Oligocene Research Articles

Review of morphotectonic studies in the East African Rift System: Insights from legacy research to modern advances

The East African Rift System (EARS) is a prime example of continental rifting, characterized by its iconic topographic features, which are significantly shaped by the interplay of tectonic and climatic forces. This review synthesizes past decades of morphotectonic studies (recent studies) on the plateau landscapes of the EARS, focusing particularly on their interiors and margins, and highlights their crucial role in advancing our understanding of landscape evolution. The paper revisits the legacy of geologic research in the EARS and delves into recent morphotectonic studies that quantify the rates and patterns of plateau incision, exhumation, and uplift through advanced thermochronological and topographic analyses. Additionally, the review identifies key unresolved questions, such as the origins of the plateau topography–whether from volcanic activity or mantle plume influence and explores how these plateaus maintained high elevations during the humid Oligocene Epoch without significant incision. The recent work described in this paper alongside legacy perspectives, continues to refine our geological understanding of the EARS, with broader implications for studies worldwide.

First record of an Eomysticetidae from the El Cien Formation (late Oligocene), “Ten Minute” locality, Baja California Sur, Mexico

Eomysticetidae (Chaeomysticeti clade) is an archaic family of baleen whales that originated and diversified during the Oligocene epoch. The fossil record of this family is represented by at least 11 formally described species, including records in Baja California Sur (Mexico), Kaiserswerth (Germany), Kitakyushu (Japan), South Carolina (United States), and South Island (New Zealand). Baja California Sur (BCS) possesses important marine deposits from the upper Oligocene, which document a rich diversity of fossil mysticetes, including members of the family Eomysticetidae. In this work, we report a whale specimen collected in the upper Oligocene San Juan Member, El Cien Formation, from the so-called “Ten Minute locality”, San Hilario, about 100 km NW of La Paz, Baja California Sur. The studied specimen consists of an incomplete skull, in which most of the rostrum and part of the occipital shield are missing but preserving one tympanic bulla in situ. This specimen is identified as belonging to the family Eomysticetidae based on both an elongated skull and nasals and transversely narrow and long intertemporal region with a wide exposure of parietals and frontals. The lack of more diagnostic regions precludes its identification at generic/specific levels. The specimen represents the first description of an eomysticetid for the San Hilario area, increasing our knowledge of the evolution of this group of mysticetes and its record in the Pacific basin from the late Oligocene of BCS.

Geological History Reconstruction using Stratigraphic Analysis: A Case Study of Kampung Baru, West Sumatra

The reconstruction of geological history includes the development of rocks and volcanic deposits on a depositional pattern consisting of various types of sedimentary materials accumulated over a long time, which involves depositional changes. Depositional changes require geological comprehension in analyzing and understanding the geological processes involved in their evolution. For this reason, stratigraphic analysis is a very relevant approach to discussing rock layers, including significant environmental changes during their geologic history. The purpose of this research is to find out the geological processes that have contributed to the formation of the earth in the past. The method in this research is a stratigraphic analysis carried out from field observation data carried out directly at the outcrop. Deposition began with the re-deposition of the Pre-Tertiary (Triassic)-aged Porphyry Quartz Dasite Formation. In the Late Oligocene Epoch, the Sawahtambang Formation was deposited, which formed in the braided river characterized by the presence of sedimentary structures in the form of lamination and cross lamination. In the Early Miocene to Middle Miocene, the transgression process occurred, where the change in deposition was caused by the supply of larger land sediments that caused the deposition of the Ombilin Formation to be deposited in the marine environment of the Transition-Nerritic Edge based on the bathymetry results. The research results are expected to enable geological information to gain in-depth knowledge and experience about the history of deposition and assist in geological modeling.

Shear strength and compressibility of reconstituted Boom clay, a stiff clay from the Paleogene

Boom clay is a stiff marine clay from the Oligocene epoch of the Paleogene period. It outcrops along the rivers Rupel and Scheldt in Northern Belgium, nearby the city and the port of Antwerp. Most of the existing geotechnical investigation around the Antwerp area, where the clay is located at relatively shallow depths, consists of standard laboratory tests on undisturbed samples performed at stress levels around the estimated in situ stress. Although the geological preconsolidation pressure of this clay is relatively well known, the clay yields at much higher stresses in 1D compression. Moreover, due to the brittle behavior of the stiff clay, it is often difficult to evaluate its shear strength behavior beyond peak strength. To this end, evaluating intrinsic parameters of compressibility, critical state shear angle and residual strength is of great relevance. In the present research, the mechanical behavior of reconstituted normally consolidated Boom clay was examined in a series of oedometer, CRS, CU triaxial, and multi-reversal direct shear tests. The mechanical behavior of the reconstituted clay was compared to the behavior of undisturbed clay samples. The results allow for a better evaluation of the structure effects of the natural samples on their mechanical response.

