Naslov
Petrography and clay mineralogy of Eocene clastic rocks from the Tremp-Graus-Ainsa basin, Southern Pyrenean Foreland (Spain) - Reconstruction of sediment provenance and burial diagenesis

Autori
Perret, M ; Šegvić, Branimir ; Puigdefabregas, C ; Moscariello, A ; Clark. J ; Fildani, A ; Dykstra, M ; Castelltort, S

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Skup
14th Swiss Geoscience Meeting

Mjesto i datum
Ženeva, Švicarska, 18.11.2016. - 19.11.2016

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
provenance ; source to sink ; Pyrenean foreland basin

Sažetak
Provenance studies improve our understanding of terrigenous material delivery and deposition into sedimentary basins. This information addresses problems of sediment distribution patterns and mountain range morphological evolution. Thanks to excellent outcrop continuity and stratigraphic constraints, the South Pyrenean foreland basin provides an ideal natural laboratory to study sediment pathways and provenance in a tectonically active basin during a period of important climate variations (e.g. early and middle Eocene climatic optima, Eocene-Oligocene transition). Previous provenance studies have focused on sandstone petrography of the fluvial-alluvial deposits of the Tremp-Graus sub-basin and, separately, on the deep-marine turbiditic environment of the Ainsa sub-basin, without considering the continuum of their source-to-sink relationships. Herein we present results from a range of analytical techniques on clastic and basement rocks from the whole source-to-sink system of the Tremp-GrausAinsa basin (Southern Pyrenees, Spain): optical mineralogy (OM), automated and classical electron microscopy (QEMSCAN© and SEM-EDS), and X-ray diffraction (XRD) on clay fraction. These tools allow us to study the sediment mineralogy and sediment provenance in order to understand the paleogeographic evolution of the Tremp-Graus- Ainsa basin during Eocene time. In addition, our results on clay mineral dynamics and albitization of K-feldspars shed new light on the thermal and geodynamic evolution of the basin and its diagenetic history. Overall, we show that most of analysed clastic rocks are hybrid arenites consisting of high contents of quartz (up to 51%), feldspar (5-18%) and carbonate (11-86%), whereas mica (up to 7%) and clay minerals (up to 8%) are reported as minor phases. Clay parageneses in the different basins are similar in terms of smectite, mixed-layer illite- smectite (I-S), kaolinite, chlorite, and illite. Still, we observe that the total smectite content diminishes westwards, i.e. downstream the sediment routing system, with a lack of discrete smectite in the Ainsa basin, while I-S is progressively transformed into high-charged illite compositions. Heavy-mineral (HM) content of analysed clastic rocks is dominated by pyrite, rutile, titanite, and apatite. Based on the composition of the adjacent crystalline basement (granites and gneisses) and pre- Mesozoic sedimentary or low-grade metamorphic rocks (sandstones, schists, and phyllites) no single source region or lithology, which might have served as a material feedstock, can be identified.However, the HM ratios (zircon-tourmaline-rutile vs. apatite vs. other heavy minerals) vary downstream deposition. The ratio of unstable heavy minerals (apatite and other heavy minerals) decreases whereas the proportion of stable heavy minerals remains equivalent or increases along the system indicating fractionation due to weathering and diagenesis. The I-S proportions within the clay fraction of analysed sediments suggest that smectite was the starting material at deposition time, formed by weathering of feldspars and mica in the parent material. Once buried, the illitization of smectite is depth dependent, suggesting that the Tremp-Graus sediments were buried under 3-3.2 km of sediment, assuming a standard geothermal gradient of 25ºC/km (foreland basin). Conversely, illitization of smectite in the Ainsa basin reveals that burial there exceeded 4 km of depth. Such a finding is consistent with the westwards deepening of the whole foreland depositional system in the Pyrenees. This result is corroborated by the observed prograde albitization of detrital K-feldspars, which is found to be total (100%) in the Ainsa basin, indicating burial temperatures higher than 100ºC. By contrast, in the TrempGraus basin, albitization remained incomplete and simply shows a depth dependence relation. This comprehensive petrographic and diagenetic study allowed an in-depth comprehension of the properties and development of the potential sandstone reservoir rocks. Still, in order to better understand the complex provenance and interactions between sediment sources, more effort will be placed on the comparative geochemical – whole rock and phase chemistry – inquiry of basin sediments and of their surrounding basement rocks.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

POVEZANOST RADA