Georges Gorin

Organic facies as a tool for recording eustatic variations in marine fine-grained carbonates—example of the Berriasian stratotype at Berrias (Ardèche, SE France)

Abstract Sediments of the Berriasian stratotype in southeastern France consist of marine, fine-grained carbonates deposited on an outer shelf at the western edge of the Vocontian Trough. The limestones are monotonous, but display some erosional surfaces and a condensed interval. They are well dated by ammonites and calpionellids, except at the base of the section where the boundary with the Tithonian is not clearly established. Despite their very high carbonate content, the limestones yielded enough organic residue to allow a study of palynofacies variations in time. Organic content consists of two fractions: one is made of terrestrial components, represented essentially by humic fragments of the inertinite and vitrinite maceral groups and accessorily by pollen and spores; the other comprises marine components, essentially dinocysts and accessorily foraminiferal linings. Absence of amorphous organic matter and the oxidized nature of dinocysts (observable in UV light) indicate that aerobic conditions prevailed throughout the Berriasian, probably associated with a low sedimentation rate. The following qualitative and quantitative criteria, indicating distal-proximal trends, were used to convert palynofacies observations into a tentative palaeoenvironmental curve: ratio of continental to marine organic constituents, diversity of dinocysts, degree of biodegradation affecting humic fragments, proportion of blade-shaped inertinite, and finally size, sorting and rounding of equidimensional inertinite particles. The palaeoenvironmental curve derived from organic matter shows oscillations between more proximal (i.e. shallower water) and more distal (i.e. deeper water) conditions. These variations, combined with biostratigraphic information and the few significant sedimentological data, can be interpreted in terms of sequence stratigraphy and compared with the global sea-level curve published earlier. The fairly good correlation between this global curve and that established from palynofacies demonstrates the influence of relative sea-level on the organic facies of the Berriasian carbonates, and the contribution of palynofacies studies to sequence stratigraphy.

Organic-rich biolaminated facies from a Kimmeridgian lagoonal environment in the French Southern Jura mountains—A way of estimating accumulation rate variations

Abstract During Kimmeridgian times, the present-day Southern Jura was a carbonate platform. In the palaeolagoon at the village of Orbagnoux, organic matter was deposited, giving birth to calcareous laminites, with organic carbon reaching 12%. Three facies may be distinguished; two types of laminites are encountered: one made of parallel laminae and the other made of undulating laminae. The laminites alternate with massive limestone beds, in which no internal structure may be seen. The lithology, sedimentology and the faunal, floral and organic content of laminites and of massive limestones was thoroughly investigated at different scales using various methods: field observation, transmitted-light and scanning electron microscopy, Rock Eval pyrolysis, biomarkers, infrared study of kerogen, palynofacies observation. It is suggested that the different types of laminites (parallel or undulating) may reflect variations in the accumulation rates, linked with the carbonate and organic matter productions. Undulating laminae would correspond to periods of lack of sedimentation whereas parallel laminae would reflect alternations of periods of sedimentation and periods of non-sedimentation. These variations might in turn reflect climatic changes affecting the emerged lands supplying the lagoon with detrital products (organic or inorganic). The accumulation and preservation of organic matter were made possible by the development of microbial mats, building “oxygen-tight” barriers at the water-sediment interface. These barriers separated overlying, oxygenated water from anaerobic sediments. These phenomena may explain one way in which potential hydrocarbon source-rocks can form under a (poorly ?) oxygenated water column.

Bacterial Activity and Preservation of Sedimentary Organic Matter: The Role of Exopolymeric Substances

Although exopolymeric substances (EPS) are associated with the microorganisms contributing to the production/degradation of sedimentary organic matter, their role in theses processes have so far never been mentioned. Using high-resolution microscopical tools (scanning and transmission electron microscopy, atomic force microscopy), fossil organic matter in the Miocene Monterey Formation (California) and Kimmeridgian laminites (France) has been compared with its present-day analogs, i.e., respectively sulphuroxidizing bacteria and cyanobacterial biofilms. This comparison shows that, particularly in the case of Kimmeridgian cyanobacterial mats deposited in a shallow back-reef environment, organic matter preservation is conditioned by exopolymeric substances secreted by bacteria. A model is proposed for the evolution through time of exopolymeric substances in relation to the mechanical constrains they have been exposed to, during lithification and diagenesis. This model is based on the microscopical observation of sulphuroxidizing bacteria and could explain the morphology of fossil organic matter usually referred to as “amorphous” in standard light microscopy. The highly hydrated nature of exopolymeric substances helps to protect organic matter from degradation and remineralization. These substances can be observed only in microscopy and are undetectable through organic geochemical methods, hence the need to combine these two methods in organic matter studies. Consequently, exopolymeric substances must be considered as an important contributing agent to organic matter preservation. These results confirm the complexity of the bacterial role in geoenvironments and add a new parameter in the productivity-vs-preservation debate.

