Paleozoic Series Research Articles

Structural styles at the junction between the Eastern Atlas Fold and Thrust Belt and the northern Sahel-Gulf of Hammamet foreland basin (Tunisia)

The Tunisian Eastern Atlas Fold and Thrust Belt is characterized by NE-SW fold-thrust structures derived from intracontinental basin inversion during the convergence between Africa and Eurasia. In this paper, we study the transition from this folding-thrusting domain to its foreland basins by discussing a regional structural transect based on field investigations, well data correlations and, seismic lines interpretation. Well correlations highlight significant thickness and facies variations particularly affecting the Cretaceous units, deposited during the Tethyan rifting. Successive compressional events during the Eocene and the upper Miocene-Quaternary have controlled the inversion process, thus shaping the overall present-day structure. Along the Atlas, we observed several large anticlines controlled by deep NE-SW thrusts that root down into a regional-scale major décollement that corresponds to Triassic evaporites. In the field, contractional structures such as fault propagation folds and pop-ups are identified. Unlike the Atlas Fold and Thrust Belt, the Sahel Mesozoic and Paleogene series are buried under thick Neogene to Quaternary syntectonic deposits with sediments supplied from the erosion of the uplifted Atlas structures, recording a high subsidence rate. The Sahel foreland is characterized by buried fold and thrust structures, which define an outer deformation front. Besides contraction, the Sahel Foreland basin is also affected by extension related to the Strait of Sicily rift. Seismic profile interpretation highlights the presence of a large scale rollover developed over a listric fault controlled by a deeper extensional system affecting the Paleozoic series.

Petroleum system analysis of the Paleozoic series in the Fars Platform of Iran

The Paleozoic petroleum system of the Fars Platform of Iran is analyzed using 1D petroleum system models of 17 deep wells drilled in the study area. The main aim was to investigate the hitherto undocumented influence of salt tectonics on the essential elements and processes of this petroleum system. For the first time, a better understanding about the geohistory evolution of this prolific petroleum system is provided over the Fars Platform. Modeling results indicate that hydrocarbon generation from the Silurian source rock was associated with salt withdrawal synclines. Salt tectonics further controlled the entrapment styles and the geometric distribution of carrier/reservoir rocks and their overlying evaporitic seals. Our findings provide new insights for identifying the remaining exploration potential in the Paleozoic petroleum system of the Fars Platform and suggest that the role of salt tectonics should be considered in the future basin and petroleum system modeling studies in this area.

Stratigraphy, sedimentology and paleogeography of a Paleozoic succession, Ghadames and Jefarah basin, Libya and Tunisia

The sedimentology and tectonics and their relationship of the Paleozoic series that fill the Ghadames basin and its northern extension consisting of the Jefarah basin in Libya and Tunisia was studied. These formations belong to the Gondwana cycle and are typically interpreted as being deposited in a cratonic basin truncated by the Hercynian unconformity. This study is based on wells. The stratigraphic correlation between the wells is based on the definition of second order cycles, electrofacies and electrosequences notion. This allowed to produce isopach maps of facies distributions and to reconstruct the paleogeography of the different stratigraphic units. These data allow to address the nature of the deformation and also clarifies the behavior of active high regional areas during the Palaeozoic. The Paleozoic succession in the Ghadames and Jefarah basins can be divided into five first order sequences, bounded by major tectonic unconformities with sequence durations of 40–70 Ma. Within these five sequences 18 s order sequences (10–40 Ma) were differentiated, describing Sequence Boundaries (SB), Maximum Flooding Surfaces (MFS) and sedimentological characteristics.

Three-dimensional seismic analysis of Late Paleozoic coal-bearing series reflections in the Hangjinqi, North Ordos Basin, China

