--> Abstract: Aragonitic Pennsylvanian Phylloid Algae from New Mexico: The Missing Link, by B. L. Kirkland, C. H. Moore, Jr., and J. A. D. Dickson; #91004 (1991)

Datapages, Inc.Print this page

Aragonitic Pennsylvanian Phylloid Algae from New Mexico: The Missing Link

KIRKLAND, B. L., and C. H. MOORE, JR., Louisiana State University, Baton Rouge, LA, and J. A. D. DICKSON, University of Cambridge, Cambridge, U.K.

Remarkably well-preserved codiacean algae (Eugonophyllum and Anchicodium) retaining original aragonite are present in the Virgilian Holder Formation, Sacramento Mountains, south-central New Mexico. The algae are preserved in a 20-cm-thick packstone between two thick (> 5 m) shale beds. Aragonite is preserved as a felt-like mesh of needles in the algal skeletons, in the shell fragments of molluscs, in the walls of sponges, and in botryoidal and isopachous marine cements. The aragonite is confirmed by X-ray diffraction, by visual inspection of pristine aragonite needles with SEM, and by a high content of Sr as revealed by microprobe analysis. The average Sr content of the algae (9091 ppm, n = 21) is comparable to modern codiaceans.

Preservation of internal structure in Eugonophyllum was previously unknown. The medullary (interior) region of the Eugonophyllum thallus is composed of an aragonite felt punctuated by small (20 um diameter), parallel utricles. As in modern codiaceans, the utricles in the cortical (exterior) region of the thallus increase in diameter and their bulbous tips coalesce to form the outer cortex of the plant. In some samples the cell structure of this outer cortex is preserved by silicification and is similar to the cortex of modern codiaceans (Halimeda and Udotea).

This occurrence provides a key piece of evidence in support of hypotheses concerning the nature and origin of phylloid algal bioherms. Because the internal structure of most fossil phylloid algae is replaced by sparry mosaic calcite, taxonomic classification has been difficult even at the fundamental level of division (phylum). Our discovery confirms that at least some ancient phylloid algae resembled the modern green algae Halimeda or Udotea, and lends credibility to the suggestion that ancient phylloid algal mounds are analogous to modern Halimeda mounds of the South Pacific.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)