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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 55 (1971)

Issue: 2. (February)

First Page: 333

Last Page: 334

Title: Recent Silica Gel from Saline Lake in Galapagos Islands: ABSTRACT

Author(s): Paul A. Colinvaux, Daniel Goodman

Article Type: Meeting abstract

Abstract:

A 4-m drill core of undisturbed sedimentary rock from the crater lake on Isla Genovesa (Tower) has well-defined banding, revealing a complex depositional

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history spanning at least 5,000 years. Amidst the bands are intervals of translucent green gel, with white granular inclusions in places. One section of gel occupies an interval of nearly 10 cm, and the uppermost is within 20 cm of the Previous HitrockNext Hit/Previous HitwaterTop interface. Three hypotheses describing the structure of the gel seemed plausible: that it was a proteinaceous gel, a polysaccharide gel, or a silica gel. Infrared spectroscopy of a sample which had separated during storage revealed no absorption at 6 µ, precluding the presence of peptide bonds. There was little in the spectra to suggest that saccharides were present, but strong absorption between 9 and 10 µ suggested abundant silicon. X-ray diffraction of samples soaked in distilled water gave strong indications of halite, which wou d mask the presence of silicon. D.C. arc emission spectroscopy showed the sample to be rich in silicon, sodium, calcium, and magnesium, and to contain trace amounts of other elements normally present in seawater. We conclude that the sample is a silica gel, hydrated with seawater and containing microcrystals of sodium and magnesium salts.

The gels formed on the bottom of a lake which is now 30 m deep and with a salinity of 52 ^pmil, or half again as salty as seawater. The lake is highly productive so that reducing conditions with free sulfides prevail at the bottom. This fertile condition is apparently maintained by a large input of guano from the immense colonies of seabirds which inhabit the island. Study of diatoms and other fossils incorporated in the gels, and of the pigments preserved in them, is enabling us to reconstruct the environment in which a silica gel forms under the waters of this crater lake. These conclusions may prove useful when seeking to explain the origin of cherts.

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Copyright 1997 American Association of Petroleum Geologists