This is a repository copy of Contrasting mixed siliciclastic-carbonate shelf-derived gravitydrive... more This is a repository copy of Contrasting mixed siliciclastic-carbonate shelf-derived gravitydriven systems in compressional intra-slope basins (southern Hikurangi margin, New Zealand).
Concepts of the interaction between autogenic (e.g., flow process) and allogenic (e.g., tectonics... more Concepts of the interaction between autogenic (e.g., flow process) and allogenic (e.g., tectonics) controls on sedimentation have advanced to a state that allows the controlling forces to be distinguished. Here we examine outcropping and subsurface Neogene deep-marine clastic systems that traversed the Hikurangi subduction margin via thrust-bounded trench-slope basins, providing an opportunity to examine the interplay of structural deformation and deep-marine sedimentation. Sedimentary logging and mapping of Miocene outcrops from the exhumed portion of the subduction wedge record heavily amalgamated, sand-rich lobe complexes, up to 200 m thick, which accumulated behind NE–SW-oriented growth structures. There was no significant deposition from low-density parts of the gravity flows in the basin center, although lateral fringes demonstrate fining and thinning indicative of deposits from low-density flows. Seismic data from the offshore portion of the margin show analogous lobate refle...
First EAGE Workshop on Unmanned Aerial Vehicles, 2019
The investigation of outcrop analogues using Unmanned Aerial Vehicle (UAV) has become increasingl... more The investigation of outcrop analogues using Unmanned Aerial Vehicle (UAV) has become increasingly popular to improve the understanding of subsurface heterogeneity. Although seismic data offer an invaluable method to characterize the stratigraphic architecture of deep-water systems and therefore help unlock their reservoir potential, the scale of observations cannot capture detailed vertical extent of the sediment distribution and the related processes responsible for the deposits. This work uses a combination of photographic data acquired from a UAV using Ground Control Points, and traditional fieldwork data to better characterize the gravity-driven systems of the Hikurangi subduction wedge (North Island, New Zealand). Their best onshore expressions are found along the coastline: a challenging working environment which evolves every day leading to multiple acquisitions (tides will affect wind variation and wave action, impacting sand coverage; sun and clouds will impact light exposure). It is therefore essential to adapt the way we acquire and model data to ensure that all the information are gathered into one single comprehensive model. Our study proposes to expand on the traditional Structure from Motion workflow to account for such settings and help create more accurate models. We will present this approach applied to three different coastal outcrops.
The study of geomagnetic excursions is key for understanding the behavior of the magnetic field o... more The study of geomagnetic excursions is key for understanding the behavior of the magnetic field of the Earth. In this paper, we present the geomagnetic record in a 2.29-m-long continuous core sampled in a flowstone in Liguria (Italy) and dated to the Lower Brunhes. The cored flowstone developed from Marine Isotopic Stage (MIS) 13 to MIS 7, according to 21 U-series dates. The mean growth rate is closely related to glacial and interglacial isotopic stages. Magnetic remanence was measured using u-channel and deconvolved. Four geomagnetic excursions were recorded at the same location, in a single flowstone, during interglacial MIS 11 and 13; Basura 1, 2, 3 and 4, at depths of 213 cm, 181, 160 and 92 cm, respectively. Due to the uncertainties of U-Th dating, the timing of the three events, namely Basura 1, 2 and 3 overlaps. The Basura 4 is well-dated to 417 + −7/8 ka and is clearly distinguishable from the others. It should therefore be considered as a possible excursion.
ABSTRACTClay minerals and organic matter occur frequently in fault zones. Their structural charac... more ABSTRACTClay minerals and organic matter occur frequently in fault zones. Their structural characteristics and their textural evolution are driven by several formation processes: (1) reaction by metasomatism from circulating fluids; (2)in situevolution by diagenesis; and (3) neoformation due to deformation catalysis. Clay-mineral chemistry and precipitated solid organic matter may be used as indicators of fluid circulation in fault zones and to determine the maximum temperatures in these zones. In the present study, clay-mineral and organic-matter analyses of two major fault zones – the Adams-Tinui and Whakataki faults, Wairarapa, North Island, New Zealand – were investigated. The two faults analysed correspond to the soles of large imbricated thrust sheets formed during the onset of subduction beneath the North Island of New Zealand. The mineralogy of both fault zones is composed mainly of quartz, feldspars, calcite, chabazite and clay minerals such as illite-muscovite, kaolinite, ...
Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science, 1999
Very high-resolution seismic data were obtained from a Kimmeridgian-Tithonian submarine series lo... more Very high-resolution seismic data were obtained from a Kimmeridgian-Tithonian submarine series located off the Roulonnais (northern France). The data were compared with high-resolution sequence stratigraphic results obtained on the same formations cropping out on the adjacent coastal cliffs. The seismic data provide better defined geometrical relationships between the sequences and the surfaces ('toplap', 'downlap') than the ones identified in the field. Based on the identification of downlap and toplap surfaces? one of the major contributions of the seismic data is to point out that the sandstone bodies isolated in offshore shales are sharp-based shoreface deposits induced by forced regression. (0 Academic des sciences / Elsevier, Paris.) sismique du Jurassique supbieur du Bouionnais ations. This geometrical information, inferred only from seisdeposits (Posamentier et al. 1992). The seismic stratigraphic mic data, allows the lowstand systems tracts to be interpreted analysis completes the field study and refines the sequence as sharp-based shoreface considered as forced regression stratigraphic analysis.
The development and evolution of slope basins on accretionary wedges are strongly related to the ... more The development and evolution of slope basins on accretionary wedges are strongly related to the activity of subduction. Yet, the relationships between the parameters of subduction and the sedimentary/tectonic records within these basins are poorly constraints. We carried out a high resolution study of slope basins across an oblique convergent margin in order to precise these relationships. The North Island
Page 1. Chapter 18 18 Offshore Frontal Part of the Makran Accretionary Prism: The Chamak Survey (... more Page 1. Chapter 18 18 Offshore Frontal Part of the Makran Accretionary Prism: The Chamak Survey (Pakistan) Ellouz-Zimmermann N. · Lallemant SJ · Castilla R. · Mouchot N. · Leturmy P. · Battani A. · Buret C. · Cherel L. · Desaubliaux ...
In Late Pleistocene times, during Oxygen Isotopic Stage (OIS) 5, aeolian calcareous sand dunes we... more In Late Pleistocene times, during Oxygen Isotopic Stage (OIS) 5, aeolian calcareous sand dunes were deposited along the northern coast of Crete, where they now discontinuously crop out wedged between thin shallow marine Tyrrhenian deposits. Late Pleistocene ...
A geophysical and geological survey (CHAMAK) has been carried out on the Makran accretionary wedg... more A geophysical and geological survey (CHAMAK) has been carried out on the Makran accretionary wedge off Pakistan in order to understand the structure of the margin and the recent sedimentary processes in this selfmaintaining prism disconnected from the modern Indus inputs (Qayyum et al., 1997; Gaedicke et al., 2002a; Schluter et al., 2002). Morphostructural analysis, based on the interpretation of bathymetric data and backscatter imagery, as well as a 3.5 kHz echo-character mapping, allow us to distinguish three structural domains, from north to south, where sedimentary processes differ: (1) the accretionary wedge to the north, (2) the trench and (3) the northern Murray Ridge at the seaward edge of the trench. The accretionary wedge is cut by canyons responsible for an important erosion of the prism especially in the eastern part of the wedge. Within the trench, sediments transported by the canyons generate sediment waves and are transported westward, parallel to the E-W axis of the trench. The eastern part of the abyssal plain is eroded by strong turbidity currents whereas important sediment deposition occurs in the western part of the abyssal plain, as a consequence of a decrease in the current energy. Nearly no mass transport deposits are recognized in the study area except near the ridges forming the accretionary wedge. Small-scale slope failure scars are described. The prevalence of turbiditic processes and the existence of a morphological barrier formed by the Murray Ridge allow the confinement of turbidites within the trench. Migrating sediment waves seem to be common sedimentary structures in this setting. These features might be produced by important velocity decrease of turbidity currents when reaching the trench.
