Geoscience Reference
In-Depth Information
10 Directional Data
Orthoceras
fossils from an outcrop at Neptuni
Acrar near Byxelkrok on Öland, Sweden.
Orthoceras
is a cephalopod with a straight
shell that lived in the Ordovician era, about
450 million years ago. A statistical analysis of
cephalopod orientations at Neptuni Acrar has
revealed a signii cant southerly paleocurrent
direction, which is in agreement with
paleogeographic reconstructions.
10.1 Introduction
Methods for analyzing circular and spherical data are widely used in earth
sciences. For instance, structural geologists measure and analyze the
orientation of slickensides (or striae) on fault planes. Circular statistics is
also common in paleomagnetic applications. Microstructural investigations
include the analysis of grain shapes and quartz c-axis orientations in thin
sections. Paleoenvironmentalists also reconstruct paleocurrent directions
from fossil alignments (Fig. 10.1). h ere are theoretically two types of
directional data in earth sciences: directional data
sensu stricto
, and oriented
data. Directional data, such as the paleocurrent direction of a river as
documented by l ute marks, or the l ow direction of a glacier as indicated by
glacial striae, have a true polarity. Oriented data describe axial data and lines,
such as the orientation of joints, without any sense of direction.
h ere are a number of useful publications available on the statistical
analysis of directional data, such as the topics by Fisher (1993) and Mardia
and Jupp (2000), and the chapters on the subject in topics by Swan and
Sandilands (2000), Davis (2002), and Borradaile (2003). Furthermore,
Chapter 9 of the topic by Middleton (1999) and two journal articles by
