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Abstract

The geologic literature on Pennsylvanian limestones is replete with references to algal banks, algal buildups, and algal mounds. Phylloid algae are alleged to have been the principal cause for these positive topographic prominences. This concept, from its advent during the late 1950s, has been further popularized by speculation that such banks, buildups, or mounds should be potentially recognizable and mappable petroleum reservoir targets. A reappraisal is warranted.

Suppositions made concerning the growth habits and the paleogeographic settings of the phylloid algae initially were based on an assumed analogy to Holocene marine grasses, and have led to either conclusions or inferences such as: (1) phylloid algae grew as “meadows” or “thickets” capable of baffling and trapping significant amounts of carbonate sediment, mainly carbonate mud, and thereby seeding the growth of banks, buildups, or mounds; (2) the binding action of some forms of phylloid algae caused development of depositional topographic rises; and (3) the areal proximity to shelf edges of many so-called algal banks, buildups, and mounds indicates a direct relationship between such shoals and large-scale depositional-slope breaks.

These conclusions and inferences are either misleading or erroneous. By analogy with modern Halimeda, the phylloid algae probably contributed considerable volumes of carbonate mud and larger detritus to the sediment store; this is considered to be their principal contribution to carbonate sedimentation. Lamellar growth forms of phylloid algae appear to have bound loose sediment into seafloor irregularities measured in centimeters. We know of no places in the subsurface where phylloid algae controlled the development of positive depositional topography of reservoir-target scale. Abundant remains of phylloid algae are common in some good reservoir rocks. Abundant remains of phylloid algae characterize a far greater volume of nonpermeable limestone of poor reservoir character. Insofar as can be demonstrated or inferred, shape and porosity-permeability characteristics of such limestones result from physical processes affecting the algal remains, and not from algae controlling physical processes. The mere presence of phylloid algae should not excite the explorationist more than that of brachiopods, fusulinids, or any other grain type. If it is evident that presence of phylloid algae enhances secondary porosity, the interest of the explorationist will be aroused.

Although the geologic literature on phylloid algae champions the importance of biologic processes relative to physical processes, objective outcrop observations and subsurface data indicate rather that primary and secondary physical processes affecting fossils as a whole are more important. As regards the phylloid algae, we have not observed even one example of their fossil record which unequivocally records growth attitude. Therefore, we have never seen an unequivocal record of control of any physical process by what may have been the growth habit of any phylloid algae exclusive of the lamellar growth forms. The least equivocal example of phylloid algae preserved in a probably upright growth position shows no evidence of resultant depositional topography. The principal conclusion of this report is that the organisms grouped as phylloid algae were not builders of depositional topography, but rather were a source of building material.

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