This study is an attempt to interpret very high resolution seismic data, usually devoted to shall... more This study is an attempt to interpret very high resolution seismic data, usually devoted to shallow marine surveys, in terms of very high resolution seismic and sequence stratigraphy at the scale of the reservoir. Seismic data are compared with outcrop-based observations in order to discuss the geometrical relationships between rock bodies, and the nature of the seismic reflections in terms of seismic facies related to type lithologies. Two high-resolution seismic surveys, using a sparker, were performed in the Dover Strait area some 100s of metres away from the Boulonnais coastal cliffs. The aim was to look to the 130 m-thick, mixed siliciclastic-carbonate deposits of Upper Jurassic age (Kimmeridgian-Tithonian) cropping out at the sea floor and to compare them to the coastal cliffs exposures. The 100 m apart seismic profiles exhibit a 100 m deep penetration and 1 m in vertical resolution (upper 30 ms part of the profiles). Four seismic facies are recognised on the seismic profiles. They comprise ten seismic units which can be grouped into three broad families. Group 1 has a parallel/aggradational reflection configuration and conformable boundaries which may correspond to the transgressive and highstand systems tracts. The seismic facies exhibit either a high continuity-high amplitude (corresponding to clayey-mudstone lithologies of the mid ramp environment) or a high continuity-low amplitude (corresponding to shaly lithologies of the outer ramp environment). Group 2 has a sigmoid-progradational reflection configuration, bounded at the base by a downlap surface and at the top by a toplap surface. These boundaries correspond respectively in the field to a marine regressive surface of erosion and a marine transgressive surface of erosion. The seismic facies is cross-layered and exhibits a moderate continuity, low to moderate amplitude and moderate frequency. Group 2 corresponds to the bioclastic, sand-prone facies of the inner ramp environment. It is interpreted as lowstand sharp-based shorefaces. Group 3 exhibits a progradational, complex to chaotic configuration. It lies on an erosional unconformity with paleogullies. The seismic facies shows discontinuous reflections with moderate amplitude and high to moderate frequency. It corresponds in the field to cross-bedded sandstones in channel fill arrangements of fluvial origin (Wealdian deposits) marking the top of the section. The different systems tracts, sequence boundaries and depositional sequences identified onshore can be recognised as well on the seismic profiles, but the comparison demonstrates that seismic data complement the onshore sequence stratigraphic analysis by providing the geometrical relationships between systems tracts (onlap, downlap, toplap .) that cannot be observed onshore. The maximum flooding surfaces however present no specific seismic signature on such a low angle ramp profile. It can only be identified precisely in the field, which also help to relate lithologies to seismic facies. This study represents one of the first attempts to interpret very high resolution seismic data at the outcrop scale. It will be improved again in the future by the collection of impedance logs and synthetic seismic traces from cored sections on the coastal cliffs for comparison with the seismic data.
