Late Cretaceous Early Paleogene Research Articles

Evaluation of the endocranial anatomy of the early Paleogene north African gavialoid crocodylian Argochampsa krebsi and evolutionary implications for adaptation to salinity tolerance in marine crocodyliforms.

Argochampsa krebsi is a gavialoid crocodylian from the early Paleogene of North Africa. Based on its recovered phylogenetic relationship with South American species, it has been inferred to have been capable of transoceanic dispersal, but potential anatomical correlates for a marine lifestyle have yet to be identified. Based on CT scans of a mostly complete and well-preserved skull, we reconstruct the endocranial anatomy of Argochampsa and compare it to that of other gavialoids. We demonstrate that Argochampsa possesses concave depressions on the internal surface of the prefrontals and lacrimals, which have been inferred to represent osteological correlates for salt glands in unequivocally marine metriorhynchoid thalattosuchian crocodyliforms. The presence of these salt glands suggests that Argochampsa likely frequented pelagic environments and provides additional support for the capability of transoceanic dispersal within Gavialoidea. We also newly interpret osteological correlates for salt glands in the Miocene north African gavialoid Sutekhsuchus dowsoni, providing further support that saltwater tolerance was widespread and possibly ancestral in Gavialoidea, given that they have been previously reported in the Late Cretaceous-early Paleogene species Eosuchus lerichei and Portugalosuchus azenhae. In addition to these gavialoids, as well as metriorhynchids, we also identify these osteological salt gland correlates in the Paleocene northwest African dyrosaurid Rhabdognathus aslerensis, which represents another crocodyliform lineage thought to be capable of transoceanic dispersal. Given that dyrosaurids, gavialoids, and metriorhynchoids are distantly related lineages, the evolution of salt glands is likely a convergent ecological adaptation to the occupation of pelagic environments. Nevertheless, we demonstrate limited evaluation of the presence of these osteological correlates across Crocodyliformes, including within most extant species, such that it remains possible that they are much more widespread.

The anatomy and taxonomy of the North African Early Miocene crocodylian ‘Tomistoma’ dowsoni and the phylogenetic relationships of gavialoids

For decades, the interrelationships of the extant crocodylians Gavialis gangeticus and Tomistoma schlegelii have been debated, with only recent morphological phylogenetic analyses recovering the sister-taxon relationship between these two gavialoid species that has long been apparent in topologies based on molecular data. Several extinct species from the Mediterranean region are currently assigned to Tomistoma; however, their phylogenetic placement is labile, and often they do not form a clade with Tomistoma schlegelii. Here, we present a revision of Tomistoma dowsoni from the Early Miocene of Egypt and Libya, based on the type specimen (a partial snout and mandible) and referred material, including a nearly complete skull. These specimens show no notable anatomical differences and diagnostic features include: (1) a heart-shaped naris; (2) a prefrontal and lacrimal that are equidimensional in anteroposterior length; and (3) a prominent posterior process of the supraoccipital with a convex posterior margin. Maximum parsimony analysis, under both equal and extended implied weighting, recovers Tomistoma dowsoni as a phylogenetically nested gavialine, distantly related to Tomistoma schlegelii and other Mediterranean species currently referred to Tomistoma. Instead, Tomistoma dowsoni forms a sister-taxon relationship with Eogavialis andrewsi, from the Late Miocene of Kenya. Given that this clade does not consistently cluster with the type species of Eogavialis, and that Tomistoma dowsoni is diagnostic and clearly not referrable to Tomistoma, we herein erect the genus Sutekhsuchus, with the new combination Sutekhsuchus dowsoni. Our phylogenetic analyses recover the European Miocene gavialoids in a monophyletic group with the North American Thecachampsa, forming a clade of early-diverging gavialines that underwent transoceanic dispersal. We also recover a monophyletic group of thoracosaurs, comprising Eothoracosaurus, Portugalosuchus, and Thoracosaurus, which is expanded to include other Late Cretaceous–Early Paleogene European, North American, and North African species under extended implied weighting. https://zoobank.org/urn:lsid:zoobank.org:pub:2DE71417-8088-4749-8672-72E4A40C0B39

Late Cretaceous–Early Paleogene Volcanic Rocks of Eastern Kamchatka (Compostional Features and Isotopic-Geochemical Heterogeneity)

