Foreland Succession Research Articles

Sedimentology and structure of the Paleogene Nummulitic series of Corsica: Reconstruction of the southern termination of the western Alpine foreland basin

AbstractNummulitic Limestones deposits are preserved along the tectonic contact between the Variscan basement and Alpine units of Corsica. These marine carbonates, dated from the Late Palaeocene to the Middle Eocene, were deposited within a foreland flexural basin that is considered to be the southern continuation of the Alpine foreland basin of southeast (SE) France. However, in contrast with the Nummulitic Limestones of SE France, those of Corsica are far less documented. This field‐based study constrains the sedimentology, stratigraphy and structure of the Nummulitic Limestones of Corsica in three localities (Balagne, Corte and Sari‐Solenzara) to identify factors that controlled foreland basin development and to clarify its significance within the early alpine orogen. The microfacies, microfaunal assemblages and siliciclastic fractions are characterised throughout the succession at each locality. The results indicate the existence of an important Variscan basement relief to the west of the basin (West Corsican Massif) that supplied early alluvial fans found at the base of the foreland succession in the northernmost Balagne area. Continuous high clastic input strongly reduced the development and diversity of the overlying Nummulitic Limestones facies and fauna. Further south, limestones in the Corte and Sari‐Solenzara areas are thicker and contain richer fauna. Three depositional models corresponding to the carbonate ramp system are proposed for the Nummulitic Limestones and used to construct paleogeographic maps illustrating the transgressive evolution of the Corsican foreland basin from the Early to the Late Eocene. Based on our results and available regional tectonic data and LT thermochronological data, we propose that the Nummulitic marine transgression took place within a continuous foreland basin encompassing southern Corsica and SE France during the early development of the western alpine arc.

Changes in orogenic style and surface environment recorded in Paleoproterozoic foreland successions

The Earth’s interior and surficial systems underwent dramatic changes during the Paleoproterozoic, but the interaction between them remains poorly understood. Rocks deposited in orogenic foreland basins retain a record of the near surface to deep crustal processes that operate during subduction to collision and provide information on the interaction between plate tectonics and surface responses through time. Here, we document the depositional-to-deformational life cycle of a Paleoproterozoic foreland succession from the North China Craton. The succession was deposited in a foreland basin following ca. 2.50–2.47 Ga Altaid-style arc–microcontinent collision, and then converted to a fold-and-thrust belt at ca. 2.0–1.8 Ga due to Himalayan-style continent–continent collision. These two periods correspond to the assembly of supercratons in the late Archean and of the Paleoproterozoic supercontinent Columbia, respectively, which suggests that similar basins may have been common at the periphery of other cratons. The multiple stages of orogenesis and accompanying tectonic denudation and silicate weathering, as recorded by orogenic foreland basins, likely contributed to substantial changes in the hydrosphere, atmosphere, and biosphere known to have occurred during the Paleoproterozoic.

Depositional and tectonic influences on preservation of Milankovitch record during long-term global cooling: Middle and Upper Ordovician convergent foreland, eastern USA

Influences of depositional setting and tectonics on recording Milankovitch (orbital) signals within evolving Ordovician foreland successions during arc-continent collision are poorly documented. To address this, we analyzed downhole gamma ray vs depth time series from three wells on the Central Appalachian foreland, eastern USA for evidence of Milankovitch forcing during the 25 Myr duration Middle and Upper Ordovician Darriwilian to Katian interval. Relatively slowly accumulating Darriwillian sabkha anhydritic dolomites (up to 450 m thick), are dominated by eccentricity and obliquity forcing whereas Darriwillian to Sandbian restricted inner ramp carbonates (350 m thick) show eccentricity forcing only as do the overlying basal Katian (80 m thick) mid-ramp units. Rapidly accumulating lower Katian deeper ramp, shale-prone sediments (400 to 500 m thick) and upper Katian peritidal siliciclastic units (∼300–550 m thick) show a full suite of short eccentricity, obliquity and precession forcing. Best recording of the astronomical signal occurred during Katian rapid tectonic subsidence whereas a much poorer record was preserved during the Darriwilian-Sandbian. Milankovitch forcing of climate influenced input of siliciclastics as well as controlling sea-level fluctuations and position of storm wave-base. Initially sea level changes may have been driven by greenhouse aquifer eustasy but following later Darriwilian cooling, glacio-eustasy was dominant.