Contrasting Styles of Salt‐Tectonic Processes in the Ionian Zone (Greece and Albania): Integrating Surface Geology, Subsurface Data, and Experimental Models

AbstractThe Ionian Zone (IZ) is one of the key elements of the fold and thrust belt (FTB) of the Albanian and Hellenides orogen and contains large outcrops of Triassic evaporites. The IZ consists of various thrust sheets with a general westward vergence, stacking over the Apulian and Pre‐Apulian zones, and repeating a thick carbonate sequence of Upper Triassic to Eocene age. Thrusting becomes younger toward the west with a piggyback sequence, starting during the latest Oligocene Epoch in the Internal Ionian and ending in the Pliocene in the External Ionian. We have studied the IZ in southern Albania and northwestern Greece using field observations and borehole data and by fully interpreting a recently acquired 2D seismic data set. Our objectives are to establish the geometry and nature of the contacts associated with the major Triassic outcrops, to unravel precursor salt diapirs, and to assess their role during the Alpine contraction. Salt structures include gentle salt pillows, isolated salt plugs and diapirs, thrust welds, and salt walls. Combining these observations with experimental modeling results, we show how these structures control the geometry and kinematics of the Alpine thrusts or the location and kinematics of recent strike‐slip faults. Salt minibasins have also been identified, demonstrating salt mobility conditioned Mesozoic sedimentation in the Ionian Basin. The use of salt‐tectonics principles to evaluate the structural style and evolution of the IZ FTB also opens new directions for interpreting the subsurface structure and evolution of the region.

A reassessment of the geochemical, phylogeny, morphology, and taxonomy of two crocodilian jaw remains of Paleogene from central Europe (Germany and Austria)

Crocodiles are a conservative group of reptiles regarding their morphology and behaviour. Available Fossils are hence important to be studied for phylogeny, taxonomy and morphology. A fossil fragmentary crocodilian skull was found in lignite clay in the Paleogene sedimentary area from the Middle Oligocene Epoch of Sieglitz near Camburg at Saale river in Central Germany. This 20th century discovery documents a new species of Diplocynodon (Diplocynodon Haeckeli, described by Seidlitz in 1917). Another crocodile fossil was unearthed in 1878 from the Eocene Epoch, of Haunsberg near Sankt Pankraz in the province of Salzburg in Austria, which had been missing since 1970 and was recently rediscovered and identified. This rediscovered specimen, representing a crocodile mandible and an isolated tooth, which could belong to Asiatosuchus and not Diplocynodon. However, the morphology of isolated tooth is not a reliable source to identify it up to the generic level, so open taxonomy is used here for identification, it may be associated to the same or a new species. The new discoveries of fossils provide new characters enhancing our knowledge on a particular taxon and on the whole group.

Comparatively analyzing of chloroplast genome and new insights into phylogenetic relationships regarding the genus Stephania

The medicinal plant of the genus Stephania holds significant economic importance in the pharmaceutical industry. However, accurately classifying and subdividing this genus remains a challenge. Herein, the chloroplast (cp) genomes of Stephania and Cyclea were sequenced, and the primary characteristics, repeat sequences, inverted repeats regions, simple sequence repeats, and codon usage bias of 17 species were comparatively analyzed. Twelve markers were identified through genome alignment and sliding window analysis. Moreover, a molecular clock analysis revealed the divergence between subgenus (subg.) Botryodiscia and the combined Cyclea, subg. Stephania and Tuberiphania during the early Oligocene epoch. Notably, the raceme-type inflorescence represents the ancestral state of the Stephania and Cyclea. The genetic relationships inferred from the cp genome and protein-coding genes exhibited similar topologies. Additionally, the paraphyletic relationship between the genera Cyclea and Stephania was confirmed. Bayesian inference, maximum likelihood, and neighbor-joining trees consistently showed that section Tuberiphania and Transcostula were non-monophyletic. In conclusion, this research provides valuable insights for further investigations into species identification, evolution, and phylogenetics within the Stephania genus.

