Abstract

Mechanical (physical) compaction of sedimentary carbonates is defined as the volume decrease in unlithified to partially lithified sediments due to pressure produced in a column of sediment by the weight of the overlying material. It is accompanied by expulsion of water from grain interstices and concomitant porosity reduction, grain reorientation, grain breakage, and plastic deformation of grains. Early mechanical compaction produces an array of early diagenetic features recognizable at outcrop and hand-specimen scales. Investigation of Lower Ordovician platform carbonates from the El Paso Gp (west Texas), the Arbuckle Gp (Oklahoma), and the Beekmantown Gp (central Appalachians) reveals that early mechanical compactional features are widespread, particularly in platform facies rich in carbonate mud.

Differential compaction is evidenced by: (1) bending of thin beds (cm-scale) around meter-scale early cemented algal bioherms (thrombolites), from both above and below. One observes increased dips of compacted thin beds and lateral thinning over crests and thickening down the sides of incompressible bioherm cores; (2) sedimentary boudinage and pinch-and-swell features in interlayered thin beds (cm-scale) of carbonate grainstone and mudstone. Upon compaction, the carbonate sand-rich layers deformed in a more-competent to brittle manner (cracking and or yielding boudins) while the muddy layers behaved in a ductile fashion (flowing and bending about boudins), indicating that at the time of burial, sand-rich layers were partially to wholly lithified while muddy layers were unlithified. Burrows and skeletal debris within continuous sand-rich layers and isolated boudins are preserved, whereas in muddy layers burrows are deformed and skeletal debris is often broken and/or strongly oriented to the horizontal.

Pervasive compaction in mud-rich facies (mudstones to packstones) in contrast to differential compaction is evidenced by: (1) flattened, tubular burrows, (2) rotation of flat platy allochems towards the horizontal, (3) telescoping of mudcracks and prism cracks, (4) the presence of anastomosing wispy argillaceous seams and the draping of wispy seams about shells. Pervasive compaction in thin beds of early lithified peloidal carbonate sand is evidenced by warping and cracking of thin beds. Notably, coarse-grained skeletal-intraclastic grainstones, ooid grainstones and algal bioherms rarely contain compaction features, attributed to the fact that these lithologies underwent early (marine) cementation.

Studies of Lower Ordovician platform carbonates around North America indicate that this enormous passive margin system is represented by a zoned distribution of major facies belts across depositional strike. A reconstructed facies mosaic for this shallow-marine flat-topped shelf system consists of four major facies belts, from updip to downdip: (1) Tidal Flat Facies - cyclical laminite facies interpreted as shelf lagoon-peritidal deposits, located at the updip edge proximal to exposed cratonic basement; (2) Inner-Shelf Facies - thin bedded grainstone facies interpreted as shelf-lagoon deposits; (3) Outer-Shelf Facies - thrombolitic bioherm facies (Renalcis algal bioherms) interpreted as shelf-lagoon-patch reef deposits; (4) Shelf-Edge Facies - intercalated algal bioherms (Epiphyton framestone reef complexes) and shelf margin ooid grainstone shoals interpreted as a shelf edge reef system.

Outer-shelf and shelf-margin facies suffered relatively minor early mechanical compaction due to the abundance of early-lithified algal bioherms and carbonate grainstones. Tidal flat facies and inner-shelf facies experienced significant early mechanical compaction owing to the higher percentage of mud-rich lithologies. Thus a preferential gradient of volume reduction via early mechanical compaction resulted from platform-scale (km-scale) differential compaction of updip facies (tidal flat, inner-shelf) versus downdip facies (outer-shelf, shelf margin). This is suggested by reconstructed facies mosaics from the central Appalachians which show a convergence of time lines from downdip to updip along depositional strike.

In an attempt to better understand compactional features, quantitative assessment of the mechanical compaction behavior of shallow marine carbonate sediments has been investigated to develop carbonate compaction algorithms and decompaction curves that can be used in stratigraphic restoration, backstripping analyses, and in quantitative models for simulation of carbonate stratigraphy. Deep-water chalks and calcareous oozes are not treated here. Empirical porosity-depth data for carbonate sediments have been compiled and compared with experimentally derived compaction curves to derive appropriate porosity-depth curves for shallow-water carbonate muds and sands. The curves are exponential in form.

This review indicates that mud-rich shallow marine carbonates behave differently than sand-rich, grain-supported sediments with regard to porosity and thickness reduction. Quantitative analysis of compaction behavior in carbonate sediments reveals that, barring early cementation, carbonate muds suffer significant thickness reduction early in their burial history and at shallow depths, for example 50% thickness reduction with about 150 - 200 m overburden. However, primary porosities of carbonate muds, approximately 70-80%, are reduced only to about 42% under that overburden. Thus, the potential exists for significant retention of primary porosity in carbonate muds in the form of microporosity. This indicates that interparticle cementation, with or without chemical compaction, is required to transform muddy carbonate sediments into lithified rock. In the absence of early lithification, carbonate sands compact more slowly than muds. Primary interparticle porosities of about 40-45% can be maintained in the first 200-300 m of burial, with little thickness reduction.