Testing Tectonostratigraphic Hypotheses of the Blountian Phase of the Taconic Orogeny in the Southern Appalachians through an Integrated Geochronological and Sedimentological Study of Ordovician K-Bentonites and Quartz Arenites

The tectonic setting of the southernmost part of the eastern margin of Laurentia during the Blountian tectophase (~472–452 Ma) of the Ordovician Taconic Orogeny remains unresolved. Tephras produced by explosive volcanism during this early phase of the orogeny are now K-bentonites, and in many locations, they are interbedded with mature to supermature quartz arenites. We conducted U-Pb analyses of detrital zircons from the sandstones, and of zoned magmatic zircons from the K-bentonites, to constrain the tectonostratigraphic setting with more precision. We also used geochemical fingerprinting of apatite phenocrysts to correlate the K-bentonites in these sandstones along the tectonic front, and we then integrated these results with a depositional systems study of the quartz arenites to further constrain and test competing models of the regional tectonomagmatic setting during that time. The general dearth of detrital zircons that have ages contemporaneous with the volcanic activity, coupled with the predominantly Precambrian ages of the zircons in these Lower Paleozoic quartz arenites that otherwise lack volcaniclastic components—such as detrital VRFs or a muddy matrix derived from an eroding volcanic arc—suggests that magmatic zircons from the tephra layers entered the depositional system only occasionally, and that the volcanic centers were separated geographically from where these quartzose sands were being deposited. Our findings support a tectonostratigraphic and tectonomagmatic model analogous to a combination of select modern settings in the western Pacific and Indonesia, specifically (1) New Guinea, where mature quartz arenites occur in the Cenozoic foreland succession, and (2) Sumatra, where the enormous Toba caldera formed in association with subduction beneath the Cretaceous-aged continental crust of Sumatra.

From rift to foreland basin: A case example from the Magallanes‐Austral basin, southernmost Andes

AbstractThis contribution characterizes primary lithologic and depositional components of the Magallanes‐Austral basin and defines infill geometries and stacking patterns from seismic and well data. An integrated seismic model is proposed for recognition of rifting, thermal sag and foreland tectono‐stratigraphic phases based on depositional geometries and its relation with the evolving deformational and geodynamic framework. Above a Middle–Late Jurassic extensional phase, evidenced by synrift depositional geometries, follow marine successions representing the subsidence thermal sag phase (Tithonian–Early Cretaceous) characterized by concordant and laterally extensive seismic reflectors. The following foreland phase is described through the evolution and lateral migration history of the foredeep depocentre and concomitant forebulge development. The foreland phase is represented by different stages characterized by asymmetric sedimentary wedges bounded by basal surfaces and/or major unconformities recording transitions from underfilled to overfilled conditions. The accumulated thickness due to lithospheric flexure reflects different foreland subsidence profile patterns across the southern depocentre of the Magallanes‐Austral basin, producing asymmetrical westward and southward thickening wedges. The first Foreland I stage (Coniacian?–Maastrichtian) is recorded as an asymmetric wedge infill, that thins cratonward, with a NW‐trending foredeep axis. The erosive basal foreland surface (BF) at its base deepens towards the west and south along the active margin of the basin, where subsidence was maximum. On top of it, along the western portion of the basin and with a source area from the north, deep‐marine slope deposits and turbiditic complexes were deposited; while on the forebulge to the east, a clastic platform developed. The Foreland II stage (early‐to‐middle Palaeocene–middle Eocene) is characterized by renewed uplift and flexure, and increasing tectonic subsidence rates, building a new clastic wedge‐shaped foreland succession next to the orogenic belt, and a well‐represented forebulge to the east. Subsequently, an extensive diachronous G7 unconformity was generated, eroding locally the previous foreland deposits towards the eastern margin. A pronounced and continuous NW‐SE trending deflection is established subtly to the east. The following Foreland III stage (middle to late‐Eocene–Oligocene) is characterized by a reduction in thrust load along the western active margin, and progradational systems towards the NE, a time during which the subsidence rate decreased and accommodation space was reduced. Deposition occurred within a wide and continuous NW‐SE trending foredeep without a marked forebulge. The top of this stage is the A1 unconformity, marking the beginning of the Foreland IV stage (early Miocene–Neogene), regarded as an overfilled basinal stage without a marked foredeep and major variations in thickness across the extent of the basin. The depositional pattern in this stage is largely conformal and tabular. The proposal model represents an evolutionary example for the internal geometry of deep‐marine foreland basin system, including variables such as tectonic load and flexural subsidence, accommodation space, sediment supply variation, and relative sea‐level fluctuations.

