Early Miocene Succession Research Articles

Depositional architecture of the tide-dominated late Oligocene hydrocarbon reservoirs in the Tapti-Daman block, Mumbai Offshore basin, India

The intricate interaction between tidal and river currents across a gently sloping shelf area complicates the geological interpretation of sedimentary deposits in subsurface sections. This study comprehensively analyzes the depositional framework and facies architecture in the late Oligocene siliciclastic succession of the Daman Formation, Mumbai Offshore Basin, which shows subtle changes in depositional stacking pattern in response to shoreline shifts. The late Oligocene siliciclastic succession being a major hydrocarbon producer in the Mumbai Offshore Basin; this research also aims to enhance the optimization of reserve exploitation and prospectivity evaluation. A detailed analysis was conducted using well-logs and seismic data integrated with available geological data. The Oligo-Miocene sequence demonstrates a coarsening-upward facies trend until the late Oligocene, followed by a fining-upward facies trend during the early Miocene and characterized by numerous stacked sandstone layers. The facies distribution pattern, sedimentary characteristics, and progradational geometry suggest a deltaic system dominated by tides for the late Oligocene succession and the latter part of the early Oligocene succession. The early Miocene succession shows a reversal in depositional trend with a retrogradation pattern. The late Oligocene deltaic deposits show a very high sediment load deposited during the peak river discharge from the Narmada-Tapti River system. The gentle depositional slope allowed redistribution of sediments by tidal currents. The deltaic system was further resolved into subaqueous mouth bar, delta front, and prodelta facies associations based on facies modeling. The correlation of the facies model with the regional depositional setup indicates that the late Oligocene succession corresponds to a third-order highstand system tract (HST) cycle.

Implication of middle Eocene to early Miocene ostracodes from the N. El Faras-1X Well, Qattara Depression, Egypt, for paleobathymetry and paleobiogeographic reconstruction

The subsurface middle Eocene - early Miocene succession in the N. El Faras - 1X Well, drilled in the Qattara Depression at the northern part of the Western Desert, Egypt (Southern Tethyan Province, STP), was studied for its ostracode content to infer changes in paleobathymetry as well as paleobiogeographic reconstructions. This succession is composed mainly of three lithostratigraphic units, as the upper part of the Apollonia Formation (middle to late Eocene), the Dabaa Formation (late Eocene and Oligocene) and the lower part of the Moghra Formation (early Miocene). The stratigraphical distribution, paleoenvironmental and paleobiogeographical significance of the ostracode faunas from the Oligocene interval of the study area are documented for the first time. Fifty seven ostracode species belonging to 36 genera were identified in this succession. Among them, two new species were described. The vertical distribution of these ostracode species enabled the recognition of seven local biozones. Qualitative and quantitative statistical analysis of ostracode associations revealed a depositional depth ranging from the inner to outer neritic setting. Considering the CONISS cluster analysis, six ecozones were established. During the middle and late Eocene, the paleobiogeographical affinities of the Egyptian ostracodes from N. El Faras-1X× Well succession with that of the Southern Tethyan countries suggest a direct connection between Egypt and the countries of the Southern Tethyan Province. This connection facilitated the dispersal and exchange within this province. The low ostracode similarity between the STP and the Northern Tethyan Province (NTP) suggests a slight connection and exchange between them. The in common occurrence of these ostracodes with that of Somalia is attributed to the connection between the STP and Somalia via the Tethyan Seaway, which facilitated the dispersal and exchange of the middle Eocene ostracodes. The similarity between the Egyptian ostracodes (STP) and that of South America and the Caribbean Sub-Province indicates a direct connection between those provinces across the Gibraltar Strait and through the North Equatorial Oceanic Water Current during that time. During the Oligocene, the low ostracode similarity between the STP and the NTP may be attributed to a more or less restricted connection. Also, the low similarity indicates a continuation of the mild connection between the STP and the South America and Caribbean Sub-Province across the Gibraltar Strait and through the North Equatorial Current. During the early Miocene, the similarity of the recorded ostracodes of the STP with that of the NTP suggests a continuous connection between the two provinces. Also, there was a connection between the South America and Caribbean Sub-Province and the North American Province through the North Equatorial Current and the warm Gulf Stream within the Caribbean Sea.

