Lower Cambrian Shale Research Articles

Natural gas potential of Neoproterozoic and lower Palaeozoic marine shales in the Upper Yangtze Platform, South China: geological and organic geochemical characterization

In this article, we describe the geological features of the Ediacaran (upper Sinian), lower Cambrian and lower Silurian shale intervals in the Upper Yangtze Platform, South China, and report on the gas potential of 53 samples from these major marine shale formations. Reflected light microscopy, total organic carbon (TOC) measurement, Rock-Eval, carbon isotope ratio analysis, thermovaporization gas chromatography (Tvap-GC), and open pyrolysis gas chromatography (open py-GC) were used to characterize the organic matter. Measured TOC in this research is normally >2% and averages 5%. TOC contents are roughly positively correlated with increasing geological age, i.e. lower Silurian shales exhibit generally lower TOC contents than lower Cambrian shales, which in turn commonly have lower TOC contents than Ediacaran shales. Kerogen has evolved to the metagenesis stage, which was demonstrated by the abundant pyrobitumen on microphotographs, the high calculated vitrinite reflectance (Ro = 3%) via bitumen reflectance (Rb), as well as δ13 C of gas (methane) inclusions. Pyrolysates from Tvap-GC and open py-GC are quantitatively low and only light hydrocarbons were detected. The lower Silurian shale generally exhibits higher generation of hydrocarbon than the lower Cambrian and Ediacaran shale. Cooles’ method and Claypool’s equations were used to reconstruct the original TOC and Rock-Eval parameters of these overmature samples. Excellent original hydrocarbon generation was revealed in that the original TOC (TOCo) is between 5% and 23%, and original S1+S2 (S1o+S2o) is ranging from 29 to 215 mg HC/g rock.

Pore characterization of organic-rich Lower Cambrian shales in Qiannan Depression of Guizhou Province, Southwestern China

Nine organic-rich shale samples of Lower Cambrian black shales were collected from a recently drilled well in the Qiannan Depression, Guizhou Province where they are widely distributed with shallower burial depth than in Sichuan Basin, and their geochemistry and pore characterization were investigated. The results show that the Lower Cambrian shales in Qiannan Depression are organic rich with TOC content ranging from 2.81% to 12.9%, thermally overmature with equivalent vitrinite reflectance values in the range of 2.92–3.25%, and clay contents are high and range from 32.4% to 53.2%. The samples have a total helium porosity ranging from 2.46% to 4.13% and total surface area in the range of 9.08–37.19 m2/g. The estimated porosity in organic matters (defined as the ratio of organic pores to the volume of total organic matters) based on the plot of TOC vs helium porosity is about 10% for the Lower Cambrian shales in Qiannan Depression and is far lower than that of the Lower Silurian shales (36%) in and around Sichan Basin. This indicates that either the organic pores in the Lower Cambrian shale samples have been more severely compacted than or they did not develop organic pores as abundantly as the Lower Silurian shales. Our studies also reveal that the micropore volumes determined by Dubinin–Radushkevich (DR) equation is usually overestimated and this overestimation is closely related to the non-micropore surface area of shales (i.e. the surface area of meso- and macro-pores). However, the modified BET equation can remove this overestimation and be conveniently used to evaluate the micropore volumes/surface area and the non-micropore surface areas of micropore-rich shales.

Shale gas potential of the major marine shale formations in the Upper Yangtze Platform, South China, Part II: Methane sorption capacity

