Geomorphology and tectonics of uplifted coasts: New chronostratigraphical constraints for the Quaternary evolution of Tyrrhenian North Calabria (southern Italy)

The Tyrrhenian coastal sector of North Calabria, stretching between Torre S. Nicola and the Lao river, belongs to the inner extensional sector of the Neogene Apennines thrust belt. It is characterised by a stair of Quaternary marine and fluvial terraces representing the geomorphic response to the interaction between the Quaternary sea level fluctuations and the regional trend of tectonic uplift experienced by the margins of the Tyrrhenian back-arc basin. Since the last century, several authors studied the North Calabria coasts, where the flight of terraces preserves significant marine and continental successions, and proposed several paleo-geomorphological and tectonic reconstructions. In this paper we present a new stratigraphic and morphostructural setting of the North Calabria coasts based on both chronostratigraphical constraints obtained from marine deposits and detailed geomorphological analysis. A ten order stair of marine terraces, stepping between 240 and 0 m a.s.l., was recognized and time-constrained by the age of the Fornaci S. Nicola marine succession which was ascribed by integrated paleoecological, biostratigraphical and paleomagentic analyses to the early Middle Pleistocene (MIS 19–15). In particular, the 240, 200 and 160 m a.s.l. high strandlines were ascribed to the Early Pleistocene and the ones between 100 and 15 m a.s.l. to the Middle Pleistocene. The total amount of the vertical motion experienced by the studied area was estimated, and evaluation of the average rates of uplift for the Middle and Late Pleistocene times were also given. Considering the elevation a.s.l. of the oldest terraces, a tectonic uplift of at least 240 m was calculated for the North Calabria coasts since the Early Pleistocene times, 100 m of which gained from the beginning of the Middle Pleistocene. On the other hand, the 8-m high Late Pleistocene strandlines display a negligible vertical displacement affecting the area during the last 130 ka. The entire staircase of terraces preserves a record of slowing down in the rate of uplift, which attained an average value of 0.15 mm/year during the Middle Pleistocene.     

Simulating the development of Martian highland landscapes through the interaction of impact cratering, fluvial erosion, and variable hydrologic forcing

On the highlands of Mars early in the history of the planet precipitation-driven fluvial erosion competed with ongoing impact cratering. This disruption, and the multiple enclosed basins produced by impacts, is partially responsible for a long debate concerning the processes and effectiveness of fluvial erosion. The role of fluvial erosion in sculpting the early Martian landscape is explored here using a simulation model that incorporates formation of impact craters, erosion by fluvial and slope processes, deposition in basins, and flow routing through depressions. Under assumed arid hydrologic conditions, enclosed basins created by cratering do not overflow, drainage networks are short, and fluvial bajadas infill crater basins with sediment supplied from erosion of interior crater slopes and, occasionally from adjacent steep slopes. Even under arid conditions adjacent crater basins can become integrated into larger basins through lateral erosion of crater rims or by rim burial by sediment infilling. Fluvial erosion on early Mars was sufficient to infill craters of 10 km or more in diameter with 500–1500 m of sediment. When the amount of runoff relative to evaporation is assumed to be larger, enclosed basins overflow and deeply incised valleys interconnect basins. Examples of such overflow and interconnection on the Martian highlands suggest an active hydrological cycle on early Mars, at least episodically. When fluvial erosion and cratering occur together, the drainage network is often disrupted and fragmented, but it reintegrates quickly from smaller impacts. Even when rates of impact are high, a subtle fluvial signature is retained on the landscape as broad, smooth intercrater plains that feature craters with variable amounts of infilling and rim erosion, including nearly buried “ghost” craters. The widespread occurrence of such intercrater plains on Mars suggests a strong fluvial imprint on the landscape despite the absence of deep, integrated valley networks. Indisputable deltas and alluvial fans are rare in the crater basins on Mars, in part because of subsequent destruction of surficial fluvial features by impact gardening and eolian processes. Simulations, however, suggest that temporally-varying lake levels and a high percentage of suspended to bedload supplied to the basins could also result in poor definition of fan–delta complexes.     

Late Pleistocene–Holocene landscape evolution in Fossa Bradanica, Basilicata (southern Italy)

Studies in the middle Basento river basin supported by reliable chronological data (tephra layers and a number of absolute datings) have allowed the reconstruction of Late Pleistocene–Holocene geomorphological evolution of the middle to low Fossa Bradanica area (Basilicata, southern Italy). The original Upper Pleistocene hillslope has been dissected by deep gullies leaving relict slope pediments. Holocene filling of the Basento river valley and gullies occurred as a succession of downcut and fill episodes. A first phase of accumulation occurred in the Late Neolithic, which was followed by a downcutting between 4500 and 3700 cal. yr BP. A second deposition phase took place in the Greek–Roman period between 2800 and 1620 cal. yr BP, which was interrupted at around 2500 cal. yr BP. Another downcutting phase took place between 1620 and 1500 cal. yr BP, followed by a deposition phase between 1440 and 1000 cal. yr BP. After 1000 cal. yr BP a deep downcutting took place. Evidence collected with this study, coupled with climate data recorded in other Italian and European locations, suggests that filling and downcutting episodes in Fossa Bradanica were predominantly climate-driven. Anthropogenic impact only intensified or weakened these processes.     

Pliocene to Holocene geomorphic evolution and paleogeography of the El’gygytgyn Lake region,NE Russia

Geomorphic, lithologhic, and stratigraphic field studies as well as pollen data and mineralogical study have been used to propose Pliocene and Pleistocene paleogeographic reconstructions of the El’gygytgyn meteorite crater area. The moment of impact is recorded above the early Pliocene hill denudation plain as a “chaotic horizon” consisting of fragments of impactite rocks. This chaotic horizon lies between layers of late Pliocene alluvial sediments. During the second half of the late Pliocene, the region was tectonically active, when the Anadyr lowland was uplifted causing alluvial sediments to accumulate in the basins to the south of the crater. Regional climatic cooling, which supported the spread of tundra and the formation of permafrost is characteristically to late Pliocene. The 35–40 m high terrace that roughly follows the 530 m contour interval along the Enmyvaam River formed during the middle Pleistocene. This terrace represents the maximum lake level. Erosion and incision of the upper Enmyvaam River increased due to another wave of uplift. Additionally, El’gygytgyn Lake discharge increased causing lake level to begin to drop in the Middle Pleistocene. Cooling continued, which led to the development of herb-dominated arctic tundra. middle and late Pleistocene glaciations did not reach the El’gygytgyn lake region. The 9–11 m high lacustrine terrace was formed around the lake during the late Pleistocene and the 2–3 m high lacustrine terrace formed later during the Holocene. During the last 5000 years, the lake level has continued to drop as the modern coastline developed. This is the third in a series of eleven papers published in this special issue dedicated to initial studies of El’gygytgyn Crater Lake and its catchment in NE Russia. Julie Brigham-Grette, Martin Melles, Pavel Minyuk were guest editors of this special issue.     

Middle-Late Pleistocene polycyclic evolution of a stable coastal area (southern Apulia, Italy)