The study of sand bodies and coalbeds that formed during strong events is conducive to understand the relationship between source rocks and reservoirs. Two sets of Late Paleozoic coal-bearing sequences including the Late Carboniferous Taiyuan Formation and Early Permian Shanxi Formation, with an accumulated thickness of more than a half wavelength (90 m), were deposited in the Hangjinqi region on the northern margin of the Ordos Basin. These strata appear as two peaks and two troughs, three peaks and three troughs in the 3D seismic data. We analyzed this coal-bearing seismic response through a combination of well-data statistical analysis, the establishment of forward modeling, and the creation of crossplots. The coal-seam thicknesses of the Taiyuan and Shanxi Formations and the spacing between the two sets of coal indicate the major roles in the merging and bifurcation of events. The formation of three peaks and three troughs requires that the thicknesses of the two sets of coal are greater than 2 m. The spacing between the two sets of coal must also exceed 10 m with a detectable lateral extension, which would otherwise generate two peaks and two troughs. The two peaks and two troughs indicate a combined relationship between the thin sandstones at the bottom of the Shanxi Formation and the thick coalbeds in the Taiyuan and Shanxi Formations. The three peaks and three troughs suggest a combined relationship between the thick sandstones at the bottom of the Shanxi Formation and the thin coalbeds in the Taiyuan and Shanxi Formations. The drilling results clarified that the well situated at the two peaks and two troughs regions is accompanied by higher gas productivity, revealing that the coal-seam thickness has a significant influence on gas accumulation. The presence of two peaks and two troughs indicates favorability for natural gas exploration.

The Anti-Atlas Pan-African Belt (Morocco): Overview and pending questions

Between the High Atlas and the Saharan platform, the Anti-Atlas of Morocco offers large exposures of Precambrian rocks beneath the moderately folded Paleozoic series. These inliers allow reconstructing a segment of the Pan-African Belt and of its foreland at the northern outskirts of the West African Craton (WAC). From ∼885 Ma to ∼540 Ma, three periods are recognized in the Pan-African cycle. The Tonian–Cryogenian period ends with the obduction of supra-subduction ophiolite and oceanic arc material at ∼640 Ma. The Early Ediacaran period is marked by the development and subsequent closure of a wide marginal basin next to a likely Andean-type arc. The Late Ediacaran period is recorded by subaerial molasse deposits associated with post-collisional high-K calc-alkaline to shoshonitic magmatism. Although a wide consensus has been reached based on the number of new robust datings, several questions still remain pending, which we address taking into account relevant African and European correlations.

U‐PB LA‐ICP‐MS DATING OF ZIRCONS FROM SUBVOLCANICS OF THE BIMODAL DYKE SERIES OF THE WESTERN TRANSBAIKALIA: TECHNIQUE, AND EVIDENCE OF THE LATE PALEOZOIC EXTENSION OF THE CRUST

Our study investigates a possibility of using isotope ratios of U‐Th‐Pb system in zircon analysis by the LA‐ICP‐MS technique and monitoring of precision, convergence and accuracy. New U‐Pb isotope‐geochronological data has been obtained for the salic and mafic members of the bimodal Late Paleozoic series of subparallel dykes in the central part of the Western Transbaikalia. It is shown that a series of subparallel dykes (290–280 Ma) gives evi‐ dence of extension of the continental crust at the final stage of the Late Paleozoic granitoid magmatism. Similar ages of zircons in the salic and mafic subvolcanic rocks confirm the geological indicators of the coexistence and interactions between the magmas of contrasting compositions.

Isotopic and geochemical constraints on lead and fluid sources of the Pb[sbnd]Zn[sbnd]Ag mineralization in the polymetallic Tighza-Jbel Aouam district (central Morocco), and relationships with the geodynamic context