Middle Eocene–early Miocene planktonic foraminiferal biostratigraphy, chronostratigraphy, sea‐level reconstruction and sequence biostratigraphy at N. ElFaras‐1Xwell, Qattara Depression, Western Desert, Egypt

The planktonic foraminiferal assemblages are tracked in the middle Eocene–early Miocene succession within N. El Faras‐1X Well, Qattara Depression, Egypt. This succession is composed mainly of three lithostratigraphic units the upper part of the Apollonia Formation (middle to late Eocene), Dabaa Formation (late Eocene and Oligocene) and the lower part of the Moghra Formation (early Miocene). Biostratigraphically, the recorded 77 planktonic foraminiferal species enabled the recognition of 12 planktonic foraminiferal biozones; 4 biozones belong to middle to late Eocene, 7 biozones belong to the Oligocene and 1 belongs to the early Miocene. The proposed biozones were compared with their equivalents in other Egyptian regions and other schemes. Eleven Lowest Occurrence and two Highest Occurrence planktonic foraminiferal bioevents were also recognized and correlated all over the world. The chronostratigraphy and the stage boundaries were mainly delineated according to the diagnostic planktonic foraminifera. The middle–late Eocene boundary (Bartonian–Priabonian boundary), The Eocene–Oligocene boundary (Priabonian–Rupelian boundary), the early Oligocene–late Oligocene boundary (Rupelian–Chattian boundary) and the Oligocene–Miocene boundary (Chattian–Aquitanian boundary) were accurately determined. Based on the % P, the palaeobathymetry was estimated and hence the sea‐level fluctuation. The reconstructed sea‐level curve revealed two major transgressive–regressive cycles with minor oscillation within them. These events were globally correlated and match in most parts with that of the worldwide sea‐level curves. The sequence stratigraphy framework consists mainly of two transgressive–regressive (T–R) depositional sequences (DS1 and DS2). The DS1 includes the TST1 below which is capped by mfs1 and the HST1 above. The DS2 involves the TST2 below that was capped by mfs2 and the HST2 above.

Litho- and biostratigraphy of a late Oligocene–Early Miocene succession in the Weber area, southern Hawke’s Bay, and implications for early Hikurangi subduction-margin evolution

ABSTRACT A deep-marine sedimentary succession of Whaingaroan to Altonian age (Early Oligocene–Early Miocene), cropping out near Weber, southern Hawke’s Bay, records abrupt changes in depositional paleoenvironments and sedimentary lithofacies. Highly calcareous early Waitakian (latest Oligocene) Weber Formation (Mangatu Group/Waka Supergroup) is unconformably overlain by terrigenous-dominated late Waitakian–Otaian (Early Miocene) Coast Road or Whakataki formations (Tolaga Group/Māui Supergroup). Most notably, in some localities, lowermost parts of the Coast Road Formation (Mangapuku Mudstone Member) contain prominent matrix-supported extra-formational olistoliths of centimetre to decametre scale derived from the underlying Weber Formation. Deposition of these blocks is attributed to deep-water collapse of parts of rapidly growing, over-steepened, reverse-fault-controlled thrust ridge(s). The onset of Coast Road and Whakataki formation deposition and associated abrupt changes in sedimentary lithofacies correlate to prominent Early Miocene lithological, mineralogical, and paleoenvironmental changes elsewhere along the Hikurangi Margin. We attribute these bio- and lithostratigraphic changes as evidence for significant shortening and upper-plate reverse faulting marking the onset of subduction beneath eastern North Island during the mid-Waitakian (c. 23 Ma). Proposed lithostratigraphic revisions presented herein help clarify depositional events and correlative sedimentary packages within the Miocene Hikurangi margin.

