Marine Rocks Research Articles

Tafonomia dos Invertebrados do Sítio Canastra, Formação Romualdo, Cretáceo Inferior, Bacia do Araripe, Araripina, Pernambuco, Brasil

The Romualdo Formation, Araripe Basin, corresponds to sedimentary rocks of shallow marine to transitional environment deposited during the late Aptian-early Albian. In the Sitio Canastra section, municipality of Araripina, Pernambuco State, are observed mudstones grading to calcarenites at the base of the outcrop. Calcareous nodules bearing fossil fishes are frequently found in the mudstones. The paleoenvironment is interpreted as transitional with fluctuating energy, suggesting successive transgression cycles. Towards the top the section, there are calcareous sandstones with fossils molluscs and echinoids, allowing the stablishment of different taphofacies, supporting the paleoenvironmental interpretation of the area. Through the taphonomic analysis, three taphofacies (T.I, T.II, T.III) were identified. The presence of marine organisms in the taphofacies, such as the echinoids of the species Pygurus tinocoi and Bothryopneustes araripensis in association cassiopid gastropods and mitilid bivalves, suggest a deposition in a of the species marine environment. Well preserves articulated equinoid shells ocurring in association to articulated and closed bivalves in T.I indicate a mostly biogenic living community generated by obrution deposits. In T.II and T.III, fragmentation is frequent, with bioclasts of several sizes. The fossil concentrations are poorly sorted with complex and chaotic packing, both in plant and vertical section, the above features are typical of tempestitic events in proximal areas. We concluded that the Canastra fossil site, represents the evolution of the Araripe paleolagoon, with an initial transgression gradating to a shallow marine environment with transport and burial of macrobenthic organisms between the beach and the platform (Araripina), through the action of storms, in agreement with data obtained in other areas of the basin.

Organic petrology and geochemistry of mudrocks from the lacustrine Lucaogou Formation, Santanghu Basin, northwest China: Application to lake basin evolution

Exploration for tight oil in the frontier Santanghu Basin of northwest China has resulted in recent commercial discoveries sourced from the lacustrine Upper Permian Lucaogou Formation, already considered a “world class source rock” in the Junggar Basin to the west. Here we apply an integrated analytical program to carbonate-dominated mudrocks from the Lucaogou Formation in Santanghu Basin to document the nature of organic matter (OM) in the context of an evolving lake system. The organic-rich samples (TOC 2.8–11.4wt%; n=10) were widely spaced from an ~200m cored section, interpreted from textural and mineralogical evidence to document transition from a lower under-filled to an overlying balanced-filled lake. Organic matter is dominated by moderate to strongly fluorescent amorphous material with Type I geochemical signature (HI values 510–755; n=10) occurring in a continuum from lamellar stringers, 10–20μm thick, some ≥1mm in length (possible microbial mat; preserved only in lower under-filled section) to finely-disseminated amorphous groundmass intimately intermixed with mineral matrix. Biomarkers for methanotrophs and photosynthetic cyanobacteria indicate a complex microbial consortium. A unicellular prasinophyte green alga(?), similar to Tasmanites in marine rocks, is present as discrete flattened discs 50–100μm in diameter. Type III OM including vitrinite (some fluorescent) and inertinite also is abundant. Solid bitumen, indicating local kerogen conversion, fills voids and occurs throughout the cored section. Vitrinite reflectance values are 0.47–0.58%, consistent with strong OM fluorescence but may be “suppressed”. Other proxies, e.g., biomarker parameters, indicate the Lucaogou Formation is in the early oil window at this location. On average, slightly more amorphous OM and telalginite are present in the lower section, consistent with a shallow, stratified, saline environment with low sediment dilution. More inertinite is present in the upper section, indicating greater terrestrial influx and consistent with higher quartz and plagioclase content (dominantly authigenic chalcedony and albite). Laminated mudstones in the upper section indicate anoxia prevented bioturbation from benthic grazing, also indicating stratified water column conditions. A decrease upsection in authigenic dolomite with reciprocal increase of ankerite/siderite is consistent with decreasing salinity, as is an overall decrease in gammacerane index values. These observations suggest evolution from a shallow, stratified evaporative (saline) setting to a deeper, stratified freshwater basin with higher water input during Lucaogou deposition. The evolution from an under-filled to balance-filled lake in Santanghu Basin is similar to Lucaogou deposition in Junggar Basin, suggesting similar tectonic and climatic controls. Paleoclimate interpretations from other researchers in this area suggested an evolution from semi-arid to humid conditions during the Roadian; we interpret that the evolution from an under-filled to balanced-filled lake seen in our data is in response to climate change, and may represent increased groundwater delivery to the Santanghu Basin.

