Porosity Development and Diagenesis in the Orfento Supersequence and Its Bounding Unconformities (Upper Cretaceous, Montagna Della Maiella, Italy)
M. Mutti, 1995. "Porosity Development and Diagenesis in the Orfento Supersequence and Its Bounding Unconformities (Upper Cretaceous, Montagna Della Maiella, Italy)", Unconformities and Porosity in Carbonate Strata, David A. Budd, Arthur H. Saller, Paul M. Harris
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This paper discusses the development and evolution of porosity associated with different subaerially exposed unconformities of different hierarchical significance in the Orfento Formation (Campanian-Maastrichtian), integrating depositional facies, duration of exposure, and paleoclimate. The unit documents an early aggradational and a later progradational stage and is composed of rudist debris ranging in size from silt to rudite, associated with megabreccias. Two major unconformities, subaerially exposed on the shelf, bound the unit. The prograding sediment bodies consist of shingled, coarse-grained prograding units which contain several minor-order unconformities.
The strata of the Orfento Formation are characterized by high depositional porosity (average 20–30%), which reflects distribution of depositional facies. Primary porosity is increased by moldic porosity (average 15–20%), selectively on aragonitic grains. Early meteoric diagenesis was responsible for aragonite dissolution and precipitation of calcite cements. Calcite cementation is scarce and has a very heterogeneous distribution. The occurrence of moldic porosity and calcite cements is maximum in the progradational units and was controlled by stratigraphy, as related to minor-order unconformities, and by facies distribution. Cathodoluminescence patterns and stable isotopes suggest that precipitation of calcite cement occurred in localized freshwater systems, associated with minor-order erosional unconformities within the progradational unit.
The lower and upper supersequence boundaries were both associated with prolonged subaerial exposure (ca. 5–6 and 8–10 m.y, respectively), but responded differently with respect to porosity formation and preservation. Beneath the lower supersequence boundary, fabric-selective aragonite dissolution and extensive meteoric calcite cementation decreased the porosity. Cathodoluminescence patterns and stable isotopic compositions of the cements indicate precipitation in a stable freshwater system. The unconformity is capped by a calcrete soil typical of semi-arid conditions. Beneath the upper supersequence boundary, silica cements precipitated in lenses with an irregular distribution and do not significantly reduce the porosity. Cement textures and distribution suggest that silica precipitated either in a vadose/phreatic meteoric environment associated with a silcrete soil, typical of semi-arid conditions, or in the mixing freshwater/sea water zone.
Although not directly responsible for generating porosity, the unconformities bounding the Orfento Formation are significant, as they act as breaks on the regional distribution of porosity.