Continental Landforms Research Articles

Geomorphological Evolution of the Patagonian Coast Surrounding the Golfo Nuevo (Chubut, Argentina) during Late Quaternary: Overview and New Proposal

Bunicontro, M.P. and Marcomini, S.C., 2021. Geomorphological evolution of the Patagonian coast surrounding the Golfo Nuevo (Chubut, Argentina) during Late Quaternary: Overview and new proposal. Journal of Coastal Research, 37(3), 528–543. Coconut Creek (Florida), ISSN 0749-0208.The main contribution of this paper is a new interpretation of the geomorphological evolution of the North Patagonian coast surrounding the Golfo Nuevo from Mid-to-Late Pleistocene until present. The proposed model shows how continental and marine landforms evolved during the Late Quaternary. According to this, the Golfo Nuevo depression formed during the Middle Pleistocene (>130 ka) above previous fluvial terraces, which are attributed to different stages of the Chubut river. Several pediment types and levels were also identified: (1) flanking pediment (level 1) associated with an ancient fluvial valley; (2) convergent pediment related to a previous depression of Golfo Nuevo (level 2), both assigned to Middle Pleistocene; and (3) a littoral pediment (level 3) linked to a Holocene highstand level. The height and position of accretion marine terraces are discussed, in addition to their previous dating. It can be assumed that Golfo San Matías was flooded twice as a consequence of marine transgressions (marine isotopic stage [MIS] 5 and MIS 1). Likewise, Golfo Nuevo and Golfo San José were affected only once, during the maximum Holocene transgression (MIS 1). This paper highlights the importance of reconstructing the geomorphological evolution of a coastal area of the Patagonia as a result of a combination of changes linked to sea-level variations (mainly associated with glacio-eustasy) and climate, paleo-shorelines records, and continental landforms through Late Quaternary. The proposed geomorphological model can be considered a novelty because of the complex integration of continental and marine landforms and the study of their relative relation.

Geochronology of pediments and marine terraces in north-central Chile and their implications for Quaternary uplift in the Western Andes

In north-central Chile, a wide shore platform is morphologically connected with a high fluvial terrace and a pediment. The eastward extension of Quaternary coastal uplift in the Southern Central Andes is poorly constrained since no age correlation between marine and continental landforms has been reported. We use 26Al and 10Be concentrations to constrain the geomorphic evolution of these marine and continental landforms near the Choapa valley (31.6° S). 10Be ages for the shore platform indicate that this surface was repeatedly reoccupied during sea-level highstands between ~800 and 500ka and uplifted after 500ka. While ‘zero erosion’ ages for the pediment between ~600 and 300ka only partly overlap the shore platform age range, more realistic exposure ages calculated for an erosion rate of 1m/Ma are between ~945 and 475ka, fitting the age range of the correlated shore platform. 10Be concentrations of the high fluvial terrace are highly scattered evidencing vertical mixing of clasts probably due to slow lowering of the surface. Although it is not possible to determine an age for this landform, the scattering among its 10Be concentrations implies that this marker is several hundreds of thousands of years old and that the high fluvial terrace began to form at ~1200ka or after. Finally, 10Be concentrations of the high fluvial terrace, the pediment and the shore platform are of the same order of magnitude, which is consistent with the clear morphologic correlation between these three types of landforms. These data suggest that the marine and continental landforms studied formed synchronously, with some local differences, during a long period of relative tectonic stability between ~(1200?) 800 and 500ka and uplifted after 500ka. Our results confirm recent studies showing a post-400±100ka renewal of uplift along the Pacific coast after a Lower to Middle Pleistocene period of slow uplift. Moreover, the extension of the surfaces suggests that a broad region of ~40km has been uplifted ca. 150m during the Quaternary.