Late Cretaceous–Early Paleogene Volcanic Rocks of Eastern Kamchatka (Compostional Features and Isotopic-Geochemical Heterogeneity)

A Composite Section of Fossiliferous Late Cretaceous–Early Paleogene Localities in Senegal and Preliminary Description of a New Late Maastrichtian Vertebrate Fossil Assemblage

A Composite Section of Fossiliferous Late Cretaceous–Early Paleogene Localities in Senegal and Preliminary Description of a New Late Maastrichtian Vertebrate Fossil Assemblage

Late Cretaceous–Early Paleogene tectonic events in the “North–South Axis” of Central Tunisia

In order to better constrain the structural evolution of the North–South Axis (NOSA) running through central Tunisia, a multidisciplinary approach based on geological mapping, field observations and paleostress analysis was used. The geological study of the middle part of the NOSA including the Gadoum, Akrouta, Sidi Khalif, Khechem El Kaleb and Faïd structures, showed the predominance of N–S and E–W fault sets. Movement on the faults of this fault network caused the formation of depositional areas and the collapsed and tilting of fault bounded blocks located in the Southern part of the Gadoum–Akrouta sector. The Gadoum and Akrouta Jebels formed as a result of slip and rotation on a single N–S trending listric fault in the Cenomanian during which time reactivation of both the N–S and E–W fault sets occurred. During Coniacian–Santonian times, when the Aleg Formation was being deposited, the study area was affected by a transtensive regime. This regime led to the division of the area into blocks (e.g., the Gadoum–Akrouta block and the Wadi El Abiod Syncline) and this resulted in the Aleg Formation being deposited with variable thicknesses. During the Campanian–Early Maastrichtian, a N–S transpressive regime was established, and this regime, coupled with the salt tectonics, resulted in the formation of an angular unconformity, subsidence inversion and lateral thickness variations of the Abiod Formation. During the Early Eocene, an E–W fault network affected the sedimentary basin. These faults, arranged in steps, generated accommodation spaces for sediments which increase in thickness along the North–South Axis.

Phosphate δ13Corg chemostratigraphy from the Gantour basin, Morocco: A proof of concept from the K–Pg transition to mid-Thanetian

The Late Cretaceous–early Paleogene interval is globally associated with transient to long-term changes in the stable carbon isotopic composition of marine carbonates (δ13Ccarb). Based on biostratigraphic reconstruction, this critical period of Earth's history is thought to coincide with the deposition of world heritage Paleocene phosphate deposits (phosphorites) in northwestern Morocco. However, the detailed stratigraphy of the Gantour basin, one of the most important Moroccan phosphate deposits, has not yet been constrained. For instance, the former “Montian” Stage has been used to tentatively approximate the Danian, whereas the succeeding Selandian Stage remains to be identified. Here, we develop a detailed organic carbon isotopic (δ13Corg) curve from phosphorus-rich horizons of the western Gantour sedimentary sequence in an attempt to constrain their stratigraphic placement and depositional age model. Upsection, these strata host long-term negative and positive δ13Corg trends that tend to correlate with global δ13Ccarb records of the Cretaceous–Paleogene and mid-Thanetian transitional boundaries. The data support the presence of Danian and Selandian rocks in the Gantour basin, which are succeeded by strata containing characteristic signatures of the well-known Cenozoic δ13C maximum at 58–57.5 Ma (the Paleocene Carbon Isotope Maximum). Our results shift the previously proposed Cretaceous–Paleogene transition in the Gantour basin further down into the older sediment C2M layer without interfering with recorded massive biological turnover in faunal diversity and abundance. Moreover, the refined stratigraphy suggests that the deposition of the Gantour phosphorites spanned ∼8.5 Myr. Our results confirm the utility of δ13Corg chemostratigraphy for dating and correlating phosphate-bearing deposits of the Tethyan province. They have important implications for deciphering Paleocene phosphogenesis, the co-evolution of associated vertebrate groups, and for prospecting phosphorus-rich mineral deposits.

