White sand vegetation in an Amazonian lowland under the perspective of a young geological history

What controls the formation of patchy substrates of white sand vegetation in the Amazonian lowlands is still unclear. This research integrated the geological history and plant inventories of a white sand vegetation patch confined to one large fan-shaped sandy substrate of northern Amazonia, which is related to a megafan environment. We examined floristic patterns to determine whether abundant species are more often generalists than the rarer one, by comparing the megafan environments and older basement rocks. We also investigated the pattern of species accumulation as a function of increasing sampling effort. All plant groups recorded a high proportion of generalist species on the megafan sediments compared to older basement rocks. The vegetation structure is controlled by topographic gradients resulting from the smooth slope of the megafan morphology and microreliefs imposed by various megafan subenvironments. Late Pleistocene-Holocene environmental disturbances caused by megafan sedimentary processes controlled the distribution of white sand vegetation over a large area of the Amazonian lowlands, and may have also been an important factor in species diversification during this period. The integration of geological and biological data may shed new light on the existence of many patches of white sand vegetation from the plains of northern Amazonia. © 2019, Academia Brasileira de Ciencias. All rights reserved

Unfolding long-term Late Pleistocene–Holocene disturbances of forest communities in the southwestern Amazonian lowlands

Linking the distribution of plant species to geology has generally been biased by the over-simplification of landscape evolution and the lack of understanding of complex geological processes. The Amazonian lowlands have forests in different successional stages, and a growing perception is that such heterogeneity results from long-term environmental changes. This hypothesis is investigated by designing an analytical model based on past and present-day vegetation and successions of the plant communities, combined with an advanced understanding of geological history. An area of southwestern Amazonia was selected for floristic inventories, and we interpreted the paleovegetation based on C/N and δ13C analyses of sedimentary organic matter. These data were examined in the context of the geological evolution on the basis of new sedimentological and chronological data. The topographically high Late Pleistocene deposits had continuous and highly diversified late-successional terra firme forests as well as local fluvial paleolandforms of younger ages with less diversified campinarana forests. Late Pleistocene–Holocene terrains in intermediate elevations had terra firme forests, but shorter trees with lower basal areas and ecotonal forests appeared near the confines of the forest–savanna, while Holocene deposits recorded only seasonally flooded varzea forests. Several deposits of Late Pleistocene and Late Pleistocene–Holocene age recorded an expansion of C4 terrestrial plants before the establishment of the forest from ~20,000 cal yr BP to 7578 cal yr BP, which is not related to past arid episodes. We recorded forests with onsets at 6130–3533 cal yr BP, 3402–2800 cal yr BP, and 1624–964 cal yr BP to terra firme, varzea, and ecotonal forests, respectively. However, not all forests have reached maturity stages due to their location on terrains with a diverse history of terrace downcutting and deposition, which had a direct impact on local hydrology with the interaction of topographic gradients. The hydrology of the study area was also controlled by the distance from the main river valley. Capturing long-term disturbances over this region of still pristine forests may help elucidate the potential mechanisms that also determine trends in tree growth and forest diversity in other Neotropical regions. © 2018 The Authors

Survival of ancient landforms in a collisional setting as revealed by combined fission track and (U-Th)/He thermochronometry: A case study from Corsica (France)

The age of high-elevation planation surfaces in Corsica is constrained using new apatite (U-Th)/He data, field observations, and published work (zircon fission track, apatite fission track [AFT] data and landform/stratigraphical analysis). Thermal modeling results based on AFT and (U-Th)/He data, and the Eocene sediments uncomformably overlapping the Variscan crystalline basement indicate that present-day elevated planation surfaces in Corsica are the remnants of an erosion surface formed on the basement between ∼120 and ∼60 Ma. During the Alpine collision in the Paleocene-Eocene, the Variscan crystalline basement was buried beneath a westward-thinning wedge of flysch, and the eastern portion was overridden by the Alpine nappes. Resetting of the apatite fission track thermochronometer suggests an overburden thickness of >4 km covering Variscan Corsica. Protected by soft sediment, the planation surface was preserved. In the latest Oligocene to Miocene times, the surface was re-exposed and offset by reactivated faults, with individual basement blocks differentially uplifted in several phases to elevations of, in some cases, >2 km.Currently the planation surface remnants occur at different altitudes and with variable tilt. This Corsican example demonstrates that under favorable conditions, paleolandforms typical of tectonically inactive areas can survive in tectonically active settings such as at collisional plate margins. The results of some samples also reveal some discrepancies in thermal histories modeled from combined AFT and (U-Th)/He data. In some cases, models could not find a cooling path that fit both data sets, while in other instances, the modeled cooling paths suggest isothermal holding at temperature levels just below the apatite partial annealing zone followed by final late Neogene cooling. This result appears to be an artifact of the modeling algorithm as it is in conflict with independent geological constraints. Caution should be used when cross-validating the AFT and (U-Th)/He systems both in the case extremely old terrains and in the case of rocks with a relatively simple, young cooling history

GONDWANA PALEOLANDSCAPES: LONG-TERM LANDSCAPE EVOLUTION, GENESIS, DISTRIBUTION AND AGE

“O conceito de “Paisagem Gondwânica” foi definido por Fairbridge (1968) como uma “paisagem ancestral” composta por “séries remanescentes de planícies” “que registram traços de episódios de aplainamento mais antigos”, durante o Mesozóico superior (localmente Jurássico ou Cretáceo). Este conjunto de planícies foi chamado de superfície terrestre cíclica gondwânica nos continentes do hemisfério sul. Remanecentes destas superfícies são encontrados também na Índia, no hemisfério norte, e se assume também terem sido presercadas na Antártida Oriental, por baixo da camada de gelo, que cobre a região com espessura média de 3000 metros. Estas paleopaisagens foram geradas quando o antigo supercontinente Gondwana estava agrupado e condições tectônicas similares em seus fragmentos à derica teriam permitido sua preservação. Restos de superfícies equivalentes, embora em condições muito fragmentadas, têm sido descritas na Europa e nos Estados Unidos. Estas superfícies gondwânicas planas são características de regiões cratônicas, às quais tem sobrevivido na paisagem sem ter sido recobertas por sedimentos marinhos ao longo de tempos muito prolongados, tendo sido expostas a intemperismo subaéreo e denundação. Sua gênese está relacionada com paleoclimas extremamente úmidos e quentes de natureza "hiper-tropical", com solos permanentemente saturados de água, ou talvez climas paleomonçônicos extremos, com flutuações cíclicas ou estacionárias, desde extremamente úmidos a extremamente secos. Meteorização química profunda é o processo geomorfológico dominante, com o desenvolvimento de perfís de alteração extremamente profundos, talvez de até centenas de metros de profundidade. Os produtos de alteração são argila, caulinita, quartzo puro e outras formas de areias silicosas, com a eliminação de todos os outros minerais e formação de duricrostas, tais como ferricretas (ferro), silcretas (de sílica) e calcrete (carbonato de cálcio). A precipitação anual nestes períodos talvez tenha sido mais elevada do que 10.000 mm, com temperaturas médias anuais extremamente altas, talvez entre 25 e 30oC. Isto poderia ter sido alcançado sob condições tectônicas e climáticas extremamente estáveis. Os processos geomorfológicos incluem pediplanização extensa sob climas úmidos/semi-áridos e/ou com mudaças sazonais. Finalmente, sua evolução continuou com a remoção fluvial dos produtos de intemperismo em climas úmidos e com deflação hidro-eólico nas áreas com ambientes semi-áridos ou forte sazonalidade climática. Os produtos finais da paisagem destes sistemas de meteorização profunda/pediplanação são superfícies aplaindas, planícies gravadas, inselbergs, bornhardts, duricrostas remanescentes que cobrem nesetas e pediplanos associados, paisagem de granitos meteorizados, etc. Alguns conceitos relativos a esses sistemas de paisagem antigos foram desenvolvidos teoricamente por Walther Penck no início do século 20. As paleopaisagens do Gondwana foram estudadas por Alexander Du Toit e Lester C. King, na África e, mais recentemente, por Timothy Partridge e Maud Rodneyna, na África do Sul, C. Rowland Twidale e Cliff Ollier na Austrália, e Lester C. King e João José Bigarella, no Brasil, entre muitos outros. Tanto na Austrália como na África do Sul estes sistemas de paisagem foram identificados como sendo formados no Jurássico médio ao superior, por todo o Cretáceo e, em alguns casos, estendidos no Paleógeno, quando o Gondwana ainda era apenas parcialmente desmembrado. Palavras chave: Supercontinente Gondwana, paleopaisagens, evolução da paisagem a longo prazo, Hemisferio Sul, Jurássico, Cretácico