The Otranto–Leuca coastal tract is marked by the presence of numerous sea caves placed close to present sea level. They are located generally at the back of a shore platform covered by a sequence of breccia deposits, marine sediments and speleothems. At Grotta di Masseria dell'Orte, marine cemented sands rest on a narrow shore platform at about 6.2 m above mean sea level and are covered by speleothems older than 185 ka. At Grotta del Diavolo, which is mostly filled by breccia deposits, three beach levels have been detected at about 3.0, 3.5 and 5.9 m above msl. They are either covered by or overlie speleothems that yield an U/Th age of 340, 78 ka and between 170.3 and 146.5, respectively. Geomorphological evidence and radiometric ages indicate that the area after a period of uplift has been tectonically stable since the last part of the Middle Pleistocene so that marine landforms close to the present shoreline underwent a polycyclic evolution. The sedimentary fills of sea caves formed during Middle-Late Pleistocene glacial stages, when arid or semiarid conditions promoted the removal of regolith and the development of thick breccia deposits. During Marine Isotope Stages (MIS) 9.3, 5.5 and 5.1, cave sediments were partially eroded whereas beach layers and related speleothems developed. These are, in fact, the only marine isotope stages marked by a sea level position which in this Mediterranean region was either close to, or slightly higher than, the present one.     

澧阳平原晚更新世晚期至全新世早中期环境演变及其对人类活动的影响

通过对澧阳平原野外调查,多剖面地层对比分析及岩板垱（YBD）剖面样品的粒度、地球化学元素分析和年代测定,揭示该区晚更新世晚期至全新世早中期环境演变,并探讨其对人类活动的影响。研究结果表明,30~6 ka B.P.间,澧阳平原由冲沟发育的黄土台地向河湖发育的平原丘岗地貌过渡;气候环境经历了弱暖湿-气候变差-凉湿-波动回暖-暖湿的变化;更新世末至全新世早期稻作农业的出现与该区的环境条件和文化基础密切相关;环境演变也推动了古人类活动范围和聚落特征的变化。     

Using fluvial terraces to determine Holocene coastal erosion and Late Pleistocene uplift rates: An example from northwestern Hawke Bay, New Zealand

In order to make robust predictions of future coastal processes and hazards, historical rates of coastal processes such as coastal erosion need to be put into a long-term (Holocene) context. In this study a methodology is proposed that uses fluvial terraces to construct longitudinal profiles which can be projected offshore to infer paleo-coastline positions. From these positions, an average Holocene coastal erosion rate can be calculated. This study also shows how constraints can be placed on sea level changes and Late Pleistocene uplift rates using fluvial terraces, and by assuming the latter has been constant since  55–37 ka, these constraints feedback into the coastal erosion rate calculations. For the northwestern Hawke Bay (North Island, New Zealand) coastline, Late Pleistocene uplift rates of 0.6 ± 0.2, 0.6 ± 0.2, and − 0.1 ± 0.1 (i.e., stable or subsiding) mm/yr have been determined for the Waikari, Mohaka, and Waihua River mouths, respectively. These rates are consistent with previous interpretations of subsidence to the northeast and uplift being the result of regional, subduction-related processes. A Holocene coastal erosion rate of 0.5 ± 0.1 m/yr was determined for the Waikari River mouth, which is at the higher end of the calculated historical ( 1880–1980) rates (0.02–0.5 m/yr). If this difference is significant, then two possible reasons for this difference are: (i) the historical rate is affected by events such as the 1931 Napier earthquake, and (ii) the Holocene rate is the average of a steadily declining rate over the last 7.3 ka.     

Monsoon triggered formation of Quaternary alluvial megafans in the interior of Oman

A vast bajada consisting of coalescing low-gradient (< 0.3°) alluvial fans exceeding 100 km in length formed along the southwestern margin of the Oman Mountains. It comprises an old fan sequence of inferred Miocene to Pliocene age termed Barzaman Formation, diagenetically highly altered to dolomitic clays, and a thin veneer of weakly cemented Quaternary gravels. A combination of remote sensing, lithological analyses and luminescence dating is used to interpret the complex aggradation history of the Quaternary alluvial fans from the interior of Oman in the context of independent regional climate records. From satellite imagery and clast analysis four fans can be discerned in the study area. While two early periods of fan formation are tentatively correlated to the Miocene–Pliocene and the Early Pleistocene, luminescence dating allows the distinction of five phases of fan aggradation during the Middle–Late Pleistocene. These phases are correlated with pluvial periods from Marine Isotope Stage (MIS) 11 through 3, when southern Arabia was affected by monsoonal precipitation. It is concluded that the aggradation of the alluvial fans was triggered by the interplay of increased sediment production during arid periods and high rainfall with enhanced erosion of hillslopes and transport rates during strong monsoon phases. However, the lack of fine-grained sediments, bioturbation and organic material implies that although the Quaternary fans are sourced by monsoonal rains they formed in a semi-arid environment. Thus, it appears that, in contrast to the Oman Mountains, the interior was not directly affected by monsoonal precipitation.     