Migrations, diversifications and extinctions: the evolutionary history of crocodyliforms in Australasia

Crocodyliform palaeontology in Australasia has a productive research record that began in the late nineteenth century and continues today. In this study, we summarize the current understanding on the taxonomic diversity and phylogenetic relationships of Australasian crocodyliforms based on first-hand knowledge of relevant fossil material and a review of the published literature. The currently known fossil record of Crocodyliformes in Australasia spans more than 113 million years, from the Early Cretaceous to the Holocene, and largely consists of body fossils discovered on continental Australia. Whilst only two crocodyliform genera are recognized from Australasia’s Mesozoic, the Cenozoic is distinguished by a remarkable taxonomic diversity of crocodylian crocodyliforms. By far the most common crocodylians from Australasia are members of Mekosuchinae, whose fossils are unambiguously known from the early Eocene until the Holocene. In addition to mekosuchines, during the Cenozoic Australasia was also inhabited by gavialoids and species of Crocodylus, with four extant species of the latter being the only surviving crocodylians in Australia and New Guinea. The phylogenetic relationships of Australasia’s crocodylians, particularly mekosuchines, have been a topic of interest to palaeontologists for over two decades. We performed several phylogenetic analyses to test the relationships of Mekosuchinae and other extinct crocodylians. Most results from our analyses found Mekosuchinae as a basal crocodyloid clade within Longirostres. However, some of the results recovered an alternative position for the majority of mekosuchines outside of Longirostres and the Late Cretaceous–early Paleogene Orientalosuchina as its deeply nested subclade. These results suggest that Mekosuchinae had its origins in Asia during the Cretaceous, and that mekosuchines arrived from southeast Asia into Australia no later than the late Paleocene. If this hypothesis is correct, then Mekosuchinae would no longer be an Australasian endemic clade since mekosuchines also seem to have persisted on continental Asia until the late Eocene.Jorgo Ristevski [jorgo.ristevski@gmail.com], School of Biological Sciences, The University of Queensland, Brisbane, 4072, Queensland, Australia; Paul M. A. Willis [paul@palaeopictures.com], School of Biological Sciences, Flinders University, Adelaide, 2100, South Australia, Australia; Adam M. Yates [Adamm.Yates@magnt.net.au], Museum of Central Australia, Museum and Art Gallery of the Northern Territory, Alice Springs, 0870, Northern Territory, Australia; Matt A. White [fossilised@hotmail.com], Palaeoscience Research Centre, University of New England, Armidale, 2351, New South Wales, Australia, Australian Age of Dinosaurs Museum of Natural History, The Jump-Up, Winton, 4735, Queensland, Australia; Lachlan J. Hart [l.hart@unsw.edu.au], Earth and Sustainability Science Research Centre, School of Biological, Earth and Environmental Sciences (BEES), University of New South Wales, Kensington, 2052, New South Wales, Australia, Australian Museum Research Institute, 1 William Street, Sydney, 2010, New South Wales, Australia; Michael D. Stein [michael.stein@unswalumni.com], Earth and Sustainability Science Research Center, School of Biological, Earth and Environmental Sciences (BEES), University of New South Wales, Kensington, 2052, New South Wales, Australia; Gilbert J. Price [g.price1@uq.edu.au], School of Earth and Environmental Sciences, The University of Queensland, Brisbane, 4072, Queensland, Australia; Steven W. Salisbury [s.salisbury@uq.edu.au], School of Biological Sciences, The University of Queensland, Brisbane, 4072, Queensland, Australia.

СТРОЕНИЕ ЗЕМНОЙ КОРЫ И ЛИТОСФЕРНОЙ МАНТИИ ЦЕНТРАЛЬНОЙ ЧАСТИ НИЖНЕАМУРСКОЙ МИНЕРАГЕНИЧЕСКОЙ ЗОНЫ И ЗАКОНОМЕРНОСТИ РАСПРЕДЕЛЕНИЯ РУДНОГО ЗОЛОТА В ЕЕ ПРЕДЕЛАХ

In this study, we consider the features of the deep structure of the Earth’s crust and the lithospheric mantle in the central part of the Lower Amur mineragenic zone and in the area that flanks on the zone from the west, including the Albazinsky gold ore cluster. Zones with a sharp change in density and magnetic properties associated with Late Cretaceous-Early Paleogene magmatism were established based on the calculated density and magnetic depth models of the territory. A ring structure ~200 km across was identified, which is characterized by the low-density lithospheric mantle and density and magnetic inhomogeneities in the Earth’s crust associated with a wide development of intrusive bodies within it, as well as their attitude and composition. The spatial relation of gold ore districts, clusters and deposits to density and magnetic inhomogeneities in the crust and lithospheric mantle is analyzed and the main patterns in their occurrence are shown. As a rule, they are confined to the low-density zones in the crust and marginal areas of deep (12–20 km) magnetic intrusions. Based on the findings, new promising areas to prospect for gold deposits are proposed. In particular, it was concluded that the potential to host gold mineralization was underestimated for the western part of the ring structure south of the Albazino deposit.

