Coastal landforms of 'Meso-Afro-American' and 'Neo-American' landscapes in the periglacial South Atlantic ocean: With special reference to the clast orientation, morphology, and granulometry of continental and marine sediments

摘要

In the periglacial South Atlantic Ocean five coastal landform series (CLFS): CLFS1 (Falkland Islands/Islas Malvinas), CLFS2 (Tierra del Fuego), CLFS3 (South Georgia + Scotia Ridge), CLFS 4 (South Shetland Archipelago) and CLFS 5 (Antarctic Peninsula) have been established and studied with regard to their geomorphology and sedimentary petrography. Although being several hundreds of kilometers apart from each other the majority of sites belong to the same climate zone, the polar tundra climate excluding the CLFS 1 which is more humid and the CLFS 5 on the opposite side that passes into the polar frost zone. The hydrographic regime is in the majority of cases microtidal, excluding some coastal regions in CLFS 1 and CLFS 5 where micro-mesotidal conditions exist around the volcanic edifices.The climate variation has an impact on the weathering of the landforms, which abruptly changes from chemical to physical between CLFS 3 and 4, and gradually from CLFS 1 to CLFS 3 (oxidic double right arrow reducing regime, pH double right arrow more acidic meteoric fluids). The mineral assemblages representative of the supergene mineralization reflect an incipient stage of on "icehouse-hothouse" transition.The CLFS 1 to 3 evolved in a geodynamically stable setting attested to by a high landscape maturity, whereas CLFS 4 and 5 are situated in a geodynamically mobile regime leading to structural landforms instead of sculptural ones of lower maturity.The CLFS1 to CLFS 3 are characterized by landscape types of plains, channels, fjords and bays which incise or truncate highly to partially eroded mountain blocks which were uplifted near the coast and thereby overprint relic landforms such as peneplains and pediments. The three geomorphological processes shaping the area between land and sea are (1) coastal marine - landforms given in brackets (beach, cliffs - beach scarps, dunes plus aeolian sand sheets, fiord remodeled into channels, bays (drowned cirque glaciers), wave-cut platforms, tidal flats (rocky), tombolo), (2) glacial (boulder trains, cirque-tarn-lip, cryopediments (?), glacial-fluvial channels, moraines, outburst valleys and spillways, pattern grounds (plus stone runs), roches moutonnees through-to U-shaped valleys, and (3) mass wasting (rock fall transitional into -slide, talus plus soil creep). While mass wasting is a rather conservative process, the glacial land-forming processes increase in quality and intensity from CLFS 1 to CLFS 3. The coastal marine processes are rather conservative, excluding those processes which interdigitate with glacial marine processes, e.g., evolution of fjords and when strongly controlled by the magmatic and metamorphic Ethology. The CLFS 4 and CLFS 5 are magmatic-arc, rift-, and fold belt-related. Both reference sites mark a volcanic landscape arising from the sea, the first one under subaerial, the second one under subglacial conditions. In these modern geodynamic settings structural volcanic landforms (cones, maars, craters, pyroclastic fans of flow and surge deposits, flat-topped volcanic plateaus, tuya) predominate. The coastal marine, glacial and mass wasting induced landform types resemble those of the CLFS 1 to CLFS 3. Among the mass wasting slide and flow deposits are more common near the beach. Among the glacial deposits moraines, aretes and nunataks are more common in the coastal hinterland. The wave-dominated coastal marine landforms (beach, cliffs-beach scarps, wave cut platforms) become more variegated when the tidal range increases (tidal channel double right arrow ephemeral stream, tidal flats). There are also mixed types between alluvial-fluvial and glacial marine named embryonic glacial-marine fan deltas.Based upon the current study an approximation of the relief generations or palaeo-landscapes can be achieved for the coastal region (hinderland + beach) in the S Atlantic Ocean: Peneplanation (relic form) - Oligo-Miocene double right arrow pedimentation (relic form) - Neogene double right arrow Glaciation and deglaciation conducive to depositional and erosive sculptural landforms = Volcanic activity conducive to structural landforms -Pleistocene-Holocene double right arrow Coastal marine and mass wasting processes conducive to depositional and erosive sculptural landforms - Pleistocene-Holocene. By analogy with similar geodynamic landscape types elsewhere the two principal landscapes are denominated as "Meso-Afro-American "(CLFS 1 to 3) and "Neo-American" (CLFS 4 and 5). These two technical terms are self-explanatory as to the geodynamic parent material and eligible for a maturity-based correlation of landscapes.The combination of the clast orientation, granulometry, grain morphology and shape in combination with sorting and clast mineralogy (visual lithological inspection double right arrow X-ray diffraction double right arrow scanning electron microscope supplemented with WDX/EDX double right arrow electron microprobe double right arrow micro-Raman spectrometry) has p