However, this Mo was trapped in the sediments by incorporation into s. Thus, the sediments are enriched in Mo, but not or very slightly in U, because the latter element is not sensitive to the particulate shuttle. This interpretation is reinforced by the absence of a marked enrichment in V, which also indicates that bottom water conditions were not reducing. However, even if the water column was oxic, the fact that Mo was retained within the sediments indicates that reducing conditions developed at shallow depth below the sediment-water interface; otherwise, Mo would have escaped back into the water column together with pore waters during incipient sediment compaction.
The Mo-rich samples also show enrichments in As and Sb. Co-enrichment in Mo, As and Sb has already been observed associated with the particulate shuttle Tribovillard et al. The present study further supports the idea that a co-enrichment in Mo, As and Sb, but with no marked enrichment in U and V, is the signature of the iron shuttle process. The samples from the top part of the section 41 to 59 show no significant authigenic enrichment in trace elements, which suggests deposition in fully oxygenated seawater.
Mo—EF crossplot of samples from the Tithonian-Berriasian. The top inset diagram illustrates the general patterns of U—EF vs. Mo—EF covariation in modern marine environments. In the description of sections presented here, we use the concepts of sequence stratigraphy developed by Embry and Johannessen transgressive-regressive sequence. A depositional sequence is considered as the record of a cycle of relative sea-level fluctuation, and is made up of two superposed units: the transgressive systems tract TST and the regressive systems tract RST. The top of a deepening transgressive sequence is interpreted as a maximum flooding surface MFS , while the top of the regressive unit is considered as a sequence boundary SB.
Based on these observations and on the evolution of facies associations, two orders of depositional sequences are recognized Fig. This facies corresponds to cross-bedded sandstones attributed to the shoreface environment. Correlation is based on key markers shown on satellite images, sequence stratigraphy and facies distribution.
The TST is represented by grey silty shales that overlie the continental deposits of the Tordillo Formation. The total organic content of these shales reaches In its lower boundary, the TST can be erosive as described by Freije et al. The RST corresponds to fine-grained deposits deposited after the transgression of the early Tithonian Spalletti et al.
The RST is represented by offshore facies F. The top of the RST corresponds to shoreface sandstones F. Sequence 1: This sequence is represented at its base by the thin TST described above and is bounded at its top by the ammonite-rich horizon that corresponds to an MFS Fig. The RST is essentially composed of the offshore silty grey shales F. The top of this sequence corresponds to the occurrence of turbidites F. Sequence 2: The base of these sequence corresponds to a transition to deeper facies offshore silty grey shales interpreted here as a flooding surface. The maximum flooding surface is placed at around the level of the maximum shale content.
The overlying RST exhibits a general shallowing trend with the occurrence F. The thickness trend associated to the occurrence of slumps in the west induced by a destabilization probably related to a tectonic origin suggests a gently prograding geometry Fig. In the eastern part of the anticline logs 1 to 6 , the base of sequence 3 corresponds to the transition from slumped facies F. The general trend is marked by the increasing thickness of sandstone-rich intervals toward the top of the sequence Fig.
The MFZ is placed in the interval with the maximum shale content. In some section logs 4 and 5 , the MFZ coincides with horizons containing septarian concretions. The RST corresponds to an enrichment in sand content with the occurrence of shoreface deposits F. The RST is a mixed carbonate-siliciclastic sequence with a coarsening-upward stacking pattern.
The top of the RST is marked by pure carbonates of F. This hardground marks the base of the next major transgressive-regressive sequence exposed on the back of the anticline but which is not discussed further in this study. In the second major T-R sequence, two higher order transgressive-regressive sequences can be recognized. The correlation of the different measured sections shows a general aggrading pattern Fig.
Sequence 4: This sequence starts with the first occurrence of the green silty shales of F. The TST is recognized by a deepening of depositional environments from the shoreface F. The TST is covered vegetation, scree in three logs 3, 6 and 8. In the other logs, the top of the TST corresponding to the maximum flooding zone is defined as the interval of maximum shale content. The RST shows a gradual increase in the development of sand beds, which evolve towards the top of the sequence into thick inner ramp proximal deposits with tidal influence F.
The top of the sequence corresponds to an unconformity observed at outcrop and on high-resolution photographs Fig.
It corresponds to a basal erosional unconformity truncating the tidal deposits. Sequence 5 : This sequence comprises heterogeneous deposits and is marked by the development of lagoonal pure carbonates F. When the TST is identified, the maximum flooding zone is placed at around the maximum shale content. The RST is represented by shallow mixed siliciclastic-carbonate deposits shoal [F. The upper boundary of the TST corresponds to the ammonite-rich horizon described in the north sector. The RST is represented by F. The top of the RST is marked by upper shoreface sandstones F. Sequence 1: This sequence has an ammonite-rich horizon at its base MFS.
