EVIDENCES FROM HISTORICAL DOCUMENTS OF LANDSCAPE EVOLUTION AFTER LITTLE ICE AGE OF A MEDITERRANEAN HIGH MOUNTAIN AREA,SIERRA NEVADA,SPAIN (EIGHTEENTH TO TWENTIETH CENTURIES)

The Sierra Nevada is the highest mountain system on the Iberian Peninsula (Mulhacén 3482 m; Veleta 3308 m) and is located in the extreme SE region of Spain (lat 37°N, long 3°W). Bibliographic resources, particularly from the eighteenth to twentieth centuries, provide insights into the changing summit landscape as the effects of cold, ice, snow and wind shaped its morphology. The selected references emphasize the Sierra's evolving climate reflected in the glaciers and snow hollows, and in the sparse vegetation above certain altitudes. Scientists had established bioclimatic conditions for the entire range in the early nineteenth century, and their works reflect the progression of ideas, particularly in the area of natural sciences, that influenced the period chosen for this study. This information, in addition to current knowledge about the morphogenetic dynamics of the Sierra Nevada, provides the basis for a comparison of the dominant environments from the Little Ice Age to the present, using the most significant high mountain morphological features as a guide. The most relevant findings indicate that cold climate processes (soli‐gelifluction, frost creep and nivation) were more predominant during the eighteenth and nineteenth centuries than they are today.     

LATE PLEISTOCENE ICE FLUCTUATIONS AND GLACIAL GEOMORPHOLOGY OF THE ARCHIPIÉLAGO DE CHILOÉ, SOUTHERN CHILE

Most of the last glacial maximum (LGM) glacier record west of the southern Andes (40–55° S) is today submerged under the Pacific Ocean and therefore the Archipiélago de Chiloé (42–43° S) provides an unusual opportunity to study local sediment and landform associations to help understand paleoglacial features of the former Patagonian ice sheet (PIS). In this context, this work presents the first comprehensive glacial geomorphologic mapping of the central region of the Archipiélago de Chiloé, which is located in a transitional geomorphic region between the Chilean Lake District (CLD, 39–41° S, 73° W) and northwest Patagonia (～43–48° S, 74° W). The Chilotan glacial geomorphology and sediment associations resulted from a warm‐based glacier that characterizes a typical active glacial temperate landsystem, which in central Chiloé combines deposits and landform units originated in subglacial and subaerial environments. Paleoglacial features that occur in central Chiloé are characteristic of an ice‐sheet style of glaciation, which differentiates it from a typical Alpine glacial style defined previously for the CLD. Therefore, the Archipiélago de Chiloé represents a geographical break point where the PIS became the large ice mass that occupied the Patagonian Andes during the last glacial period (Llanquihue Glaciation). A double ice‐contact slope on the east face of the Cordillera de La Costa provides evidence for the most extensive Early Llanquihue glacial advance on Isla Grande de Chiloé. The most prominent LGM advance in the area occurred at 26 000 cal yr BP, coincident with regional stadial conditions, and is defined by a big moraine along the east coast of the island.     

Storms Generator Method That Preserves Their Historical Statistical Characteristics. Application To Mexico City Basin Daily Rainfall Fields

A first procedure to generate synthetic storms, which preserves the statistical characteristics of the historical daily precipitation events registered in 49 stations located within the basin of Mexico City is presented. This procedure (a variation of the Svanidze method) implicitly supposes that the correlation between the maximum rainfall and its spatial distribution is meaningless. However, the obtained results did not validate that hypothesis. For this reason a second procedure was developed, which allows to consider the correlation between maximum rainfall and spatial distribution. This second procedure allowed reproducing the statistical characteristics of the daily rainfall for each station and also its spatial distribution, as shown in this work.     

A CONTRIBUTION TO THE DESIGN OF RECORDING RAIN GAGE NETWORK

Abstract

For needs of planning and design, data on rainfall duration-intensity-frequency relations are necessary. For this purpose, a network of recording raingages has to be designed. Czechoslovakia has a relatively very dense network with considerably lengthy records. Analysis of this network and particularly of parameters of a new formula for rain intensity-duration-frequency relation, abstracted from this network observations, show that in conditions of a mountainous terrain, no geographical pattern related to topographical characteristics can be evaluated to limit the density of the network.