The marine black shale formations on the Upper Yangtze Platform (UYP), South China are exploration targets for shale gas. Here, we report on the methane sorption capacity of thermally overmature samples from the Lower Silurian and Lower Cambrian black shale intervals in the UYP (UYP-samples). Two immature shale samples from the Middle Cambrian formation in the Georgina Basin, North Australia (AU samples) were also tested to investigate the effect of thermal maturity on sorption isotherms. Excess sorption isotherms were performed over a pressure range of 0–25MPa at 46°C. The effects of TOC content, thermal maturity, clay minerals, moisture content, pore properties, particle size, temperature, and pressure on methane sorption capacity were analysed. In addition, thermovaporisation gas chromatography (Tvap-GC) was used to measure the residual gas that is stored in the samples under atmospheric pressure and temperature conditions.The results indicate that the maximum methane excess sorption of the Lower Silurian samples is between 0.045 and 0.064mmol/g rock and that of Lower Cambrian samples is between 0.036 and 0.210mmol/g rock. The Langmuir sorption capacity of the Lower Silurian samples ranges from 0.096 to 0.115mmol/g rock, whereas that of the Lower Cambrian shale ranges from 0.077 to 0.310mmol/g rock. These results are close to the sorption capacities of the Barnett (U.S.), Devonian–Mississippian (Western Canada), and Alum (Southern Scandinavia) shale samples. The shape of the sorption isotherms and methane sorption capacity vary from sample to sample. Under the measured pressure range, the isotherms of the selected immature AU Cambrian samples increase monotonously with pressure, whereas the overmature UYP samples exhibit maxima. The methane sorption capacity of the measured samples positively correlates with TOC content and exhibits a distinct linear relation. The TOC-normalised sorption capacity shows a positive correlation with thermal maturity; however, the corresponding pressure of maximum excess sorption and Langmuir pressure decrease substantially with increasing thermal maturity. The clay minerals show a positive effect on the TOC-normalised sorption capacity. The sorption capacity of clay minerals, however, should have been reduced by the moisture content. The two Lower Cambrian samples that have similar maturities were measured for porosity and pore-size distribution. The sample with a high TOC content shows a high total cumulative pore volume, surface area, total porosity and thus a higher sorption capacity than the sample with less TOC. In addition, larger-sized particles show slightly less sorption capacity than smaller-sized particles. The Tvap-GC results show that the residual gas content of core samples is evidently higher than that of the outcrop samples, which implies a remarkably negative effect of the weathering process.

Shale Gas Potential of the Major Marine Shale Formations in the Upper Yangtze Platform, South China, Part III: Mineralogical, Lithofacial, Petrophysical, and Rock Mechanical Properties

The marine black shale formations on the Upper Yangtze Platform, South China, are currently exploration targets for shale gas. Here, we report on the mineralogy, lithofacies, petrophysics, and rock mechanics of samples collected from the Ediacaran (Upper Sinian), Lower Cambrian, and Lower Silurian black shale intervals. All three formations are composed of high proportion of quartz, low content of clay, and rare or nonexistent content of carbonates. The Ediacaran and Lower Cambrian shales deposited in restricted deep water marine platform to marine basin environments are characterized by a higher quartz content and lower clay content than the Lower Silurian shales that were deposited in a more restricted marine basin environment. The carbonate content varies from 0 to over 50%, with the higher values measured in the Lower Silurian samples. These stratigraphic units were formed during bottom water anoxic conditions; therefore, they were rarely influenced by bioturbation. Lithologically, laminated and nonlaminated siliceous mudstones predominate, with minor contributions of other lithotypes. Pores generally have diameters in the nanometer (nm) to micrometer (μm) range, and numerous pores occur in organic matter. Most of the measured samples have porosities less than 4%, although a few samples show porosity in excess of 10%. Pores with radii less than 50 nm contribute significantly to total pore volume and total porosity. Permeability is extremely low, and helium permeability coefficients (Klinkenberg corrected permeability coefficient) are less than 20.2 nD (nano-Darcy, ∼2 × 10–20 m2). The rock mechanical properties of the samples are characterized by high brittle behavior, which coincides with their high compressive and tensile strengths and elastic properties. The Lower Cambrian shale is generally more brittle than the Lower Silurian shales, which possess a relatively higher content of clay minerals. The rock mechanical properties of the measured samples, however, depend on the overall mineral compositions and physical properties.

