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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 22 (1972), Pages 222-222

Abstract: Chertification Phenomena in Antarctic and Pacific Deep Sea Sediments - A Scanning Electron Microscope/X-Ray Diffraction Study

F. M. Weaver (1), S. W. Wise (1)

ABSTRACT

Diagenetic sequences leading to the formation of deep sea chert have been studied in drill and piston core samples taken in the Antarctic and Pacific Oceans. A pure white chert cored at the base of a Neogene diatom/radiolarian ooze sequence on the Kergulean Plateau (Southern Ocean) contains no volcanic alteration products and was derived solely from the solution and reprecipitation of biogenous opaline silica. The chert consists entirely of alpha-cristobalite spherules (5 to 10 microns in diameter) with some strongly etched fragments of siliceous Previous HitmicrofossilsNext Hit.

Cristobalite spherules identical to those reported from the Antarctic as well the Atlantic and Caribbean Ocean basins (Wise & Kelts, 1972, these proceedings) occur in Upper Cretaceous and Tertiary cherts and silicified limestones of the equatorial Pacific Ocean Basin (DSDP samples 7/64/1/11 cc and 7/64.1/1 cc). Diagenetic sequences involving the replacement of carbonate Previous HitmicrofossilsNext Hit and ooze matrix have been traced via scanning electron microscopy. A typical replacement sequence is as follows: 1) Cristobalite spherules are deposited within the interstices of coccolith oozes and chalks. 2) Concomitantly, Previous HitcalcareousNext Hit nannofossils begin to dissolve, thereby producing additional pore space. Some calcite is lost to the rock, but some is reprecipitated within the interstices as euhedral crystals up to 10 microns in diameter. 3) More cristobalite is precipitated while more calcite is dissolved. The process continues until the calcite matrix is completely replaced by cristobalite.

Foraminifera tests in the Previous HitcalcareousTop ooze apparently recrystallize or are dissolved outright. Cristobalite spherules are deposited within molds or within the recrystallized test. Finally, the recrystallized tests are dissolved leaving a cast of cristobalite.

In all examples studied, cristobalite is deposited as an authigenic mineral by means of inorganic chemical precipitation. In no instance was a gel phase observed, and none is thought to exist in the formation of deep sea chert.

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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES

(1) Department of Geology, Florida State University

Tallahassee, Florida

Copyright © 1999 by The Gulf Coast Association of Geological Societies