The WAu, PbZnAg, and SbBa mineralizations of the polymetallic Tighza-Jbel Aouam district (central Meseta, Morocco), are hosted in Paleozoic rocks surrounding late-Carboniferous granitic stocks. The PbZnAg Tighza deposit formed at 254 ± 16 Ma, and is clearly disconnected from the late-Variscan WAu deposit (295-280 Ma). The PbZnAg mineralization precipitated from a complex hydrothermal fluid. It displays air-normalized 3He/4He ratio (0.018–0.103) typical of the upper crust. This crustal component is confirmed by the oxygen and carbon isotope compositions (δ18O = +19 to +25‰; δ13C = −3.6 to −11.2‰) and the ɛNd values (−4.84 to −9.01) of gangue carbonates, which show mixing of (i) fluids that have interacted with late-Carboniferous magmatic rocks, and (ii) fluids in equilibrium with the Paleozoic metasediments. In addition, the PbZnAg mineralization has 40Ar/36Ar values in the range 284–315 typical of a meteoric fluid. The radiogenic Pb isotopic compositions (207Pb/204Pb = 15.70–15.80 and 206Pb/204Pb = 18.30–18.50) suggest leaching of Pb from the surrounding Paleozoic metasediments and late-Variscan granites, whereas the low radiogenic signatures (207Pb/204Pb = 15.40 and 206Pb/204Pb = 18.05) provide evidence of a deeper source attributed to the lower crust.Crustal thinning related to extensional tectonics in late-Permian and Early-Triassic lead to high-K calc-alkaline to alkaline magmatic activity, which is evidenced by a dense SW-NE-trending dike network that pre-dated the Atlantic Ocean opening (early Liassic times). This magmatic event induced a regional heat flux increase that triggered the circulation of a complex hydrothermal fluid, which has a strong crustal component, but also a meteoric and a lower crustal components. The polymetallic district of Tighza-Jbel Aouam thus results from superposition of an intrusion related porphyry-gold mineralization (WAu, 286 Ma) followed by a PbZnAg epithermal mineralization (254 Ma), during two distinct magmatic-hydrothermal events.The proposed metallogenic model for the PbZnAg Tighza-Jbel Aouam deposit provides new constraints for the PbZnAg exploration in the Moroccan Meseta. Exploration targets must take into account the following geological features: (i) Permo-triassic high-K calk-alkaline to alkaline dikes, (ii) extensional tectonics and reactivation of ancient crust-scale faults and shear zones, and (iii) Paleozoic series containing organic matter (e.g., black shales) subjected to low grade metamorphism (e.g., greenschist facies).

PERMO-TRIASSIC PALYNOLOGY OF THE WEST TIMOR

Fifteen surface samples were examined to analyze palynology of the Permo-Triassic sediments of West Timor. The studied samples were collected from the clastic sediment of Bisane Formation which is considered to be the oldest formation. It derives from the Australian continent (Gondwana) following thecollision with the Banda volcanic arc. The Bisane Formation generally comprises thick calcareous sandstone (0.3-5 meters) with shale alternation and abundant marine microfossil of Chrinoid. The appearance of Chrinoid may indicate Permian age and shallow marine environment. Meanwhile, other Bisane sediment shows different lithology in which it is composed of the intercalation of non-calacreous, dark gray to black shale and siltstone showing papery structure and rich in sulphur. Generally, palynological assemblage of the studied samples characterises Permo-Triassic age as indicated by the existence of common striate-bisaccate pollen including Protohaploxypinus samoilovichii, P. fuscus, P. goraiensis, Striatopodocarpidites phaleratus, Pinuspollenites globosaccus and Lunatisporites pellucidus. However, the appearance of trilete-monosaccate spores of Plicatipollenites malabarensis and Cannanoropollis janakii within the non-calcareous shale samples de􀂿 nes the age as Permian or older for these samples. Interestingly, marine dino􀃀 agellates appear to mark calcareous samples suggesting the in􀃀 uence of a marine environment. They disappear from the non-calcareous samples indicating a freshwater environment. By integrating this palynological analysis and Permian tectonic event which is marked by rifting, it can be interpreted that the non-calcareous samples were formed during early syn-rift as evidenced by the occurrence of freshwater deposit (may be lacustrine). Subsequently, following sea level rises during post rift, the depositional environment shifted to shallow marine as indicated by the existence of calcareous Permo-Triassic samples. If this is the case, the appearance of Permo-Triassic sediments provides an opportunity to 􀂿 nd a new petroleum system in the Paleozoic series of West Timor. Source rock is represented by black shale, whereas reservoir is represented by thick sandstone.

Evidence of the diagenetic history of sediment composition in Precambrian-early Paleozoic rocks: a systematic study from the Southeast Anatolian Autochthon, Mardin (Derik-Kızıltepe), Turkey

