Depositional Model for Freshwater Limestones of Upper Allegheny Group (Pennsylvanian System) of Western Pennsylvania

Suzanne D. Weedman

A depositional model is proposed for the freshwater limestones of the upper Allegheny Group of western Pennsylvania, specifically the Upper Freeport limestone. The model is based on a facies sequence analysis of core logs through the limestone-bearing interval, on isopachs of limestone and sandstone of the same coal-to-coal interval, and on microfacies analysis of the limestone itself.

The facies sequence analysis (a Markov chain analysis with the Harper's significance test) is based on lithologic transitions recorded in 423 core logs from Indiana and Armstrong Counties, Pennsylvania. The statistically significant facies sequence is coal ^rarr black shale ^rarr dark sandy shale ^rarr sandstone ^rarr light sandy shale ^rarr silty claystone ^rarr fireclay ^rarr limestone ^rarr fireclay ^rarr coal. Because sites with thick sandstone (> 6 m) were excluded from the analysis, the resulting sequence is interpreted as the record of cyclic flood plain aggradation. The cycle from the coal to the sandstone is a coarsening-upward sequence and is interpreted as the deposits of overbank flows and crevasse splays. The cycle from the sandstone to the fireclay is a fining-upward equence, deposited after the abandonment of the crevasse system or perhaps the main distributary, being deposits of biogenic lacustrine carbonate, suspended silts and clays from distal flood waters, organic matter, and perhaps wind-blown detritus. Isopachs of the sandstone and limestone show an inverse relationship between the distribution and thickness of the limestone with the underlying sandstone.

The limestone is divided into five microfacies and three microfacies associations. Gray-green silty shale and limestone breccia comprise the shoreline facies assemblage. The shallow water association is a gray-green silty shale, a breccia, a clastic limestone, an encrusted grain (oncolite) microfacies, and a massive pelmicrite. A third assemblage consisting of an encrusted grain microfacies, a pelmicrite, a delicately laminated shaly micrite, and a thinly bedded black shale, is found in the thickest accumulations of limestones and is interpreted as a deeper water deposit.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.