Resumen

NET, Laura Inés. Factors controlling cementation patterns in Carboniferous sandstones of the Paganzo Basin, Northwest Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2002, vol.9, n.1, pp.1-30. ISSN 1853-6360.

Petrographic sandstone analysis of 192 thin sections belonging to the lower section of the Paganzo Group (Upper Carboniferous; Lagares, Malanzán, Loma Larga, Guan-dacol, Tupe, Punta del Agua and Río del Peñón Formations) cropping out in the Paganzo Basin (Figs. 1, 2) was carried out in order to define cementation styles (Primmer et al., 1997) and reveal the main controlling factors, such as clast composition, matrix content, sedimentary environments and burial depth. The lower sedimentary section of the Paganzo Group (200 to 1200 m thick) was deposited in alluvial fan, glacial, deltaic, marine shoreline, marine shelf and fluvial environments (Fig. 3). Three distinct structural and paleogeographical areas of the basin were distinguished and sampled: a) the eastern area (Olta and Malanzán valleys, Las Mellizas mine and Bum Bum localities, Fig. 1), corresponds to the stable foreland sector of the basin, dominated by thick fluvial deposits whose provenance region is the plutonic-metamorphic basement of the Sierras Pampeanas located to the east, and only registering a short marine event from the west. This region records minimum relative burial depths. b) the central area (Cuesta de Huaco and Cerro Guandacol localities, Fig. 1), is composed of thick continental deposits from the Sierras Pampeanas that alternate with two main marine episodes flooding the basin from the west; the latter carries fragments of fine marine sedimentary and volcanic rocks from the Precordillera. Burial depths in this area are intermediate. c) the western area (Punta del Agua locality, Fig. 1), transitional to the Río Blanco Basin, is composed of a thick conglomeratic and volcanic sequence followed by marine and deltaic sediments from the Precordillera but also including some fluvial deposits from the Sierras Pampeanas area. Maximum thickness of the postcar-boniferous sedimentary pile in this area accounts for maximum relative burial depths. These sandstones exhibit large textural and compositional variations (Tables 1 to 4), depending on basin floor lithologies, sedimentary environments and temporal changes of provenance areas. Petrographically, most of them are feldsarenites, subfeldsarenites and lithic feldsarenites, with subordinated feldspathic litharenites and litharenites (Fig. 4). Common cements include quartz and feldspar overgrowths, calcite as single phase or hematite-zoned, kaolinite, illite and chlorite; less frequent authigenic minerals are albite, microquartz, ankerite, hematite and gypsum. Petrographic characteristics seems to be the main control on cementation types in these sandstones (Fig. 9). Three main sandstone petrographic associations were recognized based on clast composition and matrix content (Table 5), and each one of them was assigned to a particular cementation style defined by cement distribution, textures and stratigraphy (Fig. 12). Feldspar-rich arenites (Fig. 12a) comprise feldsarenites, subfeldsarenites and lithic feldsarenites whose matrix content is less than 15% and show more feldspar than lithic fragments (F>L). They typically have high monocrystalline quartz percentages, K-feldspars dominant over plagioclases and scarce lithic fragments of plutonicmetamorphic affinity. Feldspar-rich arenites are cemented by abundant quartz and feldspar overgrowths, large kaolinite pore fillings related to feldspar breakdown, and late sparry pure or hematite-zoned calcite. Lithic-rich arenites (Fig. 12b) include litharenites, sublitharenites and feldspathic litharenites that have amatrix content of less than 15%, and lithic fragments predominate over feldspars (L>F). In spite of a common low total quartz content, significant proportions of chert are not unusual; plagioclases dominate over K-feldspar grains and a large variety of volcanic, plutonic-metamorphic and sedimentary lithic fragments can predominate. Lithic-rich arenites show a limited development of quartz and feldspar overgrowths, but radial fibrous and massive chlorite is abundant, particularly when basic and intermediate volcanic fragments are present; sparry calcite and rare micritic ankerite are late cementing phases. Muddy arenites (Fig. 12c) grouped all psamites with more than 15% of matrix, regardless of their composition. In muddy arenites the high matrix content has inhibited quartz and feldspar overgrowths (Fig. 5); illitic clays, hematite and micritic calcite dispersed into matrix micropores (pores between 4 and 62 ìm) constitute the more conspicuous cementing phases. Sedimentary environments have also played a significant role in cement distribution (Fig. 10) through their influence on the textural and compositional maturity of sandstones (Primmer et al., 1997; Stonecipher, 2000). Highly mature arenites from littoral marine and fluvial environments show maximum quartz and feldspar overgrowths development, while these cements are strongly inhibited in more immature arenites from alluvial fan and glacial environments. The local influence of sedimentary environments can also explain the high rate of quartz cement in mica-rich fine arenites from distal deltaic mouth bars or the origin of hematite concretions in high sinuosity fluvial deposits. An increasing burial depth in the basin from east to west is then correlated with a parallel rise of diagenetic temperatures, a fact that has also entered into cement formation and stability (Fig. 11). While kaolinite is abundant in the eastern area characterized by relatively low diagenetic temperatures (55-120°C), it progressively disappears towards the west, where a quartz, illite and chlorite-rich authigenic association reflects higher diagenetic temperatures (~125°C; Boles and Franks, 1979; Worden and Morad, 2000).Other external factors such as ash falls, dissolution of gypsum beds, local increase of diagenetic temperature caused by intrusives, or climatic changes have also resulted in cements of limited distribution (microquartz, some chlorite, gypsum veins, albite as mosaics, hematite as coatings), whose main characteristics show no relation with the clastic fraction.

Palabras clave : Cements; Sandstones; Diagenesis; Carboniferous; Paganzo Basin.

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