Microfacies Analysis and Diagenetic Assessment of the Late Oligocene- Early Miocene Succession in Khabaz Oilfield, Northern Iraq

The Jeribe and Azkand formations, which were deposited during Late Oligocene-Early Miocene are a major reservoir interval in the Khabaz oilfield where it is, for the most part, a carbonate reservoir. The study area is located in the north of Iraq within the Foothill Zone. Through the thin-section examination for Azkand/Anah carbonates in the Khabaz oil field (Kz-3, Kz-4, Kz-10, and Kz-43), the petrographic and microfacies have been studied, which show six major carbonate microfacies, each of them is representing a distinctive deposition environment, and these are: restricted and semi-restricted for the Jeribe Formation, and semi-restricted, back reef, reef, fore reef, shallow open marine and slope for Azkand and Anah formations. There are several types of diagenetic processes; some of these processes enhance the porosity and the other reduces it. Porosity reduction by: Cementation (Isopachous, meniscus, syntaxial calcite, Drusy calcite, botryoidal, blocky calcite), Recrystallization; Compaction and late stage of dolomitization. This appeared near the upper and lower contact of the Jeribe Formation and in the lower part of the Azkand Formation. While the porosity enhancement which represented by depositional process (primary porosity) and dissolution (secondary porosity), in addition to the early and middle stage of dolomitization. This appeared in the middle part of the Jeribe Formation and the upper part of Azkand.

Oligocene Palynological Zonation Scheme From East Java Sea

Systematic biostratigraphic analyses have been undertaken on the Oligocene clastic and carbonate Kujung Formation from the East Java Sea, North of Madura. The succession has been examined mainly using cutting samples in two wells, using a combination of foraminiferal, nannofossil and palynological analyses at regular spacing. Nannofossil analysis indicates that the Late Oligocene to basal Early Miocene succession is more or less complete, with zones NP24, NP25 and NN1 are all being well developed. In addition, the Early Oligocene is indentified by larger foraminifera indicating the Tc/ Td Letter Stage. Because the traditional palynological zonation of Morley (1978) does not work well in this area, the succession has been divided into broad assemblage zones, which appear to be controlled mainly by climate. These zones are OL-1, OL-2, OL-3, OL-4 and OL-5. Zone OL-1 is based essentially on the absence of seasonal climate and riparian elements, whilst zone OL-2 is characterized by the regular occurrence of seasonal climate elements, especially of Malvacipollis diversus. Zone OL-3 is indicated by common to abundant Dacrydium and Casuarina pollen, with a strong acme of dinoflagellate cysts dominated by Operculodinium spp. and Spiniferites spp., whereas zone OL-4 is marked by abundant Dacrydium and regular Casuarina pollen, but low representation of riparian elements. Finally, the youngest zone is OL-5 which is characterized by reduced Dacrydium and Casuarina pollen, and increased riparian elements. The above palynological zonation suggests climate change, which closely parallels the climate succession from West Java Sea (but with changes less pronounced). Therefore, this study provides a well dated Oligocene palynological zonation which can be applied across Java.

Foraminiferal patterns and palaeoenvironmental inferences from DEL-1 well, offshore Niger Delta

The Miocene Epoch (23.3–5.3 Ma) represents a remarkable time in the Earth's geological history with significant palaeoclimatic and palaeoenvironmental relevance. In this study, microfossils recovered from the DEL-1 (4600–9460 ft) well in the western Detachment Fold Zone, offshore Niger Delta in the Gulf of Guinea were used to construe the age, palaeoenvironment, and palaeoecology of the penetrated successions. Standard foraminiferal analyses were carried out on one hundred and forty-eight (148) ditch cuttings retrieved from the studied interval following standard laboratory and identification procedures. Globigerinatella spp./Catapsydrax dissimilis Concurrent-range Zone (M3/N6), Fohsella birnageae Lowest Occurrence Subzone (M4b/N7), and Orbulina suturalis Lowest Occurrence Zone (M6/N9) of early to middle Miocene were identified. An overall progradational coastal and shelfal facies dominated by argillaceous and arenaceous sediment in the early and middle Miocene is construed, whereas interpreted palaeoenvironment of deposition indicates fluctuation between coastal deltaic, inner, middle, and outer neritic system with some fluvio-marine incursions between 4600 and 9460 ft intervals. The identified early Miocene succession is characterised by significantly high ecological parameters (diversity, evenness, richness), low salinity, and relatively high occurrence of foraminifera faunas compared to the middle Miocene. The relationships between the foraminiferal faunas and environmental factors suggest the local ecologic niche exhibits environmental heterogeneity that might have influenced even distribution of different microhabitat faunal preferences. Whereas the contributing influence of arenaceous sediment on the general dearth of microfossils in the upper section of the well cannot be neglected, the foraminiferal signatures reflect the ubiquitous middle Miocene carbonate crash event, and thus confirm its occurrence in the Gulf of Guinea.

