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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 66 (1996)No.
5. (September), Pages 928-934
On the Interpretation of Subglacial Till Fabric
Stephen R. Hicock (1), James R. Goff (2), Olav B. Lian
(1)(*), Edward C. Little (3)
ABSTRACT
The modal distribution of stone long-axis fabrics and their respective
eigenvalues can be used to infer the genesis of subglacial till. In this
paper we offer a two-axis diagram that compares fabric modality to fabric
isotropy (S3/S1) and addresses the problem of eigenvectors
falling between the modes of some well-developed till fabrics with low
eigenvalues. Our simple five-fold scheme of modality categories includes:
(1) unimodal clusters, (2) spread unimodal, (3) bimodal clusters, (4) spread
bimodal, and (5) polymodal to girdle-like fabrics, and requires the analyst
to study equal-area, lower-hemisphere (Schmidt) plots of the fabric data.
After assigning the fabric to a morality category, isotropy is calculated
and both results are plotted on the graph, whic helps to separate two main
fields of subglacial till: (1) lodgement and subglacial meltout tills,
and (2) deformation fill. On the basis of selected published fabrics from
tills at modern glaciers, as well as our own Pleistocene till data, lodgement
and subglacial meltout tills tend to have unimodal or bimodal fabrics.
In contrast, deformation tills and tills that experienced multiple processes
tend to have polymodal to girdle-like fabrics. Some overlap occurs between
fields be. cause of the complex nature of till formation (i.e., because
pure end-member till facies are rare and most tills are hybrids).
We strongly recommend that Schmidt plots be visually analyzed and
used in conjunction with eigenvalues when studying till. However, fabric
data alone is not enough. Multiple criteria including structural, lithologic,
and stone morphologic data from the till must also be considered before
drawing conclusions on till genesis. Furthermore, if eigenvectors fall
between fabric modes, then they cannot be used to indicate former ice movement
directions. Finally, our new modality-isotropy diagram may have wider applications.
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