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Lithostratigraphy from downhole logs in Hole AND-1B, Antarctica

Geosphere

By:
, , , , , , , , ,
DOI: 10.1130/GES00655.1

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Abstract

The ANDRILL (Antarctic Drilling Project) McMurdo Ice Shelf (MIS) project drilled 1285 m of sediment in Hole AND–1B, representing the past 12 m.y. of glacial history. Downhole geophysical logs were acquired to a depth of 1018 mbsf (meters below seafloor), and are complementary to data acquired from the core. The natural gamma radiation (NGR) and magnetic susceptibility logs are particularly useful for understanding lithological and paleoenvironmental change at ANDRILL McMurdo Ice Shelf Hole AND–1B. NGR logs cover the entire interval from the seafloor to 1018 mbsf, and magnetic susceptibility and other logs covered the open hole intervals between 692 and 1018 and 237–342 mbsf. In the upper part of AND–1B, clear alternations between low and high NGR values distinguish between diatomite (lacking minerals containing naturally radioactive K, U, and Th) and diamictite (containing K-bearing clays, K-feldspar, mica, and heavy minerals). In the lower open hole logged section, NGR and magnetic susceptibility can also distinguish claystones (rich in K-bearing clay minerals, relatively low in magnetite) and diamictites (relatively high in magnetite). Sandstones can be distinguished by their high resistivity values in AND–1B. On the basis of these three downhole logs, diamictite, claystones, and sandstones can be predicted correctly for 74% of the 692–1018 mbsf interval. The logs were then used to predict facies for the 6% of this interval that was unrecovered by coring. Given the understanding of the physical property characteristics of different facies, it is also possible to identify subtle changes in lithology from the physical properties and help refine parts of the lithostratigraphy, for example, the varying terrigenous content of diatomites and the transitions from subice diamictite to open-water diatomite.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Lithostratigraphy from downhole logs in Hole AND-1B, Antarctica
Series title:
Geosphere
DOI:
10.1130/GES00655.1
Volume
8
Issue:
1
Year Published:
2012
Language:
English
Publisher:
The Geological Society of America
Publisher location:
Boulder, CO
Contributing office(s):
Branch of Regional Research-Central Region
Description:
14 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
First page:
127
Last page:
140
Other Geospatial:
Antarctica