Sequence biostratigraphy of the late Oligocene to Miocene succession in the northern Song Hong Basin, offshore Vietnam

The Cenozoic sedimentary succession of the Song Hong Basin, offshore North Vietnam is poor in marine microfossils, especially in the early Miocene and Oligocene and subsequently is poorly dated. However, it is rich in terrestrially derived pollen, spores, and freshwater algae. Biostratigraphic assemblages from two wells, termed Well A and B, have been evaluated using the approach of sequence biostratigraphy. The Oligocene fluvio-lacustrine succession through the Dinh Cao Formation displays multiple transgressive-regressive cycles comparable with those reported from the Cuu Long Basin to the south. By applying the local Cuu Long Oligocene palynological zonation (CLO zones), these cycles can be correlated directly with the cycles from the Cuu Long Basin, showing that sedimentation began during sequence 19, at about 29 Ma or a little earlier off-structure, over basement. The deepest interval within Well A yielded abundant pollen of the montane tree Alnus, and with the position of the basin within the northern tropics at 20oN, this suggests that in the mid Oligocene, the basin was intermontane, with Alnus widespread in swamps after the lake shallowed, at an altitude of at least 1000 m. The Miocene succession has been interpreted by reference to SEA (‘Southeast Asia’) depositional cycles following Morley et al. (2021). The evaluation suggests a complete early Miocene succession, varying from fluvial with low sedimentation rates for the youngest part of the Dinh Cao and Phong Chau Formations to shallow marine with increased sedimentation rates after 17 Ma through the Phu Cu Formation which correlates to the middle Miocene climatic optimum. There was a significant period of non-deposition from 13.8 to 9.5 Ma in Well A and from 13.5 to 11.8 Ma in well B, corresponding to the middle Miocene unconformity seen in other nearby basins.

BIOSTRATIGRAPHIC ANALYSIS OF THE OLIGOCENE-EARLY MIOCENE SUCCESSIONS FROM SULAIMANI AREA, KURDISTAN REGION, IRAQ

Three sections (Darzila, Hazar Kani and Basara) of the Oligocene –Early Miocene successions from Sulaimani area, Kurdistan region-NE Iraq, were measured and studied in order to interpret the biostratigraphic zonations at the Low and High Folded Thrust zones boundary, Low Folded Thrust zone, which are integrated with strontium stratigraphic dating. Accordingly, five biozones have been identified and they are:1- Nummulites vascus- Nummulites intermedius- Nummulites fichteli Assemblage Zone (Shallow Benthic Zone, SBZ 22A) represents the Rupelian age, 2- Lepidocyclina dilatata- Nummulites vascus - Nummulites fichteli Assemblage Zone (SBZ 22B-Early Chattian), 3- Praerhapydionina delicata- Archaias kirkukensis Assemblage Zone (SBZ23) representing the Late Chattian age, 4- Austrotrillina howchini- Peneroplis evolutus Assemblage Zone (SBZ24), almost point to the Aquitanian age and finally 5- Austrotrillina asmariensis- Dendritina rangi Assemblage Zone (equivalent to SBZ25) representing the Burdigalian age. Based on the distribution of the larger foraminifera and strontium stratigraphic dating, the Baba Formation is of Early to Late Oligocene age (Rupelian- Early Chattian), while the Bajawan Formation is of Chattian age, in addition to that, the Anah Formation is of Early Miocene age (Aquitanian) and the Jeribe Formation is of Burdigalian age. It is worthy to mention that the Oligocene-Miocene boundary has been identified in the studied area as well as, the gap duration between different formations of the Kirkuk Group, and the gap durations due to the absences of Serikagni, Euphrates and Dhiban formations.

