectonic model includes a rifting of continental basement forming a basin with a lower Palaeozoic carbonate bank on the NW margin and an island arc to the SE. Compression produced major nappes and possible thrusts during the last regional metamorphism. Vertical movements associated with major block faulting marked the end of the orogenic activity

Structural geology of central Peary Land, North Greenland

The region investigated in 1978 between Jørgen Brønlund Fjord - Øvre Midsommersø in the south and Frederick E. Hyde Fjord in the north consists of c. 1½ km thiekness of Lower Palaeozoic carbonates overlain by an up to 1 km thick series of Silurian flysch (Christie & Peel, 1977). In general the region is part of the Palaeozoic platform that ean be divided into three major E-W trending physiographic belts: a southern belt where the carbonates form plateaus incised by steep valleys, a central belt consisting of characteristic conical mountains marked by concentric terraces of resistant sandstone beds in the flyseh (Dawes, 1976, fig. 249), and in the fold belt to the north the mountains rise up to altitudes of 1000-1500 m and are covered by extensive ice caps (fig. 17). The southern border of the region, the Øvre Midsommersø - Jørgen Brønlund Fjord area, was mapped by Jepsen (1971) while the Lower Palaeozoic stratigraphy in the central part af the region has been established by Christie & Peel (1977) at Børglum Elv. Some structural aspects af the gealogy at Frederick E. Hyde Fjord are briefly described by Dawes (1971) and Dawes & Soper (1973). Prior to the field work a comprehensive photogeological interpretation was carried out on vertical aerial photographs (seale c. 1:60000) and compiled on six uneontoured l: 100000 photomosaic maps. The 1978 field work and laboratory studies using a Kern PG 2 photogrammetric instrument form the first detailed study of the fold belt margin. For descriptive purposes the region investigated in 1978 has been subdivided into seven areas (fig. 17), the main struetural features of which form the basis of this report.

Geochemical response to Upper Precambrian rift basin sedimentation and Lower Palaeozoic epicontinental sedimentation in south Norway

Geochemical analyses of 138 samples from the Upper Precambrian and Lower Palaeozoic are presented in diagrams showing the stratigraphical distribution of each element. The analyses include the major elements and the following trace elements: Sc, U, B, Ba, Co, Cr, Cu, Ni and Ga. The Upper Precambrian sedimentary rocks were deposited in an Upper Precambrian rift valley basin on the Baltic Shield and are mineralogically immature and controlled by the composition of the local basement. The Middle Cambrian to Upper Ordovician Alum Shale was deposited in a stagnant epicontinental sea covering most of the Baltic Shield. These shales are very fine-grained illite shales derived from distant sources and probably deposited to a large extent as eolian dust at 1 mm per thousand years. Ordovician and Silurian shales, however, have a high content of chlorite and are enriched in Mg, Fe, Cr, Ni and Co, and indicate derivation from Lower Ordovician igneous rocks, probably ophiolites, which were exposed following abduction along the continental margin. While the Upper Precambrian sparagmites, which have local sources, show rapid lateral variation in mineralogy and geochemistry, the Lower Palaeozoic sediments show a laterally very persistent but vertically more abrupt geochemical pattern which now has been confirmed in different parts of the Oslo region. The Lower Palaeozoic carbonate rocks in the Oslo region are mostly calcite limestone. It is concluded that epicontinental sea sediments probably had a very low mobility of their pore water due to low pressure gradients (low relief) and sealing shales. This diagenesis has been essentially isochemical.

The petrology of sands in the uppermost Cretaceous and Palaeocene of southern Saskatchewan: a study of composition influenced by grain size, source area, and tectonics

The sands of the Frenchman Formation (Maastrichtian) and the Ravenscrag Formation (Palaeocene) in southwestern Saskatchewan are mineralogically immature lithic arenites and wackes. Volcanic rock fragments, which in the Frenchman Formation make up to 30% of the fraction coarser than 0.0625 mm, are less common in the Ravenscrag Formation, where quartz and carbonate rock fragments are significantly more abundant in the same size fraction. Sand composition is influenced by grain size in both formations. Over the range from 0.59–0.074 mm there is, in general, a decrease in the percentage of rock fragments and an increase in the percentage of quartz, with decreasing grain size. Although the Ravenscrag sands mostly are finer grained than those of the Frenchman, compositions of the fraction coarser than 0.0625 mm fall into separate but overlapping fields in a conventional quartz – feldspar – rock fragment (QFR) diagram. Differences, in terms of QFR ratios, between the formations, are clearer if compositions of limited size ranges (six size ranges between 0.59 and 0.074 mm) are considered, but there is still overlap. Significant compositional differences are obvious only when the relative amounts of different types of rock fragment are taken into account.Grain-size distributions of different components in the sands are, for the most part, the same in both formations, and reveal trends that are anticipated by studies of sand transport in modern streams. The proportions of quartz and detrital carbonate grains increase with decreasing grain size, whereas relative amounts of nonvolcanic rock fragments and chert decrease with decrease in grain size. Volcanic rock fragments and feldspar show an irregular increase with decreasing grain size in the Frenchman sands, and an increase followed by a decrease in the Ravenscrag sands. The differences in the latter two components between the sands of the two formations are attributed to a reduction in both the abundance and the variety of volcanic detritus supplied to the area in Palaeocene times.The bulk of the volcanic material in both formations likely was derived from western Montana. In particular, the Elkhorn Mountain Volcanics (Campanian) and the Adel Mountain Volcanics (Maastrichtian to ?Palaeocene) probably were important sources. The uplift of Palaeozoic carbonate rocks and Mesozoic clastic deposits in the Front Ranges of the Rocky Mountains may account for the relative reduction in supply of volcanic rock fragments and feldspar, and the increased contribution of detrital carbonate and quartz, to the sands of the Ravenscrag Formation.

