So. Moshier et Bl. Kirkland, IDENTIFICATION AND DIAGENESIS OF A PHYLLOID ALGA, ARCHAEOLITHOPHYLLUMFROM THE PENNSYLVANIAN PROVIDENCE LIMESTONE, WESTERN KENTUCKY, Journal of sedimentary petrology, 63(6), 1993, pp. 1032-1041
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.