Soil microbial co-occurrence networks become less connected with soil development in a high Arctic glacier foreland succession

Classically, ecologists have considered that biota becomes more integrated and interdependent with ecosystem development in primary successional environments. However, recent work on soil microbial communities suggests that there may in fact be no change in network integration over successional time series. Here, we performed a test of this principle by identifying network-level topological features of the soil microbial co-occurrence networks in the primary successional foreland environment of the retreating high-Arctic glacier of Midtre Lovénbreen, Svalbard. Soil was sampled at sites along the foreland of inferred ages 10–90 years since deglaciation. DNA was extracted and amplicon sequenced for 16 s rRNA genes for bacteria and ITS1 region for fungi. Despite the chronologically-related soil pH decline and organic C/N accumulation, analysis on network-level topological features showed network integration did not change with inferred chronological ages, whereas network integration declined with decreasing pH and increasing total organic carbon (TOC) – both factors that can be viewed as an indicator of soil development. We also found that bacteria played a greater role in the network structure than fungi, with all keystone species in the microbial co-occurrence network being bacteria species. Both number and relative abundance of the keystone species were significantly higher when soil pH increased or TOC decreased. It appears that in the more extreme and less productive conditions of early primary succession, integration between members of soil biota into consortia may play a greater role in niche adaptation and survival. Our finding also emphasizes that ecosystem development is not simply a product of time but is influenced by locally heterogeneous factors.

Multi‐taxa colonisation along the foreland of a vanishing equatorial glacier

Retreating glaciers, icons of climate change, release new potential habitats for both aquatic and terrestrial organisms. High‐elevation species are threatened by temperature increases and the upward migration of lowlands species. Improving our understanding of successional processes after glacier retreat becomes urgent, especially in the tropics, where glacier shrinkage is particularly fast. We examined the successional patterns of aquatic invertebrates, ground beetles, terrestrial plants, soil eukaryotes (algae, invertebrates, plants) in an equatorial glacier foreland (Carihuairazo, Ecuador). Based on both taxonomical identification and eDNA metabarcoding, we analysed the effects of both environmental conditions and age of deglacierization on community composition. Except for algae, diversity increased with time since deglacierization, especially among passive dispersers, suggesting that dispersal was a key driver structuring the glacier foreland succession. Spatial β‐diversity was mainly attributed to nestedness for aquatic invertebrates, terrestrial plants and soil algae, likely linked to low environmental variability within the studied glacier foreland; and to turnover for soil invertebrates, suggesting competition exclusion at the oldest successional stage. Pioneer communities were dominated by species exhibiting flexible feeding strategies and high dispersal ability (mainly transported by wind), probably colonising from lower altitudes, or from the glacier in the case of algae. Overall, glacier foreland colonisation in the tropics exhibit common characteristics to higher latitudes. High‐elevation species are nevertheless threatened, as the imminent extinction of many tropical glaciers will affect species associated to glacier‐influenced habitats but also prevent cold‐adapted and hygrophilous species from using these habitats as refuges in a warming world.

Progressive evolution of thrust fold topography in the frontal Himalaya

The bulk of the crustal shortening across the Himalayas is accommodated across the Main Frontal Thrust (MFT), forming the topographic ridges of the Siwalik Hills. Here, we quantify topographic metrics to document the geomorphic response to progressive shortening. We use the integral method of channel profile analysis to assess the topographic response to the development of thrust anticline ridges in the eastern and western Churia, Dundwa, and Mohand Ranges. River channel steepness (ksn), the distribution of knickpoints, and channel concavity (θ) in the Siwalik Hills indicate geomorphic evolution during ongoing shortening across the deformation front. Mean channel steepness indices increase with increasing shortening over the Quaternary amongst the eastern Churia (~3.8 km shortening), Mohand (~6.9 km shortening), and Dundwa Ranges (~9 km shortening). Higher channel steepness at the western tip of the Churia Range (~0.4 km shortening) than in the eastern Churia and Mohand Ranges may indicate that the topography is still responding to the initial stages of shortening and relative uplift. Balanced cross sections reveal that there is spatial variability in the growth of these ranges, and as shortening increases, the drainage divide migrates southward (away from the mountains) relative to the flat-ramp transition of the underlying thrust; this results in steepening of the southward-draining catchments (basinward facing), and elongation of the northward flowing catchments. Basinward facing channels also have more prominent knickpoints and greater channel concavities caused by ongoing shortening between their fluvial base level and the drainage divide; this contrasts with the mountain facing channels where there is net growth of the distance between the drainage divide and the base level. Despite the asymmetry of erosional processes, the southward displacement of the drainage divide over the underlying thrust ramp drives topographic advection into the foreland succession which eventually leads to the propagation of new thrust-cored ridges.

