Time-Correlative Recovery of Milankovitch-Scale Cyclicity From An Eocene Fluvial-Deltaic System, Southern Pyrenees, Spain
Abstract
Given the controversial state of cyclostratigraphy and the important contributions it provides to the construction of the geologic time scale it is important to know what processes encode environmental changes and what data sets best preserve the integrity of the stratigraphic signal. We test cyclostratigraphy based on rock magnetic data to calibrate time at sub-magnetic chron scales to reconstruct a ~100 kilometer-long transect of an ancient fluvial-deltaic system. Rock magnetic data provides an objective record of multi-frequency cyclicity we interpret to be climate cycles preserved in the Tertiary Ainsa-Jaca-Pamplona Basins. Anhysteretic Remanent Magnetization (ARM) from rocks dominated with low coercivity magnetite and Fe-sulfides and Isothermal Remanent Magnetization (IRM) from rocks dominated by high coercivity hematite and goethite shows the potential to trace correlative beds from terrestrial watersheds to the deep marine basin across facies boundaries. The fluvial Escanilla Fm.-Campodarbe Gp. along the Eripol-Olson section varies from sheet sandstones and overbank siltstone to amalgamated gravels up-section, is 910 m long, records chrons C19r to C15n, and was deposited at a rate of 14cm/kyr. A modified S-ratio, which takes advantage of the differences in coercivities of goethite and hematite was used to assess relative mineral abundance and shows variations in the ratio of goethite : goethite + hematite concentrations around the frequencies expected for short eccentricity (~100 kyr) and obliquity (~40 kyr). A 5 T field saturation IRM (SIRM) was first applied followed by alternating field (af) demagnetization in a 100 mT field (=goethite + hematite concentration), followed by the application of a 1 T backfield and 100 mT af demagnetization (= goethite concentration only). The correlative deltaic Arguis Fm. section coarsens up section from a condensed middle-outer neritic glauconite sandstone and marl sequence to proximal prodeltaic shales and siltstones, to nearshore delta front sandstones of the Belsue-Atarés Fm. We recognized C19n to C16r in the 840 m section which was deposited at rates of 20 cm to >1 m/kyr (Kodama et al. 2010). The deposits record eccentricity and precessional Milankovitch frequencies in ARM data collected with a peak alternating field of 100mT and a DC field of 0.97μT (=fine-grained (~<3μ) ferromagnetic mineral content). Further west at Berdun, the Pamplona Fm. represents distal turbidites of the prodelta. The Pamplona Fm. is also of Baratonian and Priabonian in age and includes chrons C17 and C18. The primary paleomagnetic directions are recorded by both magnetite and Fe-sulfides in the section. Time frequency analysis of ARM data collected with a peak af of 25 mT and DC field of 0.5<μT shows cyclicity at short eccentricity and precessional frequencies. Changes in rainfall and river output modulated bed thickness, sediment grain size, and the concentration of magnetic minerals reaching the delta to encode climatic changes.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMGP13B1117A
- Keywords:
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- 1535 GEOMAGNETISM AND PALEOMAGNETISM / Reversals: process;
- timescale;
- magnetostratigraphy;
- 1540 GEOMAGNETISM AND PALEOMAGNETISM / Rock and mineral magnetism;
- 9335 GEOGRAPHIC LOCATION / Europe