Cambrian–Silurian stratigraphy of Børglum Elv, Peary Land, eastern North Greenland

A sequence of Lower Palaeozoic carbonate and clastic rocks is described from Børglum Elv, Peary Land, eastem North Greenland, and briefly compared to Lower Palaeozoic sections elsewhere in Greenland and in Spitsbergen. Lower Cambrian clastic rocks of the Buen Formation are followed by dolomite of the Lower Cambrian Brønlund Fjord Formation (125 m). Succeeding dolomite and dolomitic limestone of the Wandel Valley Formation (320 m) of Early to Middle Ordovician age are overlain by limestone of the Børglum River Formation (430 m) of Middle to Late Ordovician age. Un-narned Early Silurian dolomite and limestone formations (150 m and 320 m respectively) are followed by an un.narned Middle Silurian black shale formation (c. 100 m) and at least 800 m of a late Middle Silurian and younger un-named flysch formation. Carbonate mounds, originating in the highest beds of the un-named Silurian limestone formation, occupy stratigraphic levels through the overlying black shale formation and into the flysch formation.

Tellurium, a guide to mineral deposits

Te dispersion patterns are useful in exploring for different types of mineral deposits and in providing additional information about known ore deposits. The Te content of rocks is given for five mining districts in the western United States: Coeur d'Alene, Idaho; Robinson, near Ely, Nevada; Montezuma, Colorado; Crater Creek area, Colorado; Cripple Creek, Colorado. Many of the analyses were obtained by use of a new analytical method sensitive to 0.001 ppm Te. The principal ore deposits in the Coeur d'Alene district, Idaho, are Pb-Zn-Ag replacement veins in Precambrian rocks of the Belt Supergroup. Te dispersion patterns show the outlines of the original mineral belts, the effects of intrusive events, the location of ore deposits, the displacements caused by post-ore faulting, and the borders of the 780-km 2 district. The disseminated porphyry Cu deposits of the Robinson mining district, Nevada, are associated with Cretaceous quartz monzonite stocks that have intruded Palaeozoic carbonate rocks. Te is present in rock samples in concentrations as high as 10,000 ppm and forms a halo around the areas containing the Cu deposits. The alteration zones in the porphyry Mo district near Montezuma, Colorado, are developed around several small Tertiary intrusions occurring along a regional shear zone. Te haloes reflect the locations of porphyry intrusives, individual deposits and their ore shoots, and the pattern and intensity of adjacent alteration. The Te content of soils over the Montezuma stock is higher than, and varies independently from, the Te content of adjacent outcrops. Soils generally contain more Te than adjacent outcropping rocks. Soil may collect gaseous Te compounds from mineral deposits. The Crater Creek area is a northwestern extension of the Summitville mining district, Colorado. Te dispersion patterns radiate out from exposed Cu-Pb-Zn veins, from an outcrop of molybdenite stockwork veins and from associated iron-stained altered rock. Te haloes intensify exponentially with proximity to known ore and suggest the presence of Summitville-type chimney deposits. Most of the gold- and silver-telluride ore in the Cripple Creek district, Colorado, is found in fracture fillings within a volcanic subsidence basin. Haloes of Au, Ag and Te all define the mineralized portions of the fissure veins.

New Canadian Cryptoexplosion Crater at Lake St Martin, Manitoba

LAKE ST MARTIN (51° 43′ N, 98° 30′ W) forms a broad shallow depression on nearly flat lying Palaeozoic carbonate rocks between Lake Winnipeg and Lake Manitoba. The occurrence of crystalline Pre-Cambrian rocks, fresh looking vesicular volcanic rocks and extensive post-Palaeozoic gypsum deposits within an area of about 200 square miles on the north side of the lake suggests that unusual structural complexities are present. The relations between these rocks are obscured by poor outcrop and extensive swamp, but a series of shallow bore holes in the anomalous area, together with re-examination of the surface outcrop, have recently made it apparent that they are the surface expression of a resurgent cryptoexplosion crater of Permian or Triassic age.