Sequence stratigraphy of a Middle to Upper Ordovician foreland succession (Ottawa Embayment, central Canada): Evidence for tectonic control on sequence architecture along southern Laurentia

AbstractThe Middle to Upper Ordovician foreland succession of the Ottawa Embayment in central Canada is divided into nine transgressive‐regressive sequences that defines net deepening of a platform succession over ~15 m.y. from peritidal to outer ramp settings, then a return to peritidal conditions over ~3 m.y. related to basin filling by orogen‐derived siliciclastics. With a backdrop of net eustatic rise through the Middle to Late Ordovician, there are several different expressions of structural influence on sequence development in the embayment. During the Middle Ordovician (Darriwilian), foreland‐basin initiation was marked by regional onlap with abundant synsedimentary deformation across a faulted trailing‐margin platform interior; subsequent craton‐interior uplift resulted in voluminous influx of siliciclastics contemporary with local structurally influenced local channelization; then, a formation of a platform‐interior shale basin defines continued intrabasin tectonism. During the Late Ordovician (Sandbian, early Katian), structural influence was superimposed on sea‐level rise as indicated by renewed local development of a platform‐interior shale basin; differential subsidence and thickness variation of platform carbonate successions; abrupt deepening across shallow‐water shoal facies; and, micrograben development coincident with foreland‐platform drowning. These stratigraphic patterns are far‐field expressions of distal orogen development amplified in the platform interior through basement reactivation along an inherited buried Precambrian fault system. Comparison of Upper Ordovician (Sandbian‐lower Katian) sequence stratigraphy in the Ottawa Embayment with eustatic frameworks defined for the Appalachian Basin reveals greater regional variation associated with Sandbian sequences compared to regional commonality in base level through the early Katian.

Provenance evolution of Oligocene–Pliocene foreland deposits in the Dezful embayment to constrain Central Zagros exhumation history

This paper presents a detailed provenance analysis of the 6.5 km thick Oligocene–Pliocene foreland succession in the north Dezful embayment in Zagros, Iran. Using petrography, geochemistry and palaeoflow analysis, we determined three provenance shifts during the exhumation of the Zagros orogen. Above the Ahwaz sandstones, sourced from the Arabian craton, a sharp decrease in SiO 2 /Al 2 O 3 , Zr/Sc, Th/Sc, Th/Co and Th/Cr within the Gachsaran Fm. and the appearance of first-cycle sands derived from metamorphic and volcanic rocks at the base of the Aghajari Fm. characterize the onset of exhumation, rising topography and denudation of the Zagros orogenic system during the latest Oligocene and constitute the first shift. The second shift, in the middle Aghajari Fm., is marked by a distinct increase in radiolarian cherts and compatible/incompatible elements, indicating the exhumation of Neotethyan radiolarite–ophiolite allochthonous slices in the High Zagros Zone during the Tortonian ( c . 9 Ma). Large quantities of carbonate clasts derived from the nearby growing Izeh Zone anticlines characterize the Messinian ( c . 7 Ma) Bakhtyari conglomerates marking the third shift. In conclusion, a detailed hands-on provenance analysis of the clastic sedimentary sequences in the north Dezful embayment enabled us to determine catchment areas and shifts hitherto unstudied, thus contributing to the exhumation history of the Central Zagros orogenic system. Supplementary material: Details of the petrography and geochemistry results are available at https://doi.org/10.6084/m9.figshare.c.4905558

Provenance of the Newfoundland Appalachian foreland basins

The tectonic history of the Appalachian orogen is recorded in the adjacent foreland. Offshore seismic data suggest that an initial, Middle Ordovician foreland basin was filled by sources in Newfoundland, whereas the Late Ordovician foreland basin was loaded and filled by sources to the SW, in the Quebec segment of the orogen, where NW-vergent Taconian arc-continent collision may have continued later than in Newfoundland. We test this hypothesis using U/Pb ages of detrital zircon within foreland basin successions in western Newfoundland. Previously published results from the oldest foreland succession, the Middle Ordovician Goose Tickle Group, demonstrate ages similar to units preserved with the Humber Arm Allochthon, emplaced during Middle Ordovician Taconic orogenesis, including a predominant Paleoproterozoic peak at 1.85 Ga. In this study we investigated U/Pb geochronology of detrital zircon from younger foreland successions from Upper Ordovician to Devonian. The largest proportion of analyses within all foreland successions fall between 0.95 and 1.3 Ga, with largest peaks occurring between 1.0 and 1.1 Ga, typical of zircon derived from the Grenville Orogen. Earlier Mesoproterozoic and Paleoproterozoic ages range from 1.3 to 2.0. The abundance of Mesoproterozoic grains and conspicuous lack of 1.85 Ga Paleoproterozoic zircon in the overlying Upper Ordovician Long Point Group and latest Silurian to Early Devonian Clam Bank Formation indicates that these sediments were not derived from units within the Humber Arm Allochthon. Probability density plots of continental margin units in the Quebec/New England segment of the orogen demonstrate a similar strong Mesoproterozoic and weak Paleoproterozoic signature, suggesting derivation of the Long Point Group from the Quebec segment of the orogen. Within the mid-Paleozoic Clam Bank - Red Island Road succession, typical Gondwanan ages are absent and 1.0 Ga grains derived from the Grenville Orogen are abundant. This is consistent with underthrusting of the Gondwanan microcontinents Ganderia and Avalonia during Salinic and Acadian orogenesis. Only Mesoproterozoic zircon grains were found in the Early Devonian Red Island Road Formation, consistent with derivation from Mesoproterozoic Grenville massifs in western Newfoundland or Cape Breton Island which were exhumed during Devonian Acadian inversion.