The calcareous nannofossil record of the uppermost Maastrichtian-lower Palaeocene in the Kırıkkale Basin, in the Central Anatolian Region (Turkey)

The Late Cretaceous–Early Paleogene (K–Pg) was a critical period of transition in geological time. This period encompassed short-term climatic fluctuations on a global scale, changes in ocean circulation, and sudden and large extinctions of marine and terrestrial organisms. In the study area, located in the mid to low latitudes, the Late Cretaceous and Early Paleogene were very tectonically active due to the positioning of the site close to the collision zone of two large continents. The impacts of the global K–Pg crisis can be observed in the study area. In this study, the calcareous nannofossil contents of late Maastrichtian–Danian sediments were studied, and the nannofossil biostratigraphy determined, from samples from the Samanlık and Dizilitaşlar Formations, deposited in the Kırıkkale Basin. From three stratigraphic sections, 26 nannofossil genera and 36 nannofossil species were identified from the Late Maastrichtian UC20aTP and UC20bTP biozones and the NP1 and NP2 biozones of the Danian. Additionally, it was determined that the K–Pg boundary was not continuous in the study area. In the Kırıkkale Basin, relatively low abundances of Micula decussate Vekshina, 1959 signals a diagenetic effect and stressful environment in the Late Maastrichtian, whereas the relatively low abundances of Thoracosphaera operculata Bramlette & Martini, 1964, Braarudosphaera bigelowii (Gran & Braarud, 1935) Deflandre, 1947 and Futyania petalosa (Ellis & Lohmann, 1973) Varol, 1989 in the Danian assemblages indicate unstable environmental conditions and major environmental perturbations that reflect tectonic activity in the region. No nannofossils were encountered in those samples taken from turbiditic levels, which contained high proportions of sand.

Late Cretaceous-early Paleogene magmatism in the Gyeongsang basin, southeast Korea and its implications for middle Paleogene climate change

• The youngest volcanic rocks in the Gyeongsang basin were formed at ∼ 56 Ma. • Paleogene magmatic activities indicate a transformation in tectonic regime. • Magmatic lull along east Asian and global cooling start from Cenozoic. The Gyeongsang basin is the largest volcanic-sedimentary basin in Korean Peninsula. It is characterized by widespread late Cretaceous-early Paleogene plutonic and volcanic rocks called Gyeongsang Supergroup, with its uppermost part of Yucheon group consisting mainly of andesitic to rhyolitic lavas and their derivative pyroclastic rocks. However, the precise geochronological framework of this supergroup remains elusive. In this study, we present new SIMS zircon U-Pb and whole-rock 40 Ar/ 39 Ar ages for the volcanic rocks of the Yucheon group and associated Bunam granites in the Juwangsan area, north of the Gyeongsang basin. Seven volcanic samples range in age from 71 to 56 Ma, while a granite sample has an age of 74 Ma. Combined with literature data, the Yucheon volcanic activities can be divided into two stages: 94–78 Ma and 71–56 Ma, comparable to 91–48 Ma of the granites in the basin. Actually, a gigantic igneous belt developed along the continental margin of NE Asia during the Mesozoic-Cenozoic, with a length over 10000 km and a width ∼ 1500 km. Exhibiting an eastward migration trend during 145–48 Ma and a magmatic tempo from flare-up to lull, this magmatic belt experiences a transformation in tectonic regime from active continental margin to back-arc due to subduction retreating of the Pacific subduction. The continental arc magmatic flare-up and associated contact metamorphism during Jurassic-early Paleogene may provide a huge amount of CO 2 for the global warming. With the magmatic lull after about 50 Ma, the cool Earth may be caused by less CO 2 inputs.

