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GEOLOGIA DE L'illa DE LIVINGSTON (SHETLAND DEL SUD, ANTÀRTIDA) Del Mesozoic Al Present

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GEOLOGIA DE L'illa DE LIVINGSTON (SHETLAND DEL SUD, ANTÀRTIDA) Del Mesozoic Al Present Departament de Geologia Dinàmica, Geofísica i Paleontopgi^' UNIVERSITAT DE BARCELONA GEOLOGIA DE L'ILLA DE LIVINGSTON (SHETLAND DEL SUD, ANTÀRTIDA) Del Mesozoic al Present Treball fet per RAIMON PALLAS i SERRA dins del Departament de Geologia Dinàmica, Geofísica i Paleontologia de la Universitat de Barcelona sota la direcció del Dr. Francesc Sàbat i el Dr. Joan Manuel Vilaplana per optar al grau de Doctor en Geologia. Barcelona, Gener de 1996 El Doctorand Els Directors Raimon Pallas i Serra Francesc Sàbat Joan Manuel Vilaplana OHb PAULAS ¿ERRR Aquest treball ha estat finançat per una beca de Formació de Personal Investigador del Ministeri d'Educació i Ciència i s'emmarca dins de les Accions Especials ANT89-822E i ANT90-1095E i el Projecte d'Investigació ANT91-1270 de la Comissió Interministerial de Ciència i Tecnologia (CICYT) GEOGACETA, 14,1993 Geomorfología de la península del Cabo Shirreff. Isla Livingston. Shetland del Sur. Antártida Ceomorphology of Cape Shirreff. Livingston Island. South Shetland. Antartica J.M. Vilaplana, R. Pallas y J. Calvet Dept. de Geologia Dinàmica, Geofísica i Paleontologia. Universitat de Barcelona. Zona Universitaria de Pedralbes. 08071 Barcelona. ABSTRACT The geomorphological exploration and mapping of Cape Shirreff area was carried out during the 1992/93 Spanish Antàrtic Expedition in Livingston Island. In this area two main systems oflanforms were recognized. A) Marine landforms mostly constituted by three groups of raised platforms and by five levels of raised beaches. B) Glacial landforms conformed by recent (noraines related to the present ice-margin and by older glacial deposits located on the different marine levels. These landforms and the relathionsbip between them allow us to reconstruct the recent deglaciation of this uplift controlled area. Keywords: erosion platforms, raised beaches, moraines, eustatism, uplift. Ceogaceta, 14 (1993), 52-54 ISSN: 0213683X Introducción que se han desarrollado calas de dimen­ gua plataforma marina. A cota 50 - 55 m siones variables. En ellas se conservan existe otra plataforma que seguramente Durante la primera fase de la Cam­ distintos niveles escalonados de playas. corresponde a la que cita Hobbs (1968). paña Antartica 1992/93 se ha realizado En el istmo de la península del Los restos de esta antigua plataforma la exploración y la cartografía geomor- Cabo Shirreff se encuentra el margen son muy abundantes y extensos en toda fológica del Cabo Shirreff, en la costa del gran glaciar de plataforma con mor­ la península. Los límites de dicha plata­ norte de la Isla Livingston. En esta nota fología dómica, que cubre la zona cen­ forma coinciden, en la mayona de los se presentan los resultados de dicho tra­ tral de la Isla Livingston. Dicho margen casos, con la presencia de un escarpe bajo que se concretan en el levanta­ se subdivide en dos lóbulos glaciares que destaca en el paisaje. Dicho escarpe miento del primer mapa geomorfológi- que llegan hasta el mar: al NE, el de la corresponde al paleoacantilado de una co de esta zona. Este trabajo se enmarca playa de Media Luna, y al NW, el de antigua línea de costa. El trazado del es­ en un estudio regional de la Isla San Telmo. carpe presenta tramos rectilíneos de di­ Livingston que se inició en la campaña rección NNW - SSE, coincidente con la 1989/90. orientación de la actual costa oeste del El Cabo Shirreff es una península Plataformas de erosión marina Cabo Shirreff. Dicha orientación viene rocosa, descubierta de hielo glaciar, for­ condicionada por diques y fracturas. mada por una serie de rocas volcánicas Hobbs (1968) ya cita la existencia Plataformas intermedias: por debajo (lavas y brechas) y diques subvolcáni- de una plataforma marina levantada en de la gran plataforma superior hay va­ cos similares a las formaciones que Cabo Shirreff (también descrita en rios niveles de terrazas marinas inter­ afloran en Península Byers (Smellie et Península Byers) con una elevación es­ pretados fundamentalmente como ero­ a\., 1984). Morfológicamente, cabe dis­ timada entre 46.2 m y 53.3 m sobre el sivos; la mayona de ellos tienen tinguir la presencia de superficies erosi­ nivel del mar. También indica la pre­ recubrimiento de gravas rodadas. vas o plataformas en las que se han en­ sencia de otras plataformas inferiores Tienen una extensión muy reducida y cajado unos pequeños valles en forma (7.6 - 9.1 m y 12,2 - 15.2 m) en las que una disposición escalonada. Hay un de corredores que las disectan. Estas va- encuentra, esparcidos sobre su superfi­ tínico resto del nivel de 22 m en la costa Ilonadas presentan unas laderas con cie, gravas y cantos rodados. oeste (San