Martín Eduardo Morosi

Petrology of continental pyroclastic and epiclastic sequencesin the Chubut Group (Cretaceous): Los Altares-Las Plumas area, Chubut, Patagonia Argentina

Las sucesiones volcaniclasticas de la Formacion Cerro Barcino (Grupo Chubut, Aptiano-Cenomaniano), afloran en ambas margenes del rio Chubut con 200 m de espesor. Los miembros medio: Cerro Castano (sineruptivo, 90 m) y superior, Las Plumas (intereruptivo, 110 m) son descritos con el mapeo fotogeologico, estudios de campo y laboratorio (mineralogico, petrografico y geoquimico). Las paleocorrientes se orientan hacia el noreste (Cerro Castano) y hacia el norte (Las Plumas). Se ha reconocido, por primera vez, un limite erosivo en la base del Miembro Las Plumas. Las rocas piroclasticas presentan vitroclastos planares, curviplanares, ramificados y en forma de Y, y pumicitas fibrosas. Los depositos epiclasticos incluyen fragmentos de tobas vitreas, con pigmentacion hematitica (Las Plumas). Los vitroclastos estan alterados, por procesos diageneticos, a arcillas (esmectita y rara illita) o a zeolitas. En el Miembro Cerro Castano predomina analcima, asociada a clinoptilolita, mientras que en el Miembro Las Plumas la clinoptilolita se asocia a analcima y mordenita. La sedimentacion piroclastica (lluvias de cenizas) ocurrio en extensas llanuras onduladas, donde habia discretos canales arenosos. En la base del Miembro Las Plumas, la aparicion de valles amplios poco profundos, por los cuales corrieron rios entrelazados que depositaron conglomerados y areniscas, esta relacionada a actividad tectonica-climatica. Las erupciones fueron del tipo magmatico explosivo y los intensos procesos de fragmentacion estuvieron relacionados a magmas viscosos, ricos en volatiles de composicion riolitica-dacitica. La abundancia de vitroclastos planares, curviplanares y ramificados finos, sugieren que el centro explosivo se localizaba a una distancia aproximada de 500 km probablemente hacia el oeste.

General Pueyrredón, Balcarce, and Necochea Counties

In the Sierra Bachicha and Sierra del Volcan, near the Balcarce city, clay deposits are the result of basement rock hydrothermal alteration. The two best-known ones are called Cerro Segundo and Maria Eugenia, respectively. At Maria Eugenia mine the basement rocks have been obliterated by kaolinite formation and it is only possible to recognize the original foliation in reduced sectors. Cerro Segundo deposits also revealed the dominant presence of kaolinite, in this case, as a product of superimposed weathering argillization. Clays from Cerro Segundo are hard, with low plasticity, high content of alumina, and white cooking color. They are classified as refractory clays. They have been regularly used as an additive to lime. The Balcarce Formation outcrops discontinuously in almost all the Tandilia System, except in the Sierras Bayas Sector, but it is better exposed in the SE extreme of the range. Based on trace fossils its age is considered Cambrian-Early Ordovician. It may overlay uncomformably, weathered basement rocks; the Metapelitas Punta Mogotes or either sediments of the Las Aguilas or the Cerro Negro formations. The clay strata intercalated in the Balcarce Formation reaches only 1 m in thickness in the eastern area of Tandilia and are recovered, in general, as a by-product of the exploitation of quartzite levels, historically used for typical constructions of the zone (and named “Mar del Plata stone”). The mineralogical composition of the Balcarce Formation clays is fundamentally kaolinitic. The San Ramon clay deposits are classified as Flint Clays. They are intercalated between thick quartzite strata (Balcarce Formation). The mineralogical composition is mainly kaolinitic, with dickite associated in minor proportions. These clays are hard, whitish, plastic, or refractory (San Ramon), the latter used as chamotte and for the production of refractory materials. Based on previous research a geodynamic evolution of the Tandilia basin is offered.

Benito Juárez County

In the Benito Juarez County, four sectors will be considered: El Ferrugo and Constante 10-El Canon Sector; Villa Cacique Sector; Sierra La Juanita Sector and Cuchilla de Las Aguilas-Sierra de La Tinta Sector. Crystalline basement rocks have been altered by weathering processes, resulting, from bottom upwards in: bedrock, saprock, saprolite and, occasionally, in two superimposed paleosols. Argillized basement rocks are covered by a highly resistant conglomerate of the Balcarce Formation. Weathering profiles are analized in detail. Mineralogical composition, by X-ray diffraction of the clays of El Ferrugo and Constante 10 is similar. Also, these deposits are similar to those of La Veronica and Santa Maria, described in Chap. 2. According to the technological characteristics of the clays of El Ferrugo and Constante 10-El Canon Sector they are classified as “Fire clays”. In the Villa Cacique Sector the Olavarria Formation, followed by the Loma Negra Formation and overlaid by the Cerro Negro and the Balcarce Formation, are described. The clays of the Cerro Negro Formation are composed of detrital illite and diagenetic clay minerals. Chemical and technological analyses attest to low values of PCE. The clays are classified as varied clays (wide-ranging). At the Sierra La Juanita Sector, the Villa Monica Formation overlies unconformably the crystalline basement rocks and has been exploited for the ceramic industry. In the last years the Villa Monica Formation has been redefined as carbonate, mixed, both with quartz megacrystals, and hetherolitic facies; their origin is explained and a paragenetic sequence is proposed. MISS are described in siliciclastic and mixed facies of the Villa Monica Formation. SEM of the clay deposits and paleoenvironmental conditions of the Villa Monica Formation are discussed. The Villa Monica Formation age is considered to be Riphean, on the basis of the type of stromatolites. Technologically, clays from the Villa Monica Formation are classified as plastic clays. In the Cuchilla de Las Aguilas and Sierra de La Tinta Sector the sedimentary sequence overlying the basement rocks is represented by the Sierras Bayas Group covered by the Las Aguilas Formation and the latter, in turn, by the Balcarce Formation. Alunite provided a Middle Permian age according to K–Ar dating (telogenetic stage). MISS are described in the Las Aguilas Formation. Plastic clays, with refractory and semiplastic varieties, are used in red ceramic and cement industry.