Abstract

One type of distorted ooliths and pisoliths is characterized by structures such as notched and stretched grains, grains connected by narrow apophyses, and series of grains linked in long zigzag chains. Origin of these structures has been attributed to rupture of soft grains in a turbulent environment at the time of deposition or to effects of compaction before cementation. The Plattsburg Limestone includes lenses of distorted pisoliths, as well as genetically related breccias and collapse fabric. Similar fabric, associated with distorted ooliths, is reported in the literature and was observed in other stratigraphic units. The complex diagenetic history of the Plattsburg suggests the following sequence of events: (1) deposition of normal spheroidal pisoliths, (2) cementation by first generation calcite spar, (3) solution to create a high percentage of pisomoldic porosity, (4) compaction that caused local collapse of the cement framework, (5) cementation by ferroan calcite spar, and (6) cementation by coarsely crystalline ferroan dolomite. This sequence varies between outcrops and samples. During compaction the intergranular cement framework failed at narrow interconnections between pisomolds. Slight collapse of the cement framework produced individual distorted pisoliths and chains of linked, distorted pisoliths. These chains delineate solution breccia fragments that encompass one or more undistorted pisoliths. Greater collapse of the cement framework produced "grain inversion." That is, former intergranular cement shards became "grains" which were packed to constitute a new framework. Greatly distorted and now unrecognizable pisomolds became the new "intergranular" spaces which were subsequently filled by cement. This fabric indicates major solution and compaction in limestone and may be the only clue to the original depositional character of the sediment. It may also furnish a clue in the search for porous reservoir facies.

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