The mineralogical properties of the Precambrian-early Paleozoic series have been investigated in the Southeast Anatolian Autochthon (SEAA) and are represented by generally clayey siliciclastic and rarer volcanogenic rocks. These include basalt, andesite, and volcanic sandstones of the Precambrian period; siliceous sandstone, dolomite, shale, and siltstone of the Cambrian period; and sandy dolomite, shale, siltstone, sandstone, and limestone with red Fe nodules of the Lower-Upper Ordovician period. The rock-forming minerals determined in order of abundance are feldspar, pyroxene, quartz, and olivine in the Precambrian unit; quartz, moganite, calcite, dolomite, feldspar, and phyllosilicates (illite, chlorite, mixed-layered illite-smectite/I-S, smectite) in the Cambrian units; and phyllosilicates (kaolinite, illite, chlorite, I-S, mixed-layered chlorite-vermiculite/C-V, smectite), quartz, feldspar, calcite, dolomite, moganite, and goethite in the Lower-Upper Ordovician units. Short prismatic and partly radiating moganites discovered in the early Paleozoic sediments in Turkey are present between euhedral and coarse-grained quartz and feldspar in the sandstones. Thin platy illites have undergone a transition that introduces a fibrous/thread-like mixed-layer I-S through their edges. The rhombohedral dolomites have dissolution traces associated with siliceous balls, long thin filament-shaped illites, and acicular-radial chlorites. Illites usually show coarse and tiny plates parallel to each other and partly radiating flakes, and chamosite-type chlorites form thick plates in the siliciclastics. Kaolinites consist of typical pseudohexagonal sheets with parallel or accordion-like booklets in the siltstones. The Kubler index indicates low- to high-grade diagenesis to low-grade diagenesis, signifying that the location partially differs from the other units of the SEAA and Taurus Belt. In addition, Paleozoic rocks reflect no maturation characteristics with respect to the petroleum system.

Homogenity of geological units with respect to the radon risk in the Walloon region of Belgium

In the process of mapping indoor radon risk, an important step is to define geological units well-correlated with indoor radon. The present paper examines this question for the Walloon region of Belgium, using a database of more than 18,000 indoor radon measurements. With a few exceptions like the Carboniferous (to be divided into Tournaisian, Visean and Namurian-Westphalian) and the Tertiary (in which all Series may be treated together), the Series/Epoch stratigraphic level is found to be the most appropriate geological unit to classify the radon risk. A further division according to the geological massif or region is necessary to define units with a reasonable uniformity of the radon risk. In particular, Paleozoic series from Cambrian to Devonian show strong differences between different massifs. Local hot-spots are also observed in the Brabant massif. Finally, 35 geological units are defined according to their radon risk, 6 of which still present a clear weak homogeneity. In the case of 4 of these units (Jurassic, Middle Devonian of Condroz and of Fagne-Famenne, Ordovician of the Stavelot massif) homogeneity is moderate, but the data are strongly inhomogeneous for Visean in Condroz and in the Brabant massif. The 35 geological units are used in an ANOVA analysis, to evaluate the part of indoor radon variability which can be attributed to geology. The result (15.4–17.7%) agrees with the values observed in the UK.

Late Ediacaran–Cambrian structures and their reactivation during the Variscan and Alpine cycles in the Anti-Atlas (Morocco)

The post-Pan-African evolution of the northern border of the West African Craton is largely controlled by the remobilisation of Late Neoproterozoic basement faults. The Upper Ediacaran volcanic and volcano-sedimentary sequences of the Ouarzazate Group show dramatic and rapid thickness changes, consistent with active extensional faulting associated with post-orogenic collapse and incipient continental rifting. The geometry and kinematics of these faults differ from west to east in the Anti-Atlas. N- to NE-trending faults dominate in western Anti-Atlas in response to E–W to NW–SE pure extension, while a transtensive opening regime characterize the central (Bou Azzer) and eastern (Saghro–Ougnate) Anti-Atlas.The marine incursion in the west-central Anti-Atlas during the late Ediacaran–Early Cambrian occurred without major geodynamical break between the continental Ouarzazate Group and marine sediments of the Adoudou Fm. Extensional tectonics went on during the Early Cambrian, being concentrated in the western and central parts of the belt. From Middle Cambrian to Lower Devonian and mainly due to thermal subsidence, the Anti-Atlas basement was buried under marine sediments with dominant south-derived detrital input. Basement faults control the distribution of subsiding versus shallow areas. During the Middle–Late Devonian, the dislocation of the Saharan platform occurred, mainly in the eastern Anti-Atlas where Precambrian faults were also remobilized during the Early Carboniferous.During the Variscan orogeny, the Paleozoic series of the Anti-Atlas basin were involved in folding tectonics, concomitant with the uplift of Proterozoic basement blocks bounded by inherited basement faults. The pre-existing rift-related faults were variably inverted across the Anti-Atlas. In the westernmost part of the belt, Variscan shortening induced positive inversions along the remobilized basement faults, but in some cases, some faults preserved an apparently normal throw. Some hidden basement faults accommodate the Variscan shortening by strike-slip movement expressed by en echelon fold pattern in the overlying cover.The Mesozoic-Cenozoic evolution of the Anti-Atlas is again marked by the reactivation of the basement faults. During the Triassic, the Anti-Atlas belongs to the uplifted shoulder of the Atlasic-Atlantic rift zone. Remobilisations of paleofaults again play a significant role in the weak burial of the Anti-Atlas during the Late Cretaceous–Eocene before the Neogene exhumation related to the Africa–Europe collision. Hence the structural evolution of the Anti-Atlas during the Paleozoic to Present times has been heavily dependent on the fault pattern inherited from the Late Ediacaran–Cambrian rifting evolution at the northern fringe of the WAC, already deeply affected by the Pan-African orogeny.

