David A. Katz1, John D. Humphrey1
(1) Colorado School of Mines, Golden, CO
ABSTRACT: Earliest Diagenesis of Calcareous Red and Green Algae, San Blas, Panama, and Caicos Platform, southern Caribbean
Characterization of early diagenesis in red and green algae (Corallinaceae and Halimeda spp., respectively) was approached through comparison of Recent living and Holocene postmortem specimens. Living specimens were collected from the Caicos platform, whereas Holocene samples derive from near surface diagenetic environments of both the San Blas Islands and the Caicos platform.
Diagenesis in live Halimeda and Corallinaceae is biologically mediated. SEM and EMP analyses show calcification occurs in two stages: (1) fibrous isopachous aragonite needles (1-5 microns long) precipitate in inter-utricle spaces, experiencing competitive growth and altering to (2) anhedral equant aragonite (0.1-0.5 microns). Also, aragonite needles precipitated in open inter-utricle spaces can be up to 15-20 microns long. This early alteration should be distinguished from post-depositional diagenesis in the sediment. Early alteration such as the precipitation of anhedral equant aragonite is responsible for the micritic fabrics commonly found in Halimeda. Micritic fabrics are further enhanced during postmortem diagenesis by endolithic algae, Cliona, or other boring organisms on the sea floor.
This is the first study to document diagenesis in living Corallinaceae. Preliminary data suggest that porosity within the hypothallus skeleton is totally occluded by high-Mg calcite (HMC) cements (8-16 mole % MgCO3). Perithallus cements contain multiple generations of isopachous aragonite and HMC zoned cements (0.3-5 microns thick).
Integrated analyses of post-depositional diagenesis in red and green algae indicate pervasive recrystallization, neomorphism, and dissolution of these grains. Scanning electron microscopy shows recrystallization of both algae within the marine environment to varieties of equant micrite (0.1-1 microns) and cementation by isopachous fibrous to bladed fabrics (3-10 microns long). Loss of Mg in Corallinaceae indicates neomorphism is pervasive in the fresh water environment. Fibrous, bladed, and equant HMC and LMC cements are common diagenetic products in Recent vadose zones from the San Blas Islands.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado