Jean-Loup Rubino

Lower Palaeozoic unconformities in an intracratonic platform setting: glacial erosion versus tectonics in the eastern Murzuq Basin (southern Libya)

The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidié and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidié (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.

Testing the Application of Sequence Stratigraphy to Aptian Deposits in Southeast France

The Aptian section of siliciclastics interbedded with marls and limestones of the southern Subalpine Basin has been chosen as a good example to test sequence stratigraphy concepts and the synchronism between local depositional sequences and global eustatic cycles. Three sequence boundaries and three events identified as condensed sections have been correlated with the global cycle chart of Haq (1987) but within the sequences there are several problems with applying sequence stratigraphic concepts. Parasequences are present that do not have a consistent systems tract location; some of the turbidites are not characteristic of any sequence (“out-of-phase”). Future research should focus on refining the chronostratigraphy as well as expanding the data on the shelf to basin transition.

A Methodological Approach to Clastic Injectites: From Field Analysis to Seismic Modeling—Examples of the Vocontian Aptian and Albian Injectites (Southeast France)

Hardly detected with logs and recognized with difficulty on cores, clastic injectites (sills and dikes) can be troublemakers in oil-field development. Moreover, they provide a precious record of early fracturation. To predict their geometry, extension, and relationship to their feeders, field analysis of selected analog outcrops is conducted to propose some simple rules. In southeast France, the Aptian–Albian formation provides exceptional outcrops (Bevons, Rosans, and Nyons) where it is possible to characterize large sets of injectites: dikes and sills are associated in the same metric-to-kilometric network. The injection occurred per ascensum (more frequently) or per descensum, during or after the sand deposition. Specific geometric-based field methods have been developed to analyze the geometry based on the best conditions. A three-dimensional (3-D) model of the Rosans area injectite network has been built through gOcad™ tool using outcrop analysis and an original, very high-resolution two-dimensional seismic acquisition (0.6 km2; 0.23 mi2). This field analysis, the seismic survey, and the 3-D modeling provide some keys to consider possible occurrences of injectites and associated facies related to a turbiditic channel fill. We dedicate this article to the memory of our colleagues and friends G. P. Allen and D. Claude.

Interactions climat-eustatisme-tectonique. Les enseignements et perspectives du Crétacé supérieur (Cénomanien-Coniacien)

Abstract A recent detailed analysis of southeastern France upper Cretaceous series has evidenced that Cenomanian-Turonian and Turonian-Coniacian boundaries are marked by quick and large amplitude relative sea level changes. Comparisons with other sections in different worldwide basins are coincident with the observations pointed out in southeast France. We establish: (1) high-frequency synchronous events in different geodynamic settings, (2) the superposition of various hierarchically eustatic cycles i.e. the superimposition of high-frequency oscillations on a third-order trend. Two possible mechanisms responsible of these observations are emphasized and discussed: glacio-eustasy versus high-frequency tectonics.

Sedimentology and sequence stratigraphy of the Devonian to lowermost Carboniferous succession on the Gargaf Uplift (Murzuq Basin, Libya)

Publisher Summary This chapter discusses that in the Murzuq Basin of Libya, the Devonian succession diachronously overlies the Caledonian unconformity. The lower Devonian second order transgressive-regressive sequence includes the fluvial to shallow marine Tadrart Formation and the prograding shallow marine Ouan Kasa Formation is restricted to the western margins of the basin. Although, at present, poorly constrained by datings essentially based on macrofauna, these depositional sequences of the Gondwanan realm may be compared with the eustatic cycles and the relative coastal on lap curve for Euramerica. Therefore, a refined stratigraphy of the Devonian to lowermost Carboniferous succession of Libya should permit integration of these depositional sequences into a larger, worldwide framework. This facilitates regional correlation and helps in a better understanding of the Devonian geological evolution of western Libya.

Large-Scale Physiography of the Murzuq Basin Shelf during Hirnantian Ice-Sheet Final Retreat and Silurian Transgression

The Hirnantian glaciation strongly affected the lower Palaeozoic sedimentary succession of the Murzuq Basin (Libya). After final deglaciation, the remnant topography displays imprints of most of the ice-sheet retreat evolution. One large ice stream pathway, in the form of a wide erosional trough including surbordinate higher areas, has been recognized. Several ice-fronts and related proglacial fan-deltas are localized at basin scale. These architectural elements are closely associated with the position of the Ordovician reservoir relative to the Silurian “Hot Shale”.

The Cabao Fm.: A Transgressive Cretaceous Tidal Dunes Complex Along Djebel Nafusah, North-Eastern Libya

The Djebel Nafusah Cliff forms very continuous outcrop (300km Long) and can be traced from Libyan Mediterranean coast to southern Tunisia. The cliff is armed by Mid Cretaceous limestone’s which prevent underlying Mesozoic series from erosion. The Mesozoic series starts with Triassic deposits and ends in Maaestrichtian. In the central part of the djebel, a homogeneous sandy series unconformably rests over the pre-existing Mesozoic series and corresponds to the so called Cabao Formation. It is exposed in numerous quarries as well as in natural outcrops. It unconformably overlies the Shekshuk Fm. most likely of Oxfordian age. In Tunisia, additional stratigraphic units onlap the Austrian unconformity such as the Bouloula and Douiret Formations, Ouaja et al (2002); laterally equivalent units also occurs in western Libya near Wazin. The Cabao sandstones seems to grade into Chenini sandstones in southern Tunisia, however the relationship is not well constrains and ancient data tends to suggest a north westward pinchout of the lower part of this formation, Bouaziz et al. (1989). Age of the series The Cabao Fm was previously attributed to the Late Jurassic in Libya, Hammuda et al. (1985), Hallet (2002). However it was not firmly dated and the age derived from the age of underlying and overlying units; the underlying Shekshuk Formation ranges from Bathonian, Novocic (1977), El Zouki (1980) to Kimmeridgian, Burollet (1963), Hammuda et al (1985) and the overlying Kiklah Fm is usually considered as Albian. New Fossil vertebrates discovered in the Cabao Formation in the area of Nalut including the hybodont shark Priohybodus, the crocodilian Sarcosuchus, an abelisaurid, a baryonichine spinosaurid and a large sauropod with spatulate teeth indicate clearly a Lower Cretaceous age, Le Loeuff et al. (2010). However the range of the fauna is not precise enough, therefore, the Cabao Formation might be Hauterivian to Barremian in age, although an earlier Berriasian to Valanginian age cannot be excluded, but we are now confident that the Jurassic age can be rule out. This new datation also tends to confirm that the Cabao sandstones in Libya are not strictly coeval with the Cheneni sandstones of Tunisia which are well dated as Aptian to Albian, Cunny et al (2004). Marine evidences The Cabao Fm is classically interpreted as fluvial deposits and considered as a continental unit because of the reddish colour of the sandstones and the occurrence of soils. However marine evidences have been described since a long time by El Zouki (1980), including shark teeth and turtle plates most likely of marine origin. Our recent investigations including the measurements of numerous sedimentological sections located along an east-west profile, have first revealed that the series shows almost all conventional tidal diagnostic features and second that soil developed within the sandstones correspond to wheatering profile related to the emergence of tidal sandstones during the lowstand period and do not correspond to flood plain interbedded with channelized fluvial sandbodies.