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.