Tectonic and orbital forcing of the South Asian monsoon in central Tibet during the late Oligocene

The modern pattern of the Asian monsoon is thought to have formed around the Oligocene/Miocene transition and is generally attributed to Himalaya-Tibetan Plateau (H-TP) uplift. However, the timing of the ancient Asian monsoon over the TP and its response to astronomical forcing and TP uplift remains poorly known because of the paucity of well-dated high-resolution geological records from the TP interior. Here, we present a precession-scale cyclostratigraphic sedimentary section of 27.32 to 23.24 million years ago (Ma) during the late Oligocene epoch from the Nima Basin to show that the South Asian monsoon (SAM) had already advanced to the central TP (32°N) at least by 27.3 Ma, which is indicated by cyclic arid-humid fluctuations based on environmental magnetism proxies. A shift of lithology and astronomically orbital periods and amplified amplitude of proxy measurements as well as a hydroclimate transition around 25.8 Ma suggest that the SAM intensified at ~25.8 Ma and that the TP reached a paleoelevation threshold for enhancing the coupling between the uplifted plateau and the SAM. Orbital short eccentricity-paced precipitation variability is argued to be mainly driven by orbital eccentricity-modulated low-latitude summer insolation rather than glacial-interglacial Antarctic ice sheet fluctuations. The monsoon data from the TP interior provide key evidence to link the greatly enhanced tropical SAM at 25.8 Ma with TP uplift rather than global climate change and suggest that SAM's northward expansion to the boreal subtropics was dominated by a combination of tectonic and astronomical forcing at multiple timescales in the late Oligocene epoch.

Stratigraphy, provenance, and timing of Neogene sedimentation in the western Valdés Basin, Patagonia. Accurate paleogeographic reconstructions as a key piece for andean-passive margin integration

With the aim to better understand the timing and geological evolution of the late Cenozoic stratigraphic record of the onshore, western Valdés Basin in northeast Chubut province (Patagonia, Argentina), we perform sedimentological, stratigraphic, and zircon U–Pb geochronologic analyses. The study includes the terrestrial Early Miocene Trelew Member of the Sarmiento Formation, the shallow marine Early Miocene Gaiman Formation, the shallow marine to estuarine Late Miocene Puerto Madryn Formation, and the Plesitocene fluvial conglomerates of the Rodados Patagónicos. Based on our data set, together with the recognition of relevant stratigraphic surfaces, we divide the succession into stratigraphically bounded units and define three cycles of sedimentation. The Lower Cycle rests on top of Surface 1, which comprise a long hiatus including the entire Oligocene Epoch. This cycle is in turn divided into two subunits separated by the transgressive, ravinement Surface 2. The Lower Cycle is separated from the overlying Middle Cycle (=Puerto Madryn Formation) by the flat and regionally extensive Surface 3. Finally, the latter is separated from the Upper Cycle (=Rodados Patagónicos) by Surface 4, a complex surface at the base of gravel deposits forming several terrace levels. Maximum depositional ages calculated from 10 zircon U–Pb samples indicate the succession between surface 1 and 4 extends from 21.5 to 2.7 Ma (Aquitanian-Piacenzian). The Lower Cycle is restricted to the 21.5–16 Ma interval (Aquitanian-Burdigalian); the Middle Cycle between 12 and 2.7 Ma (latest Serravalian-Piacenzian), with most of the new and previous ages clustering in the Tortonian; whereas the Upper Cycle is restricted to the Pleistocene. Although most of the analyzed sediments show fresh volcanic glass particles, older zircon ages are dominant, representing detrital components with a dominant peak of Permian-aged grains, with subordinated Devonian, Jurassic, Cretaceous, and Paleogene peaks. We interpret the analyzed succession as a condensed section compared to the offshore Valdés Basin depocenter area. Sedimentation in the study area was controlled by periods of slow subsidence, probably related to thermal subsidence and sediment compaction in the depocenter. Two main sources of sediments for the analyzed succession are envisaged: one represented by explosive volcanism located either in the Andes or in the North Patagonian Massif, the other through erosion of older rocks surrounding the North Patagonian Massif where late Paleozoic plutonic rocks are abundant. Particularly for the Middle and Upper cycles, an increase in Mesozoic contribution suggests erosion from sediments of the Cañadón Asfalto Basin. The new ages and stratigraphic analyses here reported are of major relevance for the integration of the onshore and offshore stratigraphies of the Valdés Basin which is key to connect the geologic evolution of the Andean and passive margin areas of southern South America. They are also of major relevance for the calibration of fossil communities, particularly for the abundant and diverse marine vertebrates preserved in the Burdigalian Gaiman Formation.

