SEDIMENTOLOGICAL AND MAGNETIC ANISOTROPY ANALYSES ON PALEOCURRENT DIRECTIONS IN TURBIDITES FROM THE NORTHERN APENNINES (ITALY)
Creators: name: DALL'OLIO, ELEONORA
Publisher: name: Universit� degli Studi di Milano
Year: 2012

description: This PhD thesis focuses on the validation of an objective method to define paleocurrent directions in turbiditic systems: the anisotropy of magnetic susceptibility (AMS). The final purpose of this study is to calibrate paleocurrent directions estimated with this geophysical method to directions estimated from classic sedimentological indicators, and therefore verify the applicability of this method to cases where sedimentological paleocurrent indicators are absent such as in drill cores.
Two selected turbiditic units outcropping in the northern Apennines, the Marnoso Arenacea Formation (Miocene, northern Apennines) and on the Castagnola Formation (Oligo-Miocene, Tertiary Piedmont Basin), were investigated. These basins have different depositional settings: the Marnoso Arenacea Formation filled a foredeep basin nearly 200 km long and 60 km wide, whereas the Castagnola Formation filled an episutural basin 6 km long and 4 km wide. AMS analyses were successfully applied to both formations on a total of 853 samples taken in a wide range of depositional intervals selected by means of detailed sedimentological analyses, and were successively cross-validated by direct estimates of flow directions from sedimentological indicators (ripple marks, flute marks, etc.).
In the Marnoso Arenacea Formation, a robust correlation between magnetic fabric and paleocurrent directions obtained from sedimentological indicators was found in massive, parallel-laminated, and cross-laminated sandstones. These depositional intervals show well-clustered AMS data with an overall flow-aligned fabric. Instead, highly dispersed AMS fabrics are apparently common in convoluted and undulated sandstones as well as in debrites, suggesting depositional processes that partially prevented grains? orientation (e.g., en masse freezing) or post-depositional processes that disrupted the original current-induced fabric (e.g., post-depositional dewatering). AMS fabrics typical of deposition in standing water was observed in the hemipelagites of the Castagnola Formation. Instead, in the fine-grained sediments of the Marnoso Arenacea Formation (White Marlstone beds), an AMS fabric interpreted as current-induced was observed and interpreted as due to muddy contourites.
The study on the Castagnola Formation was carried out primarily to evaluate the effects of basin confinement on turbidity flow dynamics. In this small, confined turbidite system, we observed a strong correlation between magnetic fabric and bed-thickness distribution whereby beds thicker than ~1.20 m show high magnetic fabric variability and maximum susceptibility axes dispersion, whereas beds thinner than ~1.20 m show better developed magnetic fabrics with maximum susceptibility axes oriented consistently parallel to the mean paleoflow direction from flute casts. We believe that ~1.20 m represents a thickness threshold separating large flows that covered the entire basin floor interacting in a complex fashion with the basin?s margins from small volume flows that did not interact with the basin?s margins and produced better-defined flow-aligned AMS fabrics.
Potential future developments of this thesis are:
(1) deepening of our understanding on the relationships between grains? orientations and magnetic minerals that contribute to the AMS signal by means of textural analyses, neutron diffraction and x-ray tomography. Preliminary results indicate that paramagnetic muscovite principally controls the observed current induced AMS fabric;
(2) testing the AMS method on drill core samples, where flow marks (i.e., flute casts) are either absent or non-observable.