Quaternary landscape evolution and erosion rates for an intramontane Neogene basin (Guadix–Baza basin, SE Spain)

The landscape evolution in Neogene intramontane basins is a result of the interaction of climatic, lithologic, and tectonic factors. When sedimentation ceases and a basin enters an erosional stage, estimating erosion rates across the entire basin can offer a good view of landscape evolution. In this work, the erosion rates in the Guadix–Baza basin have been calculated based on a volumetric estimate of sediment loss by river erosion since the Late Pleistocene. To do so, the distribution of a glacis surface at ca. 43 kyr, characterised by a calcrete layer that caps the basin infilling, has been reconstructed. To support this age, new radiometric data of the glacis are presented. The volume of sediment loss by water erosion has been calculated for the entire basin by comparing the reconstructed geomorphic surface and the present-day topography. The resulting erosion rates vary between 4.28 and 6.57 m³ ha^− 1 yr^− 1, and are the consequence of the interaction of climatic, lithologic, topographic, and tectonic factors. Individual erosion rates for the Guadix and Baza sub-basins (11.80 m³ ha^− 1 yr^− 1 and 1.77 m³ ha^− 1 yr^− 1 respectively) suggest different stages of drainage pattern evolution in the two sub-basins. We attribute the lower values obtained in the Baza sub-basin to the down-throw of this sub-basin caused by very recent activity along the Baza fault.     

Late Pleistocene tectonic-geomorphological development within a passive margin — The Cariatá trough, northeastern Brazil

Studies on the geomorphological evolution of the South American passive margin have been based on the pediplanation model, which predicts that its morphology is a response to regional uniform uplift and concomitant development of erosion surfaces. We combined remote sensing, geological mapping, lithostratigraphic and facies analyses, and luminescence dating in the Cariatá trough, northeastern Brazil, in order to determine how brittle tectonics and climate influenced colluviation and the shaping of local landforms in the Quaternary. Our work indicates that Cariatá is an asymmetrical trough  40 km long,  25 km wide, 250–550 m deep, and delimited by ENE–WSW-trending faults to the north and south. We recognized an ENE–WSW-oriented compression related to a strike-slip faulting regime, which corresponds to the present-day stress field in the region. This faulting event led to the deposition of colluvial fans, shed from adjacent uplifted crustal blocks, in a tectonically controlled depression. The colluvial succession is  45 m thick and presents two facies assemblages that filled the southern and, in particular, the northern borders of the trough: non-cohesive debrisflow and mudflow deposits. Optically stimulated luminescence dates of the sedimentary infill yielded ages at 224–128 ka and 45–28 ka, dominated by debrisflow and mudflow deposits, respectively. These ages may be over-estimated due to poor bleaching of colluvium, but they and our field data suggest that the margins of the trough were tectonically uplifted and eroded twice in the Late Pleistocene. The spasmodic colluvial accretion reflects the occurrence of high-magnitude, low-recurrence episodes probably associated with climate shifts in a semi-arid hillslope system. It follows that the present-day low-lying piedmont in which the Cariatá trough occurs is a juxtaposition of surfaces of various ages. This trough may have counterparts across the region. These conclusions do not corroborate the application of the cyclical pediplanation hypothesis in the area.     