Late Mesozoic-Cenozoic multistage exhumation of the central Bangong-Nujiang Suture, Central Tibet

The Cenozoic rise of the Tibetan Plateau has significantly altered global climate change and the biological redistribution of Asia. However, there remain great controversies about its detailed topographical evolution, especially in the hinterland of the Plateau, such as around the Bangong-Nujiang Suture (BNS). Here, we choose the Gaize Basin, which is located in the central BNS, as our target research area. New low-temperature thermochronology data have been used to quantify the cooling history of the intrusive rocks and sedimentary strata from the N-S edges of the Gaize Basin. Our thermochronology results show that 1) zircon (UTh)/He ages range from 54 ± 3.7 to 113 ± 8.4 Ma, 2) apatite fission track ages mostly cluster around ~55 Ma, and 3) apatite (UTh)/He ages are between 34 ± 3.3 and 63 ± 4.4 Ma. Combined with the results from thermochronology modelling, we suggest that there were three cooling stages recorded around the Gaize Basin: 1) the late Mesozoic (120–100 Ma), which was probably due to the BNS closure and subsequent Qiangtang-Lhasa collision, 2) the Late Cretaceous-early Paleogene (85–55 Ma), which was perhaps related to the far-field effects of the Neo-Tethyan oceanic subduction, and 3) 47–30 Ma, in response to the continuous India-Asian collision. Similar to today, low relief topography has finalized in the central Tibetan Plateau after the last cooling stage and is mainly aided by river incision and erosion on both sides of the BNS.

Mesozoic-Cenozoic exhumation history and its implications for the uranium mineralization in the southern Junggar Basin, North China

The phases of tectonic activity and exhumation of the southern margin of the Junggar Basin during the Cretaceous through Cenozoic shows significant relationships with the timing of uranium mineralization episodes. To clarify the succession of metallogenetic stages that were controlled by different phases of uplift and exhumation, apatite fission track (AFT) thermochronology was performed on the Jurassic through Cenozoic sediments from the Manas section in the southern Junggar Basin. The mean ages of the AFT suites range from 63 Ma to 133 Ma, and are younger than the host sediment for the Mesozoic samples and older than the host for the Cenozoic samples. The mean track lengths (MTLs) are between 10 and 11 μm. These results indicate that most of the apatite grain assemblages in these samples experienced complicated annealing histories. AFT ages were combined with thermal history modeling to unravel the timing of the multi-phase regional uplift and exhumation history and to reveal the tectonic constraints on the uranium mineralization. The populations of AFT single-grain ages display four peaks, which are interpreted as regional uplift and cooling episodes: mid-Early Jurassic (189–180 Ma), late Jurassic–early Cretaceous (157–116 Ma), late Cretaceous–early Paleocene (88–59 Ma) and late Eocene–early Oligocene (37–28 Ma). Thermal history modeling indicates two episodes of rapid cooling of the basin sediment deposits during the late Cretaceous–Paleocene (80–50 Ma) and the Pliocene (5 Ma)–Present. A synthesis of the temporal relationship between AFT age peaks, of the modeled time–temperature paths, of a compilation of age data and regional geologic events from the Tianshan mountain belt, suggests a close relationship between tectonic evolution and uranium mineralization. Uranium metallogenesis episodes occurred in conjunction with tectonic uplift and exhumation episodes along the southern margin of the Junggar Basin during the late Cretaceous–early Paleogene and during the latest Miocene-Present. The earlier episode was the primary mineralization stage, and the later episode was a partial transformation stage superimposed on the earlier one.

PALEONTOLOGY AND EVOLUTIONARY LINEAGES OF THE DIAGNOSTIC BENTHIC FORAMINIFERAL GENUS ORTHOKARSTENIA

This work is focused on the members of the Late Cretaceous-Early Paleogene (K-Pg) diagnostic benthic foraminiferal genus Orthokarstenia which is regionally important in paleontology and stratigraphic correlations. The large number of tests available and the rapid morphologic changes, offer an opportunity to study evolutionary changes in these foraminiferal taxa over a time span of about 25 m. y. (75-50 Ma). Six species of the genus Orthokarstenia are presented: O. applinae, O. eleganta, O. esnehensis, O. higazyi, O. nakkadyi and O. oveyi, which were recorded in eight localities in the Southern Tethys: Nigeria, Tunisia, Egypt (central and north Africa), Jordan, Saudi Arabia, UAE, Iran and Pakistan (southwest Asia). Evolutionary changes of them are indicated by such criteria, such as changes in the test-size, chambers arrangement, type of sutures, periphery or surface ornamentation. These changes help to define the major faunal change of the Campanian/Maastrichtian (C/M) boundary, K/Pg boundary, and can used in biostratigraphic subdivisions and correlations based on benthic foraminifera, beside planktic foraminiferal zonation.