Vitrinite and vitrinite like solid bitumen reflectance in thermal maturity studies: Correlations from diagenesis to incipient metamorphism in different geodynamic settings

Vitrinite reflectance (VR) is a useful and reliable parameter to monitor the level of organic matter maturation from the immature stage to graphite. Additionally, VR is a useful and essential tool in modelling the temperature–maturation time path. Huminite/vitrinite-bearing sedimentary and meta-sedimentary rocks can be used to reconstruct the thermal history of basins or orogenic terranes throughout their geological history. The usefulness of VR evidently depends on the presence of huminite/vitrinite phytoclasts. However, pre-upper Silurian rocks and many organic-rich marine rocks (e.g. sapropelic sediments) lack of huminite/vitrinite particles. Furthermore, vitrinite is sparse in carbonate sediments.However, the presence of bitumen in rocks lacking Type III kerogen provides a valuable aid via its reflectance (vitrinite like solid bitumen reflectance — VlBR, and/or solid bitumen reflectance — BR) to fulfil the above-described tasks that VR enables. This study presents basically a relationship between BR/VlBR and VR for various types of geodynamic settings and over the largest possible range of maturation levels so that VlBR may be used as a substitute for VR.Whereas the effort to correlate VR with BR is not novel, it is the first time that the VR/BR correlation is re-judged, discriminating bitumen from VlB. Moreover for the first time the correlation is established for various geodynamic settings. The investigated geological contexts are:(1)sedimentary basins (e.g., the Saar, Ruhr, Ohio, West Virginia, and Petrosani basins);(2)metasedimentary terranes occurring as inverted basins in orogenic belts (e.g., from the Subalpine-Molasse, Helvetic, Austroalpine and Danubian nappe systems) and having suffered from very low to low grade metamorphism (from anchizone to greenschist facies);(3)metasedimentary terranes (e.g., slates and schists from the Franciscan and Alpine belts) that were subducted at anchizone to blueschist facies conditions; and(4)metasedimentary rocks affected by contact metamorphism.Together, the frequent geological settings are referred and most of the earth geothermal conditions and heat fluxes considered. Despite temperature, pressure, and heating time, also heat flow gradient dependences cannot be ignored.

Marine organic matter cycling during the Ediacaran Shuram excursion

Ediacaran (ca. 635–541 Ma) marine carbonates capture a global δ^(13)C carbon isotope excursion to extremely negative values (∼–12‰)—known as the Shuram excursion (SE)—that cannot be explained by conventional mass balance scenarios. Furthermore, the carbon isotopic variation of bulk organic matter (OM) does not mirror that of carbonate through the excursion, suggesting that the OM reflects a mixture of different sources. To evaluate this hypothesis, we investigated thermally immature marine sedimentary rocks that record the SE from the Sultanate of Oman. Compound-specific carbon isotopic analyses of the extractable hydrocarbons reveal low δ^(13)C values of long-chain (>C_(20)) n-alkanes and mid-chain monomethyl alkanes as low as −40‰. Such light signatures are rare in marine rocks of any age and provide evidence that the SE reflects a primary carbon cycle perturbation. The magnitude of the SE recorded in these organic phases is smaller than observed in carbonate and implies that the primary perturbation to dissolved inorganic carbon (DIC) was at least 5‰−7‰, and more likely 7‰−12‰, in magnitude when correcting for end-member source mixing. Due to isotopic differences in stratigraphic patterns of the different organic compounds, we propose that bulk organic carbon (both bitumen and kerogen) reflects source mixing between two distinct pools that previously masked the excursion in bulk δ^(13)C_(org) measurements. OM sources were derived both from autotrophs fixing ^(13)C-depleted DIC and from a less ^(13)C-depleted heterotrophic microbial biomass feeding on a marine OM pool sustained by petroleum expelled from older sedimentary OM. Expulsion of these sedimentary fluids also helps explain both the duration and magnitude of the SE.