δ13C stratigraphy of a Turinian-Chatfieldian (Upper Ordovician) foreland succession, Ottawa Embayment (central Canada): resolving local and inter-regional isotope excursions in a tectonically active basin

Positive δ13Ccarb excursions are correlated through an upper Turinian to lower Chatfieldian carbonate-platform succession along the axis of the Ottawa Embayment and into outliers of the northern Ottawa-Bonnechere graben in central Canada. Successive Turinian excursions (E1 and E2) are lithostratigraphically constrained by erosional surfaces and hosted within the Watertown and overlying L'Orignal formations, respectively, the latter coeval with the Selby Formation in the adjacent northern Appalachian Basin. The excursions coincide with periods of regional transgression, but geographic patterns of 13C depletion versus enrichment coincide with structurally defined areas of stratigraphically condensed and preferentially thickened formation successions, respectively. Differential subsidence is interpreted to have created bathymetric variation resulting in intrabasinal restriction of seawater exchange between these areas, with preferential Corg recycling with stratigraphic condensation. By early Chatfieldian time, segmentation of the once regional carbonate platform (L'Orignal Formation) produced a regional mosaic of low-energy muddy carbonate banks (Rockland Formation) and a deeper water platform (lower Hull Formation) settings subject to fluctuating high to low energy current flow. Excursion E3 occurs in both successions, but 13C enrichment is associated only with the bank-top muddy facies. This may identify preferential photosynthetic drawdown of 12C across the bank tops due to limited seawater exchange across the bank-deeper platform boundaries. Excursions E1 to E3, and a younger excursion (E4) in the Hull Formation, are correlated with varying confidence with excursions across southern Laurentia, excursion E3 being the local expression of the Guttenberg δ13C excursion. Our study supports local modulation of regional, if not global, δ13C excursions arising from structurally controlled changes in oceanography and productivity.

Significant Techno-stratigraphic development of the Caroni Basin, recorded in outcrops of the late Neogene Manzanilla and Springvale Formations, NE Trinidad

The Manzanilla and Springvale Formations outcrop along a 1.9 km stretch on the eastern coast of Trinidad, and provide vital information about the development of the Caroni Basin and north-central Trinidad. The Manzanilla Formation contains deposits that range from deepwater to delta plain facies. Sandstone turbidites are the oldest deposits and these transition upward into muddy slope deposits then shelf mudstones, siltstones and sandstones. The occurrence of sporadic conglomerate beds suggests debris flows or other coarse-grained sediment gravity flows additionally entering the basin at this time. The upper parts of the Manzanilla succession reflect marginal marine environments with delta front (including mouth-bar), crevasse splay, bay fill and marsh deposits. Most of these deposits originated as the northernmost part of the east to northeast flowing paleo-Orinoco Delta. The anomalous interfingering of coarse-grained lithic-rich debris flow deposits with the delta-derived deposits, strongly suggests a derivation directly from the steep-sloped alluvial-fan and fan-delta deposits eroding off of the Northern Range orogenic belt, some 10 km to the north. The facies associations of the Springvale Formation reflect channelized estuarine environments strongly influenced by tidal processes. Subtidal channels associate with and are succeeded by wave and tidal influenced intertidal facies and their overlying supratidal mudstones, paleo-sols and lignite beds.Examination of these two formations reveals that the Neogene Caroni Basin developed quite differently from the coeval Southern and Columbus basins to the south. The Southern and Columbus Basins are true foreland basins with growing accommodation space and expanded shelf section due to the movement of huge listric faults. When compared to similar aged formations in the Southern Basin, the Caroni Basin succession is relatively thin, starved of sediment, contains internal discontinuities and tectonically derived coarse-grained conglomerates. This contrast is due to the fact that the Caroni Basin is sited immediately adjacent to the northern Trinidad zone of Caribbean and South American plate-margin convergence. The compressional forces acting on the basin produced alternating periods of uplift/erosion and subsidence which inhibited continuous deposition. The outcrops examined herein present new detail of the tectonic impact on the evolving Caroni foreland basin within northern Trinidad, where compression-driven base level changes left their signals in the foreland succession.