Mesozoic and Cenozoic uplift and exhumation of the Bogda Mountain, NW China: Evidence from apatite fission track analysis

Apatite fission track (AFT) analysis on samples collected from a Paleozoic series is used to constrain the cooling history of the Bogda Mountain, northwest China. AFT ages range from 136.2 to 85.6 Ma and are younger than rock depositional ages and the mean confined track lengths (11.0–13.2 μm) mostly showing unimodal distribution are shorten, indicating significant track-annealing. Thermal histories modeling based on the distribution of fission-track lengths combined with the regional geological data show that two rapid cooling phases occurred in the latest Jurassic–early Cretaceous and the Oligocene–Miocene. Those new data together with previous published data show that the AFT ages become younger from the southwest to northeast in the western Bogda Mountain and its adjacent areas. The fission-track ages of the southwest area are relatively older (>100 Ma), recording the earlier rapid uplift phase during the late Jurassic–Cretaceous, while the ages in the north piedmont of the Bogda Mountain (namely the northeast part) are younger (<60 Ma), mainly reflecting the later rapid uplift phase in the Oligocene–Miocene. The trend of younger AFT ages towards the northeast might be explained by post-Cretaceous large-scale crustal tilting towards the southwest. In the thrust fault-dominated northern limbs of the Bogda Mountain, AFT ages reveal a discontinuous pattern with age-jumps across the major fault zones, showing a possible strata tilting across each thrust faults due to the thrust ramps during the Cenozoic. The two rapid uplift stages might be related to the accretion and collision in the southern margin of the Asian continent during the late Jurassic and late Cenozoic, respectively.

Post‐Variscan exhumation of the Central Anti‐Atlas (Morocco) constrained by zircon and apatite fission‐track thermochronology

AbstractThe origin of the Anti‐Atlas relief is one of the currently debated issues of Moroccan geology. To constrain the post‐Variscan evolution of the Central Anti‐Atlas, we collected nine samples from the Precambrian basement of the Bou Azzer‐El Graara inlier for zircon and apatite fission‐track thermochronology. Zircon ages cluster between 340 ± 20 and 306 ± 20 Ma, whereas apatite ages range from 171 ± 7 Ma to 133 ± 5 Ma. Zircon ages reflect the thermal effect of the Variscan orogeny (tectonic thickening of the ca. 7 km‐thick Paleozoic series), likely enhanced by fluid advection. Apatite ages record a complex Mesozoic–Cenozoic exhumation history. Track length modelling yields evidence that, (i) the Precambrian basement was still buried at ca. 5 km depth by Permian times, (ii) the Central Anti‐Atlas was subjected to (erosional) exhumation during the Triassic‐Early Cretaceous, then buried beneath ca. 1.5 km‐thick Cretaceous‐Paleogene deposits, (iii) final exhumation took place during the Neogene, contemporaneously with that of the High Atlas.

Diagenetic and very low-grade metamorphic characteristics of the Paleozoic series of the Istanbul Terrane (NW Turkey)

