Abstract: Comparison of Endolith Floras from Holocene-Pleistocene (Bahama-Florida) Ooids

Paul M. Harris, Karen J. Lukas, Robert B. Halley

Holocene ooids from Joulters Cays (Bahamas) sand shoals are bored in various ways by blue-green algae that (a) groove along the grain surface (Hyella caespitosa), (b) reside just beneath the grain surface (unidentified coccoid forms), and (c) tunnel extensively a few tens of microns within the grain (Hyella caespitosa). Borings attributable to fungi, sponges, or other endolithic organisms seldom are present in studied material. The microborings are morphologically distinctive and are documented with scanning-electron micrographs of open borings and resin casts. Microborings commonly fill with precipitated aragonite needles, obscuring the morphology of the voids. Gentle dissolution of ooid aragonite permits identification of several algal genera and their colony patterns.< P>

Most microborings penetrate only a few ooid lamellae, suggesting that boring takes place predominantly at and near the surface of the grains. Entire ooid micritization therefore may occur during stages of ooid growth and boring, rather than during a single episode of alteration.

Ooids from the Pleistocene Miami Oolite contain natural casts of microborings, some of which are similar in form to Holocene examples. These aragonite casts are more resistant to solution than surrounding ooid aragonite. They remain after most of the ooid is leached away and survive replacement of the ooid by low-magnesium calcite. Preservation of algal borings includes the following steps: (1) boring by the endolith; (2) infilling of the microboring by aragonite; (3) dissolution of the ooid aragonite, leaving natural aragonite casts; (4) precipitation of low-magnesium calcite around the casts; and (5) solution or replacement of the aragonite cast. Morphologic alteration of the microborings may take place during any or all of steps 2 through 5 and points out the difficulty of specific identification of endoliths in ancient rocks on the basis of fossilized microborings.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC