Linking alkaline shallow-lakes and travertine depositional systems with pre-salt carbonates: lessons from arid northwest Argentine

ASTINI R.A.; GOMEZ, F.J.; MORS, R.A.

Rio de Janeiro

Simposio; Carbonate Advances and New Challenges at E&P, Brazilian Petroleum Conference; 2018

UFRJ-Petrobras

Northwest Argentina (22º-32º SL) is a region where the Andes show a marked east-west weather gradient due to orographic effects on atmospheric circulation and influence of the arid diagonal crossing Argentina from northwest to southeast. Additionally, to having segments with intense volcanism and seismicity, being extremely arid and tectonically compartmentalized, helps developing internal drainages with alluvial fans terminating in a variety of shallow lakes. These may range from mud-flats to saline-pans and saline to locally alkaline shallow-water lakes that yield little vegetation and are mostly dry, although in few cases they develop palustrine rims surrounded by reeds and poorly developed soils. This particular landscape, that mimics underfilled basin conditions known as intermontane basins and ranges, extends from the Puna Plateau into the broken foreland of the Sierras Pampeanas and is locally influenced by low and high enthalpy hydrothermal surface manifestations that often give rise to ?travertine depositional systems? (TDS). Travertines (and their mild-equivalents-tufas) are defined by a set of textural, mineralogical, geochemical, petrophysical, geomorphological, sedimentological and stratigraphic criteria. Of course, like about any other basin filling materials TDS should have subsurface seismic expression and yield lateral relationships with other time-correlative stratigraphies (pertaining to particular systems tracts). However, because models are still scarce, it seems difficult to analyze time-correlative depositional systems and discern on whether or not they represent truly time-equivalent (synchronous) or slightly time-transgressive (diachronous). This is mainly due to time-and space-scale resolution limitations. In the present talk, based on recent and modern analogues from Argentina, I will speculate on what we can expect in terms of potential stratigraphic stacking and lateral relationships with coeval depositional systems in the pre-salt carbonates. We will show you that ?grainstones? (as a product of rework) should be expected as the most common ?preserved record? of TDS, along with other lacustrine and alluvial systems within the broad framework of fragmented landscapes. On the base of accommodation potential and hydrological grounds, these can somehow be compared with rift-settings and thus may serve as modern analogues to the pre-salt carbonate settings.Because travertines and tufas are born as rocks and not as unconsolidated sediments, their late compaction might be limited. Consequently, reworking products may retain many of the primary fabrics and textural characteristics (a variety of variously recrystallized ?cruststones?, ?spherulitestones?, wackestones and boundstones) inherent of the typical constructive stage carbonates. Moreover, their recycling during degradation stages might generate thick packages of peculiar cloudy grainstones-packstones (and coarser equivalents), which indicate the unsteadiness of these continental chemoherms and their relatively low-preservation potential in the rock record. Hopefully, because within rift-sag frameworks both tectonics and sedimentation increase accommodation, TDS can exceptionally be preserved, but the fact that they represent topographic highs promotes partial or total cycling within largely contemporaneous alluvial and lacustrine settings.