Rock-physics characterization of chalk by combining acoustic and electromagnetic properties

The characterization of shallow subsurface formations is essential for geologic mapping and interpretation, reservoir characterization, and prospecting related to mining/quarrying. To analyze elastic and electromagnetic (EM) properties, we characterize near-surface chalk formations deposited on a shallow seabed during the Late Cretaceous-Early Paleogene (Maastrichtian-Danian). EM and elastic properties, both of which are related to mineralogy, porosity, and water saturation, are combined to characterize the physical properties of chalk formations. We also perform rock-physics modeling of elastic velocities and permittivity and analyze their relationships. We then use measured ground-penetrating radar and P-wave velocity field data to determine the key model parameters, which are essential for the validity of the models and can be used to evaluate the consolidation degree of the rocks. Based on the models, a scheme is developed to estimate the porosity and water saturation by combining the two rock-physics templates. The predictions are consistent with previous findings. Our templates facilitate fast mapping of near-surface porosity and saturation distributions and represent an efficient and cost-effective method for near-surface hydrologic, environmental, and petrophysical studies. In the current formulation, the method is only applicable to rock type (chalk) comprising a single mineral (pure calcite). It is possible to tailor the formulation to include more than one mineral; however, this will increase the uncertainty of the results.

New fossils shed light on the Late Cretaceous terrestrial community in the Caribbean and the First American Biotic Interchange

The Caribbean islands are one of the most important hotspots of endemism and biodiversity globally, and the scenario of unique examples of biological radiations. Although our knowledge of the current and recently extinct diversity in the area is strong, the origin and evolution of most groups in the region remain obscure because of the absence of fossils from deep time periods. The existence of temporal islands on the Caribbean plate can be traced back to the late Mesozoic, but little evidence of the paleo-communities that once inhabited the archipelago and their relationship with the older lineages in the region has been discovered. Simultaneously, the relationship of the early Caribbean archipelago with the Late Cretaceous–early Paleogene biotic interchange between the Americas has remained unsolved. Here we describe the first evidence of a Late Cretaceous terrestrial community in the region based on several remains recovered at three upper Campanian–lower Maastrichtian localities in Central Cuba. The fossil assemblage includes four specimens referable to a midsize pterosaur on the base of morphological and paleohistological characters, as well as seeds and casts of leafy shoots of plants of the families Cupressaceae and Lauraceae. Fossils fruits of a new taxon closely related to Chlorocardium are of particular interest because they correspond to the first direct evidence of the role played by the Caribbean seaway and islands in the First American Biotic Interchange.

Campanian-Eocene dinoflagellate cyst biostratigraphy in the Southern Andean foreland basin: Implications for Drake Passage throughflow

The tectonic opening of the Tasmanian Gateway and Drake Passage represented crucial geographic requirements for the Cenozoic development of the Antarctic Circumpolar Current (ACC). Particularly the tectonic complexity of Drake Passage has hampered the exact dating of the opening and deepening phases, and the consequential onset of throughflow of the ACC. One of the obstacles is putting key regional tectonic events, recorded in southern Patagonian sediments, in absolute time. For that purpose, we have collected Campanian-Eocene sediment samples from the Chilean sector of Southern Patagonia. Using U-Pb radiometric dating on zircons and dinoflagellate cyst biostratigraphy, we updated age constraints for the sedimentary formations, and the hiatuses in between. Thick sedimentary packages of shallow-marine and continental sediments were deposited in the foreland basin during the early Campanian, mid-Paleocene, the Paleocene-Eocene boundary interval and the middle Eocene, which represent phases of increased foreland subsidence. We interpret regional sedimentary hiatuses spanning the late Campanian, early-to mid-Paleocene, mid-Eocene and latest Eocene-early Oligocene to indicate times of reduced foreland subsidence, relative to sediment supply. We relate these changes to varying subduction rates and Andean orogeny. Dinoflagellate cyst assemblages suggest that the region was under the influence of the Antarctic-derived waters through the western boundary current of the Subpolar Gyre, developed in the southwest Atlantic Ocean and thus argues for limited throughflow through the Drake Passage until at least the latest Eocene. However, the proliferation of dinoflagellate endemism we record in the southwest Atlantic is coeval with that in the southwest Pacific, and on a species level, dinoflagellate cyst assemblages are the same in these two regions. This suggests that both regions were oceanographically connected throughout the early Paleogene, likely through a shallow opening of a restricted Drake Passage. This implies a continuous surface-water connection between the south Pacific and the South Atlantic throughout the late Cretaceous-early Paleogene.