Evolution of overmature marine shale porosity and implication to the free gas volume

Abstract The porosities of the Lower Cambrian shale and Upper Ordovician-Lower Silurian marine shale from the Sichuan Basin were analyzed using He-Hg porosimetry and field-emission scanning electron microscopy, to discuss the relationship of porosity to the organic matter maturity, total organic carbon (TOC) and free gas volume. Overmature (Ro greater than 2%) shale samples with a TOC less than 5% display a positive correlation between TOC and porosity, but samples with a TOC greater than 5% display a limited increase in porosity with increasing TOC. The porosity of shale decreases generally with increasing maturity in the overmature stage. The free gas in porosity is the key factor for successful development of shale gas, and free gas volumes are controlled by porosity and gas saturation. Higher TOC shale has higher porosity and gas saturation. The Lower Silurian Longmaxi Formation shale has a higher TOC, Ro ranging from 2.0% to 2.3%, its porosity and gas saturation are significantly higher than the Lower Cambrian shale. The high free gas volume results in high production in the Lower Silurian Longmaxi Formation shale pay.

Physical properties of petroleum formed during maturation of Lower Cambrian shale in the upper Yangtze Platform, South China, as inferred from PhaseKinetics modelling

Lower Cambrian shale in the Upper Yangtze Platform (UYP), South China, is an important source rock of many conventional petroleum fields and was recently recognized as a promising unconventional shale reservoir. In this paper, hydrocarbon generation kinetics and petroleum physical properties were investigated using the PhaseKinetics approach (di Primio and Horsfield, 2006) and a Cambrian shale sample from the Georgina Basin, North Territory Australia (NTA), as similar paleogeological and sedimentary environments in Cambrian are found for the UYP and NTA.The source rock comprises type II kerogen and belongs to an organofacies generating Paraffinic–Naphthenic–Aromatic low wax oil. Bulk petroleum generation can be described by a single frequency factor A = 8.43E + 14 (1/s) and a dominant activation energy at 56 kcal/mol, which is characteristic for sulphur-poor organic matter deposited in an anoxic marine environment. Onset (transformation ratio TR = 10%) and end (TR = 90%) of bulk hydrocarbon generation was calculated to take place at 120 °C and 165 °C respectively for an assumed average geological heating rate of 1.5°C/Ma. Based on the thermal history of a local “model”-well, onset temperature was not reached until the Middle Triassic (241 ma) when sediments were buried more than 2000 m and basalt eruptions caused enhanced heat flows. The main generation stage of primary petroleum took place during the Middle–Late Triassic and ended in the Early Jurassic (187 ma) for burial depths exceeding 4000 m (TR 90%; 165 °C). Temperatures increased to more than 200 °C in the Middle–Late Jurassic leading to secondary cracking of primary products.Hydrocarbons formed at the onset (TR = 10%) of petroleum generation can be characterized by a gas-oil-ratio (GOR) of 63 Sm3/Sm3, a saturation pressure (Psat) of 101 bar, and a formation volume factor (Bo) of 1.2 m3/Sm3. Those parameters stay low during primary petroleum generation before 203 ma, at temperatures < 145 °C, and at burial depths <3400 m (GOR = 176 Sm3/Sm3, Psat = 189 bar, Bo = 1.6 m3/Sm3 at 90% TR). However, predicted parameters increase rapidly (GORs >> 10,000 Sm3/Sm3, Psat > 250 bar and Bo > 2.0 m3/Sm3) during secondary cracking starting roughly at 200 ma, 152 °C and 3500 m burial. Assuming zero expulsion, the shale reservoir position within the sedimentary basin indicates that bubble point pressure was always below reservoir pressure, and fluids in the shale reservoir occurred only as a single, undersaturated phase throughout maturation history. Black oil and volatile oil phases dominated during the primary cracking period, whereas wet gas and dry gas phases dominated during the secondary cracking period.

