Jean-Pierre Peulvast

Palaeolandforms and morphotectonic evolution around the Baie des Chaleurs (eastern Canada)

Abstract A morphological study of the Baie des Chaleurs area, between northern New Brunswick and the Gaspe Peninsula (eastern Canada), leads to the identification of several types of palaeolandforms in the landscapes of the northeastern Appalachians. One of them, the exhumed sub-Carboniferous palaeosurface, was recognized along the shores of the Baie des Chaleurs, around the western part of the Carboniferous basin — the Maritimes Basin — which underlies the Gulf of Saint Lawrence. By analyzing the conditions of its exhumation and its relationships with escarpments limiting the higher planation surfaces of inner Gaspesie and western New Brunswick, it was possible to identify recent deformations. These are en bloc tilting and uplift, flexuring, and faulting; they partly reflect the reactivation of Carboniferous or older structures. The study of palaeolandforms has already proved to be an appropriate method for reconstructing the morphological evolution of basement areas. In this paper, it is applied to the study of the evolution of an emerged part of the eastern Canada rifted margin after the opening of the North Atlantic Ocean.

Landforms and Neotectonics in the Equatorial Passive Margin of Brazil

Neotectonic movements have been proposed in the literature in order to explain some landforms of the equatorial passive margin of northeastern Brazil. Its seismo-tectonic activity is concentrated in a few sectors located on or near the coast. Active or recently active structures are mainly identified in Neogene deposits. Identifying a contribution of neotectonics to the morphogeny is difficult in a context where most major morphostructural patterns are explained by Cretaceous tectonics related to oceanic opening and by differential erosion induced by Tertiary epeirogenic uplift. We aim to assess the nature of features considered as possibly neotectonic in origin. Seismogenic faults are not related to significant topographic breaks, except on the coast, where they usually reach only a few meters in height. A study of landforms located near zones of seismo-tectonic activity indicates a possible, probably weak, contribution of neotectonics to the formation of a few high scarps. These scarps occur along or near fault zones reactivated in Cretaceous times. We conclude that neotectonic movements are the result of ongoing deformation along predominantly strike-slip fault zones, with long term deformation rates similar to those recorded by dated landmarks (0.01 mm.yr -1). Despite reported deformation rates that can amount in places to 0.4 mm.yr -1, neotectonic rates are lower than erosion rates. The consequence is that major structural landforms in the region mainly originated in Cretaceous to early Tertiary events.

Late Cenozoic and present-day hillslope erosion dynamics in a passive margin context: stability or instability? Case studies in Northeast Brazil

Steep slopes form the marginal scarp of the semi-arid northern Brazilian “Nordeste”, above depressions or corridors connected to a low erosion plain merging seaward with a coastal erosional-gradational piedmont. Dissected pediments and sparse debris fans visible at the base of some escarpments suggest recent erosive activity and possible slope retreat, although most of these forms are decoupled from major valleys. We describe the morphostructural patterns of chosen escarpments, in the basement as well as in the sedimentary cover, and the morphodynamic conditions of their evolution since the Miocene. Only a few scarps show strong structural control. Many of them are mainly inherited landforms, initiated during the Early Cretaceous rifting or the later margin uplift, a situation which suggests long-term stability. Later on, except in one case (the Araripe scarp), only slight or local backwearing took place, associated with downwearing on low surfaces and pediments, probably in diachronic ways. The Neogene clastic sedimentation on piedmonts and coastal areas mainly reflects the occurrence of dry periods inducing widespread stripping of deep soil horizons and erosion of bare rock slopes and surfaces. Dissection stages occurred in periods of more humid climate and/or low sea level. Marks of strong recent of present activity are mainly registered in the rims of the Chapada do Araripe, owing to favourable structural, hydrogeological and climatic conditions. The moderate volumes of Neogene clastic sediments imply overall low uplift and erosion rates until the Present, favourable to morphological and lithological resistance effects in the landscapes. However, slope instabilities are not uncommon, locally leading to well characterized processes, landforms and deposits of gullying and mass wasting. Therefore, hazards related with slope processes never may be neglected, although only small scale events were recorded in recent times, and times of recurrence of larger events are probably much longer than the historic times.

