Facies and architecture of a Carboniferous grounding-line system from the Guandacol Formation, Paganzo Basin, northwestern Argentina

SERGIO A. MARENSSI, ALFONSINA TRIPALDI, CARLOS O. LIMARINO Y ALBERTO T. CASELLI

GONDWANA RESEARCH

Elservier

Lugar: Trivandrum, Kerala, India; Año: 2005 vol. 8 p. 187 - 202

1342-937X

New outcrops of Middle Carboniferous glacigenic deposits found in the Guandacol Formation (western Paganzo Basin) are described in this paper. The study locality of Los Pozuelos Creek (northwestern Argentina) includes coarse-grained diamictites, rhythmites, laminated pebbly mudstones and shales that represent an expanded column of the Gondwanic glaciation in this region. Thirteen lithofacies recorded at the measured section have been grouped into three facies associations. Facies association I is composed of coarse-grained massive and stratified diamictites (lithofacies Dmm, Dms, Dmg, Dcs), laminated siltstones with dropstones (Fld) and interstratified  sandstones and mudstones (Fl, Sr). These rocks represent both tillites and resedimented diamictites closely associated to small water bodies where laminated siltstones with dropstones and stratified sandstones and mudstones were deposited. Facies Association II comprises couplets of matrix-supported thinly bedded diamictites (Dmld) and laminated mudstones with dropstones (Fld). This facies association results from the combination of three different processes, subaqueous cohesionless debris flows, coeval rainout of ice-rafted debris and settling of fine-grained particles from supension. Finally, Facies Association III is made up of laminated mudstones without dropstones, thin marl levels and scarse fine- to very fine-grained sandstones. This assemblage clearly suggests sedimentation in a deep marine environment below the wave base.        The architecture of the glacigenic deposits has been investigated using photomosaic panels. The geometry of the depositional bodies and facies suggest that Los Pozuelos Creek outcrops exhibit a well preserved three-dimensional example of a grounding-line system. In particular, three different subenvironments of a morainal bank were interpreted: a bank-front, a bank-core and a bank-back. The bank-front assemblage is characterized by coarse-grained, mainly resedimented, diamictites grading laterally to prograding clinoforms composed of interbedded matrix-supported thinly bedded diamictite and mudstones. The bank-core assemblage is formed by a stacking of coarse-grained diamictites where at least five major erosional surfaces, bounding four multistory diamictite bodies, can be recognized. Finally, the bank-back assemblage corresponds to discontinuous intervals of striated lodgement till, and coarse-grained resedimented diamictites showing important postdepositional deformation. The retrogradational stacking of the morainal banks indicate an overall glacial retreat and a glacioeustatic sea-level rise. Erosional surfaces at the base of each morainal bank suggest intervening short term episodes of ice advance.       The new data presented here confirm the existence of "true" tillites in western Paganzo Basin and suggest several (at least four) pulses of glacial advance and retreat during the Namurian glaciation in the region and permit a more refined interpretation of the glacial deposits in the Huaco area.