Evaluation of Lower Cambrian Shale in Northern Guizhou Province, South China: Implications for Shale Gas Potential

The Lower Cambrian shale is an important source rock for conventional oil and gas in northern Guizhou province (NGP), south China, but has recently been considered as a potential shale gas reservoir. In this study, characterization of this target shale was determined through a systematic series of measurements on core samples. According to total organic carbon (TOC) and Rock-Eval data, the Lower Cambrian shale is overmature but still contains high amounts of organic matter, which is composed mainly of alginite. The TOC content is enriched up to 10.4% at the base of the shale. There are two different correlation patterns between trace elements and TOC in the Lower Cambrian shale. A good correlation present in the upper part indicates a non-sulfidic anoxic sedimentary environment. However, higher enrichments of U, V, and Mo in the basal part exhibit sulfidic euxinic conditions. Brittle minerals are quite abundant and have a critical effect on artificial fracture. Gas sorption volume increases with an increasing TOC, pore volume, and surface area, indicating that pores at the microscale associated with the organic matter fraction are an important control on storage capacity. The contribution of clay minerals to the sorbed gas may be irrelevant because of the presence of moisture. Analysis of data indicates that the basal part of the Lower Cambrian shale demonstrates the greatest potential for gas content and can be explored for production.

Reduction in the number of Early Cambrian helicoplacoid species

Helicoplacoid echinoderms occur in Lower Cambrian shales and mudstones along the northern coast of Laurentia, now western North America. The group originally contained four genera and nine species, diagnosed by differing ambulacra, plate length-to-width ratios, and plate ornament. Ambulacra are identical in all specimens except for the single example of Waucobella nelsoni, and the remaining morphologic differences are caused by ontogeny. The characters used to diagnose seven of the original nine species are therefore invalid, reducing the group to three genera and three species. These are: Helicoplacus gilberti Durham and Caster, 1963, Waucobella nelsoni Durham, 1967, and Polyplacus kilmeri Durham, 1967, which has a test composed of ambulacra, and likely represents a developmental oddity.

Possible developmental mechanisms underlying the origin of the crown lineages of arthropods

The extraordinarily preserved, diverse arthropod fauna from the Lower Cambrian Maotianshan shale, central Yunnan (southwest China), represents different evolutionary stages stepping from stem lineages towards crown arthropods (also called euarthropods), which makes this fauna extremely significant for discussion of the origin and early diversification of the arthropods. Anatomical analyses of the Maotianshan shale arthropods strongly indicate that the origin of crown arthropods involved three major evolutionary events, arthrodisation, arthropodisation and cephalization. We try to explore possible evolutionary changes of the developmental mechanism that may have underlain origins of euarthropod appendage and head. Fossil evidence suggests that the formation of a jointed limb known as arthropodisation and formation of multi-segmented head (called cephalization), which characterize euarthropods, is an event after arthrodisation characterized with the formation of segmented-exoskeleton and the joint membrane between tergites. We propose that the Hox complex was already operating at least as early as in the Early Cambrian and is responsible for the formation of the joint membrane between two semgents through Hox gene regulation along the D-V and P-D axis. Fossil evidence indicates that the head in ground state of arthropods consists only of two segments, an ocular and an antennal one. The formation of multiple segmented, euarthropod head (called syncephalon) from the two-segmented head was a separate event, which is called cephalization. Presence of the Hox gene head expression domain and change of developmental mechanism in head segments might be responsible for the formation of the syncephalon and this event has been broadly finished in the Early Cambrian arthropods. The post-oral limbs in the early syncephalons as evidenced from the Lower Cambrian Maotianshan shale arthropods however were almost identical to those in trunk. Therefore we proposed that the Hox genes specifiedexpression in individual segment was a late evolutionary step postdating the formation of syncephalon.

Contrasting geochemical features of the Central Iberian Zone shales (Iberian Massif, Spain): implications for the evolution of Neoproterozoic–Lower Cambrian sediments and their sources in other peri-Gondwanan areas

