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Abstract
Chapter from: M
63: Unconformities and Porosity in Carbonate Strata
Edited By
D.A. Budd, A.H. Saller, and P.M. HarrisAuthor:
M. Mutti Carbonate Reservoirs
Published 1995 as
part of Memoir 63
Copyright © 1995 The American Association of Petroleum
Geologists
All Rights Reserved |
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Chapter 7
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Porosity Development
and Diagenesis in the Orfento Supersequence and Its Bounding Unconformities
(Upper Cretaceous, Montagna Della Maiella, Italy)
M. Mutti
Swiss Federal Institute
of Technology
Zurich, Switzerland
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ABSTRACT
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
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