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El cretacico superior y la morfología estructural en la costa sur del Seno Skyring (Magallanes)


  • Giovanni Cecioni Departamento de Geología, Universidad de Chile


Seno Skyring is located in the northuest political division of the chilean Magellan
Province, and geographically belongs to the eastern Andean region. Comprised
within the mountain region, the western area of Seno Skyring offers a matked contrust with the morphology of the eastern region.

The hills, though not very high, present the characteristics of high mountain;
some little glaciers are preserved during the summer and forests cover the western, more humid parts of the hills. Feature of intense glacial erosion predominate here, in addition to the marine erosion forms. The fiords are best developped near the Pacific Ocean, where the coast is very irregular, presenting a multitude of islands, inlets and peninsulas that are referred to the Patagonian Archipelago.

The landscape of the eastern region, or extra Andean geographical division,
put in evidence fairly extensive plateaus, which reflect the rnonoclinal structure,
stepping down toward the Atlantic Ocean, the dryness of the climate , and the predominance of deflaction over water erosion.

No human settling have been found in the studied area, excepting Estancia
(Farm) Miric. The region is impervious by land and has been studied by means of a cutter.

Acid igneous rocks prevail in the Andean region, as well as old sediments
ranging from Paleozoic to Cretaceous; generally they are very hard rocks that have been greatly eroded by glaciers. On the contrary, in the extra Andean region younger sediments belonging to the Uppermost Cretaceous and to Tertiary, generally prevail. There are clay soft rocks, with well preserved meanders in the rivers, and sandy rocks in the hills, that have suffered but little erosion and have been covered by remarcably thick glacial, marine and fluvial pleistocenic sediments.

An attempt is made here to give an account of the geological papers, which
a critical analysis of those studies concerning the southwestern coast of Seno
Skyring, unpublished or have been pubtished by HEMMER (1935, 1937) y THOMAS (1949), and resumed by HOFFSTETTER et al. (1957).

lt is intented in this paper to establish the following:

1° to prove the presence of sediments, Senonian in age, which may be correlate to the following Formations: Lower Fuentes (shales), San Isidro (sandstones, comglomerates), Upper Fuentes (shales), Rocallosa (sandstones), and Chorrillo Chico (siltstones), this late without visible top.

2° the sandstones and conglomerates (respectively flysch and wildflysch) were
deposited by turbidity currents which originated in the western coast of the Magellan Cretaceous Basin, flowing eastward, as result from the stratigraphic location of the faunal zones.

3° in the studied area, sixth contiguous synclinals are present, separated by thrust faults, which plain is dipping westward; two incomplete upward concave monoclinals may be two synclinals.

Tectonically this late may represent here some little "decollements", or
lateral compressive components of a vertical uplift of the Paleo-Andes, which may have produced eastward oversthusters.

Recent gravimetrical studies in the northern coast of Seno Skyring, put in
evidence the possibility of a more dense and thin crust without root; this crust is
sinking producing intra-mountain depressions with sediments and volcanics, Upperrnost Miocene - Pleistocene in age.

It is possible that this area may represent the transition between the Pacific
Active Margin and the Atlantic-type. Passive Margin; this late without endogenous
vertical crustal mobility.

The presence in soutbem Tierra del Fuego of some basic rocks is well in
line with the hypotesis of a thin crust. and may represent a tantalizing prospect
for copper deposits different from the famous disseminate cbilean porphyry - copper.