Shallow‐burial dolomite cement: a major component of many ancient sucrosic dolomites

Abstract

Dolomite cement is a significant and widespread component of Phanerozoic sucrosic dolomites. Cements in dolomites that were never deeply buried are limpid, have planar faces (non-saddle forms), often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores. Five samples of sucrosic dolomites, interpreted as having had mostly limemudstone or wackestone precursors in four carbonate aquifers, provide insights into the abundance of planar cements in sucrosic dolomites. Such cement comprises 11% to 45% (32% mean) of peritidal to sub-tidal dolomites on an outcrop in the Edwards aquifer (Early Cretaceous) of central Texas; 19% to 33% (25% mean) of ramp dolomites in the Hawthorn Group (Oligo-Miocene) and 50% to 70% in shelf dolomites of the Avon Park Formation (Eocene) in the Upper Floridan aquifer of sub-surface peninsular Florida; 18% to 45% (32+% mean) of sub-tidal shelf dolomites in quarry sections of the Burlington-Keokuk Formation (Early Mississippian) in south-eastern Iowa; and 18% to 76% (50% mean) in shallow cores and outcrops of outer-shelf dolomites from the Gambier Limestone (Oligo-Miocene) of South Australia. Backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening, pore-space reduction, induration and general 'maturation' of these dolomites. Most pre-Holocene dolomites are multiphase crystalline rocks composed of: (i) seed crystals or 'cores'; (ii) crystal cortices that concentrically enlarged the cores; and (iii) free-space, syntaxial precipitates of limpid cement around the crystals. Remaining CaCO 3 grains and micrite can be replaced by dolomite, but typically they are dissolved between stages (ii) and (iii), creating systems of intercrystal and mouldic pores typical of sucrosic dolomites. Networks of cement overgrowths, aided by water-filled pore systems under hydrostatic to lithostatic pressure, are judged to slow or prevent compaction in sucrosic dolomites. It can be argued that cortex growth involves both replacement of CaCO 3 particles and microcementation of their interparticle pores. This interpretation, and the abundance of cements in so many dolomites, would obviate the controversy over the volumetrics of 'replacement dolomitization'. Limpid, planar and syntaxial dolomite cements of early diagenetic origin are interpreted to have precipitated from clear pore waters, at low temperatures (<30 to 35°C) and shallow burial depths (<100 m), in water-saturated networks of dolomite 'silt' and 'sand'. Cements in many dolomites in island and continental-aquifer systems appear to result from event-driven processes related to sea-level highstands. Cementation events can follow 'replacement 1

Hydrothermal dolomitization of basinal deposits controlled by a synsedimentary fault system in Triassic extensional setting, Hungary

“…The CL pattern indicates that the euhedral medium-sized crystals nucleated as replacive crystals and are enlarged successively by further replacement and in the latest phase as syntaxial overgrowth cement, similarly to the alteration stages described by Choquette and Hiatt (2008). Laminae of aphanocrystalline clot-clusters have been likely formed via shallow subsurface mineralization of bacterial biofilms (Riding 2000).…”

The role of bacterial sulfate reduction during dolomite precipitation: Implications from Upper Jurassic platform carbonates

“…2A) the cloudy-center-clear-rim texture (Tucker & Wright, 1990;Budd, 1997), with Na-, Sr-and S-rich cores and partly Fe-rich rims ( Fig. 4D and Electronic Appendix B), suggests that the dolomite cement has been formed by the replacement of pre-existing magnesian calcite cement, and subsequent continuous growth of dolomite crystals under reducing conditions (Blake & Peacor, 1985;Budd, 1997;Reinhold, 1998;Warren, 2000;Choquette & Hiatt, 2008;Rameil, 2008). The results from field studies support such a dolomitization pathway in open marine, high-energetic environments such as that of recent ooid shoals, as early void-filling HMC cement is usually dolomitized with good fabric retention, in contrast to fine-grained micrite (Sibley, 1982;Tucker & Wright, 1990).…”

Early diagenetic dolomitization and dedolomitization of Late Jurassic and earliest Cretaceous platform carbonates: A case study from the Jura Mountains (NW Switzerland, E France)

“…The cloudy center often represents penecontemporaneous or (very) early post-depositional dolomite precipitation, while the clear rim forms by continuing growth during later diagenesis (Machel, 2004). Alternatively, the clear rim could also be interpreted as a low-temperature, early-burial syntaxial overgrowth cement (Choquette and Hiatt, 2008). …”