月表形貌特征研究进展及趋势分析

在月球探测及科学研究过程中,对月表形貌特征的认识和分析是月球探测计划的关键任务之一,有助于理解和揭示月球的形态特征及空间分异规律,对于分析月球岩石构造、估算月表年龄、反演月壤厚度、恢复月球的起源和演化历史等都具有重要意义。从月表形貌特征的塑造及表达、宏观特征的分析与定量刻画、月表撞击坑的识别与分类及空间分异特征等几方面进行了归纳和分析,提出了未来月表形貌的研究方向,包括全月球形貌特征的多级分区划分、撞击坑类型的划分、全月球撞击坑空间分异特征及规律的研究、基于撞击坑空间分布探讨月球演化、基于形貌特征的比较行星学研究等。     

黄河中游河流阶地的形成与水系演化

本文据黄河中游各流域大量地质地貌实测资料，以连通各河流全线的各级阶地中,沉积物与古土壤的层位关系为主线，用年代学和古土壤断代法确定了阶地年代，阐述了四期九亚期水系演化史，并指出黄河中游及其主要支流诞生于更新世早中期167－145万年之间，之后发育了六级阶地和两级河漫滩。     

LIDAR monitoring of mass wasting processes: The Radicofani landslide, Province of Siena, Central Italy

The Radicofani Basin, stretching about 30 km NW–SE, is an intra-Central Apennine basin connected to Pliocene–Pleistocene extensional tectonics. It consists of an Early to Middle Pliocene succession including essentially shelf pelites. In the Radicofani area, province of Siena (Tuscany region), morphodynamic processes are very frequent with widespread badlands and rapidly evolving mudflows. In order to evaluate the general instability of the Radicofani area, geological and geomorphological surveys were carried out. The 1954, 1990 and 2003 aerial surveys allowed a comparison of the changes in the various morphological aspects of the study area, which suggested an increase in slope instability with time. A new complex translational landslide evolving into mudflows, activated during the winter of 2003, was monitored using an experimental system based on terrestrial LIDAR (Light Detection and Ranging) and GPS (Global Positioning System) technologies. This system allowed the monitoring of the morphologic and volumetric evolution of the landslide. A comparison of the monitoring data of October 2004, June 2005, May 2006 and May 2007 points out that the evolution is characterised by the sliding of displaced materials. A volume of about 1300 m³ of materials was removed during the period 2004–2005, 300 m³ for 2005–2006, and 400 m³ for 2006–2007. The greater initial mass movement probably reflects a greater static imbalance during the early period of landslide movement and increased rainfall. Therefore, the proposed monitoring system methodology allows the numerical evaluation of the landslide morphological evolution and to validate the landslide evolution model based on geological and geomorphological field surveys.     

Late Cenozoic geomorphologic signal of Andean forearc deformation and tilting associated with the uplift and climate changes of the Southern Atacama Desert (26°S–28°S)

We analyze remarkable examples of the large ( 10,000 km²) and local-scale ( 100 km²) landscape forms related to Late Cenozoic geomorphologic evolution of the Andean forearc region in the Southern Atacama Desert. We also consider the continental sedimentary deposits, so-called “Atacama Gravels”, which are related to the degradation of the landscape during the Neogene. Our analysis integrates 1:50,000 field cartography, Landsat TM images observations,  1:1000 sedimentary logging data, and 50 m horizontal resolution topographic data to reconstruct the Late Cenozoic geomorphologic evolution of this region and discuss the factors that control it, i.e., Miocene aridification of the climate and Neogene Central Andean uplift. We determine that the Precordillera was already formed in the Oligocene and most of the present-day altitude of the Precordillera was reached before that time. Afterward, five episodes of geomorphologic evolution can be differentiated: (1) the development of an Oligocene deep incised drainage system cutting the uplifted Precordillera (up to 2000 m of vertical incision) and connecting it to the Ocean; followed by (2) the infilling of deep incised valleys by up to 400 m of Atacama Gravels. This infill started in the Early Miocene with the development of fluvial deposition and finished in the Middle Miocene with playa and playa lake depositions. We propose that playa-related deposition occurs in an endorheic context related to tectonic activity of the Atacama Fault System and Coastal Cordillera uplift. However, the upward sedimentologic variation in the Atacama Gravels evidences a progressive aridification of the climate. Subsequently, we have identified the effects of the Middle–Upper Miocene slow tectonic deformation: the Neogene Andean uplift is accommodated by a tilting or flexuring of the inner-forearc (Central Depression and Precordillera) related to some hundreds of meters of uplift in the Precordillera. This tilting or flexuring results in (3) the Middle Miocene re-opening of the valley network to the Pacific Ocean. Upper Miocene aridification, from arid to hyperarid, induces alluvial fans backfilling in the Central Depression (4) resulting in up to 50 m of Atacama Gravel deposition. Finally, in response to an increase in the rate of tilting, a new phase of vertical incision (up to 800 m in the Precordillera) allows the development of the canyon that crosses the forearc (5).     

