Print this pageApplication of 
Anisotropy
of Isothermal Remanent Magnetization (IRMA)
in High Resolution Facies Analysis: Examples from Delta and Submarine Fan
Systems
BASU, DEBNATH, and GANGOPADHYAY TANWI
Samples from deltaic and submarine fan deposits were used to determine
magnetic grain anisotropy due to deposition as part of facies analysis studies.
Anisotropy of isothermal remanent magnetization (IRMA) has been commonly used
previously in regional tectonic studies. IRMA's potential as a facies analysis
tool is only now being realized and the results of its application to two
separate basins, the Aikoma Basin and the Karoo Basin are presented here.
Oriented plugs are subjected to an artificially induced magnetic field along
multiple axes in the laboratory and an anisotropy ellipsoid is calculated. This
ellipsoid reflects either a depositional or a tectonic fabric. A depositional
fabric is characterized by the long axis of the anisotropy ellipsoid lying on
bedding and commonly directed parallel to paleocurrent while the short axis is
perpendicular to bedding. Anisotropy percent=100 (K[max]-K[min])/K[int], where
K[max], K[int], and K[min] are the maximum, intermediate and minimum axis
vectors representing the anisotropy ellipsoid. Anisotropy percent is a value
that is directly proportional to depositional flow and hence distinguishes
between various subenvironments within a depositional system.
IRMA studies have been conducted on deltaic deposits of the Pennsylvanian
Atoka Formation from the Arkoma Basin, Arkansas, and on submarine fan deposits
of the Permian Skoorsteenberg Formation in southwestern Karoo Basin, South
Africa. In the deltaic deposits percent anisotropy progressively increases
upsection within major deltaic progradational units. Low values (~10 percent)
represent delta-front facies while the overlying distributary mouth bar deposits
have characteristically higher values (~24 percent) with transitional values in
between. Crevasse splay subdeltas show a similar trend upsection. In the
submarine fan deposits various facies were tested. Layered sheet sandstones and
channel abandonment facies have the lowest percent anisotropy (~3 percent) while
levee-proximal overbank deposits and amalgamated sheet sandstones have the
highest values (~7 percent). Channelized facies have the highest scatter around
the mean anisotropy
value of ~12 percent.
The IRMA technique is a strong tool for high resolution facies analysis.