More than 90 samples of shales have been collected for geochemical purposes from Neoproterozoic and Lower Cambrian sedimentary series in an extensive area of the Central Iberian Zone (CIZ). For similar SiO 2 and Al 2O 3 abundances, the Neoproterozoic shales (NS) are characterized by relatively higher TiO 2 and Zr abundances and by higher Cr/Th and Ti/Nb ratios. By contrast, the Lower Cambrian shales (LCS) have higher Al 2O 3/TiO 2, Rb/Zr, (La/Yb) cn and Th/Yb ratios. Diagrams such as Al 2O 3–TiO 2, Al 2O 3/TiO 2–Ti/Nb, TiO 2–Zr, TiO 2–Zr/Nb and Ti/Nb–Rb/Zr are useful to define two distinct fields that discriminate the Neoproterozoic from the Lower Cambrian compositions. These and other geochemical features suggest that these rocks were derived from different source compositions and that the Lower Cambrian shales (LCS) were chemically more fractionated than those of the Neoproterozoic. These distinctive chemical features are also recorded by Neoproterozoic and Cambrian fine-grained rocks in other areas in the Central Iberian Zone and in other zones within Iberia. Published neodymium isotopic results consistently distinguish both compositions and, together with the geochemical data, support a relatively fractionated nature for the Lower Cambrian sediments compared to the Neoproterozoic ones. It is proposed that continental deposits derived from the interior orogens in North Gondwana would have been the sources for Neoproterozoic sediments in the Iberian zones, whereas the Lower Cambrian sediments in these zones would have been derived from older crust beneath those continental deposits. Thus, the Neoproterozoic–Lower Cambrian stratigraphic succession in Iberia reflects an inverse stratigraphy of the source area and contrasts with zones of Central and Western Europe where equivalent sedimentary series record important coeval igneous contributions and show less fractionated geochemical characteristics. Finally, the possible connection between the Central Iberian Zone and the Meguma Zone (MZ) in Canada is discussed on the basis of certain sedimentological and geochemical features. It is proposed that the sedimentary series of the Central Iberian Zone and of the Meguma Zone represent, respectively, proximal and distal North Gondwana-derived sediments.

Provenance of Upper Precambrian-Lower Cambrian shales in the Central Iberian Zone, Spain: evidence from a chemical and isotopic study

Geochemical and isotopic (Sr, Nd, O) data are presented for Upper Precambrian and Lower Cambrian shale sequences from the Central Iberian Zone, Spain, in order to define geochemical discriminants between them, and to assess the nature of the source regions supplying these basins. Elemental data, especially Th/Sc ratios, coupled with ε Nd t values of ∼ −2.5 suggest that the Upper Precambrian rocks contain a greater proportion of a much younger, more mafic component than the Lower Cambrian sequence which has high Th/Sc and ε Nd t of ∼ −7.3. Coherent trends between elemental and O isotope data imply a homogeneous, recycled source as the dominant provenance for the Upper Precambrian shales, whereas a lack of such coherence for the Lower Cambrian sediments suggests a more heterogeneous provenance. The sedimentological, geochemical and isotopic data are consistent with the Upper Precambrian succession being derived ultimately from a polycyclic mixture of young (pre-Cadomian), mafic sources, and older (> 2 Ga) crust. In contrast, the Lower Cambrian sequence was mainly derived from old, more felsic crust which would have contributed increasingly greater proportions of material as the younger sources were progressively removed by erosion.

Barite and witherite deposits in Lower Cambrian shales of South China; stratigraphic distribution and geochemical characterization

