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Abstract

This chapter deals only with the diagenesis of calcitic components of limestones — the formation of dolomite, silica and other minerals is covered in subsequent chapters.

Burial diagenesis represents alteration that occurs below the zone of near-surface water circulation (i.e., below the meteoric phreatic mixing zone or below the zone of active seawater circulation). Burial diagenesis plays a major, often THE major role, in the diagenesis of sediments from the point of view of length of time spent in that environment (commonly millions to hundreds of millions of years) and in terms of porosity changes.

Burial diagenetic features are among the most difficult to identify with assurance for a variety of reasons: 1. the transition between surficial (meteoric or marine) pore fluids and burial realm fluids is ill-defined, variable, indistinct, and rarely well understood (so often it is not clear where surficial diagenesis ends and burial diagenesis begins); 2. the burial realm is “out of sight and out of mind”, which means that the processes and products formed there can only be remotely and incompletely observed; 3. deposits found in the burial diagenetic zone must have passed through marine or meteoric diagenesis zones (or both), making it difficult to determine precisely whether a particular fabric is exclusively a product of burial diagenesis.

Several factors mitigate for and against extensive burial diagenesis. Burial diagenesis is hindered by water circulation rates that typically are lower in subsurface settings than in near-surface environments (because of slower circulation mechanisms as well as reduced permeabilites). Higher temperatures and increased pressures at depth, however, tend to accelerate many diagenetic processes. Elevated pore-fluid pressures (reducing grain-to-grain stress) and early hydrocarbon input retard mechanical and chemical burial diagenesis.

Statistical evidence (top diagram, facing page) indicates that burial diagenesis is very important in porosity reduction. Most rocks, especially limestones, show a consistent loss of porosity with progressive burial.

The burial-diagenetic zone is characterized by a mix of physical and chemical diagenetic processes, most leading to porosity destruction, but in some cases yielding net porosity increases.

Burial-related mechanical compaction features include dewatering structures, compactional drape around shells and nodules, plastic or brittle grain deformation, and fractures.

Embayed grain contacts, fitted fabrics, solution seams, and stylolites are common chemical compaction features that form mainly in burial settings.

Burial-stage calcite cements are low-Mg calcite. Most crystals grew slowly, and thus are relatively imperfection-free, clear (limpid) crystals as compared with marine and even meteoric precipitates. Morphologies include bladed, prismatic overgrowths of earlier cement crusts; equant calcite mosaics; drusy calcite mosaics with crystal sizes increasing toward pore centers; very coarse to poikilotopic blocky calcite spar; and outer, inclusionpoor stages of syntaxial overgrowths. Although these fabrics are common in mesogenetic precipitates, none is unequivocally or exclusively formed during burial diagenesis. Bathurst (1971, and 1975,) and Dickson (1983) provide more detailed discussion of geometric criteria for recognition of burial cements.

Many burial-stage cements are formed from relatively reducing pore fluids and, thus, may have elevated Mn2+ and Fe2+ contents. The iron is easily detected with staining techniques; the manganese/iron ratio is qualitatively identifiable with cathodoluminescence (CL). The typical CL pattern found in burial stage calcite cements is a transition from nonluminescent to brightly luminescent to dully luminescent response. This is generally interpreted as a transition from oxidizing (pre-burial or early burial) conditions with little or no Mn2+ or Fe2+ incorporation into the calcite lattice, to reducing conditions with Mn2+ and Fe2+ incorporation, and finally to reducing conditions in which Fe2+ availability and incorporation exceed Mn2+ availability and incorporation. More complex CL stratigraphies, however, are common.

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