Abstract

MAZZONI, Mario Martín  and  MEZA, Juan Carlos. Sedimentología de debritas volcaniclásticas en la Formación Yacimiento Los Reyunos (Pérmico): Sierra Pintada de San Rafael, Mendoza. Rev. Asoc. Argent. Sedimentol. [online]. 1997, vol.4, n.2, pp.59-77. ISSN 1853-6360.

Matrix-supported breccia deposits (Debrita Vieja Gorda -DVG-) associated with Permian ignimbrites in the Sierra Pintada de San Rafael (MTVG) are first described(Figs. 1 and 2). A field study reveals that DVG deposits appear at the boundary between ignimbrite cooling units (c.u.), and are specially conspicuous among the younger units (Fig. 3). The identification of the DVG in previous field studies has been probably hampered by its ressemblance with the ignimbrites (i.e. apparent lack of stratification surfaces, and similar color, texture, massive interior, see wackes, Table 1, Fig. 4). A detailed field work of the DVG indicates however that the breccias are not primary but the result of mass-wasting processes acting upon degraded ignimbrites. Thickness of DVG depositsis generally less than a few meters, withplanar and non-erosive contacts in most localities (Fig. 5). However those in proximal settings,as the stacked units infilling channelized surfaces on top of the c.u. 6 unit (Fig. 7), are up to 50 m thick. The DVG deposits are massive (Fig. 8) and approximately half of the observed units show inversely graded bases. They are often interfingered with eolian arkosic arenites (MAA)and ignimbrites (Figs. 3, 4, 5, 6 and 7) Typical deposits include isolated subrounded blocks up to 50 cm (Figs. 6 and 8). Average abundance of pebblesize clasts (5-12 cm) is about 20%,varying generally between 5% in wackes to 50% in paraconglomerates. Larger and abundant lithic fragments are a useful field characteristic to distinguish DVGfrom primary ignimbritic deposits, which generally contains less than 5% of pebbles. Quantitative petrography and determination of maximum and mean grain-size in thin sections of MAA, wackes (matrix of DVG units) and ignimbrites has been aimed to analyze provenance and genetic processes involved in the generation of DVG deposits, and also to test the sensitivity of hand estimations. Columns A and B of table 2 represent mean grain-size eye determinations, to be compared with thin section determinations (columns C, D, and E). Column C is a semiquantitive mean grain-size mainly based in the mode, but modified by values in the coarse and fine grain-size tails found in each thin section. Eye and hand-lens determinations of the average grain-size by mean of comparison with grain-size charts aredisplaced towards coarser sizes in columns A and B, as a result of the visual bias imposed by larger clasts. Columns D and E (Table 2), representing the averageof the largestfive clasts, show that minimum and maximum sizes of quartz and feldspar are in MAA (arenites) and MTVG (ignimbrites) respectively, and are intermediate for DVG (wackes). Ratios D/E show that quartz is systematically bigger and that the mean size difference is higherin arenites and intermediate in wackes. Thin section petrography was performed in the same samples as in textural analysis. No major differences in mineralogical components have been detected. Conspicuous crystals of feldspar, quartz and biotite are clearly recognized in the ignimbrites, ignimbritic lithics, and also in the matrix (wackes) of DVG. Feldspars -mainly plagioclase and scarce sanidine- are the more abundant components (Tables 3 and 4). They are often engulfed and zoned and exhibit different degrees of alteration. They are a common matrix component of ignimbrites and paraconglomerates, and also as phenocrysts phase in pumice fragments. Authigenic feldspar overgrowths are common in thin sections of arenites, wackes and pumice (Fig. 9). Syntaxial cementation develops in cases sand-crystals up to several centimeters, that often weather in strikingly big, sometimes Carlsbad twinned, euhedral crystals. Quartz is remarkably equant, euhedral or subhedral, non-ondulatory, and frequently engulfed. Bipyramidal quartz, several millimeters long is frequent in the MTVG ignimbrites. All these features, attenuated by abrasion, can also be recognized in the wackes (DVG) and, to a lesser extent, in the MAA, specially in the coarser ones. Brown basal sections of biotite are conspicuos in hand and this section specimens. It is common as a phenocryst phase in the ignimbrites and asa matrix component of the DVG units where is oftenaltered to chlorite. Magnetite is abundant whereas zircon and piroxene arescarce. The composition of lithic fragments ordered by abundance are welded ignimbrites (specially from c.u. unit 6 in DVG), acidic volcanic fragments, andesites,and lesser amounts of sedimentary rocks (siltstones, quartz arenites and shales). The matrix of the DVG (in part pseudomatrix), generally deeply dyed with hematite,is composed of clay minerals(chlorite and illite), biotite, feldspar, quartz, and isotropic glass fragments. Calcite is common as cement, usually as a penetrative and massive replacement in feldspars and also as patchy replacements in lithics and matrix. Argillic alteration showssimilar characteristics. Rhyodacitic calcalkaline ignimbrites of the MTVG are crystal-rich and pumice-poor (below 10% in volume). Pumice is very scarce or absent in DVG unitswhere recognition is difficult due to diagenetic compaction and alteration. Volumetric relationships of metastable components to quartz (Table 4) are coherent with the hypotesis that MTVG is the source rock of DVG and MAA units, the latter representing the highest degree of weathering and/ or reworking. The origin of the DVG depositsinferred from the geological features, fabric and compositional data is depicted in figure 10. The main source of sediments is connected with recurrent and intermitent episodes of explosive volcanism, resulting in the production of voluminous primary pyroclastics (MTVG, mainly ignimbrites) and discontinuous interbeds, represented by paraconglomerates-wackes (DVG), and eolian arkosic sandstones (MAA), or both. The combined evidence indicate that the DVG deposits are the result of geomorphic en masse agents, mostly debris flows and lahars, and mudflows for the wackes, which areenvisagedas distal and more hydrated facies. These flows have mobilized weathered ignimbrite outcrops and/or redeposited pyroclastic rocks of MTVG, and that the same has happened through the action of more fluid agents depositing the arenites ofthe MAA units. The following observations support this hypothesis and lead to the conclusionthat the interbeds are redeposits of the ignimbrites: 1) Similar color due to the same components in matrix, gradational to paler hues in MAA. 2) Predominance of lithic fragments belonging to the sixth cooling unit. 3) Crystal clasts with similar morphological and optical features, and the same mean abundance (F>QZ>BT) relationships. 4) Relative increase in quartz content in the sequence MTVG ÔDVG ÔMAA, result of diminishing content of metastable feldspar and biotite in the exogenous cycle (mainly weathering and abrasion). 5) Decrease ofthemaximum diameter of feldspar and quartz in the sequence of point 4, which is interpreted in the same way. 6) Greater reduction in the maximum size of feldspar compared to quartz in the point 4 sequence, here attributed to lesser abrasive resistance of feldspar. The occurrence of these volcaniclastic debris flowsmakes more coherent the association of ignimbrites and eolian sandstones previously known for this stragraphic unit. DVG units are envisaged as an intermediate group between the source pyroclastic rocks, and the more mature reworked deposits of turbulent and fluid flows (MAA). Mass-wasting deposits like those of DVG are compatible with general arid conditions, mentioned locally and regionally for the interval of time represented when special regimes of pluvial precipitation have been invoked in relation to unusual continental masses. Finally, and though more detailed and regional studies will be needed, it is consideredthe association of MTGV, DVG and MAA deposits represents the accumulation in intermediate to distal settings of extracaldera facies.

Keywords : Stratigraphy; Petrography; Lithofacies; Tuffs.

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