Out of southern Africa: Origins and cryptic speciation in Chamaeleo, the most widespread chameleon genus

Molecular phylogenetics and the application of species delimitation methods have proven useful in addressing limitations associated with morphology based taxonomy and have highlighted the inconsistencies in the current taxonomy for many groups. For example, the genus Chamaeleo, which comprises 14 species with large distributions across mainland Africa and parts of Eurasia, exhibits relatively minor phenotypic differentiation between species, leading to speculation regarding the presence of cryptic diversity in the genus. Therefore, the aims of the present study were to construct a robust and comprehensive phylogeny of the genus and highlight potential species-level cryptic diversity. Additionally, we sought to ascertain the most likely biogeographic origin of the genus and understand its spatio-temporal diversification. Accordingly, we made use of species delimitation methods (Bayesian and divergence based) to investigate the extent of cryptic diversity in Chamaeleo, and applied an ancestral area reconstruction to examine the biogeographic origin of the group. Our phylogenetic analyses suggested the presence of at least 18 taxa within Chamaeleo. Notably, three taxa could be recognised within C. dilepis, none of which are equivalent in context with any of the currently described subspecies. There were also three taxa within C. gracilis and two within C. anchietae. The single available tissue specimen identified as C. necasi was embedded within the C. gracilis clade. Our ancestral area reconstruction points to a southern African/Zambezian origin for Chamaeleo, with diversification beginning during the cooling and aridification of Africa that characterised the Oligocene Epoch, ca. 34–23 Mya (Million years ago). Species-level diversification began in the Miocene Epoch (ca. 23–5 Mya), possibly tracking the aridification that triggered the shift from forest to more open, mesic savanna for most clades, but with tectonic events influencing speciation in a Palearctic clade. These findings lay the foundation for a future integrative taxonomic re-evaluation of Chamaeleo, which will be supported with additional lines of evidence before implementing any taxonomic changes.

A molecular phylogeny and phylogeography of Greek Aegean Island sand flies of the genus Phlebotomus (Diptera: Psychodidae)

The genus Phlebotomus (Diptera: Psychodidae: Phlebotominae) comprises a group of small winged insect species of medical importance. To date, ten species of Phlebotomus are known to be present in Greece; yet their evolutionary history is poorly studied due to the lack of comprehensive phylogenetic and phylogeographic studies. Herein, we aim to clarify the phylogenetic relationships amongst the local species collected from 12 Aegean Islands, Cyprus and Turkey; and to identify which of the palaeogeographic events may have influenced their biogeographic history. Our analyses revealed for the first time the presence of P. cf. major and P. sergenti in the Aegean Islands. All studied local species were retrieved as monophyletic and the mtDNA and nDNA phylogenetic trees indicated a plausible mitochondrial introgression between the closely related species of the P. major complex. From a palaeogeographic viewpoint, the major driving force that shaped the biogeographic history of the studied Phlebotomus species seems to be the dispersal that started in the Oligocene epoch, followed by several speciation events that occurred at the end of Miocene and the Plio-Pleistocene, including multiple dispersal events of Asiatic origin. The Messinian Salinity Crisis, the bimodal Mediterranean climate, and the glacial and interglacial periods were identified as key drivers for the diversification of the local species of Phlebotomus.

Comparing between permeability prediction by using classical and FZI methods/Tertiary Reservoir in Khabaz Oil Field / Northern Iraq

This study are developed to predict the permeability of Tertiary reservoir in Khabaz oil field northern of Iraq. The Tertiary reservoir include many geological formations (Aaliji, Jaddala, Oligocene system, Jeribe, Fatha, Muqdadia formations). Actually Seven wells (Kz-1, Kz-2, Kz-3, Kz-4, Kz-9, Kz-14, Kz-15) used for this study in Khabaz oil field. Two methods are used, which are classical methods depending on core test and well log interpretations and FZI methods, the results show that the permeability estimated by core and well log interpretation method which depending on just the porosity are not close enough for the real values because it’s not toke in the consideration all the parameters that effect the values of permeability. The FZI method are show more real values and provides best correlation coefficient in comparing with the classical methods. The must conclusions of the result refer that the FZI method results are more accurate because of the high correlation coefficient (R2) values.