Eruptive chronology of monogenetic volcanoes northwestern of Morelia – Insights into volcano-tectonic interactions in the central-eastern Michoacán-Guanajuato Volcanic Field, México

Twelve monogenetic volcanoes formed during the last 7 Ma within the Tarímbaro graben, northwest of the city of Morelia, in the central-eastern part of the Michoacán-Guanajuato volcanic field (México). These include four scoria cones (Pelón, Tetillas, Jamanal, and Parastaco), four lava domes (La Cruz, Divisadero, Estadio, and Tetillas), two small shield volcanoes (Quinceo and Tetillas) and two lava flows (Cuto and Cerritos). Based on a detailed geological map and stratigraphy aided by new 40Ar/39Ar and 14C radiometric dates, and whole-rock chemical analyses, we established the eruptive chronology of these monogenetic volcanoes. These volcanoes were built upon four early Miocene successions of regional volcanism named Cuitzeo lavas (18.7 Ma), Cuitzeo ignimbrite (17.4 Ma), Atécuaro ignimbrite (16.8 Ma), and Punhuato lavas (16.3 Ma). Late Miocene to Pliocene activity consisted of the La Cruz, Divisadero, and Estadio domes and the Cuto lava flow (6.7–3.1 Ma). Pleistocene activity included the Cuitzeo fallout pyroclastic sequence (1.48 Ma), the Quinceo small shield volcano (1.36 Ma), the Pelón scoria cone (0.84 Ma), the Tetillas small shield volcano, two lava domes and a scoria cone (0.56–0.34 Ma), the Jamanal and Parastaco scoria cones and the Cerritos lava flow (0.11 Ma). Magma volumes from these volcanoes vary from <0.1 to 4 km3. We ascribe their variety of magma volumes, morphology, volcanic styles, and spatio-temporal distribution to tectonic changes and evolution of the ENE Tarímbaro graben, whose faults belong to the Tarímbaro-Álvaro Obregón fault segment in the central part of the Morelia-Acambay Fault System. Quinceo (2 km3) and Tetillas (4 km3) small shield volcanoes are by far the most voluminous centers dominating the landscape of the city of Morelia. All rock samples have compositions varying from 49.3 to 61.6 wt % in silica with medium-K contents (0.43–1.83 wt %).

An Exceptional Large Sample of the Early Miocene Ctenodactyline Rodent Sayimys Giganteus, Specific Variation and Taxonomic Implications

Abstract An assemblage of the early Miocene Sayimys giganteus (Ctenodactylinae, Rodentia) from its type locality Keseköy, and smaller assemblages of Yapıntı and Horlak (Anatolia, Turkey) are described. Almost all Sayimys species are known from small collections of isolated cheek teeth. The very large assemblage from Keseköy is of special interest, because it allows the study of the intra-specific variation of dental morphology: size, the morphological changes through wear and the criteria for recognition of the tooth position of molars. The results have been used to review the formally named Sayimys species and to evaluate the dental features used to define the species. The review of the early and middle Miocene succession of Sayimys in the Siwaliks (Pakistan) resulted in the resurrection of S. minor; S. baskini is considered to be a junior synonym of S. minor. S. sivalensis and S. obliquidens are nomina dubia, and restricted to their holotypes, S. hintoni n. sp. is established for mediumsized Sayimys from the Kamlial, lower Chinji and Manchar Formations and S. chinjiensis is resurrected for Sayimys from the upper Chinji and Nagri Formations. It is suggested that the middle Miocene species Sayimys intermedius and S. assarrarensis from Saudi Arabia are synonymous.

High frequency (4th order) sequence stratigraphy of Early Miocene deltaic shorelines, offshore Texas and Louisiana

Abstract Well logs and 3D seismic reflection data are integrated to image Lower Miocene depocenters of the upper Texas and westernmost Louisiana coastal and offshore areas. Although previously interpreted at a large scale (3rd order cycles), the detailed stratigraphy and depositional history of the early Miocene succession has not been fully developed. The upper lower Miocene interval from Robulus L (MFS10) to Amphistegina B (MFS9) has been further subdivided into five 4th order cycles (each 200–300 m thick) to provide finer scale stratigraphic detail and interpretation of depositional environments. Results of the finer scale correlations are presented in a series of sandstone percent maps. Overall, the maps display a strike-elongate (sub-parallel to the present day coastline) orientation for the sandstone bodies which thin to the southeast (basinward). Net sandstone thickness is relatively low in older cycles and increases in younger cycles suggesting an overall progradation of the deltaic system into the mapped area through time. Continued marine regression and southward movement of the shoreline are also confirmed by seismic amplitude maps. After a major transgression associated with Robulus L deltaic progradation occurred under rising sea level conditions. Deltaic sediments prograded southward and sandstone brought to the upper Texas coast was distributed laterally by longshore currents to form strandplain coast lines. Maximum regression occurred during the interval MFS9_2 to MFS9_3 when a deltaic depocenter formed offshore Texas in east High Island area. However, sandstone thickness progressively decreased laterally to the east (offshore Louisiana) where more shale was present and marine processes reworked deltaic derived sandstones into shore parallel bars. There was a clear retreat of the shoreline during the youngest cycle marking the beginning of transgression associated with Amphistegina B. Detailed stratigraphic interpretation (at a 4th order scale) shows more variability in the dominant shoreline processes during early Miocene than previously thought. Recognition of sedimentary systems variability at a 4th order scale is critical for improved hydrocarbon exploration and in understanding potential future CO2 storage in the area.