Sharp-based shoreface sandstones are of considerable interest because of their potential as hydro... more Sharp-based shoreface sandstones are of considerable interest because of their potential as hydrocarbon reservoirs and because they play an important role in the stratigraphic analysis of basin fills. The sharp-based shoreface sandstones studied herein are Upper Jurassic (Kimmeridgian-Tithonian) and exposed along the coastal cliffs of the Dover Strait in northwestern France. These series consist of tens of meter-thick alternations of sandstones bodies and organic-rich shales that can be correlated for over 30 km along coastal cliff exposures and tied to high-resolution (ca. 1 m of sediment) marine-seismic profiles obtained several hundred meters offshore. The units described here comprise two sharp-based sandstone bodies. Each is composed of a basal progradational set of shoreface parasequences overlain by a progradational-aggradational shoreface succession. Each sharp-based sand body lies on a marine regressive surface of erosion and is truncated by a marine transgressive surface of erosion, which in turn is overlain by a thin retrogradational ravinement lag or coarse-grained, planar-laminated bedset. The two progradational packages are separated by a third surface, a subaerial exposure surface that is interpreted as a sequence boundary. Two distinct types of seismic units, referred to as type A and type B, have been identified in the study area. Seismic unit A has conformable upper and lower boundaries and parallel (aggradational) configurations; seismic unit B is characterized by downlap and toplap boundaries and simple or compound, sigmoid and oblique-tangential (progradational) configurations. A single sharp-based shoreface sandstone body makes up the type B unit and typically consists of two compound superimposed progradational sets (B1, B2). The lowermost set, B1, corresponds in the field to the progradational set sensu stricto, whereas B2 corresponds to a progradational-aggradational set. B1 and B2 are separated in outcrop by a sequence boundary. These observations led us to reevaluate the sequence stratigraphic interpretation of sharp-based shoreface sandstones. It is proposed here that complete, single, sharp-based shoreface sandstones bodies can be separated into two different systems tracts: (1) a progradational set (B1 seismic body) at the base, which corresponds to the forced regressive wedge systems tracts (FRWST) of Hunt and Tucker (1992), and (2) a progradational-aggradational set (B2) at the top, above the sequence boundary, which corresponds to the lowstand systems tract (LST) of Posamentier et al. (1992). A complete sharp-based shoreface sandstone body is bounded below the FRWST by a regressive surface of marine erosion caused by the downward shift of wave base, and by a transgressive surface of marine erosion, or ravinement surface, at the top of the LST.
ABSTRACT This paper presents a lithologic and stratigraphic description of the Neoproterozoic (an... more ABSTRACT This paper presents a lithologic and stratigraphic description of the Neoproterozoic (ante- or syn- Pan-African orogeny) Mintom Formation (new) of southeastern Cameroon, and provides a new facies and geochemical analysis of the sedimentary succession, formerly referred to as the upper Dja series. The Mintom Formation can be subdivided from base to top into four members that record a general increase in carbonate content. The members (all new) from lower to upper are: Kol Member (diamictite and pelite), Metou Member (dolostone), Momibolé Member (calcareous pelite), and Atog Adjap Member (limestone). Although the lithostratigraphic architecture looks very similar to that of well-documented syn- and post-glacial Neoproterozoic deposits, physical evidence of glacial influence is absent.By contrast with other Central African Neoproterozoic carbonates deposited in ramp settings, the succession does not contain open marine facies. Limestones consist of monotonous subhedral microsparitic calcite mosaics and display occasional microbial laminae. These observations force reevaluation of both previous paleoenvironmental interpretations of the deposits and their comparison with neighboring Ediacaran carbonates.We assume that the graded basal succession from diamictite to laminated pelitic facies is compatible with emplacement of mass flow deposits in toe-of-slope setting during regional uplift. Interpretation of the overlying Métou dolostone is uncertain though sedimentological and geochemical properties point to a likely quiet depositional setting. The upper part of the Formation, including the Momibolé and Atog Adjap Members, is conspicuously laminated, in places rhythmically and ripple-bedded, suggesting shallow subaqueous and calm depositional conditions only interrupted by occasional slumps indicative of a locally steepened bottom topography. Evaporitic fabrics and fenestral pores further indicate shallow water, possibly peritidal, environmental conditions. In spite of indications of shale and post-depositional contamination, rare earth elements (REE) plus yttrium (Y) patterns obtained from carbonate samples point to a non-marine origin for the Atog Adjap limestone, but instead deposition in lacustrine or lagoonal settings under freshwater influence.This interpretation suggests that the Mintom Formation formed in a small-scale palaeodepression, isolated from the open marine environment, where confined lagoonal or lacustrine sedimentation developed.The final Neoproterozoic evolution of the Mintom Formation was dominated by erosional features, including striations and stair-cased groove structures reported for the first time here, and revealing the passage of glaciers of likely Ediacaran age.