The RST is composed of offshore silty grey shales F. The top of this sequence corresponds to the occurrence of F. This interval is correlated between the four cross-sections in this sector and also in the northern part of the study area. It is mainly composed of silty grey shales. The maximum flooding zone is placed at around the maximum shale content. The TST is composed dominantly of silty shales of F. The MFZ is placed at around the maximum shale content. The RST corresponds to relative upward enrichment in sand content with the occurrence of shallower facies F.
In this sector, the RST is also composed of mixed carbonate-siliciclastic deposits. The top of the RST is marked by pure carbonates F. In this major T-R sequence, two higher order transgressive-regressive sequences are recognized. The correlation of the four measured sections shows a general aggrading pattern Fig.
It corresponds to an erosional unconformity truncating the tidal deposits.
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The TST is recognized by the facies deepening from shoreface deposits F. The TST is covered vegetation, scree on three logs 3, 6 and 8. In the other logs, the top of the TST that corresponds to the maximum flooding zone is defined as the interval of maximum shale content. The RST shows a gradual development of sand beds which evolve to the top of the sequence into thick proximal deposits of the inner ramp with tidal evidence F. The top of the sequence corresponds to the unconformity observed on the high-resolution photography.
Sequence 5 : The sequence is marked by the development of lagoonal carbonates F. The RST is well developed and is represented by shallow mixed siliciclastic-carbonate deposits that evolve towards lagoonal carbonates at the top Fig. The western outcrops Fig.
The RST is marked by high carbonate content. The five higher-order T-R depositional sequences allow us to determine the geometry of the depositional profile as follows Fig. Indeed, the lateral thickness variability and absence of any general trend suggest a pre-depositional topography inherited from the Tordillo Formation Fig. The first sequence is dominated by siliciclastics with shelf morphology Walker and Plint, , whereas the second sequence is composed of mixed siliciclastic-carbonate deposits with ramp morphology Burchette and Wright, The lower interval of the Vaca Muerta Fm. The sandy accumulations formed in the distal part of the shoreface by longshore currents have the potential to serve as storage zones for the supply of sediments to distal areas.
The slumps and turbidity flows could have originated from shallower domains; destabilization of material on the slope would be followed by basinward transport of sediment and the deposition of turbidites Fig. The stratigraphic position of the shoreface sand bodies F. A relationship is proposed between these facies, processes of initiation of destabilization and the sea-level fluctuations. A fall in relative sea level during deposition of the regressive system tract equivalent of the LST in Exxon's model can create favorable conditions for destabilization of the shoreface zone leading to the basinward formation of slumps and turbidites Posamentier and Martinsen, Other mechanisms could be invoked, such as a major increase in sedimentation rate.
It begins with a transgressive period leading to deeper conditions with the deposition of outer ramp green shales F. The profile is exaggerated to highlight sand storage on the lower shoreface zone. The Tithonian to Berriasian-Valanginian interval over most of the basin is interpreted as the result of a second-order eustatic cycle, combined with persistent regional subsidence regulated by cooling and thermal contraction phenomena.
The much-reduced clastic sediment supply is associated with condensed sedimentation under anaerobic-dysaerobic conditions Legarreta and Gulisano, ; Legarreta and Uliana, , ; Legarreta et al.
THE TITHONIAN FACTOR by Richard Cowper | Kirkus Reviews
Biostratigraphic data from the southern part of the basin show that the studied interval is Tithonian-Early Berriasian in age Leanza, , ; Leanza and Zeiss, , The basin fill geometry has been correlated with eustatic changes and three geometrical groups can be defined. The Tithonian-Berriasian interval studied by Mitchum and Uliana was correlated to the eustatic curve Vail et al.
The sigmoidal pattern indicates moderate aggradation and progradation, whereas the oblique pattern corresponds to low aggradation and high progradation Mitchum and Uliana, The sequences and geometric pattern defined by Mitchum and Uliana cannot be directly correlated with our sequences and geometric patterns because of the different methods used for delineating sequence boundaries seismic vs. The concordance between the geometric pattern of Mitchum and Uliana and the global coastal encroachment curve Vail et al. Tectonics can play a role in determining facies and sequence architecture.
Hallam et al. The role of the Huincul Ridge has been specified by Spalletti et al. The confinement of the deposit area during deposition of the lower part of the Vaca Muerta Formation probably favoured the development of transverse marine currents and a zone of sand storage in shallow environments.