Sedimentology and Facies Analysis of the Early Miocene Succession in the Zurbatiya Area, Eastern Iraq

Sedimentology and Facies Analysis of the Early Miocene Succession in the Zurbatiya Area, Eastern Iraq

An Active East Asian Monsoon at the Oligocene-Miocene Boundary: Evidence from the Sikouzi Section, Northern China

AbstractThe East Asian monsoon forms a significant element of the global climatic system. We here present detailed sedimentologic investigations of the Oligocene to early Miocene succession in the Sikouzi section, northern China, to determine monsoon evolution during this time. The particle-size characteristics and grain surface structures, in combination with the large-scale cross-bedding arrangements, graded bedding laminations, and small-scale deformation structures, type the Oligocene red beds in association I as eolian, attributable to large dune forms. Rare earth element results imply that the Oligocene sediments are possibly sourced from a local provenance in the West Qinling Mountains, excluding a more regional wind system during this time. Packages of thick-bedded dark-red mudstone/silty mudstone as well as subordinate medium-bedded pale-gray fine- to medium-grained sandstone and thin-bedded gypsum, with the massive mudstone/silty mudstone taking dominance, point to a lacustrine deposition with d...

Tectonic heritage in drainage pattern and dynamics: the case of the French South Alpine Foreland Basin (ca. 45–20 Ma)

AbstractThe spatial and temporal organization of depositional environments in drainage networks of foreland basins reflect the tectonic and erosional dynamics associated with the development of mountain belts. We provide field evidences for the initiation and evolution of a complex drainage system in the French South Alpine Foreland Basin related to Western Alps exhumation. Sedimentological and structural analyses of the Eocene–Early Miocene succession were investigated in the (1) Argens/Peyresq, (2) Barrême/Blieux/Taulanne and (3) Montmaur/St‐Disdier sectors. Combined with the existing structural data set, we propose a new model that integrates the regional tectonic activity, the palaeovalley orientation and their dynamics through time. The Eocene–Miocene deposits clearly show the existence of N–S‐oriented palaeovalleys. The systematic presence of early NE–SW‐ to N–S‐oriented strike‐slip and extensional faults in the palaeovalleys suggests that these tectonic structures were responsible for the formation of the initial N–S‐oriented basin‐floor topographies. The vertical offset of the strike‐slip faults induced sufficient accommodation space for the Cenozoic sedimentation since the Middle Eocene. It implies the creation of N–S‐oriented palaeovalleys during the northward Pyrenean‐Provençal phase, pre‐dating westward Alpine compression. Later, the Oligocene Alpine tectonic phase induced drainage expansion toward the orogenic wedge and the erosion of the exhumed internal massifs by transverse streams. The establishment of new connections between the old topographic lows formed a longitudinal drainage pattern that remains the locus of deposition in a regional sedimentary routing system. In this model, former strike‐slip faults correspond to weakness zones overprinted by the westward Alpine shortening that allowed the formation of the modern piggyback basin structure of the foreland and the long‐time preservation of the palaeovalley geometry.

