Stratigraphy and depositional architecture of lobe complexes across a range of confinements: Examples from the Late Paleozoic Paganzo Basin, Argentina

Abstract

The well-exposed, Late Paleozoic glacially-influenced Paganzo Basin offers a unique opportunity to compare and contrast the stratigraphy and depositional architecture of turbidite sheet/lobe complexes with distinct degrees of basin-scale confinements. These include sandstones deposited in narrow paleofjord (Quebrada Las Lajas), moderately narrow paleovalley (Quebrada Grande) and broad depression (Cerro Bola) that are part of the same synchronous transgressive-regressive succession recorded in the basin. This work emphasizes the importance of scales of observation and the importance in make the distinction between the degree of confinement experienced by individual flows and the degree to which the system as a whole is limited by the basin size and geometry. In Quebrada Las Lajas, sheet-like sandstones form lobe complexes largely reflecting the basin physiography. The succession was subdivided into two turbidite stages (Ts1 and 2), separated by a finer-grained interval (F1) that together represents lobe complexes and interlobe systems. Individual sandstone beds are laterally and longitudinally continuous and onlap the paleofjord walls, suggesting vertically-stacked beds. In Quebrada Grande paleovalley, the succession was subdivided into three stages (Ts1 to Ts3), representing lobe complexes, also separated by finer-grained packages (F1 and 2) ascribed to interlobe phases. Individual sandstone beds are truncated, thinning and pinching out, making them hard to correlate between logged sections. This depositional pattern may suggest that lateral stacking dominates during the construction of lobe complexes. At Cerro Bola, the succession was subdivided into five stratigraphic units (Ts1 to Ts5). These units also onlap the basin margin. Individual sandstone beds of Ts1 and Ts2 are similar to those recorded in Quebrada Grande area, also suggesting compensational stacking dominated systems. However, individual sandstone beds within Ts4 are thicker relative to the other units and more continuous longitudinally, reaching the basin margin thus pointing to larger volume flows ascribed to a vertically-stacked pattern. On the other hand, Ts3 and Ts5 correspond to the thinner sandstone beds in the succession and, as these intervals are as thick as the others, they may be attributed to lobe off-axis to fringe settings. In all three areas, the lobe complex geometry is determined by the basin-scale confinement and, at this scale, all of them are represented by more or less tabular, large sandstone bodies showing more similarities than differences. Each of these three successions record an upward increase in the sand-to-mud ratio, which may denote flow ponding or progradation of up-dip deltas that fed these systems.