Lithofacies palaeogeography of the Late Permian Wujiaping Age in the Middle and Upper Yangtze Region, China

The lithofacies palaeogeography of the Late Permian Wujiaping Age in Middle and Upper Yangtze Region was studied based on petrography and the “single factor analysis and multifactor comprehensive mapping” method. The Upper Permian Wujiaping Stage in the Middle and Upper Yangtze Region is mainly composed of carbonate rocks and clastic rocks, with lesser amounts of siliceous rocks, pyroclastic rocks, volcanic rocks and coal. The rocks can be divided into three types, including clastic rock, clastic rock–limestone and limestone–siliceous rock, and four fundamental ecological types and four fossil assemblages are recognized in the Wujiaping Stage. Based on a petrological and palaeoecological study, six single factors were selected, namely, thickness (m), content (%) of marine rocks, content (%) of shallow water carbonate rocks, content (%) of biograins with limemud, content (%) of thin-bedded siliceous rocks and content (%) of deep water sedimentary rocks. Six single factors maps of the Wujiaping Stage and one lithofacies palaeogeography map of the Wujiaping Age were composed. Palaeogeographic units from west to east include an eroded area, an alluvial plain, a clastic rock platform, a carbonate rock platform where biocrowds developed, a slope and a basin. In addition, a clastic rock platform exists in the southeast of the study area. Hydrocarbon source rock and reservoir conditions were preliminarily analyzed based on lithofacies palaeogeography. Sedimentary environments have obvious controls over the development of the resource rocks. With regard to the abundance of organic matter, the hydrocarbon potential of the coastal swamp environment is the best, followed by the basin environment and the carbonate rock platform. The gas reservoir types of the Wujiaping Stage can be classified as conventional and unconventional gas reservoirs, like coal bed gas and shale gas; all of them have well exploration prospects.

The rise of fire: Fossil charcoal in late Devonian marine shales as an indicator of expanding terrestrial ecosystems, fire, and atmospheric change

Fossil charcoal provides direct evidence for fire events that, in turn, have implications for the evolution of both terrestrial ecosystems and the atmosphere. Most of the ancient charcoal record is known from terrestrial or nearshore environments and indicates the earliest occurrences of fire in the Late Silurian. However, despite the rise in available fuel through the Devonian as vascular land plants became larger and trees and forests evolved, charcoal occurrences are very sparse until the Early Mississippian where extensive charcoal suggests well-established fire systems. We present data from the latest Devonian and Early Mississippian of North America from terrestrial and marine rocks indicating that fire became more widespread and significant at this time. This increase may be a function of rising O2 levels and the occurrence of fire itself may have contributed to this rise through positive feedback. Recent atmospheric modeling suggests an O2 low during the Middle Devonian (around 17.5%), with O2 rising steadily through the Late Devonian and Early Mississippian (to 21–22%) that allowed for widespread burning for the first time. In Devonian-Mississippian marine black shales, fossil charcoal (inertinite) steadily increases up-section suggesting the rise of widespread fire systems. There is a concomitant increase in the amount of vitrinite (preserved woody and other plant tissues) that also suggests increased sources of terrestrial organic matter. Even as end Devonian glaciation was experienced, fossil charcoal continued to be a source of organic matter being introduced into the Devonian oceans. Scanning electron and reflectance microscopy of charcoal from Late Devonian terrestrial sites indicate that the fires were moderately hot (typically 500–600 °C) and burnt mainly surface vegetation dominated by herbaceous zygopterid ferns and lycopsids, rather than being produced by forest crown fires. The occurrence and relative abundance of fossil charcoal in marine black shales are significant in that these shales may provide a more continuous record of fire than is preserved in terrestrial environments. Our data support the idea that major fires are not seen in the fossil record until there is both sufficient and connected fuel and a high enough atmospheric O2 content for it to burn.