Organic facies of the Early Albian Niveau Paquier, a key black shales horizon of the Marnes Bleues Formation in the Vocontian Trough (Subalpine Ranges, SE France)

Abstract The marnes Bleues Formation was deposited in the central, hemipelagic part of the Vocontian Trough during Aptian-Albian times. It contains several levels of black shales which are characterized by abundant organic matter of both continental and marine origin, as well as by some qualitative and quantitative variations in planktonic and benthic foraminifera contents. We concentrate here upon the most striking of these horizons: the Niveau Paquier (early Albian). The Niveau Paquier has a laminated, paper shale facies. Varying in thickness from a few decimeters to 5 m, it can be traced over a large part of the Alpine Tethys. It is one of the most organic-rich levels in the Marnes Bleues Formation. It displays a subtly fluctuating organic content of both marine (dinocysts, algae, foraminifera linings, amorphous organic matter or AOM) and terrestrial origins (inertinite, vitrinite, cutinite, spores and pollen). Many characteristics of the Niveau Paquier (lithology, microfauna, organic matter, presence of resident dysaerobic faunas, etc.) suggest frequent oscillations between anaerobic, dysaerobic and aerobic conditions. A model involving subtle modifications in the water stratification can be suggested: the fluctuating position of the redox-potential-discontinuity layer (RPD-layer) or of the pycnocline would have affected epi-meso-planktonic, nectonic and epi-endo-benthic life and the nature and preservation of organic matter (mainly of AOM). Organic parameters (palynofacies and Rock Eval data) of the Niveau Paquier seem to have recorded some four ca. 100,000-yr long cycles. These cycles may be related to climatic variations affecting both the emerged lands supplying the Vocontian Trough in clay minerals, organic residues and nutrients and the conditions in the marine milieu. These variations seem to be in the Milankovitch frequency band.

A record of the Jurassic/Cretaceous boundary climatic variation on the southern margin of the Tethys : clay minerals and palynofacies of the early Cretaceous Jebel Meloussi section (Central Tunisia, Sidi Kralif Formation)

In order to precise the paleogeographic extension of the climatic variation known at the Jurassic/Cretaceous boundary, the sedimentary organic matter (palynofacies and Rock-Eval) and the clay minerals content of Berriasian sediments of the Sidi Kralif Formation are studied on the Jebel Meloussi section, central Tunisia. Standard sedimentological and palynofacies analysis allow to reconstruct the bathymetric curve and the sequence stratigraphic scheme. Using existing biostratigraphy based on calpionellids and ammonite zonation, the sequence stratigraphic interpretation can be correlated with the established eustatic chart. Clay mineral assemblages are characterized by a shift in the kaolinite content, recorded at the end of the calpionellid zone B, at the early/middle Berriasian boundary, at a time of high long-term sea-level (MFS Be2, second order eustatic peak). A contemporary change in the clay mineral assemblages, interpreted as a climatic change, is known from the boreal area, and from the northern margin of the Tethys Sea. That change is also documented southerly in southern Morocco (Agadir area), on the Atlantic domain. A late Tithonian to early Berriasian dry and cooler phase is replaced by a middle to late Berriasian more humid phase, indicated by a general increase in kaolinite in the clay mineral assemblages. The trend from a dry climatic phase to a more humid one, recorded on the boreal domain and along the northern margin of the Tethys is also recorded in lower paleolatitudes of Tunisia, on the southern margin of the Tethys, in better dated outcrops than the ones of Morocco. The results obtained in Tunisia show that the beginning of the climatic change was precisely synchronous on both margins, and occurred within the same long-term high sea-level context.

High-resolution seismic imaging of outcrop-scale channels and an incised-valley system within the fluvial-dominated Lower Freshwater Molasse (Aquitanian, western Swiss Molasse Basin)

Abstract High-resolution seismic reflection surveys on Lake Geneva near Lausanne, Switzerland, allow the interpretation of outcrop-scale sedimentary features within the fluvial-dominated Lower Miocene, Lower Freshwater Molasse bedrock. Seismic modelling of an outcrop is used to calibrate the seismic data and assist with interpretation. Seismic resolution is sufficient to image sandstone channel-fill complexes 4–5 m thick. Five seismic facies are recognized and interpreted as fluvial architectural elements of the Lower Freshwater Molasse. Two of these facies are characterized by fairly continuous reflections of variable frequency and low-to-moderate amplitude, interpreted as metre-thick channel-fill complexes. The lateral extent of such channels ranges from 100 to 170 m. The other three seismic facies are characterized by moderate-to-high amplitude, sub-parallel continuous reflections. The three facies are mainly distinguished using reflection frequency and continuity. They correspond to distinct floodplain settings, i.e. proximal, intermediate and distal, with respect to the active channel belt. On a larger scale, a portion of a ca. 50-m deep incised-valley system, probably caused by a relative base-level fall, was identified. Two distinct valley-fill units are recognized. The lower unit was interpreted as comprising valley-edge slump units by comparison with outcrop data. The upper unit is interpreted as the fluvial fill deposited in the most landward portion of the valley.

