Zagadka dolomitów

Marek Narkiewicz



The long disputed "dolomite question" consists of two broad problems: 1) How do vast volumes of (particularly sedimentary) dolostone form? and 2) Why there are more dolostones in ancient carbonates than in younger ones?
Some clues to the first part of the dolomite puzzle are offered by the resu.lts of extensive studies of recent and ancient marine carbonates. These results seem to show that actually there are four geologically significant diagenetic environments of dolostone formation: 1) marginal meteoric environment influenced by sea-derived brines in subsurface of carbonate banks and platforms ("dorag" model), 2) submarine eogenetic environment (volumefrically insignificant for dolostone formation), 3) marginal marine sabkha in arid tropical climate, 4) deeper subsurface (mesogenetic) zone (see Fig. 1).
In recent and ancient open marine facies the penecontemporaneous dolomite (i.e. eogenetic) is but rarely found, usually in small amounts as dispersed euhedral crystals or concretions. The marine sabkha deposits appear to be volumetrically much more important among the eogenetic, facies controlled dolostones. Thus the occurrence of a dolostone lithofacies in the stratigraphic column should be controlled by two primary factors influencing the establishment and extent of the sabkhas: 1) degree of continental flooding i.e. total area of epeiric seas (12, 20, 23) and 2) distribution of epeiric seas relative to the arid tropical climatic zones. Both these factors do not seem to show any declining or rising trend through the entire Phanerozoic. Consequently, also the sedimentary dolostone contribution to the total amount of global sediments should oscillate without any definite tendency. The larger proportion of dolostones in ancient rocks may be explained not as much by more extensive dolostone lithofacies in the past but rather as a cumulative effect of pervasive meso- and telogenetic replacement of older limestones. Considering dolomitization as a rare event (see 10) one may assume that the older the limestone the larger number of trials, i.e. opportunities to be dolomitized (see also 11). Consequently, there is a better chance to encounter secondary dolostone in the older geologic system than in the younger one (see schematic Fig. 3). If we should be able to plot two separate curves showing the global contributions of both the major varieties of dolostone, . i.e. irregular, facies controlled eogenetic versus declinig, pervasive meso and telogenetic one, and then sum them np, the final effect would be most probably comparable to the presently observed curve, e.g. that from Fig. 2.

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