Datation de surfaces geomorphologiques reperes par le 10 Be produit in-situ; implications tectoniques et climatiques

Abstract The evolution of continental landforms is mainly modulated by the impact of climatic and tectonic processes. Because of their distinctive morphology and the periodicity of their deposition, climatically induced landforms such as alluvial fans or terraces are well suited to infer rates of tectonic and continental climatic processes. Within tectonically active regions, an important step consists in dating displaced geomorphic features to calculate slip rates on active faults. Dating is probably the most critical tool because it is generally much more simpler to measure deformation resulting from tectonic activity than it is to accurately date when that deformation occurred. Recent advances in analytical chemistry and nuclear physics (accelerator mass spectrometry) now allow quantitative abundance measurements of the extremely rare isotopes produced by the interaction of cosmic rays with surface rocks and soils, the so-called in situ-produced cosmogenic nuclides ( 3 He, 10 Be, 21 Ne, 26 Al, 36 Cl), and allow to directly date the duration that a landform has been exposed to cosmic rays at the Earth's surface [Lal, 1991; Nishiizumi et al., 1993; Cerling and Craig, 1994; Clark et al., 1995]. In fact, the abundance of these cosmonuclides is proportional to landscape stability and, under favorable circumstances, their abundance within surface rocks can be used as a proxy for erosion rate or exposure age. These cosmonuclides thus provide geomorphologists with the opportunity to constrain rates of landscape evolution. This paper presents a new approach that combines cosmic ray exposure (CRE) dating using in situ-produced 10 Be and geomorphic as well as structural analyses. This approach has been applied on two active strike-slip and reverse faults located in the Andean foreland of western Argentina. These two case studies illustrate how CRE dating using in situ-produced 10 Be is particularly well suited for geomorphic studies that aim to estimate the respective control of climate and tectonics on morphogenesis.

Cosmogenic dating ranging from 20 to 700 ka of a series of alluvial fan surfaces affected by the El Tigre fault, Argentina

It is crucial to date continental landforms to quantify processes involved in terrestrial surface evolution, especially in regions affected by active tectonics. Andean quaternary alluvial fan surfaces affected by the El Tigre strike-slip fault have been studied using combined geomorphic and 10 Be exposure age approaches. Field observations and SPOT (French acronym for “Satellite for Observation of the Earth”) image analysis enable the identification of six alluvial fan units. Measurements of in situ–produced cosmogenic 10 Be concentrations in quartzite boulders exposed on the top of fan surfaces show that the depositional periods ended during successive major interglacial stages. The calculated minimum exposure ages date the abandonments of the alluvial fan surface from 41 000 ± 8500 yr for the youngest to 670 000 ± 140 000 yr for the oldest unit. When linked to the measured maximum cumulative right-lateral displacement of stream channels, the exposure ages yield a horizontal slip rate of about 1 mm/yr on the El Tigre fault. This study shows that for arid regions, where fan surface erosion is minimal, in situ–produced 10 Be can be used to constrain the age of stratigraphically separate alluvial fan surfaces. These fan surface exposure ages can be further used to calculate slip rates on active faults and infer depositional periods correlative with climatic events.

Continental Landforms: Geomorphic Systems of North America.

Continental Landforms: Geomorphic Systems of North America.

Rhone Deep-Sea Fan: Morphostructure and Growth Pattern

A detailed bathymetric survey of the Rhone deep-sea fan and its feeder canyon using Sea-Beam, reveals morphologic features such as very tight meanders of the canyon and channel courses, cutoff meanders, and downslope narrowing of the inner channel floor. Striking similarities exist between these deep-sea features and some continental landforms, especially in alluvial plain areas or desert environments. Sea-Beam also reveals evidence of huge slump scars affecting the slope and fan. The superficial structure of the Rhone Fan results from the stacking of numerous lenticular acoustic units displaying specific seismic characters in which we recognized channel and levee facies. Except in the upper fan area, these units have not been constant; they have generally migrated, owing to shifting of the channel throughout fan evolution. Construction of the fan probably began as early as the early Pliocene and continued to the close of the Wurmian (late Wisconsinian). In our opinion, the fan's growth pattern could be associated with climatic fluctuations. The principal sedimentary mechanism responsible for the growth of the fan appears to be turbidity currents, but mass gravity flows have also been an important factor in building the fan by occasionally blocking the main channel and forcing it to migrate.

Detailed morphology, structure and main growth pattern of the Rhône deep-sea fan

A sea-beam survey of the Rhône deep-sea fan and its vector canyon shows unexpected morphological features such as striking similarities with continental landforms encountered especially in alluvial plain areas or desertic countries. The structure of this fan, as shown by continuous seismic profiles, results from the stacking up of numerous lenticular acoustic units displaying specific seismic characters. It is suggested that the building of the fan began as early as the lower Pliocene with a growth pattern tightly associated with climatic fluctuations.