Telmo). Todos los demás ni­ fuertes pendientes y fondos cóncavos, Plataformas altas: existen restos de veles se encuentran a cota II- 15 m so­ casi planos. Una intensa dinámica crio- una plataforma situada a una altura de bre el nivel del mar que podrían corres­ nival ha degradado fuertemente las for­ 65 - 68 m sobre el nivel del mar. Estos ponder a retazos de rasas relacionables mas y depósitos, y es la respon.sable de retazos de superficie .son de poca exten­ con algunas de las playas levantadas co­ la existencia de una formación superfi­ sión y aparecen únicamente en la mitad mentadas más adelante. cial sobre las plataformas y las laderas. sur de Cabo Shirreff. De este nivel so­ Plataforma actual: corresponde a la La línea de costa tiene una geome- bresalen algunos relieves de orden de- plataforma de abrasión marina cubierta tna enormemente recortada y configura camétrico que podnan corresponder a por el mar y en bajamar emerge parcial­ una serie de salientes rocosos entre los paleoislotes pertenencientes a esta anti­ mente. Ha sido interpretada y cartogra- 52 GEOGACETA, 14,1993 Fig.1.— Mapa geomorfológico del Cabo Shirreff 55 m) se encuentran, diseminados en su supeifície, gravas y cantos poligénicos Fig. 1.— Ceomorphological map of Cape Shirreff rodados por la acción litoral. Localmen- te hay gran cantidad de cantos. La lito- logia de dichos cantos y gravas corres­ ponde, en parte, a rocas no aflorantes en Shirreff tales como granitoides. Estos clastos provinientes de áreas fuente ale­ jadas de Shirreff , así como una buena parte de los locales.conservan, facetas y estrías, lo cual nos permite interpretar su origen glaciar. Muchos de estos clas­ tos prodrían proceder de morrenas de­ gradadas por la acción marina que las retrabaja como playas. Se han localizado tres acumulacio­ nes de depósitos morrénicos encajados en la plataforma de 50 - 55 m (M en el mapa geomorfológico) retrabajados por acción marina y, posteriormente, por la dinámica crionival. La fracción fina de estas acumulaciones ha sido lavada y sólo se conservan los cantos y bloques cuyas dimensiones llegan a ser métri­ cas, especialmente en los granitoides. Estos clastos tienen estrías, facetas pla­ nares y morfología en "punta de nariz" pero se aprecia un cierto grado de roda­ dura. Niveles de playa Tal como se ha descrito en otros lu­ gares de las Shetland del Sur y muy es­ pecialmente en la Isla Livingston (Hobbs, 1968; John and Sugden, 1971; López et al., 1992) se encuentran dis­ tintos niveles de playas elevadas escalo­ nadas. En la zona del Cabo Shirreff es­ tán muy bien representados en la playa de la Media Luna, dónde aparece una secuencia con seis niveles de terraza si­ tuados a: 4.2 m; 6.0 m; 8.8 m; 15.8 m y 17.2 m. Los materiales que constituyen Leyenda: estas playas están formados por gravas 1. Cima; 2. Pendientes fuertes; 3. Pendientes suaves; 4a. Escarpe abrupto; 4b. Escarpe sua­ muy rodadas con una matriz arenosa. ve; 5. Crestón; 6. Laguna; 7. Plataforma de abrasión marina (P.a.m.) de 65-68 m; 8. P.a.m. de La litologia de los clastos es muy varia­ 50-55 m; 9. P.a.m. de 22 m; 10. P.a.m. de 11 -15 m; 11. P.a.m. actual; 12. Nivel de playa (a) da y no corresponde tínicamente al sus­ elevado ,(b) actual; 13. Islote, "stack"; 14. fk^argen glaciar (a) con escarpe, (b) en rampa; 15. trato volcánico de Shirreff, también Morrena con núcleo de hielo y crestones; 16. Depósitos morrénicos; 17. Flujo glaciar. aparecen granitoides, y rocas cuarcíti- cas. Muchos de estos clastos conservan Lsssnsí: aún indicios de su origen glaciar (for­ 1. Peak; 2. Steep slope; 3. Gentle slope; 4a. Scarp with break of slope; Ab.Scarp with smooth ma, estrías). Estas características son change of slope; 5. Crest; 6. Lagoon/lake; 7. Marine platform (M.p.) to 65-68 m; 8. M.p. to SO­ BS m; 9. M.p. to 22 m; lO.M.p. to 11-1S m; 11. Present M.p.; 12a. Raised tieach level; 12b. las mismas que se observan en aquellas Present beach; 13. Stack; 14. Glacial margin (a) ice-cliff; (b) ice-ramp; IS. Ice-cored moraine playas actuales donde el frente glaciar and morainic ridges; 16. Morainic deposits (M); 17. Ice flow. se dispone en contacto con el mar. fiada a partir de las fotografías aéreas nuoso y su anchura oscila entre 100 y El margen glaciar del vuelo británico realizado en diciem­ 700 m. bre de 1956 y enero de 1957. Esta plata­ Depósitos asociados: en todas las El glaciar, subdividido en dos lóbu­ forma tiene un límite enormemente si­ plataformas superiores (65 - 68 m y 50 - los, se ha instalado sobre la plataforma 53 GEOGACETA,14,ig93 de abrasión subactual; el frente glaciar a) el glaciar avanzó inicialmentc so­ de tasas de levantamiento propuesto por configura un escarpe de 10 a 15 m de al­ bre la plataforma de abrasión marina su­ otros autores en otros sectores antes tura, al pie del cual se dispone una pe­ bactual, probablemente incorporando apuntados.
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