Controls on Composition and Distribution of Pennsylvanian and Lower Permian Mounds and Reefs

Gregory P. Wahlman
BP America, Houston, TX

Pennsylvanian-Lower Permian organic carbonate buildups consist of a spectrum of mound and reef types. Community evolution altered buildup compositions through time. Paleoenvironmental factors controlled buildup compositions through space. Of prime importance were seawater temperature and light availability, which are related directly to palaeolatitude, water depth, and growth position. Other important paleoenvironmental factors were frequency of glacioeustatic sealevel fluctuations, and seawater chemistry, the latter being influenced by tectonic activity, continental glaciation, paleoclimate, and terrestrial sediment and organic input.

Pennsylvanian tropical shallow-water buildups were mostly bafflestone mounds composed of low-diversity opportunistic communities dominated by phylloid algae, which could rapidly recolonize during high-frequency sealevel fluctuations. In deeper-water flank and intermound areas, calcisponges, laminar red algae and bryozoans built small boundstone patch reefs. During the Early Permian (Wolfcampian, Asselian-Sakmarian), those two buildup communities integrated and constructed larger bafflestone-boundstone platform-margin reefs composed of frameworks of erect phylloid algae, calcisponges, and fenestrate and branching bryozoans, that were encrusted by Tubiphytes, laminar red algae, and fistuliporid bryozoans. Syndepositional originally-aragonitic botryoidal radial fibrous cements contributed to the cohesion and mass of many buildups. Microbial (automicrite) and peloidal cementation were often significant.

In subtropical to temperate paleolatitudes, mid-Pennsylvanian to Lower Permian shallow-water buildups were constructed by Palaeoaplysina and varying amounts of intermixed phylloid algae. In deeper-water upper slope areas, buildups were constructed by fenestrate bryozoan- Tubiphytes-radiaxial cementstones. At intermediate depths, the two buildup communities intermixed, and fenestrate bryozoan-Tubiphytes reefs commonly shallow upward into Palaeoaplysina facies. In deep-water lower slope-to-basin areas, buildups were dominated by siliceous sponges, sometimes with intermixed brachiopods and bryozoans. Many cooler-water calcitic biotic elements are also present in warm-water reefs, but there they are usually diluted by the more prolific tropical aragonitic biota.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005