Age, internal stratigraphic architecture and structural style of the Oligocene–Miocene Numidian Formation of northern Tunisia

The stratigraphy of the Numidian Formation of northern Tunisia and its internal organization are updated. Planktonic foraminifera point to a mainly Oligocene–Early Miocene age of this formation in the majority of the sections studied. Some key lithological horizons are recognized within the early Miocene succession, allowing lateral correlation between the outcrops of the Mogod and Kroumirie mountains. These include: (1) a conglomeratic interval, up to 1–2 m thick and rich in reworked glauconitic boulders, limestone fragments of the Tellian (Eocene) and pectinid bivalves; and (2) a relatively continuous glauconitic level. In the new stratigraphic scheme, the Zouza, Ben Metir and Sejnene sections represent the entire Numidian Formation. A lower unit (200–700 m thick), highly pelitic with subordinate sandstone beds was distinguished, overlain by an upper unit which is sandier, especially in its uppermost part (1,000–1,500 m thick). In the present study, these have been stratigraphically dated as Oligocene–Early Miocene p.p. (Rupelian–Chattian; P19–P21 zones) to Aquitanian (N4 Zone) and Miocene (Aquitanian–early Burdigalian; N4–N5 zones), respectively. In the areas studied, the sandy succession assigned to the Kroumirie Member begins with a sandstone unit with an erosional base or a major discontinuity, locally marked by conglomerates made up of various reworked components. It rests generally on a thick shale unit that characterises the upper part of the Zouza Member. The thinner, uppermost succession of the Numidian Formation represents the Babouch Member, dated as Burdigalian (N6–N7 zones). Within the framework of the new proposal, the total stratigraphic thickness of the Numidian Formation in northern Tunisia does not exceed 2,200–2,600 m. Internally, the Numidian Formation is transected by the Intra-Numidian Thrust and back-thrust faults, associated with faulted folds that are recognized in outcrop on different scales and in seismic sections. Along these thrust and/or reverse faults, the middle to upper Eocene deposits have undergone uplift and are exposed at the surface (e.g., Dowar Larmel in Meloula-Tabarka, Gaâret Sejnene and Sidi M’chreg sections).

The Miocene of Pianosa Island: key to understanding the opening of the Northern Tyrrhenian back-arc basin (Central Mediterranean)

The only place where Neogene–Quaternary rocks crop out for the entire Tuscan Archipelago in the Northern Tyrrhenian Sea is the island of Pianosa. In particular, the Miocene deposits record the depositional and tectonic evolution of the Northern Tyrrhenian region during this time period. These deposits are subdivided into two successions separated by a low-angle unconformity. The older, middle Burdigalian succession represents a calciturbidite shallow marine system, whereas the younger late Tortonian–early Messinian succession comprises a continental alluvial system that evolves upwards into a lagoonal–marginal marine environment. Here we present sedimentological, palaeontological and petrographical data that support a new stratigraphic and palaeogeographical framework for reconstructing the opening of the Northern Tyrrhenian back-arc basin. The early Miocene succession records a pre-rift marine depositional phase followed by a late Burdigalian–Langhian erosional phase. This was followed by a period of synrift continental-marginal deposition, as recorded by the late Miocene succession, terminated by an important phase of uplift, probably induced by the start of magmatic activity in the Tuscan Archipelago area.

Reworked pyroclastic beds in the early Miocene of Patagonia: Reaction in response to high sediment supply during explosive volcanic events

Two meter-scale pyroclastic levels are interbedded within the early Miocene succession of the Estancia 25 de Mayo (Patagoniense transgression) and Santa Cruz formations in the foreland Austral (or Magallanes) Basin, Argentina. The Lower Pyroclastic Level (LPL) is a tabular body interbedded within offshore marine deposits, laterally continuous for 30km and varying in thickness from few centimeters to around 4m. Grain-size grades from coarse to extremely fine ash with upward-fining along with a northeastern-fining trends. Structureless fine to very fine tuffs dominate and rare parallel laminations are the only tractive sedimentary structures. The Upper Pyroclastic Level (UPL) lies within low energy fluvial deposits and is laterally discontinuous, and it is composed by lenticular bodies reaching a maximum of 15m thick and 100m wide, with a concave-up base and a plane top. Grain-size range is similar to the LPL but it coarsens upward. The lower portion of the UPL shows parallel lamination, current ripple lamination and mud drapes with large pumice lapilli and plant debris, whereas the upper portion shows parallel lamination and trough cross-stratification. Both pyroclastic levels are composed mainly of pumice grains and glass shards with minor proportions of quartz and plagioclase crystals and lithic fragments. The LPL shows no mixing with epiclastic material whereas the UPL shows an upward increase in epiclastic material, and an upward increment in the scale of cross-bedding.The large thickness in relation to the possible emission center and the content of plant debris of the LPL does not suggest a direct, submarine, ash-fallout origin. The LPL is interpreted as a deposit of hyperpycnal-flows generated at the coastal zone when tephra-laden rivers plunged into the ocean. Large amounts of well preserved plant debris support the hypothesis of a terrestrial source of the sediments. The UPL is entirely composed of tractive deposits, so an ash fallout origin is disregarded. This, together with the lenticular shape and the alluvial plain origin of the encasing sediments, suggests accumulation within fluvial channels. Cycles of upper-flow-regime parallel lamination, current-ripple lamination and mud drapes at the lower portion, suggest short-lived turbulent flows that initially filled semi-abandoned channels. They were followed by sheet floods and channel reactivation, expressed by large-scale cross-bedding. The low degree of particle mixing observed in both levels is explained by the inability of streams to erode the substrate as they are suddenly over-saturated with pyroclastic sediments during and after the eruption. The grain-size distribution of the LPL and geochemical data indicate a contemporaneous volcanic source located to the west/southwest in the Andean ranges, where the South Patagonian Batholith is presently located.