Burial and Exhumation History of the Lujing Uranium Ore Field, Zhuguangshan Complex, South China: Evidence from Low-Temperature Thermochronology

The Zhuguangshan complex hosts the main uranium production area in South China. We report (U-Th)/He and fission track thermochronological data from Triassic–Jurassic mineralized and non-mineralized granites and overlying Cambrian and Cretaceous sandstone units from the Lujing uranium ore field (LUOF) to constrain the upper crustal tectono-thermal evolution of the central Zhuguangshan complex. Two Cambrian sandstones yield reproducible zircon (U-Th)/He (ZHe) ages of 133–106 Ma and low effective uranium (eU) content (270–776 ppm). One Upper Cretaceous sandstone and seven Mesozoic granites are characterized by significant variability in ZHe ages (154–83 Ma and 167–36 Ma, respectively), which show a negative relationship with eU content (244–1098 ppm and 402–4615 ppm), suggesting that the observed age dispersion can be attributed to the effect of radiation damage accumulation on 4He diffusion. Correspondence between ZHe ages from sandstones and granites indicates that surrounding sedimentary rocks and igneous intrusions supplied sediment to the Cretaceous–Paleogene Fengzhou Basin lying adjacent to the LUOF. The concordance of apatite fission track (AFT) central ages (61–54 Ma) and unimodal distributions of confined track lengths of five samples from different rock units suggest that both sandstone and granite samples experienced a similar cooling history throughout the entire apatite partial annealing zone (~110–60 °C). Apatite (U-Th-Sm)/He (AHe) ages from six non-mineralized samples range from 67 to 19 Ma, with no apparent correlation to eU content (2–78 ppm). Thermal history modeling of data suggests that the LUOF experienced relatively rapid Early Cretaceous cooling. In most samples, this was followed by the latest Early Cretaceous–Late Cretaceous reheating and subsequent latest Late Cretaceous–Recent cooling to surface temperatures. This history is considered as a response to the transmission of far-field stresses, involving alternating periods of regional compression and extension, related to paleo-Pacific plate subduction and subsequent rollback followed by Late Paleogene–Recent India–Asia collision and associated uplift and eastward extrusion of the Tibetan Plateau. Thermal history models are consistent with the Fengzhou Basin having been significantly more extensive in the Late Cretaceous–Early Paleogene, covering much of the LUOF. Uranium ore bodies which may have formed prior to the Late Cretaceous may have been eroded by as much as ~1.2 to 4.8 km during the latest Late Cretaceous–Recent denudation.

Regional sandstone-type uranium mineralization rooted in Oligo–Miocene tectonic inversion in the Songliao Basin, NE China