Climatically controlled river terrace staircases: A worldwide Quaternary phenomenon

A comparison of fluvial terrace sequences from around the world, based on data collected as part of International Geoscience Programme (IGCP) Project No. 449, has revealed significant patterns. River terraces provide important records of uplift, which is essential for their formation, and of landscape evolution. Their cyclic formation, however, almost invariably seems to have been a response to climatic fluctuation. Sequences in the European core area of IGCP 449, which has the longest and most extensive research history, have been used as templates for worldwide comparison. There is evidence for a global acceleration of uplift at the time of, and perhaps in response to, the Mid-Pleistocene Revolution, when climatic fluctuation switched to 100 kyr Milankovitch cycles. Terraces formed prior to this generally consist of wide aggradational sheets that probably each represent formation over several 41 kyr cycles. Subsequently, river valleys became more steeply entrenched and terraces formed in response to the stronger 100 kyr climatic forcing, in many cases at approximately one per cycle. This paper uses the new data resource to explore differences between records in different climate zones, between sequences with variable numbers of Middle–Late Pleistocene terraces and between systems in which the all-important incision event has occurred in different parts of climatic cycles. Key records discussed include European examples from the Rhine, Thames, Somme, Dniester, Dnieper, Don, Volga and Aguas; from Asia the Gediz (Turkey) and Orontes (Syria); from North America, the South Platte and Colorado; from South Africa the Vaal and Sundays; from Australia the Shoalhaven; and from South America, the Amazon, Paraguay and tributaries of the Colorado and Negro.     

Constraining provenance,thickness and erosion of nappes using low‐temperature thermochronology: the Northland Allochthon,New Zealand

The Northland Allochthon, an assemblage of Cretaceous–Oligocene sedimentary rocks, was emplaced during the Late Oligocene–earliest Miocene, onto the in situ Mesozoic and early Cenozoic rocks (predominantly Late Eocene–earliest Miocene) in northwestern New Zealand. Using low‐temperature thermochronology, we investigate the sedimentary provenance, burial and erosion histories of the rocks from both the hanging and footwalls of the allochthon. In central Northland (Parua Bay), both the overlying allochthon and underlying Early Miocene autochthon yield detrital zircon and partially reset apatite fission‐track ages that were sourced from the local Jurassic terrane and perhaps Late Cretaceous volcanics; the autochthon contains, additionally, material sourced from Oligocene volcanics. Thermal history modelling indicates that the lower part of the allochthon together with the autochthon was heated to ca. 55–100°C during the Late Oligocene and Early Miocene, most likely due to the burial beneath the overlying nappe sequences. From the Mesozoic basement exposed in eastern Northland, we obtained zircon fission‐track ages tightly bracketed between 153 and 149 Ma; the apatite fission‐track ages on the other hand, generally young towards the northwest, from 129 to 20.9 Ma. Basement thermochronological ages are inverted to simulate the emplacement and later erosion of the Northland Allochthon, using a thermo‐kinematic model coupled with an inversion algorithm. The results suggest that during the Late Oligocene, the nappes in eastern Northland ranged from ca. 4–6‐km thick in the north to zero in the Auckland region (over a distance >200 km). Following the allochthon emplacement, eastern Northland was uplifted and unroofed during the Early Miocene for a period of ca. 1–6 Myr at the rate of 0.1–0.8 km/Myr, leading to rapid erosion of the nappes. Since Middle Miocene, the basement uplift ceased and the erosion of the nappes and the region as a whole slowed down (ca. 0–0.2 km/Myr), implying a decay in the tectonic activity in this region.     