Investigation of the possible role of the Central Paratethys as a migration route and speciation area of the ancestors of Mediterranean Larroussius, Paraphlebotomus and Phlebotomus species

The Oligocene and Miocene tectonic and biogeographical alterations of the peri-Mediterranean area could strongly impact the speciation processes and migrations of Mediterranean sandfly species. To understand the possible former role of this palaeobiogeographic factor on ancestral sandfly species, the potential suitability values of five Larroussius, two Paraphlebotomus and one Phlebotomus species were modelled from the Rupelian to the Tortonian stage in the Central Paratethys. The bioclimatic extrema of this sandfly species and the Coexistence Approach-based palaeoclimatic reconstructions made the basis of hypothesis testing. In the case of the Tortonian stage, a georeferenced climatic model was created. The models indicate that the suitability values could notably vary by species and periods. The monsoon-influenced humid subtropical climate of the Oligocene epoch could be less suitable for the ancestors of Mediterranean sandfly taxa than the later, drier humid subtropical climates-characterised Miocene stages. The Central Paratethys area could be less suitable for ancestors of the extant Paraphlebotomus, and Phlebotomus in the Miocene epoch compared to Larroussius species. It may indicate that the Central Paratethys formed a barrier against the east-to-west migration of the ancestors of Phlebotomus and Paraphlebotomus species. In contrast, Larroussius species could prefer the Miocene climate of the Central Paratethys. In the Tortonian stage, mainly the coastal areas of the sea could be colonised by sandflies. These results indicate that the coasts of the Central Paratethys should be not only considered as a potential former migration route but could be a part of the speciation area of Larroussius sandflies.

Structural evolution of southern Sichuan Basin (South China) and its control effects on tectonic fracture distribution in Longmaxi shale

We aimed to reveal the structural styles and evolution sequences of the superimposed folding zone in the southern Sichuan Basin, South China, and to predict the distribution rules of tectonic fractures in the Longmaxi shale. The field survey and geological section analysis showed that the structural pattern is characterized by the superposition of near-N–S-trending folds and near-E–W-trending compressive structures. The two structural systems experienced multistage development, and the finalization of the E–W structures was later than the N–S ones. By combining these findings with the back-stripping method and fluid inclusion analysis, the burial, thermal, and structural evolution histories were reconstructed. The results indicate that the E–W-trending compressive structures were finalized at the end of the Late Cretaceous whereas the N–S-trending folds were formed by the Oligocene Epoch. A comparative analysis was conducted to reveal the relationship between tectonic fractures and fold structures. The analysis revealed that the Longmaxi shale in the core zones of the N–S-trending anticlines and the cores and near-core zones of the E–W folds is more likely to show a well-developed natural fracture system, which was verified by fracture observations on shale cores and outcrops. These places have potential for the exploration and development of shale gas resources.

Phylogenetic relationships, origin and historical biogeography of the genus Sprattus (Clupeiformes: Clupeidae).

The genus Sprattus comprises five species of marine pelagic fishes distributed worldwide in antitropical, temperate waters. Their distribution suggests an ancient origin during a cold period of the earth’s history. In this study, we evaluated this hypothesis and corroborated the non-monophyly of the genus Sprattus, using a phylogenetic approach based on DNA sequences of five mitochondrial genome regions. Sprattus sprattus is more closely related to members of the genus Clupea than to other Sprattus species. We also investigated the historical biogeography of the genus, with the phylogenetic tree showing two well-supported clades corresponding to the species distribution in each hemisphere. Time-calibrated phylogenetic analyses showed that an ancient divergence between Northern and Southern Hemispheres occurred at 55.8 MYBP, followed by a diversification in the Oligocene epoch in the Northern Hemisphere clade (33.8 MYBP) and a more recent diversification in the Southern Hemisphere clade (34.2 MYBP). Historical biogeography analyses indicated that the most recent common ancestor (MRCA) likely inhabited the Atlantic Ocean in the Southern Hemisphere. These results suggest that the ancestral population of the MRCA diverged in two populations, one was dispersed to the Northern Hemisphere and the other across the Southern Hemisphere. Given that the Eocene was the warmest epoch since the Paleogene, the ancestral populations would have crossed the tropics through deeper cooler waters, as proposed by the isothermal submergence hypothesis. The non-monophyly confirmed for the genus Sprattus indicates that its systematics should be re-evaluated.