Organic facies of lower to middle Jurassic sediments in the Jura Mountains, Switzerland

Abstract A section over 200 m thick spanning the Rhaetian, Liassic and lower Dogger has been cored in the central part of the Swiss Jura Mountains. A combination of sedimentological, geochemical (Rock-Eval pyrolysis), palynofacies and biostratigraphic observations leads to the interpretation of different palaeoenvironments. When compared with sequence stratigraphy and eustatic sea-level changes, these results provide a stratigraphy of organic events. Environmental conditions vary from fully aerobic in the Rhaetian to Sinemurian interval (low sea-level stand), to dysaerobic in the thin condensed Pliensbachian section and fully anoxic in the lower Toarcian Posidonia Schiefer (an excellent immature oil-prone source rock). Dysaerobic conditions are re-established in the late Toarcian, prior to the onset of a fully aerobic environment in the early Middle Jurassic (high sea-level stand).

Viruses as new agents of organomineralization in the geological record.

Viruses are the most abundant biological entities throughout marine and terrestrial ecosystems, but little is known about virus-mineral interactions or the potential for virus preservation in the geological record. Here we use contextual metagenomic data and microscopic analyses to show that viruses occur in high diversity within a modern lacustrine microbial mat, and vastly outnumber prokaryotes and other components of the microbial mat. Experimental data reveal that mineral precipitation takes place directly on free viruses and, as a result of viral infections, on cell debris resulting from cell lysis. Viruses are initially permineralized by amorphous magnesium silicates, which then alter to magnesium carbonate nanospheres of ~80-200 nm in diameter during diagenesis. Our findings open up the possibility to investigate the evolution and geological history of viruses and their role in organomineralization, as well as providing an alternative explanation for enigmatic carbonate nanospheres previously observed in the geological record.

Bedrock, Quaternary sediments and recent fault activity in central Lake Neuchâtel, as derived from high-resolution reflection seismics

A high-resolution seismic reflection survey (ca. 65 km) was conducted in the central part of Lake Neuchâtel in order to trace the continuation of the major strike-slip fault of La Lance across the lake. On seismic data, this dextral fault appears at the top Molasse/Mesozoic bedrock as a ca. 1 km wide zone affecting the top Molasse and offsetting the Mesozoic. These seismic observations can be correlated with onshore data. Four seismic facies are distinguished and interpreted from bottom to top within the Quaternary: glacial, subglacio-lacustrine, glacio-lacustrine and lacustrine. They show similarities with other perialpine lakes (Lake Geneva, Lake Annecy, Lake Le Bourget). Slumps and faults affecting the upper part of the Quaternary may be associated with the recent activity of the La Lance fault zone. Moreover, glacial erosion of the Molasse bedrock seems to have been locally guided by active tectonics.

(Plio-)Pleistocene alluvial-lacustrine basin infill evolution in a strike-slip active zone (Northern Andes, Western-Central Cordilleras, Colombia)

The (Plio)-Pleistocene Zarzal Formation was deposited in the Cauca Depression and Quindio-Risaralda Basin between the Western and Central Cordilleras (Northern Andes). This area is structurally located on the transcurrent Romeral Fault System (RFS). Because of the interaction between the Nazca plate and the Choco-Panama block (an active indenter), the RFS strike-slip component changes direction around the study zone (dextral in the south, senestral in the north). Zarzal sediments are the oldest ones not to have been affected by the Pliocene Andean tectonic phase. Their study aims at better understanding the subsidence of these two interandean sedimentary basins within a regional compressional regime. The two basins are separated by the the Serrania de Santa Barbara (SSB), made of Tertiary sediments thrusted during the Pliocene tectonic phase. Zarzal sediments are fluvio-lacustrine: diatomites are encountered on both sides of the SSB and are alternating with braided stream or alluvial deposits. Sedimentation was strongly influenced by volcanic processes which led to the deposition of the Quindio-Risaralda and Cartago volcaniclastic fans sourced from the Central Cordillera. These volcaniclastic mass flows mixed with the Zarzal sediments, and even dammed and infilled a lake in the Quindio-Risaralda Basin (east of the SSB). Numerous extensional features affect Zarzal sediments with a mean extensional trend subparallel to the SSW-NNE trending cordilleras. The syndepositional tectonic activity is demonstrated by numerous seismites. In the Cauca Depression, sediments are infilling the basin more rapidly than it subsides or than the relief lifts up, thereby drowning the topography. This might be related to the tectonically-induced, downstream damming of the Cauca River valley to the north by the Choco-Panama block.