The Istanbul Terrane along the Black Sea coast in NW Anatolia, is a Gondwana-derived continental microplate, comprising a well-developed Paleozoic succession. Petrographic and X-ray diffraction studies were performed on rock samples from measured sections throughout Ordovician–Carboniferous sedimentary units. Diagenetic-very low-grade metamorphic clastic (shale/mudstone, siltstone, sandstone) and calcareous rocks (limestone, dolomite) mainly contain phyllosilicates, quartz, feldspar, calcite, dolomite, hematite and goethite minerals. Phyllosilicates are primarily represented by illite, chlorite, mixed-layered chlorite–vermiculite (C–V), chlorite–smectite (C–S) and illite–chlorite (I–C). Feldspar is commonly present in the Ordovician and Carboniferous units, whereas calcite and dolomite are abundant in the Silurian and Devonian sediments. The most important phyllosilicate assemblage is illite + chlorite + I–C + C–V + C–S. Illite and chlorite-bearing mixed layer clays are found in all units. The amounts of illites increase in the upper parts of the Silurian series and the lower parts of the Devonian series, whereas chlorite and chlorite-bearing mixed-layers are dominant in the Ordovician and Carboniferous units. Kubler index values of illites reflect high-grade anchimetamorphism for the Early Ordovician rocks, low-grade metamorphism to high-grade diagenesis for the Middle Ordovician–Early Silurian rocks and high-grade diagenesis for the Late Silurian–Devonian units. The K-white micas b cell dimensions indicate intermediate pressure conditions in the Early Ordovician–Early Silurian units, but lower pressure conditions in the Middle Silurian–Devonian units. Illites are composed of 2M1 ± 1Md polytypes in all units, except for Upper Ordovician–Lower Silurian units which involve 1M polytype in addition to 2M1 and 1Md polytypes. The 2M1/(2M1 + 1Md) ratios rise from Devonian to Ordovician together with the increasing diagenetic-metamorphic grade. Chlorites have IIb polytype. In general, crystal-chemical data of clay minerals in the Istanbul Terrane show a gradual increase in the diagenetic/metamorphic grade together with increasing depth. The new data presented in this work indicate that the diagenetic/metamorphic grade of the Paleozoic of the Istanbul Terrane is higher than that of the neighboring Zonguldak Terrane and generated by a single metamorphic phase developed at the end of Carboniferous. This finding contrasts with the metamorphic history of the neighboring Zonguldak Terrane that displays a distinct Early Devonian unconformity and a thermal event.

Successive deformation episodes along the Lungmu Co zone, west-central Tibet

Field study, thermochronology and geochemistry of the east Lungmu Co (LMC) range highlight some of the geological events that shaped western Tibet. The LMC fault zone has long been interpreted as the boundary between the Tianshuihai terrane of Laurasian affinity and the Qiangtang block of Gondwanian affinity. In the LMC range, the Paleozoic series is intruded by the Mangtsa leucogranite whose zircon have a U/Pb age of 116.9 ± 1 Ma and by mafic rocks with U/Pb zircon ages ranging from 116.9 ± 1 to 95.1 ± 1.7 Ma. Geochemistry of the mafic rocks indicates that they have been emplaced in a supra-subduction zone setting, probably the north dipping Nujiang suture zone. 40Ar/ 39Ar micas ages of the granite indicate that cooling below ~ 350 °C occurred between 105 and 85 Ma. 40Ar/ 39Ar K-feldspar data suggest a fast cooling event at 60–55 Ma, which we relate to the reactivation of the LMC suture zone as a thrust at the onset of the India–Eurasia collision. The last, and still active, deformation event corresponds to left-lateral strike-slip faulting along the ENE–WSW LMC fault.

Who needs (lower Paleozoic) biostratigraphy?

In the 1970s, comic genius George Carlin pushed the limits of censorship with a popular routine regarding the seven words you cannot say on television. Similarly, one can argue that geologists currently seeking grants from major funding sources, at least in the USA, are well advised to avoid using certain words or phrases in their proposals: regional , taxonomy , geologic mapping , and, perhaps the worst of the lot, biostratigraphy . This is quite unfortunate because, in reality, we have barely begun to tap the correlation potential of species range data within Cambrian and Ordovician strata, especially those provided by the rapid evolution of trilobites, conodonts, and graptolites through the early Paleozoic. Other groups have considerable biostratigraphic value in certain intervals and paleogeographic settings, but none rivals the aforementioned big three in their broad temporal and spatial range of utility. Over 95% of the lower Paleozoic system, series, and stage boundaries in current geologic time scales are zonal boundaries based on key trilobite, conodont, or graptolite species (Gradstein et al., 2004; Webby et al., 2004; Babcock and Peng, 2007). Consequently, the examples I provide to argue that the need for focused biostratigraphic research has never been greater, all involve use of the big three (especially trilobites, my specialty) in the Cambrian–Ordovician of Laurentian North America. A biostratigrapher specializing in a different taxonomic group, paleogeographic setting, or time interval, however, could probably make at least as strong a case for the untapped potential within his or her group. If biostratigraphy is so important, then why has it fallen into such disfavor, as reflected in limited prospects for funding of purely biostratigraphic research and diminishing representation of this discipline in the geoscience faculty of academic institutions and government agencies? Flessa and Smith (1997) and Plotnick (2008) presented disconcerting data …