Lithostratigraphy of the upper Turinian – lower Chatfieldian (Upper Ordovician) foreland succession, and a U–Pb ID–TIMS date for the Millbrig volcanic ash bed in the Ottawa Embayment

Three stages of carbonate-platform development are preserved in the upper Turinian – lower Chatfieldian succession of the Ottawa Group in the Ottawa Embayment and represent deposition along the Late Ordovician Taconic foreland interior of paleo-southern Laurentia. Compared with contemporary stratigraphy in the adjacent northern Appalachian (southern Ontario, New York state) and western Quebec basins, the intermediate Stage 2 succession, which brackets the Turinian–Chatfieldian boundary, preserves embayment-specific stratigraphic patterns. These include: (i) dramatic west-to-east thickening of the upper Turinian Watertown Formation that defines differential subsidence along the present axis of the embayment, (ii) post-Watertown base-level fall defined by appearance of shoreface siliciclastics, (iii) early Chatfieldian marine transgression represented by the proposed L’Orignal Formation that is coeval with but lithologically distinct from the Selby Formation in the northern Appalachian Basin, and (iv) platform segmentation that resulted in a depositional mosaic of shallow banks (Rockland Formation) and equivalent deeper water mico-seaways (lower Hull Formation). The latter event immediately follows accumulation of the Millbrig bentonite, here dated at 453.36 ± 0.38 Ma. Bracketing these local stratigraphic patterns are the bounding stages (1 and 3) represented by the upper Turinian Lowville Formation and middle Chatfieldian Hull Formation, respectively, that contain facies attributes in common with the adjacent basins and characterize inter-regional depositional systems of first warm, then cooler oceanographic conditions. Stage 2 identifies a structurally controlled transition between these end-member stages: a far-field response in the foreland interior, localized along the axis of a late Precambrian fault system, to contemporary change in subsidence rates and tectonomagmatic events along the Laurentian margin.

Tectonostratigraphic history of the southern Tian Shan, western China, from seismic reflection profiling

The modern Tian Shan formed due to the India-Eurasia collision. Uplift of the Tian Shan is recorded notably by the sedimentary sequences and subsidence history of the coupling foreland basins. We analyzed a 353-km-long seismic profile and logging data of four wells in the southern Tian Shan foreland area to decipher its tectonic, stratigraphic, and subsidence history. The sedimentary sequences comprise the Cambrian-Silurian, Devonian-Permian, Triassic, Jurassic-Cretaceous, and Paleogene tectonostratigraphic units, overlain by the late Oligocene-Quaternary foreland basin unit. The Jidike Formation is the oldest sedimentary sequence of the foreland succession, deposited at ∼26 Ma based on magnetostratigraphic constraints, indicating that uplift of the southern Tian Shan was initiated by at least ∼26 Ma. In addition, the tectonic subsidence rate of the southern Tian Shan foreland basin increased significantly since ∼26 Ma due to lithospheric flexure caused by building of the topography of the southern Tian Shan. The envelope line of the forelandward onlaping points within the foreland unit indicates that the forebulge of the southern Tian Shan foreland basin migrated southwards at 1.6 ± 0.1 mm/yr between ∼26 Ma and ∼12 Ma and at 14.6 ± 0.1 mm/yr after ∼12 Ma. The increase of migration rate at ∼12 Ma suggests an accelerated convergence between the Tarim Basin and the Tian Shan.

Limitations and Relevance of Biological Nitrogen Fixation during Postglacial Succession in Cordillera Darwin, Tierra del Fuego, Chile

ABSTRACTWe tested the main hypothesis that nutrient accumulation during late stages of postglacial succession would decrease nutrient limitation of diazotrophic activity. We tested this hypothesis by adding carbon (C), phosphorus (P), and molybdenum (Mo) independently or in combination, and nitrogen (N) only to symbiotic, epiphylls on bryophytes, and free-living diazotrophs in three stages of glacier foreland succession in Cordillera Darwin (55°S), southern South America. Experiments were run in spring 2013 and 2014 and in autumn 2015. Diazotrophic activity (DA) was assessed by the acetylene reduction assay. Results showed no effect of C, P, or Mo added either singly or in combination in the spring incubations. During autumn, DA was enhanced by adding a mix of C, P, and Mo to the symbiotic N2-fixing Gunnera magellanica from young successional sites, while in the late successional sites, adding C and Mo alone to the diverse bryophyte carpet on the forest floor enhanced DA. Nitrogen added as ammonium sulfate had a strong negative effect on N2 fixation by free-living diazotrophs in the spring and autumn samples from the late successional site, in the bryophyte carpet from the early successional site (autumn), and in Pseudocyphellaria freycinetii of the midsuccessional site (spring). As in other high-latitude biomes, symbiotic and epiphyllous associations and free-living diazotrophs play a crucial role in the incorporation of new N to postglacial subantarctic forest ecosystems, especially in recently exposed substrates that are strongly limited by nutrient availability in soils. The increasing rates of glacier melting in southern South America is exposing new substrates to microbial colonization, including diazotrophic bacteria. In this environment, largely free of reactive N from atmospheric sources, new ecosystems are rapidly developing on deglaciated surfaces, provided that key elements such as Mo and P and C are present in the substrates.