On the other hand, the use of adequate statistical procedure can reduce the necessary lenght of records for the evaluation of reliable data of low frequency, which permits a rational and economical design of permanent and non-permanent networks.     

Sedimentary processes in a submarine canyon excavated into a temperate‐carbonate ramp (Granada Basin,southern Spain)

During the Late Tortonian, shallow‐water temperate carbonates were deposited in a small bay on a gentle ramp linked to a small island (Alhama de Granada area, Granada Basin, southern Spain). A submarine canyon (the ‘Alhama Submarine Canyon’) developed close to the shoreline, cross‐cutting the temperate‐carbonate ramp. The Alhama Submarine Canyon had an irregular profile and steep slopes (10° to 30°). It was excavated in two phases reflected by two major erosion surfaces, the lowermost of which was incised at least 50 m into the ramp. Wedge‐shaped and trough‐shaped, concave‐up beds of calcareous (terrigenous) deposits overlie these erosional surfaces and filled the canyon. A combination of processes connected to sea‐level changes is proposed to explain the evolution of the Alhama Submarine Canyon. During sea‐level fall, part of the carbonate ramp became exposed and a river valley was excavated. As sea‐level rose, river flows continued along the submerged, former river‐channel, eroding and deepening the valley and creating a submarine canyon. At this stage, only some of the transported conglomerates were deposited locally. As sea‐level continued to rise, the river mouth became detached from the canyon head; littoral sediments, transported by longshore and storm currents, were now captured inside the canyon, generating erosive flows that contributed to its excavation. Most of the canyon infilling took place later, during sea‐level highstand. Longshore‐transported well‐sorted calcarenites/fine‐grained calcirudites derived from longshore‐drift sandwaves poured into and fed the canyon from the south. Coarse‐grained, bioclastic calcirudites derived from a poorly sorted, bioclastic ‘factory facies’ cascaded into the canyon from the north during storms.     

Aeolian sand sea development along the mid‐Cretaceous western Tethyan margin (Spain): erg sedimentology and palaeoclimate implications

The existence of a mid‐Cretaceous erg system along the western Tethyan margin (Iberian Basin, Spain) was recently demonstrated based on the occurrence of wind‐blown desert sands in coeval shallow marine deposits. Here, the first direct evidence of this mid‐Cretaceous erg in Europe is presented and the palaeoclimate and palaeoceanographic implications are discussed. The aeolian sand sea extended over an area of 4600 km². Compound crescentic dunes, linear draa and complex aeolian dunes, sand sheets, wet, dry and evaporitic interdunes, sabkha deposits and coeval extradune lagoonal deposits form the main architectural elements of this desert system that was located in a sub‐tropical arid belt along the western Tethyan margin. Sub‐critically climbing translatent strata, grain flow and grain fall deposits, pin‐stripe lamination, lee side dune wind ripples, soft‐sediment deformations, vertebrate tracks, biogenic traces, tubes and wood fragments are some of the small‐scale structures and components observed in the aeolian dune sandstones. At the boundary between the aeolian sand sea and the marine realm, intertonguing of aeolian deposits and marine facies occurs. Massive sandstone units were laid down by mass flow events that reworked aeolian dune sands during flooding events. The cyclic occurrence of soft sediment deformation is ascribed to intermittent (marine) flooding of aeolian dunes and associated rise in the water table. The aeolian erg system developed in an active extensional tectonic setting that favoured its preservation. Because of the close proximity of the marine realm, the water table was high and contributed to the preservation of the aeolian facies. A sand‐drift surface marks the onset of aeolian dune construction and accumulation, whereby aeolian deposits cover an earlier succession of coastal coal deposits formed in a more humid period. A prominent aeolian super‐surface forms an angular unconformity that divides the aeolian succession into two erg sequences. This super‐surface formed in response to a major tectonic reactivation in the basin, and also marks the change in style of aeolian sedimentation from compound climbing crescentic dunes to aeolian draas. The location of the mid‐Cretaceous palaeoerg fits well to both the global distribution of other known Cretaceous erg systems and with current palaeoclimate data that suggest a global cooling period and a sea‐level lowstand during early mid‐Cretaceous times. The occurrence of a sub‐tropical coastal erg in the mid‐Cretaceous of Spain correlates with the exposure of carbonate platforms on the Arabian platform during much of the Late Aptian to Middle Albian, and is related to this eustatic sea‐level lowstand.