Diagenetic Evolution and Reservoir Quality of Sandstones in the North Alpine Foreland Basin: A Microscale Approach

Siliciclastic reservoir rocks of the North Alpine Foreland Basin were studied focusing on investigations of pore fillings. Conventional oil and gas production requires certain thresholds of porosity and permeability. These parameters are controlled by the size and shape of grains and diagenetic processes like compaction, dissolution, and precipitation of mineral phases. In an attempt to estimate the impact of these factors, conventional microscopy, high resolution scanning electron microscopy, and wavelength dispersive element mapping were applied. Rock types were established accordingly, considering Poro/Perm data. Reservoir properties in shallow marine Cenomanian sandstones are mainly controlled by the degree of diagenetic calcite precipitation, Turonian rocks are characterized by reduced permeability, even for weakly cemented layers, due to higher matrix content as a result of lower depositional energy. Eocene subarkoses tend to be coarse-grained with minor matrix content as a result of their fluvio-deltaic and coastal deposition. Reservoir quality is therefore controlled by diagenetic clay and minor calcite cementation.Although Eocene rocks are often matrix free, occasionally a clay mineral matrix may be present and influence cementation of pores during early diagenesis. Oligo-/Miocene deep marine rocks exhibit excellent quality in cases when early cement is dissolved and not replaced by secondary calcite, mainly bound to the gas-water contact within hydrocarbon reservoirs.

Isthminia panamensis, a new fossil inioid (Mammalia, Cetacea) from the Chagres Formation of Panama and the evolution of 'river dolphins' in the Americas.

In contrast to dominant mode of ecological transition in the evolution of marine mammals, different lineages of toothed whales (Odontoceti) have repeatedly invaded freshwater ecosystems during the Cenozoic era. The so-called ‘river dolphins’ are now recognized as independent lineages that converged on similar morphological specializations (e.g., longirostry). In South America, the two endemic ‘river dolphin’ lineages form a clade (Inioidea), with closely related fossil inioids from marine rock units in the South Pacific and North Atlantic oceans. Here we describe a new genus and species of fossil inioid, Isthminia panamensis, gen. et sp. nov. from the late Miocene of Panama. The type and only known specimen consists of a partial skull, mandibles, isolated teeth, a right scapula, and carpal elements recovered from the Piña Facies of the Chagres Formation, along the Caribbean coast of Panama. Sedimentological and associated fauna from the Piña Facies point to fully marine conditions with high planktonic productivity about 6.1–5.8 million years ago (Messinian), pre-dating the final closure of the Isthmus of Panama. Along with ecomorphological data, we propose that Isthminia was primarily a marine inhabitant, similar to modern oceanic delphinoids. Phylogenetic analysis of fossil and living inioids, including new codings for Ischyrorhynchus, an enigmatic taxon from the late Miocene of Argentina, places Isthminia as the sister taxon to Inia, in a broader clade that includes Ischyrorhynchus and Meherrinia, a North American fossil inioid. This phylogenetic hypothesis complicates the possible scenarios for the freshwater invasion of the Amazon River system by stem relatives of Inia, but it remains consistent with a broader marine ancestry for Inioidea. Based on the fossil record of this group, along with Isthminia, we propose that a marine ancestor of Inia invaded Amazonia during late Miocene eustatic sea-level highs.

Jonoichthys challwa gen. et sp. nov., a new Aspidorhynchiform (Osteichthyes, Neopterygii, Teleosteomorpha) from the marine Upper Jurassic sediments of Argentina, with comments about paleobiogeography of Jurassic aspidorhynchids

Aspidorhynchiformes as a whole are incompletely understood. Here, a new aspidorhynchiform, Jonoichthys challwa gen. et sp. nov., is described based on a single three-dimensionally preserved specimen recovered from Upper Jurassic marine rocks of the Vaca Muerta Formation, Neuquén, southern Argentina. The new taxon is based on a unique combination of characters (e.g., large skull roof plate composed by the fusion of certain bones; ‘L’-shaped preoperculum with a single main preopercular sensory canal that lacks sensory tubules; three predentary tooth rows; an accessory cusp in the teeth of the median predentary tooth row, and a robust and deep body). It is expected that the morphological information provided here could be useful to further understanding of aspidorhynchiform phylogeny and Neuquén Basin biodiversity.