This is a repository copy of Contrasting mixed siliciclastic-carbonate shelf-derived gravitydrive... more This is a repository copy of Contrasting mixed siliciclastic-carbonate shelf-derived gravitydriven systems in compressional intra-slope basins (southern Hikurangi margin, New Zealand).
Concepts of the interaction between autogenic (e.g., flow process) and allogenic (e.g., tectonics... more Concepts of the interaction between autogenic (e.g., flow process) and allogenic (e.g., tectonics) controls on sedimentation have advanced to a state that allows the controlling forces to be distinguished. Here we examine outcropping and subsurface Neogene deep-marine clastic systems that traversed the Hikurangi subduction margin via thrust-bounded trench-slope basins, providing an opportunity to examine the interplay of structural deformation and deep-marine sedimentation. Sedimentary logging and mapping of Miocene outcrops from the exhumed portion of the subduction wedge record heavily amalgamated, sand-rich lobe complexes, up to 200 m thick, which accumulated behind NE–SW-oriented growth structures. There was no significant deposition from low-density parts of the gravity flows in the basin center, although lateral fringes demonstrate fining and thinning indicative of deposits from low-density flows. Seismic data from the offshore portion of the margin show analogous lobate refle...
First EAGE Workshop on Unmanned Aerial Vehicles, 2019
The investigation of outcrop analogues using Unmanned Aerial Vehicle (UAV) has become increasingl... more The investigation of outcrop analogues using Unmanned Aerial Vehicle (UAV) has become increasingly popular to improve the understanding of subsurface heterogeneity. Although seismic data offer an invaluable method to characterize the stratigraphic architecture of deep-water systems and therefore help unlock their reservoir potential, the scale of observations cannot capture detailed vertical extent of the sediment distribution and the related processes responsible for the deposits. This work uses a combination of photographic data acquired from a UAV using Ground Control Points, and traditional fieldwork data to better characterize the gravity-driven systems of the Hikurangi subduction wedge (North Island, New Zealand). Their best onshore expressions are found along the coastline: a challenging working environment which evolves every day leading to multiple acquisitions (tides will affect wind variation and wave action, impacting sand coverage; sun and clouds will impact light exposure). It is therefore essential to adapt the way we acquire and model data to ensure that all the information are gathered into one single comprehensive model. Our study proposes to expand on the traditional Structure from Motion workflow to account for such settings and help create more accurate models. We will present this approach applied to three different coastal outcrops.
The study of geomagnetic excursions is key for understanding the behavior of the magnetic field o... more The study of geomagnetic excursions is key for understanding the behavior of the magnetic field of the Earth. In this paper, we present the geomagnetic record in a 2.29-m-long continuous core sampled in a flowstone in Liguria (Italy) and dated to the Lower Brunhes. The cored flowstone developed from Marine Isotopic Stage (MIS) 13 to MIS 7, according to 21 U-series dates. The mean growth rate is closely related to glacial and interglacial isotopic stages. Magnetic remanence was measured using u-channel and deconvolved. Four geomagnetic excursions were recorded at the same location, in a single flowstone, during interglacial MIS 11 and 13; Basura 1, 2, 3 and 4, at depths of 213 cm, 181, 160 and 92 cm, respectively. Due to the uncertainties of U-Th dating, the timing of the three events, namely Basura 1, 2 and 3 overlaps. The Basura 4 is well-dated to 417 + −7/8 ka and is clearly distinguishable from the others. It should therefore be considered as a possible excursion.
ABSTRACTClay minerals and organic matter occur frequently in fault zones. Their structural charac... more ABSTRACTClay minerals and organic matter occur frequently in fault zones. Their structural characteristics and their textural evolution are driven by several formation processes: (1) reaction by metasomatism from circulating fluids; (2)in situevolution by diagenesis; and (3) neoformation due to deformation catalysis. Clay-mineral chemistry and precipitated solid organic matter may be used as indicators of fluid circulation in fault zones and to determine the maximum temperatures in these zones. In the present study, clay-mineral and organic-matter analyses of two major fault zones – the Adams-Tinui and Whakataki faults, Wairarapa, North Island, New Zealand – were investigated. The two faults analysed correspond to the soles of large imbricated thrust sheets formed during the onset of subduction beneath the North Island of New Zealand. The mineralogy of both fault zones is composed mainly of quartz, feldspars, calcite, chabazite and clay minerals such as illite-muscovite, kaolinite, ...
Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science, 1999
Very high-resolution seismic data were obtained from a Kimmeridgian-Tithonian submarine series lo... more Very high-resolution seismic data were obtained from a Kimmeridgian-Tithonian submarine series located off the Roulonnais (northern France). The data were compared with high-resolution sequence stratigraphic results obtained on the same formations cropping out on the adjacent coastal cliffs. The seismic data provide better defined geometrical relationships between the sequences and the surfaces ('toplap', 'downlap') than the ones identified in the field. Based on the identification of downlap and toplap surfaces? one of the major contributions of the seismic data is to point out that the sandstone bodies isolated in offshore shales are sharp-based shoreface deposits induced by forced regression. (0 Academic des sciences / Elsevier, Paris.) sismique du Jurassique supbieur du Bouionnais ations. This geometrical information, inferred only from seisdeposits (Posamentier et al. 1992). The seismic stratigraphic mic data, allows the lowstand systems tracts to be interpreted analysis completes the field study and refines the sequence as sharp-based shoreface considered as forced regression stratigraphic analysis.
The development and evolution of slope basins on accretionary wedges are strongly related to the ... more The development and evolution of slope basins on accretionary wedges are strongly related to the activity of subduction. Yet, the relationships between the parameters of subduction and the sedimentary/tectonic records within these basins are poorly constraints. We carried out a high resolution study of slope basins across an oblique convergent margin in order to precise these relationships. The North Island
Page 1. Chapter 18 18 Offshore Frontal Part of the Makran Accretionary Prism: The Chamak Survey (... more Page 1. Chapter 18 18 Offshore Frontal Part of the Makran Accretionary Prism: The Chamak Survey (Pakistan) Ellouz-Zimmermann N. · Lallemant SJ · Castilla R. · Mouchot N. · Leturmy P. · Battani A. · Buret C. · Cherel L. · Desaubliaux ...
In Late Pleistocene times, during Oxygen Isotopic Stage (OIS) 5, aeolian calcareous sand dunes we... more In Late Pleistocene times, during Oxygen Isotopic Stage (OIS) 5, aeolian calcareous sand dunes were deposited along the northern coast of Crete, where they now discontinuously crop out wedged between thin shallow marine Tyrrhenian deposits. Late Pleistocene ...
A geophysical and geological survey (CHAMAK) has been carried out on the Makran accretionary wedg... more A geophysical and geological survey (CHAMAK) has been carried out on the Makran accretionary wedge off Pakistan in order to understand the structure of the margin and the recent sedimentary processes in this selfmaintaining prism disconnected from the modern Indus inputs (Qayyum et al., 1997; Gaedicke et al., 2002a; Schluter et al., 2002). Morphostructural analysis, based on the interpretation of bathymetric data and backscatter imagery, as well as a 3.5 kHz echo-character mapping, allow us to distinguish three structural domains, from north to south, where sedimentary processes differ: (1) the accretionary wedge to the north, (2) the trench and (3) the northern Murray Ridge at the seaward edge of the trench. The accretionary wedge is cut by canyons responsible for an important erosion of the prism especially in the eastern part of the wedge. Within the trench, sediments transported by the canyons generate sediment waves and are transported westward, parallel to the E-W axis of the trench. The eastern part of the abyssal plain is eroded by strong turbidity currents whereas important sediment deposition occurs in the western part of the abyssal plain, as a consequence of a decrease in the current energy. Nearly no mass transport deposits are recognized in the study area except near the ridges forming the accretionary wedge. Small-scale slope failure scars are described. The prevalence of turbiditic processes and the existence of a morphological barrier formed by the Murray Ridge allow the confinement of turbidites within the trench. Migrating sediment waves seem to be common sedimentary structures in this setting. These features might be produced by important velocity decrease of turbidity currents when reaching the trench.
