A phylloid algal-mound complex is a local to subregional thickening of limestone attributed chiefly to the presence of a distinctive suite of rock types containing leaflike or phylloid algae. Twenty-three such mound complexes are present at or near the southern ends of most limestone units in Missourian and lower Virgilian (Upper Pennsylvanian) rocks exposed in eastern Kansas, northeastern Oklahoma, and northwestern Missouri. Mound complexes are composed of two facies, (1) the mound, consisting primarily of massive algal calcilutite to algal sparite, and (2) mound-associated facies, consisting primarily of thin- and cross-bedded skeletal and oolitic calcarenite capping and flanking the mound. Overlying shale beds thin across the tops of mound complexes. Most mound complex s grade northward into thinner, more diversely fossiliferous, open marine limestone beds, and grade abruptly southward into thin limestone beds and lenses and thick terrigenous clastic strata.

Pennsylvanian phylloid algae are comparable with Holocene calcareous codiacean green algae and coralline red algae, and flourished in shallow sunlit water where they were sediment suppliers and stabilizers. Mounds probably began on topographic highs favorably situated between a region of great clastic influx and the open sea, and grew as the algae proliferated and produced sufficient sediment to compensate for subsidence. Mound growth allowed the algae to continue to flourish in their optimum sunlit environment. Stacking of mound complexes may reflect positive topographic influence of underlying mounds on the sea bottom, and shifts in the stackings probably resulted from shifts in northward extent of the clastic influx.