ABSTRACT: Origin of Whitings: A Biologically Induced Nonskeletal Mechanism

L. L. Robbins, P. L. Blackwelder

The origin of lime mud is an important geological problem because much of the earth's limestones were composed of lime mud prior to lithification. Yet a complete understanding of the potentially many modes of origin of this material are not known. The study of whiting sediments, drifting patches of muddy water in shallow tropical seas, has shed light onto one significant mode of carbonate production.

Whiting sediments were collected August 1988 adjacent to Andros Island, Bahamas (25°10.55^primeN; 78°32.37^primeW) and immediately preserved in glutaraldehyde. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of preserved and nonpreserved samples clearly demonstrate a relationship between cellular organic and CaCO₃ crystals. Oblong to semispherical organics, approximately 20-30 µ in diameter, covered with randomly oriented crystals, were observed in the SEM. TEM analyses of the samples show that cells and organics in varying stages of degradation are associated with needles and seed crystals. Crystals exhibit a random array, having both perpendicular and parallel orientations to the organic membranes. Further data on the biochemic l nature of the organics indicate the presence of highly negatively charged proteins and macromolecules. Clear water adjacent to the whiting cloud showed fewer organics, none of which were associated with crystals.

These data suggest the precipitation of crystals in whitings are a geologically induced, but not mediated, event which occurs as a result of highly charged organics acting as nucleation substrates. The formation of CaCO₃ crystals, therefore, is dictated by the organic microenvironment. This model helps to reconcile the differing hypotheses concerning the origin of whitings.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990