Andean stratigraphic record of the transition from backarc extension to orogenic shortening: A case study from the northern Neuquén Basin, Argentina

The temporal transition from backarc extension to retroarc shortening is a fundamental process in the evolution of many Andean-type convergent margins. This switch in tectonic regime is preserved in the 5–7 km thick Mesozoic-Cenozoic stratigraphic record of west-central Argentina at 34–36°S, where the northern Neuquén Basin and succeeding Cenozoic foreland succession chronicle a long history of fluctuating depositional systems and diverse sediment source regions during Andean orogenesis. New findings from sediment provenance and facies analyses are integrated with detrital zircon U-Pb geochronological results from 16 samples of Jurassic through Miocene clastic deposits to delineate the progressive exhumation of the evolving Andean magmatic arc, retroarc fold-thrust belt, and foreland province. Abrupt changes in provenance and depositional conditions can be reconciled with a complex Mesozoic-Cenozoic history of extension, postextensional thermal subsidence, punctuated tectonic inversion, thick- and thin-skinned shortening, overlapping igneous activity, and alternating phases of basin accumulation, sediment bypass, and erosion.U-Pb age distributions constrain the depositional ages of Cenozoic units and reveal a prolonged late middle Eocene to earliest Miocene (roughly 40–20 Ma) hiatus in the retroarc foreland basin. This stratigraphic gap is expressed as a regional disconformity that marks a pronounced shift in depositional conditions and sediment sources, from (i) slow Paleocene–middle Eocene accumulation of distal fluviolacustrine sediments (Pircala and Coihueco Formations) contributed from far western magmatic arc sources (Cretaceous–Paleogene volcanic rocks) and subordinate eastern basement rocks (Permian-Triassic Choiyoi igneous complex) to (ii) rapid Miocene–Quaternary accumulation of proximal fluvial to megafan sediments (Agua de la Piedra, Loma Fiera, and Tristeza Formations) recycled from emerging western thrust-belt sources of Mesozoic basin fill originally derived from basement and magmatic arc sources. The mid-Cenozoic stratigraphic gap signified ∼20 Myr of nondeposition, potentially during passage of a flexural forebulge or during neutral to extensional conditions driven by mechanical decoupling and a possible retreating-slab configuration along the Nazca-South America plate boundary. Neogene eastward propagation of the Malargüe fold-thrust belt involved basement inversion with geometrically and kinematically linked thin-skinned shortening at shallow foreland levels, including late Miocene deposition of accurately dated 10.5–7.5 Ma growth strata and ensuing displacement along the frontal emergent and blind thrust structures. Subsequent partitioning and exhumation of Cenozoic clastic fill of the Malargüe foreland basin has been driven by inboard advance of arc magmatism and Pliocene-Quaternary uplift of the San Rafael basement block farther east.

Des​criptive text to the Geological map of Greenland, 1:500 000, Lambert Land, Sheet 9

The Lambert Land 1:500 000 scale geological map sheet covers a segment of the East Greenland Caledonian orogen extending between latitudes 78°–81°N and longitudes 13°–29°W. The region was geologically mapped in the summers of 1978–1980 and 1993–1995 as part of a regional Survey mapping programme, and the map sheet was printed in 2000. The region covered by the Lambert Land map sheet comprises a northern segment of the East Greenland Caledonides that is in thrust contact with the Caledonian foreland. The contact is an Edipping, NNE–SSW-trending thrust that runs through Kronprins Christian Land from west of Blåsø in the south to the region east of Sjælland Fjelde in the north. The Caledonian foreland succession is exposed in Mylius-Erichsen Land and Amdrup Højland on the west side of Danmark Fjord, and in western Kronprins Christian Land on the east side of Danmark Fjord. The several kilometre thick foreland succession ranges in age from Palaeoproterozoic to Silurian. In western Kronprins Christian Land the undeformed Ordovician–Silurian carbonates of the foreland pass eastwards into the so-called Thinskinned fold-and-thrust belt developed in the same strata that farther eastwards are structurally overlain by the allochthonous Vandredalen thrust sheet. This thrust sheet is in its turn overlain by a group of thrust sheets that make up the mountainous high ground of central parts of Kronprins Christian Land, and structurally form the so-called Western thrust belt; these rock units have a deeper crustal origin. In the extreme east the Nørreland thrust sheet is exposed in a coastal strip that comprises Palaeoproterozoic crystalline gneisses that include eclogite-facies enclaves and units of meta-igneous and metasedimentary rocks.
 Post-Caledonian rocks crop out in the north-eastern coastal parts of the map sheet area, and include sedimentary rocks of early Carboniferous to late Jurassic age that form part of the Wandel Sea Basin succession. Undifferentiated Quaternary deposits, ground moraine, alluvial fans and talus are distinguished where they obscure significant areas of bedrock.