Snakes on an African plain: the radiation of Crotaphopeltis and Philothamnus into open habitat (Serpentes: Colubridae).

BackgroundThe African continent is comprised of several different biomes, although savanna is the most prevalent. The current heterogeneous landscape was formed through long-term vegetation shifts as a result of the global cooling trend since the Oligocene epoch. The overwhelming trend was a shift from primarily forest, to primarily savanna. As such, faunal groups that emerged during the Paleogene/Neogene period and have species distributed in both forest and savanna habitat should show a genetic signature of the possible evolutionary impact of these biome developments. Crotaphopeltis and Philothamnus (Colubridae) are excellent taxa to investigate the evolutionary impact of these biome developments on widespread African colubrid snakes, and whether timing and patterns of radiation are synchronous with biome reorganisation.MethodsA phylogenetic framework was used to investigate timing of lineage diversification. Phylogenetic analysis included both genera as well as other Colubridae to construct a temporal framework in order to estimate radiation times for Crotaphopeltis and Philothamnus. Lineage diversification was estimated in Bayesian Evolutionary Analysis Sampling Trees (BEAST), using two mitochondrial markers (cyt–b, ND4), one nuclear marker (c–mos), and incorporating one fossil and two biogeographical calibration points. Vegetation layers were used to classify and confirm species association with broad biome types (‘closed’ = forest, ‘open’ = savanna/other), and the ancestral habitat state for each genus was estimated.ResultsPhilothamnus showed an ancestral state of closed habitat, but the ancestral habitat type for Crotaphopeltis was equivocal. Both genera showed similar timing of lineage diversification diverging from their sister genera during the Oligocene/Miocene transition (ca. 25 Mya), with subsequent species radiation in the Mid-Miocene. Philothamnus appeared to have undergone allopatric speciation during Mid-Miocene forest fragmentation. Habitat generalist and open habitat specialist species emerged as savanna became more prevalent, while at least two forest associated lineages within Crotaphopeltis moved into Afromontane forest habitat secondarily and independently.DiscussionWith similar diversification times, but contrasting ancestral habitat reconstructions, we show that these genera have responded very differently to the same broad biome shifts. Differences in biogeographical patterns for the two African colubrid genera is likely an effect of distinct life-history traits, such as the arboreous habits of Philothamnus compared to the terrestrial lifestyle of Crotaphopeltis.

New species, revision, and phylogeny of Ronzotherium Aymard, 1854 (Perissodactyla, Rhinocerotidae)

Ronzotherium is one of the earliest Rhinocerotidae in Europe, which first appeared just after the Eocene/Oligocene transition (Grande Coupure), and became extinct at the end of the Oligocene. It is a large-sized rhinocerotid, with a special position in the phylogeny of this group, as being one of the earliest-branching true Rhinocerotidae. However, its intra-generic systematics has never been tested through computational phylogenetic methods and it is basically unknown. Its taxonomical history has gone through numerous complications, and thus we aim to provide here a complete revision of this genus, through phylogenetic methods. After a re-examination of all type specimens (five supposed species) as well as of most well-preserved specimens from all over Europe and ranging through the complete Oligocene epoch, we performed a parsimony analysis to test the position of some problematic specimens. According to our results, five species can be distinguished, Ronzotherium velaunum (type species), R. filholi, R. elongatum and R. romani as well as a new species: R. heissigi sp. nov. We also drastically re-interpret its anatomy and show that the ‘short-limbed’ “Diaceratherium” massiliae, described from Southern France, can be considered as a junior synonym of R. romani. Finally, we exclude the Asian species “Ronzotherium” orientale and “Ronzotherium” brevirostre from Ronzotherium and we consider R. kochi as a junior synonym of R. filholi.