Meso-Cenozoic Paleotopographies and Paleolandscapes in the Deseado Massif (Santa Cruz Province, Argentina)

The Deseado Massif is the southernmost part of a continent, outside of Antarctica, where Gondwana Landscapes may be observed and investigated. This chapter presents preliminary observations and field data about the Gondwana Landscapes of this cratonic area of Southern Argentina, one of the most remote, isolated, and less populated places on Earth. Under extreme cold–arid climate conditions, the region presents very scarce vegetation cover, which further enables the geomorphological observations. Remnants of planation surfaces of undisputable Late Mesozoic age, developed on Jurassic volcanic units and covered by Late Cretaceous and Paleogene sedimentary rocks, are exposed along tens of thousands square kilometers of this cratonic unit. In those remote times, the climate of this portion of Patagonia was very wet and warm, allowing the development of extensive chemical weathering.

Landforms and Landscape Evolution of the Equatorial Margin of Northeast Brazil

More than a simple monograph, the authors present a comprehensive geomorphic overview of a large tropical region where they show how deciphering the long-term landform evolution helps understanding the present set of landscapes and morphodynamic environments. The Equatorial margin of the Brazilian “Nordeste” displays stratigraphic landmarks whose interpretation reveals the age and nature of landforms, leading to a reconstruction of the geomorphic history by the means of combined morphostratigraphic and morphopedological approaches. Beyond the role of differential erosion related to moderate post-oceanic opening uplift, the plain and upland landscape reflects a juxtaposition of landform and soil generations related to a shallow basin inversion, the last stages of which occurred in semi-arid conditions since the Oligocene. These results throw light on old debates on models of long-term landform development in platform areas, and also help evaluating recent models of denudation and burial based on thermochronological methods

Morphostructural patterns and landform generations in a glaciated passive margin: the Kobberminebugt-Qaqortoq region of South Greenland

On the west coast of South Greenland, the passive continental margin displays a left-lateral step which is not apparent on the east coast and coincides with a topographic change that marks the transition between Archaean basement rocks to the north and the Ketilidian orogenic belt to the south. Two issues are considered: (1) the origin of the regional topographic asymmetry between the mountains of southeast and southwest Greenland; (2) the origin of the Kobberminebugt-Qaqortoq lowlands (differential early Cenozoic tectonic movements and/or subsequent differential erosion in the uplifted landmass). By identifying and mapping stepped landforms, morphostructural patterns and weathering features, we discriminate between control by differential erosion and tectonic movements. We suggest that the stepped patterns sof erosion surfaces represent a pre-glacial landform evolution related to the uplift of two antiforms along the west and east coast of southern Greenland. In addition, late fault reactivation may have favoured the development of a low coastal surface in less uplifted blocks. A relative chronology of landform development in the study area is given. Although implying a strong involvement of differential erosion, our interpretation of the geomorphology is compatible with a late reactivation of an oblique transfer zone formed during the early Cenozoic rifting.

Araripe Basin: A Major Geodiversity Hotspot in Brazil

In addition to a Fossil-Lagerstätte of international significance for the Lower Cretaceous, the Araripe basin is one of the richest and most threatened reservoirs of geodiversity in Brazil. Far from being limited to its palaeontological heritage, the major importance of geodiversity in the Araripe region is also related to high levels of geomorphodiversity, pedodiversity, and hydrodiversity, as evidenced by recent research. However, numerous threats and severe damages were identified in the field, affecting all the components of the abiotic nature. As a major geodiversity hotspot, the Araripe basin requires greater attention along with an urgent need for conservation in areas without adapted protection tools. The creation of the Araripe UNESCO Global Geopark in 2006 was a crucial step toward geodiversity conservation, but its present borders are far from covering the Araripe basin as a whole. This implies the search for new solutions or alternatives, mainly in the field of geoeducation, to raise geodiversity awareness among the municipal authorities as well as the local population, in a predominantly rural region today affected by rapid and poorly planned urban growth.