Des​criptive text to the Geological map of Greenland, 1:500 000, Lambert Land, Sheet 9

NOTE: This Map Description was published in a former series of GEUS Bulletin. Please use the original series name when citing this series, for example: Higgins, A. K. (2015). Des​criptive text to the Geological map of Greenland, 1:500 000, Lambert Land, Sheet 9. Geological Survey of Denmark and Greenland Map Series 7, 1-29. https://doi.org/10.34194/geusm.v7.4563 _______________ The Lambert Land 1:500 000 scale geological map sheet covers a segment of the East Greenland Caledonian orogen extending between latitudes 78°–81°N and longitudes 13°–29°W. The region was geologically mapped in the summers of 1978–1980 and 1993–1995 as part of a regional Survey mapping programme, and the map sheet was printed in 2000. The region covered by the Lambert Land map sheet comprises a northern segment of the East Greenland Caledonides that is in thrust contact with the Caledonian foreland. The contact is an Edipping, NNE–SSW-trending thrust that runs through Kronprins Christian Land from west of Blaso in the south to the region east of Sjaelland Fjelde in the north. The Caledonian foreland succession is exposed in Mylius-Erichsen Land and Amdrup Hojland on the west side of Danmark Fjord, and in western Kronprins Christian Land on the east side of Danmark Fjord. The several kilometre thick foreland succession ranges in age from Palaeoproterozoic to Silurian. In western Kronprins Christian Land the undeformed Ordovician–Silurian carbonates of the foreland pass eastwards into the so-called Thinskinned fold-and-thrust belt developed in the same strata that farther eastwards are structurally overlain by the allochthonous Vandredalen thrust sheet. This thrust sheet is in its turn overlain by a group of thrust sheets that make up the mountainous high ground of central parts of Kronprins Christian Land, and structurally form the so-called Western thrust belt; these rock units have a deeper crustal origin. In the extreme east the Norreland thrust sheet is exposed in a coastal strip that comprises Palaeoproterozoic crystalline gneisses that include eclogite-facies enclaves and units of meta-igneous and metasedimentary rocks. Post-Caledonian rocks crop out in the north-eastern coastal parts of the map sheet area, and include sedimentary rocks of early Carboniferous to late Jurassic age that form part of the Wandel Sea Basin succession. Undifferentiated Quaternary deposits, ground moraine, alluvial fans and talus are distinguished where they obscure significant areas of bedrock.

Paleoenvironments of the evolving Pliocene to early Pleistocene foreland basin in northwestern Taiwan: An example from the Dahan River section

AbstractThe overriding of the Luzon volcanic arc atop the underlying Chinese rifted‐continental margin has caused the formation of the Taiwan mountain belts and a peripheral foreland basin west of the orogen since the late Miocene. In this study, lithofacies analysis and calcareous nannofossil biostratigraphic investigations of the Dahan River section in northwestern (NW) Taiwan were performed. Our results offer insights into the temporal evolution of the sedimentary environments and the competing effects of the sedimentation and basin tectonics of the NW Taiwan foreland basin from the Pliocene to early Pleistocene. Nannofossil biostratigraphic studies showed that the upper Kueichulin Formation and the overlying Chinshui Shale can be assigned to the NN15 biozone of the Pliocene age, and the Cholan Formation pertains to NN16–NN18 of the early Pleistocene. The NN15–NN16 boundary coincides roughly with the boundary of the Chinshui Shale and Cholan Formation. We recognized three major sedimentary environments in the studied foreland succession comprising the upper Kueichulin Formation, Chinshui Shale, Cholan Formation and Yangmei Formation, in ascending order. During the deposition of the upper Kueichulin Formation in the early Pliocene, the dominant environment was a wave‐ and tide‐influenced open marine setting. During the late Pliocene, the environment deepened to an outer‐offshore setting when the sediments of Chinshui Shale were accumulated. In the Pleistocene, the environment then shallowed to wave‐dominated estuaries during the deposition of the lower Cholan Formation, and the basin was rapidly filled, generating a meandering and sandy braided river environment during the deposition of the upper Cholan to the Yangmei Formation. In summary, the evolution of sedimentary environments in the studied succession shows a deepening then a shallowing and coarsening upward trend during the period from the Pliocene to the Pleistocene, spanning the age from approximately 4 to 1 Ma.