Paleozoic structural controls on shortening transfer in the Subandean foreland thrust system, Ene and southern Ucayali basins, Peru

The Neogene evolution of the Ene and southern Ucayali basins of the Subandes has been controlled by two stacked thrust wedges that differ in terms of tectonic styles. The lower thrust wedge is formed by deep‐seated décollements within the basement related to thick‐skinned foreland structures inherited from an Early Carboniferous thrust system. Seismic reflection data show that this Paleozoic compressional system has been eroded and unconformably covered by Late Carboniferous clastic sediments. It generated an irregular Paleozoic sedimentary architecture controlling the Neogene thrust propagation. The upper thin‐skinned thrust wedge developed within this Paleozoic sedimentary series and constitutes the Subandean zone. Cross‐section balancing shows an along‐strike homogenous horizontal shortening of ∼56 km (∼30%) across the Ene−southern Ucayali thrust system. This amount of shortening was vertically partitioned onto the two stacked thrust wedges. The N‐S thickness variations of the Paleozoic sedimentary prism controlled the eastward propagation of the upper thrust wedge. The southern thickening of the Paleozoic series generated major décollements and the shortening excess is of 7 km (16%) in comparison to the north. Consequently, the northern lack of shortening onto the upper thrust wedge was transferred to the Early Carboniferous compressional structures of the lower thrust wedge. We suggest that this vertical partitioning of the shortening was accommodated by a regional oblique ramp: the Tambo transfer zone. This geometrical analysis of the Ene−southern Ucayali thrust system provides new perspectives for future hydrocarbon exploration in this region.

On the non-existence of a Cadomian basement in southern France (Pyrenees, Montagne Noire): implications for the significance of the pre-Variscan (pre-Upper Ordovician) series

Abstract The deepest Hercynian metamorphic terrains in the Pyrenees and in the nearby Montagne Noire are made up of medium-grade orthogneisses and micaschists, and of high-grade, often granulitic, paragneisses. The existence of a granitic-metamorphic Cadomian basement and of its sedimentary Lower Paleozoic cover was advocated from the following main arguments: (i) a supposed unconformity of the Lower Cambrian Canaveilles Group (the lower part of the Paleozoic series) upon both granitic and metamorphic complexes; (ii) a ca. 580 Ma U-Pb age for the metagranitic Canigou gneisses. A SE to NW transgression of the Cambrian cover and huge Variscan recumbent (“penninic”) folds completed this classical model. However, recent U-Pb dating provided a ca. 474 Ma, early Ordovician (Arenigian) age for the me-tagranites, whereas the Vendian age (581 ± 10 Ma) of the base of the Canaveilles Group was confirmed [Cocherie et al., 2005]. In fact, these granites are laccoliths intruded at different levels of the Vendian-Lower Cambrian series. So the Cadomian granitic basement model must be discarded. In a new model, developed in the Pyrenees and which applies to the Montagne Noire where the orthogneisses appear to be Lower Ordovician intrusives too, there are neither transgression of the Paleozoic nor very large Hercynian recumbent folds. The pre-Variscan (pre-Upper Ordovician) series must be divided in two groups: (i) at the top, the Jujols Group, mainly early to late Cambrian, that belongs to a Cambrian-Ordovician sedimentary and magmatic cycle ; the early Ordovician granites pertain to this cycle; (ii) at the base, the Canaveilles Group of the Pyrenees and the la Salvetat-St-Pons series of the Montagne Noire, Vendian (to earliest Cambrian?), are similar to the Upper Alcudian series of Central Iberia. The Canaveilles Group is a shale-greywacke series with rhyodacitic volcanics, thick carbonates, black shales, etc. The newly defined olistostromic and carbonated, up to 150 m thick Tregurà Formation forms the base of the Jujols Group, which rests more or less conformably on the Canaveilles Group. The high-grade paragneisses which in some massifs underlie the Canaveilles and Jujols low- to medium grade metasediments are now considered to be an equivalent of the Canaveilles Group with a higher Variscan metamorphic grade; they are not derived from metamorphic Precambrian rocks. So, there is no visible Cadomian metamorphic (or even sedimentary) basement in the Pyrenees. However, because of its age, the Canaveilles Group belongs to the end of the Cadomian cycle and was deposited in a subsident basin, probably a back-arc basin which developed in the Cadomian, active-transform N-Gondwanian margin of this time. The presence of Cadomian-Panafrican (ca. 600 Ma) zircon cores in early Ordovician granites and Vendian volcanics implies the anatexis of a thick (&amp;gt; 15 km?) syn-Cadomian series, to be compared to the very thick Lower Alcudian series of Central Iberia, which underlies the Upper Alcudian series. Nd isotopic compositions of Neoproterozoic and Cambrian-Ordovician sediments and magmatites, as elsewhere in Europe, yield Paleoproterozoic (ca. 2 Ga) model-ages. From the very rare occurrences of rocks of this age in W-Europe, it can be envisionned that the thick Pyrenean Cadomian series lies on a Paleoproterozoic metamorphic basement. But, if such a basement does exist, it must be “hidden”, as well as the lower part of the Neoproterozoic series, in the Variscan restitic granulites of the present (Variscan) lower crust. So a large part of the pre-Variscan crust was made of volcano-sedimentary Cadomian series, explaining the “fertile” characteristics of this crust which has been able to produce the voluminous Lower Ordovician and, later, Upper Carboniferous granitoids.

