Evoluzione olocenica e aspetti geomorfologici dell'area costiera della Peninsula Juan Mazia (Fogli "Puerto Percy", "Bahia Felipe" e "Puerto Sara") (Tierra del Fuego - Stretto di Magellano, Cile)

Abstract

The Juan Mazfa Peninsula is located in the Segunda Angostura of the Magellano Strait and, from the lower Holocene onwards, has been an important morphodynamic unit in the history of this Strait. This paper reports the results of a geomorphoiogical study (Sheets No. 69/524500-700730, No.70/534500-694500 and No. 51/523000-700730 of the Chile Map, Section L), the aim of which is to illustrate the recent evolution of the Juan Mazfa Peninsula on the basis of marine and transitional terrace sequences that shaped its coast The origin of this Peninsula can be dated to the last glaciation, which produced the morainic deposits forming the substratum of Juan Mazfa (Juan Mazfa Stage, 29,000 ÷ 24,000 years B.P.). With the exception of the Segunda Angostura zone, the coasts of the Juan Mazfa Peninsula and the coastline of the eastern part of the Strait, show evidence of a different morphological setting in the lower Holocene. The western part of the peninsula shows a regular sequence of mainly marine terraced deposits, forming 3 orders of terraces at elevations of 18-5-25 m, 6÷11 m and 3÷5 m and testifying as many sea level stands. In the eastern area, the 2nd order terrace (elevation 6÷11 m a.s.l.) is the best represented whereas the 1st order terrace cannot be reliably identified. On the basis of stratigraphic considerations and 14C age determinations, the terrace at 18÷25 m a.s.l. is undoubtedly lower Holocene in age, although its base may be dated to the late Pleistocene. Its formation occurred in a lacustrine environment passing to marine through transitional conditions. The 2nd order terraces are certainly linked to a marine enviroment; they too date to the Holocene (6.000÷7.000 years B.P., on the average). Terraces of the 3rd order are also marine and have an average age of 4.000÷5.000 years B.P. Another terraced sequence is located at elevations greater than 25 m; its is highly probable that they are of fluvial and/or glaciofluvial origin. Along the studied coastal stretch another series (4th order) of terraces of marine origin is visible at less than 3 m a.s.l.; these terraces have not been mapped because their outcrops are too small for the map scale used. The well defined altimetric position of the 3 younger orders of terraces may be used as an indication that positive eustasy overlapped the isostatic component of the Holocene uplift, even if its action is thought -at the present stage of the research- to have been mild. The scarce presence or even the absence of the 1st order terraces in the eastern area suggests that a tectonic activity (of which, however, no evidence has been identified to date) occurred, causing the tilting of terraces with their subsequent erosion and dismantling.