Refinement of late-Early and Middle Miocene diatom biostratigraphy for the east coast of the United States

Integrated Ocean Drilling Program (IODP) Expedition 313 continuously cored Lower to Middle Miocene sequences at three continental shelf sites off New Jersey, USA. The most seaward of these, Site M29, contains a well-preserved Early and Middle Miocene succession of planktonic diatoms that have been independently correlated with the geomagnetic polarity time scale derived in studies from the equatorial and North Pacific. Shallow water diatoms (species of Delphineis , Rhaphoneis , and Sceptroneis ) dominate in onshore sequences in Maryland and Virginia, forming the basis for the East Coast Diatom Zones (ECDZ). Integrated study of both planktonic and shallow water diatoms in Hole M29A as well as in onshore sequences in Maryland (the Baltimore Gas and Electric Company well) and Delaware (the Ocean Drilling Program Bethany Beach corehole) allows the refinement of ECDZ zones into a high-resolution biochronology that can be successfully applied in both onshore and offshore regions of the East Coast of the United States. Strontium isotope stratigraphy supports the diatom biochronology, although for much of the Middle Miocene it suggests ages that are on average 0.4 m.y. older. The ECDZ zonal definitions are updated to include evolutionary events of Delphineis species, and regional occurrences of important planktonic diatom marker taxa are included. Updated taxonomy, reference to published figures, and photographic images are provided that will aid in the application of this diatom biostratigraphy.

New data on the Vrancea Nappe (Moldavidian Basin, Outer Carpathian Domain, Romania): paleogeographic and geodynamic reconstructions

A study has been performed on the Cretaceous to Early Miocene succession of the Vrancea Nappe (Outer Carpathians, Romania), based on field reconstruction of the stratigraphic record, mineralogical-petrographic and geochemical analyses. Extra-basinal clastic supply and intra-basinal autochthonous deposits have been differentiated, appearing laterally inter-fingered and/or interbedded. The main clastic petrofacies consist of calcarenites, sub-litharenites, quartzarenites, sub-arkoses, and polygenic conglomerates derived from extra-basinal margins. An alternate internal and external provenance of the different supplies is the result of the paleogeographic re-organization of the basin/margins system due to tectonic activation and exhumation of rising areas. The intra-basinal deposits consist of black shales and siliceous sediments (silexites and cherty beds), evidencing major environmental changes in the Moldavidian Basin. Organic-matter-rich black shales were deposited during anoxic episodes related to sediment starvation and high nutrient influx due to paleogeographic isolation of the basin caused by plate drifting. The black shales display relatively high contents in sub-mature to mature, Type II lipidic organic matter (good oil and gas-prone source rocks) constituting a potentially active petroleum system. The intra-basinal siliceous sediments are related to oxic pelagic or hemipelagic environments under tectonic quiescence conditions although its increase in the Oligocene part of the succession can be correlated with volcanic supplies. The integration of all the data in the “progressive reorientation of convergence direction” Carpathian model, and their consideration in the framework of a foreland basin, led to propose some constrains on the paleogeographic-geodynamic evolutionary model of the Moldavidian Basin from the Late Cretaceous to the Burdigalian.