163-G 2.3

Depositional and exhumation history of the Cretaceous Lomas de Olmedo rift basin

The Salta Rift in NW Argentina is an intracontinental rift with thick syn- and post-rift deposits that have been extensively explored for petroleum. The Lomas de Olmedo and the Tres Cruces sub-basins are E-W and N-S trending portions of the rift, respectively. The majority of the former sub-basin lies in the subsurface, to the east. The transition zone between these rift segments outcrops  ~20 km east of Humahuaca, where the thick Cianzo syncline provides stunning exposures of the entire syn- and post-rift section. The Hornocal fault bounds the syncline and formed the northern margin of the rift. Published structural mapping and unpublished apatite fission track data show that the Cretaceous Hornocal normal fault has been inverted in the Miocene. There are prominent angular unconformities in the syncline that separate syn- and post-rift strata; these appear to delineate growth strata associated with the fault. Therefore, this locality provides an excellent opportunity to study proximal rift-margin sedimentary deposits as well as to study the exhumation history of the adjacent rift shoulder. We plan to conduct zircon (U-Th)/He thermochronology to constrain the Cretaceous exhumation of the rift shoulder. We will also examine post-rift, Cenozoic reactivation of structures using apatite fission track thermochronology. In the basin, the syn-rift sequence reportedly contains carbonate nodule-bearing paleosols. We will analyze these using both stable isotopes for paleoclimate analysis and U/Pb dating for age control. Stratigraphic, sedimentologic, provenance, and structural studies will characterize the basin fill. We plan to examine along-strike variations of rift shoulder exhumation with the aid of subsurface data from the Lomas del Olmedo sub-basin to the east.


10|2015 – 09|2018

The influence of inherited extensional structures on the growth of basement-cored ranges and their foreland basins

The reactivation of pre-existing normal faults during subsequent contractile deformation can exert a profound influence on both exhumation of ranges and sedimentary basin formation. This topic has been addressed in the well-exposed, arid intermontane basins and ranges of NW Argentina as well as more humid sectors of the broken foreland farther east. However, the more humid eastern flank of the Sierras Pampeanas has been less well-studied, despite its simpler structural history. In the Tucumán and Choromoro basins, structures that deform the foreland basins as well as their sedimentary thicknesses have been well analyzed using industry seismic reflection data. However, to date, a quantitative source-to-sink approach has not been applied in this area. Therefore, we will integrate thermochronology, structural data, and provenance and basin analysis to test whether Cretaceous normal faults exert a first-order control on the pattern and magnitude of Cenozoic contractile deformation in basement-cored ranges as well as subsidence patterns in the adjacent foreland basins. The ages of the largely continental foreland-basin fill remain imprecisely constrained, with the exception of the strata corresponding to the Middle Miocene Paraná marine transgression, reflecting both the difficulty of dating non-marine sequences and the poor outcrop quality. To address this problem, we will constrain depositional ages by dating interbedded volcanic ashes using either zircon U/Pb or 40Ar/39Ar dating. Furthermore, we will collect sandstone samples for detrital zircon geochronology using LA-ICP-MS to provide maximum depositional ages. Together, this approach will enable us to derive realistic models of flexural basin subsidence, especially in the vicinity of spatially disparate basement uplifts. Reconstructions of range exhumation and sediment deposition will be accomplished by using apatite fission-track dating on detrital sandstones to determine changes in lag time. Where possible, we will conduct double dating (fission track and U/Pb) to better constrain a local versus distal source for these sediments. We will also extend the database of basement thermochronologic data in Sierra Aconquija and the Cumbres Calchaquies to evaluate the timing and magnitude of exhumation of both Cretaceous and Cenozoic structures. These two ranges are along strike from each other and bound the Tucumán and Choromoro basins, respectively used thermochronology to delineate the extent of Cretaceous rift structures within these ranges. As the Sierra Aconquija was strongly exhumed in the Neogene while the Cumbres Calchaquíes were primarily exhumed during the Cretaceous, this region provides an ideal setting to examine the influence of pre-existing normal faults on Cenozoic evolution.