Fossil asterozoa (Echinodermata) of Argentina

Thefossil Asterozoa of Argentina have received scant attention. Marine rocks of Early Devonian-Late Miocene age yield ten species of Asterozoa (four Asteroidea and six Ophiuroidea), including the new Neogene record of Astropecten sp. presently introduced. Due to homonymy, Marginix nomen novum is proposed as a substitution of Marginura (Ophiuroidea, Encrinasteridae)

A 425-Million-Year-Old Silurian Pentastomid Parasitic on Ostracods

Pentastomids (tongue worms) are worm-like arthropods known today from ∼140 species [1]. All but four are parasitic on vertebrates. Their life cycle typically involves larval development in an intermediate host followed by maturation in the respiratory tract of a definitive terrestrial host. Fossil pentastomids are exceedingly rare and are known only from isolated juveniles [2-6]. The identity of the possible hosts of fossil pentastomids and the origin of their lifestyle have generated much debate. A new, exceptionally preserved species, described based on adults from 425-million-year-old marine rocks, is the only known fossil pentastomid associated with a host, in this case a species of ostracod crustacean. The pentastomids are preserved near eggs within the ostracod and also, uniquely for any fossil or living pentastomid, are attached externally to the host. This discovery affirms the origin of pentastomids as ectoparasitic on marine invertebrates. The terrestrialization of pentastomids may have occurred in parallel with the vertebrate invasion of land.

The first dinosaur from Washington State and a review of Pacific coast dinosaurs from North America.

We describe the first diagnostic dinosaur fossil from Washington State. The specimen, which consists of a proximal left femur, was recovered from the shallow marine rocks of the Upper Cretaceous (Campanian) Cedar District Formation (Nanaimo Group) and is interpreted as pertaining to a large theropod on the basis of its hollow medullary cavity and proximally placed fourth trochanter. The Washington theropod represents one of the northernmost occurrences of a Mesozoic dinosaur on the west coast of the United States and one of only a handful from the Pacific coast of Laramidia during the Cretaceous. Its isolated nature and preservation in marine rocks suggest that the element was washed in from a nearby fluvial system. If the femur pertains to a tyrannosauroid, which seems likely given its size and the widespread occurrence of the group across Laramidia during Late Cretaceous times, then it would represent an earlier occurrence of large body size than previously recognized (complete femur length estimated at 1.2 meters). Uncertainty surrounding the latitude of deposition of the Nanaimo Group (i.e., the Baja-British Columbia hypothesis) precludes assigning the Washington theropod to either of the putative northern or southern biogeographic provinces of Laramidia.

Seasonal and Cyclical Changes in Genetic Composition of the Marine Intertidal Rock Pool Copepod Tigriopus brevicornis.

Tigriopus brevicornis is a marine rock pool copepod widely distributed along Atlantic coasts. Due to the absence of a known dispersal mechanism by free swimming stages, exchanges between populations over long distances are questionable. In order to analyse the evolution of an isolated supralittoral rock pool population, sampling of the copepod was performed monthly during 1 year and compared to samplings over 5 years in the same rock pool, as well as from other rock pools. Using ITS1 analysis, cyclical changes in genetic composition were detected. Our results give clear indications concerning the segregation of the rock pool population and a lack of gene flow among outside populations. A network analysis shows the presence of several shared dominant haplotypes and also singletons differing by one mutation point. F(st) analyses indicate that the main changes occur in autumn and winter. The few analogies of ITS1 sequences with nearby populations may indicate that new migrants must re-colonise the pools from surrounding rock crevices in the intertidal habitat where they may have found a refuge after bad weather conditions.

Marine rock physical flume experiment: The method of seafloor shallow sediment recognition by ultrasonic physical attributes

It's a key problem to recognize the kinds of sediments in the ocean bottom during an acoustic survey or seismic exploration. In this paper, an ultrasonic rock physics experiment was introduced in the flume, which simulated different sedimentary types of ocean bottom, such as stone, fine sand, coarse sand, silt and cement. After processing the simulated acoustic data, some physical attributes, which are sensitive to different types of sediment, was found. In which, the weighted average frequency attribute can distinguish the coarse sand, silt and stone substrate if combined with sweetness attribute. The instantaneous quality factor (Q) attribute highlights the cement restrained with the weighted average frequency attribute. Through analyzing the relationships between porosity, density and these sensitive acoustic attributes, the types of sediment in the flume can be deduced and inversed by artificial neural network method.

