Il Quaternario costiero della Peninsula Brunswick nei fogli "Villa Kon Aiken" e "Isla Isabel" (Stretto di Magellano - Cile)

Abstract

: The geomorphological- lithostratigraphic map of the coastal strip of the 'Villa Kon Aiken" and "Isia Isabel" Sheets of the Chilean I.G.M. Map (No. 67/524500-705230 and No.68/524500-70300 of Section L) at the scale 1:5Q,000 is presented in this paper. Primary objective of the research is the mapping of the most recent marine and transitional terraces. The outcropping substratum - subdivided into depositions units (ground, terminal and lateral moraines, kame terraces, drumlins, eskers, etc.) and erosion units (marginal and submargsnal streams, kettle-holes, etc.) - is prevalently linked with the last glaciations of the southern Andean Cordillera (Darwin Cordillera) and adjacent coastal areas of Patagonia and Tierra del Fuego. Three orders of mostly marine terraces lie over this substratum in the present coastal areas. The 1st order terraces - the oldest - include deposits and surfaces located at elevations between 18 and 25 m a.s.l. which formed under lacustrine to transitional to marine conditions. The 2nd order terraces, located between 6 and 11 m a.s.l, are formed by erosion surfaces often with associated gravel and sand, locally also with fossiliferous deposits. Terraced deposits at the elevations between 3 and 5 m a.s.l have been attributed to the 3rd order terraces. Inland, numerous older sequences of terraces are visible at more than 25 m a.s.l.: These are, however, mainly to be linked to a continental environment. Locally, below 3 m a.s.l, there is a very young 4th order of terraces which are of marine origin. Their areal extent is very small and not mappable at the 1:50,000 scale. Mapped terraces are attributed to the Holocene on the basis of their stratigraphic position and radiometric 14C ages. In particular, the oldest terraces can be dated to the lower Holocene, although their base may be slightly older (late Pleistocene); the intermediate order is 6,000-7,000 years old B.P., and the youngest terraces (3rd order) are dated to 4,000-5,000 years B.P. The present morphological setting of the coast is the product of the evolutionary dynamics of Holocene marine and transitional terraces. Lakes {lagunas) characterizing the coastal areas and which formed as a result of morphogenetic processes occurring in a glacial enviroment, evolved in many cases as backshore basins in a littoral environment. Locally they are suspended (palaeobackshore) over the present sea level because of isostatic phenomena following the last glaciation, associated with a mild eustatic phase; beach forms such as beach ridges, berms, spits, etc. are present in these latter cases. These forms are also the most common paleo-landforms on the terraced surfaces. The western section of the studied coastal area ("Villa Kon Aiken" Sheet) is characterized by a large number of 2nd order terraces which, during uplift, trapped lagunas and a series of minor lakes along the coastal strip of Seno Otway. The eastern coast ("Isia Isabel "Sheet) shows a more complete terraced sequence, especially along the coast facing Isia Isabel, which was affected by a more homogeneous uplift than the western part of the studied coastal area and where all the terrace orders are present. The remarkable altimetric correspondence between 2nd and 3rd order terraces in the two sections of the surveyed coast of the Brunswick Peninsula suggests a prevalent and homogeneous isostatic uplift. The scarce outcrops of eroded 1st order terraces in the western coastal section may be explained with tilting phenomena during uplifting.