Geomorphological survey in the area between río Destierro and río Reventazón in Costa Rica

anglais. A study was done to investigate the area between the rivers Destierro, Parismina and Reventazon in Costa Rica. It was the aim of this study to investigate the processes which have determined the facies and geomorphology of the area. By that, the question was whether the area can be considered to be an alluvial fan. The area is situated on the transitions of the Talamanca mountain range and the Cordillera Central, and its footslopes to the Cuenca de Limon, a subsidence basin. To get started with the investigation, literature about the geology had to be studied, but not much detailed information was available. Aerial photos and topographical maps, available at the Atlantic Zone Program in Guapiles, provided a good view of the area. With the aid of these photos and maps, and with fieldwork, a geomorphological map was made. Due to the very different scales of the photos, the map had to be made mostly on scale 1:25,000 and after that reduced to 1:50,000. To get some insight in the slopes of the area, gradient reconstructions of three rivers were made, based on the topographical maps. In the field, many (natural) exposures and some augerings were described and the geomorphological map was checked. The legend of the constructed geomorphological map is divided into a facies and a morphology legend. Facies will be described first. Lavas form relatively steep, lobate hills due to their high viscosity during the eruption in this region. When lavas cool down after deposition, they crack due to shrinkage. Weathering follows these cracks. In this stadium, lavas can be recognized easily. When weathering proceeds, deeper exposures are needed to recognize the lava. Fluvial deposits consist of layered, well sorted material, which is coarser higher up on the slopes, and fine to very fine more downstream in the more flattened areas. A river deposits coarse sediments like sands and stones when the velocity is relatively high. When the velocity is much lower, clays can be deposited. But, in order to be deposited, the sediment has to be present. In the investigated area, weathering makes recognition difficult. Ash flow deposits are formed when fragments of a volcano are blown into angular elements during an eruption. They are deposited well-sorted, and sometimes layered. Lahars are mudflows related to volcanic eruptions. They consist of ashes and water and can be divided into cold and hot lahars. Cold lahars pick up a lot of present material while flowing down, due to their low consistence and stickiness, whereas hot lahars do not. So the facies of cold lahars are rather heterogeneous, while those of hot lahars only consist of volcanic ashes and blocks. Complexes are deposits as described above, but situated over each other. They have to be distinguished, because of the importance of those deposits to the understanding of the area. Peat is to be found in the lower parts of the downstream areas. It forms a layer of ten to eighty centimeters in thickness. The morphology part of the legend is described with the help of cross sections, and an explanation is given about a few units to create better understanding. To describe the morphology of the area in relation to the facies, the facies legend are used because a part of the morphology is already situated in these units. They are described from upstream to downstream as much as possible. The most upstream part of the investigation area consists of a valley, in which lahar and fluvial facies are present. Downstream, lobate hills indicate lavas and lahar facies can be found on top of lavas in the south. The next unit, the slightly dissected cold lahar deposits, is a rather flat area with a regular slope. Streams flow in it regularly. The facies consists of brown soil, gradually changing into lahar facies. More downstream, the landscape is not as flat the unit before, but hillier. The degree of dissection increases downstream. A large flat hill can be seen on the 1:80,000 aerial photos, in which river Destierro is very deeply incised in a rectangular bend. On the hill and downstream of it, facies of ash flows can be found overlying fluvial facies. In the southeast, nearer to the river Reventazon, lahar facies are present, sometimes dissected and/or covered by fluvial facies. In the southern corner in the river Reventazon, part of the area seems to have been lifted up. The most downstream part, consisting of several units (strongly dissected fluvial and ash flow deposits) and is a very hilly area. These hills are the highest upstream but fade away downstream. Small rivers flow in the valleys, draining into the Destierro or Reventazon, or seem to disappear into the flat plains downstream. In the south, fluvial facies are present and here and there terraces with lahar facies can be seen. In the valleys, peat overlying clays and sands are to be found. The following interpretation was made: The valley at the top of the investigation area consists of a cold lahar. This lahar is incised. Downward, lavas of different ages lie over each other and in the southeast a cold lahar is deposited over a lava. The flat area just under the lava-region consists almost completely of cold lahar deposits, which flattened the area during deposition. The more undulating area further downstream is a zone in which tectonics play a role in the flat hill in the west and in the corner in the Reventazon in the southeast. Cold lahar and pyroclastic ashflow deposits were deposited in this zone, after which sedimentation and incision has taken place. The lowest part have been influenced by tectonics and by incision and infilling by rivers, by lahars and by ash flows. An indication of sea level changes can be found as well. The conclusion is, that this area has been influenced by very different processes and that the relation to an alluvial fan, certainly in the lowest parts, can be made. It has been disturbed heavily disturbed by volcanic and tectonic features in the upstream parts.

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