Determining Flow Directions in Turbidites: An Integrated Sedimentological and Magnetic Fabric Study of Two Miocene Turbiditic Systems (Northern Apennines, Italy)

Abstract

Turbiditic systems are characterized by a great variability in size, geometry, facies, and stacking patterns. The development of depositional models at the basin scale is essential to understand this variability. Models require an accurate knowledge of the paleocurrent directions within the turbiditic systems. Traditionally, sedimentological current indicators (flute marks, ripple marks, etc.) are used to obtain paleocurrent directions, but these are not always present in outcrop sections and are virtually absent from drill cores. This limitation raises the need to identify an alternative, objective method to define paleocurrent directions in turbiditic successions. The anisotropy of magnetic susceptibility (AMS) is a useful tool to estimate paleocurrents in sedimentary rocks (e.g. turbiditic, fluvial, tide-dominated deltaic and estuarine environments). This method is based on the fact that a current is able to orient para- and ferromagnetic grains and minerals. The AMS ellipsoid often reflects the orientation imparted by the current to such grains. We experiment this method, in concert with classic sedimentological analyses, in two well-exposed Miocene turbiditic systems cropping out in the Northern Apennines (Italy): the Castagnola turbidite system (Tertiary Piedmont Basin) and the Marnoso Arenacea Formation (Northern Apennines foredeep). They are both characterized by well exposed strathigraphic sections and by the presence of evident sedimentological indicators of paleocurrent at the base of the beds, that have been used to validate the AMS measurements. As we were interested to calibrate this method and to determine which sediment composition and texture (grain size and sedimentary structures) work best for the application of the AMS methodology, numerous turbiditic sandstone beds have been sampled (nearly 900 samples collected) into different depositional intervals (e.g, fine- to medium-grained massive sands, fine- to medium-grained parallel-laminated sands and fine-grained cross-laminated sands). AMS fabrics have been compared to sedimentological indicators of paleocurrent direction at the base of turbidite beds; a good agreement between paleocurrents from flute casts and AMS measurements has been observed, even if a relatively small but consistent offset of ~15–20° seems to be present. Nonetheless, these data confirm the substantial validity of the AMS method as a tool to estimate flow directions in absence of sedimentological indicator.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015