Tectonic Evolution of Southern part of the Mesopotamian Foredeep Basin

This study investigated the tectonic evolution of the southern part of the MesopotamianForedeep basin. Subsidence and sedimentation rates were calculated for six oil wellsdistributed on the tectonic subzones of the sedimentary basin through the use of restoredthickness rates according to Backstripping Method for Cretaceous and Tertiary sequences. Theconsequence of this study indicates the subsidence and sedimentation rates changing verticallyand laterally through geologic time. As shown, the sequences of Albain subcycle are widesubsidence and sedimentation rates especially for the Mauddud Formation at the Am-1wellwhich located in the eastern part of the sedimentary basin (Tigris subzone), as well as for thesequences of Cenomanian-Early Turonian subcycle especially for the Ahmadi and Mishrifformations were increase in subsidence and sedimentation rate toward (Am-2 and Mj-3) wells.Both wells located within the Tigris and the eastern part of Zubair subzone. This is inaccordance with the closure of the South Tethys Sea because of the influences of the Austrianand Subhersynian orogenies. Three unconformity surfaces determine clearly in the depositinalbasin of incompatibility during the Early Turonian, Danian and Oligocene epochs. Thoseunconformities affects all tectonic subzones except for the Am-1well where it was notinfluences by the unconformity during the Oligocene epoch. The subsidence and sedimentationprocess continued at high rates, reflecting the large thickness of the eastern part of the basin inthe Tigris subzone area. Sedimentation rates augment significantly at the well (AG-19), whichindicates the effect of faults on the Tigris subzone. This affects the degree of maturation andthe source of hydrocarbons in the sedimentary basin.

Foraminiferal Biozonation of Late Eocene - Early Oligocene sediments of BC-1 well, Onshore, Western Niger Delta Basin, Nigeria

Foraminiferal analysis was carried out on forty (40) ditch cutting samples from BC-1 well in the Onshore, Western Niger Delta at depth intervals of 5590 ft. - 8300 ft. Lithologic description of the samples together with the gamma ray log analytical data indicated the presence of four lithostratigraphic units composed of shale, sandstone, sandy shale and shaly sand corresponding to the continental Benin and paralic Agbada Formations. A total of twelve (12) species defined into eleven (11) genera, six (6) subfamilies and eleven (11) families were recovered. Benthonic and planktonic foraminiferal species constitute approximately 82 % and 18 % respectively. The calcareous benthics make up 44.4 % while the arenaceous benthics representing 37.7% of the total foraminiferal assemblages. Foraminiferal index marker species - Globorotalia opima nana, Nonion oyae, Cassigerinella chipollensis and Spiroplectammina wrightiirevealed that the age of the studied well is from Late Eocene - Early Oligocene epoch. This indicated that both the planktonic and benthonic foraminiferal recovered from the well contained the transition between the Priabonian and the Rupelian age. Three (3) informal foraminiferal zone were established - Nonion oyae zone, Cassigerinella chipollensis zone and undiagnostic zone. The zones compares with foraminiferal markers species whose stratigraphic ranges are well established in the Niger Delta and globally.
 Keyword: Lithostratigraphic units, Agbada Formation, Eocene, Oligocene and Biozone.

Snout shape and masticatory apparatus of the rodent‐like mesotheriid ungulates (Notoungulata, Typotheria): exploring evolutionary trends in dietary strategies through ancestral reconstructions

AbstractMesotheriidae were small to medium‐sized native South American ungulates. Mesotheriids radiated from the early Oligocene epoch to the early–middle Pleistocene, in two subfamilies: Trachytheriinae and Mesotheriinae. Beyond the conspicuous fossil record, little is known about mesotheriid palaeobiology, and particularly, the dietary habits. To date, different studies suggest grazing habits, similar to some extant ungulates, but recent contributions are challenging this vision. Here, we explore dietary preferences in mesotheriids, focusing on food intake and oral processing strategies. We performed a shape analysis of the snout and reconstructed ancestral shapes to assess evolutionary trends within the clade. Also, we considered other morphological variables of the masticatory apparatus to perform a functional characterization. We recognized three snout configurations: Trachytherus, ‘Intermediate’ and Mesotherium morphotypes that can be associated with different food intake capabilities and, considering other morphological proxies, with the consumption of different food items. Taking into account palaeoenvironmental conditions, the evolution of the masticatory apparatus is interpreted in the context of a consumption gradient from coarse and tough foods (roots, tubers, tough leaves) to grasses. We propose that during the ecomorphological evolution of mesotheriids, an initial phase took place during the late Oligocene and Miocene, consisting of diversification and specialization for consumption of hard‐to‐break food items. A second phase began incipiently during the late Miocene, and became well‐defined during the Pliocene and Pleistocene, characterized by an ecomorphological displacement towards grazing habits. Inferred dietary habits in Mesotheriidae could provide light on palaeoecological evolution and niche partitioning within South American herbivorous mammalian communities.