Geological Setting: The Borborema Province

The study area is part of the Brazilian Equatorial continental margin, formed after the Equatorial Atlantic opening in Aptian times. Onshore, a Precambrian basement—the Borborema structural province—is subdivided into several geological domains by a wide system of Late Proterozoic shear zones partly reactivated in the Cretaceous. The present structural pattern is organized around a discontinuous NE–SW set of basins and half grabens —the intracratonic Cariri-Potiguar rift zone . Representing failed rift structures of Early Cretaceous age with remnants of their post-rift sedimentary cover, it is intersected by the transform margin in the Potiguar Basin area. Remains of the pre-rift cover of the basement are preserved outside these structures (the Parnaiba Basin , to the west) and inside the rift zone in southern Ceara (Araripe Basin ). Along the coast, a thin dissected layer of Cenozoic continental sediments (the Barreiras Group) underlies the low-lying “tabuleiros ” between the marginal escarpment and the shallow continental shelf.

Stepped Surfaces, Palaeolandforms and Morphostratigraphy: Reconstructing the Long-Term Landscape Evolution Since the Mesozoic

The present relief reflects a juxtaposition of highly contrasting elements of various ages. The landscape formed in the context of a fluctuating but continuously falling base level since the Cenomanian. After formation of the transform passive margin in Aptian times, landscape development was further driven by regional swell-like uplift . The seaward flank of this broad monocline was eroded, whereas the post-Cenomanian uplift caused an inversion of the Cretaceous basins and generated a landscape in which the most elevated landforms correspond either to resistant Mesozoic sedimentary caprock, or to eroded stumps of syn-rift Cretaceous footwall uplands. Some topographic surfaces at low elevations are Mesozoic land surfaces that became re-exposed in Cenozoic times. Denudation in the last 90 Myear never exceeded mean rates of 10 m Myear−1 and exhumed various Cretaceous stratigraphic unconformities. Obtained by morphostratigraphic methods, these results differ significantly from published AFT-derived estimates reported from Palaeozoic and Mesozoic sandstones of the Araripe basin, implying burial by a considerable thickness of younger section and, later, 1.5 km of post-rift denudation, i.e. two to three times as much as our maximum 0.6–0.7 km estimation. The likelihood of such a scenario is discussed, in the light of the exhumation of surrounding basement surfaces, which probably began very early, as shown to the northwest of the Chapada do Araripe by the presence of widespread laterites of probable Paleogene age. The distribution of these paleosoils indicates an early beginning of the basin topographic inversion , which might correspond to the first post-rift denudation episode reported by the AFTA. The second one, in the Oligocene and later, would correspond to the major stage of river incision and basin inversion . The origin of the discrepancy between the two sets of results is discussed, as well as the causes and exact timing of post-Cenomanian crustal upwarping.

Late Cenozoic Evolution of Landforms: The Influence of Neotectonics and Climatic Changes

Forming a continuous clastic apron near the coast, the mainly Neogene Barreiras Formation records the last peak of erosion in the hinterland, which coincided with the onset of more arid climates at ~13 Myear or earlier. Dissected pediments and sparse debris fans visible at the base of some escarpments suggest recent erosive activity and possible slope retreat during the Late Cenozoic. A study of landforms located near zones of seismotectonic activity indicates a possible, probably weak, contribution of neotectonics to the formation of a few high scarps. Many of them are mainly inherited landforms, initiated during the Early Cretaceous rifting or the later margin uplift , a situation which suggests long-term stability. Later on, except in high inner tablelands (Chapada do Araripe), only slight or local backwearing took place, associated with downwearing on low surfaces and pediments, probably in diachronic ways. The Neogene clastic sedimentation on piedmonts and coastal areas mainly reflects the occurrence of dry periods inducing widespread stripping of deep soil horizons and erosion of bare rock slopes and surfaces. Dissection stages occurred in periods of more humid climate and/or low sea level. Marks of strong recent or present activity are mainly registered in the rims of the Chapada do Araripe, owing to favorable structural, hydrogeological and climatic conditions. The moderate volumes of Neogene clastic sediments imply overall low uplift and erosion rates until the present, favorable to morphological and lithological resistance effects in the landscapes. However, slope instabilities are not uncommon, locally leading to well-characterized processes, landforms, and deposits of gullying and mass wasting. Therefore, hazards related to slope processes should not be neglected in large parts of the study area.