This study is an attempt to interpret very high resolution seismic data, usually devoted to shall... more This study is an attempt to interpret very high resolution seismic data, usually devoted to shallow marine surveys, in terms of very high resolution seismic and sequence stratigraphy at the scale of the reservoir. Seismic data are compared with outcrop-based observations in order to discuss the geometrical relationships between rock bodies, and the nature of the seismic reflections in terms of seismic facies related to type lithologies. Two high-resolution seismic surveys, using a sparker, were performed in the Dover Strait area some 100s of metres away from the Boulonnais coastal cliffs. The aim was to look to the 130 m-thick, mixed siliciclastic-carbonate deposits of Upper Jurassic age (Kimmeridgian-Tithonian) cropping out at the sea floor and to compare them to the coastal cliffs exposures. The 100 m apart seismic profiles exhibit a 100 m deep penetration and 1 m in vertical resolution (upper 30 ms part of the profiles). Four seismic facies are recognised on the seismic profiles. They comprise ten seismic units which can be grouped into three broad families. Group 1 has a parallel/aggradational reflection configuration and conformable boundaries which may correspond to the transgressive and highstand systems tracts. The seismic facies exhibit either a high continuity-high amplitude (corresponding to clayey-mudstone lithologies of the mid ramp environment) or a high continuity-low amplitude (corresponding to shaly lithologies of the outer ramp environment). Group 2 has a sigmoid-progradational reflection configuration, bounded at the base by a downlap surface and at the top by a toplap surface. These boundaries correspond respectively in the field to a marine regressive surface of erosion and a marine transgressive surface of erosion. The seismic facies is cross-layered and exhibits a moderate continuity, low to moderate amplitude and moderate frequency. Group 2 corresponds to the bioclastic, sand-prone facies of the inner ramp environment. It is interpreted as lowstand sharp-based shorefaces. Group 3 exhibits a progradational, complex to chaotic configuration. It lies on an erosional unconformity with paleogullies. The seismic facies shows discontinuous reflections with moderate amplitude and high to moderate frequency. It corresponds in the field to cross-bedded sandstones in channel fill arrangements of fluvial origin (Wealdian deposits) marking the top of the section. The different systems tracts, sequence boundaries and depositional sequences identified onshore can be recognised as well on the seismic profiles, but the comparison demonstrates that seismic data complement the onshore sequence stratigraphic analysis by providing the geometrical relationships between systems tracts (onlap, downlap, toplap .) that cannot be observed onshore. The maximum flooding surfaces however present no specific seismic signature on such a low angle ramp profile. It can only be identified precisely in the field, which also help to relate lithologies to seismic facies. This study represents one of the first attempts to interpret very high resolution seismic data at the outcrop scale. It will be improved again in the future by the collection of impedance logs and synthetic seismic traces from cored sections on the coastal cliffs for comparison with the seismic data.