The tectonic inversion of the Songliao Basin during the Oligo–Miocene may have played an important role in controlling the development of sandstone-type uranium deposits (SUDs). Here we investigate drill holes along a southeast to northwest section in this basin based on apatite fission-track (AFT) and zircon fission-track (ZFT) techniques. We present 50 data from 15 deep boreholes at different depths between 665 and 3956 m and different structural units including grabens and horsts formed in the Early Cretaceous beneath the basin. The results of the effective AFT ages are 100 ± 11 to 2.3 ± 0.4 Ma (P(x2) > 5%) and ZFT ages are 97.5–20.4 Ma (including binomial peak ages). These results reveal that the basin underwent two distinct stages of rapid cooling after Late Cretaceous. In the first stage, during the Late Cretaceous–Early Paleogene (~80–50 Ma), tectonic uplift occurred in all of the structural units including grabens and horsts, which was marked by an unconformity between the latest Cretaceous Mingshui and the Eocene Yi'an formations. In the second stage, during the Oligo–Miocene (~40–10 Ma), tectonic uplift occurred mainly in the grabens but not in the horsts, corresponding with a few sediments of the Neogene Da'an and Taikang formations. We propose that the folds and the thrust faults mostly characterize in the second stage indicating a major tectonic inversion in the basin. The shifting of the two stages was probably in response to differences in the subduction angles and directions of motion of the Paleo-Pacific Plate from the southeast. Combined with previous information, it was demonstrated that most of the U mineralization ages are younger than 40 Ma, with a peak in the Miocene or later (<20 Ma). We thus propose that the SUDs have been redistributed and redeposited locally in successive stages during and after the Oligo–Miocene tectonic inversion.

Intracontinental uplift of the Brazilian Central Plateau linked to continental breakup, orogenies, and basin filling, supported by apatite and zircon fission-track data

Fission-track analysis in zircon and apatite detrital grains from Mesoproterozoic sedimentary sequences show evidence of cooling episodes in central Brazil from Neoproterozoic to Cenozoic. Apatite fission-track ages reveal a main cooling episode during the Late Cretaceous-Early Paleogene, providing important clues on cooling by uplift and denudation during drifting of South America and Africa. Zircon data also suggest 3 distinct cooling events: (i) a Neoproterozoic-Cambrian event, probably related to orogenic exhumation of the Brasilia Belt and the thermal decay of West Gondwana assembly, (ii) a Devonian-Carboniferous event, probably related to the Late Famatinian orogeny and early stages of the Gondwanide orogeny, and (iii) an Early Permian to Triassic event, linked to the Late Gondwanide orogeny. These results indicate that the processes of assembly and breakup of supercontinents might significantly affect the plate interior, especially along with ancient orogenic belts. Zircon data has shown evidence of thermal decay during and after the West Gondwana assembly. Apatite data, on the other hand, has shown evidence of thermal decay during and after the breakup of Pangaea. The processes of assembly as well as the breakup of supercontinents can be tracked in the intracratonic setting, especially along with ancient orogenic belts. These assumptions take us back to the old concept of “mobile belts” as old orogenic regions that represent weak portions of the continental crust, assumed to have a greater tendency for mobility when solicited. In this case, the Brasilia Belt has shown to present great vertical mobility, accommodating tectonic far-field forces by uplift and as a result, feeding the Phanerozoic basins around it.

Astronomically forced hydrology of the Late Cretaceous sub-tropical Potosí Basin, Bolivia

AbstractPeriodic variations in Earth’s orbital parameters force climate on local and global scales, with global responses particularly sensitive to the presence of ice sheets and their associated feedbacks. Therefore, determining whether orbital forcings influenced sedimentary records of the past, and if so, which had such an effect, can shed light on Earth’s climate sensitivity and global ice volume. To this end, we present a field- and drone-based cyclostratigraphy of the predominantly lacustrine El Molino Formation of the Late Cretaceous–Early Paleogene Potosí Basin in present day Bolivia, which contains carbonate mud parasequences that record fluctuating hydrological conditions, including ephemeral marine connections, from 73 Ma to 64 Ma. We introduce a novel methodology for incorporating drone imagery into a quantitative, three-dimensional stratigraphic model that generates an upward-younging quantity comparable to stratigraphic height, and we find that our model outperforms our own field measurements of stratigraphic height. We project drone imagery at two sites into the stratigraphic model to construct time series of outcrop color, which vary systematically with facies and track basin water depth. Spectral analysis of these time series reveals sedimentary periodicities corresponding to short eccentricity, precession, and semi-precession, which are corroborated with measurements of magnetic susceptibility from mudstones. We generate independent age models at both study areas from four new U-Pb chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA–ID–TIMS) ages, which are consistent with an orbital interpretation for observed sedimentary periodicities. Importantly, we observe the presence of obliquity-scale periodicity in sedimentation during a period of marine connection, suggesting that sea level oscillations were driven by obliquity. This observation is consistent with previous claims about the presence of a small, orbitally forced Antarctic ice sheet during the latest Cretaceous.