Abstract

LOPEZ GAMUNDI, Oscar R.. The Majaditas Formation (Carboniferous), western Precordillera (San Juan province): Lithostratigraphy and Facies. Rev. Asoc. Argent. Sedimentol. [online]. 2001, vol.8, n.2, pp.57-85. ISSN 1853-6360.

The Majaditas Formation (Carboniferous) [Amos and Rolleri, 1965] is exposed along the western flank of the Precodillera in the San Juan Province of western Argentina (Fig. 1). Its exposures are flanked to the south and north by discontinuous outcrops of Carboniferous deposits of similar age. The unit has been subdivided in six members (Basal Member, Diamictítico Member, Rojo I Member, Areniscas y Pelitas Moteadas Member, Rojo II Member and Superior Member) [Figs. 3 and 5]. Nine facies were identified, namely: Facies 1: deposits, confined to the basal part of the section (Basal Member), consist of clast-supported conglomerates and subordinate coarse- to medium- grained sandstones. Clasts in the basal conglomerates display flow-parallel long axis orientation (Fig. 7B), indicative of suspension rather than traction. The abundance of conglomerates and maximum clast size decrease up section. Overall, the basal third of the section shows a clear fining-upward trend (Fig. 6). These deposits were deposited by high-density gravity flows and tractional flows in a coarse-grained delta (fandelta) setting. Facies 2: deposits are massive pebbly mudstones intercalated within the facies 1 deposits. The exposures are areally limited. Some of the clasts show striations. Based on contextual evidence, the pebbly mudstones may be interpreted as part of the spectrum of shallow gravity flows that dominate the basal part of the section. Facies 3: is made up of laminated mudstones and interbedded lenticular diamictitites. Dropstones, indicative of ice-rafted material, are present in the laminated mudstones. The lenticular bodies of diamictites show in some cases two distinct lithologies (pebbly mudstone and pebbly sandstone) with sharp contacts surrounded by variably deformed mudstones (Fig. 9). Facies 4: consists of thin-bedded, low-density turbidites with complete (Ta-e) and base-cut (Tb-e) Bouma cycles. These facies deposits were locally affected by centimetric to decametric scale soft-sediment deformation (slumps, Fig. 10B and C). Flute casts and tool marks indicate a SWdirected paleoflow. Facies 5: deposits are dominated by well sorted, coarse- to medium-grained sandstones with 3D ripples and crossbedding stacked into multistorey (5 to 20 m thick), channelized complexes (Fig. 13). Occasionally, coarse lags drape the erosional base of the channelized bodies. Facies 6: is characterized by a heterolithic interbedding of fine-grained, current-ripple-laminated sandstones and subordinate mottled (bioturbated) mudstones (Fig. 15). This facies was produced by episodic sand deposition from unidirectional (seaward-directed) tractional currents with fine-grained sand - mud from weaker and less frequent currents. Facies 7: is made up of laminated mudstones with scarce, thin interbeds of mostly massive sandy mudstones and finegrained sandstones deposited in inner to outer shelf conditions mostly below wave base. Facies 8: deposits consist of thinly laminated, mica-rich, sandy mudstones with occasional current ripples and soft sediment deformation by loading. They are confined to the upper third of the section and exclusively associated with sandstones of the facies 9. Facies 9: is dominated by very well sorted, medium- to coarse-grained sandstones with planar and trough crossbedding and weakly wave-influenced current-ripple lamination. It is interpreted as the product of the migration of straight and sinuous dunes and ripples in response to dominantly unidirectional, seaward directed tractional currents, developed between delta plain and a waveinfluenced shoreface. Several facies associations (FA) have been defined. The FA 1+5 is characteristic of the basal fill of the section (Basal Member). Its fining upward trend reflects a period of initial coarse fill in a fan delta setting with increasing accommodation space. The FA 2+3+4, only present in the Diamictítico Member, represents transgressive conditions of flooding with settling of fine-grained material, ice-rafting and subordinate gravity flows. The FA 5+7, common in the Rojo I Member, is characterized by fining upward facies successions capped by flooding/abandonment surfaces. The basal succession of the FA 5+7, incised on FA 2+3+4 deposits, reflects the facies dislocation of greatest magnitude in the section via a basinward shift in facies where deltaic distributary channels rest on fine-grained sediments deposited below wave base. The upper half of the section (Areniscas y Pelitas Moteadas Member, Rojo II and Superior Members) is dominated by coarseningupwards facies successions or parasequences (FA 8+6+9a) interpreted as mouth bars in a river-dominated delta overlain by sand-dominated sediments deposited in transitional conditions between delta front and shoreface (FA 9b+8). All paleocurrents from a-axis clast imbrication, current ripples, flute casts, tool marks and cross-bedding invariably indicate a SW-directed paleoflow pattern (Fig. 18). This orientation remains constant throughout the section, irrespective of sedimentary structure or facies, reflecting a rather simple paleogeography for this basin margin. The dynamics of this basin margin was modified by changes in accommodation space probably regulated by a combination of subsidence and glacially influenced sea level changes.

Keywords : Majaditas Formation; Carboniferous; Precordillera; San Juan province; Paleogeography; Calingasta-Uspallata basin; Facies.

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