In south China, black shales of the Lower Cambrian series cover approximately one million square kilometers. Barite and witherite deposits in this black shale series are closely associated with black cherts and belong to a class of chert-hosted bedded barium deposits. Witherite deposits are confined to the Daba Mountains in the Qinling area, whereas barite deposits are found in both the Qinling area and the Jiangnan area to the south. The barium ore beds are usually underlain by beds with prominent phosphatic concretions and overlain by black shales.The barite ores can be divided into four textural types: massive, laminated, banded, and nodular; witherite ores are generally compact, massive, and nodular. There are carbonaceous mudstone interlayers in the barium ore beds, and organic carbon content may reach as high as 14 wt percent. Radiolarians have been found in the black cherts associated with witherite and barytocalcite ores.From the geosyncline to the platform, delta 34 S values of the barites increase, reaching values as high as 50 per mil. The lowest delta 34 S value of barite (8ppm) is within witherite ore beds, suggesting that some sulfur was derived from volcanic sources. However, most delta 34 S values either are close to the sulfur isotope composition of Cambrian seawater or are significantly higher. The highly positive values suggest that bacterial sulfate reduction in closed basins led to strong fractionation of sulfur isotopes and concentration of delta 34 S in residual sulfates. Carbon and oxygen isotopes of the witherite and barytocalcite have mean values of delta 13 C PDB and delta 18 O PDB of-18 and -11 per mil in the Ziyang witherite deposit, and -14 and -13 per mil in the Chengkou deposit. The light carbon values indicate that the carbon in the barium carbonates was derived from organic matter decomposition, and thus, that these minerals formed during early diagenesis.Based on regional geology and geochemical analyses, a genetic model of mineralization is suggested that includes submarine eruption and hot spring activity during late-stage volcanism that provided barium to seawater in the basin of deposition. Deep, euxinic conditions prevailed in the late Sinian, which changed in the Early Cambrian to even greater restriction with a failure of resupply of sulfate by the open ocean. Decomposition of plankton settling through the water column may also have released barium either to the basin waters or to the pore water of the sediment, leading to deposition of barite and witherite in different sedimentary and geochemical environments.

Correlation of Ogden Thrust, Durst Mountain Thrust, and Allochthonous Precambrian Farmington Canyon Complex in North-Central Wasatch Mountains, Utah: ABSTRACT

The Precambrian Farmington Canyon complex crops out in the Wasatch Mountains between Ogden and Bountiful, Utah. Additional exposures are present at Durst Mountain and at Antelope Island. East of Ogden, between Ogden Canyon and Strongs Canyon, the Farmington Canyon complex has been thrust eastward over Lower Cambrian Tintic Quartzite and Middle Cambrian shales and limestones of the Ophir Formation and Maxfield Limestone. This thrust is named the Ogden thrust. Similarly, at Durst Mountain, east of Morgan Valley, the Farmington Canyon complex has been thrust over Lower Cambrian Tintic Quartzite and Middle Cambrian shales and limestones. This thrust fault is the Durst Mountain thrust. If subsequent vertical offset of 2,000-4,000 ft (610-1,220 m) down to the west on the Morgan Valley normal fault is restored, it appears that the two similar thrusts involving the Farmington Canyon complex could once have been continuous. Such a reconstruction requires a minimum of 12 mi (19 3 km) of thrust displacement of the complex eastward over the sequence of basal Cambrian rocks. This evidence is significant for two reasons: (1) the Farmington Canyon complex of the Wasatch Mountains may not be in place, but may be allochthonous above a decollement at depth; and (2) the Paleozoic-Mesozoic sequence east of Morgan may also overlie the same decollement, which would increase the potential for petroleum plays in the area north of Croyden. Thus, the northern Utah uplift, proposed for this area by A. J. Eardley and discussed by M. D. Crittenden, may result from a sequence of Farmington Canyon complex thrust upsection eastward, rather than a vertical uplift of basement. End_of_Article - Last_Page 948------------

Cranmore Cove–Chattanooga fault system: A model for the structure along the Allegheny front in southern Tennessee

Three thrust blocks, which represent the broken elements of a once continuous thrust sheet, can be used as a model to explain the general structure, sequence of thrusting, and mechanics of thin-skinned tectonics along much of the Allegheny structural front in eastern Tennessee. The master fault developed in Lower Cambrian shale, ramped upward about 1,800 m stratigraphically into Silurian shale, and then cut some 240 m into Upper Mississippian and Pennsylvanian shale. As the sheet moved to the northwest, it broke into three elongate blocks. The structure of each block reflects its position along the thrust prior to movement.