The role of neotectonics and glaciation on terrace formation along the Nysa K odzka River in the Sudeten Mountains (southwestern Poland)

The Nysa K odzka river drainage basin in the Sudeten Mts., SW Poland, preserves a complex late Cainozoic succession that includes eight fluvial series or terraces and deposits from two glacial episodes as well as local volcanic rocks, slope deposits and loess. Fluvial sedimentation took place during the Late Pliocene and from the early Middle Pleistocene (Cromerian), with a long erosion phase (gap) during the Early Pleistocene. Fluvial series are dated to the Late Pliocene, Cromerian, Holsteinian, late Saalian/Eemian, Weichselian, and the Holocene. Glacial deposits represent the early Elsterian and early Saalian stages. Almost all these stratigraphic units have been observed in all geomorphic zones of the river: the mountainous K odzko Basin, the Bardo Mts. (Bardo gorge) and in the mountain foreland. The main phase of tectonic uplift and strong erosion was during the Early Pleistocene. Minor uplift is documented also during the post-early Saalian and probably the post-Elsterian. The post-early Saalian and post-Elstrian uplift phases are probably due to glacio-isostatic rebound. The Quaternary terrace sequence was formed due to base-level changes, epigenetic erosion after glaciations and neotectonic movements. The Cromerian fluvial deposits/terraces do not indicate tectonic influence at all. All other Quaternary terraces indicate clear divergence, and the post-early Saalian terraces also show fault scarps. The fluvial pattern remained stable, once formed during the Pliocene, with only minor changes along the uplifted block along the Bardo gorge, inferring an antecedent origin for the Bardo gorge. Only during the post-glacial times, have epigenetic incisions slightly modified the valley.     

Shaking and splashing—A case study of far-field effects of the Mjølnir asteroid impact on depositional environments in the Barents Sea

The Mjølnir impact crater in the Norwegian Barents Sea features among the 20 largest impact craters listed in the Earth Impact Database. The impact is dated to 142 ± 2.6 Ma, corresponding closely to the Jurassic/Cretaceous boundary in the Boreal stratigraphy. Multidisciplinary studies carried out over the last three decades have suggested that the up to 40 km wide crater was created by a 1–3 km diameter impactor colliding with a shallow epicontinental sea, causing regional havoc and a regional ecological crisis that followed in its wake. Only minor evidence for the consequences of the impact for the surrounding depositional basins has been documented so far. This study describes a large submarine slump penetrated by hydrocarbon exploration well 7121/9-1, located in the southern Hammerfest Basin and approximately 350 km away from the impact site. The slump is dated by a black shale drape, which contains characteristic impact-related biotic assemblages and potential ejecta material. This precise dating enables us to associate the slump with large-scale fault movements and footwall collapse along the basin-bounding Troms-Finnmark Fault Complex, which we conclude were caused by shock waves from the Mjølnir impact and the passage of associated tsunami trains. The draping black shale is interpreted to represent significant reworking of material from the contemporary seabed by tsunamis and currents set up by the impact.     

