Sedimentary features and paleoenvironments of the Late Miocene volcanic setting in the Famatina Ranges, Central Andes

MARTINA, F.; DÁVILA, F.M.; PETRINOVIC, I.; ASTINI, R.A.

Belem. Brasil

Congreso; III Latinamerican Congress of Sedimentology; 2003

Between 27º and 33º S latitude, the modern Central Andes shows a flat-slab segment that corresponds with a gap in arc volcanism between the latest Miocene to Recent. According to previous work, Neogene volcanism suggests that flat-slab shallowing would have started at ~18 Ma. A well-constrained Late Miocene-Pliocene explosive volcanic activity is recorded in Famatina, located in the Central Andes of western Argentina. Thick Cenozoic synorogenic strata (> 3700 m) crop out in the central part of Famatina (28º30´ -29º15´S), embracing Early Miocene Del Crestón Formation, Mid Miocene-Pliocene Angulos Group, and the overlying Pleistocene Santa Florentina Formation. The Angulos Group (“Estratos Calchaqueños”) is divided into three units, the uppermost, which is the focus of the present study. The El Durazno Formation is approximately 1100 m and can be divided into four members. The Río Blanco member (~350 m), superbly exposed along the homonymous creek and toward the upper section, is composed of alternating light gray to pinkish sandstones and conglomerates with interbedded tuffs of up to 10 m thick dated between 6.9±1.2–4.0 ±0.8 Ma. Five facies can be differentiated: (1) massive tuffs, (2) volcanogenic conglomerates, (3) volcanogenic-arkosic sandstones, (4) sandy conglomerates, and (5) sandy siltstones and shales. 1) Massive tuffs are composed of fine- to medium-grained pale tuffs with lapilli, pumiceous blocks and lithics supported in a cineritic matrix, arranged into sharply defined sheet-like (> 1 km.), 1-10 m thick deposits, that commonly yield positive relief. Their bases are non-erosive and their tops are strongly irregular with cut & fill structures that frequently yield vertical walls. Internally they are massive or graded and do not show traction structures. Reverse grading in the pumiceous blocks and normal grading in the lithics is common. Local development of differentially accumulated coarse material (relatively clean and grain-supported) vertically arranged in the upper third of some deposits has been observed. An outstanding feature is the extremely poor sorting where crystals, crystal fragments, glass shards and pumiceous fragments are supported in a fine-grained ash matrix. Lithics are subrounded to angular, reach 4 cm maximum size, and are composed by rhyolites, granites and metapelites, whereas pumiceous clasts are rounded to subrounded, reach ~15 cm, and yield abundant biotite and hornblende. This facies is interpreted as slightly welded tuffs (visibly more resistant than the surrounding facies) indicating nearby explosive centers. Notable, cut & fill structures with vertical walls allow interpreting early welding compaction. 2) Volcanogenic conglomerates are lenticular to intermediate with sharp erosive boundaries and have between ~2 a 40 cm thick. They range from clast- to matrix-supported and show crude stratification. They are composed by 100% poorly sorted, subangular to subrounded pumiceous blocks (~20 cm maximum size), frequently flattened, suspended in an epiclastic sandy-muddy matriz. Concentration of pumiceous blocks toward the top is a common feature. These conglomerates are interpreted as reworked volcaniclastic deposits. Local accumulations of relatively soft pumiceous pebbles and cobbles toward the top of the beds indicate hydrodynamic buoyancy during transport by stream flow. Flattening is considered to be a product of compaction. 3) Volcanogenic-arkosic sandstones are poorly sorted laterally persistent normally graded sandy beds with good internal lamination (parallel with low-angle crosscuts grading up into ripple lamination), distinct planar contacts and thickness of up to 30 cm. Heavy mineral concentration is locally important and remarks lamination. Compositionally they range from pure arckosic to pumiceous rich. The latter, are commonly oversized, reaching granule size. These sandstones are epiclastic deposits that may be interpreted as event beds. Thin beds with erosive bases, internal grading and sedimentary structures point to decreasing energy flows, compatible with waning stage deposits. Frequent well-developed parallel lamination and low-angle truncations indicate predominance of high-regime flows. Thicker beds with medium size sets of cross lamination with higher repose angles are interpreted as possible fillings of migrating dunes in shallow high-sinuosity channels. 4) Sandy conglomerates are mostly lenticular, clast-supported, subrounded to subangular, coarse- to medium-grained conglomerates. Although most are poorly sorted some show good segregation. Thickness reach over 1 m and maximum clast sizes exceed 30 cm. Some beds are deeply incised into facies 1 and 3. Crude lamination dominates although normal grading is present. Local imbrication and grouping is also common. They are composed of in a variety of granites and volcanic rock (moslty andesites) pebbles, cobbles and boulders. Matriz is sandy to granule-rich, and has little interstitial fines. Its composition is compatible with that of the coarser fraction. They are interpreted as stream flow deposits in alluvial fan settings. Frequent channeling indicates a relatively high surface gradient. Sorting and roundness are compatible with relatively proximal environments. 5) Sandy siltstones and shales are a minor component in the section. Wave-ripple trains, profuse mud cracks and local bioturbation are present. They represent settling of fines from suspension in ephemeral stream settings. Occasional wave action has allowed preservation of wave ripples and later dissecation favored mud shrinkage Volcaniclastic rocks (slightly welded tuffs) together with a variety of volcanogenic-epiclastic deposits allow reconstructing the history of explosive volcanism in the foreland basin record preserved in Famatina. Facies association indicates recurrent volcanic-fluvial interaction. Although incompletely preserved due to the inferred high-gradient topography, as shown by frequent incision and grain size changes, textures and composition of the massive tuffs and volcanogenic conglomerates indicate a proximity to the volcanic centers. This interpretation is consistent with the flat-subduction geometry suggested by previous work to explain the coeval volcanism in a wide volcanic zone between the Famatina and the Sierras Pampeanas, more than 200 km east.