Assessing the distribution of sedimentary C40 carotenoids through time

A comprehensive marine biomarker record of green and purple sulfur bacteria (GSB and PSB, respectively) is required to test whether anoxygenic photosynthesis represented a greater fraction of marine primary productivity during the Precambrian than the Phanerozoic, as current models of ocean redox evolution suggest. For this purpose, we analyzed marine rock extracts and oils from the Proterozoic to the Paleogene for C40 diagenetic products of carotenoid pigments using new analytical methods. Gas chromatography coupled with tandem mass spectrometry provides a new perspective on the temporal distributions of carotenoid biomarkers for phototrophic sulfur bacteria, specifically okenane, chlorobactane, and paleorenieratane. According to conventional paleoredox interpretations, this revised stratigraphic distribution of the GSB and PSB biomarkers implies that the shallow sunlit surface ocean (<24 m) became sulfidic more frequently in the geologic past than was previously thought. We reexamine whether there is evidence supporting a planktonic source of GSB and PSB pigments in marine systems or whether additional factors are required to explain the marine phototrophic sulfur bacteria record. To date, planktonic GSB and PSB and their pigments have been identified in restricted basins and lakes, but they have yet to be detected in the unrestricted, transiently sulfidic, marine systems. Based on modern observations, additional environmental factors, including basin restriction, microbial mats, or sediment transport, may be required to fully explain GSB and PSB carotenoids in the geologic record.

The age of the Pleistocene raised beach near Fethard, County Wexford, using infra red stimulated luminescence (IRSL)

Stratified sediments, interpreted to be part of the Courtmacsherry Formation Pleistocene raised beach that overlies a marine rock platform near Fethard. south Wexford, were sampled and dated using the technique of Infra Red Stimulated Luminescence (IRSL). The technique yielded ages for the formation of most of the raised beach of between c. 162 ka BP and c. 129 ka BP. These arc the first age determinations for the Courtmacsherry Formation itself and. as such, represent an important advance in the development of a chronostratigraphy of the Pleistocene in Ireland. These dates are consistent with results obtained by others using UTD dating of sediments overlying the Courtmacsherry Formation in County Kerry and, therefore, suggest that IRSL holds great promise for the dating of hitherto updateable sediments.

Effects of nanoporosity and surface imperfections on solid bitumen reflectance (BRo) measurements in unconventional reservoirs

Bitumen reflectance (BRo) is an alternative maturity indicator to vitrinite reflectance (VRo) in Lower Paleozoic and marine rocks where few higher plant macerals can be found. In unconventional petroleum rocks, locally exuded and/or migrated oil are retained in the tight rocks and thermally and diagenetically consolidated into solid bitumen and pyrobitumen. Finding suitable bitumen macerals for BRo measurements pose a challenge in indicating the proper thermal maturity. This study reports the effects of surface imperfections caused by nanoporosity, surface granularities, and impurities on measuring BRo in core samples from the Jurassic Nordegg Shale (shale oil) and the Triassic Montney siltstone (tight gas) in the Western Canada Sedimentary Basin. The degree that bitumen's surface imperfections influence any given reflectance measurement was controlled by changing the probe size area. The larger the measuring probe, the higher the degree of surface imperfections is included in BRo readings. The effect is quantified in this paper, showing up to 27% diminishment of mean BRo values when the probe size increased from 0.31 to 5.85μm2. The mode in which bitumen accumulates within the rock has a strong influence on the degree of surface imperfections. Bitumen which is disseminated within clay matrix introduces a high degree of nanoporosity and mineral interference and hence will result in lowest BRo. In contrast, inclusions of high reflecting micrinite or diagenetic minerals such as pyrite result in higher BRo. These effects can vary at the microscopic scale leading to multiple populations of reflectance readings and difficult interpretations of thermal maturity.