Lower Cretaceous magnetic stratigraphy in Umbrian pelagic carbonate rocks

Summary. A record of geomagnetic field polarity for the Barremian, Aptian and Albian stages of the Early Cretaceous has been derived in three over-lapping sections of pelagic carbonate rocks in the Umbrian Apennines of northern Italy. The remanence carrier in the greyish-white Majolica limestone and Fucoid Marls is magnetite, with haematite also an important constituent in a zone of 'couches rouges' within the Fucoid Marls. The weak remanent magnetizations were measured with a cryogenic magnetometer. Alternating field or thermal demagnetization was used to isolate the characteristic remanent magnetization (ChRM) in 655 specimens from 248 stratigraphic levels. The samples respond positively to a tectonic fold test, indicating that the ChRM predates the Late Tertiary folding of the Umbrian sequence. The magnetic stratigraphy derived from variations of virtual geomagnetic pole latitude clearly defines the recognizable reversal pattern associated with Mesozoic marine magnetic anomalies M0 to M4. The sections have been zones palaeontologically on the basis of planktonic foraminifera and calcareous nannofossil assemblages. The ages of magnetic anomalies M0 to M4 determined in this way are somewhat older than those in the reversal time scale of Larson & Hilde (1975). Anomaly M0 is located in the Early Aptian, close to the Aptian/Barremian boundary. A long period of normal polarity in the Aptian and Albian corresponds to the early part of the Cretaceous magnetic quiet zone. It is interrupted in the Late Aptian by a reversal which we find in only one of the Fucoid Marl sections, and which has not been reported in oceanic magnetic anomaly investigations.     

Post‐breakup burial and exhumation of the southern margin of Africa

Despite many years of study, the processes involved in the development of the continental margin of southern Africa and the distinctive topography of the hinterland remain poorly understood. Previous thermochronological studies carried out within a monotonic cooling framework have failed to take into account constraints provided by Mesozoic sedimentary basins along the southern margin. We report apatite fission track analysis and vitrinite reflectance data in outcrop samples from the Late Jurassic to Early Cretaceous sedimentary fill of the Oudtshoorn, Gamtoos and Algoa Basins (Uitenhage Group), as well as isolated sedimentary remnants further west, plus underlying Paleozoic rocks (Cape Supergroup) and Permian‐Triassic sandstones from the Karoo Supergroup around the Great Escarpment. Results define a series of major regional cooling episodes. Latest Triassic to Early Jurassic cooling which began between 205 and 180 Ma is seen dominantly in basement flanks to the Algoa and Gamtoos Basins. This episode may have affected a wider region but in most places any effects have been overprinted by later events. The effects of Early Cretaceous (beginning between 145 and 130 Ma) and Early to mid‐Cretaceous (120–100 Ma) cooling are both delimited by major structures, while Late Cretaceous (85–75 Ma) cooling appears to have affected the whole region. These cooling events are all interpreted as dominantly reflecting exhumation. Higher Late Cretaceous paleotemperatures in samples from the core of the Swartberg Range, coupled with evidence for localised Cenozoic cooling, are interpreted as representing Cenozoic differential exhumation of the mountain range. Late Cretaceous paleotemperatures between 60°C and 90°C in outcropping Uitenhage Group sediments from the Oudtshoorn, Gamtoos and Algoa Basins require burial by between 1.2 and 2.2 km prior to Late Cretaceous exhumation. Because these sediments lie in depositional contact with underlying Paleozoic rocks in many places, relatively uniform Late Cretaceous paleotemperatures across most of the region, in samples of both basin fill and underlying basement, suggest the whole region may have been buried prior to Late Cretaceous exhumation. Cenozoic cooling (beginning between 30 and 20 Ma) is focussed mainly in mountainous regions and is interpreted as representing denudation which produced the modern‐day relief. Features such as the Great Escarpment are not related to continental break up, as is often supposed, but are much younger (post‐30 Ma). This history of post‐breakup burial and subsequent episodic exhumation is very different from conventional ideas of passive margin evolution, and requires a radical re‐think of models for development of continental margins.