Distinctions Between Reefs and Bioherms Based on Studies of Fossil Algae: Mizzia, Permian Capitan Reef Complex (Guadalupe Mountains, Texas and New Mexico) and Eugonophyllum, Pennsylvanian Holder Formation (Sacramento Mountains, New Mexico).


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January 1992


The udoteacean alga Eugonophyllum is well-preserved in the Virgilian Holder Formation, Sacramento Mountains. Original aragonite occurs as a felt-like mesh of needles in algal thalli, mollusc fragments, sponge walls, botryoidal, and isopachous marine cements. In the basal, heavily calcified portion of Eugonophyllum the interior of the thallus is composed of aragonitic felt punctuated by 20 $/mu$m diameter, parallel siphons. In the distal, less calcified portion of the thallus, irregularly arranged siphons are larger. This study substantiates suggested similarities between Pennsylvanian/Permian phylloid algal mounds and Holocene Halimeda mounds. The Permian dasyclad Mizzia is important for paleoecological study in the Guadalupe Mountains, because it has a modern, morphologically similar analog--Cymopolia. Mizzia dominates grainstone, packstone, and wackestone shelfward of the Capitan reef. Infrequent broken segments (average 10%) and discovery of intact and branched sections of thallus suggest that wave energy did not limit diversity. Instead, an arid climate and the euryhaline tendency of modern dasycladacean algae suggest that hypersalinity limited diversity. This conclusion is incompatible with the widely accepted marginal mound hypothesis for deposition of the reef complex, but is compatible with a barrier reef model. The Capitan Formation of the Guadalupe Mountains is dominated by a diverse assemblage of calcareous sponges that acted as framebuilders and encrusting organisms that acted as binders. Reef framework contains internal sediment and marine cement. All aspects of Capitan lithology are analogous to modern reef lithology. Relict spur and groove structures are preserved along the escarpment between Walnut and Rattlesnake canyons. In situ reef framework occurs on spurs, while carbonate sands with small (10 cm) bedforms occur in grooves. Diverse reef boundstone is abruptly transitional shelfward to bioclastic packstone and grainstone. The latter lithofacies probably represents a backreef apron deposit. She

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