Scarcity of the C30 sterane biomarker, 24-n-propylcholestane, in Lower Paleozoic marine paleoenvironments

24-n-Propylcholestane (24-npc), a C30 sterane compound derived from sterol precursors which are the major sterol constituents of modern pelagophyte microalgae, occurs in certain Neoproterozoic rocks and oils and throughout the Phanerozoic rock record. This broad distribution leads 24-npc to be widely considered a reliable indicator of open to partially restricted marine depositional conditions for source rocks and oils. Here we report two significant hiatuses in the occurrences of 24-npc in the Lower Paleozoic marine rock record: the first in the Middle–Late Cambrian and the second in the Late Ordovician–early Silurian transition for a range of lithofacies (carbonates and siliciclastic rocks), organic carbon contents (both organic-lean and organic-rich), and paleoceanographic environments (shelf and deeper water marine settings) and observed offshore of two paleocontinents, Laurentia and Baltica. The Ordovician–Silurian gap is at least 9millionyears, and possibly up to 20millionyears, in duration. Robust older occurrences of 24-npc steranes in some Neoproterozoic rocks and oils suggest that oceanographic conditions in our intervals of Lower Paleozoic time were unfavorable for the proliferation of pelagophyte algae as phytoplankton. Caution should therefore be applied when interpreting a lacustrine versus marine depositional environmental setting for source rocks and oils in these intervals of Early Paleozoic time using lipid biomarker assemblages.

Paleobiogeography of the Danian molluscan assemblages of Patagonia (Argentina)

A detailed quantitative analysis of bivalves and gastropods reported from Danian marine rocks of Patagonia (Argentina) allowed the identification of three molluscan biogeographic units that, from north to south, are identified as the Rocaguelian, Salamancan, and Dorotean Bioprovinces. Molluscan assemblages comprise Cosmopolitan, Paleoaustral, Gulf Coastal Plain and Endemic genera, which collectively give this fauna a distinctive signature, preventing them to be considered as related to any other assemblages recorded in the high latitudes of the Southern Hemisphere. Results obtained through the present research support the idea that the Weddellian Province did not extend to Patagonia during Danian times, proving that the geographic isolation of this region during that interval was enough to allow the development of separate biogeographic units from other austral regions. Moreover, it is demonstrated that, although significant even in northern Patagonia, Paleoaustral taxa were not dominant elements of Danian faunas. Composition of the molluscan faunas reflects the presence of warm-temperate waters in the region, and records a latitudinal temperature trend with slightly higher values in northern Patagonia than in the south. In addition, a discussion of the age of the Paleocene faunas has been incorporated. The assemblages contained in the Dorotea formation (Austral Basin), Lefipán and Roca formations (Cañadón Asfalto Basin), and Salamanca formation (Cañadón Asfalto and Golfo San Jorge basins) have been placed in the early Danian, based on the correlation with the accurately dated molluscan faunas of the Roca formation exposed in the Neuquina–Colorado Basin, and that of the Sobral and López de Bertodano formations (Antarctica).

Primera datación radiométrica (U-Pb, LA-ICP-MS, en circones detríticos) de la Formación Punta Topocalma: observaciones sobre la sedimentación marina durante el Cretácico Tardío en Chile central.

Upper Cretaceous marine rocks crop out along the Pacific coast of central and south-central Chile between 33° and 37°S. These strata constitute an important reference for the Upper Cretaceous of South America due to their diverse fossil fauna and flora. The type unit of these deposits is the Quiriquina Formation, near Concepcion. This unit is considered Maastrichtian in age based on ammonites. Upper Cretaceous marine strata from other localities of central and south-central Chile are largely unstudied and their biostratigraphic ages are not precisely known. We present the first radiometric dating (U-Pb on detrital zircons) for Upper Cretaceous marine strata of the Chilean forearc at Punta Topocalma that indicates a probable depositional age of 71.9+0.9 Ma (latest Campanian-earliest Maastrichtian). Provenance analysis indicates that the source of sediments of the Punta Topocalma Formation was plutonic and volcano-sedimentary rocks from the Coastal Cordillera and the Central Depression of central Chile. The Lo Valle Formation, a volcano-sedimentary unit in the Central Depression, recorded deposition of the Upper Cretaceous volcanic arc that was coeval with marine sedimentation in the Topocalma area.