IDENTIFICATION AND DIAGENESIS OF A PHYLLOID ALGA, ARCHAEOLITHOPHYLLUMFROM THE PENNSYLVANIAN PROVIDENCE LIMESTONE, WESTERN KENTUCKY

Phylloid algae in the Pennsylvanian Providence Limestone (Desmoinesian ) in western Kentucky are typical in appearance: thin, arcuate, leafli ke forms with a calcite mosaic interior. Cathodoluminescence and backs cattered electron imaging reveal well-preserved skeletal structure and their identity as Archaeolithophyllum sp. Johnson, 1956 and A. missou riensum Johnson, 1956. Thin skeletal walls are composed of nonluminesc ent neomorphic calcite. Polygonal hypothallial and perithallial cells in the skeletons are filled by luminescent calcite cement. Chemical st aining also distinguishes cell morphology when ferroan calcite cement is present in skeletal pores, in contrast to the nonferroan calcite fo rming skeletal walls. Hypothallial cells in Archaeolithophyllum are 25 mu m x 50 mu m and are commonly arranged in arcuate layers. Skeletal walls (5 mu m thick) are composed of calcite and lack distinctive reli ct features, although some are microporous and contain fine, dissemina ted pyrite. Some thalli contain large irregularly shaped hypothallial cells (50-100 mu m) partitioned by skeletal walls. These are interpret ed as poorly calcified thallus fragments. Neomorphic calcite of the sk eletons contains high concentrations of Sr (average 3702 ppm, range 33 9-10,549 ppm), strongly suggesting that the original Archaeolithophyll um skeleton was aragonitic. Archaeolithohyllum shares morphological, s tructural, and compositional characteristics with Holocene peyssonneli d algae. This evidence substantiates earlier hypotheses that Archaeoli thophyllum may be an ancestral peyssonnelid alga. Calcite cements fill ing skeletal cells include a bright-yellow-CL phase and two subsequent phases of dull-red and dull-yellow luminescence. Mg, Mn, and Fe are e nriched in these calcites, reflecting reducing diagenetic fluids parti cularly enriched in Fe. Cementation of skeletons by ferroan calcite du ring early diagenesis appears to have protected skeletons from dissolu tion and promoted slow rates of neomorphism. Conditions of a nearly cl osed system are constrained by the high retention of Sr in the skeleta l calcite.