Paleogeography and Depositional Environments of the Cambro-Ordovician Shallow-Marine Facies of the North Atlantic

Detailed sedimentological and petrological investigations of the Cambro-Ordovician shallow-marine facies of western Newfoundland, northwest Scotland, and central East Greenland reveal arresting similarities in their histories. Faunal, stratigraphic, sedimentologic, geochemical, and diagenetic comparisons of the thick sandstone, shale, and carbonate successions indicate contemporaneous development on a contiguous broad marine shelf bordering the western margin of a “proto-Atlantic” basin. Sedimentation dominated by intertidal to shallow subtidal environments is indicated by tidal sand bodies, ichnofossils, desiccation phenomena, oolitic lithologies, and stromatolitic algal structures. Complex diagenetic processes probably account for anomalous potash concentrations in correlative Lower Cambrian shale horizons and demonstrably effect replacements by feldspar, silica, dolomite, and calcite in the sandstones and carbonates. Parallel trends of recrystallization, dolomitization, silicification, calcitization, and dolomitization of the oolitic carbonates in each of the three study areas suggest correspondent post-depositional diagenetic environments. Independently the similarities are inconclusive, but collectively they are compelling in their suggestion of common depositional and diagenetic histories for the Cambro-Ordovician successions of western Newfoundland, northwest Scotland, and central East Greenland, ostensibly on the western shelf of a “proto-Atlantic” basin.

Geology of Belle Isle—northern extremity of the deformed Appalachian miogeosynclinal belt

Belle Isle, situated between northern Newfoundland and the southeast coast of Labrador, consists of an uplifted block of Precambrian plutonic rocks intruded by northeast-trending diabase dikes and uncomformably overlain by Lower Cambrian and earlier (?) sedimentary and volcanic rocks. The Precambrian rocks lie along strike and are similar to Grenville gneisses of the Long Range Complex of western Newfoundland. In the southwest part of Belle Isle, the cover rocks are gently dipping basaltic flows and agglomerates that are succeeded conformably by arkosic sandstones and fossiliferous upper Lower Cambrian shales. In the northeast, the basement rocks are overlain by steeply dipping boulder conglomerates and arkosic sandstones, followed conformably by white quartzites.Diabase dikes are inseparable from overlying flows, but do not penetrate higher sedimentary strata of the southwestern Lower Cambrian succession. Toward the northeast, plutonic boulder conglomerates and quartzites are cut by the dikes.The distribution of supracrustal rocks around the periphery of the island, combined with local steeply inclined surfaces of unconformity between basement and cover rocks, indicate a major anticlinal structure produced by Paleozoic deformation. The study also shows that at Belle Isle the established Lower Cambrian succession of southeast Labrador and western Newfoundland is locally underlain by basalts and conglomerates and quartzites that thicken southeastward and northeastward.

A radiometric estimate of the duration of sedimentation in the Adelaide geosyncline, south Australia

Abstract The Adelaide System forms the uppermost Precambrian sequence in South Australia and the Wooltana Volcanics lie near its base. Though affected by Palaeozoic metamorphism, the least‐altered samples give a minimum age of 850 ± 50 m.y., so that the base of the System is about 900 m.y. old or more. The unmetamorphbsed Roopena Volcanics of northeastern Eyre Peninsula are 1,345 ± 30 m.y. old and if correlated with the Wooltana Volcanics the base of the system becomes about 1,400 m.y. old. The data for the Wooltana Volcanics are consistent with this, provided that even the least‐altered total‐rock samples were open systems during the later metamorphism. Ages of basement in the Mount Painter and Olary districts (1,600 m.y.) and data for Willouran shales overlying the Wooltana Volcanics can fit both minimum and maximum estimates for the Volcanics. Lower Cambrian shales give a range of 530–690 m.y.; though some Palaeozoic isotopic movement occurred, the ages are approximately correct. Shales from the top of the Torrensian Series range from 660–840 m.y. (700 m.y. preferred value). If the base of the system is at 1,400 m.y., this is surprisingly young. It suggests either a hiatus between the Wooltana Volcanics and the Torrensian or that the correlation of the former with the Roopena Volcanics is wrong (and that the base is at about 900 m.y.). Alternatively, the shales may be abnormally updated. The Gawler Range Volcanics of Eyre Peninsula have been dated accurately at 1,535 ± 25 m.y. and illitic shale from the penecontemporaneous Corunna Conglomerate gives nearly the same value. These ages indirectly set a maximum for the age of the base of the system, as stratigraphy suggests that they are older. Granites underlying the Gawler Range Volcanics are about 1,600 m.y. old; some may be 1,800 m.y. old. Final Palaeozoic metamorphism in the northern Flinders Ranges was at 465 m.y. The ages of several post‐orogenic intrusions are given.