Long-term landscape evolution, cooling and exhumation history of Variscan rocks in the western Cantabrian Mountains (NW Spain)

The present study introduces the first comprehensive regional research of apatite-fission track (AFT) and apatite (U-Th-Sm)/He(AHe) thermochronology including state-of-the-art 3D thermokinematic modelling of longterm landscape evolution in the western termination of the Cantabrian Mountains (NW Spain). The study sets out to analyse and interpret thermochronological data to constrain the pattern and history of cooling and exhumation in the NW Iberian Massif. This allows to better resolve the potential coupling of tectonic and climatic events and their impact on long-term landscape evolution. In terms of tectonic evolution the Cantabrian Mountains are a well-studied area, providing a well-suited domain to examine the impact of tectonic events on the topography. The mountains reach elevations of more than 2,600 m along the northern coast of Spain and are composed of a Variscan crustal section uplifted in the course of Cenozoic shortening along the northern Iberian Plate. Since the end of the Variscan orogeny in Late Palaeozoic, the Iberian Massif was affected by two major tectonic episodes, (1) Mesozoic rifting that lead to continental break-up by 115 Ma and opening of the Atlantic and Bay of Biscay to the West and North, and (2) limited convergence along the Bay of Biscay margin from Middle Eocene times onwards. The study constrains the pattern and history of exhumation within the Paleozoic bedrock and Variscan intrusions over the past c. 250 Ma. Thirty-five AFT samples reveal ages from 246.7 (26.9) to 68.1 (5.0) Ma, with mean track lengths between 10.4 (1.2) and 12.6 (1.8)µm. Six AHe samples range in age from 70.6 (5.2) to 114.4 (14.2) Ma. Timetemperature path modelling of the data indicates that continuous cooling at different rates took place during the main tectonic events that affected the area. A rapid cooling event that ended by Late Jurassic corresponds to topographic decay during unroofing of the Variscan orogen and the break-up of Pangea, and is responsible for the largest amount of exhumation at a rate of c. 0.3 km/Ma. Samples in Galicia cooled contemporaneously with rifting in the North Atlantic and Bay of Biscay during Late Jurassic to Early Cretaceous at exhumation rates of c. 0.25 km/Ma. By about 80 to 100 Ma most samples cooled below 60 °C, indicating that regional denudation has not exceeded c. 1.7 km since then, for geothermal gradients ≥ 27 ºC/km and a surface temperature of 15 °C. An extensive, low relief area in Central Galicia underwent very slow exhumation (0.02 km/Ma) since post-rift stage (80 to 100 Ma), and is interpreted as the remains of a pre-Eocene paleolandscape. Surface uplift of a ridge next to the northern coastline since late Middle Eocene caused minor exhumation during activation of the North Iberian margin. This ridge that reaches heights up to 1,000 m seems to be associated to reactivation of an inherited 60 to 80 Ma old escarpment. An average exhumation rate between 0.02 to 0.07 km/Ma reflects latest denudation as the new mountainous relief developed since incipient subduction along the northern Iberian Margin by 46 Ma due to shortening associated with convergence along the northern Iberian Plate. Reasonable estimates on the initial maximum mean elevation of the area after the end of the Variscan orogeny are determined between 2,400 and 3,400 m by 3D thermokinematic modelling. The strong coincidence between timing of major tectonic events and changes in topography and exhumation rates suggest that the major controlling factor of landscape evolution in this area is tectonic forcing while climatic effects have probably only a second order impact

11- Ville de Goyet F. de, Breuer P., Gerrienne P., Prestianni C., Streel M. & Steemans P. (2007).- Middle Devonian (Givetian) megaspores from Belgium (Ronquières) and Libya (A1-69 borehole).

A set of 11 abstracts and short papers (extended abstracts) of presentations given at a meeting organized by the NFSR Working Group, "Micropaléontologie végétale et Palynologie (MVP)", held on May 24, 2006 at the University of Liège, Belgium. Most contributions are available only in English