Very fast exhumation of high-pressure metamorphic rocks with excess 40Ar and inherited 87Sr, Betic Cordilleras, southern Spain

In order to attempt to further constrain the age of the early Alpine tectonic evolution of the Mulhacén Complex and to explore the influence of inherited isotopes, micas from a small number of well-characterised rocks from the Sierra de los Filábres, with a penetrative tectonic fabric related to the exhumation of eclogite-facies metamorphic rocks, were selected for 40Ar/ 39Ar and Rb–Sr dating. A single phengite grain from an amphibolite yielded an 40Ar/ 39Ar laser step heating plateau age of 86.9±1.2 Ma (2 σ; 70% 39Ar released) and an inverse isochron age of 86.2±2.4 Ma with an 36Ar/ 40Ar intercept within error of the atmospheric value. Induction furnace step heating of a biotite separate from a gabbro relic in an eclogite yielded a weighted mean age of 173.2±6.3 Ma (2 σ; 95% 39Ar released). These ages are diagnostic of excess argon ( 40Ar XS) incorporation, as they are older than independent age estimates for the timing of eclogite-facies metamorphism and intrusion of the gabbros. 40Ar XS incorporation probably resulted from restricted fluid mobility in the magmatic rocks during their metamorphic recrystallisation. Rb–Sr whole-rock–phengite ages of graphite-bearing mica schists from Paleozoic rocks (Secano unit) show a dramatic variation (66.1±3.2, 40.6±2.6 and 14.1±2.2 Ma). An albite chlorite mica schist from the Mesozoic series of the Nevado–Lubrı́n unit has a whole-rock–mica–albite age of 17.2±1.9 Ma, which is within error of an 40Ar/ 39Ar plateau age published previously and of the youngest Rb–Sr age of the Paleozoic series obtained in this study. The significant spread in Rb–Sr ages implies that progressive partial resetting of an older isotopic system has occurred. The microstructure of the samples with pre-Miocene Rb–Sr ages reveals incomplete recrystallisation of white mica and inhibited grain growth due to the presence of graphite particles. This interpretation agrees with previously published, disturbed and slightly dome-shaped 40Ar/ 39Ar age spectra that may point similarly to the presence of an older isotope component. The progressively reset Rb–Sr system is a relic of Variscan metamorphism of the Paleozoic series of the Mulhacén Complex. In contrast, the origin of the ca. 17.2 Ma old sample from the Mesozoic series precludes any isotopic inheritance, in agreement with its pervasive tectono-metamorphic recrystallisation during the Miocene. Exhumation of the eclogite-facies Mulhacén Complex occurred in two stages with contrasting rates of about 22.5 mm/year during the early phase and 9–10 mm/year during the late phase; the latter with a cooling rate in the order of 330 °C/Ma.

Possible glaciomarine diamictites in Lower Paleozoic series of the Southern Black Forest (Germany): implications for the Gondwana/Laurussia puzzle

Possible glaciomarine diamictites in Lower Paleozoic series of the Southern Black Forest (Germany): implications for the Gondwana/Laurussia puzzle