A Suspected Spilitic Rock from the Lower Cambrian Manganese Shale Group of the Harlech Dome, North Wales

AbstractMineral and chemical evidence suggest that an unusual rock from the Upper Manganese Shales of Diphwys is of spilitic origin, despite certain geochemical anomalies. Some similarities of the rock to the shales themselves are only superficial, and a metasomatic origin is precluded; the rock is also very distinct from the “greenstone” sills of the region.

STRATIGRAPHY AND STRUCTURE OF THE KINDERHOOK QUADRANGLE, NEW YORK, AND THE “TACONIC KLIPPE”

Detailed mapping of the predominantly argillitic Cambro-Ordovician “Taconic sequence” in the Kinderhook quadrangle, New York, indicates that the Chatham thrust divides these rocks into two distinctive sequences. To the east slates predominate and form a hilly upland, but west of this thrust less resistant shales underlie a gently rolling lowland. Stratigraphically, the eastern sequence is composed of a transgressive black Trentonian (Middle Ordovician) slate, locally with a thin basal limestone, resting upon a thick series of Lower Cambrian green slate, graywacke, quartzite, limestone, and limestone conglomerate (Rensselaer-Nassau-Schodack). Canadian carbonate rocks crop out intermittently along the Chatham thrust, and they are interpreted as slivers trapped during thrusting. The western sequence is primarily Normanskill (Middle Ordovician) and Deepkill (Lower and Middle Ordovician) shale, unconformably underlain by Lower Cambrian shale and overlain by Siluro-Devonian limestone. Though many small thrusts are present, folding is the principal mode of deformation. The Chatham thrust, however, is a major structural break and continues at least 20 miles southwest of the quadrangle and an unknown distance northward. It is a post-middle Trentonian feature, and maximum displacement is a few miles rather than tens of miles. Unconformities are very important in the geologic history of this area, but some have gone undetected because of the structural complexity and poor exposure. The following formations are believed to have basal unconformities in the Kinderhook quadrangle: the Upper Silurian Manlius limestone, the Trentonian black slate (locally with a basal limestone), the Deepkill shale, and the Canadian carbonate rocks. Evidence for the existence of the “Taconic klippe” was not found in mapping this quadrangle. Analysis of the development of the klippe hypothesis indicates that it is based principally upon stratigraphic considerations; available structural evidence weighs against this interpretation. While the klippe hypothesis seems to explain well the relations at the north end of the Taconic Range, the problem of adequately defining the boundaries of this “klippe” causes serious doubt about its existence. An alternative interpretation of the regional relations is suggested, involving unconformities and facies changes in a single indigenous sequence. Trentonian rocks lie unconformably on rocks as old as Precambrian from Vermont to Pennsylvania and pass indiscriminately in and out of the “Taconic klippe.” The Normanskill and Deepkill rocks (mainly shale) are interpreted as passing transitionally into limestone to the west; an analogous situation exists in the Middle Ordovician rocks of Virginia. The Deepkill is believed to rest unconformably on rocks of Early Cambrian to middle Canadian age. Middle Canadian formations in the Kinderhook quadrangle are carbonate rocks and appear to rest unconformably upon Lower Cambrian slates; their striking similarity to equivalent rocks in the near-by “autochthonous” series suggests they have not been displaced any great distance. The Lower Cambrian is a thick series of argillite, gray-wacke, and quartzite with some thin carbonate rocks near the top. The thick, lower part of this series is considered a southward continuation of the Mendon Series of Vermont. The upper strata are interpreted as the offshore equivalents of shallow-water quartzites and carbonate rocks deposited marginal to an eastern welt in later Early Cambrian time.