Sharp-based shoreface sandstones are of considerable interest because of their potential as hydro... more Sharp-based shoreface sandstones are of considerable interest because of their potential as hydrocarbon reservoirs and because they play an important role in the stratigraphic analysis of basin fills. The sharp-based shoreface sandstones studied herein are Upper Jurassic (Kimmeridgian-Tithonian) and exposed along the coastal cliffs of the Dover Strait in northwestern France. These series consist of tens of meter-thick alternations of sandstones bodies and organic-rich shales that can be correlated for over 30 km along coastal cliff exposures and tied to high-resolution (ca. 1 m of sediment) marine-seismic profiles obtained several hundred meters offshore. The units described here comprise two sharp-based sandstone bodies. Each is composed of a basal progradational set of shoreface parasequences overlain by a progradational-aggradational shoreface succession. Each sharp-based sand body lies on a marine regressive surface of erosion and is truncated by a marine transgressive surface of erosion, which in turn is overlain by a thin retrogradational ravinement lag or coarse-grained, planar-laminated bedset. The two progradational packages are separated by a third surface, a subaerial exposure surface that is interpreted as a sequence boundary. Two distinct types of seismic units, referred to as type A and type B, have been identified in the study area. Seismic unit A has conformable upper and lower boundaries and parallel (aggradational) configurations; seismic unit B is characterized by downlap and toplap boundaries and simple or compound, sigmoid and oblique-tangential (progradational) configurations. A single sharp-based shoreface sandstone body makes up the type B unit and typically consists of two compound superimposed progradational sets (B1, B2). The lowermost set, B1, corresponds in the field to the progradational set sensu stricto, whereas B2 corresponds to a progradational-aggradational set. B1 and B2 are separated in outcrop by a sequence boundary. These observations led us to reevaluate the sequence stratigraphic interpretation of sharp-based shoreface sandstones. It is proposed here that complete, single, sharp-based shoreface sandstones bodies can be separated into two different systems tracts: (1) a progradational set (B1 seismic body) at the base, which corresponds to the forced regressive wedge systems tracts (FRWST) of Hunt and Tucker (1992), and (2) a progradational-aggradational set (B2) at the top, above the sequence boundary, which corresponds to the lowstand systems tract (LST) of Posamentier et al. (1992). A complete sharp-based shoreface sandstone body is bounded below the FRWST by a regressive surface of marine erosion caused by the downward shift of wave base, and by a transgressive surface of marine erosion, or ravinement surface, at the top of the LST.
ABSTRACT This paper presents a lithologic and stratigraphic description of the Neoproterozoic (an... more ABSTRACT This paper presents a lithologic and stratigraphic description of the Neoproterozoic (ante- or syn- Pan-African orogeny) Mintom Formation (new) of southeastern Cameroon, and provides a new facies and geochemical analysis of the sedimentary succession, formerly referred to as the upper Dja series. The Mintom Formation can be subdivided from base to top into four members that record a general increase in carbonate content. The members (all new) from lower to upper are: Kol Member (diamictite and pelite), Metou Member (dolostone), Momibolé Member (calcareous pelite), and Atog Adjap Member (limestone). Although the lithostratigraphic architecture looks very similar to that of well-documented syn- and post-glacial Neoproterozoic deposits, physical evidence of glacial influence is absent.By contrast with other Central African Neoproterozoic carbonates deposited in ramp settings, the succession does not contain open marine facies. Limestones consist of monotonous subhedral microsparitic calcite mosaics and display occasional microbial laminae. These observations force reevaluation of both previous paleoenvironmental interpretations of the deposits and their comparison with neighboring Ediacaran carbonates.We assume that the graded basal succession from diamictite to laminated pelitic facies is compatible with emplacement of mass flow deposits in toe-of-slope setting during regional uplift. Interpretation of the overlying Métou dolostone is uncertain though sedimentological and geochemical properties point to a likely quiet depositional setting. The upper part of the Formation, including the Momibolé and Atog Adjap Members, is conspicuously laminated, in places rhythmically and ripple-bedded, suggesting shallow subaqueous and calm depositional conditions only interrupted by occasional slumps indicative of a locally steepened bottom topography. Evaporitic fabrics and fenestral pores further indicate shallow water, possibly peritidal, environmental conditions. In spite of indications of shale and post-depositional contamination, rare earth elements (REE) plus yttrium (Y) patterns obtained from carbonate samples point to a non-marine origin for the Atog Adjap limestone, but instead deposition in lacustrine or lagoonal settings under freshwater influence.This interpretation suggests that the Mintom Formation formed in a small-scale palaeodepression, isolated from the open marine environment, where confined lagoonal or lacustrine sedimentation developed.The final Neoproterozoic evolution of the Mintom Formation was dominated by erosional features, including striations and stair-cased groove structures reported for the first time here, and revealing the passage of glaciers of likely Ediacaran age.
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Papers by G. Mahieux